Plug Assisted Forming

Plug assist forming is a widely used forming technique and requires the use of a female (cavity) mold. The limited depth of draw of female molds is improved by the use of plug assist. With plug assist the plastic sheet is mechanically pre-stretched by a plug that is pushed into the hot plastic before the application of vacuum to the mold. The plug has a geometry that is usually 10 - 30 percent smaller than the interior of the female mold cavity. The plug is constructed of materials with low thermal conductivity or is heated. Low thermal conductivity plugs or heated plugs must be used to keep the plastic sheet from cooling when the sheet comes in contact with it. Materials such as wood, syntactic foam, and cast thermoset plastics can be used to make a low thermally conductive plug. This insulator type plug can be covered with felt to reduce mark-off. Aluminum with temperature controlled electric heaters can also be used. Aluminum plugs produce excellent results but are usually more costly than insulator type plugs. Different wall and bottom thickness can be produced by controlling how deep the plug goes into the mold and by controlling and varying plug temperatures.

The steps in plug assist forming are:

  • After the sheet is heated and the sheet cart moves back to the forming area, the bottom platen moves up to the plastic sheet and seals.
  • The top platen with the plug moves down pushing the plug into the hot plastic.
  • After the plug reaches the required depth, vacuum is applied to the female mold forming the plastic to the contours of the mold.
  • The top platen moves back up and cooling fans cool the plastic covering the inside of the female mold.


  • better wall thickness uniformity especially for cup or box shapes
  • reduces stretching or thinning of material during forming.