M

Ja

b

c

a

ARRA

KMSA

1

rclstvmtvt2(ePo

sf

0d

Journal of Materials Processing Technology 211 (2011) 318–328

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

ulti-material stereolithography

ae-Won Choia,b, Ho-Chan Kimb,c, Ryan Wickera,b,∗

Department of Mechanical Engineering, The University of Texas at El Paso, El Paso, TX 79968, USAW.M. Keck Center for 3D Innovation, The University of Texas at El Paso, El Paso, TX 79968, USASchool of Mechanical Engineering, Andong National University, Andong-si, Gyeongbuk 760-749, South Korea

r t i c l e i n f o

rticle history:eceived 5 January 2010eceived in revised form 4 July 2010ccepted 3 October 2010

eywords:ultiple material fabrication

a b s t r a c t

A multi-material stereolithography (MMSL) machine was developed by retrofitting components from acommercial 3D Systems 250/50 stereolithography (SL) machine on a separate stand-alone system andadapting the components to function with additional components required for MMSL operation. TheMMSL machine required construction of a new frame and the development of a new rotating vat carouselsystem, platform assembly, and automatic leveling system. The overall operation of the MMSL system wasmanaged using a custom LabVIEW® program, which included controlling a new vat leveling system andnew linear and rotational stages, while the commercial SL control software (3D Systems Buildstation 4.0)

tereolithography (SL)dditive manufacturing (AM)

was retained for controlling the laser scanning process. During MMSL construction, the sweeping processcan be inhibited by previously cured layers, and thus, a deep-dip coating process without sweepingwas used with low viscosity resins. Low viscosity resins were created by diluting commercial resins,including DSM Somos® WaterShedTM 11120, ProtoThermTM 12120, and 14120 White, with propoxylated(2) neopentyl glycol diacrylate (PNGD). Several multi-material complex parts were produced providingcompelling evidence that MMSL can produce unique parts that are functional, visually illustrative, andconstructed with multi-materials.

© 2010 Elsevier B.V. All rights reserved.

. Introduction

Additive manufacturing (AM) technologies, also referred to asapid prototyping, layered manufacturing, solid freeform fabri-ation, direct digital manufacturing and other terms, utilize aayer-by-layer stacking method to produce real 3D structures fromolid computer models (Chua et al., 2003). Since their introduc-ion more than 20 years ago, AM systems have been used in aariety of applications. These applications have ranged from theore conventional prototyping and rapid tooling (Radstok, 1999)

o more advanced applications such as fabricating flexible cardio-ascular models (Arcaute and Wicker, 2008), medical implants andissue engineered constructs (Dhariwala et al., 2004; Arcaute et al.,006; Lee et al., 2007; Choi et al., 2009b), 3D electronic devices

DeNava et al., 2008), micro-channels (Wicker et al., 2005b; Choit al., in press) and various micro-systems (Mizukami et al., 2002;almer et al., 2006; Neumeister et al., 2006). There are numer-us AM technologies available in the market. These commercial

∗ Corresponding author at: Department of Mechanical Engineering, The Univer-ity of Texas at El Paso, El Paso, TX 79968, USA. Tel.: +1 915 747 7443;ax: +1 915 747 5019.

E-mail address: rwicker@utep.edu (R. Wicker).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.10.003

systems can generally be classified by the type of build mate-rial in its pre-fabricated form (powder, liquid, solid) (Bartolo andMitchell, 2003); method of bonding between the layers (chemi-cal bond, sintering, gluing) (Kulkarni et al., 2000); type of supportmaterial used during the process (same as build material, sep-arate support material, self-supported or no support material)(Pham and Gault, 1998); accuracy and resolution of the technology;and many other factors. Among these systems, representative AMprocesses include stereolithography (SL) using a laser and liquidphotocurable resin, laser sintering (LS) using a laser and powdermaterial, fused deposition modeling (FDM) using heated extru-sion nozzles and solid thermoplastic filament, and 3D printing(3DP) using inkjet printer heads and liquid jettable photocurableresin.

Most systems are designed to build parts out of single mate-rials, but there may be advantages derived from building partsout of multi-materials. Of the technologies referenced above, theFDM process certainly represents a feasible candidate for fabricat-ing parts out of multi-materials since FDM has separate extrusion

nozzles for the build and support materials. In fact, a multi-nozzle deposition system, similar to FDM, was designed and usedfor producing 3D tissue-engineered scaffolds (Khalil et al., 2005).In addition, a modified FDM system was designed and used forproducing advanced functional ceramic and composite compo-

Proces

nhtLcbtrctIpcmAimlgf

fgprfftecgtma(ri

aRarrtcistre

mmMoatpsrppmdet

J.-W. Choi et al. / Journal of Materials

ents out of multiple ceramic materials (Jafari et al., 2000). Thereave also been attempts at using LS for multi-material fabrica-ion (Jackson et al., 2000; Liew et al., 2001; Santosa et al., 2002;iew et al., 2002; Regenfuss et al., 2007). Access to the buildhamber in LS, however, is difficult due primarily to its controlleduild environment (nitrogen gas environment and particle bedemperature control) and the design of the material feed andecoating system. Although several examples exist, these buildhamber access limitations have limited multi-material fabrica-ion examples in LS. Most recently, Objet Geometries Ltd. (Rehovot,srael) launched the Connex 500TM system that employs inkjetrinting technology. This system represents the first commer-ial system on the market that can build parts out of multipleaterials, demonstrating commercial interest in multi-materialM systems. Although the use of multi-materials in inkjet print-

ng is a natural extension of this technology due to its use ofultiple inkjet printing heads, the material options available are

imited to jettable materials. As a result, other AM technolo-ies may offer improved performance options for multi-materialabrication.

