Troubleshooting Common Issues in Injection and Cast Moulding- How We Prevent Them.
Injection moulding and cast moulding are two of the most reliable and cost-effective manufacturing methods for producing a wide variety of plastic parts.
However, as with any industrial process, there are various issues which can arise. From warming and sink marks to air bubbles, even minor issues can impact the final results, so it’s vital to use a trustworthy team for this process.
At Polytech Mouldings, we specialise in delivering engineered plastic solutions using high-quality injection and cast moulding. Our in-house tooling and mould manufacturing services are suitable for a range of specifications and operating environments.
Choose us and your parts will always be consistently of high quality. Let’s look at some common moulding problems — and what we do to avoid them.
1. Warping
Warping occurs when a moulded part cools unevenly, causing it to twist or deform. This typically happens when different sections of a part solidify at different rates, especially if the wall thickness is inconsistent.
To prevent this, it helps to design parts with uniform wall thickness wherever possible. Our team can also adjust cooling times to ensure the entire part solidifies evenly. We can use materials with lower shrinkage rates to minimise stress during cooling.
At Polytech, our design engineers work closely with you from the outset to ensure the part geometry supports smooth, uniform moulding, so you won’t have to worry about costly redesigns or post-processing.
2. Short Shots
A short shot happens when the molten plastic doesn’t completely fill the mould cavity. Consequently it leaves gaps or incomplete parts. This is often caused by inadequate injection pressure, trapped air, or poor venting.
There are a number of steps which can reduce this common issue. That includes increasing the injection pressure or temperature to improve the flow.
We use advanced simulation software to analyse flow patterns before mould production begins, reducing the risk of short shots during actual runs.
3. Sink Marks
Sink marks appear as small depressions on the surface of a moulded part (often where the material is thicker). They occur due to shrinkage as the plastic cools and contracts.
At Polytech Mouldings, we help clients refine part designs during the development stage to minimise sink-prone areas and ensure aesthetic as well as functional quality.
4. Flash
Flash is basically excess plastic that seeps out of the mould cavity and solidifies along the edges of the part. It often results from worn out tooling, high injection pressures, or poor clamping.
One way to prevent this issue is to maintain and calibrate tools, so you avoid wear-and-tear. Because we handle tooling in-house, Polytech has full control over maintenance schedules and can quickly respond to wear issues before they affect product quality.
5. Bubbles and Voids
Air bubbles or internal voids reduce part strength and may signal trapped gases, moisture in the material, or uneven cooling.
To prevent it, you must dry materials thoroughly before processing, especially hygroscopic plastics. Improving venting also helps, since it allows air and gases to escape during mould filling. Adjusting the speed of the injection and pressure will also avoid turbulence.
We take material conditioning seriously — our equipment ensures plastics are processed in optimal condition, which is crucial for high-performance engineered parts.
6. Burn Marks
Burn marks appear as discoloured spots or streaks, often at the end of a flow. These are caused by trapped air overheating or degraded resin.
To prevent this issue one can improve venting at cavity ends to allow air escape. You can also reduce injection speed to prevent high shear heating. Lowering the melt temperature works if resin degradation is suspected.
Using the right mould design and processing settings is key, and our experienced technicians continuously monitor conditions to ensure a clean, high-quality finish.
Why Prevention Starts with the Right Partner
Most moulding issues stem from either design flaws or errors during processing. Both can be avoided with the right expertise and controls in place. At Polytech Mouldings, we work with you from concept to production, offering:
- Design for Manufacture (DFM) reviews
- In-house tooling and maintenance
- Material selection support
- Process optimisation and quality assurance
Whilst we specialise in engineered plastic products, our team supports a wide range of cast and injection moulded solutions across multiple sectors. Our material quality ensures that parts are moulded precisely to your specifications — ready to perform in the environment where they matter most.
Moulding problems are common, but with Polytech Mouldings, you won’t have to worry about them affecting your project. We always apply proven techniques to prevent such issues, ensuring there are no defects. This not only delivers better products faster but reduces waste
If you’re looking for a reliable partner with the expertise and facilities to get it right the first time, simply contact Polytech Mouldings.
