Mechanical Analysis of Short Fiber Composites Manufactured by Inverted Stereolithography

by

Ignace (Joe) Brazda

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Ignace Brazda 2019

ii

Mechanical Analysis of Short Fiber Composites Manufactured by

Inverted Stereolithography

Ignace (Joe) Brazda

Master of Applied Science

Chemical Engineering & Applied Chemistry

University of Toronto

2018

Abstract

Additive manufacturing, or “3D printing”, is a rapidly expanding technology that produces

components with complex geometry using computer aided design software. Unfortunately, an

intrinsic lack of mechanical properties inhibits 3D printed parts from being used in many load

bearing applications. In this study, glass fibers were introduced to reinforce an acrylic resin to

increase the elastic modulus of inverted stereolithography (ISLA) printed specimens. The elastic

modulus of the resultant short glass fiber composites increased with increasing fiber content, up to

10 vol% fiber addition. However, flow induced fiber alignment caused the high aspect ratio

reinforcements to orient transversely to the applied load. This significantly limited the

contribution of the fibers to the overall modulus of the composite. A variety of flow diversion

techniques were implemented, but the ISLA workflow did not produce a fiber reinforced part that

had a superior elastic modulus to that of existing particle reinforced composites.

iii

Acknowledgments

This work would not have been possible without the supervision of Professor Mark T. Kortschot.

My sincerest gratitude for his guidance, patience and instruction for the duration of this research.

To Shiang Law for the technical support, idea generation, and lab organization skills that made

experimentation so easy and efficient.

To other members of the Advanced Materials Lab, Fady Mettias and Jin Choi for always

volunteering to see practice presentations, generous academic support, and companionship.

Finally, to friends and family for unwavering support. I am exceptionally grateful.

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Contents

Acknowledgments.......................................................................................................................... iii

Introduction .................................................................................................................................1

1.1 Stereolithography 3D Printing .............................................................................................1

1.1.1 Fundamentals ...........................................................................................................1

1.1.2 Inverted Stereolithography.......................................................................................3

1.2 Composite Materials ............................................................................................................5

1.2.1 Fundamentals ...........................................................................................................5

1.2.2 Short Glass Fiber Composites ..................................................................................6

1.3 Objectives ............................................................................................................................6

Literature Review ........................................................................................................................8

2.1 Short Fiber-Filled Composites .............................................................................................8

2.1.1 Fiber Materials and Length ......................................................................................8

2.1.2 Mechanical Performance .......................................................................................11

2.1.3 Modulus and Strength Theories .............................................................................13

2.1.4 Stereolithography Fiber Alignment Attempts ........................................................17

2.1.5 Inverted Stereolithography Glass Fiber Reinforced Composites...........................18

2.2 Conclusions ........................................................................................................................18

Experimental Procedures ..........................................................................................................20

3.1 General strategy .................................................................................................................20

3.2 Macro-Simulations .............................................................................................................20

3.3 Specimen Development .....................................................................................................20

3.4 Materials Preparation .........................................................................................................21

3.5 Mechanical Testing ............................................................................................................22

3.6 Mechanical Characteristics Analysis .................................................................................24

Results & Discussion ................................................................................................................26

v

4.1 Specimens ..........................................................................................................................26

4.1.1 Geometric Consistency ..........................................................................................26

4.1.2 Design and Fabrication Parameters .......................................................................29

4.2 Fiber Orientation ................................................................................................................30

4.2.1 Shear and Extensional Flow in ISLA.....................................................................31

4.2.2 Quantification of Fiber Distribution ......................................................................39

4.2.3 Variation in Fiber Length.......................................................................................44

4.3 Mechanical Testing ............................................................................................................45

4.3.1 Elastic Modulus of Neat Resin ..............................................................................45

4.3.2 Elastic Modulus of Composites .............................................................................47

4.4 Predicting Elastic Modulus ................................................................................................50

Practical Implications, Limitations and Conclusions ................................................................55

5.1 Applications & Limitations of Composite Manufacturing ................................................55

5.2 Conclusions and Future Work ...........................................................................................57

References .................................................................................................................................58

Appendices ................................................................................................................................64

7.1 Elastic Modulus Calculations ............................................................................................64

7.2 Predicting Elastic Modulus ................................................................................................66

7.2.1 Calculating the Fiber Orientation Factor ...............................................................66

7.2.2 Obtaining Exact Volume Percentage .....................................................................67

7.3 Settling Experiment ...........................................................................................................68

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Figures

Figure 1: The separation of layers in an FDM printed part [3]. ...................................................... 1

Figure 2: A conventional setup for a stereolithography system. 1) Sweeper 2) Prototype 3) Resin

4) Build Platform 5) Elevator 6) Resin Tank 7) Laser Beam 8) XY Mirror 9) Lenses 10) UV

Laser. [4] ......................................................................................................................................... 2

Figure 3: An ISLA system setup. 1) Prototype 2) Scaffolding and supports 3) Resin 4) Build

platform 5) UV Laser 6) Mirror galvanometers 7) XY Scanning mirror 8) Transparent resin tank

base 9) Resin tank. [4] .................................................................................................................... 3

Figure 4: The Form 2 ISLA printer used for this research. ............................................................ 4

Figure 5: Identifying an arbitrary scan line within a sample [53] ................................................. 15

Figure 6: Fibers at different orientations will show different elliptical shapes at a specific cross

section. [44]................................................................................................................................... 16

Figure 7: The three main tensile coupon designs. Design 1: Five samples in a parallel

configuration. Design 2: Five samples with flow diversion islands. Design 3: Large rectangle to

be transversely cut......................................................................................................................... 21

Figure 8: Tensile testing setup of a short glass fiber reinforced composite. ................................. 23

Figure 9: The cantilever elastic modulus setup for Design 3, transversely cut specimens. .......... 24

Figure 10: Three standard orientations for ISLA produced tensile coupons. ............................... 26

Figure 11: Default orientation of tensile coupons using the PreForm software. .......................... 27

Figure 12: The orange standard tray shown bending as the build platform descends into the resin.

....................................................................................................................................................... 28

Figure 13: Snapshots of scaled-up experiment. As the stills progress from A to D, the

polycarbonate sheet is being pressed further into the resin bath. ................................................. 32

vii

Figure 14: A macro-experiment progression as a polycarbonate square descends into a mock

resin. .............................................................................................................................................. 33

Figure 15: The descending build platform causes a non-uniform velocity profile, which can alter

the orientation of a fiber. ............................................................................................................... 34

Figure 16: The changing Z position of the build platform causes increase volume displacement of

the resin. This also leads to increased lateral velocity. ................................................................ 35

Figure 17: The top view (looking down “through” the build platform) of an infinitely long

specimen. The velocity of the resin increases towards the nearest boundary of the specimen

causing fiber orientation. .............................................................................................................. 36

Figure 18: Blue arrows showing the direction of the resin flow during the printing process....... 37

Figure 19: A plan and elevation view of a 3D reconstructed fiber filled composite. ................... 38

Figure 20: Fibers aligned at 0° lie in the direction of the force. Fibers at 90° will lie transversely

to the applied load. ........................................................................................................................ 39

Figure 21: Slice of 5 vol% sample and the average fiber orientation counts for the specimen. ... 39

Figure 22: Slice of 10 vol% sample and the average fiber orientation counts for the specimen. . 40

Figure 23: Slice of Design 2 specimen at 5 vol % with the average fiber orientation percentage.

....................................................................................................................................................... 41

Figure 24: Post processing for tensile specimens from Design 3 samples. .................................. 42

Figure 25: Slice of Design 3 specimen at 5 vol% with the average fiber orientation percentage. 43

Figure 26: Optical microscopy of fibers at A) initial B) post mixing and C) post curing stages. 44

Figure 27: Elastic modulus results to confirm anisotropy. ........................................................... 45

Figure 28: Variation in elastic modulus due to specimen position on the standard resin tray. .... 47

Figure 29: Correlation between increasing fiber content and elastic modulus. ............................ 48

viii

Figure 30: Elastic modulus of various tensile coupon designs. .................................................... 49

Figure 31: Elastic modulus of re-oriented specimen based on position before cutting. ............... 50

Figure 32: Comparison between theoretical and experimental elastic modulus results. .............. 51

Figure 33: Design 4 specimen are cut from a circular base using the tile saw. ............................ 54

Figure 34: A) Depiction of a typical gasket used for a flange. B) The desired print orientation for

an ISLA produced gasket. ............................................................................................................. 56

Tables

Table 1: Summary of Fiber Characteristics in Reinforced Composites ......................................... 9

Table 2: Various 3D printing studies comparing how fiber characteristics effect the increase in

fiber strength ................................................................................................................................. 11

Table 3: Thickness values of adjacent samples using standard resin tray. ................................... 28

Table 4: Fiber length at 3 different stages of processing. ............................................................. 44

Table 5: Effects of ageing on the elastic modulus ........................................................................ 46

Table 6: Comparison of Krenchel factors and accompanying elastic modulus based on the design

and position of the tensile coupon. ............................................................................................... 53

Table 7: Results of Krenchel calculation. ..................................................................................... 67

1

Introduction

Additive manufacturing, or 3D printing, is a process whereby material is added in layers to make

objects from a 3D computer-aided-design model [1]. Lately, there has been great interest in

additive manufacturing technologies, largely attributed to the recent expiration of the original 3D

printing patents [2]. As this area of research is rapidly expanding, there is an opportunity to explore

a wide range of markets including healthcare, aerospace, construction, and small batch

manufacturing. This technology is extremely useful for small-scale production, but a lack of

mechanical performance and durability inhibit integration into a system as a load-bearing part. In

this study, we attempted to increase the elastic modulus of 3D printed parts by creating fiber

reinforced composites.

1.1 Stereolithography 3D Printing

1.1.1 Fundamentals

The most common type of 3D printing is Fused Deposition Modelling (FDM). In FDM,

thermoplastic polymers are extruded through a moving nozzle to build layers of a specimen. This

process is relatively fast, efficient, and user friendly, but does have limitations. Geometrically, the

models cannot have fine features as the tolerance is controlled by the diameter of the nozzle

opening. Additionally, the layers are simply placed on top of each other, and allowed to cool. This

does not strongly adhere the layers causing the prints to be anisotropic. Because of the lack of

Figure 1: The separation of layers in an FDM printed part [3].

2

bonding between the layers, delamination, or layer separation, is the most common failure

mechanism. Figure 1 shows delamination in a 3D printed part [3].

An alternate technology, stereolithography (SLA), was the original polymer additive

manufacturing system. Stereolithography uses light to initiate free radicals in a liquid resin

containing monomer chains. The free radicals will bond the chains in crosslinks which cures the

resin, creating thermosetting polymers. In a conventional SLA process, a build platform is

submerged in a resin bath. To begin, the first layer is cured directly onto the build platform by a

laser above the tank. The amount of resin between the build platform and the top of the resin bath

defines the layer height. Subsequent layers are built on top of the initial layer by the platform

descending further into the resin bath. Between every layer, a wiping mechanism levels the resin

to maintain consistent layer thicknesses. This setup of an SLA system is shown in Figure 2 [4].

A series of mirrors and lenses guide the laser to trace the outline of the part in that layer, and then

rasters to fill in each layer. The layers in SLA are under-cured creating a “green state” solid that

still contains active bonding sites. The part will remain in “green state” unless it is post-cured.

The post curing occurs in a separate machine and subjects the print to additional UV, covalently

bonding the layers together with crosslinks, making the part isotropic.

Figure 2: A conventional setup for a stereolithography system. 1) Sweeper 2) Prototype 3) Resin 4)

Build Platform 5) Elevator 6) Resin Tank 7) Laser Beam 8) XY Mirror 9) Lenses 10) UV Laser. [4]

3

The geometric tolerance of an SLA machine is defined by the spot size of the laser. A smaller spot

size correlates to finer resolution. Thus, the stereolithography system has inherent advantages for

manufacturing high strength prototypes, particularly ones requiring good geometric tolerance and

uniform mechanical properties.

1.1.2 Inverted Stereolithography

As discussed above, stereolithography is a very attractive additive manufacturing option.

However, there are some drawbacks. Predominantly, a large amount of resin is required to fill the

tank. The build volume is determined by the size of the tank. So, as larger build volumes are

needed, the amount of resin required increases. As a result, the footprint of the machine is large.