The SL process would not generally be considered a candidateor using multi-materials because it fabricates parts from a sin-le vat of liquid material, and the use of multiple materials in SLresents challenges with managing contamination between mate-ial systems. However, there may be many applications for SL toabricate multi-material parts because it offers high quality sur-ace finish, dimensional accuracy, and a variety of material optionshat includes transparent materials. In addition, SL also builds inssentially ambient conditions and provides easy access to the buildhamber, enabling fairly straightforward conversion of existing sin-le material SL systems to multiple material systems. Motivated byhe potential advantages offered by SL, a strategy for building a

ulti-material stereolithography (MMSL) machine was conceivednd has been the focus of research over the past several yearsWicker et al., 2005a; Lozoya, 2005; Inamdar et al., 2006). Mostecently, the MMSL concept successfully received patent protectionn the United States (Wicker et al., 2009).

To develop the MMSL machine, the scanning components fromn existing 3D Systems 250/50 SL machine (3D Systems, Inc.,ock Hill, South Carolina) were retrofitted on a separate stand-lone system and adapted to function with additional componentsequired for MMSL operation. This system and the developmentsequired to build multi-material parts using MMSL technology arehe focus of this paper. During these developments, the MMSL con-epts have included fabricating multi-material tissue engineeredmplants (Arcaute et al., 2007, 2010) and multi-material micro-cale parts using micro-SL technology (Choi et al., 2010). In additiono system development, a process planning and scheduling algo-ithm for MMSL fabrication has been developed as described in Kimt al. (2010a,b).

This paper describes the design and development of the MMSLachine. The components that were transferred from the com-ercial SL system were essentially the scanning system, while theMSL machine required construction of a new frame and the devel-

pment of a new rotating vat carousel system, platform assembly,utomatic leveling system, and a custom LabVIEW® control systemo provide automated control over the MMSL process. As a result ofossible interference from previously fabricated layers during con-truction, a deep-dip coating process was used with low viscosityesins. Several multi-material demonstration parts were producedroviding compelling evidence that MMSL can produce unique

arts that are functional, visually illustrative, and constructed withultiple materials. The following sections describe the design and

evelopment of the MMSL machine in more detail and provide sev-ral samples of multi-material parts that were fabricated using thisechnology.

sing Technology 211 (2011) 318–328 319

2. Multi-material stereolithography

The design of the MMSL machine (Fig. 1) consists of a man-ufacturing center and control center. The manufacturing centerincludes the optical system (optics and scanning mirrors), laser,and rim assembly retained from the 3D Systems 250/50 SL machine,and newly developed rotating platform and rotating vat carouselsystem consisting of four vats mounted on a rotary stage. Thecontrol center includes an automatic leveling system, 3D Systems250/50 controller, developed LabVIEW® program for overall pro-cess control and management, scanning mirror controller, andsignal conditioning module. The new MMSL machine containshardware (including MMSL machine frame, rotating vat carouselsystem, platform assembly, automatic leveling system, and opti-cal system) and software (including 3D Systems software, and acustom LabVIEW® program) to function as a separate stand-alonemanufacturing setup. The following section describes the hardwareof the new MMSL machine in greater detail.

2.1. MMSL machine frame

The original stand-alone MMSL machine was designed by sev-eral researchers and described in Inamdar et al. (2006). Briefly, asquare (38.1 mm × 38.1 mm) aluminum T-slotted framing system(Part #47065T122, McMaster-Carr Supply Company, Chicago, IL,USA) was used to create the overall machine frame due to its easeof construction and reconfiguration. The vat dimensions (describedin the next section) were selected based on the maximum torquecapabilities of the vat rotary stage for the four, circumferentiallylocated vats. Using the vat carousel system, the horizontal plane ofthe MMSL machine frame fits within a square 121.9 cm × 121.9 cmcross-section. A 6061 aluminum plate (12.7 mm thick) was used asthe base of the manufacturing center to accommodate the rotat-ing vat carousel system. The aluminum T-slotted framing systemwas available in 243.8 cm maximum available lengths, and theselengths were utilized to form the four vertical legs of the MMSLmachine frame. The design of the machine offers scalability byintegrating with other non-SL technologies as presented in Wickeret al. (2005a). Tee connectors and 45 degree support brackets wereused in each corner of the frame as additional support membersto reduce deflections due to loads. The four sides of the framewere enclosed with transparent, ultra violet (UV) protective Lexan®

(3.0 mm thick, GE Structured Products Department, Mt. Vernon, IN,USA). The optical system retained from the original 3D Systems250/50 SL machine was placed on another aluminum plate (6.4 mmthick) located on top of the MMSL machine.

