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Development of rapid tooling for sheet metal drawing using nickel electroforming and stereolithography processes
Journal of Materials Processing Technology 111 (2001) 286±294
Development of rapid tooling for sheet metal drawing using nickel electroforming and stereolithography processes Prasad K.D.V. Yarlagaddaa,*, Ismet P. Ilyasa, Periklis Christodouloub a
School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology (QUT), 2, George Street, GPO Box 2434, Brisbane, Qld 4001, Australia b Queensland Manufacturing Institute (QMI), Cnr. Miles Platting and Logan Road, Eight Mile Plains, Brisbane, Qld 4113, Australia
Abstract Tooling is an important area in the manufacturing of various sheet metal products. This aspect can be extremely expensive as well as time consuming. The increase in the complexity of tooling for any operation results in a corresponding increase in the time and costs required in developing such tooling. The ideal candidate operations for rapid tooling (RT) have been those for which it is dif®cult to develop tooling by the usual methods. The quest is to produce complex tooling quickly and at low cost. This paper describes the development of RT techniques for the production of sheet metal drawing tooling by using a combination of stereolithograhy and nickel electroforming processes. Two types of prototype tools have been designed and manufactured. The ®rst type is a stereolithography QuickCast pattern in®ltrated with aluminium-®lled epoxy designated as QuickTool, and the second type has been manufactured by combining stereolithography technique with the nickel electroforming process. While the QuickTool may be indeed rapidly manufactured it can be only a prototype tool, as the material it is made of does not render much durability. On the other hand, the nickel electroformed tool is far more durable and can withstand more extreme working conditions. By combining nickel electroforming and the stereolithography process, press tools for sheet metal forming have been successfully produced. Further, the developed tools have been evaluated in the press metal forming process by producing components with 0.8 mm aluminium sheets. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Rapid tooling; Nickel electroforming; Stereolithography; Sheet metal drawing
1. Introduction and background The present day trend in industry is to use short production runs in the manufacture of products. As a result the product development cycle has to be shortened. This is where rapid prototyping (RP) comes to the aid of industry. In recent years, advancements in RP technologies have led to a considerable amount of research activities in the area of tooling for which the term rapid tooling (RT) was coined. RP techniques are employed to make prototype tools. The basic idea of the RT is to produce prototype parts by using prototype tools so that the parts truly represent the future production. RP is a process by which a tangible prototype of a product is made in a relatively short time span. This prototype can then be used in analysing the product's properties and making necessary modi®cations. Tooling is an important area in the manufacturing process. This aspect can be extremely expensive as well as time consuming. The
* Corresponding author. Tel.: 61-7-3864-2423; fax: 61-7-3864-1469. E-mail address: [email protected] (P.K.D.V. Yarlagadda).
increase in the complexity of tooling for any operation results in a corresponding increase in the time and costs required in developing such tooling. The ideal candidate operations for RT have been those for which it is dif®cult to develop tooling by the usual methods. RT has found applications in a wide variety of areas. RT techniques are of two types Ð soft tooling and hard tooling. Soft tooling, is associated with low costs, used for lowvolume production, and uses materials that have low hardness levels such as silicones, epoxies, low melting point alloys, etc. [1,2]. Aluminium-®lled epoxy resins can be cast into a polished release-coated RP model to manufacture injection moulding tools that can be used to produce lowvolume parts or prototypes [3]. Spray metal tooling is a very popular and common method for producing soft tooling. A thin metal shell (of about 2 mm) is sprayed onto a pattern. This shell is then backed with epoxy [4]. Metal spray moulds have been used successfully for low-pressure processes such as vacuum forming, rotational moulding and resin injection moulding (RIM). Plaster mould casting is used as a prototype manufacturing process for simulated die castings. A silicone rubber mould is made and then plaster is poured
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over it to obtain a cavity. This serves as a die casting tool. Spin casting is yet another area where RT has great potential. High temperature vulcanising (HTV) silicone rubber has been used in the process of mould making. The mould cavities are hand carved and then cured under high temperatures and pressures to form the ®nished rubbed moulds. Finally, ingates and vents are cut out and the moulds are mounted on the spin casting machine, ready for use. They offer greater resistance to heat and chemical action than room temperature vulcanising (RTV) silicone rubber [5]. Hard tooling, on the other hand, is associated with higher volume of production, and the use of materials of higher hardness such as hardened steel. Most hardened steel tools are manufactured by conventional metal cutting processes or by electric discharge machining (EDM). The ability to produce intricate details is common to RP and investment casting. Hence rapid prototypes can be used instead of wax, in the investment casting process (as `lost' patterns) [3]. Investment casting involves making a wax pattern, around which a ceramic slurry is coated to form a shell. Once the shell is hard enough, the wax is melted away and the ceramic shell is used to cast the required shape, by pouring molten metal into the shell. Since the wax is melted away, the process is also called the `lost wax process'. The replacement of precision machined wax with rapid prototypes, results in a saving in cost of tooling and the lead time [6]. EDM seems to be an interesting area in which RT ®nds a potential application. Early research work [9] has shown the possibility of using stereolithography patterns in the manufacture of EDM electrodes. Some methods of making EDM electrodes have been arrived at 3D systems, uses a powder compaction technique to produce a sintered copper electrode. A metal powder/binder mixture with a solvent is poured around a silicone RTV cast pattern which was in turn produced from an SL negative pattern. After evaporation of the solvent, the `green' compact was sintered to burn off the binder. The porous structure was then in®ltrated with copper. The tool showed good de®nition and surface ®nish. The only disadvantages are that the part undergoes shrinkage during sintering so that proper allowance has to be made and the process is limited to small tools. Metal spraying and electroplating of SL patterns is another area that is being researched upon. The purpose is to give the SL pattern a thin coating of copper, and using this coated pattern directly as an EDM electrode. The patterns may be repeatedly used with subsequent coatings of copper. 2. Current developments in RT From the beginning of 1990s, several researchers [1±17] have applied the RP technique in various ®elds of manufacturing. In this work, Jacobs [2] classi®ed the various types of RP systems as liquid polymerisation, fused deposition manufacturing, laminated object manufacturing, selective
