The Four Common Plastic Moulding Types

There are many methods of producing thermoplastic products and the general name for these is Moulding. The products currently stocked by our business are produced by variations of 4 main methods. A simple description of each method follows but it is vital that you visit suppliers to gain a first hand understanding. Mould and Tool are the words commonly used to describe the mould which shapes the plastic.

Injection Moulding

Plastic granules are heated in a screw mechanism until they melt. The plastic is squirted or "injected", at very high pressure, into a mould which is formed from two pieces of steel. The mould is held together by a hydraulic ram which prevents the pieces coming apart while the plastic is injected. Cold water is run through the mould, cooling the plastic and allowing it to harden. The hydraulic ram then withdraws one half of the mould allowing the item to be removed. The ram pushes the mould pieces together again and the process is repeated.

Rotational Moulding

The inside of the mould creates a hollow shape (imagine a hollow sphere). The mould is able to be split in half. Very fine plastic powder is put inside the mould and the two halves are joined together. The mould is heated from the outside while it is rotated in two planes or axes. The plastic powder runs around the entire inside of the mould and slowly melts. When the inside of the mould is evenly covered with molten plastic the outside of the mould is cooled and the plastic solidifies. The two pieces of the mould are pulled apart, the product is removed and more powder put in.

Blow Moulding

Plastic granules are extruded (heated and forced through a die) into a cylindrical shape. The two halves of the mould enclose the extrusion and hot air is forced in, "blowing" the plastic into the shape of the inside of the mould. Injection/Blow Moulding is a similar method except that an injection moulded "preform" is heated and blown.

Vacuum Forming

A sheet of plastic is heated and sucked into a mould, then trimmed. Draw is the term for the depth to which the plastic is sucked.

Comparison of Plastic Moulding Methods

	Injection Moulding	Rotational Moulding	Blow Moulding	Vacuum Forming
Mould Cost	High	Low	High	Low
Unit Cost	Low. Very quick moulding times.	High. Very slow moulding times.	Low. Very quick moulding times.	Variable. Some small items are very fast.
Limitations	Can't mould spherical shapes. Largest item 1.6m x 1.3m x 1m.	All external features, e.g. nesting lugs must be hollow. Not very accurate. The inside surface is not completely smooth.	Largest item 200 litres. To get a good thread on the neck of a bottle injection/blow moulding is desirable.	Depth of draw, max 350mm. Can't mould spherical shapes. Tends to be expensive for larger items as materials have already been formed into sheet.
Typical Products	Cases, Crates Boxes, Shovels, Caps, Box Pallets, Pottles.	Water Tanks, Transit Tubs, Box Pallets, Barrels.	Bottles, Barrels, Jars, a few crates.	Trays, Pottles.
Materials	Almost all polymers.	LDPE, MDPE, EVA, Polypropylene.	Almost all polymers.	Most polymers usually amorphous.