Additionally, the surrounding resin must be either treated or thrown out, as it contains partially

cured material from the edge of the laser tracing. To avoid print malfunctions, the partially cured

coagulates must be removed. To appeal to a greater number of consumers, an SLA machine with

a small footprint and basic power voltage requirements is preferable.

Inverted stereolithography (ISLA) operates on the same principles as a conventional SLA system,

with one main exception – the parts are manufactured inverted on the build platform. Instead of

the laser scanning the surface of the resin tank, an ISLA system cures from the bottom of the

Figure 3: An ISLA system setup. 1) Prototype 2) Scaffolding and supports 3) Resin 4)

Build platform 5) UV Laser 6) Mirror galvanometers 7) XY Scanning mirror 8)

Transparent resin tank base 9) Resin tank. [4]

4

machine through a transparent window at the base of the resin tray. The build platform descends

into the resin to a height such that the distance between the base of the resin tray and the build

platform is equivalent to the layer thickness. The laser will raster an initial layer, before

incrementally ascending out of the resin tank. Each increment of ascension will be equal to the

desired layer thickness. Thus, the final print is manufactured inverted, before being removed from

the build platform. Inverted SLA systems do not have the benefit of a viscous resin supporting the

print in its specific orientation. Therefore, scaffolding and support structures are printed so the

sample does not shift positions during production. An overview of the ISLA process can be found

in Figure 3 [4].

A new ISLA printer on the market is the Form 2, manufactured by Formlabs®. The Form 2 has

advanced features that create a more streamlined workflow. For example, a resin cartridge is

installed directly to the back of the machine where a sensor in the resin tray determines when the

material volume is too low and automatically dispenses the appropriate amount. Therefore, the

manufacturing process does not need to be paused to add more resin, reducing the possibility of

defects in the final product. Additionally, a wiping mechanism was developed to agitate the resin

in between the curing of each layer. A sheet of polydimethylsiloxane (PDMS) on the base of the

resin tray aids in efficient layer separation from the tray. The wiping mechanism clears the PDMS

layer to ensure that no debris is obstructing the laser path. Formlabs has developed a large material

pipeline, with each resin having various colours, mechanical properties, and applications. One of

Figure 4: The Form 2 ISLA printer used for this research.

5

these resins is, Rigid Resin®, contains glass particles for reinforcing. The particle reinforced resin

does maintain isotropy but limits the mechanical properties of the final prototype. No fiber

reinforced resins have been developed. The Form 2 is pictured in Figure 4.

The Form 2 is not the only ISLA printer available. However, in comparison to other competitors

(Nobel Series by XYZ Printing, Peopoly Moai, Asigo Pica 2), the Form 2 provides the highest XY

resolution (140 µm), and is the only ISLA printer to include a wiping mechanism.

1.2 Composite Materials

1.2.1 Fundamentals

A composite material is a combination of at least two constituents. The composites behaviour is

based on the physical, chemical, or mechanical properties of the individual parts. The extent to

which each constituent contributes to the overall sample properties is based on the interaction

between the materials and what proportions they are present in. For the purposes of this research,

there is particular interest in the mechanical properties of composite materials. Composites are

used in a variety of products and applications including: aircrafts, bone casts and sporting

equipment.

The properties of a composite can be tailored to suit the applications. For example, fiberglass is a

resin combined with glass fibers. Fiberglass will exhibit properties of both the fibers and the

matrix resin. By varying the quantities of the constituents, the mechanical properties of the

fiberglass can be tailored for a specific application. The orientation and length of the fibers also

effect the mechanical behaviour.

There is an equation that predicts the elastic modulus of a composite. The Rule of Mixtures states

that the elastic modulus (Equation 1.1) of a composite is the summation of an individual materials

properties factored by the volume fraction they are present in the product. This simple equation

allows for reverse engineering, so if the final desired modulus of a composite is known, the exact

proportions of each material can be calculated.

𝐸𝐶 = 𝐸1𝑣1 + 𝐸2𝑣2 + ⋯ (1.1)

6

1.2.2 Short Glass Fiber Composites

A fiber reinforced plastic contains three basic phases: the fiber, the matrix material, and the

interface where adhesion occurs between the fiber and the matrix. In order for the composite to

withstand higher loads, the fiber must be stronger than the base matrix. Additionally, good

adhesion between the fiber and the matrix will yield superior mechanical performance. There are

a variety of fiber materials that are used including: carbon, metal, cellulose, and glass. Fibers come

in 3 main forms: continuous, woven, and chopped. Continuous fibers will span the entirety of the

composite. Woven fibers are manufactured into a mat and provide increased two-dimensional

strength. Chopped fibers (often referred to as short fibers) are discontinuous, with multiple fibers

spanning across a sample.

Fibers increase the modulus of a polymer matrix when they have higher stiffness. Assuming there

is sufficient adhesion at the interface, the matrix will deform around the fiber when a stress is

applied. Additionally, the orientation and length of the fiber will alter the deformation pattern.

For this research, short glass fibers were used to reinforce the standard acrylic resin with the Form

2. Short glass fibers are most often used to reinforce thermoplastic materials via injection molding.

They have high strength and modulus, and bond well to a variety of matrix materials. However,

carbon and Kevlar fibers have superior mechanical properties, but both have disadvantages,

particularly when integrating them into an SLA system. For starters, carbon and Kevlar fibers are

more expensive than glass. Additionally, both carbon and metal fibers will absorb 405 nm light,

which could cause difficulties when curing the composite. Short glass fibers come in a variety of

sizes, and if necessary, can easily be coated to ensure adhesion to the matrix. Most importantly,

the glass fibers will not interfere with the intensity of the UV laser. The laser is still able to pass

through the fibers, allowing the surrounding resin to cure.

1.3 Objectives

Currently, ISLA printed prototypes lack the strength required to be used in many load-bearing

applications. Given the benefits of ISLA for small-scale production (economical, small footprint,

high geometric tolerance, isotropic properties), there is a desire to increase the strength and elastic

modulus of photo-active thermosetting polymers. Thus, in this study, we attempted to create

specimen with superior mechanical properties by using ISLA to manufacture short glass fiber

7

composites. To our knowledge, there have been few attempts to reinforce ISLA parts, and no

attempt to use short glass fibers to increase the strength and elastic modulus. Additionally, there

have been no studies analyzing the flow of the resin during the ISLA process, and how it affects

the orientation of the fibers.

8

Literature Review

2.1 Short Fiber-Filled Composites

For many years, researchers have been experimenting with fiber reinforced plastics to increase

mechanical properties. A wide range of fibers have been used in previous experiments ranging in

size, aspect ratio, and material type. The fibers can be added in specific amounts to achieve a

desired strength or elastic modulus. Additionally, a variety of techniques are used to integrate

fibers into a system. Fibers can be coated or chemically treated to increase bonding to the matrix

material. All of these parameters will cause specific changes in mechanical properties. A variety

of texts have been published on short fiber reinforced composites. Load Bearing Fiber Composites

[5], provides a general overview of composite behaviour, including short fiber reinforcement

theory. Additionally, a book by Fu, Lauke and Mai [6] looks more at the mechanical theory of

short fiber-filled plastics. Herein, a literature review of fiber characteristics, fiber introduction

methods, and resulting mechanical properties is presented, as a foundation for understanding how

fiber addition might be beneficial in 3D printing.

2.1.1 Fiber Materials and Length

A wide variety of fiber materials are used to produce fiber-reinforced composites. The choice of

fiber depends on the composite manufacturing method, the desired mechanical and physical

characteristics, and the application of the product. Common fiber types include: glass, wood,

Kevlar and carbon. Glass fibers provide high strength and chemical resistivity. A review paper

by Sathishkumar et al. [7] explains in great detail the various methods for producing glass fiber

composites (namely extrusion and injection molding), and confirms the improved mechanical

properties of these products. Wood fibers are common, naturally occurring fibers used to reinforce

polymers. Wood fibers are more eco-friendly than other types of fibers, as the bulk of wood is

made up of organic cellulose and lignin [8]. Wood fiber composites generally exhibit high water

absorption and low fire resistivity, which significantly limit their applications [9]. Kevlar fibers

are highly flexible and have a good strength to mass ratio. The most common application of Kevlar

is for ballistic protection, but it is also used for other applications such as sporting goods and

turbine blades [10]. Carbon fibers have become very popular due to their high tensile strength and

modulus, as well as good wear resistance properties [11]. Of these fibers, glass and carbon are the

9

most popular for reinforcing plastics using conventional injection molding and extrusion, as well

as in 3D printing. Table 1 highlights some studies that use a variety of composite manufacturing

methods with carbon and glass fibers.

Carbon fiber is currently not able to be used to reinforce thermosetting polymers using SLA

techniques due to its light attenuating properties. Therefore, glass fibers were chosen as the

reinforcing agent in this study. Additionally, fiberglass reinforced plastics are used in applications

where high mechanical strength and lightweight properties are required. It has been extensively

shown that adding glass fibers to a polymer resin increases the strength, stiffness, impact

resistance, and dimensional stability of the specimen [12]. These properties are dependent on:

properties of the constituents, fiber/matrix interface strength [13], fiber/fiber interaction [14], fiber

volume fraction [15], fiber length distribution [16], and fiber orientation distribution [6].

Table 1: Summary of Fiber Characteristics in Reinforced Composites

Technique Fiber

Material

Matrix

Material

Fiber

Length (µm) Fiber Addition Author

Injection

Molding E-glass PP 1360 20-30 wt% Ho [17]

Injection

Molding Silica PP 1600 20-40 wt% Doshi [18]

Injection

Molding E-glass

Polyamide

6-6 500 0.5 wt% Curtis [19]

Injection

Molding Silica PP, PBT 760-870 15-54 vol% Denault [20]

FDM E-glass ABS 850 18 wt% Zhong [21]

FDM Carbon ABS 3200 40 wt% Tekinalp [22]

FDM Carbon ABS Continuous 13 wt% Love [23]

10

Direct Write Carbon,

Silicon Epoxy 220 35 wt% Compton[24]

SLA Silicon Epoxy 10000-

15000 5-50 wt% Sano [25]

Polymers can be split up into thermoplastic and thermosetting materials. Thermoplastics are more

widely used as a matrix material in fiber reinforced composites. Most short fiber reinforced

thermoplastic composites are produced using injection molding or extrusion. While this study

involves the use of thermosetting polymers, useful information can still be obtained from previous

research on thermoplastics. In fact, the most common type of 3D printing, FDM, is a form of

polymer extrusion. From Table 1, it can be observed that a variety of fiber lengths, weight fractions

and matrix materials are used for thermoplastic composites.

One of the defining characteristics of a fiber as a reinforcing agent is the length. The fiber length

directly affects the mechanical properties of the composite and can be influenced by damage during

preparation of the composite. Glass fibers are generally brittle and can break during processing.

In thermoplastic compounding, specific machine parameters are better suited to preserve fiber

length. Lower screw speed [26], higher barrel temperature [27] and lower mixing times [28] create

products capable of having higher average fiber lengths. Similar trends can be found for injection

molding. Manufacturing using a reduced back pressure [29], lower injection speed [30] and molds

with more generous gate and runner dimensions [31] create materials with longer fibers. Fu et al.

investigated how fiber length changes during injection molding , and how the accompanying

mechanical properties changed [32]. Fiber length measurements were taken at various stages of

processing using centrifugal separation techniques. The study concluded that glass fibers break as

a result of increased fiber-polymer interaction, fiber-fiber interaction, and fiber contact with

surfaces and the processing equipment. A variety of other studies have shown similar trends [20,

28, 33-36]. While the results of these studies were considered when mixing SLA resin and glass

fibers, the low viscosity of thermosetting resin was expected to be less damaging.

Thermosetting polymer composites are found in fewer commercial applications, as they are more

complicated to process on large scale and are not easily recycled. Stereolithography is the most

common technique used to 3D print thermosets. Conventional methods for manufacturing

11

thermosetting products include casting, wet layup, and resin transfer molding. Both chopped

fibers and woven mats are common additives to create a thermosetting composite. While

fiberglass and carbon fiber thermosetting composites are becoming increasingly popular, little

research has been put into SLA and ISLA fiber reinforced products. In fact, recent review articles

published on fiber reinforced 3D printed materials have not noted any studies referring to either

SLA or ISLA studies [1, 37].

