DOI10.1007/s00170-003-1840-6
Int J Adv Manuf Technol(2005)25:308–320
C.M.Cheah·C.K.Chua·C.W.Lee·C.Feng·K.Totong
Rapid prototyping and tooling techniques:a review of applications for rapid investment casting
Received:1April2003/Accepted:20June2003/Published online:11August2004
Springer-Verlag London Limited2004
Abstract Investment casting(IC)has bene?ted numerous indus-tries as an economical means for mass producing quality near net shape metal parts with high geometric complexity and acceptable tolerances.The economic bene?ts of IC are limited to mass pro-duction.The high costs and long lead-time associated with the development of hard tooling for wax pattern moulding renders IC uneconomical for low-volume production.The outstanding manufacturing capabilities of rapid prototyping(RP)and rapid tooling(RT)technologies(RP&T)are exploited to provide cost-effective solutions for low-volume IC runs.RP parts substitute tra-ditional wax patterns for IC or serve as production moulds for wax injection moulding.This paper reviews the application and po-tential application of state-of-the-art RP&T techniques in IC.The techniques are examined by introducing their concepts,strengths and weaknesses.Related research carried out worldwide by dif-ferent organisations and academic institutions are discussed. Keywords Investment casting·Low-volume production·Moulding·Rapid Prototyping·Rapid Tooling
List of Abbreviations
ABS Acrylonitrile-butadiene-styrene
ACES Accurate clear epoxy solid
AIM ACES injection moulding
CAM-LEM Computer-aided manufacturing of laminated en-gineering materials
CMB Controlled metal build-up
C.M.Cheah· C.K.Chua(u)· C.W.Lee· C.Feng
Systems and Engineering Management Division,
School of Mechanical and Production Engineering,
Nanyang Technological University,
50Nanyang Avenue,Singapore639798
E-mail:mckchua@https://www.doczj.com/doc/2f7164927.html,.sg
Tel.:+65-67904897
Fax:+65-67911859
K.Totong
Mechanical Engineering Division,
Ngee Ann Polytechnic,
Singapore CTE Coef?cients of thermal expansion
DMD Direct metal deposition
DMLS Direct metal laser sintering
DSPC Direct shell production casting
FDM Fused deposition modelling
IC Investment casting
LENS Laser engineered net shaping
LG Laser generating
LOM Laminated object manufacturing
LS Laser sintering
MJS Multiphase jet solidi?cation
MMA Methyl methacrylate
MM II Model Maker II
PC Polycarbonate
POM Precision optical manufacturing
PS Polystyrene
RIC Rapid investment casting
RP Rapid prototyping
RP&T Rapid prototyping and tooling
RT Rapid tooling
RSP Rapid solidi?cation process
SDM Shape deposition modelling
SGC Solid ground curing
SL Stereolithography
SLS Selective laser sintering
3D-P3D printing
1Background on investment casting(IC)and rapid prototyping(RP)
Investment casting(IC),or“lost-wax”casting,is a precision cast-ing process whereby wax patterns are converted into solid metal parts following a multi-step process[1].IC enables economical mass-production of near net shaped metal parts containing com-plexgeometries andfeatures[2,3]froma varietyof metals,includ-ing dif?cult-to-machine or non-machinable alloys.To produce precision components,the near net shape of castings can reduce machining time and cost to bring components into speci?cations.
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Despite its popularity,traditional IC suffers from high tooling in-vestments for producing wax patterns.As such,IC is prohibitively expensive for low-volume production typical in prototyping,pre-series,customised or specialised component productions.
Rapid prototyping (RP)techniques are fast becoming stan-dard tools in product design and manufacturing [4].With revo-lutionary capabilities to rapidly fabricate three-dimensional parts for design veri?cation or to serve as functional prototypes and production tooling,RP is an indispensable tool for shortening product design and development time cycles [5,6].RP,which is a powerful communication tool that bridges design,marketing,process planning and manufacturing,can facilitate the imple-mentation of concurrent engineering [7,8].The application of RP in IC is motivated by prospects of reduced tooling costs and production lead-times [9,10].This paper aims at presenting a comprehensive review on the applications and potential appli-cations of rapid prototyping and tooling (RP&T)techniques in IC.The paper concludes with a general discussion on the issues faced with the application of RP&T techniques.
2The IC process
Traditional IC consists of the block mould and the more common ceramic shell processes.The process chain for the ceramic shell process (Fig.1)consists of the tooling,shell fabrication and cast-ing stages.In the tooling stage,the mould for wax pattern produc-tion is designed and machined from aluminium stocks.For com-plex patterns,multiple parting lines and loose inserts are incorpo-rated into the mould.The completed mould is coated with release agent,assembled and injected with molten wax.Upon cooling,the mould is stripped to extract the patterns.Individual patterns are attached onto a wax sprue system to form a cluster (Fig.2)in the shell fabrication stage.The cluster is repeatedly dip coated in investment slurry containing graded suspensions of refractory par-ticles and followed by stucco application to build shell thickness and strength.When dried,the wax pattern is melted out via au-toclaving to reveal the internal cavities of the ceramic shell.The shell is ?red to build strength and remove residual volatiles.In the casting stage,molten metal is poured into the heated shells to form the castings,which are extracted after cooling by cracking the shell during the knockout process.Individual castings are sep-arated,cleansed and subjected to ?nishing processes.2.1Limitations in low-volume IC production
In IC,substantial investments are committed to prototype or production tooling development [11].The committed
resources
Fig.1.Process chain for ce-ramic shell
process
Fig.2.Wax cluster arrangement of dolphin patterns produced from silicone rubber tooling (left );Stainless steel casting of horse ?gurines (right )
increase substantially with mould complexity or lower volume production.Tooling costs for wax injection moulding range from several thousands to tens of thousands of dollars depending on size and complexity,while lead-times range between several weeks to months depending on machine shop scheduling and ca-pabilities.As such,a toolmaker has to evaluate different mould designs before committing to manufacturing since design er-rors or iterations are usually expensive and time-consuming to amend or accommodate [12].Although most RP&T applications in IC are limited to design,prototyping and tool production,such primary stages are crucial in deciding marketing strategies.Sig-ni?cant bene?ts of RP&T application are re?ected by improved mould designs and lead-time savings in the development of pro-duction tooling for high-volume production,rapidly changing high-volume production,and,emerging cost-effective solutions for low-volume production.Recent technological advances have enhanced the accuracy,performance and durability of RP&T end products,enabling some to serve as tooling for low-volume pro-duction and in some cases,high-volume production [13].
