Causes and solutions for the appearance of numerous tiny pitting spots on the surface of bright plastic parts
One common cause of tiny pitting on the surface of shiny plastic parts is excessive impurities or volatiles in the raw materials. These substances evaporate due to heat during the injection molding process or fail to fuse with the melt, ultimately forming pitting on the surface of the part. Impurities in the raw materials include dust, metal debris, and unmelted particles. If the raw materials are not stored in a clean environment or the hopper is not cleaned promptly, these impurities can mix into the melt. For example, tiny dust particles mixed into PC raw materials can appear as pitting on the surface of the plastic part after molding. Volatile substances in the raw materials come from the decomposition of additives (such as plasticizers and stabilizers) or low-molecular-weight volatiles within the raw materials themselves. For example, the HCl gas produced by the decomposition of PVC materials at high temperatures can form needle-shaped pitting on the surface of the plastic part if it is not discharged promptly. For this reason, it is necessary to strengthen the purification of raw materials and install a high-efficiency filter (filtration accuracy 5μm) above the hopper to prevent dust from entering; pre-dry the raw materials, such as drying PC at 120℃ for 4 hours to remove moisture and volatiles; regularly clean the barrel and screw, and clean them with cleaning material every 8 hours of production to avoid impurity accumulation.
Poor mold venting is a key factor leading to fine pitting on the surface of shiny plastic parts. Air and volatiles within the mold cavity cannot escape promptly, becoming trapped by the melt and forming bubbles or depressions on the part surface, which appear as fine pitting. When the melt fills too quickly or the cavity structure is complex (such as deep cavities or narrow gaps), air can be easily compressed in corners or at the base of ribs. For example, the narrow side ribs (0.8mm width) of a mobile phone case prevent air from escaping during rapid melt filling, resulting in dense pitting on the rib surface. Insufficient mold venting depth and width can also contribute to poor venting. Generally, venting grooves should be kept between 0.01-0.03mm deep (adjusted based on material viscosity, with the smaller value being used for low-viscosity materials) and 5-10mm wide. A venting groove depth of only 0.005mm is insufficient for adequate venting. Solutions include deepening the venting groove to an appropriate depth (e.g., from 0.008mm to 0.02mm), extending the venting groove to the outside of the mold, and adding venting inserts at the final fill point. For complex cavities, vacuum exhaust technology can be used to extract the air in the cavity through a vacuum pump, and the vacuum degree is controlled at -0.08MPa to -0.09MPa, effectively reducing gas residue. For example, when molding optical lenses, vacuum exhaust is used to reduce the number of surface pitting from 5 per square centimeter to 0.
Improperly set injection molding process parameters can cause pitting on the surface of shiny plastic parts. These primarily involve parameters such as melt temperature, injection speed, holding pressure, and mold temperature. Excessively high melt temperature can cause the raw materials to decompose and produce gases. For example, ABS decomposes easily above 250°C, and the resulting gases form pitting on the surface of the plastic part. Excessively low temperatures increase melt viscosity, making it easy to entrap air during filling, resulting in bubble pitting. Excessively fast injection speeds increase melt turbulence and entrap air. Excessively slow injection speeds cause the melt to cool too quickly during filling, resulting in surface depressions and pitting. Insufficient holding pressure or a short holding time fails to compensate for melt shrinkage, resulting in sink marks and pitting. Excessively low mold temperature causes the melt to cool rapidly on the cavity surface, forming a hardened layer that prevents subsequent melt from merging with the hardened layer, causing pitting. Methods for adjusting process parameters include: controlling the melt temperature within the material’s recommended range (e.g., 200-230°C for ABS), using multi-stage injection (slow-fast-slow) to reduce air entrapment, increasing the holding pressure (e.g., from 70MPa to 85MPa) and extending the holding time (e.g., from 5 seconds to 8 seconds), and raising the mold temperature (e.g., from 50°C to 70°C). For example, a glossy plastic part developed pitting due to a too-low mold temperature (40°C). Raising the temperature to 60°C completely eliminated the pitting.
Poor mold surface quality, oil stains, or scratches can cause tiny pitting on the surface of shiny plastic parts. This is because the melt cannot adhere tightly to the rough cavity surface during flow, or oil volatilizes due to heat, forming bubbles. The mold cavity surface roughness must reach Ra0.02-0.05μm (mirror finish). If the surface is not sufficiently polished (Ra0.1μm or above), tiny pits on the surface prevent the melt from completely filling the cavity, resulting in pitting. Oil stains on the cavity surface (such as mold release agent residue and finger oil) will evaporate or carbonize at high temperatures, mixing with the melt and causing pitting. To address this issue, the mold cavity needs to be re-polished using diamond paste (grit size W1-W3) for mirror polishing, ensuring a surface roughness of Ra0.03μm. Use an oil-free release agent or reduce the amount of release agent used. Wipe the cavity surface with a clean cotton cloth dipped in alcohol before each mold closing to remove oil and impurities. Regularly inspect the cavity surface and repair any scratches or corrosion, such as by filling the scratches with argon arc welding and then re-polishing. A mold for an automotive lampshade had minor scratches in the cavity, resulting in pitting. After re-polishing, the surface of the plastic part was bright and pit-free.
Poor plasticization of the melt within the barrel is also a cause of pitting on the surface of shiny plastic parts. Insufficiently plasticized raw material particles or gels within the melt are exposed on the surface after molding, resulting in rough pitting. Poor plasticization is often caused by low screw speed, insufficient backpressure, or an improper barrel temperature distribution. For example, a screw speed of 50 r/min is too low, preventing the raw material from fully melting within the barrel for an insufficient time; a backpressure of 0.5 MPa is too low, failing to eliminate bubbles and unmelted particles. Solutions include increasing the screw speed (e.g., from 50 r/min to 80 r/min), increasing the backpressure (e.g., from 0.5 MPa to 2 MPa), and optimizing the barrel temperature profile (slightly lower in the feed zone and gradually higher in the compression and metering zones). For materials prone to gel formation (such as PC and PMMA), the barrel temperature must be strictly controlled to avoid localized overheating. For example, for PMMA, the barrel temperature should be set between 210-230°C, and the barrel should be regularly flushed with cleaning material between production runs. A PMMA lens developed pitting due to poor plasticization. By increasing the back pressure to 2.5 MPa and adjusting the screw speed to 70 rpm, the pitting problem was completely resolved. Additionally, the screw and check ring should be regularly inspected for wear. If severe wear results in uneven melt formation, they should be replaced promptly to ensure melt plasticization quality.
