PROFILE OF SUSTAINABILITY IN ADDITIVE MANUFACTURING AND ENVIRONMENTAL ASSESSMENT OF A NOVEL STEREOLITHOGRAPHY PROCESS
Harsha Malshe, Hari Nagarajan Oregon State University
School of Mechanical, Industrial and Manufacturing Engineering
Corvallis, OR, USA
Yayue Pan University of Illinois at Chicago Department of Mechanical and
Industrial Engineering Chicago, IL, USA
Karl Haapala Oregon State University
School of Mechanical, Industrial and Manufacturing Engineering
Corvallis, OR, USA
ABSTRACT Additive manufacturing has emerged as an arena that is
receiving intense interest from numerous technology domains,
traditional and non-traditional manufacturers. With this
growing interest, concerns have arisen regarding the relative
performance of these novel processes compared to conventional
techniques from economic, environmental, and social
perspectives. Sustainability-related benefits can be realized
through additive manufacturing, and it is often promoted as a
sustainable technology. For appropriate future development and
application, however, it will be important to understand relative
costs, environmental impacts, and human health effects of
processes and materials. Prior research addressing sustainability
and additive manufacturing is briefly reviewed. A life cycle
assessment is then conducted to understand the environmental
performance of a novel additive manufacturing process known
as fast mask-image-projection based stereolithography (Fast
MIP-SL). In Fast MIP-SL, projection light is patterned by a
digital micromirror device as a mask image to selectively cure
liquid photopolymer resin, and a two-way movement design is
adopted to quickly recoat material. The cradle-to-gate life cycle
assessment considers the impacts related to the curing of one
resin type and the consumption of electricity in the production
of parts of various geometries. Using the ReCiPe 2008 method
(hierarchist weighting), it is found that damage to resource
availability dominates ecosystems and human health damage
types for each part assessed.
INTRODUCTION In recent years, many advantages have been championed
for additive manufacturing (also known as 3D printing, solid
freeform fabrication, and rapid manufacturing) over traditional
subtractive and formative manufacturing processes. Subtractive
manufacturing processes remove material from billets or stock
material [1], and formative processes require specialized
materials, labor, and manufacturing techniques [2]. However,
additive processes assemble material layer-by-layer from
digital inputs of computer-aided design (CAD) models to
produce final or net-shape parts [3]. The ability to produce
customizable and functional parts on demand, the elimination
of tooling, and the expansion of the product design space has
significant benefits for a wide range of applications in many
industries [4]. Progress made through research has enabled the
growth of new and innovative techniques, and functionally
viable products, framing layer-by-layer manufacturing
processes as feasible alternatives to subtractive and formative
techniques [5].
Given that additive manufacturing enables the production
of geometrically complex parts from a wide range of materials,
a tremendous advantage over traditional processes can be found
in material utilization, which is nearly one-to-one [3]. In fact,
many sustainability benefits can be realized through additive
manufacturing due to the optimization of part design, which
can lead to high-performance functional parts with minimal
mass [6]. However, in recent years, several studies [4], [5], [7]
have been conducted on the environmental impacts of additive
manufacturing and their findings have been mixed. While the
advantages provided by a reduction in material consumption,
tooling, and harmful chemicals used in machining process is
well known, the benefits have been tempered by findings that
additive processes tend to be energy inefficient and contain
hidden wastes [8]. In reality, more efforts are required to fully
understand the breadth of sustainability factors and improve the
efficiency of additive techniques to compete with traditional
manufacturing processes [5], [9]–[11]. While additive
manufacturing has key advantages over traditional
manufacturing in terms of environmental performance, it still
lacks the ability to produce products at the scale of traditional
processes [5]. Although new additive technology is being
developed to overcome these hurdles, little is known about the
Proceedings of the ASME 2015 International Manufacturing Science and Engineering Conference MSEC2015
June 8-12, 2015, Charlotte, North Carolina, USA
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environmental performance of these processes. This is vital
given the future growth of additive manufacturing.
The investigation herein reviews the complementary roles
of sustainability and additive manufacturing. The synergy of
sustainability and additive manufacturing, the role of each in
design and their benefits for society, various indicators and
factors (e.g., energy consumption), and sustainability
assessment models in additive manufacturing are considered.
This review is followed by an environmental assessment of a
novel stereolithography (SLA) process, fast mask-image-
projection based stereolithography (Fast MIP-SL), for
production of parts in a more efficient manner than traditional
SLA processes. The study reviews the motivations and method,
presents a life cycle assessment for the production of several
products, and discusses the results of the assessment. Finally,
challenges and future work are discussed.