3RP and RT technologies
RP has its beginnings in the mid-1980s with the debut of 3D Systems Inc.’s (Valencia,CA)SLA-1[8].There are currently 28manufacturers worldwide offering a total of more than 56differ-ent RP systems to meet the diverse demands of end-users [14].Contrary to conventional fabrication techniques,which involve the subtraction of materials from a stock,casting or moulding,all RP techniques are layer additive processes [15],whereby layers
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of material representing the cross-sections of the part formed by processing solid sheet,liquid or powder feed stocks,are fused to-gether to create the part.The RP process chain is presented in Fig.3.Detailed descriptions of RP techniques can be found in literature [4].
RT technology,which embodies the creation of prototype or production tooling based on RP parts,complements RP when large quantities of similar parts containing complex features,made economically utilising materials close to or identical to end production materials and with normal production processes are required [16].This is common in concurrent engineering en-vironments where prototype parts are required simultaneously by different cross-functional design teams or when performance tests has to be conducted on prototypes produced using end pro-duction materials and processes.As such,RT not only allows the prototyping of products but also the production processes.
4Novel techniques using RP&T technologies
Since the inception of RP,parts fabricated by pioneering RP sys-tems have been employed as IC patterns to cut tooling costs and lead times [17].RP application in IC is one of its more popular tooling related applications [18].However,the economic bene?ts derivable from RP patterns are limited to small quantity pro-duction due to high RP material costs [19,20].Current research focus has shifted from RP pattern fabrication to the development of RT for producing IC patterns.For higher quantities of cast-ings,RT can economically and effectively produce from tens to millions of wax patterns.RT bene?ts traditional foundries since they generate wax patterns,unlike non-wax RP patterns,which require changes in the IC process for successful runs [21].Other tangible bene?ts include:
?Rapid production:RP systems are not limited by part com-plexity.The high manufacturing ?exibility permits parts that are previously dif?cult or impossible to fabricate via machin-ing to be fabricated at a fraction of the cost and lead-time.Additionally,RP does not require tooling or ?xtures resulting in simpler set-ups and lower overheads.
?Prototyping:RP&T techniques provide affordable and fast evaluation of tooling designs and design iterations.RP parts function as design prototypes to iron out ?aws in casting or tooling design and functional prototypes to address the po-sitioning of gates,vents and runners.Problems due to wax shrinkage or feeding can be recti?ed before manufacturing the production tooling.With prototypes,optimised designs can be realised quickly and any risk of corrective rework whilst in production is eliminated.Utilising RP&T,
econom-
Fig.3.Typical RP process chain
ical pre-series castings can be produced and the high invest-ments for production tooling are deferred.
?Process optimisation:With pre-series castings,the position-ing of parting lines,ejection pins and inserts can be per-fected.Optimisation of moulding parameters and evaluation of moulded patterns can be conducted effectively.These re-sult in con?dence before the production tooling is manufac-tured.The term rapid investment casting (RIC)represents the em-ployment of RP&T techniques in IC.Figure 4presents the strate-gies and techniques introduced to shorten the production cycle for IC.The sections that follow introduce each technique.4.1Direct fabrication of IC patterns
The application of RP IC patterns stems from the fact that pat-terns of any material,which can be melted or burnt-out without damaging the ceramic shell can be employed in IC.The ?rst reported use of RP IC patterns appeared in 1989[22].Today,al-most all commercialised RP processes (systems),selective laser sintering (SLS)[23],stereolithography (SL),fused deposition modelling (FDM),ink-jet plotting (MM II),3D printing (3D-P),solid ground curing (SGC),multi-jet modelling (Actua)[24]and laminated object manufacturing (LOM),have been employed to produce IC patterns with varying success.With numerous bene?ts achievable,it is not astonishing to note that RP&T techniques are gaining widespread acceptance among traditional foundries [19,25].Some examples include Shellcast Foundries Inc.in Montreal,where the solid model casting (SMC)process is developed to directly convert RP models into castings without the application of hard tooling [26].The Cercast Group has iden-ti?ed important parameters critical in designing RP patterns,as well as the strengths and limitations of various RP patterns [27].Nuclear Metals Inc.has evaluated different RP techniques for casting Beralcast alloys [20].Table 1presents a list of RP tech-niques,the building materials utilised and the ?nal part charac-teristics.
