The extrusion process may be very easily visualized perhaps the best examples are the meat grinder and the toothpaste tube. However the very simplicity of these examples belies the complexity of the process and extrusion technology is highly developed both as a science and as an art.
The extrusion line
The extrusion line begins with the hopper which holds the plastic material (in either powder or granule form). The hopper continuously feeds the material to a heated barrel which contains a rotating screw. This screw transports the polymer to the die head and simultaneously the material is heated, mixed, pressurized and metered. Needless to say the detailed design of an extruder screw is extremely complex in order to perform all the above tasks. At the die the polymer takes up the approximate shape of the article and is then cooled either by water or air to give the final shape. As the polymer cools it is drawn along by haul-off devices and either coiled (for soft products) or cut to length (for hard products).
The layout of an extrusion line
The processes during extrusion
The typical extrusion line described above can be used. with modifications. to produce a wide variety of products. Some typical examples are:
Wire coating: for all types of wires and cables.
Monofilament: for rope, bristles and synthetic textile fibers.
Blown film: for plastic bags, plastic film and heat shrinkable film for food packaging.
Sheet extrusion: for sign production, refrigerator interiors and even small boat hulls. When a clear sheet is produced it can be used in glazing or lighting applications.
Pipe and tube: plastic tubing is used for garden hose, industrial hose, food and drink, transport and hydraulic or pneumatic control. Plastics pipe is used for water, gas, agricultural drainage, sewers and drains. New developments allow plastics pipe to replace copper pipe for heating and hot and cold water services.
This is probably the most interesting area for engineering designers and the possibilities are virtually limitless. The initial constraint of a constant cross section is overcome in many applications by fabrication techniques such as cutting, drilling welding and stamping and by innovative processing techniques such as co-extrusion of soft and hard polymers, multiple color extrusion and in-line application of decorative foils or adhesive tapes.
Most common thermoplastic polymers can be used for extrusion and the material choice is dependent on both the performance requirements and on the economic constraints.
The most commonly used material for general purpose extrusions is PVC. The wide application of this material is due to cost, chemical resistance and its availability in various hardnesses and colors. The hardness of PVC can vary from the rigid type used for windows (Shore "A" hardness of 100 or British Standard softness of 0) to the plasticized or soft version used for garden hoses (generally Shore "A" 80 deg or BSS 38) and even down to very soft materials of Shore "A" 60 deg (BSS 75) which have limited uses. The color can be either matched to a color sample or chosen from several hundred standard colors. PVC is a very versatile material but, as with all materials, there are limitations and again specialist advice should be sought for critical applications.
While plastics extrusions can be produced to consistent tolerances the designer must be aware that these are not the same as for machined parts or for metals extrusion and are generally greater. The tolerance bands applicable vary with the relevant dimension, the material used and with the manufacturer but in general BS 3734:1978 for extruded rubber products (Table 2 Class E 2) can be used as a guide. Specific tolerances for critical areas and non-critical tolerances must be discussed and agreed between customer and producer. Inevitably, the unit price increases with the number of tolerance dimensions and the tightness of the tolerances specified.