287
laser sintering and point-to-point solidi®cation based on material creation and shape building techniques. Williams et al. [10] presented the results of their investigation into the effect of build styles (ACES, WEAVE and QuickCAST) and built parameters on the performance measures of dimensional accuracy, surface roughness and build time. They built all prototypes on SLA-250 by using SL-5170 photopolymer resin. The attempt of Thomas [14] to use SL tooling for FFR parts offers a signi®cant reduction in tooling requirements. By using RT techniques, he reduced the tools required to produce the FFR parts to one, whereas in the traditional manufacturing technique ®ve different tools were required to produce the same part. Dickens and Philip [1] studied the SL photo-polymerisation process in order to improve its ef®ciency, examining the behaviour of the STARWAVETM build style technique using acrylate-based resin from Zeneca. His results showed that the build parameters such as retraction, cure depth and staggered hatching are not consistent. According to the author, this could be due to the variation in machine, process, or material parameters such as laser power, re-coating mechanism, and resin parameter. Cobb and Reeves [12] studied the factors that are associated with poor surface ®nish in using SL process. A methodology for surface ®nishing using additive processes, abrasive ®nishing and chemical etching has been discussed. The epoxy primer (Jaxacote) has been shown to be an excellent coating for SL components, either as a base for other coatings or as a surface ®nishing. Finally, dual additive and abrasive ®nishing have signi®cantly reduced the surface deviation by up to 95%, with only minor changes to the part geometry. Murakami et al. [13] developed refrigerative stereolithography for rapid product development. In this work, they described the basic concept of refrigerative SL and a refrigerative SL machine produced by embedding of a newly developed unit for supplying and cooling the liquid resin into a conventional SL machine. Bocking et al. [11] discussed recent work carried out to implement the rapid production of injection mould cavities and spark erosion tooling using electroforming and electroplating technology. One of the main problems they presented in the production of the injection mould cavities was the electroforming. The distribution of the metal, particularly within the deep cavities, recesses, bores and blind holes was inadequate. From the above review, it is clear that many researchers have made attempts in different directions to use RP technology for the manufacturing of prototype models or of the products. Technologists and researchers involved in the development of RP processes are now devoting much of their effort to pursuing the philosophy of reduced lead times and development costs in manufacturing of production tooling. Attempts are being made by many researchers to manufacture production tooling for the injection moulding of plastic products, press tools for metal products and EDM tooling for the die sinking processes. Dover et al. [9] attempted to use the electro-deposition technique to directly
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produce metal tools. They used a copper sulphate electrolyte system to produce EDM electrodes and a nickel sulphate electrolyte system to produce press tools. In order to limit the deposition to the required area they used high-speed selective jet electro-deposition. This forces the electrolyte through a small nozzle. With the anode upstream from the nozzle, the electro-deposit is limited to a small area on the cathode at the end of the nozzle. The nozzle is then moved in a vector path in a similar way to an SLA laser. In most of the manufacturing industries it is generally considered that the cost of traditional tooling (either prototype tooling or production tooling) has been a major constraint in the launching of a new product. It is also recognised that the long lead times encountered during the tool-making stage can seriously delay the product launch and so erode the market lead. However, with the development of the RP systems, master tools can be generated in a matter of hours. Depending on the manufacturing process, material, component geometry, size and the number of components required, the tool techniques used in tool production can be subdivided into resin tooling, metal faced tooling, investment casting and EDM electrode manufacturing. 3. Steps in the production of tooling for sheet metal forming In this section, the procedure used to manufacture the tools starting from conceptual design stage to the tool's fabrication stage is presented. The various stages involved in the tool production and evaluation stage are shown in Fig. 1 in the form of manufacturing ¯ow diagram. From Fig. 1, it is evident that there are four distinct activities namely: concept development, virtual prototyping, physical prototyping
and evaluation of the tool by making prototype parts; involved in the production of tooling for sheet metal forming applications. 3.1. 3D CAD solid modelling The 3D solid models of the tools were designed using IDEAS Master SeriesTM software from SDRC at the Queensland Manufacturing Institute (QMI). First the 3D solid model of the part is created, then both the punch and die are developed from the solid model of the part. In this process the solid model of the part is used to develop a master (negative) pattern that is used to manufacture the punch and die model through the nickel electroforming process. While developing the 3D CAD solid models and preparing STL ®les, two aspects need to be considered. First, the resolution of curved surfaces should be good enough to reproduce the part accurately. The CAD package estimates curved surfaces with polygons (usually triangles and rectangles). The number of polygons used affects the smoothness of the resultant SL model. I-DEAS CAD software allows the number of polygons to be set by setting `iso-line density' and a `facet deviation' for the CAD solid models. In the case of the solid model for the master pattern, the iso-line density was set to 0 and the facet deviation to 0.005: these settings produced 10,500 polygons. The iso-line density was set to 4 and the facet deviation to 0.005 in the case of QuickCAST: these produced 97,000 and 95,000 polygons for punch and die, respectively. The second consideration is that the part should be scaled to ®t the working space of the SLA. Based on the size of both tool types, the SLA 500 machine was selected to build the model. The stepwise procedure used in the development of electroformed nickel-epoxy resin (ENER) tool is shown in Fig. 2.
Fig. 1. Stages involved in the production of tooling by the RP process.
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Fig. 2. Procedure for the production of the ENER tool.
