Rapid investment casting: direct and indirect approaches via fused deposition modelling

7/29/2019 Rapid investment casting: direct and indirect approaches via fused deposition modelling

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O R IGINA L A R T IC L E

C. W. Lee C. K. Chua C. M. CheahL. H. Tan C. Feng

Rapid investment casting: direct and indirect approachesvia fused deposition modelling

Received: 11 April 2002 / Accepted: 24 February 2003 / Published online: 25 October 2003 Springer-Verlag London Limited 2003

Abstract Investment casting (IC) offers an economicalmethod for mass producing complex, shaped metalparts. However, high tooling cost and lead times asso-ciated with the fabrication of metal moulds for pro-

ducing IC wax (sacrificial) patterns result in costjustification problems for customised single casting,small- and medium-quantity production. Rapid proto-typing (RP) techniques can reduce the costs associatedwith single-part or small-quantity production as theycan be applied to the fabrication of sacrificial IC pat-terns containing complex and intricate designs withsignificant cost and lead-time savings. In this project, abenchmark model is designed to assess the fused depo-sition modelling (FDM) process for creating sacrificialIC patterns. In addition, an indirect approach towardproducing wax patterns via silicone rubber moulding isinvestigated. Cost and lead time comparisons between

the two IC pattern production methods were carried outand presented. The dimensional accuracies of metalcastings generated from the RP-produced patterns arealso presented.

Keywords Investment casting Rapid prototyping Fused deposition modelling Silicone rubber moulding

1 Introduction

1.1 Background

Investment casting (IC) is a key technique among arange of modern metal casting techniques that is capableof providing an economical means of mass producingshaped metal parts containing complex features (e.g.thin walls, undercut contours and inaccessible spaces)

which can be difficult or impossible to create using otherfabrication methods [1]. Despite the wide range ofapplications in many industries, the standard (conven-tional) IC process practiced in modern foundries has its

drawbacks. High tooling costs and lengthy lead timesare associated with the fabrication of metal moulds re-quired for producing the sacrificial wax patterns used inIC [2]. The high tooling costs involved in conventionalIC result in cost justification problems when smallnumbers of castings are required.

Rapid prototyping (RP) techniques are fast becomingstandard tools in the product design and manufacturingindustry. With the capability of rapidly fabricating 3-Dphysical objects, RP has become an indispensable toolemployed for shortening new product design anddevelopment time cycles [3, 4, 5]. RP techniques arelimited neither by the geometry nor by the complexity of

the parts to be fabricated. In addition, RP techniquesinvolve no tooling or fixtures, resulting in simpler set-up,lower overhead cost and shorter production lead timescompared to other fabrication methods. With RP, partsthat were previously impossible or extremely costly andtime-consuming to fabricate can be built with ease.

1.2 Rapid investment casting processes

The application of RP-fabricated patterns as substi-tutes for the traditional wax patterns employed in ICstems from the fact that RP materials can be melted

and burned out from the ceramic shell (ceramic shellcasting) without damaging it [6]. Most commercialisedRP techniques are capable of producing such patternsthat can be used directly in IC. Several foundries haveutilised various RP techniques to produce IC patterns.For instance, Shellcast Foundries Inc. (Montreal,Quebec) researched and developed a process calledsolid model casting (SMC) that allows the direct con-version of RP models to functional prototype castingswithout the need for tooling [7]. Cercast Group (Quebec,Canada) has identified the most important variables

Int J Adv Manuf Technol (2004) 23: 93101DOI 10.1007/s00170-003-1694-y

C. W. Lee C. K. Chua (&) C. M. Cheah L. H. Tan C. FengSchool of Mechanical & Production Engineering,Nanyang Technological University,50 Nanyang Avenue,639798 Singapore, SingaporeE-mail: [email protected]


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in designing RP models for IC applications and theadvantages and disadvantages of RP models producedby different technologies [8, 9]. Nuclear Metals Inc.(Concord, MA) has done a comparison of RP tech-niques for casting the Beralcast family of alloys. [10].

