The future of injection moulding

In this ‘ultimate guide’, you will learn about the injection moulding process, what makes a high-quality injection moulded product, find out if injection moulded components are the right choice for your needs and what considerations need to be in place before ordering.

• What is injection moulding?
  • Tooling
  • Thermoplastics
  • Process parameters and controls
• How does injection moulding work?
• What are the specific elements of the injection moulding process?
• Why you should choose injection moulding components
• What to look for when buying injection moulding components
• What are the alternatives to injection moulding?
• The future of injection moulding

Its ability to produce thousands of complex parts quickly makes it the perfect process for the mass production of plastic components. Essentially, the process involves the injection of plastic at high speed and pressure into a mould, which is clamped under pressure and cooled to form the final part.

By melting thermoplastic and injecting it into an aluminium mould at high speed and pressure, manufacturers can create multiple complex parts at once. When the parameters of the process are controlled correctly, there’s also little need for finishing and processing the manufactured part, making it more cost effective and efficient.

How does injection moulding work?

Although the process may seem simple, there are many elements involved which can alter and ruin the overall quality of the plastic component produced. In order to make a high-quality part, experienced manufacturers select the right thermoplastic (the material used to create the part), mould (which shapes the part), temperature and injection pressures to ensure the final part meets customer requirements.

Step 2: Pre-injection process

Before the molten plastic is injected, the tool, which is usually made up of a fixed half called the cavity and a moving half called the core, closes.

When closed, a clamp applies pressure to the tool, ready for the injection of the plastic.

The screw within the barrel of the machine also screws back to its set point so the plastic can enter the barrel, ready to be injected.

Step 3: Plastic injection

Once the clamp pressure is at an optimum level, the plastic is injected by the screw at high speed and pressure into the cavity. A gate inside the tool helps to control the flow of the plastic.

To make sure no damage is done to the final components, it’s important that the manufacturer monitors the injection pressure of the plastic and that they have 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 4: Forming the part

When the tool cavity is mostly full of liquid, a holding phase begins. This is where the part in held under high pressure so it can start to take its final form.

After a set holding time, the screw will screw back to its set point. This happens at the same time as the cooling phase of the cycle, which allows the thermoplastic to set in its final form.

Once the set cooling time has passed, the mould opens and ejector pins or plates push the new part out of the tool. These fall on to a conveyor belt ready to be finished and packed.

Step 5: Part finishing

Depending on the final application of the part, the moulded component may require some finishing, including dyeing, polishing, or removing of excess material.

These processes are unique to each part and are completed before they’re packed and distributed to customers.

By picking and checking products by hand, as well as performing regular quality checks, experienced manufacturers can make sure they’re producing consistent, high-quality parts for their customers.

For the final parts to meet their final specifications, manufacturers need to carefully control each of these parameters within the injection moulding process. For a detailed example, see our video

Tooling

Selecting the right tool, or mould, for a component, is key to ensure it meets a customer’s requirements. An injection moulding tool is typically made up of two halves, the core and the cavity. The cavity is the hollow section of the mould that the molten thermoplastic is fed into and the core is the solid half that fills the cavity to form the final part.

The design of both the core and the cavity will change dependent on the final form and application of the component e.g. whether it’s a cap or a plug. Within one mould tool there can also be cores and cavities available for multiple parts, so more than one component can be created in a single injection moulding cycle.

• If you’re having a custom part made, considering prototyping the part, or having test runs and asking for samples of the components will reassure you that the mould tool and thermoplastic choices are correct. Testing and prototyping will also ensure that the parameters of the process are accurate, so the final components are of a consistent high quality.

• The cost of developing, building and maintaining a tool is quite high. Offsetting this by creating or selecting a tool that creates multiple components every cycle is one way of balancing this cost. Also considering how long and how often the component will need to be produced will help to guide the tooling specification, cavity selection, cycle time and final costs.

Although it can often take some time and cost investment to find or develop the right tool, it’s key to making the process more efficient and accurate.

Selecting the right mould tool, with the correct controls and parameters, will shorten cycle times and lead to production efficiencies. Controlling elements within the process such as the dimensions of the gate and runner that feed the plastic into the tool will also make the process as efficient as possible.

Thermoplastics are plastics that become soft when they are heated and solidify when cooled. Injection moulding tends to use thermoplastics, heating up the material, then injecting it at high pressure into a mould.

Clearly, the main quality of thermoplastics is that they turn molten at high temperatures. This characteristic is a result of these polymers’ molecular structure, which has weak electrical bonds between the monomer molecules. This structure, which includes repeating units of molecular strands, means that thermoplastics are easy to soften, mould and even re-mould if necessary, as their semi-regular pattern makes them easy to recycle.

