Quality control for plastic injection molding

Quality control for plastic injection molding

Parts made in a plastic injection moulding process can have their own unique set of possible defects due to the complex technology involved.

In order to deliver quality and consistent finished products, our team is trained to spot these problems. Products are examined and checked using a thorough quality control process before any product is dispatched.

The following defects associated with the plastic injection moulding and transfer/compression moulding processes are some of the potential issues we look for:

Gas marks (burning)

Gas or burn marks are small, dark or black spots on the part surface. Air trapped in pockets may compress, heat up and cause burn marks. Strategically locating air vents of the proper depth within the mold is the best way to avoid burn marks. Adjustments injection speed and screw speed can also be altered to reduce or eliminate marks.

Short shots

A short shot is the incomplete filling of a mould cavity which results in the production of an incomplete part. Changing the plastic injection moulding parameters can correct this.

Weld Lines

Weld Lines are created when two or more melt flow fronts meet possibly causing a visible line. It can also create a weakened area in the finished moulded part. Adjusting moulding parameters such as injection pressure, temperature and speed can help avoid this problem.


Moulding flash occurs when a thin layer of material is forced out of the mold cavity at the parting line or ejector pins location. The machine clamp force must be greater than the pressure in the cavity, to sufficiently hold the mould plates shut and the mould must be properly built and maintained to avoid this problem.

Sink Marks

Sink marks are caused by localised shrinkage of the material at thick sections without sufficient compensation when the part is cooling. Parts designed with a consistent wall thickness are less prone to this issue. Adjusting moulding parameters such as injection pressure and time can often minimise the effect of the sink mark.

Flow marks

Flow marks may result if molten plastic does not properly flow as it fills the cavity. Changing moulding parameters or adjusting the mould by changing the gate location or size can usually eliminate this problem.


Bubbles may flow along the part surface during the plastic injection moulding process. The trapped air can cause incomplete filling and packing, and will often cause a surface blemish in the final part. Correct mold design and processing parameters can prevent splay.

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About Graham Dickson

Graham Dickson has been involved in the design and manufacture of tooling for the plastics industry for the past 35 years. He has extensive experience in the design, manufacture, and purchase of tooling for the plastics industry from local and overseas markets.