Extrusion of plastics parts

Highlights

Extrusion is a manufacturing method where long lengths of the same cross section can be produced quickly and repeatedly. It is usually associated with the high volume production of thin walled parts such as channels, pipes, rails, window frames, sheets and films. However the cost and time to make an extrusion die is low compared to an injection mould tool and once this is taken into account low volume quantities of parts can be extruded in short lead times and at low cost. The factor controlling part cost is related to the minimum quantity of the material that can be run through the extrusion machine at one time. This will usually be measured by weight of material used and is therefore a function of the cross sectional area. Small batches may also be subject to a setup charge each time the extrusion die is used. This means it is more cost effective to produce a larger batch in one go than several smaller batches at regular intervals.

Process Features

Accuracy

Tolerances vary depending on geometry, material, machine setup and cooling, but can be as tight as +/-0.15mm.

Minimum Feature Size

Thin films of 0.25mm can be extruded, however for engineering parts in rigid materials a minimum wall thickness of 0.5mm is recommended.

Part sizes

Plastic extrusions have been made with cross sections over 1m in diameter, but this is unusual and expensive. Commonly extruded parts do not normally have cross sections larger than 200mm in diameter.

Materials

Commonly extruded materials include Polyethylene (PE), Polypropylene (PP), and Polyvinylchloride (PVC). Engineering materials such as Acetal (POM), Polystyrene (PS), Nylon (PA) and Acrylonitrile Butadiene Styrene (ABS) can be processed but with increasing complexity, difficulty and cost; Polycarbonate (PC) being the most challenging and expensive.

Post Processing

Extruded sections are cut into individual lengths as required. Numerous different lengths can be produced from the same section. Additional machining operations such as hole drilling or milling can be carried, but this requires additional tooling to hold the extrusion and significantly increases piece part costs.

Description

Plastic pellets loaded into a hopper above the extrusion machine are fed down into the extruder screw. As the screw rotates it moves the pellets forward and compresses them; generating heat and causing the material to become molten. Additional heaters around the barrel further control the temperature of the plastic. After passing through a screen assembly that filters out contaminants and stabilises the pressure, the molten plastic is forced through a die. The die is a steel plate with a profile cut into it that matches the shape of the desired extrusion cross section. As the extruded plastic emerges from the die it is guided through a series of water baths to extract the heat and harden the material. After exiting the baths the extruded profile is cut to the correct length by a saw or coiled onto a drum.

For more information, please contact:  stefan@smartprototype.net