Common Injection Mould Defects and Their Solutions
China injection moulds can deliver efficient high-volume production, provided that you catch defects early. Left unchecked, minor flaws can ruin the aesthetics of your moulded parts, while serious issues destroy their strength and safety.
You can avoid many of these problems as long as you understand what causes them. When you know what to look for, you can reduce waste and avoid costly rework. Plus, you can prevent your products from becoming unsellable.
Here are some of the most common China injection mould defects, along with why they happen and how manufacturers can minimise them.
Flow lines
Flow lines are those wavy marks or bands you might see on the surface of your moulded part. They’re often a slightly different colour from the surrounding material and usually show up around narrow sections or near the mould gates where the molten plastic enters the cavity.
Although flow lines don’t usually affect your part’s function, they can be a problem for products where appearance matters (such as premium consumer goods).
In most cases, flow lines happen with China injection moulds because the plastic cools at different speeds as it moves through the mould. Changes in wall thickness can also cause uneven cooling, making these marks more noticeable.
Manufacturers can reduce the risk of flow lines by increasing the injection speed and pressure, or even material temperature so the mould fills before the plastic cools. You can also adjust your mould design by including rounded corners where wall thickness changes or relocating the mould gates to reduce uneven cooling.
Air pockets
Also known as vacuum voids, air pockets are essentially trapped bubbles that remain inside a moulded part after they leave your China injection moulds.
Small voids are often considered a minor cosmetic issue. However, larger or more frequent air pockets can weaken your finished product because they leave empty spaces where solid material should be.
These defects usually occur with China injection moulds when the injection pressure isn’t high enough to force trapped air out of your mould. They can also develop if the material near the mould walls cools too quickly, so it causes the outer layer to harden before the inside has fully settled. This is likely to happen to your part when it’s thicker than about 6mm. Likewise, some materials are naturally more prone to forming voids as they cool.
To reduce the risk of air pockets, manufacturers can increase the injection pressure to remove trapped air more effectively and position the mould gates closer to thicker sections of the part, where premature cooling is more likely to occur. They can also recommend that you choose a material with lower viscosity to help the plastic flow more easily and reduce the chance of bubbles forming during moulding.
Warping
Warping happens when different areas of your moulded part cool and shrink at different rates. As the material cools, uneven shrinkage creates internal stress that can cause the part to bend or twist.
This usually happens when the cooling process is too fast or uneven. High material temperatures or materials with low thermal conductivity can make the problem worse. In some cases, how you design the mould can also be a factor, especially if the part has walls with inconsistent thicknesses that cool at different speeds.
Manufacturers can reduce the risk of warping by making sure the part cools gradually and evenly. This should give the material enough time to stabilise before it’s removed from the mould. They may also adjust the mould or material temperature or recommend a material with lower shrinkage. If your design itself is contributing to the problem, you can give your mould a more uniform wall thickness and improve the part symmetry to help keep the finished component stable as it cools.
Avoid injection moulding defects
The best way to prevent moulding defects is to identify potential issues before production begins.
At HLH Prototypes, we work with you from the early design stage to make sure your mould is properly designed for efficient manufacturing. From there, our team carefully manages the production process to produce high-quality China injection moulds and moulded parts that meet your specifications.
Get in touch today to request a free project review and discuss your project with our team.