In an SLA system, the fiber length might affect the viscosity of the resin. Zak et al. examined how

the fiber length changes the viscosity of a photopolymer resin for SLA production [38]. A

fiber/polymer mixture at 15 vol% fiber was mixed using three fiber lengths: 3.2 mm, 1.6 mm and

0.8 mm. The study concluded that a longer fiber length shifts the viscosity/shear rate curve towards

increasing shear rates, assuming volume fraction remains constant. This theory is useful when

predicting the upper limit of fiber length that does not compromise processability and

manufacturing of the thermosetting composite material.

2.1.2 Mechanical Performance

Short fiber reinforced polymers are being used because of their improved mechanical properties

in comparison to pure plastics. The mechanical properties that are most important for end use

applications are elastic modulus, tensile strength, and impact strength. Some factors affecting the

mechanical properties have already been discussed. Fiber characteristics such as: length, material,

percentage content, and processing technique all effect the mechanical properties of the composite.

Table 2 presents strength increase values for a variety of 3D printed polymer composites.

Table 2: Various 3D printing studies comparing how fiber characteristics effect the increase

in fiber strength

Technique Fiber Material Fiber Addition Strength Increase (%) Author

FDM Carbon 5wt% 24 Ning [39]

FDM Carbon 13 wt% 194 Love [23]

FDM E-Glass 18 wt% 140 Zhong [21]

12

FDM Carbon 40 wt% 115 Tekinalp [22]

FDM Co-extrusion Continuous Carbon 6.6 vol% 335 Van der Klift

[40]

FDM Co-extrusion Continuous Carbon 34.5 vol% 446 Matsuzaki

[41]

Direct Write Silicon Carbide 35 wt% 127 Compton [24]

SLA Nickel Coated

Carbon

0.5 wt% 260 Yunus [42]

Mold SLA E-Glass 10 vol% 30 Zak [43]

Mold SLA E-Glass 5 vol% 11 Cheah [44]

ISLA E-Glass 1 wt% 0* Sano [25]

*indicates fibers were too long and specimen did not print effectively

Despite differences in fiber length, matrix material and layer thickness used in the various 3D

printing studies, there are still general conclusions that can be made. A specimen’s strength

increases with increasing fiber content. However, there is an upper limit to the amount fiber that

can be added. When comparing studies [22] and [23], an increase in carbon fiber addition did not

yield a larger strength increase, suggesting that 40 wt% exceeds the upper limit of carbon fiber

addition in FDM manufactured samples. Any weight percentage above this value will decrease

strength, as opposed to plateau or remain constant. This is likely due to the increase in fiber/fiber

interactions causing less adhesion to the matrix material [45].

Table 2 also confirms that the fiber material and length play a crucial role in the mechanical

properties of the sample. Carbon fibers performed better than glass fibers in each of the studies.

Additionally, the continuous carbon fibers yielded larger strength increases than shorter or

chopped fibers. This is because continuous fibers have a length efficiency factor of 1. Finally, the

13

nickel coating on the carbon fibers in the Yunus et al. study [42] showed the highest comparative

strength increase. These conductive fibers were ideally oriented for higher strength.

Another important aspect to consider is the viscosity of the matrix, and how this may affect the

fiber length distribution in the composite. A matrix with a higher viscosity leads to more breakage

during processing and shorter average fiber lengths, which in turn causes weaker final products

[6]. Viscosity also plays an important role in processability. Zak et al. [38] determined that

increasing the fiber count increases the viscosity of the fiber/resin mixture, and that there is an

upper limit to fiber addition that causes the viscous resin to be non-processible. In a mold SLA

system, it was determined that the maximum glass fiber addition is ~15 vol% before processability

is compromised.

2.1.3 Modulus and Strength Theories

Many theories predicting the modulus and strength of a fiber reinforced composite have been put

forward. The simplest approach in the “Rule of Mixtures”, which is a common equation used to

determine the elastic modulus or strength of the composite based on the contributions of the matrix

and the fibers:

𝐸𝐶 = 𝐸𝑚𝑣𝑚 + 𝜒𝐿𝜒𝑜𝐸𝑓𝑣𝑓 (2.1)

where Ec, m, f represent the elastic modulus of the composite, matrix, and fiber respectively, vm is

the volume fraction of the matrix, vf is the volume fraction of the fibers and XL and Xo are efficiency

factors based on the length and orientation of the fibers. The length factor, XL, is 1 if the fibers are

continuous, or have a very large aspect ratio [32]. This efficiency factor assumes the fibers are

uniform in terms of size. Therefore, the shorter the fiber length, the smaller the efficiency factor,

which will decrease the overall modulus or strength of the specimen. The orientation factor, Xo,

is 1 if the fibers are unidirectionally aligned such that they are oriented in the direction of the

applied force. The value of Xo will coincidently be 0 if all fibers are transversely oriented to the

load. If the fibers show a random orientation, the efficiency factor is 3/8 [46]. The orientation of

the fibers is dependent on the processing parameters and geometry of the final product.

Fiber orientation plays a particularly important role in the mechanical performance of a short fiber

reinforced polymer composite [6]. A composite will have greater stiffness and strength if the fiber

14

orientation is predominantly in the direction of the applied force. However, the specimen will only

exhibit small amounts of increased strength if the fiber orients transversely to the force.

The alignment of short fibers in both thermoplastic and thermosetting composites is highly

dependent on the flow of the liquid (whether molten or uncured) compound in the manufacturing

process. For thermoplastic composites, a variety of fiber alignments are possible based on the

thickness of the part [47, 48] and where the fiber is positioned along the boundaries of the die [49].

Generally, fibers will align in the direction of the plastic flow. The high aspect ratio of the fibers

causes a streamline effect due to the shear stress of the liquid in the direction of the flow [34].

Many theories predicting the length and orientation efficiency factors have been put forward. Cox

was the first researcher to hypothesize how the elasticity and strength of a material containing

fibers, such as paper, could be approximated [50]. From this work, Fukuda and Kawata were able

to develop a modulus theory for short fiber reinforced composites [51]. It was assumed that the

fibers had varying length and orientation within the sample. Then, Fukuda and Chou expanded the

work to include strength predictions using a probabilistic approach [52]. Both [51] and [52] were

the first theories that included a distribution of fiber length and orientation; thus creating the first

version of the Modified Rule of Mixtures. Both theories use a composite mechanics approach,

which is based on averages throughout the entirety of the sample.

To calculate the elastic modulus, first step is to determine how many fibers are crossing an arbitrary

scan line in the sample (Figure 5). Then, the axial force in each fiber of length L and orientation

θ is calculated, followed by finding the load-direction component of that axial force. This is done

for all fibers in the specimen. Next, the average load direction of all the fibers is multiplied by the

number of fibers that cross the scan line to find the overall force that the fibers are experiencing.

15

However, Jayaraman and Kortschot [53] proved that the overall axial force should only be

averaged over the fibers crossing the scan line, not all of the fibers in the sample [54].

Jayaraman suggested that the fiber orientation efficiency factor be computed using the following

equation:

𝜒𝑜 = ∫ 𝑔(𝜃)(cos4 𝜃 − 𝑣𝑠 sin2 𝜃 cos2 𝜃)𝑑𝜃

𝜋2

0

(2.2)

Zak et al. [55] estimated the orientation efficiency in a layered composite unit based on the

assumption that the specimen is modelled as a series of laminae, with each subsequent layer having

different fiber orientations [56]. If each lamina, κ, is given a thickness tκ proportional to the

fraction of fibers with a perpendicular orientation, then the orientation factor is:

𝜒𝑜 =∑ 𝐸𝜙𝜅𝑡𝜅

𝑛𝜅=1

𝐸1 ∑ 𝑡𝜅𝑛𝜅=1

(2.3)

The Krenchel factor is a common formulation to determine the fiber orientation efficiency factor

[57]. The Krenchel factor considers the portion of total fibers lying at a particular angle. It is

calculated by the summation of each proportion of fibers at every angle and factors it with a cosine

term. It is written as follows:

Figure 5: Identifying an arbitrary scan line within a sample [53]

16

𝜒𝑜 = ∑ 𝑎𝑛𝑐𝑜𝑠4𝜙 (2.4)

where an is the percentage of total fibers lying at a given angle, φ. The Krenchel factor assumes

that there is adequate fiber/matrix adhesion such that the composite will deform together, in the

direction of the applied force. If all the fibers were optimally placed parallel to the force, the

Krenchel Factor would be 1. A wide variety of studies have used the Krenchel factor as an

orientation efficiency factor [43, 44, 58-60]. It is a simplified version of the full theory developed

by Cox and later by Jayaraman et al.

2.1.3.1 Measuring Fiber Orientation

To measure the fiber orientation in a specimen, Cheah et al. [44] used an optical microscope to

geometrically examine the cross-sectional area of fibers. The fibers will form a two-dimensional

ellipse or circle, which can be correlated to the tilt angle of the fiber (Figure 6). Other studies have

used X-ray tomography to obtain 3D reconstructions of a fiber-filled composite [61]. The specific

planes of the models were analyzed to obtain fiber direction.

Figure 6: Fibers at different orientations will show different elliptical shapes at a

specific cross section. [44]

17

2.1.4 Stereolithography Fiber Alignment Attempts

Many studies have been conducted that focus on the alignment of reinforcing fibers in

thermoplastic composites. However, few studies have attempted to align fibers in an SLA

manufactured composite. Cheah et al. used a mold SLA technique to create short fiberglass tensile

specimens [44]. A random distribution was obtained by scattering fibers in each individual layer.

The orientation was not affected by the flow of the resin. Instead the orientation was dictated by

the placement of the fibers in each layer of fabrication. The study did confirm random orientation

by calculating the Krenchel factor but failed to comment on how the orientation affected the

increase in elastic modulus.

Erb et al. used super paramagnetic nanoparticles as a reinforcing agent in an SLA system [62]. By

applying a magnetic field to the resin, the particles were aligned in a variety of orientations. The

system was effective in obtaining desired strengths. The study does not attempt to use fibers,

which would further increase the mechanical properties of the specimen.

There have been several experiments using ultrasound waves to obtain fiber alignment for SLA

manufactured specimens. Llewllyn-Jones et al. demonstrated an instantaneous method to orient

short glass fibers in a photo-curable resin [63]. The glass fibers used were approximately 14 µm

in diameter and 50 µm in length. The study demonstrated sufficient increase in mechanical

properties, as the ultrasound created optimal arrangements in the 3D architecture. While this

method was effective, the print speed was much slower because of the time required to ultrasound

each layer. Additionally, the physical footprint of the machine is quite large as the SLA system

was a right side up process and included the ultrasound rig. Scholz et al. also used ultrasonic

pulses to achieve fiber alignment [64]. In this experiment, a standing wave acoustic field was

used. The ultrasound was produced by piezoelectric transducers. While this study also showed

great increases in specimen strength, the process has similar weaknesses to that of [63].

Furthermore, a layer thickness of 0.5mm was used. Most SLA systems are capable of thinner

layers which can yield more geometrically precise and accurate prints.

Finally, two studies conducted by Yunus et al. used shear forces to create alignment [42, 65]. Both

studies used aluminum oxide nanowires and an ISLA manufacturing system. In [42], a wall pattern

technique combined with lateral oscillation of the resin tank was able to generate shear flow. This

shear flow was evident in both vertical and horizontal directions. The study yielded ~28% increase

18

in tensile strength at 1.5 vol% fiber introduction. In [65], a 3D printed microchannel was

manufactured in a semi-circular formation. The study observed changing fiber orientation based

on the direction of the channel as a result of lateral tank oscillation. The results demonstrated that

the shear induced alignment followed the changing direction of the microchannel. The flexural

strength of the specimens varied between the orientations and performed better than randomly

oriented samples. While this technique was effective for fiber alignment and creating increased

strength and stiffness, there are some drawbacks. Aluminum oxide nanowires are quite expensive

in comparison to glass fibers, and the mechanical results were not incrementally higher.

Additionally, the lateral oscillation process between each layer significantly increased the print

time. Furthermore, there are limitations on the types of samples that can be printed, as the

microchannel was thin and creates boundaries for printing desired shapes.

2.1.5 Inverted Stereolithography Glass Fiber Reinforced Composites

There have been few studies evaluating the performance of glass fibers in an ISLA produced

specimen. Sano et al. presented a study that tested how the mechanical properties were affected

by the addition of glass particles, short fibers, and continuous fiberglass mats [25]. The study used

the Noble 1.0 printer to manufacture composites. The glass powder was ~15 µm in diameter and

mechanically mixed into the resin at 10, 20, 30, 40 and 50 wt%. The glass fibers measured 30 mm

in length and were added at 1 wt%. One sheet of fiberglass fabric was added to every 8 layers.