These molecular characteristics, also mean they have great electrical properties, a low coefficient of friction (COF) value and dimensional stability, which makes them an ideal choice for many functional components. However, there are different types of thermoplastics with different molecular structures and specific characteristics, so choosing the right ones is key to ensuring the final part meets its usage requirements.

The different types of thermoplastics can be broken down into two main categories as defined by their molecular structure. These are known as amorphous and semi-crystalline polymers.

The molecular chains within semi-crystalline polymers are regularly structured and tightly packed. This ‘crystalline’ quality makes the construction of these polymers very organised and susceptible to radical structural change at specific temperatures.

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 are much more flexible to temperature change than semi-crystalline thermoplastics.

Their ordered molecular construction makes them structurally strong. This means that semi-crystalline polymers are resistant under the stress of weight and repeated friction. Their ability to resist cracking and wear makes them a good choice for structural applications that require repetitive motions or bearing capabilities.

In contrast, the qualities of amorphous thermoplastics include:

• A more gradual softening from solid to molten plastic during temperature change, making it easier to mould and remould than semi-crystalline polymers.
• A more flexible structure which means they often have a translucent aesthetic quality and are easily bonded to other plastics and materials using adhesives and solvents.
• A disorganised molecular construction which makes them unable to bear significant weight or friction motions, meaning they’re best suited for structural applications that aren’t put under significant stress.

In short, semi-crystalline thermoplastics are more appropriate for heavy-duty functional components, whereas amorphous polymers suit parts that need to be more aesthetically flexible or joined to other materials.

This knowledge helps to immediately exclude certain plastics from your list based on the necessary function of your component. However, there are further classifications you can make to narrow your polymer selection further.

There are several polymers that derive from the amorphous and semi-crystalline categories, all of which have particular characteristics, as well as varied prices. Within the two main thermoplastic categories, each plastic will fall into one of three areas:

• High performance
• Engineering
• Commodity

Commonly, high performance polymers are more expensive due to their ability to withstand high temperatures and maintain their strength and chemical resistance under wearing conditions. As you move from high performance to engineering to commodity polymers, the cost, temperature resistance and strength of the plastic drops.

This means that in order to choose the right thermoplastic for your part, you need to consider the final components usage requirements and the characteristics it needs to perform its function. 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 within the injection moulding process that need to be tightly controlled to create accurate and high-quality components: temperature and pressure.

The temperature of both the thermoplastic and elements within the injection moulding machine need to be tightly controlled throughout the process. Similarly, the clamp pressure applied to the injection moulding tool and the injection pressure of the thermoplastic are key to helping the process run smoothly.

When it comes to temperature, the key controls that need to be considered are:

• The temperature of the thermoplastic, as some polymers can withstand higher temperatures than others. Ensuring that it’s at optimum temperature throughout the process will help it to be in the best material state for injection i.e. not too molten or too solid.
• The temperature of the barrel and screw where the thermoplastic is held are also important as they help to keep the polymer in an optimum state ready for injection. The screw within the barrel also causes friction with the plastic during the injection process. The heat produced by this needs to be taken into account to avoid overheating the plastic.

Why should you choose injection moulded components?

Injection moulded components are one of the main part choices for Original Equipment Manufacturers (OEM). There are many reasons why you might want to consider using them within your own product, but here are some of the main characteristics that make injection moulded components popular.

They’re high quality and strong:

• Modern plastics are strong. This means they can replace metals in some parts. Plastics can also be combined to create stronger parts.
• Controlling parameters like temperature and humidity is essential to create high-quality products. To make sure your parts are of the highest quality, it’s important that you choose an experienced manufacturer who knows how to control them.
• Many injection moulding machines are now equipped with sensors. These collect real time information and data on the machines’ performance, cycle times and send notifications when maintenance or servicing is needed.

Production is low cost and efficient:

• Injection moulding is a key mass production process. Millions of products can be made from just one single mould and a single cycle.
• The integration of automation and robotics into the process means that it has become more streamlined and cost-efficient. According to Plastikcity, there’s a 40% increase in the output of a production line if you replace one key person with a robot.
• It is also a quick process. Within one cycle, thousands of products can be moulded, solidified and ejected within a few minutes.

They offer great design flexibility:

• With injection moulds, complex shapes are easier to create compared to other moulding techniques. It’s also possible to make more than one component from a single tool, e.g. one tool can be designed to produce six of the same component within one injection cycle. The injection moulding process is also ideal for creating compact shapes such as polymer optics.
• With alternative processes like co-injection and sandwich moulding, manufacturers can combine a number of characteristics within one part for example, different colours, tactile qualities or strength.
• Modern colorants within plastics offer manufacturers and companies the best possible colour consistency. Whether it’s from a pantone chart or a custom colour, your manufacturer should be able to offer you the colour you want, consistently.

