What is plastic injection moulding and how does it work?

animated image of an injection moulding machine

Plastic components are used in many industries. From automotive to home appliances and medical devices, components in a variety of plastics are used to protect, enhance and build a huge range of products.

With its reliable, high-quality performance, injection moulding is one of the most common processes used to produce plastic components. Indeed, the compound annual growth rate (CAGR) of the global injection moulded plastics market is expected to increase by 4.6% up to 2028.

Yet, despite its ability to produce high numbers of plastic components quickly, the injection moulding process must be tightly controlled to maintain the quality of the final parts. This article will explain how injection moulding works and how experienced manufacturers control the process to produce the best quality plastic components. We'll cover:

 

What is injection moulding?

Injection moulding machine

Injection moulding is a complex manufacturing process. Using a specialised hydraulic or electric machine, the process melts, injects and sets plastic into the shape of a metal mould that’s fitted into the machine.

Plastic injection moulding is the most widely used components manufacturing process for a variety of reasons, including:

  • Flexibility: manufacturers can choose the mould design and type of thermoplastic that’s used for each component. This means the injection moulding process can produce a variety of components, including parts that are complex and highly detailed.
  • Efficiency: once the process has been set up and tested, injection moulding machines can produce thousands of items per hour. Using electric injection moulding machines and other innovative energy reduction initiatives makes the process relatively energy efficient.
  • Consistency: if the process parameters are tightly controlled, the injection moulding process can produce thousands of components quickly at a consistent quality.
  • Cost-effectiveness: once the mould (which is the most expensive element) has been built, the cost of production per component is relatively low, particularly if created in high numbers.
  • Quality: whether manufacturers are looking for strong, tensile or highly detailed components, the injection moulding process is able to produce them at a high quality repeatedly.

This cost-effectiveness, efficiency and component quality are just some of the reasons why many industries choose to use injection moulded parts for their products.

How does injection moulding work?

Although on the face of it, the injection moulding process may seem simple, there are many parameters which need to be tightly controlled to ensure the overall quality of the plastic components produced. Understanding the process and parameters in some depth will help manufacturers to identify plastic components producers who can provide the quality and consistency they need.

Step 1: selecting the right thermoplastic and mould

Before the actual process begins, it’s key that the right thermoplastics and moulds are selected or created, as these are the essential elements that create and form the final components. Indeed, to make the right selection, manufacturers need to consider how the thermoplastic and mould interact together, as certain types of plastics might not be suitable for particular mould designs.

Each mould tool is made up of two parts: the cavity and the core. The cavity is a fixed part that the plastic is injected into, and the core is a moving part that fits into the cavity to help form the component’s final shape. Depending on requirements, mould tools can be designed to produce multiple or complex components. The repeated high pressures and temperatures that mould tools are put under mean they are typically made from steel or aluminium.

Due to the high level of design and quality of materials involved, developing mould tools is a long and expensive process. Hence, before creating a final bespoke mould, it’s recommended that tools are created, prototyped and tested using computer aided design (CAD) and 3D printing technology. These tools can be used to digitally develop or create a prototype mould that can then be tested in the machine with the chosen thermoplastic.

Testing the tool with the right thermoplastic is key to ensuring that the final component has the right properties. Each thermoplastic offers different characteristics, temperature and pressure resistances due to their molecular structure. Plastics with an ordered molecular structure are called semi-crystalline and those with a looser structure are known as amorphous plastics. Find out more in The difference between amorphous and semi-crystalline plastics.

Each plastic’s properties will make them appropriate for use in certain moulds and components. The most common thermoplastics used in injection moulding and their characteristics include:

  • Acrylonitrile-Butadiene-Styrene (ABS) – with a smooth, rigid and tough finish, ABS is great for components that require tensile strength and stability.
  • Nylons (PA) – available in a range of types, different nylons offer various properties. Typically, nylons have good temperature and chemical resistance and can absorb moisture.
  • Polycarbonate (PC) – a high-performance plastic, PC is lightweight, has high impact strength and stability, alongside some good electrical properties.
  • Polypropylene (PP) – with good fatigue and heat resistance, PP is semi-rigid, translucent and tough.

The final thermoplastic selection will depend on the characteristics that manufacturers need from their final component and the design of the mould tool. For example, if a manufacturer needs a lightweight part with electrical properties, then PC will be appropriate, but only if the mould doesn’t need to operate above 135°C or at very high pressures, which the plastic won’t be able to resist.

Once the right thermoplastic and mould have been tested and selected, the injection moulding process can begin.

Step 2: feeding and melting the thermoplastic

Injection moulding machines can be powered by either hydraulics or electricity. Increasingly, Essentra Components is replacing its hydraulic machines with electric-powered injection moulding machines, showing significant cost and energy savings. At their most basic level, these machines consist of a feeder or ‘hopper’ at the top of the machine; a long, cylindrical heated barrel, which a large injection screw sits in; a gate, which sits at the end of the barrel; and the chosen mould tool, which the gate is connected to.

To start the process, raw pellets of the chosen thermoplastics are fed into the hopper at the top of the machine. As the screw turns, these pellets are fed gradually into the barrel of the machine. The turning of the screw and the heat from the barrel gradually warm and melt the thermoplastic until it is molten.

Maintaining the right temperatures within this part of the process is key to ensuring the plastic can be injected efficiently and the final part formed accurately.

Step 3: injecting the plastic into the mould

Once the molten plastic reaches the end of the barrel, the gate (which controls the injection of plastic) closes and the screw moves back. This draws through a set amount of plastic and builds up the pressure in the screw ready for injection. At the same time, the two parts of the mould tool close together and are held under high pressure, known as clamp pressure.

Injection pressure and clamp pressure must be balanced to ensure the part forms correctly and that no plastic escapes the tool during injection. Once the right pressure in the tool and screw is reached, the gate opens, the screw moves forward, and the molten plastic is injected into the mould.

Step 4: holding and cooling time

Once most of the plastic is injected into the mould, it is held under pressure for a set period. This is known as ‘holding time’ and can range from milliseconds to minutes depending on the type of thermoplastic and complexity of the part. This holding time is key to ensuring that the plastic packs out the tool and is formed correctly.

After the holding phase, the screw draws back, releasing pressure and allowing the part to cool in the mould. This is known as ‘cooling time’, it can also range from a few seconds to some minutes and ensures that the component sets correctly before being ejected and finished on the production line.

Part of the injection moulding machine

Step 5: ejection and finishing processes

After the holding and cooling times have passed and the part is mostly formed, pins or plates eject the parts from the tool. These drop into a compartment or onto a conveyor belt at the bottom of the machine. In some cases, finishing processes such as polishing, dying or removing excess plastic (known as spurs) may be required, which can be completed by other machinery or operators. Once these processes are complete, the components will be ready to be packed up and distributed to manufacturers.

Blue plastic component

How Essentra Components uses injection moulding

At Essentra Components, injection moulding is a key production process. That’s why, we have hundreds of experts in injection moulding in our manufacturing centres across the world. From developing and testing a bespoke mould to setting process parameters, the Essentra Components team has the experience you need to produce millions of consistent, high-quality parts.

Plus, with 45,000 moulds to choose from, it offers one of the largest ranges of plastic components on the market. Teamed with hassle-free customer service across 40 sales and service locations and 34 distribution centres, Essentra Components can deliver your components where you need them, when you need them.

Find out more about the injection moulded products we offer by ordering a free sample, downloading a CAD drawing or heading to our product catalogue.

Questions?

Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.