2.2. Rotating vat carousel system

The rotating vat carousel system includes four stainless steelvats (203.2 mm × 228.6 mm), located circumferentially, as shownin Fig. 2. The maximum build envelope of the MMSL machine basedon the vat design was 165.1 mm × 120.7 mm. The vertical build waslimited to 120.7 mm due to physical constraints such as interfer-ence between the platform supporting plates and the vat. The vatcross-section of the MMSL machine was limited, as compared tothe original 250/50 system vats (with a cross-section of approx-imately 292.1 mm × 368.3 mm), by the maximum torque output(425 N-m) of the ADRT-200 direct drive rotary stage (Aerotech Inc.,Pittsburg, Pennsylvania) used to rotate the vats. The volume of theresulting MMSL resin vats was 9 l as compared to the 32.21 l for

the original 250/50 vats. The MMSL vats were rotated about a ver-tical axis (using a 50.8 mm diameter aluminum shaft attached tothe rotary stage) to position a specific vat below the build plat-form. The direct drive rotary stage was selected for the vat carouselsystem due to its high accuracy (±30 arc-s) and high torque out-

320 J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328

ed M

plFacfiefisS

Fig. 1. Develop

ut along with superior angular positioning, velocity control, andoad capacities (30–173 kg). The vat carousel frame as shown inig. 2 included a 6061 aluminum plate at the center (26.0 mm thick)nd a series of aluminum support columns (25.4 mm × 25.4 mmross-section) attached to the plate designed to support each vat

lled with resin. As can be seen in the photograph in Fig. 2,ach vat incorporated a small rectangular (53 mm × 25 mm) con-nement section that housed a floating target used for levelensing. Further details of the floating target are provided inection 2.4.

Fig. 2. Rotating vat ca

MSL machine.

2.3. Platform assembly

A stainless steel build platform (165.1 mm × 165.1 mm in cross-section), as shown in Fig. 3, was designed to fit into the vats whilemaintaining the same spacing between the platform and the sides

of the vat as in the original 250/50 system. The build platform alsoprovides space within the vat for the floating target confinementsection as shown in Fig. 2. The platform assembly was mountedonto another direct drive rotary stage (ADRS-200, Aerotech Inc.,Pittsburg, PA, USA) in order to provide for angled part building, as

rousel system.

J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328 321

tform

wfsA(2brmuc

aiaomibs2at(p

lsctpchiCiabaat

2

wpflMT

Fig. 3. Pla

ell as enabling part and platform rotation that may be requiredor washing and drying of multi-material parts. The platform rotarytage was further secured to a high precision Z-stage (Model#TS10050, Aerotech Inc., Pittsburg, PA, USA) with high accuracy±20 �m) and repeatability (±1 �m). The original Z-stage of the50/50 machine had reported accuracy of ±50 �m and repeata-ility of ±7.6 �m, and it was thus determined the new Z-stage wasequired to enable accurate registration of Z-height during multipleaterial builds. In this configuration, the platform can be manip-

lated vertically as well as at any angle about a horizontal axis forustomized part fabrication.

To maintain the position of the platform during part building,mechanical brake setup was designed and fabricated as shown

n Fig. 3. The custom-made mechanical brake consisted of a pair ofluminum plates (127.0 mm × 63.5 mm in cross-section) mountedn the top and bottom of the platform rotary stage. Each plate wasachined to accept a small aluminum spacer sized to fit the rotat-

ng disc of the rotary stage. The aluminum spacer contacted the discy tightening a bolt that was threaded into the aluminum plate. Thetage was locked in position when the brake was activated. Two,41.3 mm aluminum extension posts (25.4 mm diameter) werettached to the platform rotary stage via a rectangular plate so thathe center of the platform coincided with the center of the UV opticsi.e., when the laser beam was vertical, it impacted the center of thelatform).

As a result of possible interference from previously fabricatedayers during multiple material fabrication and the physically con-trained complications due to integration of a new rotating vatarousel assembly, a sweeping recoating system was not used inhe MMSL system. Instead, a deep-dip coating approach with 3-mmre-dip using diluted resins was used to demonstrate MMSL fabri-ation. A deep-dip approach combined with low viscosity resinsas been successfully used previously as a method for building

n SL and micro-SL (Varadan et al., 2001; Tan and Gibson, 2005;hoi et al., 2006) when recoating is not possible. Although a recoat-

ng system was not used here, several methods (including contactnd/or non-contact approaches) can be employed in this machiney incorporating them individually in each vat or designing a singleutomated removable system. Research is ongoing to incorporatenew recoating system that will allow building with reduced layer

hicknesses, and higher viscosity resins.

.4. Automatic leveling system

The automatic leveling system of the existing 250/50 machine

as replaced by a custom leveling system (including a laser dis-lacement sensor, floating target, peristaltic pump, pump head,exible tubing, and resin containers) designed specifically for theMSL setup to regulate the resin level in the vat (Grajeda, 2010).

he leveling system provided high accuracy (±5 �m) control for

assembly.

maintaining resin levels in the multiple vats of the MMSL machine.A Masterflex® L/S peristaltic pump, as depicted in Fig. 4, facilitatedadding and removing desired quantities of resin to and from eachvat and allowed isolation of the resin from the moving parts of thepump. The most significant advantage of the peristaltic pump is theuse of flexible tubing (MasterFlex® Tygon® L/S 18) for the multi-pump system, as the resin being pumped remains inside the tubingat all times. This feature greatly reduced the risk of contaminationby offering control over the content and purity of the resin alongwith the integrity of the process. This resulted in minimizing themaintenance, cleanup, and resin changeover times while enablingthe pump to deliver various fluids simply by changing the tub-ing within the pump heads. Four pump heads (Model #77201-60,L/S Easy-Load® II, Cole-Parmer Instrument Company) were used tofacilitate the fabrication of complex parts using multiple materials.