4. Considerations and modi®cations of the part The `Left-hand guard' of the lawn mower was the part selected as the trial part to evaluate the prototype tools and is shown in Fig. 3. From the stereolithography process limitations point of view, several modi®cations have been made by redesigning some features on the original part by excluding cutting features such as trimming, piercing, and blanking. From the geometry of the part it can be understood that the part can be produced by stretch forming and bending, hence the arrangement of the punch, die and blank holder is similar to that of drawing operations. In both cases, QuickTool and nickel electroformed tool, the punch and the die were
designed as inserts that ®t into the die set. A nickel shell is deposited on a mandrel that has the reverse shape of the punch and die. To avoid any mismatch the mandrel for the punch and the die is formed from the same model by offsetting them by the required thickness. The die and punch prototypes produced by the negative `offset' model are shown in Fig. 4. To ensure that the punch and die are aligned two pins were added to the model. In this case the tools are generated directly as the punch and die from solid models, as shown in Fig. 5. Maestro is software that guides through the preparation stages that are required to build the stereolithography pattern using SLA. The solid model of the tools was converted into STL format and then sliced into individual
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Fig. 3. A Left-hand guard part of a lawn mower.
Fig. 4. A negative `offset' model of the Left-hand guard.
layers. A compromise between accuracy and the build time often needs to be achieved. Also, when the layers are too thin they tend to warp, adding to inaccuracy in the z direction. Taking the above issues into account the thickness of each layer was set to 0.15 mm. Any overhangs of the SL patterns need to be supported in the build process. This is done using the Maestro package that pinpoints any area of the overhanging features of the part and incorporates a support structure underneath them. The support structure is then removed before the ®nal curing.
5. Design considerations for building geometry of the tooling elements Depending on the application, the build scheme of the SL model is determined. In general, the greater the percentage of resin which is cured in the initial SLA build, the less the overall shrinkage and warpage that will occur in the postcure process. SL models for the nickel electroformed tool were built using the accurate clear epoxy solid (ACES) scheme, which assures a very smooth surface ®nish and
Fig. 5. Direct SL (QuickCAST) model for the Left-hand guard.
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Fig. 6. The proposed tool layout for production of the Left-hand guard.
warrants high accuracy. The model for the QuickTool has been manufactured using QuickCast style. The internal structure is similar to a honeycomb while the external surfaces are built in the form of a tough thin shell made of three skin layers. The QuickCAST model was then ®lled with aluminium-®lled epoxy, in order to increase the strength of the tooling elements. 5.1. Production of the nickel electroformed tools The tool consist of an electroformed nickel shell as the hard part of the forming tools (punch and die); a highstrength epoxy resin as a baking (support) material; and standard press die set components of steels for containment. The proposed tool layout for the production of the required part is shown in Fig. 6. The nickel shell provides mechanical integrity at the highly stressed area of the tool. The highstrength epoxy resin is an easily cast material which can ®ll the shell to provide a backing and uniform support to the shell. Both the punch and the dies are contained and aligned by the steel frame, forming a very rigid structure. The backing material effectively absorbs compressive load and transfers it away from the nickel shell. Ciba Geigy SL 5170 photopolymer epoxy resin has been used for prototype development of the tool. No data on the compressive strength for this material could be found, hence appropriate tests had been conducted. Test specimens were build using two build styles Ð QuickCast and ACES. The QuickCast specimens were in®ltrated with aluminium-®lled epoxy thus representing the properties of the QuickTool material. The ASTM D3410M Ð 95 test procedures have been followed. The ultimate compressive strength of SL 5170 was found to be 4.98 MPa and for the QuickTool material 27.64 MPa. Since the SL material distorts when exposed to water the plastic vacuum casting intermediate process is used to make the mandrel in the material of choice. First SL pattern of the reverse tool was produced, which in turn was employed to make a silicone tool. The silicon tools produced by RTV casting are shown in Fig. 7. In this process, liquid silicone
slurry is poured over the SL pattern. The slurry vulcanises at room temperature and the process is called RTV. The vulcanised silicone has a rubber-like properties and is translucent. Then the silicone rubber tool is cut through to remove the SL pattern. This silicone rubber mould can be used to make several parts. The silicone mould very faithfully reproduces all surface features of the SL pattern, even the ®ngerprints of the operator are re¯ected on the mould's surface and subsequently on the surface of the part. Since the silicone mandrel is not very rigid and could potentially be distorted in the electroforming process, the thickness (offset) of the mandrel was quite substantial. Additionally, two steel rods have been embedded into the body of the mandrel to increase its rigidity. Then using the locating pins, steel frames were attached to both sides of the mandrel prior to the electroforming process. These frames were used to ®x these components to the die set and to assure and maintain the alignment of both punch and die in the assembly. The sequence of operations followed in the production of tooling from part geometry to the ®nal stage of die assembly are shown in Fig. 8. 5.2. Production of direct tooling by using QuickCAST process In the case of the direct tooling, the QuickCAST technique was used to produce the punch and die for the press metal forming. The SL model is developed by implementing a ``QuickCAST'' (QC) building style. It looks like a honeycomb semi-hollow part with a tough thin shell around it. In developing the QC tool, an important consideration is made regarding model quality: liquid resin inside the semihollow must be drained out in order to overcome model deformation during post-curing. After it has been postcured, the QC model then was back-®lled with aluminium-®lled epoxy, under a vacuum, through the holes. In this stage, one important consideration is to get rid of any bubbles that may occur inside the tool because these may effect the tools in the forming process. The resultant models
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Fig. 7. Production of the silicon tooling by RTV casting.
Fig. 8. Stages involved in the production of nickel electroformed tooling.
Fig. 9. Production of direct tooling by the QuickCAST process.
