Plastic injection moulding glossary

Plastic injection moulding is a complex process with its own set of terminology. This can be a challenge if you're trying to choose an injection moulding manufacturer. To know what questions to ask and understand whether the components produced will suit your needs, it helps to have the key terms at hand.
Whether it's the process, machinery, materials or sustainability, this glossary will outline the key injection moulding terminology you need to know.

What do injection moulding process terms mean?
What are the parts of injection moulding machines?
What are the different injection moulding materials?
What are injection moulding defects called?
Is injection moulding environmentally friendly? 
Where can you learn more about plastic injection moulding?

What do injection moulding process terms mean?

Although the injection moulding process is typically completed within a few seconds or minutes, there are lots of steps and factors involved. This means there are also lots of terms around the plastic injection moulding process.

What is a two-shot or multi-shot process?

A two-shot or multi-shot injection process is when two or more materials are used to form one part. Also known as over-moulding, the exact multi-shot process changes depending on the specific requirements of a part.
Typically, one material will be injected and partially cooled in the mould. This is then transferred to another mould where the second material is injected. This process will repeat in the case of a multi-shot process until the final part is formed and ejected from the machine.

What is clamp pressure?

Clamp pressure is the amount of force that's used to hold the two parts of an injection moulding tool together. This is applied to ensure the material in the mould packs out the whole tool and doesn't leak. The clamp pressure has to be compatible with the force of injection to ensure none of the injected material escapes the tool due to the force of injection.

What is back pressure?

Back pressure is the amount of force that the screw inside the barrel needs to overcome as its screws back after injection. To avoid delays in the moving back of the screw to the starting point and damage being caused to the material, this force has to be significantly smaller than that of injection.

What is injection pressure?

Injection pressure is the amount of force required to push the molten materials out of the barrel and into the mould tool. This is calculated based on the amount of force required to send the material through the mould tool's sprues and gates. It also has to be higher than the flow of the material in the barrel as it moves through the screw.

What is injection speed?

Injection speed measures how quickly a material is pushed through the injection machine's barrel into the moulding tool. This is dictated by the movement of the screw through the barrel and is measured by the time it takes to complete each injection.
Getting injection speeds right is key to ensuring that the material flows into the tool correctly and is evenly distributed around the inside of the mould.

What is wall thickness?

Wall thickness is a measurement of how much material forms the outside of the final part. It's a key metric that manufacturers use when considering its tolerances and quality.

What is a holding phase?

The holding phase is the name given to the period of time when the moulding tool is held under pressure. This packs out the part and ensures the material forms inside the mould correctly. This happens after injection and before the final part is ejected from the tool.

What does cooling time mean?

The cooling time is the number of seconds or minutes that the material is left to cool and form into the final part. Getting this right is important to ensure there are no faults such as warpage, distortion or sink marks. The length of cooling time will depend on a range of factors including the type of material and the complexity of the mould tool.

What is the shrink rate?

The shrink rate refers to the amount that a part reduces in size as the material cools. This is important to consider when designing a mould as a part made of a certain material may end up smaller than one made from another. This will affect the quality and tolerances of the final part.

What does flow rate mean?

Flow rate is the speed at which material moves through the barrel into the mould tool. This is impacted by the pressure and temperature applied to the material. It needs to be optimised to ensure materials such as molten plastic flows at the right rate into the mould tool, causing problems with the forming of the final part.

What does shear rate mean?

Shear rate takes into account how adjacent layers of material flow at different rates, rub together and cause friction. Knowing the shearing rate is important for manufacturers as it will help to determine the properties of the product. For example, if the shear rate is high, the plastic will be more fragile and stressed.

What is the line of draw?

The line of draw is the place where the two parts of the moulding tool join together. Also known as the parting line, the direction of the line of draw is important because it determines where the mould splits and contributes both towards the strength and aesthetic qualities of a part.

What are the parts of injection moulding machines?

Injection moulding machines are as complex as the process they complete. Knowing the different elements within the machine will help you to understand exactly how production works.

What is the hopper?

The hopper is a funnel where the pellets of a chosen material are put into the machine.

What is the barrel?

The barrel is a long thin section of the machine that is used to melt and inject the chosen material into the mould tool. The material is fed in pellet form from the hopper and is moved along the barrel by a screw. The barrel is also heated, meaning the material in it becomes molten.
This, combined with the shear force caused by the movement of the screw, heats the material until it is at the optimal state for injection.

What is the screw?

The screw is a long metal tool that sits within the barrel of an injection moulding machine. This is the main element that's used to inject the molten material into the mould tool. It also helps to melt the material in the machine by moving backwards and forwards in the barrel.
It does this thanks to the pressures set within the machine. First, the screw pulls back, leaving space for the material to flow behind the moulding tool gate. Then, when this gate opens, the screw pushes the material into the mould. This needs to be set at the right speed and pressure for the final part to be of high quality. 

What is the moulding tool?

The moulding tool, otherwise known as the mould or die (when used with metal), is the element which shapes molten plastic or other materials into the final part. It is typically formed from two parts, a cavity and a core, though each mould has a different design that creates parts of varying shapes.
The final design of a mould tool will be influenced by the type of shape being created, the material being used and the pressures and temperatures required within the moulding process.
Though aluminium moulds are the most common type of tool due to their durability, they can also be built using 3D printers. This makes it quicker for manufacturers to iterate designs and adjust part shapes, though the materials they're made from mean they can't be as used for as many parts as an aluminium alternative.

What is a moulding cavity?

