Knowledge Hub
15/08/2024
Wall Thickness in Injection Moulding
What is Wall Thickness in Injection Moulding?
Wall thickness in injection moulding refers to the thickness of the walls or sections of a plastic part. It is the distance between the outer and inner surfaces of the part.
Wall thickness is an important design consideration, as it can impact the overall strength, dimensional stability, and aesthetics of the final part.
The appropriate wall thickness for a specific part depends on various factors such as the material being used, the size and geometry of the part, and the intended application.

Generally, maintaining a uniform wall thickness is preferred to ensure consistent part quality and to minimise defects such as shrinkage.
Design guidelines for injection moulding often recommend certain minimum and maximum wall thickness values based on the material properties and manufacturing capabilities. Deviations from the recommended wall thickness can result in part failure, dimensional inaccuracies, or increased production costs.
Key Considerations of Wall Thickness in Injection Moulding
Some of the key considerations of wall thickness in injection moulding are:
Flow and Fill: The wall thickness directly affects the flow and fill of molten plastic in the mould cavity. If the wall thickness is too thin, it may not adequately fill the mould cavity, leading to incomplete parts or defects in the final product. Conversely, if the wall thickness is too thick, it may cause longer cycles, reducing the overall production efficiency.
Cooling: The cooling of the plastic part is crucial in injection moulding to ensure sufficient solidification. The wall thickness affects the cooling rate, as thicker walls take longer to cool compared to thinner walls. Proper cooling time is necessary to achieve dimensional stability and to maintain the desired shape.
Part Strength: Wall thickness influences the strength and mechanical properties of the final product. Thicker walls can provide greater strength, while thinner walls may result in reduced strength and increased flexibility. The required strength and structural integrity of the part should be considered when determining the appropriate wall thickness.
Material Cost: Thicker walls require more material, leading to increased material costs. By optimising the wall thickness, manufacturers can improve cost-efficiencies.
Mould Design: Wall thickness plays a crucial role in the mould structure, gate size, runner system, and overall mould complexity. By maintaining uniform wall thickness, mould complexity is reduced, ensuring easy flow of molten plastic during the moulding process.

Why is Uniform Wall Thickness in Injection Moulding Important?
Uniform wall thickness in injection moulding refers to the consistent thickness of the plastic material throughout the moulded product. It means that the thickness of the walls, or sections, should remain constant throughout, without any significant variations.
Uniform wall thickness is recommended in injection moulding for several reasons:
Consistent filling: Uniform wall thickness ensures consistent and even flow of molten plastic material during the injection moulding process. This helps in achieving complete filling of the mould cavity without any imperfections such as short shots or voids.
Balanced cooling: Uniform wall thickness allows for balanced and uniform cooling of the injected plastic. Non-uniform wall thickness can lead to uneven cooling rates, which may result in inconsistencies or residual stresses in the final product.
Reduced cycle time: Uniform wall thickness promotes uniform heat transfer to the cooling system, leading to faster and more consistent cooling of the plastic material. This reduces the overall cycle time, enhancing both productivity and cost-effectiveness.
Material saving: Uneven wall thickness may require excessive material to compensate for thin areas, leading to higher material costs. Non-uniform walls may also cause material flow issues, such as flow marks or knit lines, which will affect the appearance and structural integrity of the part.
Improved part quality: Uniform wall thickness results in parts with better dimensional accuracy, strength, and surface finish. It reduces the risk of distortion caused by uneven shrinkage during cooling and solidification.
How is Uniform Wall Thickness in Injection Moulding Achieved?
Uniform wall thickness is achieved through several design and processing considerations:
Part design: The part should be designed with a uniform cross-sectional thickness. Avoiding sharp corners, thick sections, or abrupt changes in wall thickness is crucial.
Gate location: The gate is the point at which the molten plastic enters the mould cavity. Proper gate placement helps to achieve even filling of the mould and avoids flow imbalances.
Mould design: The mould should be designed to facilitate proper flow and even cooling. This can be achieved by using uniform mould cavities, balanced runner systems, and appropriately sized sprues, runners, and gates. Proper venting should also be provided to prevent trapped air from causing thickness variations.
Process optimisation: Injection moulding parameters like melt temperature, injection pressure, and injection speed should be properly controlled to ensure consistent filling and packing of the mould cavity. Maintaining the mould temperature within a narrow range is also important to prevent differential cooling and variation in wall thickness.
Mould temperature control: Cooling channels can be designed in the mould to maintain uniform temperature throughout the part.
Problems Arising from Uneven Wall Thickness in Injection Moulding
If a plastic part does not have uniform wall thickness, several issues can occur:
Warping: Non-uniform wall thickness can cause warping or deformation, leading to dimensional inaccuracies and potential functionality issues.
Sink marks: Uneven cooling rates caused by varying wall thickness can result in sink marks on the surface of the part. Sink marks are depressions or dents that occur when the thicker sections cool at a slower rate, causing shrinkage and uneven surfaces.
Poor strength and stiffness: If there are significant variations in wall thickness, the part may be weak, potentially leading to failure or inability to withstand required loads.
Inconsistent filling and flow: Varying thicknesses can result in different filling rates, inadequate pressure distribution, and incomplete part filling.
Increased cycle time/material wastage: Parts with varying thicknesses may require more material to be injected, resulting in higher material consumption and increased time/costs.
Mould design/manufacturing challenges: Non-uniform wall thickness can complicate mould design/manufacturing, which can delay the process and impact time to market.