One of the issues encountered by Wicker et al. (2005a) andLozoya (2005) during the first generation retrofit design, in whichmultiple smaller vats were placed inside a 3D Systems 250/50 SLmachine, was the re-registration of the part to maintain a uniformlevel of resin on the build layer in multiple vats to produce multi-material parts. In the current work, the automatic leveling systemwas used to maintain registration between the part and multiplevats of resin. The leveling system included a laser displacementsensor (MicrotrakTM 7000, MTI Instruments Inc., Albany, NY, USA)and a sensing head coupled to a floating target that was housed ina confined section of each resin vat as shown in Fig. 2. The floatingtargets were manufactured by a 3D Systems Viper si2TM SL sys-tem (3D Systems Inc., Rock Hill, SC, USA) using 14120 White (DSMSomos®, DSM Desotech Inc., Elgin, IL, USA). The leveling systemwas calibrated for each of the resins (as the height of the targetvaried slightly in each resin). Each floating target (1 mm thick) has12 wedges to avoid surface contact (generating friction) betweenthe floating target and the vat. The calibrated values were used inthe LabVIEW® control program to provide automatic leveling of theresins by calculating the error and adjusting the resin level surfaceto a calibrated (zero) value. A PID (Proportional Integral Derivative)control algorithm was used for automatic resin leveling, due to itsfunctional simplicity and robust performance in a broad range ofoperating conditions.

2.5. Transferred technologies

Several components from the existing 3D Systems 250/50 SLmachine were retained (including the optical system, rim assem-bly with beam profilers, the associated controllers and the wiring

harness) in the new MMSL machine. The basic strategy was to retainthe 3D Systems scanning system (hardware and software) so thatthe engineering development invested in the commercial systemcould be easily transferred to the new MMSL technology. The opti-cal system (including the optics plate with the UV laser, mirrors,

322 J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328

tic lev

bfpnamt(Sttm

3

tpctpnOvTtpcictnc

ccr

Fig. 4. Automa

eam expander, scanning mirrors, and focusing lens) was trans-erred in its entirety. The rim assembly (including the laser beamrofilers), and the associated controllers (computer system, scan-ing mirror controller, power supply-vat controller) were used tochieve the functionality of the laser scanning process. The MMSLachine used a 355 nm solid-state laser (JDS Uniphase Co., Milpi-

as, CA, USA) due to its high resolution and small beam diameter0.102 mm). The wiring harness was utilized to initialize the 3Dystems software and achieve successful interface with the cus-om LabVIEW® control system by acquiring the analog signal fromhe stepper motor of the recoating system in the original 250/50

achine.

. MMSL operation overview

This section describes the overall process management and con-rol software system of the new MMSL machine. The MMSL buildrocess is initialized by submerging the build platform in the vatontaining the first resin material. A normal SL build is performedo fabricate the desired part with the first resin. When finished, thelatform is raised out of the vat, and a cleaning vat is rotated under-eath the platform so that the platform and part can be cleaned.nce cleaned and dried, the platform is raised out of the cleaningat and the second material vat is rotated underneath the platform.he SL build continues with other materials until completion, andhe process continues indefinitely to fabricate multiple materialarts. The entire MMSL process is managed by the overall pro-ess management and control software using a program developedn LabVIEW®. The LabVIEW® program controls the rotating vatarousel system, the new linear (Z-stage) and rotational stages (forhe vats and platform), the automatic leveling system, and commu-icates directly with the 3D Systems software (Buildstation 4.0) for

ontrolling the laser scanning process.

The key elements for successful operation include being able toommunicate between the two systems (i.e., between the scanningomponents from the 250/50 machine and the new componentsequired for MMSL operation). The sequence of operations for suc-

eling system.

cessful MMSL fabrication is shown in Fig. 5. A signal conditioningmodule (SC-2345 carrier with configurable connectors, NationalInstruments, Austin, TX, USA) is used to acquire the analog signal(change in voltage) from the stepper motor that is used to drivethe recoating system after each laser scan in the 250/50 machine.The LabVIEW® program uses this change in voltage as a trigger toopen an interlock switch (using one of the signal wires of the wiringharness retained from the 3D Systems machine) that serves to stopthe 3D Systems program to perform the required MMSL operations.Once the MMSL operations are completed and scanning of the nextlayer can commence, the LabVIEW® program closes the interlockswitch and the 3D Systems scanning program continues.

Any CAD package (such as SolidWorks®) can be used to designthe individual parts required for a multiple material build, whilesaving each file individually in STL file format. Additional detailsfor how to prepare a process plan for the MMSL process are con-tained in Kim et al. (2010a,b). In the present work, the 3D Systemsproprietary slicing software (3D LightyearTM 1.4) was used to con-vert the individual STL files generated during the part preparationand design stages into the build file format required for the opera-tion of the MMSL machine. It should be noted that 3D LightyearTM

can also be used to generate supports as required for the individualCAD models or a single CAD model of the overall part. The supportstructures can be saved as an individual STL file or separate STL filesdepending on the material used to build the specific support struc-tures, and these files can be managed using the process planningmethods described in Kim et al. (2010a,b). In the MMSL demon-strations provided here, supports were not required; however, itshould be understood that incorporating supports using MMSL is anatural extension of the current work.

To begin the build process, the MMSL platform assembly(secured to the Z-stage) and the rotating vat carousel assembly are

initialized to their home positions. The platform is lowered to thestart position and a Masterflex® L/S peristaltic pump is activatedto level the resin surface with the platform. After the initializa-tion is complete, the 3D Systems program scans the first layer ofthe desired part. As described above, the LabVIEW® program then

J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328 323

ateri

osddasmtgsclai

4

Ztrscbtniru

Fig. 5. Process flowchart for multi-m

pens the interlock switch to stop the 3D Systems program (Build-tation 4.0) and commands the build platform to traverse verticallyownward by a distance equal to the build layer thickness using aeep-dip coating approach. The automatic leveling system is thenctivated to maintain a thickness of 0.102 mm (equivalent to thetandard layer thickness used by 3D Systems in the 250/50 SLachine), or other pre-programmed layer thicknesses, of resin on

he part for the next build layer. Once leveled, the LabVIEW® pro-ram closes the interlock switch so the 3D Systems program cancan the next layer. In addition, it should be noted that this processan be used to provide multi-material builds both within the sameayer and across layers, and several examples demonstrating thisre provided below. These procedures are continued, as depictedn Fig. 5, until a complete multi-material part is fabricated.

