Throughput and Strength Optimization for Fused Deposition...
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Throughput and Strength Optimization for
Fused Deposition Modeling of Ankle-Foot Orthotics
Project Final Report
April 22, 2016
1.1 Aim Orthotics are medical devices used to help patients with various muscle deficiencies maintain
alignment during walking or sitting. The current process for producing custom orthotics is a time-
consuming and labor intensive manufacturing process with lead times of up to two-
weeks. Additive Manufacturing (AM) has made significant impacts on our ability to quickly and
accurately prototype custom parts with high complexity and detail. Fused-deposition modeling
(FDM), an inexpensive and reliable form of AM, has been suggested as a possible solution to
reducing custom orthotic manufacturing time . One of the steps in creating a viable business
model around FDM orthotic manufacturing is minimizing the manufacturing time and maximizing
strength subjected to various manufacturing constraints.
In this project, we are optimizing the FDM manufacturing time and strength by manipulating
key variables, constraints, and parameters associated with the process. Variables such as
orientation angle, infill percentage and layer height can all affect the overall time for FDM printing.
Additionally, the type of infill plays a large role in the required AM time and strength. Jin proposed
using a wavy infill to reduce infill time ; however, the paper does not suggest the optimum
parameters of the wave. In this paper, we will attempt to answer the following two research
(1) What are the optimum wavy toolpath parameters that minimize layer time and maximize
strength (Robert Chisena)?
(2) What are the optimum set of build parameters that minimize the AM manufacturing time of
a solid ankle-foot orthotic (AFO) (Dian-Ru Li)?
1.2 Technical Features Fused-deposition modeling (or 3D printing) is a process that builds three-dimensional parts by
stacking layers one-by-one. These layers are created by depositing lines, or roads, of molten plastic
through a toolhead, which is able to travel in the XY-plane (Figure 1). This material is usually in
the form of 1.75 mm round filament wrapped around a spool. This filament is then fed into the hot
end of the toolhead with a gear drive.
Figure 1. FDM Desktop TAZ 5 3D Printer by Lulzbot.
Additive manufacturing creates objects using many successive layers. To print, slicing
software called Simplify-3D was used to convert the computer-aided design (CAD) model into
printing instructions called G-Code. After importing the CAD model, the software slices the model
into multiple layers based on a given layer height. Upon slicing the model, the infill pattern is
assigned based on user settings. Figure 2 shows the process used in Simplify-3D to create the print
parameters for this project.
Figure 2. Simplify-3D Interface with Orthotic Toolpath
The path taken by the toolhead plays an important role in the overall time and strength of the
finished part. Common toolpaths include contours, rectilinear, and honeycomb fills (Figure 3).
However, these toolpaths are used without concern for the part being created. When using a
rectilinear infill pattern, for instance, the toolhead must accelerate to its normal operating velocity
and then decelerate to a stop at the end of each road. Because there are often thousands of stops
within a large part, the required accelerations result in large inefficiencies in time and energy.
Figure 3. (Left to Right) Contour Infill made by offsetting each road. Raster fill. Honeycomb fill.
Other build parameters that are important to the time of FDM additive manufacturing include
thickness of the layers, raster width (synonymous with beadwidth and contour width), maximum
toolhead speed, and part orientation (Figure 4). Each of these parameters can be altered within a
slicing software such as Simplify 3D; however, since so many tuning parameters exist, it is difficult
to determine which parameters affect the printing time the most.
Figure 4. (a) Longitudinal and Medial Angles and Location of the Part on the Print Bed are important
considerations. (b) Layer thickness plays an important role in manufacturing time because an increased layer height
reduces the total of overall layers. (3) Various parameters associated with a raster fill layer.
2 SYSTEM OVERVIEW
2.1 Notations All symbols to be used in this project and units for each quantity are given in Table 1.
Table 1. Notations used for Optimization Project
Symbols Definition Unit
L Length of part mm
W Width of part mm
H Height of part mm
TA Actual manufacturing time min
TE Estimated manufacturing time min
TI Ideal printing time min
TPi Tool path of i layer mm
Part build orientation (medial axis) degree
Part build orientation (longitudinal axis) degree
N Number of layer -
HL Layer height mm
I Infill percentage %
Is Support infill percentage %
WE Extruder width mm
WC Contour width percentage %
V Toolhead speed mm/min
WP Weight of the part g
WS Weight of the support material g
A Infill angle (raster angle) degree
O Outline overlap %
NC Number of contours -
Flexural stress MPa
SW Flexural Strength-to-weight ratio MPa/g
ext Extension mm
f Frequency 1/mm
Int Interference %
Interference Region mm
BW Beadwidth mm
OL Overall Length of Coupon mm
OT Overall Thickness of Coupon mm
t time per layer sec
x Distance along coupon mm
Lss Length of Support Span mm
DLN Distance between load-applying noses mm
W Weight of the coupon sample mm
T Thickness of AFO mm
2.2 System Description The aim of this project is to minimize the additive manufacturing time and maximize the strength
of an AFO with a wavy infill. We approach the optimal solution by dividing an additive
manufactured AFO into two subsystem: (1) per layer parameters and (2) macro-system build
parameters. Each of these subsystems can be further divided into another two subsystems, strength
and time (Figure 5). In this project, we have limited our focus to optimizing the manufacturing
time for the macro-system and the strength for each layer.
Figure 5. System Level Design Optimization Problem for Optimizing Strength and Time of AM AFO.
Red box defines the scope of this project.
2.2.1 Subsystem 1 - Wavy Tool Path Strength Optimization In this subsystem, the goal was to design an optimized infill pattern that maximizes strength while
minimizing the time required to complete the toolpath on each layer. According to Jin, a toolpath
that uses a sine wave between two outer contours will reduce the accelerations required to start
and stop the toolhead . Reducing accelerations on the machine reduces print time, machine wear,
and energy consumption. Furthermore, the sine wave behaves similar to a truss bridge, a robust
structure that can dynamically and efficiently redistribute loads across its structure.
2.2.2 Subsystem 2 - AFO Build Parameters In this subsystem, we aim to minimize the manufacturing time by adjusting the build parameters.
Those parameters include layer height, infill pattern, contour width, etc. Altering these parameters
will generate different tool paths per layer from Simplify3D based on the part geometry, and also
change the manufacturing time. The ideal objective time function can be described as follows:
Figure 6. Example of a Warren Truss Bridge.
where V is the toolhead speed adjusted depending on users and is the tool path at i layer.
However, in the real practice, the manufacturing time actually depends on the velocity control
algorithm of the machine. More specifically, there exist lots of stopping points where the extruder
changes its printing direction to fill in the materials within the contours. The extruder will decrease
the speed down to zero on the stopping points and then accelerate to the given tool head speed (V).
Also, the algorithm implemented in Simplify3D will adjust the printing speed according to
different geometry and parameters sets for better printing quality of part, and thus the speed wont
always stay on the maximum (the V we assigned). Therefore, the above equation fails to reflect
the real manufacturing time.
Due to the difficulty of describing the subsystem in a simple physical equation, we use data-
driven modeling technique to derive our objective function. Several experiments are performed to
capture system behavior for finding the minimal manufacturing time in our design space. The
methodology and results will be discussed in section 4.
3 SUBSYSTEM OPTIMIZATION SUBSYSTEM 1 In the printing of thin-walled features such as orthotics, reducing time while maintaining overall
part strength is important. Jin proposed using a wavy structure that would use a sine wave to fill
in the areas between thin-walled features .
3.1 Design Variables and Parameters The wavy infill pattern is akin to a truss bridge that uses truss elements to dynamically support