Published ArticleIn this paper the time and cost parameters of tooling manufacturing by metal casting in rapid prototyping sand moulds are assessed and comparison is made with alternative tool making processes such as computer numerical control machining and investment casting (Paris Process). To that end two case studies obtained from local companies were carried out. The tool manufacturing was conducted according to a five steps process chain referred to as Rapid Casting for Tooling (RCT). These steps include CAD modelling, casting simulation, rapid prototyping, metal casting and finishing operations. In particular the Rapid Prototyping (RP) step for producing the sand moulds was achieved with the aid of an EOSINT S 550 Laser Sintering machine and a Spectrum 510 Three Dimensional Printer. The results indicate that RP is the rate determining step and cost driver of the proposed tooling manufacturing technique. In addition it was found that this tool making process is faster but more expensive than machining and investment casting

Bhero, S.

De Beer, D.

Nyembwe, K.

Van der Walt, K.

English

Central University Of Technology Free State - LibraryCUT, South Africa

INTERIM52TIME AND COST ASSESSMENT OF THE MANUFACTURINGOF TOOLING BY METAL CASTING IN RAPID PROTOTYPINGSAND MOULDSK. NYEMBWE, D. DE BEER, K. VAN DER WALT AND S. BHEROAbstractIn this paper the time and cost parameters of tooling manufacturing by metalcasting in rapid prototyping sand moulds are assessed and comparison is madewith alternative tool making processes such as computer numerical controlmachining and investment casting (Paris Process). To that end two case studiesobtained from local companies were carried out. The tool manufacturing wasconducted according to a five steps process chain referred to as Rapid Castingfor Tooling (RCT). These steps include CAD modelling, casting simulation, rapidprototyping, metal casting and finishing operations. In particular the RapidPrototyping (RP) step for producing the sand moulds was achieved with the aidof an EOSINT S 550 Laser Sintering machine and a Spectrum 510 ThreeDimensional Printer. The results indicate that RP is the rate determining stepand cost driver of the proposed tooling manufacturing technique. In addition itwas found that this tool making process is faster but more expensive thanmachining and investment casting.Keywords: Tooling; Metal Casting; Rapid Prototyping; CNC Machining; LaserSintering; Three Dimensional Printing.1. INTRODUCTIONMetal casting processes that have been used for tool and die making includeshell casting and investment casting. A few applications of this process arementioned in literature such as the production of wax pattern from aluminiumdies for use with the investment casting (Paris) process and the manufacturing ofsteel cast dies for forging, drawing, extrusion, die casting and glass making bythe Shaw process (Beeley, 2001).The above processes generally suffer several limitations such as coarsemetallurgical microstructure, segregation, inferior tool quality in terms of surfacefinish, dimensional accuracy and the need for a pattern that is costly and timeconsuming to produce. These short comings reduce the durability and precisionof cast tools that are furthermore expensive and take long to deliver tocustomers. Hence the use of metal casting for tool making has traditionally beenrestricted in favour of CNC machining and rapid tooling processes.New technological developments applied to the field of metal casting haveallowed the alleviation of some of its limitations thus making it a feasible andcompetitive option for tool and die making.INTERIM53One innovation worth mentioning is the direct production of sand moulds andshells by using Rapid Prototyping (RP) processes therefore completelyeliminating the patternmaking requirement (Klocke et al, 2000; Ederer, 2005;Hahn et al, 2005; Lerner et al, 2005). A successful case study on the productionof an injection moulding tool by casting in an RP sand mould is reported inliterature (Chua, Leong & Lim, 2003).Current RP processes for the manufacturing of sand moulds are subdivided intwo categories namely Laser sintering (LS) and 3-Dimentional Printing (3DP). Inthe LS process a laser beam is used to selectively fuse pre-coated foundry sandparticles into a solid part that will become a component of the