When you need to produce high volumes of plastic products or parts of products, injection moulding is a very efficient and effective method. It is naturally a machine-precise procedure that churns perfect copies of the desired plastic products every time. Well, not every single time; there can be some flaws or defects that may occur by no fault of yours.

Regardless of whose fault the defects are, you still have to deal with them. Thus, the production process needs checks or measures to ensure there are no hitches from start to finish. For a satisfactory end-product for your clients, you must prevent these defects, check for them, and correct them if they occur. 

Defects can cause considerable damage, not only to the product, but to the whole process itself. They can impede the process’s speed and incur more production costs. Below are a few defects that may commonly occur in the process of plastic injection moulding and, of course, how we at All Mould Plastics avoid them

7 Defects In Injection moulding


Flow lines

In the injection moulding process, flow lines are a particular defect that manifests in various ways. Flow lines are often off-color or faded streaks, or other irregular streaks that show on the surface of the plastic.

They are caused when the injected molten plastic moves at irregular and different speeds through the mould. Thus, different portions of the plastic solidify at different rates. Flow lines are also caused when the injection pressure is lower than normal.

At All Mould Plastics, we can avoid this by ensuring the pressure and speed of injection are uniform and high. This ensures parts that settle, cool, and solidify evenly.


Sink Marks

A sink mark is a defect in injection moulding that manifests as depressions, craters, or dents in thick sections of the plastic parts.

The cause can be attributed to the thickness of the parts, which are often the parts that are much slower to cool and solidify. When these thick or thicker parts take too long to cool but are attached to parts not as thick, it causes the former to sink into the latter as one begins to shrink quicker as it cools.

To avoid this defect, we at All Mould Plastics use thick parts with reduced thickness to ensure even cooling. We also ensure the pressure in the mould cavity is sufficient and stable.


Surface Delamination

The plastic parts’ surface is often the product’s most important and observed portion. Because of this, any damage to it is a terrible outcome.

Surface delamination describes a situation where the surface of the plastic parts refuses to fuse properly and separate into thin layers. Delamination is caused by the presence of unwanted substances that contaminate the material, impeding the bonding phase of the mould.

In order to prevent this defect, it is important to dry the plastic properly before committing it to the moulding process. At All Mould Plastics, we ensure the mould temperature is increased, and the mould ejection mechanism relies little on mould-release agents. These agents are known to increase the risk of delamination.


Weld Lines

The injection moulding process is akin to welding but uses molten plastic resin instead of molten metal ores. In plastic injection moulding, weld lines are a kind of defect that occurs at the spot where two flows of molten resin fuse as they flow through and in the mould configuration.

Weld lines are also called knit lines and are quite common if the process isn’t done correctly. It is caused when the temperature of one of the streams of molten resin isn’t in tandem with the other; this will make both cool at different rates.

In order to avoid this defect, the temperature of the molten resin is raised to prevent hastened solidification. An increase in the injection speed and pressure can also help mitigate this defect as well.


Short Shots

In the injection moulding process, there can be instances where the molten resin will not fully occupy the mould. When this happens, the resulting parts are incomplete and unviable.

The parts that are made from such anomalies are known as “short shots” since they fell short of their intended design. It is caused by restricted flow in the mould due to narrow or blocked gates, insufficient injection pressure, or concealed air pockets.

In order to avoid this defect, the temperature of the mould is increased, and venting is incorporated into the design for air to exit evenly.



When warping occurs in injection moulding, the resulting plastic parts have twists and bends that were not intended in the original design or mould. It is caused by heterogeneous and inconsistent internal shrinking as the mould begins to cool.

In order to avoid warping, the parts must be given enough time to cool evenly and at a fairly slow rate. Doing it this way prevents any internal stress or damage to the parts.

We can also prevent warping by making the thickness of the mould walls uniform. The trained and seasoned experts at All Mould Plastics make extra sure there is no warping.



Jetting is another defect in injection moulding that occurs due to inconsistent solidification of the molten resin. 

It describes a process where an initial stream of molten resin is injected and completely cools before the mould cavity is filled. This will result in visible, squiggly flow lines on the part’s surface, drastically weakening the part’s structural integrity. 

To remedy and fix the jetting defect, we increase the mould temperature and reduce the injection pressure.




Plastic injection moulding is an essential production process, helping to produce various parts needed across many industries. We must do it with care and precision to ensure the products are perfect for their areas of utilization.

You can only trust a few brands for the best plastic parts made from cutting-edge injection moulding processes.

All Mould Plastics is one of the best in the business. Call us today!