This is the most common method of blow moulding. There are two possible methods which may be used intermittent extrusion or continuous extrusion. For extrusion blow moulding materials having high hot strength and hot elongation are preferred.
The main method of extrusion blow moulding utilizes continuous extrusion. Since the parison is continuously being extruded it is necessary to devise methods of gripping and inflating the parison without interfering with the extrusion process. There is a variety of ways in which this can be achieved. In one case the parison is cut off and transferred to the mould which is remote from the extruder die. In another method the mould moves with the extrudate whilst gripping and inflating it. In most cases, several moulds are operating simultaneously on horizontal or vertical rotory table to utilise the capacity of the extruder. Another variation of the continuous extrusion method is to extrude downwards two parallel sheets, rather than a tube. The mould closes on the sheets trapping the inflation nozzle between them so that desired shape may be formed in the mould.
A possible problem with this method is non-uniform wall thickness distribution in the moulding. Some modern machines have overcome this by programming the thickness profile of the parison as it is extruded. This is done by adjusting the die gap automatically during the extrusion process. Another problem is that as the parison is usually formed vertically downwards, the self-weight of the parison needed for large mouldings causes draw-down and thinning of the parison. As a result the wall thickness of the moulding is difficult to control. A method of overcoming this is to use an accumulator cylinder. The extruder screw is used to plasticize the melt and supply it into a cylinder containing a piston. When sufficient material is available in the accumulator, the plunger pushes forward quickly to produce the large parison at a rate that minimizes sagging under its own weight. The blow moulding process then proceeds in the normal way.
The main materials used for extrusion blow moulding are PVC, HDPE, HMHDPE, polystyrene, polypropylene, and polyethylene. The biggest application area is for all types of bottles, but products range from small phials to large tanks and barrels, and can also include products such as crates and typewriter cases.