SUSTAINABLE MANUFACTURING Sustainability is a varied conception in today’s world. The
concept of sustainability was largely motivated as a result of a
series of environmental incidents and disasters, as well as fears
from chemical contamination and resource depletion [1], [12],
[13]. According to the United Nations Brundtland Report [14],
sustainable development is “development that meets the needs
of the present without compromising the ability of the future
generations to meet their needs.” Furthermore, it can be posited
that sustainable development is a function of three major
dimensions, namely economic, social, and environmental [15].
Under the sustainable development framework, the term
sustainable manufacturing can be interpreted within the
engineering contexts [16] as the “design of human and
industrial systems to ensure that humankinds’ use of natural
resources and cycles do not lead to diminished quality of life
due to either losses in future economic opportunities or to
adverse impacts on social conditions, human health, and the
environment.” Considering manufacturing systems as a
business function, the U.S. Department of Commerce [17]
defined sustainable manufacturing as “the creation of
manufactured products that use processes that are non-
polluting, conserve energy and natural resources, and are
economically sound and safe for employees, communities, and
consumers.” While these and other definitions (e.g., [1], [13])
have been proposed, they each contain the fundamental tenets
of economic, environmental, and social responsibility.
Manufacturing is the result of humanity’s rational desire
for continuous development and growth. It plays a major role in
modern socioeconomic systems. However, sustainability as a
systems approach requires a balance between consumption and
waste generation at a rate at which the environment can
assimilate and reproduce nutrients and resources [18]. For a
system to continuously develop and also constitute
sustainability, it should be considered a closed system with
system inputs and outputs in a closed loop [1]. Thus,
engineering researchers have a duty to provide advancements in
manufacturing processes, equipment, and systems, and reduce
material consumption, energy use, waste production, and
environmental impacts while simultaneously focusing on
product and process design.
ADDITIVE MANUFACTURING As defined by ASTM International [19], additive
manufacturing is a process of making objects from three-
dimensional solid model data by joining materials layer-by-
layer. While the most popular applications in additive
manufacturing still involve rapid prototyping for testing the
form, fit, and function of a design, the technology is growing as
a reliable method to design and manufacture functional
products of value [5], [20]. A key aspect of additive
manufacturing and its future success is the ability of the
technology to quickly produce parts at high volumes and
produce components customized for application- or customer-
specific needs. The layer-based process allows for the design of
almost any geometry, a drastic expansion of the previously
constrained design space.
The process begins with the designer producing a CAD file
for the specified geometry of the part to be made, which then
must be converted to a surface tessellation (STL) file. The file
is transferred to the computerized system, where the digital
representation is “sliced” into virtual horizontal layers of
varying thicknesses by computer software. The manufacturing
system then builds each layer individually, with each
successive layer added to the previous one. This bottom-up
build process is repeated until the part is completed.
Additive manufacturing systems are capable of utilizing
polymers, ceramics, metals, composites, and various other
materials. Different materials and binding processes are used,
but usually a powder of ceramic, nylon, or metal is used as a
base, which is then fused to form the desired layer geometry.
This eliminates the need for individualized molds usually
required for traditional, formative and subtractive
manufacturing methods. The process also reduces build times
for prototype or very low production volumes. ASTM
International defines seven key processes that form the set of
technologies know as additive manufacturing [19].
SUSTAINABILITY OF ADDITIVE MANUFACTURING The elements that make additive manufacturing an
advantageous method compared to traditional subtractive and
formative processes are compatible with the principles of
sustainability. The elimination of tooling, the ability to
manufacture complex geometries, and the selective placement
of material only where necessary, contribute to a reduction in
waste and an increase in process efficiency [6]. It has been
shown that the ability to update, repair, and remanufacture
tooling presents an opportunity for significant reductions in
energy consumption, emissions, and costs [2]. Additive
manufacturing, therefore, has the potential to impact the life
cycle of products [4] by both directly and indirectly reducing
the burden placed on the environment by manufacturing
processes.
Due to the layer-based nature of additive processes, the use
of raw materials and feedstocks can be minimized and made
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more efficient, as material is only placed where it is necessary
[6], and machining processes, which require solid billets, can
be displaced [5]. In addition, manufacturing waste streams can
be reduced by eliminating need for casting sands and binders
and avoiding the production of machining chips and spent
cutting fluids [5], for example. Lightweight products can
reduce carbon emissions across a product’s life cycle through
the implementation of lattice or honeycomb structures – only
made possible through additive processes [5], [6]. The optimal
design of products, which is usually constrained by traditional
manufacturing techniques, can be exploited to increase product
performance and add value through embedded functionality.
Furthermore, environmental benefits to the supply chain can
also be realized through the displacement of inefficient and
detrimental production processes, improvement of supply chain
flexibility, elimination of work-in-process and stock
obsolescence, compression of the supply chain, manufacturing
closer to the distribution location, and implementing on-
demand (just-in-time) manufacturing [6], [21].
