EXPERIENCES USING RAPID PROTOTYPING
TECHNIQUES TO MANUFACTURE SHEET METAL
FORMING TOOLS
00SE008
Prof. Dr.-Ing. D. H. Mueller and Dipl.-Ing. H. Mueller,
BIBA (Bremer Institut für Betriebstechnik und angewandte
Arbeitswissenschaft an der Universität Bremen, Germany)
Abstract
The automotive industry uses Rapid Prototyping Techniques (RPT) like Stereolithography or Selective Laser Sintering to produce plastic parts for prototypes faster and cheaper compared to the techniques used up to now. As a consequence of this process there is a need to have sheet metal parts also available in earlier product phases. This paper deals with Rapid Prototyping (RP) process chains to manufacture sheet metal forming tools. It presents a systematic of the approaches, describes them and presents the results of case studies for selected process chains.
Keywords:
Rapid prototyping, rapid tooling, sheet metal forming tools, layer milling
1 Introduction
By using Rapid Prototyping Techniques (RPT) car manufacturer today produce plastic parts for prototypes faster and even cheaper compared to the techniques used up to now. As well metal parts made either by RPT combined with metal casting processes or by laser sintering, are inserted earlier as usual into dummies and tests. As a consequence of this process the automotive industry increasingly demands to have sheet metal parts also earlier than now at their disposal. This paper deals with the question whether RP process chains are suitable to manufacture them. The ideas and results which are presented here have been worked out in the EU funded research project RAPTEC (BE-20511). A full presentation of the project results is available in the internet, http://ikppc43.verfahrenstechnik.uni-stuttgart.de:80/raptec/
Sheet metal forming tools are geometrically complex, massive and require a high surface quality. The material must be stiff and resistant to pressure and wear. According to the current industrial practise tools for cuttings larger than 250 mm are mainly made from low melting point metals. A main reason is that these metals can be reused. Head zinc or bismuth-tin alloys are used. The tools are either made by casting or NC – milling or by using both. Tools up to a size of 1000 mm do not need machining. The final shape of larger tools is produced by NC-milling. The batch-quantity for prototype parts varies from 1 to 100.
1 Acknowledgement
The work presented in this paper has been supported by the Brite–Euram programme of the EU. The authors wish to thank to the project partners and the Directorate Xll „Science, Research and Development“ of the European Commission.
2 A systematic of the Rapid Prototyping approaches
The search for new ways to produce sheet metal forming tools already has a long history [Walczyk et all 94]. The development of manual or automatically configurable tools, consisting of elements which are movable against each other and which are clamped after positioning is one approach. Publications of this kind are e.g. [Hardt et all 80], [Finkenstein et all 91] and [Ssmatloch 96]. To stack sheets is an other approach. [Sepold 94] and [Geiger et all 94] propose to join vertically laser cut laminates. [Walczyk et all 94] describes an approach using vertically stacked and clamped sheets with profiled edges. [Berger et all 93] describes an approach using a mould made by SLA to cast a sheet metal forming die out of concrete. Investigations of forming blocks for a rubber pad forming press applying SLA and resin casting have been published in [Fritz 97] and [Voelkner 98]. To manufacture formed sheets by beat out a scythe is an other research approach.
This paper will now focus on approaches using RPT. As there exist many possible approaches a systematic structure is necessary in order to give an overview. This is done by table 1. The approaches are structured by a four level subdivision. The highest level distinguishes between a direct and an indirect method. The direct method manufactures the sheet metal forming tool completely or at least the shape giving part of it using RPT. The indirect method utilises a pattern made by RPT as an aid. The steps two and three, which are called “generic technology” and “RP fabrication method” break down the solutions. Thus the direct method distinguishes two generic technologies that is “by RPT” and “RPT based”. The generic technology “by RPT” itself is broken down into the fabrication methods “extrusion” and “laminated tool”. The actual RP process chains are allocated on level four. The chapters 4 and 5 will tell more about the processes itself.
1. Method 2. Generic Technology 3. RP Fabrication Method Extrusion
RPT based Direct RPT hard structures Metal spraying on
RP model Metal Deposition on
model Non metal casting Resin Casting into
RP mould
EOSINT S DTM Sandsintering TSF
Contour Crafting
Electroforming (Ni) + Densit Epoxy casting
LLCC (Laminated Laser Cut Cavities)
Stratoconception Conveyed adherent process
Rapid Layer Milling
Most RPT combined with several metal spraying techniques
e. g. EOSINT M or RapidTool + follow up processes
Profiled-Edge Lamination method
Casting of low melting point melals
4. Process/ Process Chains
combined with sandcasting Direct
tool manufacturing
Metal shell forming on model Indirect
tool manufacturing
Casting of low melting point melals By RPT Laminated tool Contour Crafting C C C C
Table 1: RP process chains to manufacture sheet metal forming tools (Case study )
3 The case studies
The previously mentioned research project Raptec investigated promising RP process chains by case studies. Two different testparts were used. Figure 1 shows testpart 1, a B-post lower reinforced panel:
Size 180 x 125 x 75 mm Material St 1403
Sheet thickness 1,25 mm
The flangings have a 3 mm radius.
