Injection Moulding Defects and How to Prevent Them

By Protolabs
moulded part with flash

In the world of injection moulding, overlooking proper mould design and maintenance can lead to defects that can ultimately wreak havoc on your production processes and end product quality. This blog will explore some issues associated with mould maintenance and design, particularly related to cause, effect, and prevention. This includes things like surface defects on the part, weld lines, mismatch and short shots. Continue reading to learn more about how neglecting the fundamentals of mould design can result in costly defects and production delays, as well as practical insights on preventing these problems.


What is meant by the term “injection moulding defect”?

An injection moulding defect is an imperfection or unwanted mark, bump, or gap left on the final part. Various reasons, such as improper venting, cooling, or gating, insufficient draft angles, and inadequate wall thickness, can cause these undesirable blemishes.


How can injection moulding defects impact the end product?

Injection moulding defects can impact an end product in many ways. The most obvious is the cosmetic appearance of your part. Defects such as flash, sink, and warpage can detract from your part’s visual appeal. But your part can be affected in other non-visual ways, such as dimensional accuracy, mechanical properties, functionality, durability, and much more.


What are the most common injection moulding defects?


What is flash? Flash is excess plastic on your final part that has leaked out of the mould cavity.

How to identify flash: Flash is easy to spot as it is excess material. It is typically found where the mould cavity seals, at parting lines, ejector pins, venting channels, etc.

How problematic is flash? Flash is mainly a cosmetic issue; however, removing it can increase costs and potentially reduce design accuracy and consistency.

How to prevent flash: Flash can be caused by problems with the mould, such as a mismatch between the mould halves or inadequate venting. But part design can also play its part. Very thin parts require higher injection pressure to force the molten plastic into the mould, which can lead to flash. Also, making the two halves of the mould as easy as possible to mate with each other increases the likelihood of a suitable shutoff between the two halves. This can be done by trying to keep the parting line as flat as possible and avoiding the need for steep areas, which could lead to excessive wear in the mould as the two halves open and close against each other.


What is sink? A sink is a dip (or depression) on the surface of a final part. It typically occurs in areas of thicker material where cooling is inconsistent.

How to identify sink: Sinks are also an obvious cosmetic defect, as they will leave noticeable dips on the material surface. Sinks occur where there is a thicker area on the part. The thicker area of plastic takes longer to cool down than the surrounding thinner areas, so it contracts more, leading the surface of the part to be pulled inwards.

How problematic is the sink? Sinks are generally just a cosmetic issue, but they can look quite ugly and make the part look of poor quality.

How to prevent sink: Sinks can happen due to problems with the moulding process, such as using the incorrect melt temperature, pack and hold time, or mould temperature. But like flash, it can also be affected by part design. The best way to avoid sink is to prevent variations in wall thickness. Try to core out thicker areas to leave an even wall thickness. Doing this in the line of draw will add little or nothing to the mould cost. Also, ensure that ribs and bosses don’t create thick areas. As a general rule, avoid walls being less than 40-60% of the thickness of adjacent walls.

Knit Lines

What are knit lines?
Also known as weld lines, knit lines are when two separate flows of molten resin material meet in the middle. This joining is known as the “knit line.”

How to identify knit lines: They will be visible on your plastic part where the flow of material has met. They sometimes look like scratches and radiate from the gate behind obstructions like holes and slots.

How problematic are knit lines? They could be purely cosmetic issues or, in the worst-case scenario, cause serious structural problems (structural problems from knit lines can be more likely to occur if the knit line is behind a boss*).

How to prevent knit lines? The material being used can greatly impact how prominent the knit lines are. Easier flowing materials will produce less prominent knit lines, so try to avoid filled materials if the part has many knit line-causing features. You can also try to prevent the knit line causing features in the first place, such as bosses and holes through the part. But if they are unavoidable, moving these as far from the edges of the part as possible will help.

* a boss is a feature raised above the surface.

Short Shot

What is short shot?
A short shot is when an injection moulded part or feature is not completely formed. This happens when the molten plastic has cooled and solidified before it has fully filled the mould cavity.

How to identify short shot:  Short shot is easily noticeable as an unformed/unfinished part or feature.

