Moulding process of plastics

Injection Moulding Process of plastics – a simple guide

Injection moulding process of plastics

The moulding process of plastics 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..

Injection moulding process

A manufacturing process that produces parts after injecting molten material into a mould is known as Injection Moulding process. This process can be performed by using a host of material which includes glasses, confections, metals, thermosetting polymers, and thermoplastics. The material for these parts fit inside a heated battle and mixed by a helical shaped screw. Later, this is injected in a mould cavity where it can cool down and harden to how the cavity is configured.

Once a product is designed by an engineer or an industrial designer, moulds are created by a toolmaker or mould maker from metals like aluminium and steel. A precision-machine is used to form the perfect features of the desired part. The process of Injection moulding is used to manufacture a plethora of parts that range from the entire body panels of a car to its smallest of components. The advancement of 3D printing technology that used photopolymers that don’t melt during this process of some thermoplastics at low temperature is useful for simple injection moulds.

The injection moulding process requires a machine that has three parts which are the mould, the clamp, and the injection unit.  The parts which are supposed to be injection moulded should be designed carefully to hold the moulding process, the desired features and shape of the part, the mould material, and the properties of the moulding machine. The breadth of design possibilities and consideration are responsible for the versatility of injection moulding.

Injection moulding process can be used to create multiple things like packaging, automotive parts and components, pocket combs, one-piece chairs and small tables, mechanical parts, wire spools, bottle caps, tops, some musical instruments more such as plastic products which are used in today’s day and age. This process is considered ideal for manufacturing products because it allows the production of high volumes of the same object.

A screw-type plunger is used to force the plastic material inside a mould cavity. This material solidifies and conforms to the shape of the mould. Injection moulding is widely used for processing both thermosetting polymers and thermoplastic, where the volume of the latter is higher. The characteristics of thermoplastics make them suitable for injection moulding.

The first injection moulding machine was patented by an American inventor named John Wesly Hyatt. In comparison to the machines used today, this machine was quite simple. It worked like a giant hypodermic needle that used a plunger for injecting plastic, which passed through a heated cylinder and then reached the mould. The industry had started to progress after the first establishment in 1872 and started producing hair combs, collar stays, and buttons.

There are pellets and granules made of plastic materials which are first shipped as raw materials in paper bags. In injection moulding, these pre-dried granules of plastic are rammed by a hopper to be fed into a heated barrel.

The melted plastic is then forced through a nozzle and rested against the mould with the advancement of the plunger. This allows it to run through the mould cavity from a gate. As soon as the mould is filled, it shall remain cold for solidifying the plastic.

This sequence of events is known as the injection moulding cycle. It begins when the mould is closed and followed by an injection of the polymer inside a mould cavity. After filling the cavity, the material shrinkage is compensated by holding pressure. Then the screw is turned to feed the next shot into the front screw. As the next shot prepares, the screw begins to retract.

injection moulding process
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The part is ejected by opening the mould once it becomes sufficiently cool.

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 is 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