The elastic modulus of a tensile specimen increased 6.4 times at 50 wt% glass powder, but the

print failed at 55 wt%. The glass fabric caused an 11.5 times increase in the modulus. The glass

fibers sample failed to print and was never tested mechanically. The authors attribute the

malfunction to insufficient mixing of the fibers in the resin.

2.2 Conclusions

Short fiber reinforced composites have been shown to increase the elastic modulus and strength of

both thermoplastic and thermosetting polymers. Fiber reinforcement theory has been well studied

and developed. Both the fiber length and orientation play crucial roles in the mechanical properties

of the composite. While there has been extensive research on conventional manufacturing

techniques such as injection molding and extrusion, there has been few studies on 3D printed fiber

composites. Very little research has been conducted on SLA and ISLA manufactured fiber

composites.

19

Previous studies in which fibers have been added to SLA or ISLA have focused on varying

amounts of fiber addition and how the mechanical properties change. Little attention has been put

towards the fiber orientation and how the processing affects the fiber alignment in a specimen.

Thus, there is a disconnect between continuous ISLA processes and how the fibers move during

manufacturing. The goal of this research is to analyze how the fibers behave during ISLA

workflow and how the mechanical properties are affected. Flow of the fibers in the resin mixture

and print geometries will be of particular interest.

20

Experimental Procedures

3.1 General strategy

The experimental procedures performed in this study were designed to understand and optimize

how the flow of the resin affects the fibers and the resultant elastic modulus of tensile coupons.

First, a scaled-up proof-of-concept experiment was conducted to understand how the resin flow

changes the position of the fibers. Once the flow was analyzed at a macro level, fiber containing

specimens were manufactured using ISLA techniques. These samples were mechanically tested

and imaged to understand the structure-property relationships for this new method of producing

composite parts.

3.2 Macro-Simulations

To understand how the fibers and resin would flow during an ISLA process, a scaled-up benchtop

experiment was conducted. Sodium stearate (hand soap) was used as a mock resin as it has a

similar viscosity to the resins used in ISLA manufacturing. The soap was placed in a large white

tray (42 cm x 25 cm), to a depth approximately 3 cm. To mimic the fibers on a macro scale,

extruded polyvinylchloride (PVC) rope was chopped to create fibers that could easily be imaged.

These fibers measured approximately 25 mm in length. Long rectangular transparent

polycarbonate sheets were cut (20 cm x 5 cm) to mimic the print. The polycarbonate was manually

pressed into the resin, via handles attached to the sides of the ‘specimen’. This would simulate the

lowering of the build platform into the acrylic resin used in the actual manufacturing process. The

black PVC strands were randomly mixed into the sodium stearate. The polycarbonate sheets were

pressed into the mixture, and optical images were taken using a Nikon D60 camera. Photos were

taken sequentially to show how the fibers oriented as the ‘print’ descended further into the bath.

3.3 Specimen Development

Once the fiber flow was analyzed and understood, specimens were designed and uploaded to the

Form 2. Three main types of specimens were modelled using AutoCAD software: ASTM D638

(Type IV) tensile coupons, modified tensile coupons with ‘islands’ printed in between the

21

specimen, and large rectangles intended to be cut into coupons after printing. The three

arrangements are shown in Figure 7.

Design 1 conformed to ASTM D638 standard dimensions. Five coupons were printed in a parallel

configuration, spanning the length of build platform. The tabs of the specimen were separated by

1.5mm. A variety of build orientations were tested using this design (see section 4.1 for further

information). Design 2 specimens were also manufactured to ASTM D638 standard. However,

the surroundings were manipulated to modify the flow of the resin when the build platform

descended. In this case, coupons were printed 0.7 mm apart. Additional islands were printed

between the necks of the specimen. A constant channel width of 0.7 mm between the specimen

and the islands was maintained to aid in sample separation after the print had completed. Finally,

Design 3 was a simple rectangle with a length of 135 mm, width of 35 mm, and thickness of 4

mm. This rectangle was further manipulated post-printing to prepare samples for elastic modulus

testing. To create specimen that could be tested to find elastic modulus, the large rectangle was

cut across its width at 15 mm intervals. This process was conducted using a Mastercraft wet tile

saw. Final specimen had widths slightly narrower than 15 mm, as the width of the blade would

displace some of the composite material.

All samples were created in AutoCAD and uploaded in .stl format to the Formlabs software

PreForm®. The software allowed scaling, orientation manipulation and choosing design

parameters. The exact printer settings are described in Section 3.4.

3.4 Materials Preparation

The next step in the study involved preparing the resin/fiber solutions. The basic Clear Resin

(FLGPCL04) from Formlabs was used as the matrix material. The fibers were Fibertec 6608,

Figure 7: The three main tensile coupon designs. Design 1: Five samples in a parallel configuration.

Design 2: Five samples with flow diversion islands. Design 3: Large rectangle to be transversely cut.

22

which had an average length of 1500 µm and a diameter of 16 µm. Fibers were added at 5, 7, 10

and 12 vol%. For each study conducted, three replicates were printed and their properties were

averaged in the results. For every experiment where elastic modulus was tested for Design 1

specimens, three sets of the five coupons were printed. Therefore 15 samples were averaged to

obtain the elastic modulus. Fibers were weighed to the nearest thousandth of a gram and

mechanically mixed into the resin in a 250 mL beaker. The composite solution was then added to

the resin tank. For fiber filled composites, the rigid resin tank was used. No more than 200 mL

of resin could be added at a time, or the displacement of the build platform during production could

cause resin overflow and risk damaging the machine. For fiber filled samples, the printer was set

to Open Mode, which deactivated the wiping mechanism, and the automatic resin dispensing. The

wiping mechanism was found to push fibers to the sides of the tank, as opposed to mix them into

the resin homogenously. The automatic resin dispensing would have altered the fiber addition

content of the mixture, yielding inaccurate prediction of the elastic modulus. The print was paused

every 30 layers (~20 min), so the resin could be re-agitated directly in the tray. Particular caution

was taken to avoid damaging the PDMS layer.

Once the print had completed, samples remained on the build platform for 2-3 hours so residual

resin could drip back into the tank. The samples were then removed from the build platform and

placed in a 99% isopropyl alcohol (IPA) bath for 15-20 min. The first bath removed most of the

resin remaining on the surface of the parts. The samples were transferred to a second IPA bath,

where final cleaning would take place for another 10-15 min. Samples were removed from the

second bath and allowed to dry in ambient conditions for approximately 5 minutes. If the coupons

were printed with supporting structures, they would be removed using flush plyers at this time.

Then, the specimens were post-cured in the FormCure system. This apparatus subjected the parts

to additional UV at increased temperatures to bind any remaining free-radicals. For the majority

of the prints, this process occurred at 60 °C for 45 minutes.

3.5 Mechanical Testing

The main mechanical property studied in this research was elastic modulus. For Designs 1 and 2,

a tensile testing apparatus obtained force/displacement data, and the elastic modulus was obtained.

For Design 3, the specimens were too short to get accurate data using the conventional tensile

testing equipment. Therefore, a cantilever bending test was used to obtain the force/displacement

23

plots. For all tests, specimen dimensions were individually measured to ensure accurate

calculations. A Sintech 20 tensile testing machine was used to obtain the results. Coupons were

pulled at a rate of 5mm/min until failure. From the retrieved data, a stress strain curve was derived,

and the modulus was taken from this plot. Examples of the modulus calculations can be found in

Appendix 7.1. Strain gauges were not implemented to measure displacement, as the necks of the

specimen did not fit the available gauges. However, the machine compliance was measured and

included in the elastic modulus calculations. An example of the compliance calculation can be

found in Appendix 7.1. The setup for the tensile test apparatus is shown in Figure 8.

Figure 8: Tensile testing setup of a short glass fiber reinforced composite.

24

Any specimens with Design 3 geometry used cantilever testing to obtain the elastic modulus. One

end of the coupon was clamped on top of a stack of metal bars. These bars were placed in between

the sample and a rigid plate. Then, a pulling force was applied to the free-standing end of the

specimen. Similarly to the tensile testing, the force was applied at 5 mm/min. These specimens

were also too small to be monitored by a strain gauge, so the same machine compliance value was

included for this elastic modulus calculation. The experimental setup for the cantilever testing is

shown in Figure 9.

3.6 Mechanical Characteristics Analysis

The elastic modulus of a short fiber glass reinforced composite is dependent on a variety of factors.

To test these parameters, specific tests were conducted to obtain exact volume percentages, fiber

orientations, and fiber lengths.

After mechanical testing had been conducted, specimens were imaged using an x-ray tomography

system. Sections of the neck of tensile coupons were cut for imaging. Samples were analyzed

using a Skyscan 1172 micro CT. X-ray images were taken at 0.4° increments to acquire a 180°

array of images. A magnification capable of resolving 5 µm features was used to evaluate the

specimen. An x-ray voltage of 40kV was used. The images were reconstructed using NRecon

Bruker® software. This software would compile the array of snapshots and reconstruct the two-

dimensional features into three-dimensional objects. Images were compiled in ImageJ, which

Figure 9: The cantilever elastic modulus setup for Design 3, transversely cut specimens.

25

could create 3D models of the specimen, and display sections of specimen in multi-view ortho-

slices. The orientation was classified using the Distribution plugin [66]. The software identifies

individual fibers from an x-ray image and provides a histogram of fibers oriented at each angle. A

Fourier transform was chosen to identify changing intensities in neighboring pixels which could

then be interpreted as the angle at which the fiber was aligned. Orientation counts were taken at

five even thickness intervals and averaged to obtain an overall generalization of the fibers within

the tensile neck of each specimen.

To obtain exact volume percentages of printed specimens, a burn test was performed using a

Sybron Thermolyne muffle furnace. Prior to entry in the furnace, samples were gently polished to

remove any non-load bearing fibers penetrating outside the sample. The part was weighed before

and after being placed in the furnace for 6 hours at 600°C. During this time the resin was burned

away, leaving only the glass fibers remaining. The difference in mass can be equated to the mass

of the resin, and the exact volume percentage can be calculated. An example of this calculation

can be found in Appendix 7.2.2.

Size distributions were obtained using optical microscopy and ImageJ. A Nikon microscope

equipped with a TV relay lens 1x/16 took images of fibers at 3 time stages: before implementation

into the resin, after mixing but before printing, and after the burn test. The optical images were

evaluated in ImageJ using the ‘Measure’ feature. By setting a base measurement in the photo, all

other features could be measured relative to the length of the basis. Each fiber in the image was

individually identified by hand to get an overall average fiber length.

26

Results & Discussion

4.1 Specimens

Tensile coupons were printed in a variety of shapes, orientations (Figure 10), and layer thicknesses.

Generally, the printed specimen was an ASTM D638 Type IV. Modifications were made to the

design when trying to optimize fiber flow (See section 4.2.1). The tensile samples were printed at

various locations on the build platform to increase the longevity of the resin tray. By not printing

the specimen in the same location, the layer of PDMS remained in good condition as the wear was

distributed throughout the entire surface.

4.1.1 Geometric Consistency

Samples were regularly printed without any malfunctions or failures. The PreForm software

identifies the optimal orientation and scaffolding for a prototype. The software orients a tensile

specimen to a default angle (30°-70° from build platform) in the Y orientation (Figure 11). The

printed sample was completely supported by the scaffolding. This optimal orientation and support

system created 3D structures with exceptionally high geometric accuracy and repeatability. Up to

X

Y

Z

Figure 10: Three standard orientations for ISLA produced tensile coupons.

27

7 prints were manufactured in this orientation at a time. Samples were only printed in default

orientation during machine calibration and for initial anisotropy studies. For the remainder of the

testing, tensile coupons were printed flat on the build platform, in the X orientation (recall Figure

10). It was hypothesized that the X orientation would provide the greatest strength and elastic

modulus increase when adding fibers, as it would have the highest proportion of fibers oriented in

the direction of the applied tensile force. This orientation additionally manufactured prints in the

shortest amount of time, as the rate determining step in the process was the layer count, and the X

direction print had the fewest number of layers. Geometrically these tensile coupons were accurate

in the XY-plane. However, the Z-direction or thickness of the samples did vary when the standard

resin tray was used. When samples were printed next to each other, the middle samples were the

thickest. If 5 samples were printed in a parallel configuration, the third sample was invariably the

thickest. Table 3 shows how the range of thickness for linearly printed specimens.

