Thermoforming describes the process of heating a thermoplastic sheet to its softening point, stretching it over or into a single-sided mold, and holding it in place while it cools and solidifies into the desired shape. The thermoplastic sheet is clamped into a holding device and heated by an oven using either convection or radiant heat until it is softened. The sheet is then held horizontally over a mold and pressed into or stretched over the mold using vacuum pressure, air pressure, or mechanical force. The softened sheet conforms to the shape of the mold and is held in place until it cools. The excess material is then trimmed away and the formed part is released. Excess material can be reground, mixed with unused plastic, and reformed into thermoplastic sheets
Thermoforming is commonly used for food packaging, but has many applications from plastic toys to aircraft windscreens to cafeteria trays. Thin-gauge (less than 0.060 inches) sheets are mostly used for rigid or disposable packaging, while thick-gauge (greater than 0.120 inches) sheets are typically used for cosmetic permanent surfaces on automobiles, shower enclosures, and electronic equipment. A variety of thermoplastic materials can be used in this process, including the following:
- Low Density Polyethylene (LDPE)
- High Density Polyethylene (HDPE)
- Polypropylene (PP)
- Polystyrene (PS)
- Polyvinyl Chloride (PVC)
- Acrylic (PMMA)
- Acrylonitrile Butadiene Styrene (ABS)
- Cellulose Acetate
As mentioned above, there are different methods of forcing the thermoplastic sheet to conform to the mold. These types of thermoforming include the following:
- Vacuum forming - A vacuum is formed between the mold cavity and the thermoplastic sheet. The vacuum pressure (typically 14 psi) forces the sheet to conform to the mold and form the part shape.
- Pressure forming - In addition to utilizing a vacuum underneath the sheet, air pressure (typically 50 psi, but up to 100 psi) is applied on the back side of the sheet to help force it onto the mold. This additional force allows the forming of thicker sheets and creating finer details, textures, undercuts, and sharp corners.
- Mechanical forming - The thermoplastic sheet is mechanically forced into or around the mold by direct contact. Typically, a core plug will push the sheet into the mold cavity and force it into the desired shape.