. Material preparation

In conventional SL, recoating using a sweeping blade orephyrTM blade is critically important to ensuring accurate layerhicknesses without deformation due to surface tension forces. Theecoating process is equally important in MMSL. When using aingle-material, recoating using a sweeping mechanism (sweeper)an occur without interference, whereas in MMSL, a previouslyuilt layer can present an obstacle when the sweeper moves across

he build chamber. Thus, in many instances, the use of a sweeper iso longer possible in MMSL. In the present work, a deep-dip coat-

ng process in combination with low viscosity resins was used toemove the need for sweeping. This approach has been successfullysed in previous work (Choi et al., 2006, 2009a, 2010). There are

al fabrication using MMSL machine.

clearly benefits, however, with sweeping, and thus, current effortsare focused on developing integrated contact (similar to the cur-rent approach used in SL) and non-contact (for use when materialobstructions from previously fabricated layers are present) sweep-ing approaches for the MMSL technology.

To demonstrate MMSL using low viscosity resins, DSM Somos®

WaterShedTM 11120, ProtoThermTM 12120, and 14120 White (DSMDesotech Inc., Elgin, IL, USA) were selected, and propoxylated (2)neopentyl glycol diacrylate (PNGD) (Sartomer, Inc., Warrington,PA, USA) was used to dilute the resins and reduce the viscositiesof the resin solutions. To obtain viscosities according to the con-centration of the diluent and solution temperature, a Brookfieldviscometer (DV-E, Middleboro, MA, USA) was used. The commer-cial resins were mixed with the diluent with the ratio (by weight)of 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50 and solution vis-cosities were measured at temperatures from 24 ◦C to 30 ◦C. Fig. 6shows the viscosity variation of the three resins according to themixture ratio and temperature. The viscosities of the 50:50 mix-ture for each resin are less than 100 cP at room temperature, andthis value is reasonable for the deep-dip coating approach (Varadanet al., 2001; Tan and Gibson, 2005; Choi et al., 2006). As a result,the three 50:50 mixtures were used at room temperature for themultiple material fabrication demonstrations.

5. Fabrication demonstration

To demonstrate the fabrication capabilities of the developedMMSL machine, a rook or castle, which is a commonly useddemonstration part in SL, was used here for multi-material demon-

324 J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328

Fa(

s(c1d

ig. 6. Viscosity variation of diluted resins according to the concentration of diluentnd solution temperature: (a) WaterShedTM 11120, (b) ProtoThermTM 12120, andc) Somos® 14120 White.

trations. Two complex models consisting of three sub-models

three different resins) as shown in Fig. 7 were prepared. Fig. 7(a)onsists of the outer structure (using the diluted WaterShedTM

1120), staircase (using the diluted ProtoThermTM 12120), andouble spiral (using the diluted Somos® 14120 White). The outer

Fig. 7. Multi-material ‘Rook’ models: (a) model 1 and (b) model 2.

structure has a base diameter of 31.7 mm, inner diameter of15.5 mm, and height of 53 mm. Fig. 7(b) consists of two sepa-rate resins for the middle section of the outer structure (oneusing the diluted WaterShedTM 11120 and the other using thediluted Somos® 14120 White) and the remaining structure (usingthe diluted ProtoThermTM 12120). Each sub-model was separatelysliced, and process planning as shown in Fig. 8 was accomplishedusing the approach described in Kim et al. (2010a,b). The processplan results in a specific sequence of part building for a specificmaterial. For example, the sub-models 1-1 and 2-1 (Fig. 7) weredivided into Build 1 and Build 4, respectively (see Fig. 8 for thebuild sequence). Both models of Fig. 7 required three materialchangeovers. Between material changeovers, a manual cleaningprocess was accomplished, although an automatic cleaning methodusing the rotating platform is currently under development. Themanual process involved interrupting the process and manuallycleaning the platform and part with isopropyl alcohol beforeimmersing the platform into the next resin vat. Table 1 shows buildparameters for each sub-model.

Figs. 9 and 10 show the two example parts that were fabri-cated on a Mylar® sheet. Fabricating on a Mylar® sheet was usedto avoid building base supports and is a method used to provideexceptional surface finish on the bottom surface (Wicker et al.,2005b). The total build time for both parts was ∼24 h includingtime for material changeovers and cleaning between materials. Ineach build, a sub-part was built on the platform or directly on apreviously built part, and the final parts were successfully pro-duced from the designs shown in Fig. 7. Fig. 11 shows the finalfabricated rooks with various material combinations including sixvertically altered material combinations, one that includes sideswith different material combinations, and three samples showing atransparent exterior with red stairs and white internal spirals. Theexamples shown in Figs. 9–11 provide compelling evidence thatthe developed MMSL system is capable of producing high qualitymulti-material parts. There are a number of potential applicationsfor this technology, such as pre-surgical medical models with mul-

tiple colors, 3D electronic devices combining a conductive materialdispensing technology as described in DeNava et al. (2008) andtissue-engineered scaffolds with gradients of different bioactiveagents as described in Arcaute et al. (2007, 2010).