The moulding cavity is one-half of a mould tool. It is the section that the solid core is pushed into under high pressure before the material is injected to form the final part.

What is a moulding core?

The moulding core is one-half of a mould tool. It is a solid form that's pushed into the other half, the mould cavity, and placed under high pressure as a material is injected into it to form the final part.

Where is the sprue in an injection moulding machine?

The sprue is the passage through which the molten material is delivered to the mould. It has a crucial role in controlling the amount of material that passes into the mould.
‘Sprue’ is also an alternative word to flash and refers to the excess plastic left on a part.

Where is the runner in an injection moulding machine?

The runner connects the sprue and the gate in an injection moulding tool. Ensuring the runner has the right diameter and length for the part being created is essential to ensure the correct amount of material is supplied at an adequate speed.
Runners can be part of hot and cold systems. Hot runner systems keep the material molten as it's delivered to different parts of the mould. Cold runner systems enable a greater range of polymers to be used in the machine.

Where is the gate in an injection moulding machine?

The gate is where the melted material enters the mould tool. Gates can be used to adjust a part's shape or to minimise the amount of waste being created from the moulding process.

What are the ejector pins or plates?

Ejector pins or plates push the final formed part out of the moulding tool. It will push the part onto a conveyor belt ready to be packed or finished.

What is the vestige?

Vestige is the excess material that protrudes from the gate and runner after the part is formed. It is often removed by hand or a machine to finish the final part.

What are the different injection moulding materials?

Though plastic injection moulding is the most widely known form of this manufacturing process, a range of materials can be used to create parts using this method.

Thermoplastics

Thermoplastics are types of polymers that can be heated into a molten state and cooled to form a solid shape. Falling into two molecular structures of amorphous or crystalline, each type offers different material characteristics and melt temperatures.

To choose the right polymer, it's important that the mould design, process and temperature are considered alongside the desired aesthetic, function and tactile qualities. This will ensure that manufacturers choose the most appropriate polymer for the final part.
Some of the most common plastic materials used in injection moulding are:
•    High-density polyethylene (HDPE)
•    Low-density polyethylene (LDPE)
•    Polypropylene (PP)
•    Polycarbonate (PC)
•    Acrylonitrile butadiene styrene (ABS)
•    Nylon

Metals

Metal injection moulding is also a common manufacturing process. Some of the most common metals used in this process include alloys, types of steel and iron.

Rubber

Rubber injection moulding is the final most common process of this type. Natural rubber, nitrile and silicone are materials that are often used.

What are injection moulding defects called?

To make sure the components you're buying are of high quality, it's important to be aware of the most common injection moulding defects and what causes them.
 

What does the term flash mean in injection moulding?

Flash is the term for any excess material that is left after a part has been moulded. This is often left on the edge of a part. Often, the flash has to be removed (de-flashed) through cutting, breaking or grinding because it is considered a fault and caused by an error of the moulding process.
Process errors that can cause flash excess include over-moulding, the leakage of material from the mould or excessive injection pressure.

What are sink marks?

Sink marks are caused when certain areas of the part are too thick and become depressions or small recesses in the final product. They are usually caused by the plastic cooling too slowly or if there isn’t enough pressure to push the material into the mould.

What is jetting?

Jetting is caused when too much injection pressure is applied to the material. This causes wavy folds to appear in the final part where the material was injected. This doesn’t just affect the part aesthetically but makes it structurally weaker too.

How are burns caused?

Burns are caused by excessive temperature in the molten plastic or other material. It can also be caused by excessive injection speed which puts the material under extreme friction. This can cause the plastic to degrade and burn, which then appears as marks when the part solidifies.

What are shorts?

Shorts are caused when the machine fails to ‘shoot’ enough material into the mould to create the full part. Also known as a short shot, it can be caused by an overly viscous material blocking the machine. Or, if material hasn’t been heated enough, it won't be able to reach and cover the full cavity because it’s too viscous.

What does warpage mean?

Warpage means there is excessive distortion in the final plastic moulded part. Often, these are caused by uneven cooling or shrinkage rates in the part. Ejecting the part from the machine before the molten plastic or material has cooled fully can also cause warpage.

Is injection moulding environmentally friendly? 

The use of molten plastic and high-energy machinery means injection moulding has been criticised for its environmental impact. However, leading manufacturers are now taking action to maintain the sustainability of this process for customers and the planet.

What are bioplastics?

Bioplastics are polymers made from materials that are sustainably found in nature. Rather than using fossil fuels, these plastics are made from substances such as vegetable oils, cornstarch or other biomass. This means they can be renewably sourced and don't harm the environment when they break down.
 

Why are electric injection moulding machines more energy-efficient?

Electric injection moulding machines are much more energy efficient than the hydraulic machines historically used by injection moulding manufacturers. This is because they create high-quality parts more efficiently, be optimised using data systems and use electricity, which doesn't need to be disposed of and can be renewably sourced.
This means electric injection moulding machines are much more environmentally friendly than their hydraulic alternatives.

How does post-consumer polymer recycling enhance sustainability?

Post-consumer polymer recycling is when manufacturers take plastic components after they've been used and break the materials back down to put them back into the injection moulding process. This means used parts don't go to landfill and materials are used to their full potential, without sacrificing any product quality. This is part of a process called the circular economy.

Where can you learn more about plastic injection moulding?

With more than 65 years of experience in the components manufacturing sector, the Essentra Components team has a wealth of knowledge about the injection moulding process. To find out more about how it works and why choosing the right injection moulding manufacturer is key to finding high-quality parts, see the Knowledge Centre articles.