A Visual Guide to Plastic Forming Processes

Plastics can be shaped by more than twenty distinct forming processes, each suited to a different geometry, production volume and material family. This guide walks through eight of the most widely used methods, with an animated diagram of each so the mechanics are easy to follow.

1. Injection Molding

Injection molding produces parts by injecting molten material into a mold, and it is the dominant process for shaping plastics. Resin is loaded into a hopper and fed into a long heated barrel, where a reciprocating screw drives it forward. Heat and shear soften the material into a fluid state. At the end of the barrel a nozzle forces the melt into a closed mold. Once the plastic has cooled and solidified, the mold opens and the finished part is ejected from the press.

2. Foam Molding

Foam molding adds a suitable blowing agent to resins such as PVC, PE and PS to generate a microcellular structure within the plastic. Almost all thermosetting and thermoplastic resins can be foamed, and foam processing has grown into an important branch of plastics manufacturing in its own right.

3. Extrusion

Plastic extrusion is a high-volume process in which raw resin is melted and formed into a continuous profile. It is used to make film, continuous sheet, pipe, profiles, rod, wire jacketing, filament and cable. As in injection molding, dried resin is loaded into a hopper and fed into a long heated barrel. At the end of the barrel the melt is forced through a die whose opening defines the cross-section of the finished product. After leaving the die, the extrudate travels along a conveyor to cool; blowers are sometimes used to speed up cooling.

4. Blow Molding

Blow molding is the forming method used for hollow plastic articles. It is a secondary forming technique: a parison in a rubber-like state is enclosed in the mold cavity and inflated by gas pressure until it takes the shape of the cavity.

5. Thermoforming

Thermoforming is a specialized method for converting thermoplastic sheet into finished articles. The sheet is clamped in a frame and heated until it softens, then forced against the contours of a mold so that it takes a shape matching the mold surface. After cooling and trimming, the part is complete. The same principle is also applied in rubber processing. Recent years have brought further advances, such as continuous lines that run straight from sheet extrusion into thermoforming.

6. Calendering

In calendering, material is fed into a train of heated rolls where it is melted and formed into sheet or film, then cooled and wound onto rolls. PVC is by far the most commonly calendered plastic.

7. Vacuum Forming

Vacuum forming is a plastics process in which a flat, rigid plastic sheet is heated until pliable and then drawn tightly against the mold surface by vacuum. Once cooled, the part retains the molded shape. It is widely used in packaging, lighting, signage and decorative applications.

8. Rotational Molding

Rotational molding — also called rotomolding, rotational casting or rotoforming — starts by charging resin into the mold. The mold is then rotated continuously about two perpendicular axes while being heated. Under the combined action of gravity and heat, the resin gradually spreads and fuses evenly across the entire cavity surface, taking the required shape. The part is then cooled, set and demolded.

9. Ram Extrusion

In ram extrusion, a billet held under triaxial, non-uniform compressive stress is forced through the opening or gap of a die. Its cross-sectional area is reduced while its length increases, producing the required profile.