The Definitive Guide to Plastic Injection Moulding

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. Yet, despite its fast, high production abilities, the injection moulding process must be tightly controlled to maintain the quality of the final parts.

In this ‘ultimate guide’, you will learn about plastic injection moulding, what makes high-quality components, whether they are the right choice for your needs and how to find the best injection moulding partner.

• What is plastic injection moulding?
• How does plastic injection moulding work?
• What injection moulding parameters need to be controlled?
• What are the benefits of plastic injection moulding?
• What are the alternatives to injection moulding?
• Why should you choose injection moulded components?
• The future of injection moulding

What is plastic injection moulding?

Plastic injection moulding is a complex manufacturing process commonly used to create plastic components. 

The ability of injection moulding to produce thousands of complex parts quickly makes it the perfect process for the mass production of plastic components. 

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.

The process starts when steel or aluminium moulds are selected and installed into a specialised hydraulic or electric machine. This machine then melts and injects thermoplastic at high speed into the mould, which is clamped under pressure, before being cooled to form the final part.

Developed in the 1800s, the injection moulding process has since been used to quickly mass produce components and other products. Today, it is still a widely used manufacturing process. However, injection moulding manufacturers are integrating smart technologies and Industry 4.0 innovations into the process to further improve its efficiency and sustainability. 

When the parameters of the process are controlled correctly, there’s little need for finishing and processing the manufactured part, making it a cost-effective and efficient way of creating multiple complex parts at once.
 

How does plastic 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. This includes selecting the right thermoplastic, mould, temperature and injection pressures to ensure the final part meets customer requirements.

But before we talk about the specific parameters that need to be controlled within the process, how does plastic injection moulding actually work?

Step 1: selecting the right thermoplastic and mould

Before the actual process begins, the right thermoplastics and moulds need to be selected. To do this, manufacturers need to consider how the thermoplastic and mould interact together, as certain types of plastics might not be suitable for particular mould designs.

If an appropriate mould doesn't already exist for the desired component, one needs to be prototyped and tested with the right thermoplastic. Thanks to advances in computer-aided design (CAD) technology, much of the prototyping can be conducted digitally. 3D printing can also be used to create a prototype mould, though the final tools are typically steel or aluminium moulds.

Alongside the mould, the right thermoplastic needs to be chosen. Each thermoplastic offers different characteristics, including temperature and pressure resistances. These specific properties will make them appropriate for use in certain moulds and components.

The most common thermoplastics used in plastic injection moulding 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.

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

Step 2: feeding and melting the thermoplastic

At their most basic level, injection moulding machines consist of a feeder or ‘hopper’ at the top of the machine; a long, cylindrical heated barrel, in which a large injection screw sits; a gate, which sits at the end of the injection unit; and the chosen mould tool, which the gate is connected to.

To start the process, the plastic pellets of choice are fed into the hopper. As the screw turns, these pellets are fed gradually into the barrel of the machine, which is temperature controlled. The turning of the screw and the heat from the barrel gradually warm and melt the thermoplastic.

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. Otherwise, the thermoplastic may overheat and burn or scorch the final part.

Step 3: pre-injection process

Before the molten thermoplastic is injected, the mould, which is usually made up of a fixed half called the cavity and a moving half called the core, closes. These two parts are held together 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 molten polymer 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.

Step 4: injecting the plastic into the mould

Once the right pressure in the tool and screw is reached, the gate opens, the screw moves forward, and the molten polymer is injected into the mould. The gate helps to control the flow of the plastic so no excess forms.

To make sure no damage is done to the final components, it’s important that the manufacturer monitors the pressures and temperatures and has the expertise to maintain and use the mould tools correctly. This ensures they are creating high-quality and consistent parts from their injection moulding process.

Step 5: forming the part

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 packing out the tool with plastic and forming the part 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’ and can also range from a few seconds to some minutes. This time ensures the component sets correctly before being ejected and finished on the production line.

Step 6: ejection and finishing processes

After the holding and cooling times have passed and the part is mostly formed, ejector pins or plates push the components 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. This 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 customers.

What injection moulding parameters need to be controlled?

To ensure the components produced are of consistently high quality, the following process parameters need to be controlled:

• The thermoplastic used to create the part
• The pressures used in the process
• The tooling (mould) used to shape the part
• The temperature of the thermoplastic and process

Understanding how these elements need to be monitored and adjusted will help customers choose an experienced manufacturer.

Tooling

An injection moulding tool is typically made up of two halves, the core and the cavity. The cavity is the hollow section which the molten plastic flows into and the core is the solid half that fills the cavity to form the final part.

