The future of injection molding
10 minutes | 30 Jun 2021
In this ‘ultimate guide’, you will learn about the injection molding process, what makes a high-quality injection molded product, find out if injection molded components are the right choice for your needs and what considerations need to be in place before ordering.
- What is injection molding?
- Process parameters and controls
- How does injection molding work?
- What are the specific elements of the injection molding process?
- Why you should choose injection molding components
- What to look for when buying injection molding components
- What are the alternatives to injection molding?
- The future of injection molding
Injection molding is a manufacturing process which is commonly used to create plastic components.
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 mold, which is clamped under pressure and cooled to form the final part.
By melting thermoplastic and injecting it into an aluminium mold 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.
Although it’s one of the oldest manufacturing processes around, its speed and cost-efficiency is what continues to make it a popular choice with worldwide manufacturers. Today’s injection molding machines are fast, accurate and produce consistently high-quality components at scale.
How does injection molding 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), mold (which shapes the part), temperature and injection pressures to ensure the final part meets customer requirements.
Before we talk about the specific parameters that need to be controlled within the process, how does injection molding actually work?
Step 1: Feeding and heating the plastic
To start, a thermoplastic or combination of thermoplastics are fed into an injection molding machine. The plastics, which turn to liquid when heated, are fed into the hopper at the top of the machine in solid pellet form.
The pellets pass through the machine and into a temperature-controlled cylinder called the machine barrel. Here, the plastic pellets are heated until the thermoplastic is molten.
The temperature of the barrel and the plastic needs to be carefully monitored to make sure the thermoplastic doesn’t overheat and burn or scorch the final part.
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 mold tools correctly.
This ensures they are creating high-quality and consistent parts from their injection molding 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 mold 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 molded 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.
Within the process of injection molding, there are lots of parameters that need to be tightly controlled to ensure that the components produced are consistently of a high quality.
- The thermoplastic used to create the part.
- The injection pressure used in the process.
- The tooling, or mold used to shape the part.
- The temperature of the thermoplastic and process.
For the final parts to meet their final specifications, manufacturers need to carefully control each of these parameters within the injection molding process. For a detailed example, see our video
Selecting the right tool, or mold, for a component, is key to ensure it meets a customer’s requirements. An injection molding tool is typically made up of two halves, the core and the cavity. The cavity is the hollow section of the mold 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 mold tool there can also be cores and cavities available for multiple parts, so more than one component can be created in a single injection molding cycle.
As well as determining the shape of the final part, the tool also effects the type of thermoplastic used, as well as its injection pressure and temperature. The amount of clamp pressure that needs to be applied to tool will also change depending on the mold that is selected. The size of the tool, as well as the number and complexity of the cavities within the tool will affect the temperature and injection pressure of the thermoplastic being used.
To choose the right tool for a component, manufacturers need to consider:
- How the final component will be used, for what purpose and in what type of environment. This will determine the final characteristics it needs to have and therefore which thermoplastic and mold tool are used.
- 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 mold 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 mold 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.
Plus, if your tool is properly maintained, designed and built and the parameters of the injection molding process are accurate, any tool should be able to produce parts for thousands of process cycles with very few faults.
Making sure the right grade of steel is selected and the correct cooling channels are put within the mold will ensure the tool doesn’t wear or delay the molding process. This means the parts will always be accurate and consistently high quality.
What are thermoplastics and why are they important in the injection molding process?
Thermoplastics are plastics that become soft when they are heated and solidify when cooled. Injection molding tends to use thermoplastics, heating up the material, then injecting it at high pressure into a mold.
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, mold and even re-mold 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.
These different molecular structures give each type of thermoplastic different characteristics. The key characteristics of semi-crystalline polymers are:
- They change from solid to molten quickly at a specific melting point, so molding these polymers can be a temperamental process.
- Their close molecular structure means they usually have an opaque aesthetic quality and can be difficult to bond to other plastics or materials using adhesives or solvents.
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 mold and remold 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
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 molding 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 molding machine need to be tightly controlled throughout the process. Similarly, the clamp pressure applied to the injection molding 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.
Pressure is the other key parameter within the injection molding process. There are two types of pressure that need to be tightly controlled:
- Clamp pressure, which holds the core and cavity of the tool together. This needs to be correct, so the tool doesn’t open or break during injection and to ensure the component is formed correctly within the tool.