Figure 11: Default orientation of tensile coupons using the PreForm software.

28

Table 3: Thickness values of adjacent samples using standard resin tray.

Fiber Content 1 2 3 4 5

Sample

Thickness

(mm)

No Fiber 2.91 3.0 3.05 2.99 2.87

5 vol% 2.93 3.06 3.08 3.03 2.9

The thickness of specimen, both with and without fibers, showed consistent increase to the third

specimen. However, specimen 4 and 5 decrease in thickness, as compared to the middle print. The

thickness variation can be attributed to the deformation of the standard polycarbonate resin tray

(Figure 12). As the build platform descends into the tank, it is passing through the highly viscous

resin and causes a pressure buildup at the center of the tray. This leads to resin tray deformation.

The build platform will stop at a desired height relative to the nominal position of the tray that is

equivalent to the layer thickness. After it stops, the tray will remain deformed as the laser begins

curing the slice. The arc in the tray allows a greater amount of resin to be present at the middle

sample as opposed to the outside samples. This effect will be prominent in the first printed layer.

The machine will print as if all the layers are the same size, so the thicker specimen will cause

greater amounts of tray bending. Thus, completed specimens 1 and 5 will be the thinnest samples,

and the middle sample (number 3) will be the thickest as that is where the greatest tray deformation

occurs.

Thickness variation was only present in samples that were printed in close proximity to each other.

Spacing the specimen widely over the entire build platform area dissipated the resistance of the

resin, as there were larger channels for the flow of the resin. The problem could also be solved by

using the rigid tank that Formlabs has designed for their Rigid Resin® and Tough Resin®. The

rigid tank had a thicker base, meaning there was less flexing during printing and samples were

produced with uniform thicknesses.

Figure 12: The orange standard tray shown bending as the build platform descends into the resin.

29

There were two main categories of samples printed: neat resin, and resin/fiber composites. The

fiber composites had varying fiber content, ranging from 3 vol%-10 vol%. One attempt was made

to include 12 vol% fiber content however the resin became too viscous and the print failed. As

previously discussed, the standard resin tank had increased bending during fabrication. Depending

on the number of tensile coupons printed and how they were spaced, the increased and

concentrated surface area would cause delamination. The bending of the tank caused stress points

in the layer when peeling away from the PDMS, leading to delamination. However, the rigid tank

had a stiffer base and more resistant PDMS layer. The increased stiffness allowed the print to

more uniformly pull away from the PDMS and no delamination occurred.

The short fiber reinforced composites were geometrically accurate up to the cured resin

boundaries. However, because the fiber length exceeds the feature tolerance of the printer, fibers

typically protruded beyond the edges of the samples. The fibers are brittle and easily abrade via

simple polishing. After polishing, the composites were safe to handle without protection, with

little risk of skin irritation from protruding fibers.

Finally, small amounts of shrinkage were present in the samples. Post-process curing of the full

specimens (using the FormCure) caused sample bending in fiber reinforced composites. The

curvature is likely a result of random fiber orientation in the first few printed layers (see section

4.2.1 for a detailed discussion of the fiber orientation as a function of depth in the specimens). As

the sample experiences increased heat and additional UV exposure during post-processing, the

remaining free-radicals covalently bond causing slight shrinkage in the specimen. The randomly

oriented initial layers will experience less shrinkage than the layers with a more transverse

arrangement of fibers. The sample deforms away from its base, where the first layers were printed.

The deformation was very small, with the ends of tensile coupon raising no more than 0.1 mm.

Sample thickness is ~3 mm. Non-fiber samples showed little curvature due to shrinkage.

4.1.2 Design and Fabrication Parameters

The Form 2 printer does not allow the user to adjust many parameter inputs. The wavelength of

the laser is set to a constant 405 nm, with no option to alter laser intensity. The raster pattern of

the laser is determined by the PreForm software and cannot be changed. Additionally, the timing

of laser initiation is constant. It would have been preferable to stall the laser curing, allowing the

resin time to properly flow, thus restoring a flat tray. There are no options for choosing the

30

frequency or speed of the wiping mechanism. The resin tray is set to a specific temperature

depending on which resin is being used. There were only a few tunable parameters. The PreForm

software gives 3 options for layer thickness (0.025 mm, 0.05 mm, 0.1 mm). Furthermore, the

orientation, and scaffolding of the specimen can be altered and optimized. The software provides

good warning indicators if too little scaffolding is used, or if the desired orientation may be difficult

to release from the build platform. The final option is to print samples in ‘Open Mode’. This

feature deactivates the wiping mechanism, does not alter the temperature of the resin tank (which

would slightly alter the viscosity of the resin), and will not automatically dispense resin into the

tank. Therefore, in this mode manual resin addition is required.

For any samples manufactured without fibers, the suggested settings were used. A layer thickness

of 0.1 mm was chosen to reduce printing time. The printer manufactured samples in ‘Regular

Mode’ with the wiping mechanism and automatic resin dispensing feature engaged. However, for

short glass fiber reinforced composites, Open Mode was used. Initially it was hypothesized that

the wiping mechanism would help maintain homogenous fiber distribution throughout the resin.

Instead, experimentation found that wiping pushed fibers to the sides of the tank. An exact fiber

content was needed for mechanical property calculations and analysis, so this feature was

disengaged. Additionally, the automatic dispensing system would have altered the fiber content as

the print progressed, causing a gradient of fiber concentration in the samples. The initial layers

would have higher fiber content than the final layers, as the introduction of non-fiber resin would

decrease the fiber concentration in the remaining layers. A fiber containing resin was incapable

of flowing out of the control valve on the resin tanks.

Finally, the printing process was stopped every 20 minutes, so the resin could be manually agitated.

A settling experiment was conducted to observe how the fibers would move after extended periods

of time. The fibers showed very little settling before 20 mins of resting (see Appendix 7.3).

However, a significant amount of settling occurred after ~25 minutes. If the resin had not been

agitated, the fibers would have settled and caused increased fiber content in the first layers.

4.2 Fiber Orientation

As previously mentioned in Section 2.1.3, fiber alignment plays a crucial role in the performance

of reinforced composites. For randomly oriented fibers, an orientation efficiency factor of 3/8 is

used [46]. As the fibers become more aligned in the direction of the applied force, the efficiency

31

factor will trend closer to 1. There is very little known about fiber orientation in SLA manufactured

composites. Glass fiber reinforced composites manufactured using inverted stereolithography

have not been studied previously. Given the practical implications of an ISLA system mentioned

in section 1.1.2, it was hypothesized that there is great potential to increase the strength and elastic

modulus of 3D printed materials. Understanding how the workflow of ISLA affects fiber

alignment is therefore critical in optimizing the mechanical properties.

4.2.1 Shear and Extensional Flow in ISLA

The first step in manipulating fiber orientation is understanding how the fibers naturally align

during the ISLA process. As previously discussed, ISLA involves building a specimen inverted in

a shallow resin bath with a laser curing each individual layer. As the build platform descends into

the resin tray, resin is predominantly displaced laterally from the print. In right-side-up SLA, the

fiber movement is dependent on the wiping mechanism, whereas orientation from resin

displacement is minimal as the build platform slowly and continuously descends further into the

resin bath [43, 44, 55].

The flow of the resin during ISLA causes fiber movement and orientation. It was originally

hypothesized that the fiber movement would result in random two-dimensional arrangements

within each layer. Because the build platform descends to a height that creates very thin layers, it

was presumed that fibers would arrange in 2-D stacks. Any fibers aligned in the Z-direction will

be pressed into the X-Y plane when the platform descends to a height of 0.1 mm above the resin

tank. However, in each layer the fibers were presumed to be randomly oriented, but the descending

of the build platform could cause movement of the resin and shift the arrangement of the fibers. If

the fibers remained in randomly dispersed, then an orientation factor of 3/8 could be used for the

modified rule of mixtures [46].

32

To understand fiber flow during the ISLA process, a scaled-up benchtop experiment was

conducted. Extruded polyvinylchloride (PVC) rope was chopped such that the aspect ratio of the

PVC closely resembled that of the glass fibers used to create composites (~75). These fibers were

dispersed in a sodium stearate (hand soap) solution with a viscosity similar to the acrylic resin used

in the Form 2. Long rectangular transparent polycarbonate sheets were cut, so that the fiber

movement could be traced. Handles on the side of the polycarbonate were used to press the sample

into the mock resin/fiber mixture. This simulated the lowering of a print into the acrylic resin used

in the actual manufacturing process.

Figure 13 shows the fiber movement as the polycarbonate ‘specimen’ was descended into the

mixture. Progressing from left to right correlates with the sample being pushed further into the

solution. Each figure border was cropped to the edge of the polycarbonate top platen. As can be

seen from Figure 13A) a relatively random fiber orientation was initially present before the

specimen descended into the bath. As the still shots progress through B) and C) and the specimen

descends deeper into the bath, there is fiber displacement occurring. Less fibers are present in

Figure D) than in A). Additionally, the fiber orientation is shown to predominantly align

Figure 13: Snapshots of scaled-up experiment. As the stills progress from A to D,

the polycarbonate sheet is being pressed further into the resin bath.

33

transversely across the specimen. The findings from Figure D) disprove the assumption that the

fibers would be randomly oriented in the print.

Using the findings from Figure 13, it was hypothesized that in the initial stages of the print, the

square build platform distributes the resin uniformly. As the layers build up and the print becomes

larger and more defined, the resin will begin to displace around the sample, causing shear flow.

However, when it is just the platform and the initial layers, the fibers will distribute more

uniformly. To prove this, another scaled-up macro experiment was conducted. A large square

sheet of polycarbonate was cut, and handles were attached to descend into the hand soap/PVC rope

mixture. The stills of the experiment are included in Figure 14.

Figure 14: A macro-experiment progression as a polycarbonate square descends into a mock resin.

34

In Figure 14 A) the polycarbonate sheet has just come into contact with the resin but has not

descended into the solution. As the stills progress through B), C), and D), the fibers are more

dispersed. However, the orientation is not predominately in a specific direction. This shows that

the initial layers of the build will likely contain fibers of random orientation. In image D), the

fibers close to the boundaries of the square are perpendicular to the edges. This follows flow

induced alignment principles, as the resin path of least resistance near the edges will be into the

area where there is the least pressure. As the part becomes thicker, then the resin will flow around

the edges of the sample and the shear flow will cause transverse fiber alignment.

To understand why the fibers are showing preferential orientation transversely across a high aspect

ratio specimen, the flow of the ISLA was derived from first principles. It can be assumed that

before the build platform descends into the resin tray, the fibers are oriented in three-dimensions:

x, y and z. There are two likely flow patterns that can alter the fiber orientation in this situation:

shear flow and extensional flow. Both of these involve the flow of a liquid induced by a force or

pressure [67, 68]. In this case, the force is exerted on the resin/fiber mixture by the build platform

descending into the resin tank. Shear flow involves subsequent layers of the resin moving in

parallel, but at varying speeds [69]. The resin will be moving at different speeds depending on

where it is in relation to the platform and the bottom of the tray. Near these surfaces, the lateral

velocity (u) of the resin will be 0. The lateral velocity profile is shown in Figure 15.

Figure 15: The descending build platform causes a non-uniform velocity profile, which can alter

the orientation of a fiber.

35

The non-uniform velocity profile will cause shear flow, and the fiber will change orientation in

the x-z plane. The end of the fiber that is subject to the higher velocity will align in the direction

of that stream. The high velocity stream will exert more force on the fiber at that end, causing the

orientation change. In Figure 14, the fiber at stage 1 has its right side in the central faster part of

the flow, therefore the speed of that end of the fiber will cause rotation through stages 2 and 3

across a distance x. The ending position of the fiber has little orientation in the z dimension.

Therefore, the assumption that the fibers will arrange in 2D stacks in the x-y plane is valid.

To understand how the fibers are arranged in the x-y plane, extensional flow should be considered.

Extensional flow is caused by very similar conditions to shear flow, however for extensional flow

there is a stagnation point at which the lateral velocity is 0. This case still considers a platform

descending into the resin bath. Now, the amount of resin that is displaced by the platform as it

lowers into the solution must be considered. As the platform changes its position (Z) relative to

the tank, there is an incremental increase in the volume of resin that needs to be displaced. This is

highlighted in Figure 16.

