How to prevent black spots during injection moulding

Black specks in transparent injection molded parts can persist for hours or days, wasting production time and material. The following precautions from the old-site article also help with light-coloured parts.

1. Select a suitable machine. Before producing transparent or light-coloured parts, avoid machines recently used with scorch-prone materials such as POM or PVC, and preferably avoid machines recently used for dark or black parts. Worn screws or check rings can create burnt deposits. Use newer, well-maintained machines for transparent production; keep machines for PC and transparent ABS separate from those used for transparent PVC.

2. Keep regrind clean and to a minimum. Virgin material is ideal when feasible. Any reused runners or sprues must be free of black specks and oil. Screen out fines before production because powder can scorch easily, especially with transparent PVC. Immediate, clean recycling beside the machine can prevent contamination during storage and transport.

3. Heat in stages. Gradual heating helps avoid local overheating in the barrel, particularly for PVC. For transparent PVC, the old-site procedure starts near 110 °C, holds briefly, then raises the setting to processing temperature and starts production promptly.

4. Maintain a stable production cycle. Avoid repeated starts and long stops. Some materials scorch even after a short pause. Before a necessary stop, lower the temperature by about 30–50 °C and run for a few minutes; PVC may also require immediate purging.

5. Match the machine to the mould. A large machine making a small shot leaves melt in the barrel for too long and increases the chance of scorching.

6. Avoid excessive barrel temperature. A descending temperature profile from the front to the middle and rear zones may help prevent burnt deposits.

7. Check powder quality and dispersion. Poorly dispersed powder can leave coloured specks; in some cases the material needs to be compounded into pellets first.

8. Avoid an unnecessarily long moulding cycle, especially when processing materials that scorch easily, such as PVC.

The same precautions can reduce specks in light-coloured parts, although these are generally easier to manage than transparent parts.