mould or shell. In 3-DP, the principle is similar to ink printing on a piece of paper from an ink-jetprinter. In this case in a layer-by-layer fashion, a printing head selectivelydeposits or “prints” binder fluid that fuses the powder particles together in desiredareas (Palm, 2002). RP technologies locally available include the Direct CroningLaser Sintering (DCLS) process (www.actTech.com) and the Z-Corpororationprocess (www.zcorp.com).The use of direct RP sand moulds and shells to manufacture tooling has not beenfully exploited. One possible reason is that commercial RP processes are stillrelatively new and costly. In addition there is apparently no availableinvestigation relating to the impact of these technologies on metallic tooling.Stringent requirements are demanded for tooling different from ordinary castingssuch as delivery time and price as well as quality. In particular time and cost areimportant parameters of any tool manufacturing process. Time should be asshort as possible in order to allow quick turnaround to market new products. Costneeds to be as low as possible so as to increase the profit margin of massproduction processes of finished goods that use the tools. Time and costdetermine the economic viability of a tool making processes (Altan, Blaine & Yen,2001).In this paper the time and cost parameters of producing metallic tooling in RPsand moulds are assessed by means of case studies. Experiments areconducted to determine the rate determining step and cost driver of this toolmanufacturing method. Time and cost results are also compared to the ones ofconventional tool making processes such as CNC machining and investmentcasting (Paris Process). As such the study intends to provide a preliminaryunderstanding on the competitiveness of the tool manufacturing at hand. Thequality of products in terms of dimensional accuracy as surface finished will beexamined in future publications.Casting experiments are carried out according to a five step process chainreferred to as Rapid Casting for Tooling (RCT). It is an adaptation of thetraditional casting production flowsheet shown in Figure 1 where thepatternmaking step is eliminated and replaced by the RP of a sand mould.INTERIM54RCT is further refined by the use of modern CAD modelling and castingstimulation computer programs during the design stage. RCT was specificallydeveloped in order to minimise the incidence of casting defects in the metallictool. It is described in detail in the next section on the methodology.Figure 1: Flow diagram of casting production (Beeley, 2001)2. METHODOLOGYThe methodology followed in this investigation was made up of the followingtasks :• Obtaining suitable case studies• Conducting casting trials using RCT• Recording manufacturing time and costDetails of each task are provided below.2.1 Case studiesThe case studies consisted of the replication of existing tools using the route ofcasting in RP sand moulds. It would then be possible to compare experimentalcost and time parameters with the actual ones. Case studies were selected ontechnical grounds including the size, the minimum wall thickness, the shapecomplexity and the material specifications of the tool (Table 1) that had to becompatible with the available RP machines and the gravity sand castingpossibilities. The shape complexity is calculated taking into account the volume,the surface area and number of cores required. Details of the mathematicalexpression and explanation can be found in literature (Ravi, 2005).Process selectionDesign and specificationDetermination of casting techniqueMoulding material preparation Pattern making Furnace charge preparationMoulding and core making Metal meltingCastingFittingHeat treatment and finishingInspection and testingBlack castingINTERIM55Two case studies were obtained from local foundries. The first case study dealswith the manufacturing of an aluminium die for investment casting wax patternsand the second case study was the production of a cast iron sand casting patternfor a Disamatic moulding machine.2.1.1 Wax pattern dieThis tool is used for the production of wax pattern for the investment casting of asteel automotive bracket (Appendix 1). The production die was manufactured byCNC machining in a local tool room. Its actual manufacturing time and costprovided by the foundry are respectively 40 hours and 