Most of the limitations encountered with earlierapplications of RP-fabricated IC patterns include dam-age to the ceramic shells due to excessive thermal

expansion of the pattern or the release of corrosivedegradation by-products during pattern burn out, whichcracks or attacks the cavity surface of the shell. Ceramicshell cracking is attributed to a mismatch in the coeffi-cient of thermal expansion (CTE) between the RP andceramic materials. Most RP materials have CTE valuesthat are larger than the ceramic material and as such, theexpansion of the pattern during heating imposes signif-icant amounts of stress on the ceramic shell. Shellcracking occurs when the stresses imposed by theexpanding pattern are greater than the modulus ofrupture (MOR) of the shell material [11]. In some cases,residual ash can also cause defects in the final castingswhen present in relatively significant quantities. Also,due to the high operating and material costs of RPtechniques, the utilisation of RP techniques has beenreported to be beneficial only when five or fewer castingsare required [10].

The application of RP techniques to produce sacrifi-cial IC patterns can be classified under two methods: thedirect RP method, and the indirect RP method. Thedirect RP method covers the application of RP tech-niques for producing plastic, wax or paper IC patternsfor fabricating ceramic moulds that can be employeddirectly for metal casting (i.e. direct shell productionmethod) [2]. For the indirect RP method, alternativemoulds produced via silicone rubber moulding in con-junction with an RP-fabricated master pattern of thefinal desired casting are employed for the injection of ICpatterns from foundry wax. In addition to siliconerubber moulds, moulds fabricated with RP techniquescan be used to produce IC wax patterns.

For the direct RP method, the RP-fabricated patterncan only be used to produce a single casting. On theother hand, for the indirect RP method, the siliconerubber mould or RP mould can generate multiple waxpatterns to produce a number of castings. However, dueto the cost of RP materials as mentioned, silicone rubbermoulds are usually the preferred choice for indirect RPmethods. The process chain for direct and indirect RP

methods is illustrated in Fig. 1. Using either direct orindirect RP methods, hard tooling production can beavoided. This results in tremendous time and cost sav-ings, especially for projects involving a low number ofcastings. In addition, design iterations can be accom-modated without incurring the high cost of modifying orre-producing the hard tooling.

The focus of the work presented in this paper is onthe production of sacrificial IC patterns using the fuseddeposition modelling (FDM) technique. To the authorsknowledge, no attempts have been made so far to

benchmark the FDM process for the production of ICpatterns and to assess the performance of FDM-fabri-cated IC patterns. In this project, a commercialisedFDM system, FDM3000 (Stratasys Inc., Eden Prairie,MN) is evaluated for fabricating IC patterns followingthe direct RP method (Figure 1). A benchmark test partis designed to assess the performance of FDM-fabri-cated acrylonitrile butadiene styrene (ABS) patterns inthe IC process. In addition, the indirect RP method forproducing injected wax patterns via a silicone rubbermould is investigated and compared to the direct RPmethod.

2 Methodology

The implementation of a benchmark test part is anessential practice in most evaluation studies conductedon any manufacturing system or process, especiallywhen high capital investments are involved. Such sce-narios include the acquisition of expensive CAD/CAE/

CAM systems, CNC machine tools and rapid proto-typing systems [12] or major revamping of an existingprocess line and implementing a new process chain. Ingeneral, system or process evaluation using a customisedbenchmark model that is tailored to test specificrequirements will allow the strengths and weaknesses ofthe system or process to be assessed.

For the evaluations conducted, a number of criteriaare selected to assess the performance of the FDMprocess and the resulting ABS IC patterns. The list ofcriteria used in this benchmarking exercise is as follows:

Fig. 1 IC with RP patterns

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1. Cost of RP fabrication2. Time taken for part building3. Accuracy of the fabricated part compared to the de-

sign model4. Dimensional stability of RP part5. Surface finishing of the RP part

In order to facilitate smooth and accurate dataacquisition, the benchmark model should be relatively

simple in design but should contain the desired features.Such features must incur relatively low consumption ofresources, low demand on material usage and facilitatethe use of simple measuring instruments in generatinganalytical data. There are generally two primary types ofmeasurements that can be taken from a benchmarkmodel: main measurements and detailed measurements.