The FDM and MM II systems produce wax patterns that are readily accepted by foundries.For non-wax RP patterns,two signi?cant advantages are identi?ed.Firstly,the durabil-ity and strength of non-wax patterns will allow the casting of thin wall structures,which are previously dif?cult due to the fragility of wax structures.Secondly,the relatively tough non-wax patterns allow ?nishing operations to be conducted to improve surface quality,which is then transferred onto the castings.To counter the shrinkage of castings during cooling,RP patterns can be scaled up accordingly.However,with non-wax patterns,many new problems related to ceramic shells
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Fig.4.RP&T techniques for RIC
Table 1.List of RP techniques RP
Process
Build Layer Surface
Part Residual technique
materials
thickness roughness (μm)Ra accuracy ash (%)
(mm)(as processed)(mm)SL Photocuring Epoxy
0.112.5±0.05N.A.SLS Sintering of powders Polystyrene 0.07513±0.25<0.02Polycarbonate N.A.FDM Melt extrusion ABS 0.0512.5±0.1270.05Wax ≈0LOM Paper lamination Paper 0.0525±0.25N.A.SGC Photocuring Epoxy 0.06250.1%N.A.3DP Ink-jet printing Starch 0.1N.A.±0.0201-2%MM II Ink-jet printing Wax
0.013N.A.0.03%≈0Thermojet
Ink-Jet printing Organic polymer 0.04
5.090
N.A.
N.A.
Sources:[11,21]
cracking,incomplete pattern burning out and residual ash,sur-faced.Despite these earlier setbacks,the bene?ts of RP pat-terns are too signi?cant to be snubbed.Since then,extensive worldwide research has conceived a variety of strategies,spe-cialised materials and new processes to counter or eliminate the setbacks.
4.1.1Ceramic shell cracking
For wax patterns,autoclaving is employed to remove the patterns and sprue systems through melting.Remaining wax traces are vaporised in the ?ring stage.For non-wax RP patterns,stresses induced by pattern expansion during dewaxing and burning-out are major problems resulting in shell cracking.Shell cracking arising from mismatches in coef?cients of thermal expansion (CTE)between RP and shell materials is well studied and docu-mented [28,29].Most research to address shell cracking is based on SL fabricated epoxy patterns.The most successful solution arrived at to date is the fabrication of quasi-hollow structures using QuickCast build styles.The concept of QuickCast is based on the fact that hollow structures would soften at lower tem-peratures and collapse inwards upon itself before critical stress levels are developed [30].Added advantages will be a drop in material costs and lead time to build the hollow structures.QuickCast capitalises on large hatch spacing to create internal skeletons containing large inter-connected square or triangular cells supporting a dense thin external shell [12].Small holes created on the external surfaces allow internally trapped resin to be drained after part building.Early QuickCast patterns are reported to be only partially successful in countering shell crack-ing [31,32].Shell cracking is only fully addressed with the introduction of QuickCast 2.0build style [33],which provides
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an internal architecture of hexagonal honeycombs that collapses during autoclaving.
For FDM fabricated acrylonitrile-butadiene-styrene (ABS)patterns,the sparse (cross-hatched)build style is employed to create quasi-hollow structures (Fig.5).FDM fabricated hollow patterns have shell thickness of around 1.5mm and internal structures of large inter-connected quadrilateral cells with con-stant wall thickness of 0.254mm.Detailed evaluations of ABS patterns based on casting issues,ash handling,process param-eters,thermal and macroscopic properties,have been carried out [34].The promising results ascertain the potential of ABS patterns.However,some minor modi?cations to the IC process are necessary to accommodate ABS patterns.
For powder-based RP techniques (e.g.,SLS and 3D-P),fab-rication of hollow internal structures is not possible due to ma-terial entrapment.However,to optimise parts for burning-out,a “shelling”technique can be employed to fabricate only the outer shell of the pattern.The high porosity of the entrapped powders prevents excessive pattern expansion and shell cracking.For SLS,several building materials,namely,wax and polycar-bonate [27]have been introduced for IC applications.However,patterns built from these materials are plagued by distortions,poor surface ?nish and shell cracking due to foaming of PC parts [35]during autoclaving.CastForm PS,a polystyrene-based powder,is the latest material offered by 3D Systems Inc.for building IC patterns.CastForm PS patterns have characteristics similar to wax and can be adapted to standard foundry practices with minimal alterations.Post-processes necessary for CastForm PS include dipping in lique?ed wax to seal surface porosity and to increase pattern strength.
LOM paper patterns have also been utilised in IC [36].Com-pleted LOM patterns require a coat of sealant to prevent delam-ination and swelling due to moisture absorption.LOM patterns have the advantage of low CTE and are relatively cheap to pro-duce [37].However,LOM pattern intricacy is limited due to dif?culties encountered in the removal of excess paper entrapped within the recesses of completed
parts.
https://www.doczj.com/doc/2f7164927.html,parison of QuickCast (left )and sparse build used in SL and FDM,respectively (right )
4.1.2Pattern quality
The quality attributes of untreated RP patterns are indicated in Table 1.Many of the different RP patterns are well within the ac-ceptable tolerance range (±0.05–±0.254mm)and surface qual-ity (16–20μm R a )[38]requirements of most IC applications.In IC,the surface quality of patterns and castings are directly related.The relatively rough surfaces of untreated RP patterns are due to stair stepping caused by layered building,building method (laser scanning,binder spraying,etc.)and feed stock material.Patterns with porous surfaces (SLS,3D-P,FDM)re-quire the application of sealants (e.g.,wax)to prevent slurry penetration during shell production.For most RP patterns,pol-ishing can be conducted to improve surface quality.The surface of ABS patterns can be improved by sealing with wax or by brushing on a thin coat of methyl methacrylate (MMA)followed by light polishing.Measurements conducted by the authors on MMA ?nished patterns produced surface roughness of 1–2μm R a .Similarly,SLS parts impregnated with epoxy followed by light polishing yielded surface roughness of <1μm R a .Actua patterns can be ?nished with a light coat of liquid paraf?n and polishing [24]to yield improved surface roughness of 0.8μm R a .The surface of other RP patterns (e.g.,LOM paper patterns)can also be improved via polishing with ?ne abrasive papers.4.1.3Combustion properties
The use of non-wax RP patterns may lead to casting defects due to incomplete pattern burn-out or residual ash.Residual ash is usually removed by ?ushing the cooled shell with water or compressed air.However,ash trapped in deep recesses or tight cavities may prove dif?cult or impossible to remove.The use of QuickCast and sparse build styles will signi?cantly reduce residual ash content as they eliminate approximately 60-95%of the internal mass of epoxy (SL)and ABS (FDM)parts re-sulting in small residual ash contents of between 0.02-0.05%.For patterns with dense structures (e.g.,LOM),a much longer burning out time is usually required.The large amounts of ma-terials to be burnt-out will result in much higher residual ash contents [20,27].
