Plastic moulding is the process of shaping plastic using a rigid frame or mould. The technique allows for the creation of objects of all shapes and sizes with huge design flexibility, for both simple and highly complex designs. A popular manufacturing option, plastic moulding techniques are responsible for many car parts, containers, signs and other high volume items.
Because tooling can be built once and run for years, moulding is how manufacturers produce identical parts at volume — from a single-serve bottle to a 1,000 litre storage tank. The choice of process and polymer determines the wall thickness, strength, finish and cost of the finished part.
Plastic Moulding Techniques
The underlying concept of plastic moulding is placing liquid polymer into a hollow mould so that the polymer can take its shape, often with various ranges of pressure and heat required. There are different plastic moulding techniques available to accomplish this, and each has its benefits and is best suited to the creation of specific items.
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Rotational Moulding
Bi-axially rotated moulds produce large, seamless hollow parts such as tanks, bins and playground equipment. Low tooling cost.
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Injection Moulding
Molten polymer forced into a mould at 10,000–30,000 psi. The workhorse of high-volume precision parts.
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Blow Moulding
Air inflates hot polymer against the mould wall. The standard route to bottles, jars and hollow containers.
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Compression Moulding
A preheated slug pressed between heated mould halves. Suited to complex, high-strength automotive components.
Polymers Used in Moulding
Most manufacturers choose polyethylene for the moulding process because of its availability, ease of use and suitable properties. According to industry reports, 80–90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LDPE and LLDPE), although PVC, nylons and polypropylene are also widely used.
The History of Plastic Moulding
The history of rotational moulding began somewhere between 1940 and 1950 in the USA, when the process was developed for a small number of plastics. By the late 1950s, when the rotational moulding process was better understood, applications for other industries were developed including road cones, marine buoys and car armrests. A process developed in Europe in the early 1960s enabled large hollow containers to be created in low density polyethylene (LDPE) by rocking a mould on a chassis through 30 degrees over open gas jets, coating the inside of the mould with polymer.
The history of plastic injection moulding has seen steady industry growth since its beginnings in the late 1800s. The technique has evolved from the production of combs and buttons to major consumer, industrial, medical and aerospace products. In 1868, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould.