Methods to prevent black spots on transparent and light-colored plastic parts
Transparent plastic parts (such as PC lenses and PMMA lampshades) and light-colored plastic parts (such as white PP housings and light-colored ABS components) have extremely high requirements for appearance quality. Black specks are one of the main defects that affect their pass rate. Black specks are often caused by plastic degradation, foreign matter contamination, and mold impurities. They appear as dark spots on or within the plastic part, ranging in diameter from 0.1mm to 1mm, severely affecting the product’s light transmittance and aesthetics. For example, a 0.2mm black speck on a mobile phone screen protector can result in direct product scrapping, with a scrap rate of 10%-30%. Preventing black specks requires a comprehensive approach, encompassing raw material control, equipment cleaning, process optimization, and mold maintenance. Establishing a comprehensive quality control system is crucial to keeping the black speck defect rate below 1%.
Controlling the purity of raw materials is fundamental to preventing black specks, requiring strict scrutiny throughout procurement, storage, and drying. When purchasing, select raw materials of consistent quality and require suppliers to provide material impurity reports to ensure that no more than five black specks are detected per kilogram of raw material. Raw materials must be stored in sealed, moisture-proof containers to prevent the ingress of dust, metal debris, and other foreign matter. The storage environment must be kept clean, with floors and shelves regularly swept to prevent contamination. Highly hygroscopic transparent plastics (such as PC and PA) require drying, and the air filters in drying equipment must be replaced regularly (every 50 batches) to prevent airborne dust from entering the raw materials. Dried raw materials should be used immediately to avoid secondary contamination, with storage times of no more than two hours. For example, if PC raw materials are dried at 120°C for four hours and then left exposed to air for more than three hours, they will absorb moisture and potentially contaminate with impurities, increasing the risk of black specks. Recycled materials must be rigorously screened and purified. Recycled materials with black speck content exceeding the standard should not be used in transparent or light-colored plastic parts.
Maintaining equipment cleanliness is a key measure to prevent the formation of black specks. Particular attention should be paid to cleaning components that come into contact with the melt, such as the barrel, screw, and nozzle. The barrel must be thoroughly cleaned before production. Select the appropriate cleaning compound based on the material used in the previous batch. For example, when switching from dark to light-colored material, a dedicated cleaning compound (such as PE cleaning compound) should be used 3-5 times until the discharged melt is free of impurities. The screw and check ring should be regularly disassembled and cleaned (once every 1,000 mold cycles) to remove residual char and impurities. Use a copper brush, not a wire brush, to avoid scratching the surface and causing char residue. The nozzle should be equipped with a high-efficiency filter (80-120 mesh) and inspected every four hours. Replace any blockage or damage immediately. Hopper cleanliness is equally important. Before each shift, empty the hopper and blow it out with compressed air to remove dust and foreign matter. A filter should be installed at the intake port below the hopper to prevent large impurities from entering the barrel. For example, a company extended the barrel cleaning time from 10 minutes to 20 minutes by establishing an equipment cleaning SOP (standard operating procedure), and reduced the black spot defect rate by 60%.
Optimizing process parameters can reduce black specks caused by plastic degradation. This requires precise control of temperature, pressure, and screw speed. Transparent and light-colored plastics have limited thermal stability. Excessively high barrel temperatures can cause material degradation, resulting in the production of black charred material. For example, the barrel temperature for PC should be controlled between 260-300°C; temperatures exceeding 310°C can easily lead to degradation and the formation of black specks. The upper temperature limit for PMMA is 250°C; temperatures above this limit can cause molecular chain breakage and the formation of black impurities. Staged temperature control is necessary, with the feed section temperature slightly lower to prevent premature melting of the raw materials and the homogenization section temperature controlled 5-10°C below the material’s upper limit. The screw speed should be kept low (typically 30-60 rpm) to avoid localized degradation due to shear heating. For example, if the PC screw speed exceeds 80 rpm, the heat generated by shear can cause localized temperature increases of 20-30°C, increasing the risk of degradation. Back pressure should be moderate (3-8 MPa). Too high a back pressure will prolong the melt’s residence time in the barrel, exacerbating degradation. Too low a back pressure will result in uneven plasticization and the possibility of inclusion of unmelted particles. By optimizing process parameters, black spots caused by plastic degradation can be reduced by over 70%.
Mold cleaning and maintenance are crucial to preventing black specks. Regularly clean residual impurities from the cavity and runners. Before production, inspect the mold cavity and wipe the surface with alcohol to remove oil and dust. For complex cavities, use an ultrasonic cleaner (perform every 500 mold cycles) to ensure no residual char residue in corners. Runners and gates should be regularly cleared to remove solidified plastic and prevent char from entering the cavity with the melt. For example, the nozzles of hot runner molds should be disassembled and cleaned weekly to remove any degraded material. Mold vents must be kept unobstructed to prevent localized overheating and degradation caused by trapped air. Impurities within the vents should be cleaned with a fine copper wire brush to prevent blockage. Furthermore, the mold’s cooling system must function properly to ensure uniform cavity temperature and prevent localized high temperatures from causing plastic degradation. For example, temperature variations in PC molds should be controlled within ±5°C; otherwise, high-temperature areas are more likely to produce black specks. Establishing a regular mold maintenance program can reduce mold-related black speck defects by 50%.
Controlling the cleanliness of the production environment is a supplementary measure to prevent black spots. Airborne dust and impurities must be reduced. Workshops producing transparent and light-colored plastic parts should adopt cleanroom design with a cleanliness level of at least Class 100,000. The number of dust particles (≥0.5μm) in the workshop air should be controlled to below 3.5 million/m³. Air showers should be installed at workshop entrances, and operators must wear cleanroom suits, gloves, and hats to prevent contamination of raw materials and molds with impurities such as human hair and dander. Raw material delivery pipelines must be well sealed to prevent the inhalation of outside air, and filters should be installed at pipe joints to prevent the ingress of impurities. The area around production equipment should be regularly vacuumed to prevent dust from rising from the floor, and the workshop should be thoroughly disinfected and cleaned after each day’s production. For example, after upgrading its production workshop to a Class 100,000 cleanroom, one company saw the black spot defect rate for transparent plastic parts drop from 8% to 1.5%. Furthermore, the use of automated production equipment to reduce human contact with raw materials and molds can also effectively reduce the risk of black spot contamination.