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were used as inserts (punch and die) of the PMF tool to make the aluminium part. This process of producing direct tooling is illustrated in Fig. 9. The model produced by this process is to have a tendency of distorting after post-curing. The deforming features of the distorted model are machined in order to bring back the original geometry and dimensional accuracy of the model. 6. Results and discussion The ®rst tool type, the electroformed nickel-epoxy resin (ENER) tool, utilises an electroformed nickel shell, a highstrength epoxy resin, and standard press die components. In producing this tool, an indirect RP&T technique was employed due to involvement of a secondary step of using an RTV rubber mould to cast the silicon tool. This step becomes essential because the material of the stereolithography model was not recommended for the electroforming process. Therefore, a silicon tool was employed as a mandrel in producing an electroformed nickel shell. To back up this nickel shell aluminium-®lled epoxy was selected: this is a mixture of 60% aluminium and 40% plastic (polymer resin). Based on the trial assessment, it was realised that the performance of this tool in the forming of 0.5 and 0.8 mm aluminium sheet is reasonably good. The tool was able to form and produce a number of sheet metal components. However, the limitation in PMF die design caused destruction on the tool. The tool was scratched by a wrinkled blank that was forced to ¯ow into a die cavity. The wrinkle blank could not be avoided in this operation because the PMF die was operated without a blank holder. The elimination of this blank holder is required due to the development of the tool and its assembly process. The second direct SL (QuickCast) tool was developed by utilizing a direct RP&T technique. Unlike the ®rst tool type, there was no secondary step involved. The tool utilizes a direct SL model, built in QuickCast building style, and back®lled with the aluminium-®lled epoxy used in the ®rst tool type. The product quality is much better than the products produced by the ®rst tool type because the PMF die for this QuickCast tool was designed and operated with blank holder. However, there was still limitation in PMF die design, especially in spring construction and calculation. Since the shape of the part was considered irregular, the calculation of the spring forces used for holding the blank was not inaccurate. The inaccuracy in the calculation lead to the selection of springs which were not strong enough to hold and to ¯atten the wrinkled blank at the deeper side. Therefore, the spring forces applied on this side of the part should be constructed stronger than for the other sides. Examination of the ®nal products revealed that the product quality needs to be improved. As was mentioned before, the conceptual design of the PMF die is the main limitation in producing a better product quality with the ®rst
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type of the tool. However, tooling parameters such as punch radius, die ring radius, and punch±die clearance also become other limitations that have also to be taken into consideration. Furthermore, the part geometry and shape also create a dif®culty in determining the tooling parameters accurately. Nevertheless, these limitations also affected the quality of the products produced from the second tool type (QuickCast tool). This is because the 3D CAD solid model implemented for building both tool types was similar. It was evaluated that both tool types performance shows an impressive result in terms of tooling development. In terms of the tool material, both tool types seem to become potential tools in the sheet metal forming process. However, the second tool type is recommendable than the ®rst tool type. In terms of tool development and production, the QuickCast tool is more economical. The process involved in this tool development and production is more simple and shorter than for the ®rst tool. The QuickCast tool can be developed and built in SLA machine directly from a 3D CAD model, while the ENER tool requires as secondary steps other processes such as nickel electroforming. 7. Conclusions Using RP, time and cost savings can be achieved in the production of tooling of sheet metal forming applications. It can be realised that the SL parts could be used as low scale production parts, signi®cantly cutting time by byepassing the detailed process of die design and die making. The results and limitations that were found in this research brought other potential aspects that need not be addressed. The aspects that need to be addressed in future study are as follows. The ®rst aspect is in relation to the product quality improvement and the veri®cation of tool and die design, including the improvement in drawing parameters and operation. The second aspect concerns variables involved in evaluating these tools so that they can be utilized for steel sheet instead of aluminium. The outcome of this present research forms a sound basis for ongoing further development that will involve a larger variety of RP&T technologies and techniques for tooling development. References [1] P. Dickens, M. Philip, Rapid News 4 (5) (1996) 54±60. [2] P. Jacobs, Stereolithography and other rapid prototyping and manufacturing technologies, Society of Manufacturing Engineers, Dearborn, MI, 1996. [3] G. Tromans, D.I. Wimpenny, Rapid News 3 (3) (1995) 40±46. [4] R. Flint, D. Ellis, in: Proceedings of the SME Rapid Prototyping and Manufacturing Conference, Society of Manufacturing Engineers, Dearborn, MI, 1994. [5] A. Seybert, Rapid News 4 (5) (1996) 62±65. [6] B. Sarkis, Rapid News 4 (3) (1996) 48±52. [7] A. Arthur, P. Dickens, C. Cobb, Rapid Prototyping J. 2 (1) (1996) 4±12.
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[8] P.K.D.V. Yarlagadda, P. Christodoulou, V. Subramanian, J. Mater. Process. Technol. 89±90 (1999) 231±237. [9] S.J. Dover, C.E. Bocking, G. Bennett, in: G. Bennett (Ed.), Proceedings of First National Conference on Rapid Prototyping and Tooling Research, Mechanical Engineering Publications Limited, 1995, pp. 157±173. [10] R.E. Williams, S.N. Komaragiri, V.L. Melton, R.R. Bishu, J. Mater. Process. Technol. 61 (1996) 173±178. [11] C. Bocking, G. Bennet, S. Dover, A. Arthur, C. Cobb, P. Dickens, GEC J. Technol. 14 (1997) 66±74.
[12] C. Cobb, P. Reeves, in: Proceeding of the First National Conference on Rapid Prototyping and Tooling Research, UK, 1995, pp. 111±127. [13] T. Murakami, A. Kaminura, N. Nakajima, Rapid Product Dev. 3 (1997) 74±82. [14] S. Thomas, Mech. Eng. 114 (1992) 62±66. [15] T. Himmer, T. Nakagawa, M. Mohri, in: Proceedings of the Seventh European Conference on RPM, 1998, pp. 315±326. [16] B. Fritz, in: Proceedings of the Sixth European Conference on RPM, 1997, pp. 210-229. [17] E. Finkenstein, M. Kleiner, Ann. CIRP 35 (1991) 311±314.