Sustainable Design through Additive Manufacturing Diegel et al. [22] describes sustainable design as “design
which aims to achieve triple-bottom line ideals by striving to
produce products that minimize their detriment to the
environment while, at the same time, achieving acceptable
economic benefits to the company and, wherever possible,
having a positive impact on society.” In traditional
manufacturing, product design is constrained by the rules of
design for manufacturing (DFM) and rules of design for
assembly (DFA). Many products are not manufactured to their
optimal geometry, and extraneous materials are required by
casting/molding, forming, and machining processes [2]. For
example, injection molded parts must be removed from the die
and, so, are designed with a draft for ease of removal [22].
As highlighted above, additive manufacturing presents
numerous opportunities that have the potential to benefit the
sustainable design of products. Additive manufacturing is able
to produce a vast range of geometrically complex shapes
untethered from traditional manufacturing constraints. With
additive manufacturing, product designs are not constrained by
DFM and DFA. Hence, more sustainable designs can be
realized. More specifically, design for additive manufacturing
results in the generation of designs that enable more efficient
manufacturing, including hybrid manufacturing processes
(coupling additive and subtractive processes). These designs
can lead to decreased environmental burdens compared to
subtractive manufacturing [23]. Furthermore, additive
manufacturing enables extensive customization of products
exactly to customer specifications [3]. This can potentially
increase their desirability, subsequently maximizing the
resulting satisfaction and feelings of attachment from the
customer [22]. Ultimately, this can impact the longevity of
products and can therefore positively impact the sustainability
of products and their supply chain [24], [25]. In this context,
longevity is described as extending the useful life of a product,
which reduces the impact it has on the environment [26].
According to Diegel et al., most research in sustainable
product design focuses on aspects like lowering the
environmental impacts of material, resource, and energy use,
while it mostly ignores understanding “design quality” as a
method to maximize product longevity [22], [24], [25]. The
trade-offs between optimal design and manufacturability of a
product, and the trade-offs between custom fit and
characteristics of large-scale enterprise focused on process and
cost efficiency can potentially negatively impact design quality,
and subsequently longevity. Diegel et al. argues that additive
manufacturing has the potential to address both of these factors
and is, therefore, an effective tool to enable sustainable product
design [22].
In additive manufacturing, complexity and geometry have
little bearing on restrictions to design for manufacturability
[23]. Tools like topology optimization solve material
distribution problems to generate optimal geometries [27],
which can take advantage of the design freedom enabled by
additive processes. Importantly, this increased shape
complexity does not have a significant effect on the cost of the
additive manufacturing process or the finished good [27].
Maheshwaraa et al. [28] showed that topology optimization
resulted in improvement of the maximum surface deflection of
a UAV wing, compared with a non-optimized structure. Others
have shown environmental performance improvement through
topology optimization, e.g., reduced carbon impact through
reduced vehicle weight [27], [29], [30].
Societal Impacts of Additive Manufacturing Additive manufacturing, as a mainstream manufacturing
process, has made many convincing promises in the area of
social development. The potential of additive manufacturing to
optimize product design, increase product functionality, and
reduce the amount of energy or natural resources required for
the process can provide various societal benefits [31]. Part
flexibility is suited for producing customized products that
meet individual needs and, hence, play a significant role in
personalized health care [31], for example. Specifically, it has
been used to produce customized surgical implants and
assistive devices for improved health and wellbeing. With
increased needs for surgical implants, additive manufacturing
technology can be used to make custom implants in a solid or
resorbable material. Singare et al. [32] reported a new approach
to produce accurate implants that function well and are
aesthetically pleasing to the customer. Such approaches can
significantly shorten the design cycle and delivery lead-time for
custom surgical implants [33]. Another viable use of additive
manufacturing technology is custom fit, lightweight safety
equipment. Stab-resistant additive manufacturing textiles have
been investigated at Loughborough University, where additive
manufacturing technology is used to produce personal
protective equipment [34].
Apart from customized healthcare, additive manufacturing
technology also provides reduced environmental impact for
manufacturing technologies. It has been pointed out that
additive manufacturing can reduce supply chain complexity
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[35]. Specifically, this technology offers opportunities to
redesign products with fewer components and to manufacture
products near the customer (i.e., distributed manufacture) [31].
The net effect is the reduction of the need for warehousing,
transportation, and packaging.
As discussed by Tuck et al. [36], additive manufacturing
could change supply chain management thinking by improving
the efficiency of a lean supply chain through just-in-time
manufacturing and waste elimination. It reduces the material
distribution and inventory holding time for work in process by
reducing setup and changeover time. Also, additive
manufacturing can improve the responsiveness of an agile
supply chain. A build-to-order strategy can be implemented to
ensure no stockouts occur, while costs can be reduced through
distributed manufacturing and customizing products to
customers’ needs.
