Profile of Sustainability in Additive Manufacturing and ... · receiving intense interest from...

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



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



[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.



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



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



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



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



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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