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Materials Processes

Processes

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Thermoforming

The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable, then applying differential pressure to make the sheet conform to the shape of the mold or die positioned below the frame.  When the pressure is applied entirely by vacuum, the process is called vacuum firming.  When air pressure is employed to partially preform the sheet prior to application of vacuum the process becomes air assist vacuum forming.  In another variation, mechanical pressure is applied by a plug to partially preform the sheet (plug assist forming).  In the drape forming modification, the softened sheet is lowered to drape over the high points of a male mold prior to application of vacuum.  Still other modifications are plug-and-ring-forming, ridge forming, slip forming or air slip forming, and bubble forming. The term thermoforming also includes methods employing only mechanical pressure, such as matched mold forming, in which the hot sheet is formed between registered male and female molds. [LW]

 

Transfer Molding

A method of molding thermosetting materials, in which the plastic is first softened by heat and pressure in a transfer chamber, then transferred through suitable sprues, runners and gates into a closed mod for final curing.

 

Tumbling

A finishing operation for small plastic articles by which gates, flash and fins are removed and/or surfaces are polished by rotating them in a barrel together with wooden pegs, sawdust and polishing compounds.