The benchmark test part was designed using Pro/ENGINEER software. The design of the test part(Fig. 2) consisted of a rectangular base with dimensionsmeasuring 110 mm (L) 90 mm (W) 10 mm (H) toallow for the testing of linear accuracy, five squarebosses with 20 mm sides and 30 mm (H) for the testing

of squareness accuracy, four cylinders with varyingdiameters ranging from 3 to 20 mm for the testing ofradial accuracy, three through holes with diameters be-tween 3 and 20 mm to assess shrinkage rates, thin wallswith thickness ranging from 0.52 mm to test for con-sistency of wall thickness, and ribs elevated at 30 and60 to the base for testing angular accuracy. Table 1summarises the purposes of each design feature on the

test part as mentioned. The dimensions that were mea-sured to assess the accuracy of the part are clearlyindicated with appropriate denotations in Fig. 2.

3 Experimental procedures

3.1 IC pattern fabrication using FDM

Upon finalising the design of the benchmark test part, ashrinkage factor was incorporated by scaling up thedesign by 1% to compensate for shrinkage encountered

by the casting material (aluminium alloy, A356 series)during solidification. The shrinkage factor was deter-mined largely from the experience of the local foundryFig. 2 Detailed drawing of benchmark test part

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(Precision Products Singapore Pte Ltd). Aluminium al-loy (A356 series) was selected for the current work as itis commonly employed in many IC applications. Thetest pattern was built on the FDM3000 RP system usingP400 ABS modelling material (Stratasys Inc., EdenPrairie, MN). To facilitate the burning out of the ABStest pattern during the IC process, the pattern was builtto be hollow using the sparse build style supplied withthe FDM system software. With a hollow pattern, thematerial can expand inwards when heated, causing less

pressure to be exerted on the ceramic shell and therebyreducing the risk of cracking. Figure 3 shows an exam-ple of a part built with the sparse build style.

To test for repeatability, two benchmark test pat-terns, labelled BM02 and BM03 and with similar designswere built and subsequently sent to the foundry. Acorner at the base of BM03 was chamfered to distinguishthe castings produced from the two patterns. Figure 4presents the designs of the two test parts. Figure 5 showsthe FDM-fabricated ABS test patterns.

3.2 Fabrication of master pattern for silicone

rubber moulding

For the indirect RP approach, a master pattern that issimilar to the benchmark test part illustrated in Fig. 2 wasemployed with shrinkage compensation factors forfoundry wax (%1%) and aluminium alloy A356 series

(%1%) added. The master pattern was fabricated on theFDM3000 RP system using ABS material. Once com-pleted, liquid silicone tooling material (MCP VTV750,HEK GmbH, Germany) was cast around the masterpattern inside a mould box. When cured, the siliconeblock was parted into four pieces using a hand-held cutterto extract the master pattern. A gating system to allow thein-filling of the cavity within the silicone mould left by themaster pattern wascut into thesiliconeblock. Once ready,the silicone mould was re-assembled before liquid wax

was injected into itscavity to produce a wax casting, whichwas subsequently employed as a sacrificial IC pattern. Thesilicone rubber mould produced using the FDM-fabri-cated master pattern is presented in Fig. 6.

Table 1 Purposes ofmeasurements General Feature Dimension Purpose of measurement

Length, width d01, d02, d09, d11, d12, d13, d14, d17,d18, d20, d21, d23, d25, d26

Linear accuracy

Square area d07d08, d10d19, d15d16, d22d24 Squareness accuracyCylinders and

blind holesd27, d28, d29, d30, d31, d32, d33 Radial accuracy

Thin walls andthickness of base

d03, d04, d05, d06, d37 Thickness accuracy

Angular ribs d41, d42 Angular accuracyThrough holes d34, d35, d36, d38, d39 Shrinkage

Fig. 3 Part with hollow interior built using the sparse build styleFig. 4a,b Benchmark model a BM02 and b BM03 (with achamfered corner)

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3.3 Investment casting (IC) process

Two FDM-fabricated ABS patterns, BM02 and BM03,and one silicone rubber moulded wax pattern were sentto Precision Products Singapore Pte Ltd for IC. For theABS patterns, the autoclave stage was by-passed and themodels were burnt off inside a furnace set at a temper-ature of 1000C for one hour.