Figure 16: The changing Z position of the build platform causes increase volume displacement of

the resin. This also leads to increased lateral velocity.

36

Assume the build platform to have an area X-Y and is descending in increments dZ. The volume

of resin that is displaced at each increment of Z can be simply calculated as follows:

𝑑𝑉 = 𝑑𝑍 × 𝑋 × 𝑌

Therefore, the volume of displaced resin increases with increasing dZ. The volume will also

dissipate in the X direction, so dV also increases as X increases. To determine the lateral velocity

of resin in the X direction, divide the rate at which the volume is changing with time by the area

of the plane through which the displaced resin must flow. This area, A, is the ZY plane, which in

Figure 15 is vertically aligned and goes into the page. The area depends only on Y and dZ, not on

X.

�⃑�(𝑥) =

𝑑𝑉𝑑𝑡𝐴

=

𝑑𝑍 × 𝑋 × 𝑌𝑑𝑡𝐴

=𝑑𝑍

𝑑𝑡×

𝑋 × 𝑌

𝑍 × 𝑌= (

𝑑𝑍

𝑑𝑡× 𝑍−1) × 𝑋 (4.1)

It can be concluded that the lateral velocity of the resin is dependent on the rate at which the build

platform is descending but, critically, increases linearly with the distance from the center line X.

As this lateral velocity increases, the fiber will move similarly to the z alignment caused by shear

flow (Figure 17).

Figure 17: The top view (looking down “through” the build platform) of an infinitely long

specimen. The velocity of the resin increases towards the nearest boundary of the specimen causing

fiber orientation.

37

If the specimen is assumed to be infinitely long in the y direction, then the resin will flow out in

the x direction as the resin will displace on the path of least resistance. Resin at the center of the

specimen will have a low lateral velocity. However, displaced resin will travel at faster velocity as

it moves towards the boundaries of the surface. The resin near the outside of the specimen will

experience the greatest velocity. In this instance, the velocity increases linearly with the distance

X that the resin travels (Equation 4.1). This explains why the fibers predominantly align across the

images in Figure 13. The polycarbonate specimen was long and narrow, and the mock resin flows

to the closest edge, out the narrow sides. As it flows out the sides, the PVC fibers will orient in

that direction, transversely across the specimen.

It has been proven that a fiber will reorient with the motion of the resin unless perpendicular to the

flow [70]. This can explain why there are a few fibers in Figure 13D that are not oriented

transversely across the specimen. The fiber will experience no velocity gradient along its length,

so no orientation will occur.

At the initial stages of the ISLA print, the sample will be very thin, and the resin will flow radially

from the center of the square build platform surface. One can therefore assume that the first layers

of the sample will have a relatively random orientation of fibers (assuming the fibers were initially

dispersed randomly in the solution). However, as the thickness of the sample increases, the resin

flow will be affected by the shape of the print itself when the build platform descends. When

printing tensile coupons, the neck of the specimen is thin. In Figure 18, the blue arrows indicate

Figure 18: Blue arrows showing the direction of the resin flow during the printing process.

38

the likely direction of the resin motion after the specimen has achieved significant thickness. The

path of the least resistance is going to be out the sides of the neck of the specimen, as opposed to

along the length of the coupon. The velocity of the resin will be highest at the edges, away from

the body of the coupon. Therefore, it can be concluded that the majority of fibers will align

transversely across the neck of the specimen.

As previously mentioned, the initial layers will have a more random fiber orientation (recall Figure

14). Because the build platform has a large square area dispersing the fibers, there will be less

flow-induced alignment. Figure 19 is a 3D reconstruction of a glass fiber filled composite that

shows the initial layers which contain randomly oriented fiber content. Beyond the first layers,

fibers align transversely across the neck of the tensile specimen.

Figure 19: A plan and elevation view of a 3D reconstructed fiber filled composite.

39

4.2.2 Quantification of Fiber Distribution

All fiber containing specimens were imaged using X-ray tomography. An x-ray image would be

taken at an array of angles to obtain a 180º set of images. To find fiber orientation counts, each set

of X-ray images were reconstructed into 3-D models as described in Section 3.6. Individual slices

in the XY plane of the model were evaluated. The OrientationJ plugin for ImageJ® software was

used to create a color survey and count the number of pixels corresponding to each orientation.

Figure 20: Fibers aligned at 0° lie in the direction of the force. Fibers at 90° will lie transversely to

the applied load.

Figure 21: Slice of 5 vol% coupon and the average fiber orientation for the specimen.

40

The following orientation counts were taken from the middle of their respective samples and

averaged over all the samples printed in the same batch. Figure 20 provides the axis on which the

angle of the fibers was evaluated. Fibers at 0° are oriented in the direction of the applied load.

Figure 21 shows the percentage of fibers at a given angle for a 5 vol% fiber addition sample. As

can be seen from the tomography slice, most fibers are arranged at an angle of 90° to the applied

tensile force. This is confirmed by the orientation count, as the trend peaks near at 90°. It should

be noted that in this specimen the fibers appear to be quite short. However, the diameter of the

fiber (16 µm) is significantly smaller than the layer depth but larger than the 3D reconstruction

slice thickness. Some fibers are laying directly in the plane of the slice whereas others are lying

in multiple slices. The fibers are rigid, so the directionality of a specific fiber would remain

constant from layer to layer.

Figure 22: Slice of 10 vol% sample and the average fiber orientation counts for the specimen.

41

Figure 22 shows the orientation of fibers at 10 vol%. Compared to the 5 vol% specimen, there is

increased fiber content and a more random orientation. This is reflected in the orientation count.

There is a much larger area under the curve, and more peaks at values besides 90°. It can therefore

be concluded that an increased fiber content will produce a more random fiber orientation. This

is likely due to the increased fiber contact. If the fibers remain in contact throughout the printing

process, the effects of flow-induced fiber alignment will be reduced. This may also explain why

many of the fibers in the image are segmented. The increased viscosity could restrict z-direction

alignment, meaning the fiber is more likely to be oriented out of the X-Y plane, and will be more

likely to span across multiple tomography slices.

Figure 23: Slice of Design 2 specimen at 5 vol % with the average fiber orientation percentage.

42

For both previous samples, tensile specimens were printed adjacent to each other using Design 1.

To reduce the effects of flow-induced transverse orientation, later groups of coupons were printed

with ‘islands’ filling the gaps in between the specimen (Design 2). Small channels were present

to facilitate specimen separation after the print had completed. The orientation counts of this

arrangement at 5 vol% are shown in Figure 23. This set of tensile samples were expected to

produce relatively random fiber alignment. Random orientation was observed in the first few

printed layers. It was hypothesized that the random orientation would continue as thickness

increased throughout these samples as the entire print covered a low aspect ratio surface area, with

only small channels separating the tensile coupons. However, for 5 vol% specimens, the results

showed no significant orientation improvement from the initial specimens. As the specimen gets

thicker, it seems that the channels acted as a low pressure drain and the fibers flowed up through

the depth of the print, as well as along the channel.

Further attempts were made to reduce the channel size between the tensile coupons, however any

finer adjustments resulted in fiber crossover between the samples. The samples remain attached

and could not be separated without fracturing. It is likely that the minimum width of channel for

efficient separation is still too wide, resulting in resin and fibers to flow into the channel, as

opposed to travelling across it. This flow would still lead to flow-induced arrangement that causes

the fibers to predominantly orient transversely across the length of the specimen.

Figure 24: Post processing for tensile specimens from Design 3 samples.

43

In a final effort to confirm that fibers oriented at 0° would increase the mechanical properties of

the specimen - a new technique was adopted. It had been proven that the fibers would orient

transversely across a narrow specimen. So, a scaled-up version of a high aspect ratio block was

printed (Design 3). Figure 24 shows the formulation of these specimen. The block was cut using

a tile saw transversely across the width of the specimen into smaller specimens. The specimens

were too small to be tested in tension, but were sufficient for cantilever experimentation, as the

elastic modulus is determined in the initial stages of the applied force.

Figure 25 shows the orientation counts from these cut specimens. It is observed that there is a

predominant orientation at 0°. Recalling Figure 20, 0° aligned fibers are in the direction of the

applied force. Therefore, this method is effective in reorienting the specimen to have the majority

of fibers in a direction that will have significant contribution to the elastic modulus. Regardless

of the increased size of the pre-processed print, the aspect ratio causes most of the resin to flow

out the narrower sides, and flow-induced alignment will cause the fibers to orient transversely.

Figure 25: Slice of Design 3 specimen at 5 vol% with the average fiber orientation percentage.

44

4.2.3 Variation in Fiber Length

The fiber length directly effects how a specimen will behave mechanically. The fibers used in this

study initially measured ~1500 µm. This length was confirmed using optical microscopy.

However, examination of the fibers once production had completed showed a reduction in length

by almost half. Table 4 lists the average fiber length at various stages in the printing process for a

5 vol% specimen.

Table 4: Fiber length at 3 different stages of processing.

Manufacturing Stage Fiber Length (µm)

Initial 1430 ± 80

After Mixing 1210 ± 103

After Printing 656 ± 111

The results of Table 4 suggest that fiber breakage was minimized during mixing. This is a good

indication that fibers are capable of withstanding mechanical agitation, a useful quality should this

process be scaled-up. However, during printing the fiber length drastically reduces (Figure 26).

There are a few possible explanations for this. The most plausible is fiber breakage occurring as

the build platform descends into the resin. It can be assumed that after mixing but before

processing, the fibers are randomly oriented in a 3D architecture. As the build platform lowers to

a very thin height above the resin tray, it is likely that a portion of the fibers in the Z-direction will

break once in contact with the platform and the resin tank. If the tensile samples are being printed

with islands between the necks, fibers may break during flow through the small channels.

Figure 26: Optical microscopy of fibers at A) initial B) post mixing and C) post curing stages.

A B C

45

Additionally, a large amount of fiber breakage likely results from fiber-fiber interactions. Even if

the broken fibers are not immediately cured in the specimen, it is likely that subsequent layers will

contain previously broken fibers. These reduced-length fibers are a drawback of the ISLA system.

4.3 Mechanical Testing

Elastic modulus was found using tensile testing of a 3D printed specimen. Samples with varying

shapes, sizes, and orientations were evaluated to understand trends in the mechanical performance.

Reported data is the average of all specimens from a single print unless otherwise stated. An

example of the modulus calculation can be found in Appendix 7.1.

4.3.1 Elastic Modulus of Neat Resin

The first study conducted was an isotropy test to validate the claim by Formlabs® that the Form 2

produces isotropic parts [71]. Samples were printed in 4 different orientations: X, Y, Z, and in the

default orientation as determined by PreForm software (recall Figures 10 and 11). These tensile

coupons did not contain glass fibers. The results are presented in Figure 27. Each orientation

shows an elastic modulus of approximately 1.2 GPa. Since isotropy was confirmed, all the

subsequent specimens were printed in the X direction, directly on the build platform unless

otherwise stated.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

X Y Z Default

Ela

stic

Mo

du

lus

(GP

a)

Print Orientation

Figure 27: Elastic modulus results to confirm anisotropy.

46

The next mechanical study examined how curing and ageing affected the elastic modulus of the

printed neat resin. The optimal curing time and temperature for clear resin as stated by Formlabs

is 45 min at 60 °C [4]. In this experiment, five samples were printed in six groups. Three of the

groups were immediately cured at the recommended settings in the FormCure. The other three

were not cured and simply stored at room temperature. Within each group, a portion of specimens

were immediately tested. The other portions were aged in ambient conditions for 15 and 30 days

to test how basic environmental conditions would affect the elastic modulus. Once again, no fibers

were added to these specimens. Table 5 highlights the findings of this experiment.

Table 5: Effects of ageing on the elastic modulus

Curing Protocol 0 Days (GPa) 15 Days (GPa) 30 Days (GPa)

No Cure 0.24 ± 0.02 0.27 ± 0.04 0.36 ± 0.06

Recommended Cure 1.17 ± 0.04 1.15 ± 0.03 1.17 ± 0.05

The results clearly indicate a cured tensile coupon has a significantly higher elastic modulus.

However, once cured there is no change in the modulus. For the non-cured specimens, the modulus

continues to increase as the specimen is exposed to ambient conditions. Ageing the specimen

cross-links the layers over time by bonding free radicals that were initiated by the laser during the

manufacturing process. As previously discussed, during printing each layer is under-cured to

create a ‘green-state’. This green state contains many active bonding sites to cross link the

monomers. It is likely that in uncured specimen, the cross-linking is occurring at a slow rate over

time, which can explain the gradual increase in elastic modulus.

