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Table I shows a comparison of the material properties achieved with the RM with that of a tool steel which typically is used to make injection moulds. The properties in Table I result from a feed stock that contains 40 vol per cent copolymer mol per cent butyl methacrylate and 80mol per cent methyl methacrylate mixed with 60 vol per cent -325 mesh iron powder (Hoeganaes ANCOR ATW 230). The SLS produced test bars were then oxidized in an air oven, according to a proprietary temperature schedule, and subsequently infiltrated with epoxy resin (Dow DER 331). DTM Corporation, Austin,Texas, has recently announced a polymer-coated metal powder, Rapid Steel, that uses a smaller amount of a similar copolymer binder and a larger metal particle size than were used in this study. The properties obtained when using Rapid Steel powder in the RM process should be similar to those in Table I.
Because even rapid prototype moulds are expensive to build, only three have been constructed and tested. Consequently, there is very little information with which to check the veracity of the predictions above. The first mould was built to test the material, not the mould-making process. As pictured in Figure 13, a simple slab mould insert was constructed by machining a slab that had been cast from the mixed iron and binder powders in an air oven, then oxidized and infiltrated with epoxy in the usual way. The machined insert was fastened to a standard MUD mould base and knock-out pin assem-bly with two bolts and was further constrained on two opposing boundaries by close fitting metal plates. The cavity was only 0.125-0.187 inches deep. This mould was used to prepare a total of 176 parts with no mould cooling and a one minute cycle from four different plastics whose moulding conditions ranged from 200-300°C melt temperature and from 8,000-35,000psi injection pressure. Two small failures, a hairline after shot number 125 and a small chip breakout around an ejector pin at shot number 123, were observed; however, no catastrophic failure resulted. The mould was checked for wear, however none was found. Two lessons emerge from this study:
1) the RM material is machinable. It can be drilled, tapped and surface ground and behaves much like cast iron during these operations.
(2) one should probably install sleeves for the ejector pins if one needs more than 50-100 parts.
Conclusions and recommendations
The RM system shows good potential for rapidly making prototype mould inserts with sufficient accuracy and strength to provide the limited numbers of plastic parts needed for engineering evaluation and development. Two successful mould inserts have been built from the RM material, a result that seems to verify the results of stress calculations which suggest that steel boundary constraints are necessary to prevent mechanical failure. To provide some guidance to the designer, stress calculations are presented for a steel ring-reinforced RM insert with variable cavity geometry; however, given the low cost, high quality and ease of use associated with multidimensional PDE equation solvers such as PDEase2, the direct use of such numerical packages for helping to design each particular insert is highly recommended. The RM material is less conductive than steel, and when designing and using a RM mould, care should be taken to adjust conduction lengths and cycle times, respectively, if one requires that parts from the prototype mould closely simulate those from production tooling. These simulations are also relatively easy to accomplish with modern numerical methods and are recommended as part of the design process. Finally, the ability of the SLS process to prepare production moulds or prototype mould inserts with complex cooling channel geometry needs to be exploited more fully, especially for situations where careful control of mould temperature is required.
摘要：介绍了机械性能的一个新的制模材料,提出了快速原型类型的注塑模具生产塑料衬垫通过选择性激光烧结。解释说,虽然这种材料的强度是远低于工具钢，通常用于制造模具,设计计算表明,它仍然可以被用于模具生产。这种材料的导热系数低于钢但高于塑料融化。从计算看出,适当的选择和循环时间的传导长度可以减少差异,相对于钢模具,在模具的操作行为的新材料。论述了模具寿命的例子。