14 000 Rands. Thecheapest local quotation obtained to produce the same tool by investmentcasting (Paris process) was 10 000 Rands in 120 hours.2.1.2 Sand Casting plateThe pattern is used for the production of sand moulds on a Disamatic mouldingmachine for the casting of a steel engineering bonnet (Appendix 2). Theproduction plate was manufactured by CNC machining and assembly at thefoundry. The actual manufacturing time is 80 hours at a cost of 20 000 Rand. Thecheapest local quotation to manufacture this plate by investment casting (Parisprocess) was 8 000 Rand in 240 hours.Table 1 Technical characteristics of tool used for the case studies2.2 RCT casting trialsThe tool manufacturing trials were carried out according to a process chainreferred to as Rapid Casting for Tooling (RCT) that comprises five sequentialsteps described below:• CAD modelling: Conversion of 2D design drawings of tools into 3Dmodels. The tool model is further modified to include filleting andcontractions. From the tool model, the 3D models of the sand mouldparts are designed and constitute the outcomes of this step.INTERIM56• Casting simulation: Dimensioning and positioning of the gating andfeeding system of the casting. The objectives of the casting simulationdepend on the case studies and include minimisation of oxidation,elimination of shrinkage defects and complete mould filling.• RP: Production of sand moulds by AM from CAD files generated in step1.• Metal casting: Melting of the tool alloy followed by pouring of the metal inthe mould produced in step 4.• Finishing operations: Cleaning of the casting by sand blasting followedby minimal machining.Local companies assisted in conducting the various steps of the castingexperiments. These companies were selected on the basis of the cheapest priceand shortest time to execute a task. The best proposal was in the case of CAD,casting and finishing operation selected from three quotations. For the castingsimulation step only one quote was obtained because the market is stillmonopolistic in South Africa dominated by Magmasoft. The Metal CastingTechnology Station at the University of Johannesburg and the Centre for RapidPrototyping and Manufacturing at the Central University of Technology, FreeState are the only two institutions offering RP technologies in 3DP and LSrespectively.In total three casting experiments were conducted. The first two produced thealuminium dies using two different RP technologies: 3DP and LS. The thirdexperiment produced the plate by casting in a LS RP mould. Additionalexperimental conditions are summarised in Table 2.INTERIM57Table 2 Technical characteristics of RCT steps during tooling manufacturingtrials2.3 Manufacturing time and costThe manufacturing time is the actual working time devoted to a specific step andrecorded during the execution. The manufacturing cost is the invoiced price aparticular step provided at its completion.3. RESULTSThe manufacturing time and cost results of RCT steps for the three experimentsare respectively shown in Table 3 and Table 4.INTERIM58Proportions in percentage of the total time and cost of the manufacturing timeand cost of each RCT step are shown in Figures 2 and 3. It can straight away beseen from these tables and Figures the importance of the RP step with regard toits contribution to the total time and cost.Table 3 Experimental manufacturing time resultsTable 4 Experimental manufacturing costFigure 2 Contribution of RCT steps to the final manufacturing timeINTERIM59Figure 3 Contribution of RCT steps to the final manufacturing cost4. DISCUSSION4.1 RCT manufacturing time and costThe growing of the sand mould by RP is the slowest step and therefore RCT ratedetermining step (Figure 2). This step contributed 54% to 73% of the total RCTtime respectively for the die manufacturing using the Spectrum 510 printer andthe sand casting plate using the EOSINT LS machine. In the case studiesconducted layer-by-layer manufacturing is slower possibly because of theprocess technical limitations.Furthermore in all cases the manufacturing of sand moulds by RP is the mostexpensive step therefore RCT cost driver. This step contributed 61% to 78% ofthe total RCT cost respectively for the die manufacturing using the Spectrum 510printer and the sand casting plate using the EOSINT LS machine. A possiblereason for the expensiveness of