Screw-Injection Molding of Thermosetting Resins

There are several designs for the feed mechanism. The simplest one is the use of a plunger which pushes the material into the mold. However this gives rise to densification of die material. In another design the mix is forced from a cylindrical tank down to the entrance of a screw injection cylinder. The screw conveys the material to the mold at the fixmt of the machine. Short screws are used for thermoset resins, and the compression ratio is below 1.5. Otherwise, heat may be produced and hardening may take place inside the machine. The barrel of the machine needs to be cooled to prevent premature polymerization. The typical temperatures at the rear and front of the cylinder are around 50 and 65"C respectively. 

Plastication. First, the thermoplastic resin is fed from a hopper into the barrel of a single screw extruder in which the resin is softened (or plasticized) by the action of the rotating screw. Injection. The molten polymer is forced through a nozzle by the forward action of either a piston or the screw itself acting as a piston. The resin enters through the nozzle and a series of channels, the mnners, a number of cavities in a colder mold. It should be noted that in the case of thermoset injection, the mold will be hotter than the injected resin. 

Polybutadiene resins are cured to a thermosetting structure by peroxide catalysts. These catalysts produce carbon-to-carbon bonds at the double bond sites in the vinyl groups. The final product is 100 percent hydrocarbon except where the starting polymer is the —OH- or —COOH-terminated variety. Use of high-temperature peroxides maximizes the opportunity for thermoplastic-like processing, because even the higher-molecular-weight forms become quite fluid at temperatures well below the cure temperature. Compounds can be injection molded in an in-line machine with a thermoplastic screw. 

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