A simple guide to different plastic moulding processes

What is plastic moulding?

Plastic moulding is a process that injects molten plastics into the desired shape usually referred to as a cavity and core in a plastic injection mould. Sometimes the item will be hollow
The modern Moulding technologies can reproduce extraordinarily complex designs and shapes used in various industries catering to the automotive, material handling, engineering industries. They are equally useful for producing everyday shapes too, eg: Buckets, Mugs & Jars, Toothbrush handles etc..

How many types of Plastics are there?

Plastics can be classified into 2 basic forms-
1. Thermosetting plastic ( once formed in the required shape- thermoset plastic cannot be remelted, if reheated it will char and burn), Thermoset plastics usually cater to niche applications, mainly in heat resistance.
2. Thermoplastics can be remelted and reused.
Therefore, thermoplastics are much more versatile and useful than thermosets.

What are the different processes to mould thermoset plastics and thermoplastic?

There are a whole range of ways to mould plastics:

  • Compression Moulding is a commonly used process for thermoset plastics
  • Reaction Injection Moulding (RIM) is a process consisting of two-part reactive products, for Polyurethanes (PU) this reactive process creates the internal pressure that fills the mould with the polyurethane plastic
  • Blow Moulding is a process where molten plastic is extruded in the form of a tube and trapped into a Mould and upon inflating this tube it fills into the shape of the mould. Typical applications range from simple water bottles, water tanks to complex automotive fuel tanks etc.
  • Injection Moulding is the most common process in the plastic industry. This process is used for making articles for everyday used products like toothbrushes, plastic buckets, mugs etc to critical medical devices and automotive parts etc.. Basically nearly everything plastic. The injection moulding process forces molten plastic material under high pressure into a closed mould to form the part in the required shape, design and size.
  • Injection Stretch Blow Moulding– this process is a combination of injection moulding and blow-moulding. In this process, the thermoplastic (commonly PET) is injection moulded to form a preform. This preform is then transferred to a blow moulding station which consists of multi-cavities of the required shape of the bottle. Here the heated preform is inflated, cooled and removed. The best example of this process is the commonly used carbonated drinks bottles and mineral water bottles.
  • Rotational Moulding is another process, where thermoplastic powders are loaded into the interior ( cavity) of a preheated mould that revolves on the end of an articulated arm and the powder sticks to the walls of the hot mould. Rotational moulding is mostly used for manufacturing large tubs, troughs and large containers, especially custom made items. Rotational Moulding is a fairly primitive process that is great for small runs and small markets.
  • Expanded Polystyrene Moulding In this process, beads of polystyrene are treated with a blowing agent and using steam to expand many times( about 40 times) their original size and fuse together in a closed mould. Typical applications are the protective white blocks that light in weight and used for packaging of electronic, medical and high-value articles.
  • Slush Moulding where a liquid thermoplastic resin (usually Polyurethane) is poured into a hot hollow mould, here it forms a skin on contact with the walls of the cavity. The excess liquid is drained off and the cured part is removed from the mould. This is the preferred process by manufacturers as it gives greater freedom in the component design. Slush moulding is used to produce a variety of automotive interior components such as automotive door trim consoles, instrument panels, passenger airbag covers etc.
  • Dip Moulding is a process where a metal part is dipped into molten PVC which leaves a thin residue on the metal surface which is enough to make a cushioned handle grip. Typical examples are the soft feel handles on metal tools and other equipment.
  • Finally, other methods of processing plastics are, forming, which is different from moulding
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