Thermoforming

Thermoforming is a manufacturing process in which a plastic sheet or film is heated until pliable and pressed against a mold, where it cools and keeps the mold's shape. The most common variant is vacuum forming, in which a vacuum draws the sheet onto the mold. In pressure forming, compressed air assists, which allows sharper details and a texture close to that of injection molded parts. Deep shapes are pre-stretched with a mechanical plug (plug assist) so that the material thins more evenly.

There are two main branches. Thin-gauge thermoforming of film from a roll (roughly up to 1.5 mm) is typical for packaging, blisters and trays in high volumes. Heavy-gauge forming shapes thicker sheets, usually several millimeters thick, one piece at a time, and produces technical parts: machine and equipment covers, vehicle interior panels, tubs and dunnage trays for shipping. After forming, the part is trimmed, sheets usually on a 5-axis CNC router, film by die cutting.

Common materials are thermoplastics such as ABS, high-impact polystyrene (HIPS), PETG, PMMA (acrylic), polycarbonate, PP, PE or PVC, and for outdoor cosmetic parts, for example, ABS with a PMMA cap layer. Molds are single-sided and much cheaper than injection molds: wood, tooling board or 3D-printed molds for prototypes and short runs, temperature-controlled aluminum molds for production.

Only the side that touches the mold is precisely defined: with a female (negative) mold it is the outer surface, with a male (positive) mold the inner surface. Wall thickness is not constant. The material thins as it stretches, most at the bottom corners with a female mold and at the lower part of the side walls with a male mold, and the final thickness can be just a fraction of the original sheet.

Thermoforming suits large, geometrically simple, thin-walled parts in quantities from single units to thousands per year: covers, guards, trim panels, tubs, trays, dunnage and packaging. For large parts in low-volume production it is significantly cheaper than injection molding, because the mold costs a fraction of an injection mold and can be modified quickly. It also works well for prototypes and pilot production runs of plastic covers.

It is not suitable for parts with ribs, screw bosses or varying wall thickness. Such features have to be bonded on or machined afterward, and at higher volumes injection molding becomes the better option.

The most common misunderstanding is confusing the sheet thickness with the wall thickness of the finished part. In the RFQ, specify the minimum wall thickness at critical points and let the supplier choose the starting sheet. Mark the cosmetic side (it decides whether a male or a female mold is used), allow sufficient draft angles and corner radii, and keep the ratio of depth to width as low as possible, because deep narrow pockets lead to extreme thinning.

Dimensional accuracy is lower than with injection molding: for large parts, tolerances range from tenths of a millimeter to millimeters and are strongly affected by the shrinkage and thermal expansion of the plastic. Specify tight tolerances mainly on the trim line and mounting holes. Some materials, such as polycarbonate, must be dried before forming, otherwise bubbles form. For electrical or transportation parts, check the flammability requirements (for example UL 94 ratings for electrical equipment) and UV stability, and choose a matching sheet grade.