- Injection pressure, which is when the thermoplastic is injected into the tool. This needs to be accurate to ensure the component forms correctly. Too little pressure and the part won’t form fully, too much pressure and scorches or warpage can start to occur.
Both parameters need to be controlled accurately so the injection molding process can run efficiently and create high-quality components in every cycle.
Why should you choose injection molded components?
Injection molded 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 molded 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 molding 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 molding is a key mass production process. Millions of products can be made from just one single mold 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 molded, solidified and ejected within a few minutes.
They offer great design flexibility:
- With injection molds, complex shapes are easier to create compared to other molding 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 molding process is also ideal for creating compact shapes such as polymer optics.
- With alternative processes like co-injection and sandwich molding, manufacturers can combine a number of characteristics within one part for example, different colors, tactile qualities or strength.
- Modern colorants within plastics offer manufacturers and companies the best possible color consistency. Whether it’s from a pantone chart or a custom color, your manufacturer should be able to offer you the color you want, consistently.
If you think injection-molded components might be the best option for you, then it’s important you choose parts from an experienced manufacturer. They will understand the injection molding 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 molding components
If any of the parameters within the injection molding 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 molded components and advice on how to spot them.
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 mold tool join together.
This fault can be caused by a variety of problems. Lack of clamp pressure, high injection pressure, overfilling the tool and faults within the mold can lead the plastic to escape and form flash. Thermoplastics with high melt flow or lower viscosity are also more likely to escape the tool and form flash.
By carefully monitoring process pressures and the temperature of the thermoplastic, manufacturers can make sure these parameters are at the optimum level to prevent flash. Keeping mold tools well maintained is also key to preventing this.
Gassing and venting
Gassing and venting is a small explosion within a part, this can cause holes or burn marks within the final component. When air is caught in the mold tool in front of the material flow during injection, it becomes trapped. The mix between the plastic and the air causes gas to be produced and a small explosion to occur.
Monitoring injection speed so it’s not too high is key to ensuring that gassing and venting doesn’t occur. Making sure that the tool is adequately vented to prevent air being trapped and that the plastic material being injected isn’t damp are also precautions that manufacturers should take.
When the plastic stops short of filling the whole mold, this is called a short fault. This means that parts of the component are often missing or damaged. Mainly, shorts are caused by errors with the pressure or temperature levels. If the injection pressure or the thermoplastic temperature isn’t high enough then this can cause the plastic to fall short of filling the whole mold.
By closely monitoring the temperatures and pressures within the injection molding process, manufacturers can make sure they maintain consistent flow and prevent shorts.
Distortion and warpage
If there are indents or parts of a component where the plastic is thinner, weaker or bent, this is known as distortion or warpage. This is caused by a lack of hold pressure of cooling time during the injection molding process.
As the component cools within the mold, it shrinks. If the hold pressure (which packs the part) or the cooling time are wrong, it can lead to variable thicknesses within the part or internal stress that causes it to be damaged.
As well as monitoring hold pressures and cooling times, ensuring that the selection of the material is right for the complexity of the mold tool is essential to making sure that the final part cools and forms consistently.
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 molding?
There are two main alternatives to injection molding which can deliver the same level of accuracy and flexibility. These are 3D printing and blow molding.
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 molding 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 molding 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 molding is a simpler, lower cost process than injection molding. It also produces components that are formed in one piece with no parting lines.
However, although the molds used in injection molding require lots of upfront investment, they offer much more design complexity and flexibility compared to blow molding, 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 molded parts.
There are three main aspects that will influence the future of the injection molding process: plastic development, advances in machinery and customer demands.
The progression of thermoplastic science and the demands of the market have led to the development of thousands of types of thermoplastics with different qualities and characteristics.
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 molding 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 molding 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 molding will need to face.
Alongside changing market demands, shifts in customer awareness and priorities are already starting to influence the world of injection molding.
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.
Inaccurate or low-quality parts can have a huge impact on a customer’s lead times and production budgets, making it essential that their chosen manufacturers maintain high quality standards within their components consistently.
Experienced manufacturers are starting to adapt to this new customer awareness by:
- Increasing investment in upskilling their workforce and maintaining up-to-date training.
- Emphasising the importance of expertise to tightly control and maintain quality within their injection-molding processes.
- 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 molding industry. As well as making us ready for the future, it also ensures that our customers get the quality standard of parts they require when they need them with our hassle-free service.