4 Common Injection Moulding Defects and How To Remedy Them

Plastic injection moulding is a delicate process that is part chemistry and part art (learn about the process in our blog post: How Plastic Injection Molding Works.) Many things can go wrong and, if they are not caught and addressed, will waste a lot of time and money.

A reputable plastic injection moulder will note these problems and give solutions. But it is always good to know what you are looking for when inspecting a prototype part. Here are four common defects:

Flow Lines

You can’t miss flow lines. As the name describes, they are lines that look similar to the erosion water might leave on a surface: shallow grooves that seem to flow in a certain direction and are not quite the same colour as the rest of the product. They can also appear as circular patterns.

Several things can cause flow lines, but it most often comes down to the liquid plastic not moving fast and uniformly enough around the mould cavity. This is usually remedied by increasing the heat of the plastic and the pressure inside the mould. A lack of uniform wall thickness in the mould can also lead to flow lines. Another cause is low residence time, being the time it takes for the plastic resin to be heated.


Sometimes a prototype can arrive with warped sides that are supposed to be straight, and no amount of smoothing or pulling can get them back to how you need them! This is called warping, a common defect that also causes more than just cosmetic problems – and even that is an issue, as a warped product simply looks bad. That being said, not all warping is obvious and may require more than a quick inspection to spot.

Warping happens most often because parts of the plastic cool faster than others. This creates tensions in the product, a phenomenon called uneven stress. When released from the mould, the plastic will warp to release some of that tension. Several things can fix warping, including longer cooling times, more uniform moulds, and using plastic resins that are less prone to shrinkage. The pressure and fill rate can also cause warping.

Sink Marks

If you spot slight indentations that should not be there, those are called sink marks. These most often appear around thicker parts of the plastic and can be unsightly – especially since they aren’t uniform in appearance. In consumer products such as toys or electronics bezels, a sink mark is pretty much a death knell for any perception of quality.

Sink marks are usually caused by a lack of sufficient cooling. Since the thicker parts of a plastic product will take longer to cool, this is where they are most likely to appear. A lack of pressure can also cause sink marks. Lower moulding temperatures and higher pressure inside the mould are ways of avoiding sink marks. The thicker parts of a mould’s walls can also be thinned to ensure more even heat distribution. At the very least, a product with sink marks likely needed to stay in the mould for a longer cooldown period.

Weld Lines

Weld marks are incredibly common and are even found on final products. These sometimes appear as lines, but most often it looks as if two plastic planes have come together, forming a distinct division between then instead of a smooth surface. These are not only unappealing but can create weak points in the product.

Weld lines happen because of the plastic from the two planes, flowing in from two different sources, did not bond sufficiently. This can happen because there are two flow sources where the plastic comes from, or there are parts in the mould that split the molten plastic’s flow. If the various flows then cool at different rates before they bond, weld lines are the results. Higher temperatures or using plastic resin that becomes viscous at lower heats can help. The mould can also be heated more evenly and the injection rate of the plastic can be increased to help avoid weld lines.

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