The Basics of Thermoforming
Thermoforming is the process of heating thermoplastic sheet, which is extruded from pellet form, to a pliable state. When the sheet reaches its forming temperature, it is removed from the heat source. A mold then enters the softened sheet. The sheet is sealed on the edge of the mold and vacuum is applied to evacuate the air trapped between the sheet and the mold. The atmospheric pressure then forces the sheet against the mold. The cooling process begins, both internally and externally. When the part reaches a cooled state or its set temperature, it is removed from the mold, trimmed, the finishing is completed on it per print, and it is then ready for shipment.
Cast aluminum molds are generally the best for thermoforming. They offer the best heat transfer, which allows us to accurately control the cycle times, part shrinkage, and part warpage. Consistency in the forming process is essential in the final trimming and quality control.
Molds, depending on the size and geometry, can be made in single or multiple cavities. There are also male and female molds. These are determined by dimensional requirements of the customer, Aesthetic requirements, and compatibility with mating parts in assemblies.
Material comes in various types of grain. The grain is usually only on one side of the sheet and is almost always formed with the smooth side against the mold. Forming the sheet with the grain on the mold side almost always washes the grain out leaving an unpleasant appearance. For the customer that wants grain on both sides, wants to enhance the grain, or the part must have a grained finish on a female mold, textured molds are an option. Though these molds can be quite expensive, they produce parts with exceptional quality.
Variations of the basic process give us the various techniques used in thermoforming. They are: snapback forming, plug assist forming, billow forming, pressure forming, and twin sheet forming. The technique used on any particular job depends on several factors. These can be discussed on a part by part basis.