What are the injection moulding alternatives?

Conventional injection moulding is one of the most common processes used to create plastic components

However, as injection moulding technology has evolved and consumer demands have changed, alternative processes have been developed to meet them. As well as creating much smaller and more complex parts, the process has also adapted to the changes that Industry 4.0 has brought to manufacturing.

To give you an overview of the processes that are out there and the type of products they produce, we’ve outlined a few of the most useful alternatives.

Gas-assisted injection moulding

The injection of gases can help to achieve features such as hollow cores – and reduced weight – in a plastic component. Popular in the medical sector, the process introduces a gas, typically nitrogen, into the mould cavity after it has been filled with plastic. The compressed nitrogen then displaces some of the molten plastic, resulting in a hollow part.

These parts are lighter than typical injection moulded components and yet cheap to manufacture. The components can also have enhanced strength and stiffness, even at lower weights. With the added benefit of reduced cooling times, the injection moulding cycle times can also be reduced.

Water can be used in a similar process to create hollow, strong parts. Water injection moulding is said to be even more effective than gas-assisted injection moulding at reducing cooling, and thus cycle, times. This is because water directly cools the part from the inside. However, both are equally effective at creating strong, hollow components.

Co-injection moulding

Engineers may want to combine two materials for a number of reasons. For example, they may want to save money by using a cheaper fill material, or combine polymers with different properties, whether they’re different colours or alternative mechanical qualities such as shock-absorbency.

Co-injection moulding is the method through which engineers can combine these materials. The technology has developed to combine polymers in a cavity that is filled in the same amount of time as a conventional injection mould.

Some companies have also developed co-injection moulding systems with separate streams for each plastic to enter the mould, or separate streams that go through the skin and core of the component. Resins can also be combined by co-injection nozzles at the injection mould gate before entering the cavity, dependent on the specific needs of the component.

Mono-sandwich injection moulding

In the mono-sandwich method of injection moulding, two materials are injected into the mould via one passage into the cavity. The barrel of a standard injection moulding machine is filled with two types of plastic, with a different type of plastic at each end. The first enters the mould and forms a skin as it hardens on the mould surface. The second material follows and forms the core of the moulded component.

This process is great for creating rigid, strong parts that also have attractive textural surfaces, such as an elastomeric skin. It can also produce a foamed core with a solid skin, reducing material consumption, weight and cost. This means that the core can be produced from plastic that is unsightly or discoloured without damaging the aesthetics of the final component.

Two-shot injection moulding

Two-shot, or dual-shot, injection moulding produces parts that use two different polymers combined in a mould with two separate cavities and supplied by two injection units. A substrate is produced in the first cavity and then transferred to the second, where the finished part is made.
Two-shot moulding enables multi-coloured parts such as handles, grips, devices and enclosures to be created and enables the addition of soft-touch features. This process of injection moulding also allows for movable segments or components, such as hinges or springs, to be added to the finished component.

Micro-moulding

Injection moulding of very small parts requires moulding equipment that can operate with high precision. Producing injection moulded parts with a shot weight of one gram or less is classified as ‘micro moulding’. Here, the tool design can be complicated due to the minute nature of parts used.

However, it can be more cost-effective to mould small parts in a micro-moulding machine than to develop a multi-cavity tool for a standard size injection moulding machine producing very small parts. Balancing cost-effectiveness with the final component’s quality is essential to creating a customer’s perfect part, both with micro-moulding and any injection moulding process.