Several researchers have investigated the use of additive
manufacturing technology in the spare parts supply chain [37]–
[39]. Although the technology has not been fully incorporated
into the spare parts supply chain, it has been successfully used
to supply certain consumer goods. Reeves [40] described four
such businesses: (1) Fabjectory enables players of Second Life
to purchase models of individualized avatar characters,
(2) FigurePrints allows players of World-of-Warcraft to order
1/16th scale models of their online gaming characters,
(3) Landprint offers personalized 3D models of any place on
earth, and (4) Jujups offers a range of personalized gifts such as
photo frames, badges, mugs and even chocolate. The
technology has the potential to dramatically change the
landscape of the conventional supply chain and is expected to
become a key manufacturing technology in the sustainable
society of the future.
SUSTAINABILITY ASSESSMENT
Environmental Impacts Advancements in manufacturing technologies provide
ample opportunities in the product sector. Rapid prototyping or
rapid tooling has shown remarkable expansion over the past ten
years. However, despite demonstrated success of rapid
prototyping, the need for environmental impact assessment is a
challenging and necessary task [41]. Joined effort of process
control engineers with environmental specialists is essential in
understanding the fundamental impacts of newer technologies
and materials. Also, evaluation of the extent of impact of these
process and their resulting products is required to define
regulations and the spatial distribution that will enable the
control and prevention of the potential harm, along with
estimates of the cost required to deal with related issues.
Industrial ecology, also known as green manufacturing,
recognizes industrial impacts on environment and involves the
development of methods for their measurement and assessment
[41]. Environmental responsibility has become an important
issue in industry, driven by the regulations governing
manufacturing emissions and end-of-life disposal of products,
increasing demand for environmental certification requirements
(ISO 14000) worldwide. Following the call for environmental
responsibility and the demand for impact assessment, several
evaluation methods have been developed [42]–[44]. Life cycle
analysis (LCA) developed by the Society of Environmental
Toxicology and Chemistry (SETAC) is the internationally
accepted tool for evaluating the environmental impacts of
various industrial processes, products and activities [41].
Environmental impact assessment of additive
manufacturing is limited in literature. One such study,
conducted by Luo et al. [10], proposed a method for evaluating
the environmental performance of additive manufacturing by
dividing the whole process into individual elements (material
preparation, energy consumption, material toxicity, and waste
disposal). Each element was considered within the various life
stages to cope with process complexity [10]. Their method was
demonstrated for a stereolithography (SLA) process with focus
on the energy consumption rate (ECR). The environmental
impact was calculated as the ECR multiplied by electricity
consumption factor (0.57 mPts/kWh) [45], and comparisons
were made for three different equipment models used for the
SLA process. Even though the principle method is LCA, it fails
to account for the potential toxicological health and
environmental risks that can occur from handling, use, and
disposal of the photo resin materials. This is simply because the
toxicity and environmental impacts of many additive
manufacturing materials and chemical solvents used for their
removal have not been identified to date [41]. The material
safety data sheets (MSDS) available are limited to older
generation, epoxy resin materials. The majority of these data
recognize that severe eye and skin irritation and possible
allergic skin reactions might occur as a result of handling or
inhaling vapor from those materials.
Other environmental impacts, e.g., eco-toxicity and
emissions of carbon dioxide and nitrogen oxides, are less
known and are acknowledged to occur during resin
decomposition. The recommended disposal of such products by
producers is landfilling or incineration. This leads to
recognizing the importance of not only biodegradability, but
also the effects of leaching of chemicals from landfills.
Environmental effects of newer materials are unknown and
future waste treatment, storage, and transportation of such
products is limited to federal, state/province, and local
regulations [41]. A lack of information about the potential
damage of the products on the environment inhibits the
reliability of the regulations to provide for suitable waste
treatment. In addition to photopolymer resins, information
about the chemical solvents used for the removal of excess
resin on the products is unknown. Data about environmental
mobility (air, water, and soil) is unavailable [46]. Similarly,
data on human toxicity is limited [41]. Another important issue
that should be addressed is the impact and consequences of
energy consumption of these processes, as discussed in the next
section.
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Energy Consumption Energy generation and industrial activity contribute
significantly to the overall emission of greenhouse gases [11],
which are thought to be the key driver of global warming. Thus,
the reduction of energy consumption in manufacturing is key to
limiting the overall emission of greenhouse gases. Additive
manufacturing allows the production of multiple components in
a parallel manner entirely without the need for tooling [47],
[48]. It inherently offers opportunities for reducing waste. The
single-step nature of additive manufacturing also provides
transparency to the energy utilized in the process. However,
additive manufacturing has several drawbacks in terms of
product quality, processing speed, and high costs [41]. From
the perspective of energy consumption, additive manufacturing
processes are usually not as efficient as conventional
manufacturing processes [49]. For example, machine tools are
equipped with many peripheral devices; thus, basic power
consumption and processing time are the two main
considerations in energy consumption calculations [41].