The tools were tested on a rubber pad forming press. The study assessed the quality of the formed part. The working times and the manufacturing costs were determined. They were compared to the values of the actual production method used at DaimlerChryler. The cost comparison used in each case the figures of the different tool manufacturer. That means that the costs are influenced by company specific factors. The time comparison used working times. They are defined as the times during which work is actually done on the job as it goes through the system. All values were collected in the course of a normal industrial production. Therefor a tolerance range of 10 to 15 % must be allowed for. The figures 2 and 3 show these comparisons. In each diagram the reference process chain NC-milling was put to 100%. The descriptions of the corresponding process chains refer to these diagrams.
Fig. 1:Testpart 1 100 120 100 100 280 320 0 50 100 150 200 250 300 350
NC milling Sheet Metal Laminating Rapid Layer Milling (Zimmermann) Shell version of Nylon tool EOSint S and steel cast. Solid version of Nylon tool L e a d tim e [% ]
Fig. 2: Comparison of working times
100 38 248 82 91 482 0 50 100 150 200 250 300
NC milling Sheet Metal Laminating Rapid Layer Milling (Zimmermann) Shell version of Nylon tool EOSint S and steel cast. Solid version of Nylon tool Co sts [% ]
Figure 4 shows the second testpart, a shock absorber panel. It is geometrically complex and has a high drawing depth. The characteristic values are:
Part size 380 x 490 x 280 mm Cutting size 830 x 850 mm Cold rolled St 1403
Sheet thickness 1.5 mm Weight 2 kg
By means of this part a comparison of the production methods NC-milling of Zamak castings and Layer Milling was made. The description of the generic technology “Laminated tool” will refer to the results.
Fig. 4: Shock absorber panel; source: DaimlerChrysler
4 The direct method of tool manufacturing
The next two chapters deal with the direct or indirect tool manufacturing method respectively. There is a separate subchapter for each generic technology. Here the “RP fabrication methods” will be shortly described and the results of the case studies will be presented. Those RP process chains for which case studies are available are specially marked in table 1.
4.1 Tool manufacturing direct by RPT
Extrusion and laminating are the two “RP fabrication methods” which are suitable to manufacture complete tools. Both can produce large and massive parts.
RP fabrication method “Extrusion
However one must realise that the RPT which are in the market today do not fulfil the requirements which were mentioned in the introduction. Contour Crafting however, which is under development at the University of Southern California, has the required properties. It is based on the principle of extrusion. The key feature of it is the use of two towels, which in effect act as two solid planar surfaces, to create surfaces on the object being fabricated that are exceptionally smooth and accurate [Khoshnevis 99]. The main innovation of Contour Crafting compared with existing RPT are:
• Exceptionally smooth and accurate surfaces are created because of the elimination of surface discontinuities.
• Fabrication of a part is considerably faster because the layer thickness is typically much larger than layer thickness in other rapid prototyping processes.
• A wide variety of materials can be used, including thermosets, thermoplastics, metal and ceramic pastes mixed with a binder, and also materials that are not commonly used in rapid prototyping such as plaster, cement, clay, and concrete.
R P fabrication method “Laminated tool”
We know at least five different processes which are based on laminating. They manufacture an object by cutting, stacking and joining layers of solid materials. Sheet metal forming tools can be made out of thin sheets which are joined e. g. mechanically or by welding [Sepold 94], [Geiger et all 94], [Walczyk et all 94].
Laminated too case study
Figure 5 shows a laminated tool for the B-post lower reinforced panel. It consisted of 0.5 mm thick steel sheets which were joined in the way of a vice. Good parts were produced without any rework of the edges of the sheets. The laminated tool needed 120% of the working time but only 38 % of the costs of the reference process.
Fig. 5: Laminated tool; source: DaimlerChrysler Layer Milling
Layer Milling is an other member of the group “Laminated tool”. Milling thick layers is an idea which applies a basic feature of RPT to milling. The generative aspect of this approach is to make the tool by joining plates and shaping them by milling. The plates are much thicker than the layers of e.g. SLA or SLS or even laminated tools. The plate thickness may vary from 10 to 100 mm. Thus massive parts can be made rapidly. Layer milling produces exactly the required shape in the accuracy we are used to milling. The whole range of materials suitable for machining can be used. Figure 6 shows the principle.
Concerning the question of the benefit of Layer Milling in comparison to milling a casting or a solid block one can say:
• It is not necessary to produce a casting.
• The amount of metal removed is higher because always short and stiff tools are applicable
• Supposed a fastening joint is applied single plates can be replaced in case of design changes.
• Different materials can be combined in one tool. E.g. in the area of the die a steel plate may be used.
• Because it is possible to mill close to the final contour, less material has to be removed, which is only valid in comparison with milling from a solid block. This also means that the safe removal of remaining plates must be technically solved.