4.2Fabrication of wax injection moulds
For mould fabrication,two approaches,namely,direct and in-direct RT approaches [39]are followed.Utilising the direct ap-proach,the mould is fabricated by RP systems with no interme-diate https://www.doczj.com/doc/2f7164927.html,pleted moulds may require post-processing to improve strength,surface ?nish and accuracy.Metal or polymer moulds fabricated using direct approaches can usually be em-ployed for medium-to high-volume production.For the indirect approach,RP fabricated masters are employed to create the ne-cessary moulds.Materials utilised in indirect approaches include polymer or metal composites,polymers and silicone rubber.As such,the indirect approaches will result in moulds that are mech-anically weaker and mostly suitable for low volume-production.
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4.2.1Direct RT approaches for mould fabrication
Direct metal mould fabrication.Direct metal mould production is a new area in RP that has attracted substantial amounts of at-tention.To date,more than six commercialised systems are avail-able that allow metal mould production.Although pioneering processes such as RapidTool (3D Systems,Inc.)and direct metal laser sintering (DMLS)(EOS GmbH,Munich),are intended for producing prototype tooling,continued research have enhanced the performance of RapidTool or DMLS tooling,thereby,nar-rowing the performance gap between RT and conventional hard tooling.Direct metal tooling processes have proven to signi?-cantly impact the cost and lead-time required for producing prototype or production moulds.RP mould fabrication allows the incorporation of conformal cooling channels [40,41]which can reduce injection cycle times,thereby,directly affecting part cost and production rates.Although RP moulds are mechanically weaker than conventional moulds due to porosity (5–10%),their performance is expected to match conventional moulds when employed for wax injection moulding.The less vigorous wax in-jection process involves temperatures and pressures of around 55–60?C and 1–3MPa as opposed to 200?C and 100MPa for plastic injection allowing the preservation of tool life.Table 2presents a list of commercialised RT solutions applicable for wax injection moulding.
?Laser sintering (LS)of metals
LS is the pioneer and perhaps the most extensively work-on RP technique for fabricating structurally sound metal tooling.To date,two different metallic materials,namely,polymer coated and multi-phase metal powders have been commercialised under the RapidTool and DirectTool processes respectively.
For RapidTool,LS is carried out on steel-based powders coated with a thin layer of polymeric binder material.The pre-heated binder melts upon exposure to CO 2laser resulting in coagulation of powders to form the required part.The completed “green”part is ?red inside a furnace for debinding and sinter-ing.The sintered part is strengthened and densi?ed by bronze in?ltration via capillaric action.To improve part quality,?nish-ing operations can be https://www.doczj.com/doc/2f7164927.html,serForm ST-100(420stain-Table 2.List of commercialised direct RT techniques Process
Layer Surface Dimensional Final part Tool life thickness roughness accuracy density (approx.)
(mm)(μm)R a
(mm)(treated)RapidTool (steel)0.75 5.0(raw)
±0.1
≈100%100,000*DirectTool (steel)N.A.≈1(treated)12–16(raw)±0.05%+0.0595100,000*ProMetal (3DP)N.A.N.A.±0.1≈100100,000*LENS
N.A.12(raw)±0.13≈100%100,000*Direct AIM (polymer)0.1–0.212.5
±0.05N.A.N.A.
Direct shell
N.A.
30–50μm Rz
±0.020
N.A.
1(due to production Production casting (DSPC)
of expendable shell)
?Results
based on ?gures obtained for plastic injection molding;wax injection molding should produce much higher ?gures.
Sources:[14,39,44,50]
less steel-based powder),is the latest tooling material system offered to replace RapidSteel 2.0and Copper Polyamide https://www.doczj.com/doc/2f7164927.html,serForm ST-100tooling is reported to be fully dense after LS with surface roughness of 5μm R a .RapidTool moulds have been successfully employed in both plastic [40,42]and wax injection moulding.Reports claim that complex moulds are pro-duced under 2weeks and are capable of producing 50,000to 100,000parts [43].Other RapidTool related works include Pham et al.[44]who investigated factors in?uencing part accuracy and proposed an approach to optimise process parameters and inves-tigations on the strengths of RapidTool parts and tooling [45,47,56].A cost analysis by Dalgarno [48]based on RapidSteel tool-ing ascertained that signi?cant cost savings can be derived using RT over conventional tooling for injection moulding.
