OVERVIEW

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THERMOFORMING PROCESSES

• VACUUM FORMING:

The process of evacuating air from the sealed space between the hot sheet and the mold, thus allowing atmospheric pressure (14.7 p.s.i.) to force the sheet to conform with the contour of the mold.

Applications: Non-critical appearance covers, dunnage trays, internal covers. Competes with sheet metal and Fiberglass. A low to medium volume process.

• PRESSURE FORMING

The process of applying compressed air (20-120 p.s.i.) to a hot sheet, thus forcing it to conform to the contour of the mold. vacuation of the air between the sheet and the mold is required.

Applications: High appearance covers, computer housing, microprocessor based equipment, typically non-structural. Part size is usually 12" x 12" and larger. Competes with injection molding. A low to medium volume process.

• TWIN-SHEET PRESSURE FORMING

The process of injecting compressed air (20-120 p.s.i.) between two hot sheets, thus forcing it to conform to the contour of each of two molds mounted opposed to each other. Evacuation of the air between the sheet and the mold is required.

Applications: Those enclosures that require structural rigidity. Appearance items that require high surface detail and yet light weight. Competes with blow molding and rotational molding. A low to medium volume process.

• ABOUT THE SHEET

Thermoforming requires a previously extruded sheet. In the process the two operations; i.e.., sheet extrusion and forming, are uncoupled. This simplifies the process, but does add to the cost. Costs are increased because of the extra energy required to heat the polymer twice, and the fact the extruders are commonly custom processors that also need to generate profits to continue to exist. On the other hand, gauge changes, color changes, even material (i.e.., polymer) changes can be done very easily in thermoforming because neither the oven nor the mold care what the sheet looks like.

Because the final part almost always weighs less than the starting sheet, the trim must be captured and returned to the sheet extruder for reprocessing. This results in a "scrap credit" which is included in the price of every part.

• MATERIAL SELECTION

Almost any thermoplastic can be thermoformed. As with any manufacturing process, there are wide variances in the cost of different materials due to the polymer cost, temperature of processing, crystalinity of the resin, or, in general, the degree of difficulty of extruding the sheet. The most common resins (and therefore the most cost effective ones to specify) are FRABS, ABS, DKE or Kydex, HIPS, HDPE or HMWHDPE.