Development of rapid tooling for sheet metal drawing using nickel electroforming and stereolithography processes Prasad K.D.V. Yarlagaddaa,*, Ismet P. Ilyasa, Periklis Christodouloub a
School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology (QUT), 2, George Street, GPO Box 2434, Brisbane, Qld 4001, Australia b Queensland Manufacturing Institute (QMI), Cnr. Miles Platting and Logan Road, Eight Mile Plains, Brisbane, Qld 4113, Australia
Abstract Tooling is an important area in the manufacturing of various sheet metal products. This aspect can be extremely expensive as well as time consuming. The increase in the complexity of tooling for any operation results in a corresponding increase in the time and costs required in developing such tooling. The ideal candidate operations for rapid tooling (RT) have been those for which it is dif®cult to develop tooling by the usual methods. The quest is to produce complex tooling quickly and at low cost. This paper describes the development of RT techniques for the production of sheet metal drawing tooling by using a combination of stereolithograhy and nickel electroforming processes. Two types of prototype tools have been designed and manufactured. The ®rst type is a stereolithography QuickCast pattern in®ltrated with aluminium-®lled epoxy designated as QuickTool, and the second type has been manufactured by combining stereolithography technique with the nickel electroforming process. While the QuickTool may be indeed rapidly manufactured it can be only a prototype tool, as the material it is made of does not render much durability. On the other hand, the nickel electroformed tool is far more durable and can withstand more extreme working conditions. By combining nickel electroforming and the stereolithography process, press tools for sheet metal forming have been successfully produced. Further, the developed tools have been evaluated in the press metal forming process by producing components with 0.8 mm aluminium sheets. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Rapid tooling; Nickel electroforming; Stereolithography; Sheet metal drawing
1. Introduction and background The present day trend in industry is to use short production runs in the manufacture of products. As a result the product development cycle has to be shortened. This is where rapid prototyping (RP) comes to the aid of industry. In recent years, advancements in RP technologies have led to a considerable amount of research activities in the area of tooling for which the term rapid tooling (RT) was coined. RP techniques are employed to make prototype tools. The basic idea of the RT is to produce prototype parts by using prototype tools so that the parts truly represent the future production. RP is a process by which a tangible prototype of a product is made in a relatively short time span. This prototype can then be used in analysing the product's properties and making necessary modi®cations. Tooling is an important area in the manufacturing process. This aspect can be extremely expensive as well as time consuming. The
* Corresponding author. Tel.: 61-7-3864-2423; fax: 61-7-3864-1469. E-mail address: [email protected] (P.K.D.V. Yarlagadda).
increase in the complexity of tooling for any operation results in a corresponding increase in the time and costs required in developing such tooling. The ideal candidate operations for RT have been those for which it is dif®cult to develop tooling by the usual methods. RT has found applications in a wide variety of areas. RT techniques are of two types Ð soft tooling and hard tooling. Soft tooling, is associated with low costs, used for lowvolume production, and uses materials that have low hardness levels such as silicones, epoxies, low melting point alloys, etc. [1,2]. Aluminium-®lled epoxy resins can be cast into a polished release-coated RP model to manufacture injection moulding tools that can be used to produce lowvolume parts or prototypes [3]. Spray metal tooling is a very popular and common method for producing soft tooling. A thin metal shell (of about 2 mm) is sprayed onto a pattern. This shell is then backed with epoxy [4]. Metal spray moulds have been used successfully for low-pressure processes such as vacuum forming, rotational moulding and resin injection moulding (RIM). Plaster mould casting is used as a prototype manufacturing process for simulated die castings. A silicone rubber mould is made and then plaster is poured
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 5 4 2 - 8
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over it to obtain a cavity. This serves as a die casting tool. Spin casting is yet another area where RT has great potential. High temperature vulcanising (HTV) silicone rubber has been used in the process of mould making. The mould cavities are hand carved and then cured under high temperatures and pressures to form the ®nished rubbed moulds. Finally, ingates and vents are cut out and the moulds are mounted on the spin casting machine, ready for use. They offer greater resistance to heat and chemical action than room temperature vulcanising (RTV) silicone rubber [5]. Hard tooling, on the other hand, is associated with higher volume of production, and the use of materials of higher hardness such as hardened steel. Most hardened steel tools are manufactured by conventional metal cutting processes or by electric discharge machining (EDM). The ability to produce intricate details is common to RP and investment casting. Hence rapid prototypes can be used instead of wax, in the investment casting process (as `lost' patterns) [3]. Investment casting involves making a wax pattern, around which a ceramic slurry is coated to form a shell. Once the shell is hard enough, the wax is melted away and the ceramic shell is used to cast the required shape, by pouring molten metal into the shell. Since the wax is melted away, the process is also called the `lost wax process'. The replacement of precision machined wax with rapid prototypes, results in a saving in cost of tooling and the lead time [6]. EDM seems to be an interesting area in which RT ®nds a potential application. Early research work [9] has shown the possibility of using stereolithography patterns in the manufacture of EDM electrodes. Some methods of making EDM electrodes have been arrived at 3D systems, uses a powder compaction technique to produce a sintered copper electrode. A metal powder/binder mixture with a solvent is poured around a silicone RTV cast pattern which was in turn produced from an SL negative pattern. After evaporation of the solvent, the `green' compact was sintered to burn off the binder. The porous structure was then in®ltrated with copper. The tool showed good de®nition and surface ®nish. The only disadvantages are that the part undergoes shrinkage during sintering so that proper allowance has to be made and the process is limited to small tools. Metal spraying and electroplating of SL patterns is another area that is being researched upon. The purpose is to give the SL pattern a thin coating of copper, and using this coated pattern directly as an EDM electrode. The patterns may be repeatedly used with subsequent coatings of copper. 2. Current developments in RT From the beginning of 1990s, several researchers [1±17] have applied the RP technique in various ®elds of manufacturing. In this work, Jacobs [2] classi®ed the various types of RP systems as liquid polymerisation, fused deposition manufacturing, laminated object manufacturing, selective
