Ever wondered how plastic is moulded into the exceptionally useful things that we employ in our daily life? Is it as simple as melting plastic and lathering the sides of a mould with it and cooling it, much like chocolate? The answer, actually, is no. Moulding plastic is a little more complex than that. Plastic is made using a process often called plastic injection moulding.
What is this type of moulding
Plastic injection moulding is the method of manufacturing parts made of thermoplastic and thermosetting plastic by melting and forcing into moulds where they cool to form the desired object.
How does plastic injection moulding work?
The process of plastic injection moulding usually begins with an industrial designer or engineer who designs a product. This is followed up by the work of a toolmaker or mould maker who makes the mould to fit the design created. These moulds are metallic and usually made using either steel or aluminum. Using machines, they are made to acquire the exact shape desired by the design. Once this is done, the process of actually making the plastic follows. This involves thermoplastic and thermosetting plastic being fed into a heated barrel and mixed. This melted material is then forced into the cavity of a mould and there it cools and hardens to form the desired part.
Some characteristics of the process:
1. I uses melted and mixed thermoplastic or thermoset plastic as the base
2. It uses a plunger which acts like a screw or a ram to force the melted material in the mould
3. It makes a shape that is open-ended and has taken the shape of the cavity of the mould
4. It shows a parting line and gate marks on the finished products and the ejector pin marks can also usually be made out
Alexander Parkes invented plastic in 1851 in Britain. This was worked on and bettered by John Hyatt, an American inventor in 1868. He also patented, in 1872, the first injection moulding machine. In the 1940s, the need for mass production of plastic products increased and saw the invention of the first screw injection machine by inventor James Hendry of America. This increased not only the speed of production but also the amount of precise control that could be exercised on the finish of the product.
Since then, this type of moulding has been used widely in the production of everything right from milk cartons to entire car panels and automotive parts. Since it is not a very costly material, it is best suited for mass produced goods.
Advantages of this type of moulding:
1. The rate of production are very high and therefore mass production is much benefitted
2. Since tolerance levels are high, they can be repeated
3. The labour cost is very low
4. The losses in scrap are very minimal
5. The products require very minimal finishing
6. A wide range of materials can be used
Disadvantages using this type of moulding:
1. Although labor expense and scrap loses are minimal, the cost of equipment and therefore investment is high
2. The running costs have the potential to be high
3. There is also need to design the correct parts