The DirectTool process (EOS GmbH)employs the DMLS technique to sinter multiphase metal powder mixtures con-taining nickel,bronze and a copper-phosphate (Cu-P)?ux-ing/deoxidation agent.A 200W CO 2laser selectively delivers heat energy to induce liquid phase sintering via the melting of Cu-P particles [49].Molten Cu-P occupies inter-particle voids and wets the bronze/nickel powders causing them to bind.With optimised process parameters,part shrinkage during sintering is minimised resulting in high dimensional accuracy of ±0.05%plus ±0.05mm [14].Surface roughness of untreated parts range between 12–16μm R a [50].However,surface quality can im-prove signi?cantly (<1μm R a [51])by shot peening (EOS’s Micro Shot Peening),epoxy in?ltration and polishing.To in-crease part density,in?ltration with high temperature epoxy,tin,silver,etc.is conducted.Epoxy in?ltration is preferred as it has minimal effects on ?nal part accuracy unlike metal in?ltration,which may induce thermal strains.DirectSteel 50V 1,a steel-based powder mixture with no organic components,is the latest material offered by EOS in addition to DirectMetal (bronze-based)powder.With DirectSteel,in?ltration is unnecessary as parts produced are 95%dense,exhibit negligible shrinkage and are highly accurate.However,sintering of steel-based powders has to be conducted in a nitrogen atmosphere to inhibit oxida-tion.The DirectTool process is limited by the small build volume (250×250×150mm 3)of the EOSINT M RP system which lim-its part size.DirectTool has mainly been applied in fabricating
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complex mould inserts that are usually produced via conven-tional machining.DirectSteel moulds are capable of producing 100,000injection moulded components.
The immense potential of DMLS has increased research on a wide variety of metals[52]and metal compositions[53–55]. These include steels,Ni-base superalloys,Ti and its alloys,re-fractory metals,bronze-Ni and cermets[56].Other related works include Khaing et al.[50]and Agarwala et al.[57]who in-vestigated the in?uence of process parameters on physical and mechanical properties of bronze-Ni parts,Karapatis et al.[58] who demonstrated improvements in powder layer density using multimodel powders,and,Hauser et al.[59,60]who investi-gated the in?uence of scanning parameters and sintering at-mosphere on part distortion and quality for single metal alloy powders.
Another LS process developed is rapid mould(RM)[42]. RM is similar to RapidTool and uses the same powder stocks. However,unlike RapidTool,RM utilises epoxy in?ltration in-stead of bronze.Although RM production is cheaper and faster than RapidTool,RM moulds possess longer injection cycle times and are much weaker due to epoxy in?ltration and metal oxida-tion arising from the use of a simple air oven for debinding and sintering.
?3D Printing of metals
3D Printing of metal components is pioneered by Sachs et al.[41, 61].The ProMetal process(Extrude Hone Corp.,PA)is based on 3D Printing whereby an electrostatic inkjet head selectively de-posits minute droplets of binder onto successive thin layers of steel powders to form the required object.The porous“green”part is?red inside a furnace for debinding and sintering.The re-sulting steel skeleton(60%dense)is fully densi?ed via bronze in?ltration.The simplicity of inkjet printing results in relatively fast fabrication times and adaptability with a wide variety of metals including tool steel.Although inkjet printing has an ac-curacy of±0.025mm,post-processing reduces part accuracy to ±0.1mm[16].
?Laser generating(LG)
LG is based on laser cladding technology[62],originally de-veloped as a surface treatment process.LG produces metal parts by injecting a stream of metal powder into a molten metal pool at the focal zone of a high power Nd:YAG https://www.doczj.com/doc/2f7164927.html,ser scan-ning is followed by solidi?cation of molten metal to form fully dense structures that require no secondary post-processes.Accu-racy and surface?nish of untreated built parts are about0.13mm and12μm R a respectively.The use of an inert gas environment and powder delivery system prevents oxide formation.A var-iety of metals including tool steel and Ni-based super alloys can be used in LG.However,LG is limited to parts containing low geometric complexity(straight and simple features)due to the absence of a support https://www.doczj.com/doc/2f7164927.html,mercialised LG processes in-clude laser engineered net shaping(LENS)process(Optomec Inc.,NM).LENS produced tooling were reported to be capa-ble of producing above100,000injection moulded components. Other LG-based processes developed include shape deposition modelling(SDM)[63],direct metal deposition(DMD)(Preci-sion Optical Manufacturing(POM),Michigan),direct laser fab-rication[64]and controlled metal build-up(CMB)[65].Both SDM and CMB employ a milling cutter to shape each deposited layer to ensure?atness for the deposition of successive layers.In addition,SDM incorporates a sacri?cial material support system to stabilise the part during building.CMB moulds are reported to have surface roughness between3–5μm R z and are estimated to be capable of producing up to200,000injection moulded components[66].
?Others
Other direct metal tooling processes being developed include Solidica(Solidica,Michigan),laminated tooling,FDM of semi-solid metals[67]and multiphase jet solidi?cation(MJS)[68]. The Solidica process produces metal parts by fusing and process-ing0.1mm thick aluminium alloy tape using a hybrid approach that combines layered building and high-speed milling.Reported building accuracy is about±0.075mm.For laminated tooling, metal tools are built by stacking laser or water-jet cut metal lami-nates together.Fusion between adjacent laminates is achieved via spot welding,bolts and nuts,adhesives,etc.Finishing via CNC and EDM machining is conducted to improve part quality.Exam-ples of laminate tooling processes include computer-aided manu-facturing of Laminated Engineering Materials(CAM-LEM)and Stratoconception.CAM-LEM(CAM-LEM Inc.,OH)is capa-ble of fabricating metal or ceramic tooling using pre-formed stainless steel or ceramic sheet material made from steel or ceramic powders that are held together using a binder.The “green”parts are?red in a furnace to achieve full density.In the CAM-LEM process,shrinkage values of12–18%are typi-cal.As such,scaling up of part designs is necessary for shrinkage compensation.For Stratoconception(CIRTES,France),high-speed micro-milling or laser machining is used to produce a set of elementary layers which are assembled by the insertion of stiffeners and plugs to form the required part.Stratoconcep-tion parts possess good surface?nish and dimensional accuracy since stair stepping is eliminated by cutting sloped edges on each layer to conform to the shape of the required part and no shrinkage is encountered during processing.Other than these, the Stratoconception process is fast and requires no secondary processes.
