The injection molded surface is dull or has uneven gloss.
Dull or uneven gloss on injection molded parts is a common appearance defect that directly impacts the product’s visual quality and market competitiveness. This is particularly true in applications requiring high appearance standards, such as automotive interiors, appliance housings, and medical devices, where such defects can lead to product rejection. This problem involves multiple factors, including material properties, mold condition, and process parameters. It manifests as an overall dull surface, with areas of darkening, or uneven light and dark patches. The root cause is often uneven solidification of the melt on the mold cavity surface or microscopic defects (such as tiny pits and scratches) on the part surface. Addressing this issue requires a systematic analysis of all influencing factors and targeted improvement measures to achieve a uniform and stable surface gloss.
Material properties and pretreatment processes significantly influence the surface gloss of plastic parts. Different plastic materials naturally vary in gloss. Materials like ABS and PC inherently possess relatively high gloss, while polyolefins like PP and PE, due to their higher crystallinity, have relatively low surface gloss. If the proportion of recycled materials mixed into the raw materials is too high (over 30%), impurities or degraded molecules in the recycled materials can lead to uneven melt flow, resulting in uneven surface gloss on the plastic parts. Furthermore, the moisture content of the raw materials is a key factor. Hygroscopic materials like PA, PC, and PBT, if not adequately dried, can generate bubbles during the melting process. These bubbles burst, forming tiny pits on the surface of the plastic part, reducing gloss. To address these issues, the raw material purity must be strictly controlled, with the recycled content not exceeding 20%. Hygroscopic materials must also be thoroughly dried: PA66 should be dried at 80-100°C for 4-6 hours, and PC at 120-130°C for 6-8 hours, ensuring a moisture content below 0.02%. The dried raw materials should be sealed and stored to avoid moisture absorption again. At the same time, adding 0.5%-1% brightener (such as stearamide) to the raw materials can improve the fluidity and surface film-forming properties of the melt and enhance the gloss of the plastic parts.
Mold condition is a key factor in determining the surface gloss of a plastic part. The mold cavity surface quality and venting performance directly impact the part’s appearance. Cavity surface roughness is a key indicator. Scratches, rust, or machining marks on the surface can cause these defects to replicate on the part surface, resulting in a localized matte appearance. Cavity surface roughness is typically required to be Ra ≤ 0.08μm. For high-gloss parts, mirror polishing (such as diamond paste polishing) is required to achieve Ra ≤ 0.02μm to ensure the melt perfectly replicates the cavity surface condition. Oil stains or mold release agent residue on the cavity surface can also cause a dulling of the part surface. This prevents the melt from fully contacting the cavity surface, forming tiny bubbles. Regular cleaning of the cavity surface is essential. Use alcohol or a specialized mold cleaner to remove oil stains. Avoid using silicone-based mold release agents (use fluorine-based release agents instead). Poor mold venting is another major cause of uneven gloss. Air trapped in the cavity cannot be promptly expelled, forming an air barrier at the melt front. This prevents the melt from adhering closely to the cavity surface, resulting in localized gloss variations due to varying cooling rates. An exhaust groove needs to be set in the area where the cavity is last filled. The depth should be controlled at 0.01-0.03mm (adjusted according to the fluidity of the material) and the width should be 5-10mm to ensure smooth exhaust of gas. At the same time, exhaust needles should be added at the corners of complex structures to improve exhaust efficiency.
Optimizing process parameters is crucial for improving the surface gloss of plastic parts. Proper settings of melt temperature, mold temperature, injection pressure, and injection speed can significantly enhance surface quality. Too low a melt temperature results in insufficient melt fluidity, preventing it from fully filling the corners of the mold cavity, resulting in matte areas on the part surface due to material shortages. Excessively high temperatures can cause material degradation, with degradation products adhering to the mold cavity surface, forming dark spots. The melt temperature should be set appropriately based on the material’s characteristics: 200-230°C for ABS, 280-320°C for PC, and 180-220°C for PP. Localized overheating should be avoided through segmented temperature control (with the front section of the barrel slightly lower than the middle section). Mold temperature significantly affects the surface gloss of plastic parts. Too low a temperature causes the melt to solidify quickly on the mold cavity surface, preventing a smooth surface. Too high a temperature prolongs cooling time and reduces production efficiency. For plastic parts requiring high gloss, the mold temperature should be set 10-20°C above the material’s glass transition temperature: 60-80°C for ABS and 100-120°C for PC. Increasing the mold temperature slows the solidification of the melt surface, allowing the molecular chains to align and form a smooth surface. The injection pressure and speed must be matched. Insufficient pressure prevents the melt from closely adhering to the cavity surface, while too slow a speed causes the melt to cool excessively during the filling process. The combined effect of these two factors can result in a lack of gloss on the part surface. A “high-pressure, high-speed” filling strategy should be adopted, with injection pressure controlled at 80-120 MPa and injection speeds of 50-80 mm/s. This ensures that the melt fills the cavity quickly, closely adhering to the cavity surface, and minimizing surface defects.
The condition of the injection molding machine and the stability of its auxiliary systems can also affect the surface gloss of the molded part. Equipment wear or parameter fluctuations can lead to process instability, resulting in cosmetic defects. Wear of the screw and barrel can cause uneven melt plasticization, resulting in the presence of incompletely melted particles. These particles form bumps on the surface of the molded part, disrupting the uniformity of the surface gloss. Regularly check the screw-barrel clearance. If the clearance exceeds 0.3mm, replace the worn part promptly. Clean the screw after every 5,000 mold cycles to remove residual degraded material or impurities. Inadequate sealing performance of the check ring can cause melt backflow during the injection phase, resulting in unstable melt pressure and flow rate in the actual injection cavity, causing alternating light and dark streaks on the surface of the molded part. Check the check ring for wear monthly and replace any poorly sealed rings to ensure no melt backflow during the injection process. In addition, attention should also be paid to the oil temperature and oil pressure stability of the injection molding machine. Excessive oil temperature (over 55°C) will cause pressure fluctuations in the hydraulic system, affecting the stability of the injection molding speed and pressure. The cooler needs to be cleaned regularly to ensure that the oil temperature is controlled at 40-50°C. At the same time, the oil contamination level should be checked and the hydraulic oil should be replaced every six months to avoid valve block jamming due to oil impurities.
Post-processing can be used as a supplementary means to improve the surface gloss of plastic parts and is suitable for products with extremely high aesthetic requirements. Minor gloss defects caused by mold or process limitations can be corrected through surface spraying. Using a high-gloss coating (such as polyurethane), a uniform coating is applied to the surface of the plastic part via electrostatic spraying or curtain coating, enhancing gloss while concealing minor defects. For low-surface-energy materials such as PE and PP, corona treatment or flame treatment is required before spraying to increase the surface tension (to 38-42 mN/m) and ensure good adhesion of the coating. For transparent plastic parts (such as PC and PMMA), polishing can be used to enhance gloss. Mechanical polishing using a wool wheel and a polishing paste (such as chromium oxide polishing paste) removes minor surface scratches and improves gloss by 10%-20%. Additionally, plasma treatment can be used to improve the microscopic morphology of plastic surfaces. Plasma bombardment removes surface oil and oxide layers while creating a nanoscale surface texture, enhancing the uniformity of light reflection. This is suitable for surface treatment of high-end medical and electronic plastic parts. It should be noted that post-processing will increase production costs and production cycles. Therefore, priority should be given to solving the gloss problem from the source by optimizing molds and process parameters, and post-processing is only used as a supplementary means.