The design of both the mould cavity and core changes to suit the final form of the component e.g. whether it’s a cap or a plug. One mould tool can contain multiple cavities and cores, meaning more than one component can be created in a single injection moulding cycle. Other components made using injection moulding include fasteners, fibre wire, cable management, PCB hardware and high temperature masking components.

The tool selection also affects the type of thermoplastic used, the injection and clamp pressures and the plastic temperature. For example, successful injection moulding for large multiple cavity moulds requires different thermoplastics, temperatures, and pressures than a mould for a single, small part.

Selecting or developing the right steel or aluminium moulds and using the correct parameters will shorten cycle times and make production more efficient. So, although it can often take some time and cost investment to find or create the right tool, it’s key to making the process more efficient and accurate.

Thermoplastics

Thermoplastics are polymers that become soft when they are heated and solidify when cooled.

This characteristic is a result of these polymers’ molecular structure, which has weak electrical bonds between the repeating monomer molecules. These molecular characteristics also mean thermoplastics have great electrical properties, a low coefficient of friction (COF) value and dimensional stability.

Plus, different thermoplastics have specific molecular structures and material characteristics. The different types can be broken down into two main categories: amorphous and semi-crystalline polymers.

The molecular chains within semi-crystalline polymers are regularly structured and tightly packed. This ‘crystalline’ quality means these thermoplastics have an organised molecular structure that's susceptible to radical change when overheated.

The key characteristics of semi-crystalline polymers are:

• The rapid change from solid to molten states at specific melting points. This makes moulding these polymers a temperamental process.
• Tensile strength, impact resistance and durability.
• Typically opaque aesthetic qualities.
• Difficulty bonding to other plastics or materials using adhesives or solvents.

For example, polyvinyl chloride (PVC) polymer is a crystalline thermoplastic as are acrylonitrile butadiene styrene ABS and polypropylene (PP).

In contrast, amorphous polymers have a more fluid molecular structure. With chains of molecules that are randomly ordered and layered around each other, the qualities of these polymers include:

• A more gradual state change from solid to molten plastic as temperatures increase, making it easier to mould and remould.
• Greater flexibility and a translucent aesthetic quality.
• Easily bonded to other plastics and materials using adhesives and solvents.
• Lower weight and friction resistance than semi-crystalline plastics.

Amorphous and semi-crystalline plastics can be broken down further into one of three categories:

• High performance
• Engineering
• Commodity

These categories are based on the cost, temperature resistance and strength of these plastics, with high-performance having the greatest of all these features and commodities having the lowest.

To choose the right thermoplastic for your part, you need to consider the functional requirements and essential characteristics of the final component. This will help you to determine whether you need an amorphous or semi-crystalline thermoplastic and whether this needs to be a high-performance, engineering or commodity-level polymer.

Process parameters and controls

There are two main parameters that need to be tightly controlled in injection moulding: temperature and pressure.

The key controls that need to be considered are the temperature of:

• The thermoplastic, as some polymers can withstand more heat than others. Ensuring an optimum temperature will keep the polymer in the best material state for injection i.e., not too molten or too solid.
• The barrel and screw of the machine, which keep the polymer in an optimum state ready for injection. The screw within the barrel also causes friction with the plastic, producing heat that needs to be considered to avoid burnt plastic.

There are also two types of pressure that need to be tightly controlled:

• Clamp pressure, which holds the mould cavity and core of the tool together. If correct, this stops the tool from opening or breaking during injection and means the component is formed correctly within the tool.
• Injection pressure, which pushes the thermoplastic into the tool. This ensures the component forms correctly. Too little pressure and the part won’t form fully, too much pressure and plastic scorches or warpage can occur.

Controlling both parameters means injection moulding can run efficiently and create high-quality components in every cycle.

What are the benefits of plastic injection moulding?

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 a variety of components can be produced, including complex and highly detailed ones.
• Efficiency: once the process has been set up and tested, injection moulding can produce thousands of items per hour without wasting energy or materials.
• Consistency: if the process parameters are tightly controlled, injection moulding 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 they're produced in high numbers.
• Quality: whether manufacturers are looking for strong, tensile or highly detailed components, injection moulding can produce them at a high quality repeatedly.

However, customers may choose an alternative to injection-moulded parts depending on their specific requirements.

What are the alternatives to injection moulding?

There are two main alternatives to injection moulding which can deliver the same level of accuracy and flexibility: 3D printing and blow moulding.

3D printing

3D printing is a rapid manufacturing process where a specialist machine creates each part individually using a software CAD design. This makes it a great option for producing small batches of components or prototypes.

However, as each part is created one at a time, consistency can be difficult to achieve, and the process is not an efficient or cost-effective mass production process

Blow moulding

Blow moulding is a process where heated plastic is pushed into a cavity using air to form a hollow part. Without the need for a specialist tool, blow moulding is a simpler, lower-cost process than injection moulding.