Direct fabrication of polymer/wood moulds.To date,extensive amounts of research have been conducted to study and evalu-ate polymer and wood tooling produced by commercialised RP systems.Wax injection moulds fabricated from SLA,FDM and LOM systems have been employed with varying degrees of suc-cess.FDM produced ABS moulds have to be in?ltrated with epoxy or aluminium?lled epoxy before being utilised.The in?l-trant seals the porous surfaces and strengthens the ABS moulds, allowing successful employment for wax injection moulding at pressures and temperatures of1.38MPa and66?C respec-tively[69].For LOM paper or wood moulds,surface coating with a sealant is essential to improve wear and moisture resis-tance.A successful case study[70]reported the use of LOM moulds with injection pressures of0.2–0.4MPa and at a tem-
315 perature of70?C to produce a series of wax patterns of an
automotive axle bracket under a21
2week time frame.SL is
perhaps the most extensively researched RP technique for pro-ducing injection moulds.The desire for increased mould ac-curacy drove the creation of the direct AIM(ACES injection moulding)process.Direct AIM uses the ACES(accurate clear epoxy solid)build style for fabricating polymer moulds on a SLA system.The process is straightforward and produces relatively accurate moulds.Polymer moulds have the advantage of the 80?C glass transition temperature which is much higher than the melting temperature of50–55?C for IC waxes,thereby,pre-serving mould rigidity during moulding.Other processes being developed to produce direct polymer tooling include the Opto-Form process[71],which is capable of fabricating wax injection moulds from low cost acrylate?lled resin.
For purely non-metal tooling(epoxy,epoxy in?ltrated ABS, paper,etc.),their injection cycle times are much longer com-pared to conventional tooling due to the poorer thermal con-ductivity of non-metals.Although moulds with conformal cool-ing channels can be fabricated,improvements in cycle times are generally limited[37].Non-metal tooling are weaker,less wear resistant and possess much shorter tool lives compared to their metal counterparts.As such,non-metal tooling containing ?ne features can be easily damaged by excessive injection pres-sures and mishandling.A caution to heed while using non-metal moulds is the application of release agents.Aerosol-based re-lease agents containing carboxylic,ketones and aldheydes will attack the surfaces of RP parts[69]resulting in degradation of surface quality.As such,only silicone or talc-based release agents are recommended.
Direct fabrication of ceramic IC shell.Direct fabrication of ce-ramic shells carries a greater advantage in terms of lead-time and cost savings due to the elimination of pattern production. Some added advantages[11]include the fact that the process involves minimal shell transfers thereby reducing the risk of damage while preserving dimensional tolerances.Also,for cast-ing geometrically complex parts that require core inserts,any risk of core shift is eliminated as the shell and core are fabri-cated as a single structure.The?exibility to adjust the ceramic shell thickness during RP fabrication allows some changes and degrees of control over the rate of heat transfer from the casting. RT processes capable of directly producing ceramic tooling are listed below.
?Direct shell production casting(DSPC)
DSPC(Soligen Inc.,CA)capitalise on the3D printing technique for fabricating ceramic shells.In DSPC,alumina powders are held together through the spraying of colloidal silica binder.The completed shell is?red prior to casting.Typical build accuracy is within±0.02mm.DSPC allows the fast turn around of small quantities of fully functional castings produced from a wide se-lection of metals.Success achieved in using3D-P ceramic shells is reported by Sachs et al.[11]where they are utilised for cast-ing nickel superalloys at1660?C.Shell shrinkage during?ring is reported to be minimal.
?Direct sintering of ceramics
Fraunhofer-IPT is developing a direct ceramic shell production process via direct sintering of zirconium silicate powder[66]in an EOSINT M160RP system.The process selectively melts ce-ramic powder layers to form the shell.The completed shell can be directly utilised for casting upon cleaning and pre-heating. Surface quality is reported to be30–50μm R z with accuracy well below±0.6%.Analyses carried out on successful castings indicate lead-time reductions of up to95%from conventional IC.
4.2.2Indirect RT approaches for mould fabrication
All indirect RT approaches start with the fabrication of a RP pat-tern of the?nal desired casting.The?nished pattern is used to cast the required mould.As such,mould quality depends greatly on the quality of the RP patterns.Similar to conventional moulds, factors such as shrinkage of injected wax and casting requires compensation during the design and production of the RP pat-terns.These compensation factors are transferred to the mould during mould production.Table3presents the list of commer-cialised RT techniques utilising the indirect approach.?Silicone rubber tooling
In this process,a degassed liquid silicone and hardener mixture is cast around a RP pattern contained in a mould box.Runner and gating channels are incorporated by embedding ABS or per-spex rods into the liquid silicone or by cutting the channels in the cured silicone block.Upon curing,the rods are removed to form through channels and the pattern is subsequently removed to form the mould cavity by cutting along the parting line.Silicone rubber tooling allows quick production of inexpensive multiple moulds for small and large parts with good part cosmetics.The process has been applied successfully for moulding wax patterns[20].
