Injection molding white spots and solutions
White specks are a common cosmetic defect in injection molding production. They appear as white spots or streaks on or within the surface of a plastic part, affecting not only the product’s appearance but, in severe cases, its mechanical properties. The causes of white specks are complex and varied, involving various factors, including raw materials, equipment, processes, and molds. Therefore, specific solutions are needed for each cause.
Raw material issues are a common cause of white spots during injection molding. Impurities or foreign matter mixed into the raw materials are a significant factor in causing white spots. For example, during storage or transportation, other types of plastic particles, dust, or metal debris can be introduced. These impurities cannot fully blend with the melt during the injection molding process, resulting in white spots in the molded part. Raw material quality issues can also contribute to white spots, such as the presence of incompletely polymerized small molecules. These substances can volatilize or decompose at high temperatures, forming bubbles that appear as white spots. Insufficient drying of the raw materials is also a common cause, especially for highly hygroscopic plastics such as PA and PC. Incomplete drying can cause moisture in the raw materials to vaporize during the injection molding process, forming bubbles that appear as white spots on or within the molded part.
Problems with the injection molding equipment may also cause injection molding white spots. When the barrel and screw of the injection molding machine are severely worn, it will cause uneven plasticization of the raw materials in the barrel. Some of the raw materials will not be fully melted, forming solid particles. These particles enter the mold cavity during the injection process and form white spots in the plastic part. Residues in the barrel are also a hidden danger. If the previously produced plastic is incompatible with the currently produced plastic, the residues will accumulate in the barrel and form white spots in the plastic part after mixing with the current raw materials. In addition, if the nozzle of the injection molding machine is clogged or damaged, it will cause poor melt flow, local overheating or excessive shearing, causing the raw materials to decompose, producing gas or decomposition products, and then forming white spots.
Improperly set injection molding process parameters are a key factor in the generation of white spots. Both excessively high and low melt temperatures can cause problems. Excessively high temperatures can cause the raw materials to decompose, producing volatile gases. These gases form bubbles in the plastic part, appearing as white spots. Excessively low temperatures can lead to insufficient plasticization of the raw materials, resulting in unmelted particles and the formation of white spots. Improper settings for injection speed and pressure can also contribute to white spots. Excessively fast injection speeds can cause turbulence in the melt within the cavity, entraining air and forming bubbles. Insufficient injection pressure can lead to inadequate melt filling, creating voids within the part, which manifest as white spots. Insufficient holding pressure and time can also contribute, failing to fully compensate for the cooling shrinkage of the melt, leading to shrinkage cavities within the part and the formation of white spots.
Problems in mold design and use can also lead to white spots in injection molding. If the mold’s exhaust system is not designed properly, the air in the cavity cannot be discharged in time and will be compressed by the melt, forming bubbles. These bubbles appear as white spots in the plastic part. Improper design of the mold’s gate and runners can also affect the flow state of the melt. For example, if the gate size is too small, the melt will be subjected to strong shearing when passing through, causing the raw materials to decompose and produce gas. A rough runner surface will increase the flow resistance of the melt, resulting in poor melt flow and local overheating, which in turn forms white spots. In addition, oil, moisture, or impurities on the mold surface will come into contact with the melt, affecting the fluidity and adhesion of the melt and forming white spots on the surface of the plastic part.
Different solutions are needed for the various causes of white spots in injection molding. Regarding raw material control, storage and management should be strengthened to avoid contamination with impurities, while also selecting high-quality raw materials. Hygroscopic plastics should be thoroughly dried, with strict control of drying temperature and time to ensure the moisture content of the raw materials is within the permitted range. Regarding equipment maintenance, regularly inspect and replace worn screws and barrels, clean any residue inside the barrel, and ensure the proper functioning of the injection molding machine. If the nozzle is clogged or damaged, clean and replace it promptly to ensure smooth melt flow.
When optimizing process parameters, rationally adjust parameters such as melt temperature, injection speed, injection pressure, holding pressure, and holding time based on the type of plastic and product structure. For easily degradable plastics, the melt temperature should be appropriately lowered; for plastics with poor fluidity, the melt temperature and injection pressure can be appropriately increased, while the injection speed can be adjusted to avoid turbulence in the mold cavity. Ensure sufficient holding pressure and holding time to compensate for melt shrinkage during cooling and reduce the formation of shrinkage cavities and bubbles.
In terms of mold optimization, the mold’s exhaust system can be improved by adding exhaust slots to ensure timely exhaust of air within the cavity. The gate and runner design can be optimized, with gate size and location appropriately set to ensure smooth melt flow and minimize shear. Oil, moisture, and impurities should be regularly cleaned from the mold surface to keep it clean and smooth. Furthermore, the mold can be properly preheated to prevent defects caused by a sudden drop in temperature after the melt enters the mold. Implementing these comprehensive measures can effectively resolve the problem of white spots in injection molding and improve the quality of injection molded products. In actual production, specific analysis and judgment should be conducted to identify the specific cause of white spots and implement targeted solutions to achieve optimal results.
