1.2 Blowmoulding Concepts

To understand the concept of blowmoulding, imagine that this is a balloon. Further imagine a hollow plastic ball cut in half and closed around the balloon. By pinching the bottom and blowing air in the balloon, the balloon inflates and takes on the shape of the ball. Blowmoulding isn’t balloons and plastic balls - but in principle, is quite similar to what you have just seen.
Blowmoulding begins with a metal mould that is almost exactly the same size and shape as the article to be made. The mould consists of two halves, each designed to allow cold water to be circulated below its surface.

Figure 2: Parison

The mould halves close around hot plastic that has been formed into a centerless hose-like tube. This hose-like tube is called a ‘parison’. As the moulds close they pinch one end of this parison, sealing it shut. Into the other end of the parison a pin is inserted which compresses the plastic between itself and the mould. Thus creating a seal on this end of the parison as well.

Figure 3: Closed Mould

Compressed air is then blown in via a channel through the centre of this pin, generally called a ‘blow pin’. The air pressure inflates the hot plastic parison outward until it contacts the mould surface, much like the balloon did in the demonstration.

Figure 4: Blow Pin

The moulds, called ‘blowmoulds’, are cooled by the circulating water that is a temperature much lower than that of the hot plastic. Thus in contacting the cold mould surface, the hot plastic cools and sets up in the shape of the mould. The blow pin holds the object on centre as the mould halves part and break free from the plastic. The pin retracts and the hollow plastic article falls away.

Figure 5: Compressed Air from Blow Pin

Let’s review the basics of this principle. A cold two-part mould is closed around a hot plastic parison. Compressed air blows the parison out to the mould surface where the plastic is cooled into the form of the mould.

Figure 6: Product in the Form of the Mould