the RP step is the newness of theAM processesand the lack of competition with regards to providing RP services locally.4.2 Comparison of RCT with casting and machiningFigure 4 shows the comparison in terms of manufacturing time of RCT processchain versus machining and metal casting for the case studies conducted.INTERIM60RCT is the fastest process chain. A manufacturing time improvement of 68% to72% can be obtained compared to metal casting and 6% to 18% versusmachining. Casting was the slowest process chain because of the requiredmanufacture of a pattern prior to casting. On the other hand machining wasslower than RCT possibly because of the intricacy of the case study tools.Figure 5 shows the comparison in terms of manufacturing cost of RCT processchain versus machining and metal casting for the case studies conducted. It canbe seen that RCT is the most expensive compared to casting and machining.RCT was respectively 124% to 453% and 60% to 176% more expensivecompared to metal casting and machining. The possible reason for this finding isthe expensiveness of RP processes as mentioned. Manufacturing of tooling bymetal casting appeared to be extremely undervalued.Figure 4 Manufacturing time comparison between RCT and other toolmanufacturing processesINTERIM61Figure 5 Manufacturing cost comparison between RCT and other toolmanufacturing processes5. CONCLUSIONIn this investigation, three casting trials were carried out to understand themanufacturing time and cost of producing metallic tooling in RP sand moulds.Two main findings were obtained from these experiments using locally availabletechnologies:1) The manufacturing of sand moulds by RP was found to be the ratedetermining and cost driver step.2) The proposed tooling manufacturing was found to be faster thanmachining and metal casting (Paris process) but more expensive.In the present situation these results are important for future optimisation work tomake the manufacturing of tooling in sand moulds produced by RP processesfaster and cheaper than other tool manufacturing processes such as machining.This will be achieved by concentrating the effort to minimise the cost and time ofthe AM of sand mould step as it is the one that has been identified as stronglycontrolling the cost and delivery time of the final tool.INTERIM626. ACKNOWLEDGMENTSSpecial gratitude to:1) The Metal Casting Technology Station at the University ofJohannesburg2) The Centre for Rapid Prototyping and Manufacturing at the CentralUniversity of Technology, Free State3) The following foundries:a. Precision Casting Patternb. Guestroc. Ozzd. Relay Precision CastingsAppendicesAppendix 1: 2D drawing of steel bracketINTERIM63Appendix 2: 2D drawing of steel bonnetINTERIM647. REFERENCESAltan, T., Blaine, L. & Yen, Y.C. (2001). Manufacturing of dies and molds. CIRPAnnals- Manufacturing Technology, 50(2): 404-422.Beeley, P. (2001). Foundry Technology. 2nd ed. Oxford, Auckland, Boston,Johannesburg, Melbourne, New Delhi: Butterworth Heinemann.Chua, C.K., Leong, K.F. & Lim, C.S. (2003). Rapid Prototyping: Principle andApplications, 2nd ed. New Jersey, London, Singapore, Hong Kong: WorldScientific.Ederer, I. (2005). Patternless production of furan bonded sand in rapidprototyping, Casting SA, 6(4):35-37.Hahn, T., Demarzyk, N., Gantner, D. & Becker, T. (2005). Comparison betweenrapid moldmaking technologies.AFS Transactions 2005.Klocke, F., Wirtz, H. & Freyey, C. (2000). Selective laser sintering of zirconiumsilicate shells for investment casting. Production Engineering, VII(I):45 -48.Lerner, Y., Rao, P.N. & Kouznetsov, V. (2002). Rapid tooling applications in metalcasting, Foundry Management and Techonology,August: 47-55.Palm, W. (2002). Rapid Prototyping Primer: http://www.me.psu.edu/-lamancusa/rapidpro/primer/chapter2.htmRavi, B. (2005). Metal Casting Computer- Aided Design and Analysis, EasternEconomy Edition. New Delhi: Prentice-Hall of India.

Time and cost assessment of the manufacturing of tooling by metal casting in rapid prototyping sand moulds

http://ir.cut.ac.za/bitstream/handle/11462/335/%e2%80%a2Nyembwe%2c%20K..pdf?sequence=1&isAllowed=y

Time and cost assessment of the manufacturing of tooling by metal casting in rapid prototyping sand moulds

Abstract

Similar works

Full text

Available Versions

Central University Of Technology Free State - LibraryCUT, South Africa