4 Results and discussion

4.1 Direct RP method for producing IC patterns

To determine the residual ash content of ABS andfoundry wax material, a thermogravimetric analyser(TGA-7 Perkin Elmer) was employed. Test results ob-tained for TGA analysis of ABS conducted in O2 envi-

ronment at a temperature of 900C revealed residual ashcontent of 2.218%. For conventional foundry waxmaterial, the same analysis yielded a residual ash contentof 0.04%. The analysis was not repeated at temperatureshigher than 900C due to limitations of the TGA ana-lyser employed. However, it should be noted that theburning out of ABS patterns during the actual IC pro-cess was conducted at temperatures that are slightlyabove 1000C. As such, a reduction in the amount ofresidual ash to a value lower than 2.218% should beexpected.

Surface roughness measurements were conductedwith a Mitutoyo Surftest-301 system to determine the

surface quality of the RP-fabricated ABS benchmarkmodels before and after the application of surface fin-ishing processes. Surface finishing was carried out bysealing the porous surfaces of the ABS patterns with athin coat of polymer solution followed by light sandingusing 1000 grade abrasive paper. The surface roughnessof the resulting aluminium castings was also measuredand recorded. The surface roughness measurements arepresented in Table 2 as averages calculated from anumber of repeated measurements on the same surfaces(10 readings per surface). From Table 2, it is observedthat the surface roughness of the ABS patterns is greatlyimproved after surface finishing. However, it is noted

that the same surface quality was not achieved in thefinal aluminium castings. The deterioration encounteredin the surface quality of the aluminium castings may bedue to the texture imparted by the silica particles used inconstructing the ceramic shell.

During the IC process, shell cracking was notencountered during the burning out of the ABS patterns.This could be attributed to the building style (sparsebuild style) that was employed to fabricate the patternsas mentioned earlier. Since the models were hollow in-side, the ABS material could expand inwards (intoempty spaces) when heated and less pressure was exertedoutwards on the ceramic shell mould. It was also noted

that the ceramic shells were clean after the burning outprocess. However, to ensure the total removal of any

Fig. 6 Silicone rubber mould created using FDM-fabricatedmaster pattern

Fig. 5a,b FDM-fabricated test patterns a BM02 and b BM03 (witha chamfered corner)

Table 2 Surface roughness values of benchmark models

Benchmarkmodel

Beforesurface finish(I 1/4 m Ra)

Aftersurface finish(I 1/4 m Ra)

Aluminiumpart (I 1/4 m Ra)

BM02 17.895 0.550 4.633BM03 16.893 0.348 4.694

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residual traces of ash or material from within the shellcavity, the shells were flushed with water and com-pressed air. The aluminium casting obtained from theABS patterns is shown in Fig. 7. No major defects wereobserved on the aluminium castings and all the variousfeatures were well formed.

To assess the quality of the castings, fluorescent pe-netrant inspection was conducted. Casting defects de-

tected on the aluminium casting produced from theBM02 pattern include the presence of gas holes (poros-ity), misrun and metal penetration. The gas holes formedwere located inside the aluminium part and could not beobserved visually. Gas holes are formed in the castingsdue to gas bubbles entrapped there during the solidifi-cation stage. The misrun defect occurred at the 0.5 mmthin wall (Fig. 8), causing a small portion of the wall tobe missing. Misruns are usually caused by the inabilityof the molten material to completely fill up the cavitiesof the ceramic shell or by the presence of residual ash. Ametal-penetration defect (Fig. 9) is observed as excessaluminium material formed at the base of the 30 angledrib. Metal penetration defects are due to improper for-mation of the ceramic shell or can be caused by damageto the cavity surface of the ceramic shell. For the castingproduced from the BM03 pattern, the only casting de-fect observed is a misrun defect that was detected at oneof the corners of the base (Fig. 10). This defect couldhave been caused by the presence of residual ash. It

should be noted that the defects encountered in thecastings are common defects observed with the pro-duction of aluminium castings and are not only

Fig. 7a,b Aluminium castings a BM02 and b BM03

Fig. 8 Misrun defect on BM02 casting

Fig. 9 Metal penetration defect on BM02 casting

Fig. 10 Misrun defect on BM03 casting

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restricted to cases where RP-fabricated patterns areapplied in IC.