Additive manufacturing processes involve the construction
of a part that may consist of thousands of layers. Thus,
production may require significantly more time than
conventional processes. Additive manufacturing requires a
significant amount of energy, because the energy consumed per
unit volume of material is high [32]. Nevertheless, the
advantages of additive manufacturing can contribute to
improving the environmental impacts over the entire lifecycle
of the product, as discussed above. The Advanced
Manufacturing Office of the U.S. Department of Energy claims
that additive manufacturing saves energy by eliminating
distributed manufacturing processes and material waste [50].
The energy consumption in additive manufacturing is often
specified in terms of energy consumption rate (ECR), i.e., the
amount of energy consumed per unit mass of material used
[11], [10], [41], [49], [51], [52]. Previous studies suggest that
there may be an increase in ECR as productivity increases. In
SLA processes, this effect is related to the solidification rate of
the raw materials [8]. It is seen that additive manufacturing
provides an advantage for large build volumes, as well as
higher build rates for products having a small number of parts
[53]. However, additive manufacturing processes are often
inefficient compared to conventional processes. To reduce
environmental impact, optimization of energy consumption in
additive manufacturing is essential [8].
Environmental Modeling of Additive Manufacturing Until recently, little effort has been invested in the
development of environmental models representing the additive
manufacturing life cycle. In the 2009 Roadmap for Additive
Manufacturing, Bourell et al. [5] stated that achieving the
important additive manufacturing sustainability goals will
require a total life cycle analysis and a comprehensive
sustainability evaluation of each additive manufacturing
process. This includes analysis of four life cycle stages: pre-
manufacturing, manufacturing, use, and post-use. It is also
imperative to ensure the development of DFSAM (Design for
Sustainable Additive Manufacturing) [9]. As defined by Rosen
[54], Design for Additive Manufacturing (DFAM) is the
“synthesis of shapes, sizes, geometric mesostructures, and
material compositions and microstructures to best utilize
manufacturing process capabilities to achieve desired
performance and other life-cycle objectives.” However, in the
context of sustainability, a new methodology for DFSAM was
developed [9]. It includes both DFAM and environmental
impact assessment in the development of products and
processes. This is critical, as innovative product design and
manufacturing activities in the coming century will require the
integration of life cycle data and sustainable design principles
for products and processes [5]. This integration will be enabled
through material, process, and system modeling approaches.
In the past decade, however, only a few models have been
developed for assessment, prediction, and optimization of
environmental impacts and efficiency of additive processes. For
instance, Bourhis et al. [9] proposed a new LCA-based
methodology to evaluate the environmental impact of a part
from its CAD model for a direct metal deposition process. The
process model is based on electricity, fluid, and material
consumption, unlike previous energy-only assessments [4],
[10], [29], [49]. Through predictive modeling of process inputs,
the model aims to minimize consumption of all material and
energy fluxes during manufacturing by integrating the model
into design activities [55]. The model enables environmental
evaluation of different manufacturing strategies for the same
part, based on the CAD model.
Verma and Rai [56] have proposed another modeling
approach based on multi-step optimization to enable energy
efficiency of additive manufacturing technology. The model
aims to minimize material waste and energy consumption for
finished parts as well as on a layer-by-layer basis [56]. The
model is formulated for selective laser sintering (SLS), but is
claimed to be easily extended to other additive processes. The
proposed approach is generic and does not seem to require part
geometry data, including complexity, curvature, or identified
features. Development and experimental analysis demonstrates
the ability of the proposed optimization techniques to determine
manufacturing process plans and compete with current layer-
by-layer slicing approaches.
Faludi et al. [7] used LCA to conduct a comprehensive
comparison of subtractive and additive manufacturing
processes across major sources and types of ecological impacts.
The study compared fused deposition modeling (FDM), inkjet
printing, and CNC (computer numerically controlled)
machining of polymeric materials. In order to conduct a fair
comparison, part production was modeled on a part per year
basis, which was then followed by a calculation of ecological
impacts. This comprehensive cradle-to-grave study found it
cannot be unconditionally stated that additive manufacturing
technology has an advantage over subtractive processes in
terms of environmental impact, specifically material waste or
energy consumption. The relative impact of additive processes
depends primarily on machine utilization, therefore the best
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strategy for enhancing environmental performance is to have
the fewest number of machines running the most jobs each [7].
