Forming Techniques for Plastics

Quite a variety of different techniques are employed in the forming of polymeric materials. The method used for a specific polymer depends on several factors (1) whether the material is thermoplastic or thermosetting (2) if thermoplastic, the temperature at which it softens (3) the atmospheric stability of the material being formed and (4) the geometry and size of the finished product. There are numerous similarities between some of these techniques and those used for fabricating metals and ceramics. 

Fabrication of polymeric materials normally occurs at elevated temperatures and often by the application of pressure. Thermoplastics are formed above their glass transition temperatures, if amorphous, or above their melting temperatures, if semicrystalline. An apphed pressure must be maintained as the piece is cooled so that the formed article retains its shape. One significant economic benefit of using thermoplastics is that they may be recycled scrap thermoplastic pieces may be remelted and re-formed into new shapes. 

Molding is the most common method for forming plastic polymers. The several molding techniques used include compression, transfer, blow, injection, and extrusion molding. For each, a finely pelletized or granulized plastic is forced, at an elevated temperature and by pressure, to flow into, fill, and assume the shape of a mold cavity. 

In transfer molding—a variation of compression molding—the solid ingredients are first melted in a heated transfer chamber. As the molten material is injected into the mold chamber, the pressure is distributed more uniformly over all surfaces. This process is used with thermosetting polymers and for pieces having complex geometries. 

Like metals, polymeric materials may be cast, as when a molten plastic material is poured into a mold and allowed to solidify. Both thermoplastic and thermosetting plastics may be cast. For thermoplastics, solidification occurs upon cooling from the molten state however, for thermosets, hardening is a consequence of the actual polymerization or curing process, which is usually carried out at an elevated temperature. 

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