J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328 325

: (a) m

6

atmstpTpr2maaeeastarK

TB

Fig. 8. Process planning

. Discussion

Despite the potential benefits offered by this technology, therere possibly many issues associated with multi-material fabricationhat still remain to be addressed before becoming a viable com-

ercial technology. Some of these issues, for example, include laserhadowing, trapped volumes, and surface tension problems as illus-rated in Fig. 12. Laser shadowing will occur when the laser beam ishysically blocked by a previously built part as shown in Fig. 12(a).his problem can be solved during process planning by buildingarts from the outside to the inside and determining the geomet-ic constraints in the process planning algorithm (see Kim et al.,010a,b). Trapped volumes as shown in Fig. 12(b) occur when oneaterial creates a cup or region within the material that requiresseparate material. This issue can be partly resolved by splittingpart with a trapped volume to be built in several pieces that

liminate any trapped volumes between material systems. How-ver, splitting the part to eliminate trapped volumes may introducenother problem as illustrated in Fig. 12(c) and (d) in which unequal

urface energies result in non-flat interfaces between material sys-ems. In fact, uneven surfaces similar to those shown in Fig. 12(c)nd (d) were a common problem in early generation single mate-ial SL parts due to surface tension effects of the resin (Renap andruth, 1995). The interface between material systems is an impor-

able 1uild parameters.

Model Sub-model Material (diluted) Total layers Stopp

11 WaterShedTM 11120 520 4372 ProtoThermTM 12120 328 None3 Somos® White 14120 328 None

21 ProtoThermTM 12120 520 1052 WaterShedTM 11120 332 None3 Somos® White 14120 332 None

odel 1 and (b) model 2.

tant area of research and one that requires additional investment.Although Lozoya (2005) found a sufficiently strong bond betweenseveral SL materials fabricated using MMSL, unequal surface ener-gies between materials can lead to no bond between materials ata material interface. Unequal surface energies and surface tensionissues can be addressed using several means, one of which maybe the use of surface coatings or treatments that modify the sur-face energy of the previously built part. The selection, application,and response of surface coatings for use in MMSL are areas of cur-rent research. In addition, the strength of the interface betweenmaterial systems could be improved by incorporating reinforcingmaterials such as multi-walled carbon nanotubes (Sandoval et al.,2005, 2007; Sandoval and Wicker, 2006) and other reinforcing par-ticles in the resins (Bartolo and Gaspar, 2008; Griffith and Halloran,1996). These composite resins could also improve the mechani-cal performance of the SL resins, leading to a variety of additionalapplications for MMSL.

The deep-dip coating approach without sweeping leads to rel-atively long settling times and thus building times, so the surface

energies between materials and controlling them needs to be inves-tigated and optimized for MMSL. As mentioned previously, thedevelopment of successful non-contact recoating methods willalso improve the MMSL process. Advancements in these areas areunderway and will be the focus of future publications.

ing layer Settling time (s) Others

60 Layer thickness: 0.1016 mm (0.004 in.)Deep-dip coating thickness: 3 mmZ-stage decent velocity: 3 mm/sZ-stage ascent velocity: 0.5 mm/sDip delay: 3 s

2020603030

326 J.-W. Choi et al. / Journal of Materials Processing Technology 211 (2011) 318–328

Fig. 9. Fabricated multi-material rook using model 1: building (a) sub-model 1-1 using WaterShedTM 11120 (clear), (b) sub-model 1-2 using ProtoThermTM 12120 (red), (c)sub-model 1-3 using Somos® 14120 White, and (d) sub-model 1-1 using WaterShedTM 11120 (clear). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 10. Fabricated multi-material rook using model 2: building (a) sub-model 2-1 using ProtoThermTM 12120 (red), (b) sub-model 2-2 using WaterShedTM 11120 (clear), (c)sub-model 2-3 using Somos® 14120 White, and (d) sub-model 2-1 using ProtoThermTM 12120 (red). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

J.-W. Choi et al. / Journal of Materials Proces

Fig. 11. Various multi-material parts fabricated using the developed MMSL system.

Fit

7

d(sTfmpo

Choi, J.W., Wicker, R.B., Cho, S.H., Ha, C.S., Lee, S.H., 2009a. Cure depth control for

ig. 12. Several issues associated with multi-material fabrication: (a) laser shadow-ng, (b) trapped volume, (c) surface tension between two materials, and (d) surfaceension on the previously built layer.

. Conclusions

The research presented in this paper described the design andevelopment of a fully functional multi-material stereolithographyMMSL) machine and demonstrated its functionality by presentingeveral complex multi-material models fabricated by the system.he MMSL machine was developed by retrofitting components

rom a commercial 3D Systems 250/50 stereolithography (SL)

achine on a separate stand-alone system and adapting the com-onents to function with additional components required for MMSLperation. The new MMSL machine required construction of a

sing Technology 211 (2011) 318–328 327

frame and the development of a new rotating vat carousel sys-tem, platform assembly, and automatic leveling system. A customLabVIEW® control system was developed to provide automatedcontrol over the entire process, including the vat leveling sys-tem, linear and rotational stages, and commanding the 3D Systemsscanning software to scan a particular layer or sub-layer. Overallcontrol by the LabVIEW® program was enabled through interceptof a signal from the 250/50 machine that signaled the comple-tion of a scan, and so the LabVIEW® program could stop the 3DSystems program temporarily to complete processes required forMMSL operation. One significant drawback to MMSL is the inabil-ity to use a contact recoating system such as the ZephyrTM bladebecause of possible material obstruction due to previously builtlayers. As a result, the current work used a deep-dip coating pro-cess combined with low viscosity resins. The development of novelnon-contact recoating methods to be used in MMSL is currentlya focus of research. In addition to recoating, research is ongoinginto determining and improving material compatibilities and theinterfacial bonding strength between materials in MMSL. Despitethe need for further research, several multi-material complex partswere successfully produced providing compelling evidence thatMMSL can produce unique parts with enumerable applications thatare functional, visually illustrative, and constructed with multiplematerials.

