Selecting a process appropriate for both the material and part geometry is complicated. Most manufacturing equipment can use many types of material and some designs function well made of differing materials. This section defines thermoplastic and thermoset materials and the manufacturing processes for making them. The extrusion process is a continuous operation that forces hot plasticized material through a die opening to produce the desired shape. The material coming out of an extruder is called the extrudate. Extrusion processes consume more plastics materials than any other process, since large, high-speed extruders run continuously. Blow molding is a process for shaping thermoplastic materials into one-piece, hollow articles by heat and air pressure. The two principal methods are extrusion blow molding and injection blow molding. Reaction injection molding (RIM) is a form of injection molding that brings temperature and ratio-controlled, liquid reactant streams together under high-pressure impingement mixing to form a polymer directly in the mold. Thermoforming consists of heating a thermoplastic sheet to its processing temperature and forcing the hot, flexible material against the contours of a mold. Rotational molding is a process for forming hollow plastics parts. Casting processes are applicable to some thermoplastics and thermosets. In compression molding, thermoset molding compounds placed in a mold (generally hardened steel) are first heated to plasticize and cure the material, then placed under pressure to form the desired shape.
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