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Top 3 injection moulding defects and how to fix them?

Making injection moulding parts is a combination of art and science. Our team at CJ Tool and Mouldings provide high levels of technical expertise and attention to detail to prevent small mistakes when it comes to mass-production of novel parts. This article discusses some of the moulding defects that can occur in a part during injection moulding, and ways to fix and avoid them. 


Flow Lines 

Flow lines are streaks, patterns, or lines which are commonly off-toned in colour.



Flow line defects can be caused by the varying speed at which the molten plastic flows as it changes direction through the contours and bends inside the injection moulding tool. Flow Lines can also be caused by other variants during the injection moulding process.



Increased injection moulding speeds and pressure to the optimal level will ensure the cavities are filled properly. The temperature of the molten plastic or the mould itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect.


Vacuum Voids:

Vacuum voids are pockets of air trapped within or close to the surface of an injection moulded part.



Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the part. Voids can also develop from an injection moulding part that is cast from a mould with two halves that are not correctly aligned.



1. Locate the gate at the thickest part of the injection moulding.

2. Switch to a less viscous plastic. This will ensure that less gas is trapped as air is able to escape more rapidly.

3. Increase holding pressure as well as holding time.

4. Ensure that injection moulding parts are perfectly aligned.


CJ Tool and Mouldings offer all their customers a mould flow analysis which would identify the exact causes and ideal remedies for both flow lines and vacuum voids when injection moulding.  


Surface Delamination

Surface delamination is a condition where thin surface layers appear on the part due to a contaminant material.



Foreign materials that find their way into the molten plastic separate from the finished product because the contaminant and the plastic cannot bond.  



1. Pre-dry the plastic properly before injection moulding.

2. Increase the mould temperature.

3. Smooth out the corners and sharp turns in the mould design to avoid sudden changes in melt flow.

4. Focus more on the ejection mechanism in the mould design to reduce or eliminate the dependence on mould release agents.


people product partnership

CJ Tool & Mouldings offer a full range of plastic injection moulding services, with a customer centred and personal approach. We pride ourselves on delivering quality products to meet customer needs, with a strong focus on quality service and value. CJ Tool & Mouldings are a dynamic and growing business through people product partnership. 

Added: 11 Sep 2019 10:47

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