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A rapid mould-making system: material properties and design considerations
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Introduction
The engineering reasons for building a proto-type mould are several and include evaluation of the processsability of candidate materials in the mould and of the mechanical be haviour and moulded geometry of candidate parts ,prior to committing to a costly production tool. In order for the evaluation to be truly valid, moulding temperatures, pressures and cycle times should be as similar to those intended for production as possible. For example, polymers which have glass transition temperatures, T g , above ambient usually show some residual molecular orientation due to flow into the mould[1,2]. The amount of orientation depends on the cavity fill rate and on the rate of melt cooling. The part mechani- cal properties, strength for example, can be enhanced in the orientation direction and reduced transverse to the flow[3] and fracture energy can be reduced as a consequence of orientation[4]. Consequently, engineers have often concluded that even prototype moulds should be made of the same materials as pro- duction moulds to ensure a proper evaluation. When faced with the time and production costs associated with this conclusion, most
engineers have decided not to build prototype tools, sometimes with very expensive conse-quences. The cost and time savings inherent in modern rapid prototyping methods and mate-rials can help to solve this dilemma.35497
The University of Texas has developed a rapid prototyping process for preparing moulds that are suitable for injection mould-ing a limited quantity of polymeric materi-als[5-8]. In this process, selective laser sinter-ing (SLS) is used to form “green” mould cavity inserts from metal powder which is coated with fusible thermoplastic binder. In subsequent steps, the binder is thermally removed and the metal powder is oxidized to form a porous metal/ceramic cavity that shows little shrinkage and generally excellent retention of geometry, relative to the green part. T he cavity is then strengthened and sealed by infiltration and cure of an epoxy tooling resin.
The RM system is named so as to distin-guish it from the Rapid Tool mould-making system that was introduced recently by DT M Corporation, Austin, Texas. Both RM and Rapid Tool can use the same polymer-coated metal powder feed stock, and the preparation of green shapes is identical. The processes deviate from one another in the post-SLS processing steps. In the Rapid Tool process, the polymer is removed under reducing con-ditions to prevent metal oxidation, and the porosity is filled by infiltration of a lower-melting metal. The result is a mould that is much more durable and suitable for produc-tion tooling than that produced by the RM system. However this gain in tool durability comes at greater cost owing to the need for a metallurgical-grade furnace to handle reduc-ing gases and for greater care during firing and infiltration. One interesting possibility is to use the RM process to prototype the plastic part, then switch to the Rapid Tool process to build the production mould using the same powder and cavity part file.
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. T he properties in Table I result from a feed stock that contains 40 vol percent copolymer (20mol per cent butyl methacrylate and 80mol per cent methyl methacrylate (7)) mixed with 60 vol per cent -325 mesh iron powder (Hoeganaes ANCOR AT W 230). T he SLS produced test bars were then oxidized in an air oven, according to a proprietary temperature schedule, and subse-quently infiltrated with epoxy resin (DowDER 331). DT M 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.