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laser sintering and point-to-point solidi®cation based on material creation and shape building techniques. Williams et al. [10] presented the results of their investigation into the effect of build styles (ACES, WEAVE and QuickCAST) and built parameters on the performance measures of dimensional accuracy, surface roughness and build time. They built all prototypes on SLA-250 by using SL-5170 photopolymer resin. The attempt of Thomas [14] to use SL tooling for FFR parts offers a signi®cant reduction in tooling requirements. By using RT techniques, he reduced the tools required to produce the FFR parts to one, whereas in the traditional manufacturing technique ®ve different tools were required to produce the same part. Dickens and Philip [1] studied the SL photo-polymerisation process in order to improve its ef®ciency, examining the behaviour of the STARWAVETM build style technique using acrylate-based resin from Zeneca. His results showed that the build parameters such as retraction, cure depth and staggered hatching are not consistent. According to the author, this could be due to the variation in machine, process, or material parameters such as laser power, re-coating mechanism, and resin parameter. Cobb and Reeves [12] studied the factors that are associated with poor surface ®nish in using SL process. A methodology for surface ®nishing using additive processes, abrasive ®nishing and chemical etching has been discussed. The epoxy primer (Jaxacote) has been shown to be an excellent coating for SL components, either as a base for other coatings or as a surface ®nishing. Finally, dual additive and abrasive ®nishing have signi®cantly reduced the surface deviation by up to 95%, with only minor changes to the part geometry. Murakami et al. [13] developed refrigerative stereolithography for rapid product development. In this work, they described the basic concept of refrigerative SL and a refrigerative SL machine produced by embedding of a newly developed unit for supplying and cooling the liquid resin into a conventional SL machine. Bocking et al. [11] discussed recent work carried out to implement the rapid production of injection mould cavities and spark erosion tooling using electroforming and electroplating technology. One of the main problems they presented in the production of the injection mould cavities was the electroforming. The distribution of the metal, particularly within the deep cavities, recesses, bores and blind holes was inadequate. From the above review, it is clear that many researchers have made attempts in different directions to use RP technology for the manufacturing of prototype models or of the products. Technologists and researchers involved in the development of RP processes are now devoting much of their effort to pursuing the philosophy of reduced lead times and development costs in manufacturing of production tooling. Attempts are being made by many researchers to manufacture production tooling for the injection moulding of plastic products, press tools for metal products and EDM tooling for the die sinking processes. Dover et al. [9] attempted to use the electro-deposition technique to directly
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produce metal tools. They used a copper sulphate electrolyte system to produce EDM electrodes and a nickel sulphate electrolyte system to produce press tools. In order to limit the deposition to the required area they used high-speed selective jet electro-deposition. This forces the electrolyte through a small nozzle. With the anode upstream from the nozzle, the electro-deposit is limited to a small area on the cathode at the end of the nozzle. The nozzle is then moved in a vector path in a similar way to an SLA laser. In most of the manufacturing industries it is generally considered that the cost of traditional tooling (either prototype tooling or production tooling) has been a major constraint in the launching of a new product. It is also recognised that the long lead times encountered during the tool-making stage can seriously delay the product launch and so erode the market lead. However, with the development of the RP systems, master tools can be generated in a matter of hours. Depending on the manufacturing process, material, component geometry, size and the number of components required, the tool techniques used in tool production can be subdivided into resin tooling, metal faced tooling, investment casting and EDM electrode manufacturing. 3. Steps in the production of tooling for sheet metal forming In this section, the procedure used to manufacture the tools starting from conceptual design stage to the tool's fabrication stage is presented. The various stages involved in the tool production and evaluation stage are shown in Fig. 1 in the form of manufacturing ¯ow diagram. From Fig. 1, it is evident that there are four distinct activities namely: concept development, virtual prototyping, physical prototyping
and evaluation of the tool by making prototype parts; involved in the production of tooling for sheet metal forming applications. 3.1. 3D CAD solid modelling The 3D solid models of the tools were designed using IDEAS Master SeriesTM software from SDRC at the Queensland Manufacturing Institute (QMI). First the 3D solid model of the part is created, then both the punch and die are developed from the solid model of the part. In this process the solid model of the part is used to develop a master (negative) pattern that is used to manufacture the punch and die model through the nickel electroforming process. While developing the 3D CAD solid models and preparing STL ®les, two aspects need to be considered. First, the resolution of curved surfaces should be good enough to reproduce the part accurately. The CAD package estimates curved surfaces with polygons (usually triangles and rectangles). The number of polygons used affects the smoothness of the resultant SL model. I-DEAS CAD software allows the number of polygons to be set by setting `iso-line density' and a `facet deviation' for the CAD solid models. In the case of the solid model for the master pattern, the iso-line density was set to 0 and the facet deviation to 0.005: these settings produced 10,500 polygons. The iso-line density was set to 4 and the facet deviation to 0.005 in the case of QuickCAST: these produced 97,000 and 95,000 polygons for punch and die, respectively. The second consideration is that the part should be scaled to ®t the working space of the SLA. Based on the size of both tool types, the SLA 500 machine was selected to build the model. The stepwise procedure used in the development of electroformed nickel-epoxy resin (ENER) tool is shown in Fig. 2.
Fig. 1. Stages involved in the production of tooling by the RP process.
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Fig. 2. Procedure for the production of the ENER tool.
