CJ Tool and Mouldings Logo

Troubleshooting Guide Part 1 The 5 most common problems when injection moulding

When plastic injection moulding, high levels of technical expertise and attention to detail are vital to prevent avoidable mistakes. At CJ Tool and Mouldings we offer our own unique ‘innovation filter’ which is built and incorporated into each project, at no additional charge to the customer, giving an added value service and potential cost saving.

With every project, unexpected problems can occur, below are the top 5 most common problems when plastic injection moulding.  


Distortion is a widespread issue within the plastic injection moulding industry, normally caused by differences in cooling, shrinkage or orientation from one area of the part to another. Distortion often occurs in the corners and is due to poor cooling. Parts incorporating a variable wall thickness profile can exhibit distortion due to differential shrinkage.


Flash usually occurs when the mould opens slightly during filling or the holding phrase. It’s known as a small burr formed around the parting line of the plastic injection moulding. Flash can also form on a new mould during the commissioning stage if the two halves are not fully bedded out. This usually occurs when moulding very low viscosity materials as the melt can flow into very thin gaps.

Short Mouldings

Short mouldings are where a plastic injection moulding has sections missing, these are usually in regions remote from the injection point because the mould was not completely filled. This can be due to the metered shot size being too small, the non-return valve is leaking, or the pressure of plastic injection moulding is set too low.

Weld or Knit Lines

Another common problem is when visible lines are formed on the surface of the moulding, usually found between gate points or the downstream of apertures or other obstacles. Weld lines are formed when separate melt fronts travel in different directions and meet causing a reduction in strength.

Black Specs

Black/brown specs or deposits are found on the surface the plastic injection moulding and are normally caused by the contamination or degradation of the material.

CJ Tool and Mouldings have over 30 years of plastic injection moulding experience. If you have any concerns regarding the process please contact us today!


Added: 09 Apr 2019 12:30

Back to News