How is a servo-valve used in the blow moulding process?

A conventional extrusion blow moulding machine incorporates an annular die gap through which the moulding material is extruded to produce the tubular parison which is subsequently subjected to blow moulding to produce a product which is outputted from the machine.

The die gap can be controlled by a movable member which is displaceable by supply of hydraulic fluid to a cylinder under the control of an electrohydraulic servovalve. The servovalve is under the control of a parison programmer which is set by the machine operator to the desired wall thickness and weight settings and which receives a feedback signal from a transducer indicative of the actual position of the movable member, and which supplies an output signal to the servovalve to effect movement of the movable member to a desired position corresponding to the desired width of the die gap.

Furthermore the parison programmer may be programmed to vary the wall thickness during extrusion of the parison so that the wall thickness of the generally tubular parison varies along its length in the manner required by the form of the final moulded product, that is to take account of features such as the shaped bottom and the necked outlet of a container, for example.