4. Considerations and modi®cations of the part The `Left-hand guard' of the lawn mower was the part selected as the trial part to evaluate the prototype tools and is shown in Fig. 3. From the stereolithography process limitations point of view, several modi®cations have been made by redesigning some features on the original part by excluding cutting features such as trimming, piercing, and blanking. From the geometry of the part it can be understood that the part can be produced by stretch forming and bending, hence the arrangement of the punch, die and blank holder is similar to that of drawing operations. In both cases, QuickTool and nickel electroformed tool, the punch and the die were
designed as inserts that ®t into the die set. A nickel shell is deposited on a mandrel that has the reverse shape of the punch and die. To avoid any mismatch the mandrel for the punch and the die is formed from the same model by offsetting them by the required thickness. The die and punch prototypes produced by the negative `offset' model are shown in Fig. 4. To ensure that the punch and die are aligned two pins were added to the model. In this case the tools are generated directly as the punch and die from solid models, as shown in Fig. 5. Maestro is software that guides through the preparation stages that are required to build the stereolithography pattern using SLA. The solid model of the tools was converted into STL format and then sliced into individual
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Fig. 3. A Left-hand guard part of a lawn mower.
Fig. 4. A negative `offset' model of the Left-hand guard.
layers. A compromise between accuracy and the build time often needs to be achieved. Also, when the layers are too thin they tend to warp, adding to inaccuracy in the z direction. Taking the above issues into account the thickness of each layer was set to 0.15 mm. Any overhangs of the SL patterns need to be supported in the build process. This is done using the Maestro package that pinpoints any area of the overhanging features of the part and incorporates a support structure underneath them. The support structure is then removed before the ®nal curing.
5. Design considerations for building geometry of the tooling elements Depending on the application, the build scheme of the SL model is determined. In general, the greater the percentage of resin which is cured in the initial SLA build, the less the overall shrinkage and warpage that will occur in the postcure process. SL models for the nickel electroformed tool were built using the accurate clear epoxy solid (ACES) scheme, which assures a very smooth surface ®nish and
Fig. 5. Direct SL (QuickCAST) model for the Left-hand guard.
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Fig. 6. The proposed tool layout for production of the Left-hand guard.
warrants high accuracy. The model for the QuickTool has been manufactured using QuickCast style. The internal structure is similar to a honeycomb while the external surfaces are built in the form of a tough thin shell made of three skin layers. The QuickCAST model was then ®lled with aluminium-®lled epoxy, in order to increase the strength of the tooling elements. 5.1. Production of the nickel electroformed tools The tool consist of an electroformed nickel shell as the hard part of the forming tools (punch and die); a highstrength epoxy resin as a baking (support) material; and standard press die set components of steels for containment. The proposed tool layout for the production of the required part is shown in Fig. 6. The nickel shell provides mechanical integrity at the highly stressed area of the tool. The highstrength epoxy resin is an easily cast material which can ®ll the shell to provide a backing and uniform support to the shell. Both the punch and the dies are contained and aligned by the steel frame, forming a very rigid structure. The backing material effectively absorbs compressive load and transfers it away from the nickel shell. Ciba Geigy SL 5170 photopolymer epoxy resin has been used for prototype development of the tool. No data on the compressive strength for this material could be found, hence appropriate tests had been conducted. Test specimens were build using two build styles Ð QuickCast and ACES. The QuickCast specimens were in®ltrated with aluminium-®lled epoxy thus representing the properties of the QuickTool material. The ASTM D3410M Ð 95 test procedures have been followed. The ultimate compressive strength of SL 5170 was found to be 4.98 MPa and for the QuickTool material 27.64 MPa. Since the SL material distorts when exposed to water the plastic vacuum casting intermediate process is used to make the mandrel in the material of choice. First SL pattern of the reverse tool was produced, which in turn was employed to make a silicone tool. The silicon tools produced by RTV casting are shown in Fig. 7. In this process, liquid silicone
slurry is poured over the SL pattern. The slurry vulcanises at room temperature and the process is called RTV. The vulcanised silicone has a rubber-like properties and is translucent. Then the silicone rubber tool is cut through to remove the SL pattern. This silicone rubber mould can be used to make several parts. The silicone mould very faithfully reproduces all surface features of the SL pattern, even the ®ngerprints of the operator are re¯ected on the mould's surface and subsequently on the surface of the part. Since the silicone mandrel is not very rigid and could potentially be distorted in the electroforming process, the thickness (offset) of the mandrel was quite substantial. Additionally, two steel rods have been embedded into the body of the mandrel to increase its rigidity. Then using the locating pins, steel frames were attached to both sides of the mandrel prior to the electroforming process. These frames were used to ®x these components to the die set and to assure and maintain the alignment of both punch and die in the assembly. The sequence of operations followed in the production of tooling from part geometry to the ®nal stage of die assembly are shown in Fig. 8. 5.2. Production of direct tooling by using QuickCAST process In the case of the direct tooling, the QuickCAST technique was used to produce the punch and die for the press metal forming. The SL model is developed by implementing a ``QuickCAST'' (QC) building style. It looks like a honeycomb semi-hollow part with a tough thin shell around it. In developing the QC tool, an important consideration is made regarding model quality: liquid resin inside the semihollow must be drained out in order to overcome model deformation during post-curing. After it has been postcured, the QC model then was back-®lled with aluminium-®lled epoxy, under a vacuum, through the holes. In this stage, one important consideration is to get rid of any bubbles that may occur inside the tool because these may effect the tools in the forming process. The resultant models
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Fig. 7. Production of the silicon tooling by RTV casting.
Fig. 8. Stages involved in the production of nickel electroformed tooling.
Fig. 9. Production of direct tooling by the QuickCAST process.