Layer milling has the following disadvantages:
• The plates must be joint.
• There may be a loss of stiffness.
• Danger for staggered joints (danger for an offset between adjacent plates)
Two layer milling machines are offered by the market. The French company Charlyrobot sells a machine called Stratoconception®. Figure 7 shows the machine of the company Zimmermann.
Fig. 6: Principle of Layer Milling; Source: Desk Artes
Fig. 7: Layer Milling machining centre; source: Zimmermann Layer Milling case studies
Figure 8 shows the layer milled tool for the B post panel. It was manufactured out of a standard plastic material used for sheet metal forming tools and was produced as fast as the reference tool with only 82 % of the costs (refer to figures 2 and 3)
Fig. 8: Layer milling tool (material: PU)
respect to manufacturing costs and working time is shown by figure 9. The study found out that Layer Milling only needed 82 % of the time causing only 84 % of costs. The same hourly rates were used for calculating the costs. That means that the results is free of company specific factors.
0 20 40 60 80 100 120
Working time Costs
[%]
Layer Milling Zamak route
Fig. 9: Comparison of working time and costs
4.2 Tool manufacturing based on RPT
This generic technology comprises the RPT using metal and reinforced plastic material. Nylon powders filled with glass balls are on the market and Stereolithography processes working with glass fibre filled resins are under development. Currently it is not known to what extend the metal materials fulfil the requirements which were mentioned in the introduction. But at the beginning they can be rated as suitable. But because of the low building rate the application is not economic or limited to small tools respectively.
Laser sintering case study
Figure 10 shows two laser sintered nylon tools for the “ B-post lower reinforced panel”. The negative tool to the left was made completely out of nylon, the one to the right consisted of a backfilled shell. The surface quality of the tools were good, though they were not finished. No stairsteps were visible.About 25 deep drawings were produced in the negative version. The maximum pressure was 700 bar. A lubrication was used. At 450 bar a well formded part was produced, except the 3mm small radius of the flangings. The parts which were made in the positive tool even had well formed flangings. The comparison of the economically relevant data showed that the shell version needed the same working time and was 10 % cheaper as the reference process NC milling ( firgures 2 and 3). The solid version used more than three times of the lead time and caused five times of the costs.
5 The indirect method of tool manufacturing
This chapter will discuss those process chains which utilise an RP made pattern.
5.1 Tool manufacturing by forming of a metal shell
This generic technology can be split into the two “RP fabrication methods” “Metal spraying on RP model” and “Metal deposition on RP model”. In both cases a metal shell is produced which is backfilled afterwards.
It is common to produce sheet metal forming tools by metal spraying. Also processes for spraying steel are available. But they are hardly industrially applied. The main reason is the warpage or inner stresses of the shells.
Sheet metal forming tools consisting of backfilled electroformed shells are in operation. The principle design and an application of such a tool can be seen in figure 11. A model of the tool is manufactured using RPT. On this model a nickel shell is electroformed. After a sufficient shell thickness has been reached it is removed from the model and backfilled in order to get the required stiffness. The application shown by figure 11 used concrete for backfilling.
Electroformed Densit ToolCast
Fig. 11: Design and application of an electroformed shell
5.2 Tool manufacturing using non metal casting
It is current practise of industry to manufacture resin tools by casting. Supposed that the part is small and has a complex shape RPT are very useful to produce the models which are needed for casting. Whereas RPT are to expensive for the production of large tools. This was also verified by a case study which used testpart 2 of chapter 3. For detailed information please refer to
http://ikppc43.verfahrenstechnik.uni-stuttgart.de:80/raptec/.
5.3 Tool manufacturing using casting of low melting point metals
Most of the prototype sheet metal forming tools for car production are made from castings of low melting point metals. From the technically point of view the required moulds could be produced by sandsintering. But this application is not economic because the building rates are fare to low. This is confirmed by the case study of the “ B-post lower reinforced panel”. The tool made by
sandsintering and steel casting needs 2.8 times of the lead time and 2.5 times of the costs of NC milling (refer to figures 2 and 3).
Contour Crafting, the process mentioned at the beginning of the paper seem very promising also for this approach.. It can produce parts which are large and massive and it is the only RPT which shows a way how smooth freeform surfaces can be made by RPT.
6 Conclusions
Current additive processes are powerful to manufacture small, detailed parts. The processes or the machine concepts now available are not suitable to manufacture sheet metal forming tools on a large basis economically. But for selected applications the use of RPT already brings profit today. This is valid for Layer Milling, which is mature for industrial application. It can be adjusted to the production of sheet metal forming tools with low costs and development risk. It is also valid for geometrically complex small parts for which shell tools out of reinforced plastic or metal are a possibility. The authors have the opinion that laminated tooling should be developed to the state for industrial application and the development of new RPT should be pressed ahead which are suitable to make large parts with material properties which are at least equal to those of the low melting point alloys.
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