ENVIRONMENTAL IMPACT ASSESSMENT OF A NOVEL STEREOLITHOGRAPHY PROCESS
Mask Image Projection Stereolithography (MIP-SL) is part
of a class of additive manufacturing technologies that uses light
to solidify liquid resin one layer at a time, also known as
Digital Light Projection (DLP). In the process, projection light
is patterned by a digital micromirror device (DMD) as a mask
image to selectively cure the liquid photopolymer resin. A
DMD is a micro-electromechanical system (MEMS) device that
enables one to simultaneously control ~1 million small mirrors
to turn on or off a pixel at over 5 kHz. An illustration of the
DMD chip and its use in a MIP-SL system is shown in Figure
1. Similar to other additive manufacturing technologies, MIP-
SL processes start with a CAD model, which is then sliced with
a certain layer thickness. Each resulting slice is stored as a
bitmap to be displayed on the dynamic mask. The light
radiation that is reflected by the “on” micromirrors projects the
sliced bitmap image onto the resin surface to cure a layer. An
automated vertical (Z) stage is used to raise the platform in a
resin vat.
The technology addresses the need to develop additive
manufacturing machines with higher throughput [5]. Since an
entire 2D layer of the part is cured during one shot of projection
using dynamic mask images, unlike point or line processing
additive manufacturing technologies, MIP-SL has an advantage
of build speed. The MIP-SL process showed its manufacturing
capability in shortening the building time greatly without
affecting the part quality. Thus, it is desired to assess the
relative performance of a specific MIP-SL system on an
environmental impact basis. The approach for conducting the
assessment is presented below, but, first, general MIP-SL
processes and a specific MIP-SL system is described in more
detail.
In MIP-SL processes, the typical building sequence for
each layer consists of spreading liquid resin into a uniform thin
layer by stage motions and curing the formed liquid layer into a
solid layer. There are two ways of spreading liquid resin:
(1) The free surface approach, which usually uses a top-down
projection as shown in Figure 1. The material recoating is
done by lowering the cured part down into the vat of resin
and waiting for the liquid to settle down under gravity.
(2) The constrained surface approach, which usually uses a
bottom-up projection. The mask image penetrates the
bottom of a resin tank which is optic clear. After a layer is
cured at the bottom of the built part, the platform or the
tank is moved by a stage to separate the newly cured layer
from the bottom of the tank. Then a small gap is formed to
fill a new layer of liquid resin between the part and the
bottom surface of the tank for the next layer curing.
In order to achieve high-speed production, a bottom-up
projection system with a two-way movement separation
mechanism was proposed for the Fast MIP-SL system [57]. A
PDMS (polydimethylsiloxane) film is coated on the bottom
surface of the resin vat, and a slide motion in the horizontal (X)
direction is adopted. The PDMS coating and the two-way
movement design significantly reduced the separation force
between cured layers and the resin tank. Hence, the motions
can be performed quickly to separate the newly cured layer
from the bottom of the tank, and then recoat a uniform thin
liquid resin layer. The fast MIP-SL process showed the
capability of building a moderately-sized part within minutes
instead of hours that are typically required in additive
manufacturing systems.
FIGURE 1: AN ILLUSTRATION OF A MASK-IMAGE-PROJECTION-BASED STEREOLITHOGRAPHY (MIP-SL)
SYSTEM.
A set of CAD models with different complexity was used
in testing the performance of the approach [39]. Two different
layer thicknesses commonly used in the MIP-SL process were
tested. A 50 μm layer thickness was used in the production of a
gear model (Figure 2a). The mask image projection time was
0.35 s for each layer except the base. The projection waiting
time was set at 0.1 s. For all the other models (Figures 2b-f), a
100 μm layer thickness was used in their building processes.
Due to the larger layer thickness, a longer image exposure and
projection waiting times were used (0.45 s and 0.3 s,
respectively). Accordingly, the Z-stage movement took a longer
time for a larger layer thickness. The movement time in the Z
axis is 0.32 s and 0.42 s for 50 μm and 100 μm layer
thicknesses, respectively. A relatively brief waiting time (50-
100 ms) is adopted after the X sliding movement. To obtain
good surface quality, it is critical to guarantee the small gap is
filled with liquid resin completely with no high flow velocities
before curing. So a shorter time (50 ms) was used for parts with
smaller cross-sectional areas, while a longer time (100 ms) was
used for parts with bigger cross-sectional areas.
Two types of resins, SI 500 and Acryl R5 (Envisiontec
Inc.) [58], [59], with slightly different curing characteristics
were tested. For the same layer thickness, the curing of Acryl
R5 took ~0.1s longer than SI 500. The viscosities of the two
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resins are also slightly different. However, the same settings
can be used in the two-way movement design based on the two
resins. Based on the chemical and build time similarity between
both resins, only SI 500 was considered.