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were used as inserts (punch and die) of the PMF tool to make the aluminium part. This process of producing direct tooling is illustrated in Fig. 9. The model produced by this process is to have a tendency of distorting after post-curing. The deforming features of the distorted model are machined in order to bring back the original geometry and dimensional accuracy of the model. 6. Results and discussion The ®rst tool type, the electroformed nickel-epoxy resin (ENER) tool, utilises an electroformed nickel shell, a highstrength epoxy resin, and standard press die components. In producing this tool, an indirect RP&T technique was employed due to involvement of a secondary step of using an RTV rubber mould to cast the silicon tool. This step becomes essential because the material of the stereolithography model was not recommended for the electroforming process. Therefore, a silicon tool was employed as a mandrel in producing an electroformed nickel shell. To back up this nickel shell aluminium-®lled epoxy was selected: this is a mixture of 60% aluminium and 40% plastic (polymer resin). Based on the trial assessment, it was realised that the performance of this tool in the forming of 0.5 and 0.8 mm aluminium sheet is reasonably good. The tool was able to form and produce a number of sheet metal components. However, the limitation in PMF die design caused destruction on the tool. The tool was scratched by a wrinkled blank that was forced to ¯ow into a die cavity. The wrinkle blank could not be avoided in this operation because the PMF die was operated without a blank holder. The elimination of this blank holder is required due to the development of the tool and its assembly process. The second direct SL (QuickCast) tool was developed by utilizing a direct RP&T technique. Unlike the ®rst tool type, there was no secondary step involved. The tool utilizes a direct SL model, built in QuickCast building style, and back®lled with the aluminium-®lled epoxy used in the ®rst tool type. The product quality is much better than the products produced by the ®rst tool type because the PMF die for this QuickCast tool was designed and operated with blank holder. However, there was still limitation in PMF die design, especially in spring construction and calculation. Since the shape of the part was considered irregular, the calculation of the spring forces used for holding the blank was not inaccurate. The inaccuracy in the calculation lead to the selection of springs which were not strong enough to hold and to ¯atten the wrinkled blank at the deeper side. Therefore, the spring forces applied on this side of the part should be constructed stronger than for the other sides. Examination of the ®nal products revealed that the product quality needs to be improved. As was mentioned before, the conceptual design of the PMF die is the main limitation in producing a better product quality with the ®rst
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type of the tool. However, tooling parameters such as punch radius, die ring radius, and punch±die clearance also become other limitations that have also to be taken into consideration. Furthermore, the part geometry and shape also create a dif®culty in determining the tooling parameters accurately. Nevertheless, these limitations also affected the quality of the products produced from the second tool type (QuickCast tool). This is because the 3D CAD solid model implemented for building both tool types was similar. It was evaluated that both tool types performance shows an impressive result in terms of tooling development. In terms of the tool material, both tool types seem to become potential tools in the sheet metal forming process. However, the second tool type is recommendable than the ®rst tool type. In terms of tool development and production, the QuickCast tool is more economical. The process involved in this tool development and production is more simple and shorter than for the ®rst tool. The QuickCast tool can be developed and built in SLA machine directly from a 3D CAD model, while the ENER tool requires as secondary steps other processes such as nickel electroforming. 7. Conclusions Using RP, time and cost savings can be achieved in the production of tooling of sheet metal forming applications. It can be realised that the SL parts could be used as low scale production parts, signi®cantly cutting time by byepassing the detailed process of die design and die making. The results and limitations that were found in this research brought other potential aspects that need not be addressed. The aspects that need to be addressed in future study are as follows. The ®rst aspect is in relation to the product quality improvement and the veri®cation of tool and die design, including the improvement in drawing parameters and operation. The second aspect concerns variables involved in evaluating these tools so that they can be utilized for steel sheet instead of aluminium. The outcome of this present research forms a sound basis for ongoing further development that will involve a larger variety of RP&T technologies and techniques for tooling development. References [1] P. Dickens, M. Philip, Rapid News 4 (5) (1996) 54±60. [2] P. Jacobs, Stereolithography and other rapid prototyping and manufacturing technologies, Society of Manufacturing Engineers, Dearborn, MI, 1996. [3] G. Tromans, D.I. Wimpenny, Rapid News 3 (3) (1995) 40±46. [4] R. Flint, D. Ellis, in: Proceedings of the SME Rapid Prototyping and Manufacturing Conference, Society of Manufacturing Engineers, Dearborn, MI, 1994. [5] A. Seybert, Rapid News 4 (5) (1996) 62±65. [6] B. Sarkis, Rapid News 4 (3) (1996) 48±52. [7] A. Arthur, P. Dickens, C. Cobb, Rapid Prototyping J. 2 (1) (1996) 4±12.
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[8] P.K.D.V. Yarlagadda, P. Christodoulou, V. Subramanian, J. Mater. Process. Technol. 89±90 (1999) 231±237. [9] S.J. Dover, C.E. Bocking, G. Bennett, in: G. Bennett (Ed.), Proceedings of First National Conference on Rapid Prototyping and Tooling Research, Mechanical Engineering Publications Limited, 1995, pp. 157±173. [10] R.E. Williams, S.N. Komaragiri, V.L. Melton, R.R. Bishu, J. Mater. Process. Technol. 61 (1996) 173±178. [11] C. Bocking, G. Bennet, S. Dover, A. Arthur, C. Cobb, P. Dickens, GEC J. Technol. 14 (1997) 66±74.
[12] C. Cobb, P. Reeves, in: Proceeding of the First National Conference on Rapid Prototyping and Tooling Research, UK, 1995, pp. 111±127. [13] T. Murakami, A. Kaminura, N. Nakajima, Rapid Product Dev. 3 (1997) 74±82. [14] S. Thomas, Mech. Eng. 114 (1992) 62±66. [15] T. Himmer, T. Nakagawa, M. Mohri, in: Proceedings of the Seventh European Conference on RPM, 1998, pp. 315±326. [16] B. Fritz, in: Proceedings of the Sixth European Conference on RPM, 1997, pp. 210-229. [17] E. Finkenstein, M. Kleiner, Ann. CIRP 35 (1991) 311±314.