Pressure Forming
Pressure forming is a variation of vacuum forming that utilizes both vacuum and compressed air to force the plastic sheet against the mold.  As the platens are closed, the vacuum pulls on one side of the sheet and compressed air pushes on the other.  Specially shaped tooling is used to match the top and bottom halves of the mold creating a seal to maintain pressures of up to 500 psi, therefore, the platens must be locked together.  This compressed air pressure reduces the cycle time and makes it possible to run at lower temperatures, it also improves the distribution of the material creating a more even wall thickness and enhances the detail of the part to a nearly-injection-molded quality.  After the part has been formed, the platens unlock and one of the platens moves out of the way to speed up the cooling process. 

The increased air pressure will require a stronger mold and a locking device for the platens so consequently a higher tooling expense will be incurred.

Steps

  • Material is heated to proper temp then moves over the mold.  
  • Platens close and lock.  
  • Vacuum and air pressure are applied.

Materials
Theoretically, any thermoplastic material can be pressure formed. However, some materials are more difficult to work with than others. Polyethylene, for instance, flows easily and causes few problems for pressure formers. With vacuum alone, polyethylene can be intricately formed. On the other hand, polycarbonate, which chills quickly, can cause manufacturers to be concerned about tool design and plug assists.

Medical device manufacturers usually specify that their products should be formed of a material that passes the Underwriters Laboratories (UL) 94 V0 or 94 5V tests for flammability. The resins most commonly used in pressure-formed medical products are flame- retardant grades of acrylonitrile butadiene styrene (ABS).

In many cases, assists are used to help distribute material evenly and to coin it into sharp or narrow corners. Depending on its complexity, the design of a product's tooling may require the former to use matched heated molds and assists; otherwise, assists can be made of low-heat-transferring materials such as wood.

Advantages

  • Sharp, crisp lines and details
  • Low tooling costs
  • Short lead time
  • Textured surfaces and molded in colors
  • Formed in undercuts
  • Ideal for short runs
  • Zero degree draft on sidewalls
  • Embossed and Debossed areas
  • Highly detailed openings
  • Superior, uniform tolerance control

Applications
Use pressure forming when the part will be the "face" of the product expecting a long life. You can pressure form a company logo or model designation with styling lines, surface texture or other features in a light weight and durable part. Use pressure forming when you have undercuts or rims and a greater depth of draw.