A typical silicone mould can produce between100-300patterns. IMI Rapid Prototyping,UK,has successfully produced in excess of1000wax patterns from a single silicone mould[19].In an inter-esting piece of work conducted by Zhang et al.[72],ice sculptures produced using the Rapid Freezing process developed at Tsinghua University,China,are utilised as patterns to cast silicone moulds for wax pattern production.The ice sculptures can also be em-ployed as IC patterns[73]with advantages of easy removal via melting and zero residual ash content.
Table3.List of indirect RT approaches
Process Dimensional Tool life
accuracy(mm)(approx.)
Silicone rubber tooling Not applicable100to300
Epoxy resin tooling0.02%500,000to1,000,000* (PolySteel)
Spray metal tooling N.A.10,000to100,000* Cast metal tooling0.1–0.3%>200,000?
Keltool tooling N.A.>1,000,000?
?Results based on plastic injection molding
Sources:[14,39,50]
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?Epoxy resin tooling
To produce epoxy resin moulds,the RP pattern is embedded in clay up to the pre-determined parting line.Runner and gating channels are formed by attaching ABS rods onto the pattern.The assembly is spray coated with release agent before liquid resin or a resin/aluminium powder mixture is cast around the pattern. Upon curing,the pattern is separated from the hardened mould half,cleaned and re-coated with release agent in preparation for casting the second mould half.Holes forming the alignment and locking features for the mould set are milled onto the surface of the existing mould half.Matching pegs will form on the second mould half by resin occupying the empty volume of these holes during casting.The prepared pattern is replaced onto the exist-ing mould half and the procedures for casting are repeated.The formation of residual stresses between the two mould halves dur-ing curing will require the mould set to be heat treated before the halves are separated.
Epoxy resin moulds have been successfully utilised for both plastic and wax injection moulding[36,74].The use of metal ?lled epoxy tooling material increases the durability and heat transfer characteristics of the mould.Additional cooling chan-nels can be added by embedding copper coils into the resin body during https://www.doczj.com/doc/2f7164927.html,mercially available tooling resins in-clude EP250tooling resin(MCP HEK-GmbH,Germany)and PolySteel(Dynamic Tooling,CA)which contain aluminium and steel?llings respectively.PolySteel moulds are reported to be much stronger than aluminium resin tooling with a composition of approximately90%steel by weight and dimensional accuracy of0.02%.The surface?nish of epoxy resin moulds is depen-dent on the quality of the RP pattern.PolySteel moulds have been successfully employed for wax injection moulding.Moulds cast from PolySteel II+,an improved version of PolySteel with zero shrinkage,are capable of producing500,000–1,000,000plastic injected components.
EcoTool is a process being developed by Danish Techno-logical Institute(DTI)and TNO Institute of Industrial Technol-ogy,which can produce cast metal tooling for injection moulding using a tool steel powder/binder system[75].The difference be-tween EcoTool and metal?lled epoxy resin tooling lies in the incorporation of a sintering and copper-based alloy in?ltration furnace cycle.Overall shrinkage encountered by EcoTool parts after the furnace cycle is around0.1–0.3%depending on the metal powder and binder system used.
?Spray metal tooling
To create a spray metal mould,minute droplets of molten metal such as tin-zinc,or steel are sprayed onto a RP pattern using an arc spray process[76].The thin shell of deposited metal constitutes the mould surface.To strengthen the shell,a solid backing is cast around the shell using pure epoxy,metal-?lled epoxy or low-melt alloy back?ll materials.With a proper choice of back?ll material and the incorporation of cooling chan-nels,a spray metal mould exhibits good injection cycle times. Other advantages include the ability to cater for large moulds at relatively low costs,high tolerances due to minimal shrink-age encountered during and after the spraying process,and good tooling life of approximately10,000-100,000injections[3]. Developed spray metal tooling processes include Ford’s Spray-Form[77]and Idaho National Engineering and Environmen-tal Laboratory’s(INEEL)rapid solidi?cation process(RSP) tooling[78].
?Cast metal tooling
In this approach,expendable RP mould patterns are used to in-vestment cast the production moulds in aluminium or steel.All bene?ts derivable as mentioned in Section4.1can be realised with this approach.Its main disadvantages compared to other indirect RT processes are the relatively longer lead-times and lower levels of accuracy achievable due to additional steps taken in investment casting the mould.However,tight tolerances for critical features on the cast metal mould can be brought into speci?cations with secondary machining operations or local in-serts[79].Depending on the metal used,cast metal tooling were estimated to be capable of producing several thousands to be-yond200,000plastic components.Leyshon Miller Industries has utilised FDM fabricated wax patterns to successfully investment cast aluminium mould https://www.doczj.com/doc/2f7164927.html,ing the process described,the company has reported turnaround times of1–2weeks for moulds with moderate size and complexity[80].
?3D Keltool tooling
The3D Keltool process(3D Systems Inc.)starts with an im-pression of the RP pattern created using silicone rubber tooling. The silicone mould is used to cast the required mould inserts from a tool steel powder and binder mixture.The cured“green”parts are?red in a furnace to debind and sinter the steel powders. Sintered parts are in?ltrated with copper to produce fully dense structures with compositions of70%steel and30%copper.3D Keltool mould inserts are limited to small product sizes and have an estimated life exceeding1,000,000shots in plastic injection moulding.