FIGURE 2: OBJECTS MANUFACTURED USING THE MIP-SL PROCESS: (A) GEAR, (B) HEAD, (C) STATUE, (D) SHELL,
(E) BRUSH, AND (F) TEETH
Motivation for Environmental Impact Assessment As iterated above, the rapid pace of advancements in
additive manufacturing technology necessitates sustainability
assessment to ensure their responsible development. Given the
potential of these technologies to enhance environmentally
responsible manufacturing and economic development across
the world, engineering research should investigate the relative
environmental impacts of these technologies. Thus, this study
aims to conduct an environmental impact assessment of the
experimental MIP-SL technology presented in the section
above using a cradle-to-gate LCA approach.
Life Cycle Assessment LCA is useful for examining the design of products and
processes to reduce the impact on human health and the
environment [10], [60]. To evaluate the environmental
performance, a process model based on life cycle concept is
proposed. From the life cycle point of view, a part produced
with an additive manufacturing process goes through the
following key stages:
a) Production of input building material
b) Input of building material into the system
c) Building the part layer to layer
d) Post-processing of the part
e) Distribution of the part to the customer
f) Disposal of part.
To provide a more precise view of the process, material
and energy consumption, and process wastes in the production
of the part, and disposal are taken into account to calculate the
process environmental performance. In the process model, the
overall environmental performance of the process is the sum of
individual environmental performance of the various life
stages. By identifying the individual environmental impact
factors of different life stages, the overall environmental
performance can be evaluated.
Stand-alone LCA studies were conducted for six design
models (gear, head, statue, shell, teeth and brush), shown in
Figure 2. The functional unit for the analysis is 1000 units of
each manufactured part (to represent production at scale). The
process under study has several life cycle stages as follows:
a) Material preparation
b) Part building
c) Part cleaning
d) Waste disposal.
TABLE 1: FAST MIP-SL PROCESS DATA FOR ENVIRONMENTAL IMPACT ASSESSMENT
Part Gear Head Statue Shell Brush Teeth
Mass (g) 1.10 7.38 1.17 1.19 0.38 0.72
Build time (min.) 2.31 11.26 9.86 6.24 2.25 2.04
Mat. waste (g) 0.15 0.22 0.38 0.35 0.09 0.21
In the material preparation stage, the environmental
impacts are due to material extraction and production. During
the part building stage, the main source of environmental
impact is electricity use. Process residues generated during part
cleaning have environmental consequences. Finally, the wastes
generated during the process, such as material residues and
cleaning wastes, can be landfilled or incinerated. Different
disposal methods have different environmental impacts, and
landfilling is selected for the hazardous chemical waste
produced during the process. Process emissions (gaseous) are
not considered. For the analysis, the relevant data, including
electric power consumption, mass of the built object, build
time, and material waste (Mat. waste) are shown in Table 1.
RESULTS AND DISCUSSION The ISO 14040 LCA framework is utilized to conduct the
environmental impact assessment [61]. The goal of this study is
to assess the environmental performance of the Fast MIP-SL
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process in fabricating six different parts introduced above (i.e.,
gear, head, statue, shell, brush, and teeth). Environmental
impacts are assessed for each of the parts, including materials,
energy involved in the process, and waste generated by the
process. The life cycle inventory (LCI) was developed with
reference to previous studies by Luo et al. [10], who analyzed
the environmental performance of stereolithography (SLA) as a
rapid prototyping process. The inputs for the process are resin
(Perfactory SI500) for the parts, ethoxylated alcohol for part
cleaning, and electrical energy for the process. Outputs include
the finished part and process waste.
To determine the environmental impacts of the process, the
ReCiPe 2008 method with World ReCiPe H/A weighting is
selected, because of its categorization of impacts [62]. The
impact categories are addressed at the endpoint level, with three
indicators (damage to human health, damage to ecosystems
diversity, and resource depletion). In this research, the
hierarchist perspective is applied, which is based on the most
common policy principles with regard to time frame and other
issues [62]. LCI data were imported to LCA software (SimaPro
8.1), which generated the relative environmental impact results
presented below.
The results of the environmental impact assessment are
shown in Figure 3. From the endpoint perspective, damage to
human health (33%) and resource depletion (64%) represent the
vast majority of impacts. It is seen that production of the brush,
which has the lowest mass and second shortest build time, has
the lowest impact, while the head, which has the greatest mass
and longest build time, causes the highest environmental
impact. The distribution of environmental impacts for the
different elements involved in the production of the “head” is
shown in Figure 4. The impact distribution was found to be
similar for other parts evaluated. From the figure, it can be seen
that impacts are primarily attributed to MIP-SL process energy
use (electricity), followed by impacts due to production of the
resin. The majority of impacts (75%) for the “head” part
material were due to resource depletion, as well as the majority
of process energy use impacts (58% due to resource depletion).
Data from environmental impact assessment for SLA from
previous work [10] is shown in Table 2. Primarily herein, the
ReCiPe 2008 method with World ReCiPe H/A method
implemented within SimaPro 8.1, which is more accurate and
current, is used to discuss the resulting environmental impacts.
