The reason why light-colored PVC parts have many pitting spots after being stored for a few days
Pitting is a common cosmetic defect that appears on light-colored PVC parts (such as white, light gray, and beige) after a few days of storage. It manifests as tiny depressions or spots (0.1-1mm in diameter) distributed across the surface, seriously affecting the product’s aesthetics and market acceptance. The occurrence of this defect is closely related to the PVC material’s properties, molding process, storage environment, and post-processing. Effectively preventing pitting requires multi-step investigation and targeted preventive measures.
Insufficient thermal stability of PVC materials is one of the root causes of pitting. PVC is prone to decomposition during processing and storage, especially at temperatures exceeding 170°C or in the presence of impurities (such as metal ions). This releases hydrogen chloride gas, triggering an autocatalysis reaction that produces black or brown decomposition products. These products can cause spots on light-colored surfaces. Insufficient amounts of heat stabilizers (such as calcium-zinc stabilizers and organotin stabilizers) in the formulation (typically 3% to 5%) or poor compatibility between the stabilizer and the PVC resin can accelerate decomposition. For example, a white PVC pipe developed dense tan pitting after five days of storage due to only 2% heat stabilizer. Increasing the amount to 4% and replacing the stabilizer with a more compatible calcium-zinc composite stabilizer eliminated the pitting defect. Furthermore, a broad molecular weight distribution (dispersity index > 3) in the PVC resin can also reduce thermal stability, necessitating the selection of a resin grade with a narrow molecular weight distribution.
Improper molding process parameters can pose a risk of pitting. During the injection molding process, excessive barrel temperatures (over 190°C), fast screw speeds (>150 rpm), or high back pressures (>15 bar) can cause excessive shearing and heating of the PVC melt, leading to localized overheating and decomposition, resulting in tiny char particles. These char particles form pitting on the surface of the part. While initially invisible, they gradually become apparent during storage due to chemical changes. For example, a light-gray PVC profile produced at a barrel temperature of 200°C and a screw speed of 180 rpm developed numerous black pitting on its surface after three days of storage. Inspection revealed char particles with diameters of 0.2-0.5 mm. Reducing the barrel temperature to 180°C and the screw speed to 120 rpm eliminated the pitting. Poor mold venting can also lead to air entrapment in the melt, forming bubbles. These bubbles rupture during storage due to internal and external pressure fluctuations, leaving sunken pitting. This is especially true near the gate and in deep cavities. Ensure the venting groove is 0.01-0.02 mm deep and 10 mm wide.
Humidity and temperature in the storage environment are key external factors that trigger pitting. PVC products have low hygroscopicity (water absorption rate <0.5%), but in high humidity environments (relative humidity >80%), their surfaces are susceptible to moisture absorption. If there are temperature fluctuations (e.g., 30°C during the day and 15°C at night), moisture condenses on the surface and penetrates into the product, reacting with residual chloride ions in the PVC to form acidic substances, which corrode the surface and cause pitting. For example, a white PVC toy stored during the rainy season (humidity 85%-90%) developed white crystalline pitting on its surface after seven days. Testing revealed chloride salt precipitation. After controlling the storage humidity to 60%-70% and sealing the product, the pitting disappeared. Furthermore, contaminants in the storage environment (such as dust, oil smoke, and chemical gases) can adhere to the product surface and react with the PVC, causing spots. Therefore, the storage area should be kept clean and free from chemicals (such as solvents and acidic and alkaline substances).
Improper surface treatment of PVC products can accelerate pitting. If the product isn’t adequately cooled after molding (cooling time < 20 seconds), residual stress within the product can be high. Stress release during storage can cause microscopic surface cracks, which then absorb airborne impurities and form pitting. For example, a light yellow PVC sheet cooled for only 10 seconds developed a network of fine pitting on its surface after storage. Extending the cooling time to 30 seconds and employing a slow cooling process eliminated the defects. Furthermore, improper use of release agents can cause problems. Excessive amounts of grease or silicone can form an oily film on the surface, attracting dust and moisture and causing speckled pitting. A water-based release agent should be used at a dosage of 0.1-0.3g/m², and any excess should be wiped away with a clean cloth.
Impurities in raw materials and uneven dispersion of additives are other causes of pitting on light-colored PVC parts. If fillers (such as calcium carbonate) in the PVC formula have uneven particle size (particles >5μm) or are poorly dispersed, this can cause bumps or depressions on the surface of the finished product. Excessive amounts of lubricants (such as stearic acid) (over 1%) or incompatibility with other additives can precipitate onto the surface, forming oily spots that attract dust and cause pitting. For example, a white PVC gusset plate developed uneven pitting after storage due to uneven calcium carbonate particle size distribution (10% particles >10μm). However, replacing it with 800-mesh ultrafine calcium carbonate and adding a dispersant (0.5% calcium stearate) significantly improved its surface smoothness. The quality of the masterbatch is also crucial. Poorly dispersed pigments (particles >2μm) in low-quality masterbatch can cause spots on light-colored parts. Highly dispersible masterbatch should be used, with a controlled addition ratio of 2% to 5%, and premixed to ensure uniform distribution.
Comprehensive measures are needed to prevent pitting in light-colored PVC parts: Ensure the appropriate types and proportions of heat stabilizers, lubricants, and fillers in the formulation, and add 0.1% to 0.3% antioxidants when necessary. Strictly control barrel temperature, screw speed, and cooling time during processing to avoid excessive shearing and decomposition. Maintain a dry (humidity 50% to 70%), constant temperature (15 to 25°C), and clean storage environment, using sealed packaging and adding desiccant. Select high-quality PVC resins and additives for low impurity content and uniform dispersion. Through comprehensive process control, the pitting defect rate can be kept below 0.1%, ensuring the appearance quality and storage stability of light-colored PVC parts.