5Discussion
The RP&T techniques discussed have been employed or are potentially bene?cial to the IC process.As such,this paper does not constitute an exhaustive list of developed or commer-cialised techniques.The immense demands for lower produc-tion costs and lead-time,higher part accuracy and performance have driven the impressive numbers of new techniques intro-duced annually.The Rapid Prototyping&Tooling Industrial Ap-plications(RAPTIA)Newsletter indexed a staggering number of30techniques for producing injection moulds in1999[81]. Known and proven techniques are improved and enhanced for penetration into new application areas.With the current pace of developments,it is not outlandish to consider a closure in the gap and distinction between conventional and rapid tooling in future.
IC is continually being revolutionised by RP&T techno-logical breakthroughs.To date,RP&T has been applied to many IC applications ranging from the casting of jewellery(Fig.6), sporting equipment and medical implants to high performance
317
parts utilised in injection moulding,die casting or the automotive and aerospace industries (Fig.7).
Figure 8presents the process chains of conventional IC and the RP&T techniques.Most RP&T techniques described are still under development or have just been recently commercialised.As such,the success of tool production utilising any of the tech-niques relies heavily on “training”runs,accumulated experience,and the understanding and familiarisation of the toolmaker with the technique,its limitations and critical parameters.In most cases,in-depth ?eld veri?cation has to be accomplished be-fore long term con?dence in the techniques is realised.Despite the immense bene?ts that can be achieved through RP&T tech-niques,rapid tools are still behind conventional tooling in terms of quality and performance.Issues pertaining to dimensional ac-curacy,surface quality and part durability need to be further addressed and
improved.
Fig.6.Jewellery produced by IC (back )gold and platinum castings (front ,L–R)Al castings in cluster arrangement,MM II fabricated wax patterns.(Image courtesy of Ngee Ann
Polytechnic)
Fig.7.Aerospace component (left )QuickCast pattern (right )stainless steel casting.(Image courtesy of Ngee Ann Polytechnic)
5.1Dimensional accuracy
As RP techniques rely on CAD data for input,the accuracy of digital models created by the toolmaker can directly affect the outcome of the fabrication process.To create accurate patterns and tooling,a high level of pro?ciency in shrinkage compen-sation factors,post-machining allowances and foundry require-ments are pre-requisites to consider during the modelling pro-cess.Besides human factors,the CAD system used can some-times be the limiting factor in the production of accurate digital representations of the required design and the conversion of na-tive CAD data to data acceptable by RP systems (i.e.,STL for-mat).In STL format,the surfaces of modelled geometries are approximated by arrays of triangles (tessellation)whose sizes (chord length)are determined by the user.The setting of the chord length is often a compromise between the ?le size or the required amounts of computing resources necessary and the ?-nal accuracy of the converted data.Besides this,STL ?les are prone to problems related to missing or reversed surfaces.To date,much research has been conducted by the authors [82–85]and other researchers to address issues related to STL ?le re-pair and data interfacing.Alternative data formats to replace STL such as CLI [86]and SLC [87],which circumvent the tessella-tion process by directly slicing a CAD model to improve data accuracy have also been worked on.5.2Direct IC pattern production via RP
For RP fabricated IC patterns,the surface ?nish of powder-based patterns (SLS,3D-P)and residual ash content need to be im-proved.One possible solution may lie in the use of ?ner powders,which will allow thinner layers to be deposited,thereby,improv-ing surface quality and stair stepping effects.However,a com-promise between build time and part quality has to be achieved in order not to complicate the powder deposition process nor lengthen the build time.Alternatively,in?ltrants can be used to improve the properties of powder-based patterns.For patterns produced from liquid-and solid-based feed stock materials,the introduction of new build styles (QuickCast and sparse build)and building materials (waxes and organic thermoplastics)have led to casting success rate of close to 100%[27].5.3Direct RT approaches for metal tooling production
Most RP&T processes available for fabricating metal tooling require additional post-processing stages (debinding,sintering and in?ltration)to bring the metal tool to its fully dense state.To produce precise metal castings,?nishing stages (machining and polishing stages)have to be added to processes in order to yield tooling that comply with speci?cations.The ?nishing stage contributes signi?cantly to the ?nal accuracy and quality of the mould and as such,requires some amount of planning to achieve optimum results.All added processes increase the lead-time re-quired for mould production.However,continued advances in laser generating (LG)and direct metal laser sintering (DMLS)
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https://www.doczj.com/doc/2f7164927.html,parison of conventional IC and RP&T process chains
techniques coupled with the introduction of low shrinkage pow-der compositions will allow the creation of fully dense metal moulds that require no post-processes.
5.4Indirect RT approaches for non-metal tooling production Non-metal tooling produced by indirect RT processes are faster and much cheaper to fabricate compared to RP metal tooling.The indirect approaches are also generally much simpler and less stringent than direct approaches.As such,indirect RT ap-proaches can be adopted with fewer efforts.However,non-metal tooling possess longer injection cycle times due to their high heat insulation properties,and exhibit poorer tool performance.
6Conclusion
This review is aimed at presenting the various commercialised RP&T techniques and the research and development conducted to improve technologies that will directly bene?t the IC process.From the manufacturing perspective,RP,RT and IC are highly advanced manufacturing processes that can be applied in uni-son to provide product manufacturers with a competitive edge in the modern consumer market.Although RP&T technologies are developed with the hope of replacing conventional fabrication techniques,further improvements in current technologies are re-quired before such hope can be realised.From the discussion,it
319
is obvious that each RP&T approach possesses its own set of ad-vantages and disadvantages.Coupled with the fact that most of the techniques described are still in their infancy stages,there is no clear evidence as to which RP&T technique is the most bene-?cial in terms of cost and lead-time required to produce a unit of the?nal metal casting.As such,the user should evaluate his/her requirements and the capabilities of the various methods before deciding on the process.
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