How to Prevent Black Specks During Injection Molding

When producing transparent molded plastic parts, it often happens that black specks cannot be completely eliminated even after continuous molding for more than ten hours or several days. This results in substantial waste of both production time and raw materials. Therefore, it is of great significance to prevent excessive black specks. The preventive measures and precautions listed below are effective for this issue.

① For production scheduling, the injection molding machines assigned to produce transparent or light-colored parts should not have processed easily scorched materials such as POM and PVC recently. Preferably, they should not have run dark or black molded parts either. Meanwhile, high-quality or relatively new injection molding machines are recommended for transparent products. Heavily worn screws and check rings tend to cause material scorching and black specks. For better control, it is advisable to designate several good-condition, newer injection molding machines exclusively for transparent parts. It is critical to separate machines for rigid transparent resins such as PC and transparent ABS from those for transparent PVC; these machines shall not be shared.

② The usage of regrind material (commonly known as sprue material) should be minimized. 绝大部分 virgin material is the ideal option. Regrind is usually reused to save material costs, so the regrind must be kept clean, free of black specks and oil stains. Fine powder in regrind is prone to scorching and generates abundant black specks, so the powder must be sifted out thoroughly before production. This phenomenon is especially prominent with transparent PVC, so its regrind requires more careful sieving. To avoid contamination of regrind during storage, transportation and granulation, it is best to adopt the method of on-the-machine granulation and immediate reuse.

③ Adopt stepwise heating during warm-up to avoid local overheating and scorching inside the barrel, especially for easily scorched resins like PVC. For transparent PVC, set the temperature to around 110℃ first, hold for several minutes once the temperature stabilizes, and then raise it to the molding temperature. Start production immediately once the target molding temperature is reached; long idle periods will generate massive black specks.

④ Maintain a stable production cycle. Avoid frequent start-stop operation or unscheduled long shutdowns. Some scorch-prone resins can form numerous black specks even after just a few minutes of halt. For temporary stops, reduce the barrel temperature by 30~50℃, run the machine for a few shots before stopping. For PVC, purging the barrel right away is mandatory; otherwise, persistent defects will occur.

⑤ Avoid using a large machine for a small mold, which is a major cause of excessive black specks. With a relatively small shot size, the melt stays inside the barrel for too long and gets scorched inevitably. It is almost unworkable for materials sensitive to scorching.

⑥ Do not set the temperature excessively high. Set the barrel temperatures in a descending gradient from the front zone to the rear zone. Lower temperatures in the middle and rear zones help effectively prevent scorching and black specks.

⑦ Pigment quality matters greatly. Poorly dispersible pigments will leave persistent pigment spots that are hard to remove. In some cases, pre-compounding and pelletizing of raw materials in advance is required to solve the problem.

⑧ An overly long molding cycle easily triggers black specks for scorch-sensitive resins such as PVC.

The above measures and precautions for preventing black specks on transparent molded parts are also applicable to light-colored parts to a certain extent. Compared with transparent parts, black speck defects on light-colored parts are relatively easier to control.