Acknowledgements

The research presented here was performed at the University ofTexas at El Paso (UTEP) within the W.M. Keck Center for 3D Inno-vation (Keck Center). This material is based, in part, upon worksupported by the Texas Advanced Research (Advanced Technol-ogy/Technology Development and Transfer) Program under grantnumber 003661-0020-2003, research grant number CBET-0730750from the National Science Foundation, and the Mr. and Mrs. Mac-Intosh Murchison Chair I in Engineering at UTEP. There are manyindividuals who have contributed to the development of the mul-tiple material stereolithography machine described here, and theauthors are particularly appreciative of the efforts of Frank Med-ina, Asim Inamdar, Marco Magana, Yinko Grajeda, Oswaldo Lozoya,Karina Arcaute, Amit Lopes, and Armando Rivera. The opinionsexpressed in this paper are those of the authors and do not nec-essarily reflect those of any other individuals or any sponsors ofthe Keck Center or its research.

References

Arcaute, K., Mann, B.K., Wicker, R.B., 2006. Stereolithography of three-dimensionalbioactive poly(ethylene glycol) constructs with encapsulated cells. Ann. Biomed.Eng. 34 (9), 1429–1441.

Arcaute, K., Zuverza, N., Mann, B., Wicker, R., 2007. Multi-material stereolithog-raphy: spatially-controlled bioactive poly(ethylene glycol) scaffolds for tissueengineering. In: Proceedings of Annual Solid Freeform Fabrication Symposium,Austin, TX, pp. 458–469.

Arcaute, K., Wicker, R.B., 2008. Patient-specific compliant vessel manufacturingusing dip-spin coating of rapid prototyped molds. J. Manuf. Sci. Eng.-Trans. ASME130, 051008-1–051008-13.

Arcaute, K., Mann, B.K., Wicker, R.B., 2010. Stereolithography of spatially-controlledmulti-material bioactive poly(ethylene glycol) scaffolds. Acta Biomater. 6,1047–1054.

Bartolo, P.J., Gaspar, J., 2008. Metal filled resin for stereolithography metal part. CIRPAnnuals 57 (1), 235–238.

Bartolo, P.J., Mitchell, G., 2003. Stereo-thermal-lithography: a new principle for rapidprototyping. Rapid Prototyping J. 9 (3), 150–156.

Choi, J.W., Ha, Y.M., Lee, S.H., Choi, K.H., 2006. Design of microstereolithographysystem based on dynamic image projection for fabrication of three-dimensionalmicrostructures. J. Mech. Sci. Technol. 20 (12), 2120–2130.

complex 3D microstructure fabrication in dynamic mask projection microstere-olithography. Rapid Prototyping J. 15 (1), 59–70.

Choi, J.W., Wicker, R., Lee, S.H., Choi, K.H., Ha, C.S., Chung, I., 2009b. Fabrication of 3Dbiocompatible/biodegradable micro-scaffolds using dynamic mask projectionmicrostereolithography. J. Mater. Process. Technol. 209, 5494–5503.

3 Proces

C

C

C

D

D

G

G

I

J

J

K

K

K

K

L

L

L

28 J.-W. Choi et al. / Journal of Materials

hoi, J.W., MacDonald, E., Wicker, R., 2010. Multi-material microstereolithography.Int. J. Adv. Manuf. Technol. 49, 543–551.

hoi, J.W., Quintana, R., Wicker, R.B., in press. Fabrication of characterization ofembedded horizontal micro-channels using line-scan stereolithography. RapidPrototyping J.

hua, C.K., Leong, K.F., Lim, C.S., 2003. Rapid Prototyping: Principles and Applications.World Scientific Publishing, Singapore.

eNava, E., Navarrete, M., Lopes, A., Alawneh, M., Contreras, M., Muse, D., Castillo, S.,MacDonald, E., Wicker, R., 2008. Three-dimensional off-axis component place-ment and routing for electronics integration using solid freeform fabrication. In:Proceedings of Solid Freeform Fabrication Symposium, The University of Texasat Austin, Austin TX, pp. 362–369.

hariwala, B., Hunt, E., Boland, T., 2004. Rapid prototyping of tissue-engineeringconstructs, using photopolymerizable hydrogels and stereolithography. TissueEng. 10 (9–10), 1316–1322.

rajeda, Y., 2010. Automated fluid handling and leveling system for multiple mate-rial stereolithography. M.S. Thesis. The University of Texas at El Paso.

riffith, M.L., Halloran, J.W., 1996. Freeform fabrication of ceramics via stereolithog-raphy. J. Am. Ceram. Soc. 79 (10), 2601–2608.

namdar, A., Magana, M., Medina, F., Grajeda, Y., Wicker, R., 2006. Development ofan automated multiple material stereolithography machine. In: Proceedings ofAnnual Solid Freeform Fabrication Symposium, Austin, TX, pp. 624–635.

ackson, B., Wood, K., Beaman, J.J., 2000. Discrete multi-material selective laser sin-tering (M 2 SLS): development for an application in complex sand casting corearrays. In: Proceedings of Solid Freeform Fabrication Symposium, The Universityof Texas at Austin, Austin, TX, pp. 176–182.

