Extrusion and thermoforming — plastic in strips and over moulds.
Two production methods that shape plastic completely differently — one endless one over a template
Injection moulding is far from the only way to form plastic. Two other methods – extrusion and thermoforming – cover a large part of the plastic products you encounter in everyday life, from water pipes and window frames to packaging trays and bathtubs. Both methods are based on the same basic principle as all plastic processing: heat the material until it is formable, give it the desired shape, and let it cool in the new form.
§Extrusion — the endless profile.
With extrusion, granulate is fed into a heated cylinder where a rotating screw melts and transports the material forward. At the end of the cylinder the melted plastic is pressed through a die — also called a nozzle — which gives the profile its cross-section. Right after the die, the profile is typically cooled in a water bath or on a calibration table which holds the precise dimensions while the material hardens. Finally, the profile is cut or wound to the length the product requires.
- 01Pipe for water, sewer and electrical installation
- 02Window frames and profiles in PVC
- 03Cable insulation extruded directly onto a conductor
- 04Plates and foils that can later be cut or thermoformed further
§Thermoforming — the sheet is formed over a mold
Thermoforming starts with an already manufactured plastic sheet, often made by extrusion. The sheet is clamped in a frame and heated until it becomes soft and malleable without melting completely. It is then formed — either by a vacuum sucking the sheet down into or up over a mold, or by a back pressure pressing it into place. When the mold is filled, the sheet is cooled so it retains its new shape and the finished part is trimmed free from excess plastic edge.
| Extrusion. | Thermoforming | |
|---|---|---|
| Starting point. | Granulate | Already manufactured plate or foil |
| Process | Continuous, long profile | Single piece or short series of one shape |
| Typical products | Pipes, profiles, cable insulation, foil | Packaging trays, blisters, bathtubs, cabins |
| Tool cost | Relatively low (nozzle) | Can be low (vacuum form) to moderate |
Post-processing
Both extruded and thermoformed parts often require post-processing. Extruded profiles are cut to size and can be bolted, welded or glued together into finished structures. Thermoformed parts typically have a protruding edge, waste edge that is trimmed off — often with a template or robot so the edge is uniform from part to part. The waste edge from thermoforming can often be granulated and reused in production if the material is clean.
“Extrusion gives you metre after metre of the same — thermoforming gives you the shape you actually need.”
— Professional rule of thumb in plastic production.