Advantages include lower costs and one-piece construction.

Other differences between blow moulding and injection moulding include:

• Blow moulding makes one-piece components (with no parting lines) while injection moulding produces tens or hundreds of components at once.
• Injection moulding offers more design complexity and flexibility compared to blow moulding.
• Blow moulding has more process parameters to control, leading to a higher degree of inaccuracy in these parts.
• The parts created by blow moulding tend to be thin walled, meaning they’re not suitable for all applications.
• Blow moulded parts tend to need processing after production due to the presence of the parison (tube of softened plastic inside the final mould). In contrast, injection moulded parts don’t always need post-production processing.

Why should you choose injection-moulded components?

Injection-moulded components are the parts of choice for many Original Equipment Manufacturers (OEM) because of their advantageous characteristics, including:

• Consistently high-quality products from a tightly controlled process.
• Low-cost and efficient production, particularly at scale.
• Great design flexibility including a choice of mould and thermoplastic.

If you think injection-moulded components might be the best option for you, then it’s important to order and review samples from the manufacturers you're considering. This will give you to determine whether they have the quality and characteristics you require.

When inspecting your samples, keep an eye out for some of the most common faults that occur within injection moulded components.

• Flash is the name for a burr of excess material that forms on the edge of a component, usually in the split or parting line, where the two parts of the mould tool join together. This fault is caused by plastic escaping from the mould and cooling with the rest of the part.
• Gassing and venting are caused when air is trapped in the mould during injection and a small explosion occurs. This can cause holes or burn marks within the final component.
• Shorts, which is when the plastic stops short of filling the whole mould. This means parts of the component will be missing or damaged.
• Distortion or warpage, which causes indents or parts of the component to become thinner, weaker or bent. This is caused by a lack of hold pressure or cooling time after plastic injection.

As well as looking for faults within samples of a manufacturer’s components, it’s also essential that you ask them about their tolerances. Any experienced manufacturer should have narrow dimensional tolerances of between +/-0.1mm to +/-0.25mm depending on the type of part. They should also be able to tell you about their validation and quality control processes, so you can be reassured that you're getting the high-quality parts you need.

The future of injection moulding

There are three main aspects that will influence the future of injection moulding: plastic development, advances in machinery and customer demands.

Plastic development

As thermoplastic science has progressed, new injection moulding materials with a variety of qualities and characteristics have been developed.

These qualities include flexibility, the ability to resist high temperatures, varying levels of transparency, strength, durability, a wider variety of colour options and enhanced electrical conductivity and lack of friction.

As this development continues, customers will have greater choices when it comes to the characteristics of their plastic parts, such as anti-bacterial properties, for example.

Plus, polymer science is also enabling experienced manufacturers to integrate recycled plastics into their production process. This means less waste material is created and the demand for virgin plastic decreases, reducing production costs and environmental impact.

Indeed, some manufacturers can produce plastic parts with 98% recycled polymer, bringing significant sustainable benefits to them and their customers.

Advances in machinery

Changes in technology have had a real impact on the efficiency of injection moulding. In particular, Industry 4.0 technologies, including Big Data and the Industrial Internet of Things (IIoT), have helped manufacturers maximise the performance of their equipment.

Introducing automation and robotics into the process means its parameters can be closely monitored and controlled, increasing its precision and accuracy. This gives manufacturers the ability to create consistently high-quality components more easily and efficiently.

In addition, switching from hydraulic powered to electric machines has had a huge impact on manufacturers' production and energy efficiency. This has helped them reduce their operational cost and reduce their carbon emissions.

Essentra Components is embarking on an investment programme to introduce a complete electric machinery portfolio by 2031. This will cause an estimated 33% reduction in energy usage and significantly increase productivity while taking up a lower factory footprint.

Customer demands

Shifts in customer priorities are already starting to influence the world of injection moulding.

Alongside having a green and efficient supply chain, customers are starting to focus more closely on the accuracy and quality of their components. This is because inaccurate or low-quality parts can have a huge impact on their lead times and production budgets.

Experienced manufacturers are starting to adapt to this by:

• Increasing investment in upskilling their workforce and maintaining up-to-date training.
• Emphasising the importance of expertise to tightly control and maintain quality.
• Offering factory tours and inspections to customers and their in-house experts to maintain an honest, open conversation on quality standards and processes.

By making these changes, Essentra Components is working to be at the forefront of the injection moulding industry. As well as future-proofing the business, it ensures that Essentra Components' customers get the quality standard of parts they require when they need them with a hassle-free service.