Injection molding black spots and solutions
Injection molding black specks are a common cosmetic defect in plastic part production, appearing as dark spots on or within the part. These defects are often caused by raw material contamination or impurities introduced during processing. Raw materials exposed to dust, metal debris, or other non-colored particles during storage and transportation can directly lead to black specks. For example, sand particles mixed into PE raw materials when stored outdoors can appear as black specks on the surface of the molded part after molding. Improper use of recycled materials can also contribute. Recycled materials containing unremoved metal impurities or high-temperature carbonized plastic particles can easily form black specks when mixed with virgin material. To address these raw material issues, strict raw material control procedures must be established: raw materials must be stored in a sealed, moisture-proof environment, and bags must be sealed promptly after opening. Recycled materials must be screened, cleaned, and dried to ensure impurity levels are below 0.01%. Magnetic filters and screens (50-100 mesh) should be installed at the hopper inlet to intercept metal and particulate impurities. For example, a company reduced the black spot defect rate from 3% to below 0.5% by adding a three-stage filtration system (coarse filtration, magnetic separation, and fine filtration) in front of the hopper.
Residual impurities or char in the injection molding machine barrel, screw, and nozzle are key factors in causing black specks. Over time, these components can accumulate unremoved plastic debris on their inner surfaces. This char, carbonized at high temperatures, can then mix into the new melt, causing black specks. Excessive clearance between the barrel and screw allows material to accumulate within the gap and carbonize after repeated heating. For example, if an injection molding machine is over five years old and not regularly serviced, the screw-barrel clearance may increase from 0.1mm to 0.3mm, exacerbating this stagnation. Solutions include regular barrel and screw cleaning. Flushing with a specialized cleaning compound (such as PE cleaning compound) three to five times every eight hours of production or when changing raw materials, for at least 10 minutes each flush. Severely carbonized components should be disassembled and polished with a copper brush or sandpaper to remove any residual char. Regularly inspect the screw, check ring, and nozzle for wear and replace any excessive wear to ensure the clearance is maintained within 0.1-0.2mm. A factory’s PC plastic parts had black spots due to residual coke in the barrel. By disassembling, cleaning and replacing the worn check ring, the black spot problem was completely solved.
Oil, rust, or foreign matter inside the mold are also common causes of black spots in injection molding. Residual release agent, anti-rust oil, or plastic debris in the mold cavity, runners, or ejector pin holes can carbonize at high temperatures or mix with the melt, forming black spots. For example, if the mold is not thoroughly cleaned after prolonged downtime, the anti-rust oil on the cavity surface can evaporate during molding due to heat, combining with the melt and forming black spots. Plastic residue accumulated in the gap between the ejector pin and the ejector pin hole can mix with the melt as the ejector moves, causing recurring black spots. Solutions include: wiping the mold cavity and runners with alcohol or a specialized cleaner before each use to remove oil and impurities; regularly checking clearances between moving parts like the ejector pin and slider to remove any remaining debris, disassembling and cleaning as necessary; and installing filter inserts at the mold runner entrances to intercept impurities that may enter the cavity. For precision molds, ultrasonic cleaning can be used to completely remove impurities within tiny gaps. For example, ultrasonic cleaning of an optical lens mold reduced the black spot defect rate from 2% to 0.1%.
Improperly set injection molding process parameters can exacerbate the formation of black specks, especially when the melt temperature is too high or the injection speed is too fast. The raw material is susceptible to thermal degradation, forming black degradation products. Different plastics have varying heat resistance. For example, the decomposition temperature of PVC is approximately 160°C. If the barrel temperature is set to 180°C, the raw material will decompose and produce black impurities. PP will also carbonize due to overheating when the temperature exceeds 270°C. Excessively fast injection speeds can cause severe shearing of the melt within the flow channel, leading to localized temperature increases and degradation. For example, when the speed increases from 50mm/s to 100mm/s, shear heat can cause the melt temperature to rise by 30-50°C locally, exceeding the thermal stability limit of the material. Solutions include: setting an appropriate barrel temperature based on the plastic’s characteristics, such as 140-160°C for PVC and 200-240°C for PP, and monitoring the actual barrel temperature in real time using thermocouples; employing multi-stage injection to avoid high speeds throughout the injection process, such as using a low speed (30 mm/s) at the runner entrance and a medium speed (60 mm/s) during cavity filling to reduce shear heating; and appropriately reducing back pressure (for example, from 3 MPa to 1.5 MPa) to avoid heat accumulation caused by excessive melt plasticization. In an ABS plastic part, black spots developed due to an overly high barrel temperature (260°C). Reducing the temperature to 230°C reduced the number of black spots by 80%.
Environmental factors and improper equipment maintenance can also cause injection molding black spots. Excessive dust and poor air cleanliness in the production workshop can allow impurities to enter the mold cavity with the raw materials or mold. Failure of the equipment’s cooling system can cause localized overheating of the barrel, which can also lead to raw material carbonization. Addressing environmental issues requires strengthening workshop management: installing air purifiers to control dust concentrations in the workshop to below 10mg/m³; implementing sealed raw material delivery pipelines to prevent dust from entering; and regularly cleaning production equipment surfaces to prevent dust from falling into the hopper. Regarding equipment maintenance, check the cooling water lines for unobstructed flow and ensure uniform temperature across the barrel. For example, if the temperature in the middle of the barrel is abnormally high (20°C above the set point), remove scale from the water lines or replace damaged cooling coils to restore normal cooling function. Furthermore, operators must adhere to standardized procedures, avoiding direct contact with the raw materials or mold cavity to prevent the intrusion of foreign matter such as oil, hair, and so on. Comprehensive environmental and equipment management can further reduce the occurrence of black spots and ensure consistent plastic part appearance quality.
