Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mold. Cast or machined aluminum is the most common mold material, although epoxy, wood and structural foam tooling are sometime used for low volume production. The sheet is held against the mold surface unit until cooled. The formed part is then trimmed from the sheet.  The trimmed material is usually recycled - reground, mixed with virgin plastic, and reprocessed into usable sheet.  There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, and simple sheet bending.   FEA finite element analysis is used to predict localized wall thickness of the formed sheet. Heavy gauge sheet is trimmed with multi-axis routers adapted from the woodworking industry.

       Thermoforming can fabricate thin walled parts with large areas, using relatively inexpensive, single-sided tooling. Its deficiencies – variable wall thickness, added cost of sheet and trim regrind, and extensive trimming – have been offset by the ability to economically produce a few, thick walled parts or very many thin walled parts.  The forming process can be used to make product packaging, speaker casings and even car dashboards.  Normally, draft angles must be present in the design on the mold (a recommended minimum of 3°), otherwise release of the formed plastic and the mold is very difficult.    Formability is relative to the starting sheet thickness gage and the depth of the pull, tool geometry and resulting wall thickness.  Relatively deep parts can be formed if the form sheet is mechanically or pneumatically stretched prior to bringing it in contact with the mold surface and before vacuum is applied, know as Insert or Stretch Thermoforming.

        Suitable materials for use in thermoforming are conventionally thermoplastics, the most common and easiest being High Impact Polystyrene Sheeting (HIPS). Molds can be (soft) such as wood, structural urethane board or (hard) cast/machined aluminum mold.   Thermoforming is appropriate for transparent materials such as acrylic and polycarbonate which are widely used in applications for aerospace such as airline passenger cabin windows and aircraft canopies or "bubbles" for rotary wing aircraft. 

       Polystyrene is a colorless, hard plastic with limited flexibility. It can be cast into molds with fine detail. Polystyrene can be transparent or can be made to take on various colors. It is economical and is used for producing plastic model assembly kits, license plate frames, plastic cutlery, CD "jewel" cases, and many other objects where a fairly rigid, economical plastic of any of various colors is desired

Vacuum Forming

       Commonly known as thermoforming.  Vacuum forming is usually restricted to forming plastic parts that are shallow in depth. 

Twin Sheet Thermoform 

       Twin sheet thermoforming is basically a sandwiched - clam shell assembly of the thermoplastic sheet process.  The two sheets or film are heated simultaneously between the heaters, then stretched individually and pulled and vacuum fitted over the top and bottom temperature-controlled, single-surface molds. The molded sheets are then pulled off the mold and brought together at the seam line and pinched together to form a molded or melted together seam. The resulting part is a hollow wall part similar to blow in shape molding. Twin sheet thermoforming is not as limited in size as blow molding.  Twin sheet materials and tools are the same materials as the single sheet thermoform process.  The twin sheet part is then 3D trimmed or routed out, based an a 3D CAD / CAM multi-axis routers, trimming program, holding fixture, trimming off of the excess material including routing out holes, cavities and fastener locations. 

Pressure Forming 

       Pressure forming is based on single sheet thermoforming thermoplastic sheet or film under sealed or higher cavity pressures.  The sheet or film is heated then stretched over or into a temperature-controlled, male or female single-surface, cast or machined tool.  he mold is then subjected to a high  positive pressure or vacuum  to force the material into to deeper cavities and mold recess details. The process allows for heavy surface texturing and sharper corner feature versus standard thermoforming.  Then as in thermoforming, the formed part is then separated form the mold and trimmed from the sheet.  The pressure forming process allows for heavier gauge sheet a much crisper detail simulating injection mold on the outside surface allowing for some boss and rib features.  Industry applications include structures such a refrigerator panels, spas and shower enclosures, and electrical and electronic equipment.  However, as in thermo forming the inside surface is also non appearance quality.