Chapter 4: The Extrusion Blow Moulding System

4.1 Extruder

This chapter will begin with a basic description of a single-screw extruder machine (it is noted here that there are twin-screw machines mainly used in blow moulding where a powdered resin is used such as polyvinyl chloride (PVC)). Imagine a simple old fashioned mincing machine as used in a kitchen, as shown in Figure 4.1.

Figure 4.1: Basic extruder-kitchen mincer

This consists of a barrel with an arbor (screw), which turns by use of a crank handle. Chunks of meat are fed into the barrel through a hopper; at the end of the barrel is a die through which the meat is extruded. The process causes the meat to move through a forced zone, then a compression or transition zone, on to the metering zone through the spirals on the screw, and then through the holes in the die. All these basic elements are contained in an extrusion machine (see Figure 4.2).

Figure 4.2: Single screw extruder

4.2 Drive

The extruder s drive system consists of a gearbox, screw bearings, and a motor that is linked to the gearbox by gears or belts. The system is designed to support the screw in the barrel and to rotate the screw at a specified speed, this being selected from a range of speeds. Once the speed has been selected, it should remain constant despite changes in the load on the screw or the temperature of the drive system. The turning force (torque) supplied by the drive must overcome the resistance of...