It should be noted that the elastic modulus observed in this study was significantly less that that

reported by Formlabs on the standard resin datasheet (2.8 GPa). We were unable to determine the

cause of this discrepancy, which could be related to our measurement techniques or cure

conditions. However, since the focus of this study was on comparison between reinforced and

non-reinforced resins, and the modulus results were internally consistent, the discrepancy does not

influence the important findings reported here.

47

4.3.2 Elastic Modulus of Composites

Once isotropy was confirmed, fibers were introduced into the acrylic resin. The initial mechanical

tests involved printing specimen with a 5 vol% glass fiber addition. These specimens were Design

1 and printed in groups of 5 using the standard resin tray. Exact mass fractions were obtained using

a burn experiment, as described in section 3.6. In general, printed composites exhibited slightly

lower fiber content than contained in the initial resin. As previously discussed, the lateral velocity

of the resin increases away from the center line of the sample. Thus, some of the fibers are likely

carried out of the layer as dV increases. The results are shown for each individual coupon in Figure

28.

Figure 28: Variation in elastic modulus due to specimen position on the standard resin tray.

48

These results show highest elastic modulus values at the outside specimen while the center

specimen had the lowest modulus. The standard resin tray exhibits greater amounts of bending,

which can explain these findings. The standard tray will deform similarly to Figure 12, meaning

the middle specimen has a greater thickness than the surrounding specimens. To initiate the

printing of every layer, the build platform descends into the resin tank. The thicker specimen will

contact the fiber/resin mixture before the surrounding ones, allowing increased time for shear flow

to orient the fibers. The middle sample will therefore have an increased fraction of transverse

fibers, which then contribute less to the overall modulus of the composite. The same experiment

was conducted with the rigid tray, where there is less bending and more uniform thickness (see

Table 3). All 5 samples that were printed using the rigid tray had the same modulus.

A simple test was conducted to observe how increasing fiber content will increase the elastic

modulus. Figure 29 shows this increase.

Glass fibers increased the elastic modulus from 1.18 GPa to 1.7 GPa at 10 vol%. This increase is

expected. Glass fiber has an elastic modulus near 70 GPa, and the rule of mixtures states that the

increasing fiber content should increase the elastic modulus of the composite. However, the large

portion of transversely oriented fibers have little contribution to the elastic modulus of the fiber

reinforced print. While the trend is increasing, the elastic modulus values are not high enough to

justify the additional cost of the glass fibers. It should be noted that 12 vol% specimens were

0

0.5

1

1.5

2

0 5 10

Ela

stic

Mo

du

lus

(GP

a)

Fiber Content (Volume%)

Figure 29: Correlation between increasing fiber content and elastic modulus.

49

attempted but the print failed. Viscosity increases with increasing fiber content, so it is likely that

the viscosity reached the upper limit of the Form 2 printing capabilities. Reduced flow of the 12

vol% mixture might have resulted in continued motion of the resin after the build platform had

reached its desired height. If the laser started curing while the resin was moving, incomplete layers

may have been formed. These incomplete layers likely caused delamination during production.

Knowing that shear flow causes transverse fiber alignment, an attempt was made to increase the

fraction of fibers aligned along the neck of the sample in the direction of the applied force. As

previously mentioned, Design 2 tensile coupons were manufactured with islands in between that

were intended to redirect the flow. The islands were designed to create one large square print area

with small channels between the specimens that would provide boundaries for separation. These

samples were predicted to have random fiber orientation. However, orientation counts confirmed

that there was no increase in the fraction of longitudinally aligned fibers. The results of the elastic

modulus testing in Figure 30 confirm the results.

Additionally included in Figure 30 are the mechanical results of the specimens that were cut

transverse to the long edge of the large rectangle. For these Design 3 specimens, the elastic

modulus increased by ~170% from the regularly printed samples. As expected, specimens with

fibers aligned in the direction of the load had a higher modulus.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

No

Reinforcement

5 vol% Design 2 Design 3

Ela

stic

Mo

du

lus

(GP

a)

Figure 30: Elastic modulus of various tensile coupon designs.

50

While the Design 3 specimens performed to a higher level mechanically, a trend was observed in

the elastic modulus based on the position of the sample in reference to the whole print. Figure 31

highlights this theory. These tensile coupons were strips cuts from one large rectangular piece. In

the previous section it was noted that these specimens contained fibers predominantly aligned in

the direction of the applied force. However, resin flow follows the path of least resistance. In this

case, the velocity of the resin is going to be highest at the long sides of the rectangle. But, local

resin near the shorter sides will predominantly flow out at that location, causing more transverse

fibers at those locations. These transverse fibers contribute less to the overall modulus of the

composite. Therefore, a slight decrease in the modulus of the specimens at the ends of the sample

is observed. The interior specimens (samples 2-7), even those directly beside the outside

specimen, have similar moduli.

4.4 Predicting Elastic Modulus

There are several methods to predict how a fiber reinforced composite will behave. Generally, the

overall modulus of a composite is predicted using a weighted average of the matrix modulus and

Figure 31: Elastic modulus of re-oriented specimen based on position before cutting.

51

reinforcing material modulus. A detailed overview of the approaches taken by Cox, Fukuda, Chou,

Kawata, and Jayaraman can be found in Section 2.1.2. The simplest equation used to predict the

modulus is the Modified Rule of Mixtures, which states the following:

𝐸𝐶 = 𝐸𝑚𝑣𝑚 + 𝜒𝐿𝜒𝑜𝐸𝑓𝑣𝑓 (4.2)

where XL and Xo are efficiency factors that scale the contribution of the fibers. For the purposes of

this research, the fiber orientation efficiency factor (Xo) quantified the overall alignment of glass

fibers within the ISLA produced specimen. There is debate on the most accurate equations to

calculate these factors, however, based on the equipment available for fiber imaging, the Krenchel

factor was used to determine the orientation efficiency factor:

𝜒𝑜 = ∑ 𝑎𝑛𝑐𝑜𝑠4𝜙 (4.3)

where an is the percentage of total fibers lying at a given angle, φ. The OrientationJ plugin from

ImageJ was used to provide quantitative values for each fiber angle throughout the entire 3D

reconstructed sample. By knowing how many fibers were oriented at each angle, an could be

calculated using simple Excel protocols. An example of the Krenchel calculation can be found in

Appendix 7.2.1. Figure 32 shows the theoretical modulus of the composite using the Modified

Rule of Mixtures with the Krenchel Factor and compares it to the experimental results.

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

0 5 10

Ela

stic

Mo

du

lus

(GP

a)

Fiber Content (Volume%)Figure 32: Comparison between theoretical and experimental elastic modulus results.

Theoretical ■

Experimental ▲

52

Both the 5 vol% and 10 vol% samples are analyzed in the same plot. There are 5 data points for

both volume percentages, representing each individual specimen from the Design 2 print. The

Krenchel factor for each individual specimen was calculated and substituted into the rule of

mixtures (Equation 4.2). It is observed that the experimental results closely resemble the

theoretical trends for the 5 vol% specimens. However, the 10 vol% specimens show higher

variation between the theoretical calculations and experimental results. This could possibly be

due to the increased fiber interactions. These interactions could cause more fiber breakage,

potentially lowering the fiber length efficiency factor. Additionally, fibers could agglomerate at

higher volume percentages. The Krenchel factor for each individual specimen is an average of 5

different thicknesses throughout the sample, so the clumping may not be accurately represented in

the average. Overall, the data fits nicely to a general trendline that can be calculated from the

average Krenchel factor obtained over all specimens. In these calculations, XL was assumed to be

1; meaning the fibers were sufficiently long to be considered continuous. While this is likely not

the case, using just the Krenchel factor gives a close approximation of the experimental values.

Rosenthal et al. developed a technique to determine the fiber length efficiency factor [72]. Their

approach is based on the work of Cox [50], and accounts for fibers having distribution of lengths.

The overall equation states:

𝜒𝐿 =1

𝑣𝑓∑ 𝑣𝑖 [1 −

tanh (𝛽𝐿𝑖

2 )

𝛽𝐿𝑖

2

]

𝑖

(4.4)

where Li is the length of a fiber, vf is the total volume fraction of fibers, and vi is volume fraction

of fibers at length Li, and

𝛽2 =2𝜋𝐺𝑚

𝐸𝑓𝐴𝑓 ln (𝑅𝑟 )

(4.5)

where Gm is the shear modulus of the matrix, Af is the cross-sectional area of the fiber, r is the

radius of the fiber, and R is the centre-to-centre distance between the fibers. However, based on

the non-geometric packing of the fibers, calculating R was not possible given the software

available. Therefore, it was assumed to be 1 for the theoretical calculations.

53

Table 6: Comparison of Krenchel factors and accompanying elastic modulus based on the design and

position of the tensile coupon.

Design 2

Specimen 1 2 3 4 5

Krenchel Factor 0.0852 0.0764 0.08 0.0793 0.0752

Theoretical

Modulus (GPa) 1.61 1.42 1.49 1.52 1.43

Experimental

Modulus (GPa) 1.51 1.47 1.48 1.46 1.47

Design 4

Specimen 1 2 3 4 5

Krenchel Factor 0.303 0.476 0.498 0.479 0.3104

Theoretical

Modulus (GPa) 1.94 2.41 2.47 2.42 1.96

Experimental

Modulus (GPa) 1.68 2.09 2.14 2.07 1.67

Design 3

Specimen 1 2 3 4 5 6 7 8

Krenchel Factor 0.501 0.781 0.792 0.695 0.696 0.672 0.616 0.483

Theoretical

Modulus (GPa) 3.69 4.01 4.09 4.21 3.97 3.96 3.88 3.7

Experimental

Modulus (GPa) 3.48 3.89 4.07 3.89 3.81 3.82 3.71 3.46

Table 6 compares the theoretical modulus to the experimental modulus for specimen designed to

have optimal flow, reoriented specimen and specimen cut from a circular base (Design 4, Figure

33). The theoretical modulus was calculated using the Krenchel factor and assumed a fiber length

factor of 1. The Design 2 specimens have comparable moduli with no obvious trends between the

theoretical and experimental results. The specimens cut from a circular base show significantly

higher moduli for the theoretical calculations. It should be noted that the outside specimen had

the weakest elastic modulus, whereas the center sample had the highest.

54

Design 4 specimen contained fibers oriented in an array, as the flow will dissipate uniformly

around all edges. Therefore, the center specimen will have the most fibers arranged in the direction

of the applied tensile force, explaining the higher modulus. The array of fiber orientations suggests

that the outside specimen will have the highest proportion of transverse fibers.

The specimen that were cut transversely to the long edge of the large rectangle (Design 3) exhibit

similar modulus values. However, the theoretical results are consistently higher than the

experimental results. This is likely due to the fiber length factor assumption. The cutting procedure

for both the circle design and Design 3 cause more drastic range of fiber lengths, altering the

behaviour of each specimen. The shorter fibers mean the fiber length efficiency factor is lower,

so the elastic modulus of the composite also decreases. With more advanced imaging equipment

and software, it is possible to find the value of R and a fiber length factor (Equation 4.4) can be

added to the Rule of Mixtures to get a more accurate calculation of the elastic modulus.

Figure 33: Design 4 specimen are cut from a circular base using the tile saw.

55

Practical Implications, Limitations and Conclusions

5.1 Applications & Limitations of Composite Manufacturing

Generally, structural parts with high aspect ratio members are intended to support force along their

lengths. When considering fiber reinforced composites that provide increased modulus to these

structural parts, conventional manufacturing methods produce composites with fibers

predominantly aligned in the direction of the applied force. Therefore, the fibers should be aligned

longitudinally along the high aspect ratio sections of the part.

An increase in mechanical properties would allow ISLA printed parts to be used in more load-

bearing applications. Unfortunately, using the natural workflow and build parameters for an ISLA

printed sample, it was found that the fibers preferentially orient transversely across the narrow

sections of a part. The transversely aligned fibers contribute little to the strength and modulus in

the desired load direction. Therefore, this limits the applications of naturally oriented ISLA printed

fiberglass composites.

