The blowmoulding process begins in the extruder portion of the machine. An extruder is made up of a long, hollow metal tube called a ‘barrel’. On the outside of the barrel are electric heaters. Into the barrel fits a threaded shaft, which resembles a large screw, in fact it is known as the ‘extruder screw’. It is turned inside the barrel by a motor and drive system.
Plastic pellets are fed into the back end of the extruder, the turning motion of the screw pushes the pellets forward, the electric heaters and the mixing action of the screw soften the pellets as they move toward the front of the barrel. When they reach the end, the pellets have melted and mixed into a hot fluid-like matter.
Attached to the end of the extruder barrel is a unit designed to form the hot plastic mass into parison. This unit is called the ‘extruder head’ and is a series of precisely machined metal components containing flow channels and special parts. These parts, referred to as the ‘head tooling’, form the plastic mass into centerless hot plastic posts, called parisons.
When it is desirable to blowmould multiple containers simultaneously, the extrusion head is where the plastic mass can be divided into two or more streams, thus two or more parisons. This is an extruder seen from one side – it is an intricate part of extrusion blowmoulding machine. This is the motor which turns the screw, located inside the barrel. The electric heaters that help soften the pellets are these shiny silver bands.
The plastic pellets fall from the hopper into an opening in the barrel onto the screw. The pellets are melted and mixed as they push forward toward the extrusion head. Shown here from the front of the extruder, this head has divided the plastic mass into three streams and has formed three continuously extruding parisons.
Thus, the first essential component for blowmoulding is an extruder which converts plastic pellets into one or more continuous hot parisons.
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