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How to Prevent Plastic Injection Moulding Defects

Having helped hundreds of clients across various sectors over the years, we are constantly asked ‘how do you achieve the perfect part?’

Whilst plastic injection moulding is one of the most effective ways to produce highly detailed and durable parts, the process itself is complex and can present an array of challenges.

The quality of your plastic injection moulded part is very much dependent on the following four areas:

1. Part design

2. Tooling design

3. Process set up

4. Material quality

All the above factors have an impact on the quality of the moulded part, either individually or collectively.

In this blog, we will look at some of the most common plastic injection moulding issues, their most likely causes, and potential solutions.

As plastic injection moulding specialists, Paul Norman Plastics Limited has the experience to address and resolve these issues at the design stage to ensure that your part is ‘designed for manufacture’.

Flow Lines

Definition: Flow lines are streaks, or lines, often slightly different in colour that appear as a wavy pattern often and generally appear on narrower sections of the moulded part. They may also appear as ring shaped bands on a product’s surface near the entry points of the mould or gates, which the molten material flows through. Flow lines will not affect the integrity of the part, but they can be unappealing.

Causes: Flow line defects are generally caused by changes in the flow rate of the molten plastic as it moves through the injection mould tool. They are commonly found where the plastic must follow curved or shaped contours or pass-through areas with varying wall thicknesses. They can also be caused by injection mould pressures being too low.


1. Increase injection speeds and pressure to the optimal level, this will ensure that the cavities are filled properly, whilst not allowing the molten plastic time to start cooling in the wrong place. The temperature of the molten plastic or the mould itself can also be raised to ensure that the plastic does not cool down sufficiently to cause a defect.

2. Round corners of the mould where the wall thickness increases to help keep flow rate consistent.

3. Relocate mould gates to create more distance between them and the mould coolant to prevent the material from cooling too early during the flow.

4. Increase the nozzle diameter to raise the flow speed and prevent early cooling.

Weld Lines

Definition: Weld lines refer to the surface defect which is caused by the two streams welded together. Weld lines can appear on the surface of a moulded part where the molten material has come together after splitting off into two or more directions in a mould. The weld line is the consequence of weak material bonding which lowers the strength of the part.


1. If there are holes, inserts, or multi-gate injection moulding methods in the parts or the wall thickness of the parts is uneven, weld lines may result.

2. Two or more fronts of polymer or other molten material need to maintain a certain temperature when colliding, otherwise, they become partially solidified and won’t sufficiently bond where they meet, this can also result in weld lines.


1. Raise injection speed and pressure to minimize cooling before the material has filled the mould.

2. Redesign the mould to eradicate partitions.

3. Increase material temperature to prevent partial solidification.

4. Switch to a material with a lower melting temperature or viscosity to facilitate faster flow and prevent early cooling.

Sink Marks

Definition: Small depressions located in thicker areas of the injection moulding after there is shrinkage in the finished product.


1. The cooling time is incorrect, and the plastic is not cooling down enough whilst in the mould.

2. Issue with the pressure inside the cavity, or with too much heat at the gate.

3. Melt or mould temperature is too high

4. Inappropriate design of the structure of the parts.


1. Decrease the mould temperature.

2. Increase the holding pressure and time and allow for more adequate cooling and curing for longer inside the mould.

3. Increase the gate size or change the gate position.

4. Reducing the thickness of the thickest wall sections will also ensure faster cooling and help to reduce the likelihood of sink marks.


Definition: Warping is the deformation that can occur in injection moulded components when different parts of a product shrink unevenly. Plastic usually warps during the cooling process when uneven shrinkage puts undue stress on different areas of the moulded part; this excessive stress results in twisting of the finished component as it cools.

Causes: Warping is usually caused by non-uniform cooling of the mould material. Different cooling rates in different parts of the mould cause the plastic to cool differently and thus create internal stresses, which when released lead to warping.


1. Ensure that the cooling process is gradual and long enough to prevent uneven stresses on the material.

2. Lower the temperature of the material or mould

3. Choose polymers that are less likely to shrink and deform. Semi crystalline polymers are more prone to warping.

4. Design the mould with uniform wall thickness and part symmetry to ensure greater stability in the part during cooling.

Burn Marks

Definition: Burn marks are black- or rust-coloured discolorations that appear on the surface of a part.

Causes: The usual cause of burn marks in injection moulded components is trapped air, or the polymer itself, over heating in the mould cavity during injection. Excessive injection speeds or heating of the material often leads to over heating that causes burn marks.


1. Lower the mould and melt temperature to prevent overheating.

2. Shorten the mould cycle time.

3. Enlarge gas vents and gates to allow trapped air to escape the mould.

4. Reduce the injection speed to limit the risk of trapping air inside the mould.

Vacuum Voids

Definition: Air pockets trapped inside of or close to the surface of an injection moulded part.

Causes: Vacuum voids are often caused by unequal solidification between the surface and the inner sections of the part. This can be provoked when the holding pressure is inadequate to condense the molten plastic in the mould and thereby force out air that would otherwise get trapped. Voids can also develop from a part that is cast from a mould with two halves that are incorrectly aligned.


1. Increase injection pressure to force out trapped air pockets and increase holding time.

2. Ensure that the mould parts are perfectly aligned

3. Locate the gate at the thickest part of the moulding.

4. Choose a grade of material with lower viscosity to limit risk of air bubbles forming.

We hope you have found this blog post regarding injection moulding defects and associated causes and solutions of interest.

If you are considering plastic injection moulding for your component part, then why not schedule a free consultation with us. To arrange your consultation either call us on 01453 833 388 or email us at

Image depicts injection moulding defects
Common injection moulding defects


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