However, the Eco-Indicator 95 method [45] has only been used
to evaluate the environmental impact in comparison with this
previous study [10]. It is observed that the value obtained for EI
(Table 2) for the current study is lower than the best value
obtained from the previous study.
In the Fast MIP-SL process, the total building time is much
shorter compared to typical MIP-SL processes, that is, the
projector operation time and, hence, the projection power
consumption is reduced greatly. Meanwhile, the separation
force in Fast MIP-SL is much smaller than typical MIP-SL
processes, resulting a much smaller load on Z slide and, hence,
a smaller power consumption for movement. In comparison, it
appears that the Fast MIP-SL method has a lower
environmental impact than traditional SLA processes.
0
1
2
3
4
5
6
7
8
9
Gear Head Statue Shell Brush Teeth
Envir
onm
enta
l im
pac
t (m
Pts
)
Human Health Ecosystems Resources
FIGURE 3: ENVIRONMENTAL IMPACTS OF EACH MANUFACTURED PART (METHOD: RECIPE ENDPOINT (H) V1.03/WORLD RECIPE H/A, FUNCTIONAL UNIT = 1 PART).
0
1000
2000
3000
4000
5000
6000
Process Waste Perfactory Resin SI500 Electricity
Envir
onm
enta
l im
pac
t (
mP
ts)
Human Health Ecosystems Resources
FIGURE 4: RELATIVE ENVIRONMENTAL IMPACTS FOR “HEAD” PART (METHOD: RECIPE ENDPOINT (H)
V1.03/WORLD RECIPE H/A, FUNCTIONAL UNIT = 1000 PARTS).
TABLE 2: ENERGY CONSUMPTION RATEa AND ENVIRONMENTAL IMPACTb OF SLA AND FAST MIP-SL
Process Machine Material ECR a
(kWh/kg)
EI b
(mPts/kg)
SLA [10] SLA-250 Epoxy resin
SLA 5170
32.48 18.51
SLA-3000 Epoxy resin
SLA 5171
41.41 23.6
SLA-5000 Epoxy resin
SLA 5172
20.70 11.80
MIP-SL PAN-1 Epoxy resin
SI 500
13.87 7.91
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CHALLENGES AND FUTURE WORK Additive manufacturing is a class of technologies that
builds 3D objects by accumulating material, which are defined
under seven key categories by ASTM International [7]. In each
category, systems developed by different groups use different
techniques to deliver energy, and deposit and process material.
For example, vat photopolymerization includes laser scanning
SLA, MIP-SL, two-photon technology, and similar processes.
For MIP-SL, different light sources like laser, LED, or visible
light lamps are used. Some MIP-SL systems use the
constrained surface recoating technique and some use the free
surface recoating technique. Due to such big category-to-
category and process-to-process variations, it is difficult to
assess a whole category of technologies. Furthermore, material
waste, disposal information and part disposal information are
not well-documented in system/process development studies.
While developments in additive process technology have
rapidly progressed, process breakthroughs are yet to be
followed by breakthroughs in design [23]. In fact, one of the
technical barriers to the adoption of additive manufacturing for
finished part production is incomplete integration of
homogenous design with heterogeneous CAD and closed-loop
additive manufacturing [5]. This will enhance the desirability of
designed products that are unlimited by the traditional materials
selection and geometric definition approach.
Currently, Vayre et al. [23] have proposed a general
methodology to design for additive manufacturing. The method
includes generation of an initial shape, definition of a set of
geometrical parameters, attainment of an optimized shape, and
finally, validation of the shape. Finally, Ponche et al. [63]
describes work on creating a global design for additive
manufacturing methodology, which proposes to obtain an
appropriate design for additive processes. In order to prevent
“psychological inertia” which may limit design innovation, and
also to best utilize additive process capabilities, the
methodology starts directly with both functional specifications
and additive process characteristics.
Although results of this study indicate that the Fast MIP-
SL method has a lower environmental impact than traditional
SLA processes, the LCA conducted highlights several concerns
as the Fast MIP-SL method continues to develop. Further
detailed analyses of the Fast MIP-SL process, as well as other
additive manufacturing processes, would be beneficial to
predict trends and any internal relations between the individual
elements of the process and the environmental impacts. In
particular, it will be useful to better understand layer build
volume, layer thickness, and part shape effects on various
impact measures. Subsequently, the results of future studies
would be strengthened by an examination of the sensitivity of
relevant design and manufacturing parameters that influence
environmental performance, including part mass, build time,
and layer thickness. Separately, it would be informative to
examine the influence of design parameters on environmental
performance, including shape complexity, dimensional size,
and build time.
Comparison of the SLA process with other similar
processes would be preferable to determine a foundational
baseline for product and process comparisons. Defining this
information will enable engineers to suggest improvements to
products and processes to enable more sustainable additive
manufacturing.
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