How problematic is short shot: If a part or feature is not fully formed, it cannot complete its intended job. The part would likely need to be remoulded.

How to prevent a short shot?  As with knit lines, the likelihood of a short shot depends on the material being used. There will be less risk of one with an easier flowing material than a stiffer flowing one such as a glass-filled grade. Part geometry also plays a part. Avoid designing your part too thin and keep the wall thickness as consistent as possible. The moulder also has a role to play. You are ensuring that the mould has sufficient venting designed so that trapped gas can escape, which might otherwise have prevented the plastic from fully filling the cavity.


What are voids?
Voids are areas of trapped gas or air pockets in the plastic.

How to identify a void: If not at the surface or using a clear material, voids can be a little difficult to spot. If they are inside the part and your part is not clear, you may notice blemishes, lumps or bumps on the surface of your part. If you’re unsure whether your part has voids, cutting a part open through a thick section will allow you to see if any are present.

How problematic are voids? Like parts that aren’t fully formed, voids can be problematic cosmetically. They can also affect the part's mechanical properties and weaken it.

How to prevent voids: Voids are caused when the outer layers of the part cool faster than the inner layers. So, avoiding thick areas and keeping wall thickness consistent will help. Processing can also have a big impact. Ensuring the material is dried correctly and increasing injection and holding pressure will aid things.


What is jetting?
Jetting is usually caused when the material is injected too quickly into the mould cavity and starts setting before the entire cavity is filled.

How to identify jetting: Jetting leaves marks on the surface of the plastic that resembles a snake, typically starting at the gate area.

How problematic is jetting? Although it may seem purely cosmetic, it can also lead to structural weaknesses.

How to prevent jetting: Jetting is an issue that is caused and can be prevented by the processing at the press. The three leading causes of jetting are linked to the flow of the plastic into the mould cavity, the speed of injection and the temperature of the plastic. Ensuring that the flow of plastic hits a surface as it enters the mould cavity will prevent it from cooling down too much before it has properly sped out through the mould. Also, reducing the speed at which the plastic enters the mould can help. Finally, slowing the rate at which the material cools down in the mould can prevent jetting, which can be affected by ensuring the mould is at the correct temperature.


What is warping?
Warping is a plastic part's disfiguration, such as bending and twisting.

How to identify warping: Like most defects, it is quite easy to spot; the part will bow, bend or twist out of shape.

How problematic is warping? Warping can cause problems, particularly for a part that requires assembly, as it results in parts being unable to connect as planned.

How to prevent warping: Warping can be tackled by various aspects of the moulding process, such as mould and barrel temperatures, gate size, and locations. It can also be caused by part design and material choice. Like many of the defects we have discussed, trying to keep a consistent wall thickness is very important, as warp can occur when inconsistent wall thicknesses cause inconsistent shrinkage.  Some materials are also more prone to wrapping than others. For example, glass-filled materials are much more likely to warp than unfilled materials.

Burn Marks

What are burn marks?
 Burn marks are browned or blackened marks on the surface of a plastic part.

How to identify burn marks: Burn marks are undeniable due to their black/brown discolouration of the plastic surface.

How problematic are burn marks? Although they may look like a simple cosmetic issue, the degradation of the material can cause problems with structural integrity and functionality.

How to prevent burn marks: Burn marks are usually the result of suboptimal processing. They can generally be found at the end of the filling process and are caused by the injection speed being too high. They can also result from poor venting in the mould, causing air to become trapped.


What is splay?
Splay are streaks left along the surface of a plastic part.

How to identify splay: Splay presents streaks across the part surface and is fairly easy to spot.

How problematic is splay? As it is usually the result of excess moisture in the material, it can be more than just a cosmetic issue; it can also affect the material's mechanical properties.

How to prevent splay: The most common causes of splay are moisture, excessive heat, and shear. The moulder is responsible for identifying the cause and taking appropriate corrective action to prevent it.


These are just some of the most well-known injection moulding defects, how to spot them, and how to prevent them ideally. However, if you have further questions or want to know more about a particular defect, please feel free to book a meeting with one of our team of experts or email us at [email protected]