In most situations, increased strength is desired along the length of the object, not across the width

of it. However, one possible application for transversely stiff and strong composites is a gasket

for a flange (Figure 34). When the bolts are tightened, they exert a uniform compressive force

between the two flanges. A fiberglass gasket printed in the orientation of Figure 34B) would align

the fibers through the thickness of the gasket. These aligned fibers would provide increased

compressive strength, and more importantly, reduced stress relaxation [73], possibly increasing

the longevity of the connection. Inverted stereolithography would be particularly useful for

printing a gasket that may not be manufacturable by conventional sizing or shapes. Thus, a unique

gasket could be printed for a niche application.

56

Another possible application could be a foot plate for a walking cast. The foot plate provides

structure and rigidity to the cast under the weight of the patient. There are a few benefits of using

a fiberglass ISLA composite to create this plate. For starters, 3D printing is becoming common in

healthcare as the additive manufacturing is custom designed to the exact specifications of the

patient. A foot plate could be manufactured using ISLA that perfectly fits the orthotic nature of

the foot. Like the gasket, the foot plate will experience dissipated pressure over long periods of

time. To avoid extensive levels of wear or creep, a compressively resistant material is required.

A foot plate could be built using ISLA allowing shear flow to orient the fibers in the direction of

the compression. The plate would need to be built on its edge to correctly orient the glass fibers.

Additionally, this technology could be useful for building wear resistance fiber plates for shoes.

By using rigid and hard transversely oriented fibers, a soul or tread of a shoe could be developed

for applications where the shoe is experiencing rough terrain or continuous activity.

In this study, despite several attempts to align in the fibers longitudinally along a narrow specimen,

there was little success. Walls and islands were printed to redirect the flow of the resin along the

length of the specimen. However, in order to have clean separation between the walls and the

prototype, a thin channel was needed. Use of a minimally sized channel to avoid fiber crossover

still resulted in transverse alignment.

Figure 34: A) Depiction of a typical gasket used for a flange. B) The desired print orientation for an

ISLA produced gasket.

A B

57

There are other possible reinforcing agents. While conducting this research, Formlabs® released

a Rigid Resin containing class particles at approximately 40 wt%. This created clean,

geometrically accurate prints, however the elastic modulus was similar to the 10 vol% Design 1

specimens used in this research (~1.8GPa).

5.2 Conclusions and Future Work

Currently, parts produced by additive manufacturing are not strong enough for some structural

applications. Given the low strength and modulus of these 3D printed specimens, there are few

applications where it can be introduced into a system as a functioning, load-bearing part.

Stereolithography manufacturing produces isotropic parts, , however, the printed specimens are

still not strong enough to endure the force many applications require. In this study, short glass

fibers were integrated into the inverted stereolithography matrix in an attempt to significantly

increase the strength and elastic modulus of 3D printed parts. The glass fibers were successfully

integrated into the matrix by simply mixing them into the resin tank, and no special techniques

were needed to obtain good fiber dispersion. To better visualize the fiber orientation in the

resultant printed specimens, X-ray tomography was used to reconstruct 3D models. ImageJ

accurately found the fiber orientation distribution, which can be used to find the Krenchel factor.

This study showed that using the Krenchel factor in the Rule of Mixtures produced estimates of

theoretical elastic modulus, quite close to the corresponding experimental results. Unfortunately,

we have demonstrated that resin flow velocity gradients will generally result in non-ideal fiber

orientation, which may limit the applications of an ISLA produced specimen produced using the

techniques described here. The resin accelerates towards the nearest boundary of the print, which

will cause the fibers to align in the direction of the flow. This will generally be across the width a

high aspect ratio member, and causes transverse fiber orientation, which will not significantly

increase the elastic modulus of the specimen along the main axis of the member.

Future work with the ISLA should continue exploring flow induced orientation and

conceptualizing new ideas to modify the movement of the resin. To obtain longitudinally aligned

fibers, channel flow is desired. It is likely that using an ISLA printer with more open source

capabilities is preferable. Having the option to adjust printer parameters may lead to ideal

orientation. These parameters include: laser initiation time and intensity, lateral movement of the

resin tray, and time between layer separation.

58

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64

Appendices

7.1 Elastic Modulus Calculations

Elastic Modulus was derived from the force/displacement data given by the Sintech 20 mechanical

tester. Two methods were used to obtain the modulus. The first was tensile testing. The Sintech

pulled coupons at a constant rate of 5mm/min to obtain force displacement data. This data was

used to find the stress and strain of the coupon. These parameters were plotted on a stress/strain

curve, and the linear segment of the trend was taken as the elastic modulus. Examples of the

calculations are provided below.

Before deriving the strain from the grip displacement, a compliance factor was needed to account

for any machine compliance during the application of the tensile force. To find this compliance

factor, a rigid piece of steel was placed in the grips and pulled at a rate of 0.001 mm/min. This

load was only applied for a few seconds before stopping the test. Because of the high strength and

rigidity of the steel, an infinite was expected for this short test. However, the slope of the load-

displacement curve was found to have a value of 602.9kg/mm. This value was factored in to the

displacement values before any strain values were obtained.

Compliance Experimental Data

Figure A-1: Load-displacement plot, comparing experimental values with

compliance modified values.

65

In Figure A-1, the red triangle represents the area above the compliance trendline. This was

subtracted from the area above the experimental data to achieve the blue tirangle, which would be

used for calculating the strain and elastic modulus.

Consider a point on the force/displacement curve: F=31.48 kg, d=0.265 mm. The sample has a

gauge length of 30 mm, neck thickness of 4 mm and neck width of 6 mm.

𝑇𝑟𝑢𝑒 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 𝑑 − (𝐹

𝑐𝑜𝑚𝑝𝑙𝑖𝑎𝑛𝑐𝑒) = 0.265 𝑚𝑚 −

31.48𝑘𝑔

602.9𝑘𝑔𝑚𝑚

= 0.21278𝑚𝑚 (𝐴1)

Equation A1 was applied for every displacement value obtained. This true displacement was used

to calculate the strain using Equation A2:

𝜀 =∆𝑑

𝑑0=

0.21278𝑚𝑚 − 0𝑚𝑚

30𝑚𝑚= 0.00709 (𝐴2)

This strain formula was carried through for every data point. The stress was obtained using the

following equation and unit conversion:

𝜎 =𝐹

𝐴=

31.48𝑘𝑔

4𝑚𝑚 × 6𝑚𝑚×

9.81𝑁

𝑘𝑔×

𝑀𝑃𝑎

𝑁𝑚𝑚2

×𝐺𝑃𝑎

1000𝑀𝑃𝑎= 0.01286𝐺𝑃𝑎 (𝐴3)

Equation A3 was used for all data points. Following these calculations, a stress/strain curve was

obtained. The slope of the initial linear portion of the trend was evaluated and used as the elastic

modulus of the specimen.

For Design 3 specimens, the cantilever bending test was used to determine the elastic modulus.

Consider a specimen with a length, width and thickness of 30 mm, 12.5 mm, and 4 mm

respectively, securely fastened at one end, while a force pulls the open end upwards at a rate of

5mm/min. From the force displacement graph, a plot is taken at F=15 kg and d=4mm. To obtain

the correct values, the compliance needs to be factored into the displacement value using equation

A1:

𝑇𝑟𝑢𝑒 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 𝑑 − (𝐹

𝑐𝑜𝑚𝑝𝑙𝑖𝑎𝑛𝑐𝑒) = 4 𝑚𝑚 −

15𝑘𝑔

602.9𝑘𝑔𝑚𝑚

= 3.975𝑚𝑚

66

From this, the elastic modulus can be calculated using Equation A4:

𝐸 =𝐹𝐿3

3𝑑𝐼 (𝐴4)

where I is the moment of Inertia and is calculated using the following formula:

𝐼 =1

12𝑤 × ℎ3 (𝐴5)

By substituting equation A5 into A4, the elastic modulus is found.

𝐸 =4𝐹𝐿3

𝑑𝑤ℎ3=

4 × 15𝑘𝑔 × 30𝑚𝑚3

3.975𝑚𝑚 × 12.5𝑚𝑚 × 4𝑚𝑚3×

9.81𝑁

𝑘𝑔×

𝑀𝑃𝑎

𝑁𝑚𝑚2

×𝐺𝑃𝑎

1000𝑀𝑃𝑎= 5.09𝐺𝑃𝑎

This formula was used to calculate the elastic modulus at each data point. These individual moduli

were averaged to find the overall modulus of the specimen.

7.2 Predicting Elastic Modulus

To predict the elastic modulus, the Modified Rule of Mixtures was used (Equation 4.1). To use

this equation, a fiber orientation factor and exact fiber content value are required.

7.2.1 Calculating the Fiber Orientation Factor

As previously mentioned, the formula used to obtain the fiber orientation factor was derived from

Krenchel (Equation 4.2):

𝜒𝑜 = ∑ 𝑎𝑛𝑐𝑜𝑠4𝜙

To use this formula, a quantification of fibers at each angle is required. This was obtained from

the orientation counts using ImageJ. Below is a sample calculation (results in Table 7) that

generalizes fibers into angles at increments of 15°. To calculate an, the total fiber count is divided

by the fibers at each angle.

Therefore, the orientation factor for this scenario is obtained below:

67

𝜒𝑜 = ∑ 0.0734 cos4(0) + 0.0963 cos4(0.2618) + 0.1577 cos4(0.5236) + ⋯

It should be noted that the fiber angles were converted to radians for this calculation. The

summation of each an multiplied by the cos4 of the angle was used the orientation factor.

Table 7: Results of Krenchel calculation.

Fiber Angle Fiber

Count an

0 3535 0.073439

15 4638 0.096354

30 7592 0.157723

45 8759 0.181967

60 6238 0.129594

75 8643 0.179557

90 8730 0.181365

Fiber Total 48135

7.2.2 Obtaining Exact Volume Percentage

In order to accurately predict the elastic modulus of a short glass fiber reinforced composite, the

exact fiber concentrations were needed. The resin had a calculated amount of fibers introduced,

but this does not necessarily guarantee a specific fiber content. To obtain this value, a burn test

was conducted. Consider a section of the neck of a tensile coupon with initial weight of 5g and

post burn weight of 0.35g. The resin and fibers have densities of 1.1 g/mL and 2.55 g/mL

respectively. The exact volume percentage of the specimen is calculated as follows:

𝑚𝑓𝑖𝑏𝑒𝑟𝑠 = 0.35𝑔, 𝑚𝑟𝑒𝑠𝑖𝑛 = 𝑚𝑖 − 𝑚𝑓 = 5𝑔 − 0.35𝑔 = 4.65𝑔

𝑉𝑓𝑖𝑏𝑒𝑟𝑠 =𝑚𝑓

𝜌𝑓=

0.35𝑔

2.55𝑔𝑚𝐿

= 0.137𝑚𝐿, 𝑉𝑟 =𝑚𝑟

𝜌𝑟=

4.65𝑔

1.1𝑔𝑚𝐿

= 4.227𝑚𝐿

𝐹𝑖𝑏𝑒𝑟 𝑣𝑜𝑙% =𝑉𝑓

𝑉𝑇=

0.137𝑚𝐿

0.137𝑚𝐿 + 4.227𝑚𝐿= 0.0314 = 3.14 𝑣𝑜𝑙% 𝑓𝑖𝑏𝑒𝑟𝑠

It should be noted that this experiment and calculation was performed for every sample. The

densities for the fibers and resin were taken at 2.55g/mL and 1.1 g/mL respectively. For groups

of 5 samples the middle 3 specimens showed slightly fiber content than in the uncured fiber/resin

68

mixture. The outside samples contained slightly higher levels of fibers, likely caused by shear

flow.

7.3 Settling Experiment

An experiment was conducted to observe how the fibers would settle once introduced into the

resin. For this study, 5 vol% fiber was mechanically mixed into the clear resin and placed in a

small, polycarbonate vile. Using the x-ray tomography machine, images were captured at 5-minute

intervals. A voltage of 50kV was used to obtain the pictures. Figure A-2 shows the fiber settling

progression.

From Figure A-2, it can be seen that the fibers continue to settle throughout 10-minute increments.

In image A, the solution appears to be dark which suggest that the fibers are homogenously

distributed throughout the mixture. As time progresses, fewer fibers are present in the images as

they settle out of the frame. There continues to be fibers caught in the meniscus of the resin, likely

due to the surface tension encapsulating some fibers. Thus, it can be concluded that the resin

should be agitated during printed, to avoid fiber settling.

Figure A-2: Progression of fibers settling over time.