The measurements of the various dimensions forthe BM02 ABS pattern before and after the surfacefinishing process were recorded and the variations inthe actual dimensions from their design values (with acompensation of 1% to offset the shrinkage of thealuminium alloy) were calculated. To allow changes in

the dimensions to be monitored due to the surfacefinishing process, the averaged measured values beforeand after the surface finishing process were alsoacquired and tabulated. From the variations obtainedin the dimensions, the percentage errors (before andafter surface finish) can be calculated. The averagepercentage error in the dimensions before and aftersurface finishing was calculated to be 1.422% and1.533% respectively.

Similarly, the measurements conducted on BM03ABS pattern were recorded and the average percentageerror in the dimensions before and after surfacefinishing was calculated to be 0.750% and 1.178%respectively. The low percentage errors (%11.5%)observed in both ABS patterns allow users to ascertainthe capability of the FDM process of producingaccurate patterns for IC. The additional surface fin-ishing process conducted on the benchmark testpatterns did not affect the dimensional accuracy greatlyas the percentage errors of the finished parts did notdiffer much (


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the shrinkage of aluminium alloy) were calculated andtabulated. The average percentage error in thedimensions of the master pattern before and aftersurface finishing was calculated to be 1.701% and2.187% respectively.

The measurements of the wax pattern producedusing the silicone rubber mould and the final alumin-ium casting were recorded. The average dimensionalerror calculated for the wax pattern was about 2.064%,which is close to the value obtained for the masterpattern after surface finish. For the aluminium casting,

the average dimensional error was calculated to be0.908%.Both the lead-time and production cost comparison

studies for IC pattern production using the indirectRP method (via silicone rubber moulding) and con-ventional wax injection moulding via hard toolingwere conducted and the results tabulated in Tables 6and 7 respectively. Comparing the time and costrequired to produce a wax pattern via silicone rubbermoulding to employing a metal mould in the con-ventional IC process, significant time (89%) and costsavings (50%) can be realised using the indirect RPmethod.

5 Conclusion

The results of this study showed substantial advantageswhen employing ABS models as direct IC patterns or asmaster patterns for producing silicone rubber moulds to

Fig. 11 a Wax pattern produced using silicone rubber mould.b Resulting aluminium casting

Table 5 Surface roughness values of master pattern and metalcasting

Surface roughness(I 1/4 m Ra)

ABS master before surface finish 19.93ABS master after surface finish 0.38Wax pattern 0.32Aluminium casting 5.79

Table 6 Lead time comparison for IC pattern fabrication

Indirect pattern fabrication viasilicone rubber moulding

Conventional pattern fabricationvia metal mould

CAD design of pattern 5 days CAD design of mould and inserts 3 weeksFDM fabrication of master 24 h Mould production + fabrication

of 1 wax pattern6 weeks

Post processing of FDM master(i.e., removal of support structures+surface finishing)

18 h

Creation of silicone rubber mould+ fabrication of 1 wax pattern

12 h

Total time %1 week 9 weeks

Table 7 Production cost comparison for IC pattern fabrication

Indirect pattern fabricationvia silicone rubber moulding

Conventional pattern fabricationvia metal mould

Fabrication of FDM master %S$1000 Aluminium mould + inserts(Quoted by a local foundry)

%S$2000

Creation of silicone rubber mould(Quoted by a local service bureau)

Fabrication of 1 wax pattern

Fabrication of 1 wax patternTotal cost S$1000 S$2000

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cast wax IC patterns. The advantages derived includesignificant amounts of cost and time savings, relativelyaccurate final castings (average dimensional error%1.5% for the direct RP method and 0.9% for theindirect RP method) with reasonable surface quality andthe complete elimination of hard tooling required inconventional IC process. Thus, it is much more benefi-cial for foundries to employ FDM-fabricated patterns in

IC for single or small quantity production of castings(

Rapid investment casting: direct and indirect approaches via fused deposition modelling

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