注塑模具外文文献和翻译
The engineering reasons for building a proto-type mould are several and include evaluation of the processability of candidate materials inthe mould and of the mechanical behaviour 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, Tg, above ambient usually show some residual molecular orientation due to flow into the mould. The amount of orientation depends on the cavity fill rate and on the rate of melt cooling. The part mechanical properties, strength for example, can be enhanced in the orientation direction and reduced transverse to the flow and fracture energy can be reduced as a consequence of orientation. Consequently, engineers have often concluded that even prototype moulds should be made of the same materials as production 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 consequences. The cost and time savings inherent in modern rapid prototyping methods and materials can help to solve this dilemma.
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. 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. The cavity is then strengthened and sealed by infiltration and cure of an epoxy tooling resin. This mould-production process has been called Rapid Mold (RM) by the inventors. Among the advantages of the RM system are ease and speed of manufacture, low-cost post treatment in a simple air oven, and good hardness, thermal conductivity and thermal expansivity, relative to filled epoxy tooling.
The RM system is named so as to distinguish it from the Rapid Tool mould-makingsystem that was introduced recently by DTM 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 conditions 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 production 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 reducing 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 power and cavity part file.
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