If you think injection-moulded components might be the best option for you, then it’s important you choose parts from an experienced manufacturer. They will understand the injection moulding process, which means they can meet high quality standards with their products. They may also offer additional services such as easy delivery and good lead times. Ask about the standards and accreditations they work to and check their Original Equipment Effectiveness (OEE) score. For more information, see our Short guide to finding high-quality plastic components.

What to look for when buying injection moulding components

If any of the parameters within the injection moulding process aren’t controlled correctly, then faults can start to appear within the final components. Requesting samples from any manufacturer you’re considering and performing a visual inspection for any faults will help you to determine whether they’re of a high quality or not.

Here are some of the most common faults that occur within injection moulded components and advice on how to spot them.

Tolerances

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 be able to reach tolerances of between +/-0.1mm to +/-0.25mm depending on the type of part and be able to tell you how they validate them.

By examining the tolerances and the validation processes of a manufacturer, you can be assured that the components they’re producing are of the highest quality, even when they’re produced at scale. Building this level of relationship with your manufacturer will also reassure you that their supply chains can continue to deliver the parts you need in the future to the same high-quality standard.

Asking a few questions about a manufacturer’s process can also help you to determine whether they’re the right match for your business:

• Does the supplier have a good history and background in manufacturing the type of products that the customer requires?
• What are the manufacturing capabilities of the company?
• What manufacturing and quality systems are in place to ensure delivery of good quality parts at the correct costs?
• Have you addressed considerations around their manufacturing capabilities, tooling, machinery and process control?
• Do you have a full picture of their capacity, quality control measures and staff competences?
• What are their OTIF measurements and on-time-in-full delivery performance like?

These questions will help you to determine whether a manufacturer can give you the level of service you need, as well as the amount of high-quality components you require for your products.

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. These are 3D printing and blow moulding.

3D printing

3D printing is a rapid manufacturing process that uses a specialist machine to create each part individually using a software CAD design. Typically used to create prototype parts, it’s a great process for producing small batches of components for testing or design purposes.

However, unlike in the injection moulding process, each part is created one at a time, meaning that consistency can be difficult to achieve, and the process is not efficient or cost effective. This means that 3D printing is not a great option for producing large amounts of components at scale.

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. It also produces components that are formed in one piece with no parting lines.

However, although the moulds used in injection moulding require lots of upfront investment, they offer much more design complexity and flexibility compared to blow moulding, which can only be used to create hollow parts. It’s also not possible to create a multi-cavity design and there are more process parameters to control, leading to a higher degree of inaccuracy in blow moulded parts.

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

Customer demands will continue to drive this development and change. Whether that is with plastics that are less environmentally impactful or that have special material qualities, such as anti-bacterial properties, are all potential advances that polymer science could make in the future.

Increasingly, demand for plastics with a significant post-consumer recycled plastic content is growing. With the Single Use Plastics Directive from the EU and a number of US states looking to introduce Extended Product Responsibility, the eye is firmly on manufacturers to change the way they operate. At Essentra Components in the UK, we are now able to manufacture nearly all of our LDPE products using at least 40% recycled plastics.

Other measures include ongoing energy reduction, site insulation and the roll-out of energy efficient LED lighting.

Advances in machinery

Changes in technology have had a real impact on the efficiency of the injection moulding process. In particular, the introduction of Industry 4.0 approaches using data and the Industrial Internet of Things (IIoT) has resulted in manufacturers being able to maximise the performance of equipment, thereby creating efficiencies.

Within the machinery itself, the increase in levels of automation and robotics has meant that the parameters of the process, such as the temperature of the thermoplastic and the pressure of injection, can be more closely monitored. This has increased the precision and accuracy of the process. Manufacturers are now able to create more consistent, high-quality components more easily and efficiently.

As companies look to achieve greater sustainability and reduce carbon emissions, the type of machinery can contribute to this. At Essentra Components, we are embarking on an investment programme to introduce a complete electric injection moulding machine portfolio by 2031.

Not only will this result in an estimated 33% reduction of energy but predicted process and productivity improvements will enable three electric machines to do the work of four hydraulic models. Plus, the increased capacity of these fully electric machines will reduce our overall footprint.

The data produced as a result has helped to reduce downtime, apply preventative maintenance rather than having to mend a machine and aids standardisation of components. Opening up this data to be viewed by customers without putting intellectual property at risk will be one of the main challenges the future of injection moulding will need to face.

Customer demands

Alongside changing market demands, shifts in customer awareness and priorities are already starting to influence the world of injection moulding.

The need to reduce carbon footprint is key focus for customers, and we have been able to significantly increases use of post-consumer recycled plastics in LDPE-manufactured components. Since April 2021, nearly all LDPE products produced at the UK facility are now being manufactured using at least 40% recycled plastics without any drop in quality or performance.

As well as receiving their parts quickly and efficiently, customers are starting to focus more closely on the accuracy and quality of their components.