Understanding Plastic Injection Moulding Terminology

Sep 12, 2023

Plastic injection moulding is a manufacturing technique that creates incredibly precise parts and products. However, if you’re new to this process, it can be difficult to understand the terminology and language used. That’s where we can help. In this blog, we’ll delve into the most common terms used in plastic injection moulding so that you can get ahead of the curve. Whether you’re a product designer or a curious consumer, join us in understanding injection moulding terminology.


Inserts in plastic injection moulding are solid objects that are made from metal (usually brass or steel), plastics, or even thermoset composites. They’re usually needed for connecting your finished part to other components via screws, bolts, or other fasteners. Inserts can be placed strategically within the mould before the injection process begins for over moulding or post fitted after moulding, and there are several different methods for including them, such as:

  • In mould. The insert is placed into the mould tool before the molten plastic flows into the cavity so that the insert is secured in place once cooled.
  • Thermal insertion. The insert is heated and pressed into the part post moulding which allows it to be fixed into position.
  • Ultrasonic insertion. Ultrasonic vibration creates heat that melts the plastic so that the insert can be fixed in to position post moulding.
  • Press inserts. The inserts are pressed into the part whilst it is still warm post moulding.

Shut Offs

Shut offs are precise mould features designed to control the flow of molten plastic during the injection process. Two pieces of steel act as gates during the moulding process, enabling moulders to selectively open or close specific passages within the mould. This is essential for directing plastic material into precise locations in the mould cavity to create complex part geometries with fine details. By using an expert moulder with high-quality moulds, you can prevent ‘flash’, which is a small flap of thin plastic that appears on the plastic part where tool shut offs occur. This usually happens when there is a gap between the steel shut offs that can allow molten plastic to leak through.

Over Moulding

Over moulding is when you need to put one plastic over another one to create a one-piece product. One finished part is placed in the mould tool and the molten resin is injected around it to form a tight bond. You can usually find this technique used in products that are designed to have different colours, textures, or other qualities such as a plastic tooth brush that has been over moulded with different colours and textures to provide a comfortable finish.

Snap Fits

Snap fits are engineered mechanical features in plastic parts that securely interlock or ‘snap’ in place without the need for screws, clips, or adhesives. These features are carefully designed to flex and engage during assembly, providing a reliable and reversible connection. They’re usually used in consumer products like plastic enclosures and lids to simplify the assembly process.


Undercuts are features on plastic parts that create recesses or protrusions, making it challenging to remove the part from the mould. To release parts with undercuts, specialised mould design techniques like sliding cores, fabricated shut-offs, moving elements, and bump-off actions are used. These techniques allow the production of complex, multi-dimensional shapes that would otherwise be impossible to achieve in a single mould pull.

Screw Threads

Screw threads are helical ridges on the inside or outside of a plastic part that are designed to engage with corresponding threads on another component like a screw or nut. Precise engineering of threads is crucial for ensuring a secure and functional connection. This feature is commonly seen in a wide range of applications, including bottle caps, closures, and fasteners.

Draft Angle

A draft angle is a slight intentional taper incorporated into the vertical surfaces of a moulded part. This serves two purposes:

  • Aids in the easy removal of the part from the mould cavity by preventing suction forces.
  • Ensures the part demoulds smoothly without damage or distortion.

Without draft angles, parts can’t be cleanly released from the mould, and therefore risk being damaged or broken. Draft angles are calculated based on material properties, part geometry and the surface finish required.

Mould Cavity

The mould cavity is the hollow space within the mould that defines the final shape of the part. The detail and intricacies in the mould cavity dictate the part’s appearance, surface finish, structural integrity, and overall functionality. Parting lines, gating locations, and ejector pin placements are carefully planned within the mould cavity to create a successful part.


Shrinkage is a natural and predictable occurrence in plastic injection moulding. It refers to the reduction in size that the plastic material undergoes as it cools and solidifies within the mould. Accurate considerations of shrinkage are important in mould design as it ensures that the final part aligns with its intended dimensions and tolerances. Different plastic materials have different degrees of shrinkage, so it’s important that this is taken into consideration during the tool design phase.

Injection Pressure

Injection pressure is the force exerted on the molten plastic material as it’s injected into the mould cavity. This pressure is controlled to ensure the mould is properly filled to reduce any risk of defects like air traps or voids. Injection pressure balances against the machine’s clamping pressure and is calculated based on the size and shape of the part as well as material viscosity, part geometry, mould design, and processing parameters.


Venting Is a crucial aspect of plastic injection moulding as the air inside of the mould must be able to escape so that the plastic can fill the entire space. Without these vents, defects can occur such as burn marks, gas traps, and incomplete filling. Venting ensures that the moulded part is free from surface imperfections and maintains structural integrity. Generally, a vent should be as large as possible without allowing the plastic to escape; if the vent is too shallow, the clamping pressure could force it to close.

The size of the vent is based on the material being usedduring injection moulding. They should be at the end of flow paths, or whereflow paths combine.


Plastic warpage is deformation or distortion that occurs in a plastic part after moulding because of uneven cooling. This can result in dimensional inaccuracies, surface imperfections, and compromised part functionality such as unintentional bending and twisting. Addressing and minimising warpage involves adjusting the mould design, optimising processing conditions, and selecting materials with appropriate shrinkage characteristics. Warpage is a challenge, particularly for large or thin-walled parts.


Resin is a plastic or polymer that can be heated, melted, and injected into moulds for plastic production. The choice of resin in plastic injection moulding has a huge impact on the final product’s properties, including its mechanical strength, flexibility, thermal resistance, and chemical compatibility. There is a vast array of material options available, each with unique characteristics, such as:

  • Polyethylene (PE) for its low friction properties.
  • Polycarbonate (PC) for its optical clarity and impact resistance.

The perfect resin selection is important as it directly influences the part’s performance and cost. To see the full list of materials that can be used in the injection moulding process, you can find them on our material considerations page.


Although plastic injection moulding is a relatively precise process, there may be slight variations between the design and the finished product. Tolerance represents the allowable deviation from the specified dimensions of a moulded part, expressed in numerical values such as +/- 0.1 mm. It’s defined to ensure that parts consistently meet design specifications.

Tighter tolerances are essential for precision components like when manufacturing medical or automotive parts. Looser tolerances may be acceptable for less critical parts.

Wall Thickness

Wall thickness references the thickness of the structural walls of the component, the thicker the wall the stronger the structure however a thick wall section will increase the volume of material required to manufacture the product and therefore increase the cost of the component.  Products having a thinner wall section will be lighter and require shorter cooling times.

It is very important in plastic injection moulding that the wall thickness is kept as uniform as possible in the design and that areas with a significant wall thickness change are kept to a minimum as this can affect the material flow, visual quality, and structure of the part.

These core terms help provide a better understanding of the plastic injection moulding, their functionalities, and their significance within the complex manufacturing process. A comprehensive grasp of these terms is essential for professionals and enthusiasts alike in the field of plastic injection moulding.

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