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Injection-moulding metals and ceramics with Catamold® |
Progress in engineering plastics
Injection-moulding metals and ceramics with Catamold®
Presented by Arnd Thom, Sales Catamold®, Inorganics Division
Trade Press Conference K 2004, June 22, 2004, Ludwigshafen, Germany
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Powder injection moulding (PIM) - a combination of thermoplastic injection moulding and conventional powder metallurgy - is still a relatively new process for the manufacture of precision metal and ceramic parts. BASF's own PIM product is called Catamold®, a group of injection moulding compounds consisting of very fine metal or ceramic powder, a polymer binder - mostly polyacetal (POM) - plus several additives. Apart from the powder material, BASF supplies all the ingredients found in these compounds.
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As easy as thermoplastic injection moulding
Catamold pellets, which have a powder content of over 80%, are moulded in the same way as plastics using a conventional injection-moulding machine. Once the part has been moulded, the polymer binder, which is merely there to make the powder injectable, is removed catalytically - by a process known as "debinding" - to leave a powder skeleton of the part. Debinding does not alter the part's shape or size. Finally, the metal or ceramic part is sintered at very high temperatures to make the particles fuse together to form a coherent mass (figure 1).
PIM enables extremely complex parts to be produced that require little or no finishing. PIM parts are found, for example, in motor vehicles as well as consumer and medical products. Tricky features such as undercuts, which are impossible to produce by conventional press sintering, are easy with PIM. One special advantage of the process is its ability to reproduce fine detail and sharp features - even knife blades for instance - thanks to Catamold's excellent flow properties (figure 2).
Parts made by conventional powder metallurgy usually have densities of around 90 percent of normal. By contrast, the density of Catamold parts lies between 96 and 100 percent; their properties therefore largely correspond to those of forged or fully machined products.
PIM produces a surface finish far superior to investment casting. The fact that parts are extremely smooth after sintering allows finishing and polishing costs to be eliminated or significantly reduced - as, for example, in the manufacture of jewellery from Panacea, a nickel-free stainless steel (figure 3). Dimensional tolerances of plus/minus 0.3% are attainable. BASF offers Catamold moulding compounds based on a variety of materials for a wide range of uses: stainless steels, soft-magnetic alloys, nickel steels, tool steels, and super alloys and high-performance ceramics (figure 4). Surface treatments and finishing, such as chrome and nickel plating, are also possible.
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High requirements pay off
Power injection moulding with Catamold is most economic for complex parts requiring narrow tolerances, good mechanical properties and a high surface finish. Whether the technology proves suitable for a particular application must be determined from case to case - obviously, a part usually stamped from metal would be hopelessly uneconomic if made by PIM. If the technology's potential is considered at the design stage, it is possible to make savings of up to 50% over competing technologies. In the case of a gripper blade for a cutting machine, the cost saving was as high as 54% - achieved solely through the elimination of finishing operations (figure 5). Often, costs can be saved by reducing the number of components that go to make an assembly (figure 6).
The fact that Catamold parts generally require no finishing makes the process all the more attractive, especially when the material or part is difficult to machine. This was so in the case of a part for a vehicle’s auxiliary heater. Previously the part was made by investment casting and required considerable finishing in order to meet the narrow tolerances. Today it is made much more cheaply from Catamold 316 L stainless steel (figure 8).
Something that does weigh on the economics of PIM are the comparatively high tooling, process and material costs. The high material cost can be offset somewhat by the fact that there is no wastage of the metal or ceramic powder. Having said that, PIM is still preferred for parts for which the material cost is not crucial – i.e., small parts weighing up to 100g. The upper economic limit lies at about 300g, the lower limit about three hundredths of a gram (0.03 g).
In order to amortize the relatively high tooling costs, a minimum production quantity is necessary. A general figure is difficult to give since total cost - not just the tooling cost - is the deciding factor. As a rule of thumb, 10.000 units per annum can be regarded as a minimum (figure 9).
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Intelligent product for new projects
In order to benefit from the opportunities PIM offers, the process must be considered at the very start of the development process; the more the part is designed with PIM in mind, the greater the functional advantages and economics of the end product. The design rules are generally the same as those for plastic parts.
New projects currently in progress in the automotive industry and elsewhere should boost the worldwide use of PIM technology. In high competitive environments, Catamold is the intelligent answer for the cost-effective production of complex parts.
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