Compression molding automation

Adding vacuum chambers to compression molding equipment in recent years has reduced the number of defects caused by trapped air or water in the molding compound, resulting in higher-quality finished parts. Another relatively new improvement has been the addition of various forms of automation to the process. For example, robots are used both to install inserts and remove finished parts. 

Osswald, T. A. and Tucker, C. L., An automated simulation of compression mold filling for complex parts . In Proceedings of 43rd SPE ANTEC, Washington, D.C., 1985, pp. 169-172. 

An early move toward automation and mass production was the invention of preform machines that sprayed chopped-glass strands onto a three-dimensional air screen that approximated the shape to he molded. The preformed glass mat was placed in a matched metal die in a compression molding press and liquid resin was added to form the product. Today, resin transfer molding (RTM) and resin injection molding (RIM) preform die-molding processes are used to mass-produce small and mid-size parts with two finished sides, notably automobile bodies, truck cabs, and even small boat shells. 

Compared to compression molding, injection molding of composite bipolar plates has advantages, such as automated production, short cycle time, good reproducibility, and accurate size and shape. The principle of injection molding is shown in Figure 6.4. 

There are two major allyl plastics, diallyl phthalate (DAP) and diallyl isophthalate (DAIP). Both of these are widely used in fiber RP forms. The allyl plastics are usually compression or transfer molded performing well in automated equipment (Chapter 5). They retain their physical and electrical characteristics under prolonged exposure to severe environmental conditions. They have high heat and moisture resistance, excellent electrical performance, good chemical resistance, dimensional stability, and low creep. These plastics are used where they provide environmental resistances. 

Compared to both compression and transfer CM, injection molding requires shorter vulcanization and cycle times as well as better automation. On the other hand, mold costs are so high that only large-series production is economical. 

---