afari, M.A., Han, W., Mohammadi, F., Safari, A., Danforth, S.C., Langrana, N., 2000. Anovel system for fused deposition of advanced multiple ceramics. Rapid Proto-typing J. 6 (3), 161–175.

halil, S., Nam, J., Sun, W., 2005. Multi-nozzle deposition for construction of 3Dbiopolymer tissue scaffolds. Rapid Prototyping J. 11 (1), 9–17.

im, H.C., Choi, J.W., Wicker, R.B., 2010a. Process planning and scheduling for mul-tiple material stereolithography. Rapid Prototyping J. 16 (4), 232–240.

im, H.C., Choi, J.W., MacDonald, E., Wicker, R.B., 2010b. Slice overlap detection algo-rithm for the process planning of multiple material stereolithography apparatus.Int. J. Adv. Manuf. Technol. 46 (9), 1161–1170.

ulkarni, P., Marsan, A., Dutta, D., 2000. A review of process planning techniques inlayered manufacturing. Rapid Prototyping J. 6 (1), 18–35.

ee, K.W., Wang, S., Fox, B.C., Ritman, E.L., Yaszemski, M.J., Lu, L., 2007.Poly(propylene fumarate) bone tissue engineering scaffold fabrication usingstereolithography: effects of resin formulations and laser parameters. Biomacro-molecules 8, 1077–1084.

iew, C.L., Leong, K.F., Chua, C.K., Du, Z., 2001. Dual material rapid prototyping tech-niques for the development of biomedical devices. Part I. Space creation. Int. J.Adv. Manuf. Technol. 18 (10), 717–723.

iew, C.L., Leong, K.F., Chua, C.K., Du, Z., 2002. Dual material rapid prototyping tech-niques for the development of biomedical devices. Part II. Secondary powderdeposition. Int. J. Adv. Manuf. Technol. 19 (9), 679–687.

sing Technology 211 (2011) 318–328

Lozoya, O.A., 2005. Development and demonstration of a multiple material stere-olithography system. MS Thesis. The University of Texas at El Paso.

Mizukami, Y., Rajniak, D., Rajniak, A., Nishimura, M., 2002. A novel microchip forcapillary electrophoresis with acrylic microchannel fabricated on photosensorarray. Sens. Actuators B: Chem. 81, 202–209.

Neumeister, A., Himmelhuber, R., Temme, T., Stute, U., 2006. Generation ofmicro mechanical devices using stereo lithography. In: Proceedings of SolidFreeform Fabrication Symposium, The University of Texas at Austin, Austin, TX,pp. 12–24.

Palmer, J.A., Jokiel, B., Nordquist, C.D., Kast, B.A., Atwood, C.J., Grant, E., Livingston,F.J., Medina, F., Wicker, R.B., 2006. Mesoscale RF relay enabled by integratedrapid manufacturing. Rapid Prototyping J. 12 (3), 148–155.

Pham, D.T., Gault, R.S., 1998. A comparison of rapid prototyping technologies. Int. J.Mach. Tools Manuf. 38, 1257–1287.

Radstok, E., 1999. Rapid tooling. Rapid Prototyping J. 5 (4), 164–168.Regenfuss, P., Streek, A., Hartwig, L., Klötzer, S., Brabant, Th., Horn, M., Ebert, R.,

Exner, H., 2007. Principles of laser micro sintering. Rapid Prototyping J. 13 (4),204–212.

Renap, K., Kruth, J.P., 1995. Recoating issues in stereolithography. Rapid PrototypingJ. 1 (3), 4–16.

Sandoval, J.H., Ochoa, L., Hernandez, A., Lozoya, O., Soto, K.F., Murr, L.E., Wicker,R.B., 2005. Nanotailoring stereolithography resins for unique applications usingcarbon nanotubes. In: Proceedings of Annual Solid Freeform Fabrication Sym-posium, Austin, TX, pp. 513–524.

Sandoval, J.H., Wicker, R.B., 2006. Functionalizing stereolithography resins: effectsof dispersed multi-walled carbon nanotubes on physical properties. Rapid Pro-totyping J. 12 (5), 292–303.

Sandoval, J.H., Soto, K.F., Murr, L.E., Wicker, R.B., 2007. Nanotailoring photocrosslink-able epoxy resins with multi-walled carbon nanotubes for stereolithographylayered manufacturing. J. Mater. Sci. 42, 156–165.

Santosa, J., Jing, D., Das, S., 2002. Experimental and numerical study on theflow of fine powders from small-scale hoppers applied to SLS multi-materialdeposition—part I. In: Proceedings of Solid Freeform Fabrication Symposium,The University of Texas at Austin, Austin, TX, pp. 620–627.

Tan, W., Gibson, I., 2005. Numerical study on the recoating process in microstere-olithography. In: Proceedings of Annual Solid Freeform Fabrication Symposium,Austin, TX, pp. 458–469.

Varadan, V.K., Jiang, X., Varadan, V.V., 2001. Microstereolithography and Other Fab-rication Techniques for 3D MEMS. Wiley, West Sussex.

Wicker, R.B., Medina, F., Elkins, C.J., 2005a. Multiple material micro-fabrication:extending stereolithography to tissue engineering and other novel application.In: Proceedings of Annual Solid Freeform fabrication Symposium, Austin, TX, pp.

754–764.

Wicker, R.B., Ranade, A.V., Medina, F., Palmer, J.A., 2005b. Embedded micro-channelfabrication using line-scan stereolithography. Assem. Autom. 25 (4), 316–329.

Wicker, R.B., Medina, F., Elkins, C., 2009. Multi-Material Stereolithography. U.S.Patent 7,556,490.