Color variation is a common appearance defect in injection molding, manifesting as uneven shade, gloss difference, streaks, spots, or deviation from standard samples. It seriously reduces qualification rates, increases costs, and delays deliveries. Most color variation stems from uncontrolled variables in raw materials, color masterbatch, processing parameters, molds, or equipment. Blind parameter adjustments often worsen the problem; systematic inspection and targeted solutions are essential.
Root Cause of Color VariationColor variation essentially results from instability in raw materials, color masterbatch, processing parameters, molds, and equipment. Any deviation in these links directly affects product color. Raw material/color masterbatch quality fluctuations are fundamental causes; unstable processing parameters are the most common on-site factors; mold/equipment conditions determine whether color variation can be adjusted. Identifying core causes is the first step to solving the problem.
Raw Material and Color Masterbatch Management: Foundation for Color StabilityRaw material stability is key. The same product must use the same grade, brand, and batch of base resin—different resins vary in melt flow and base color, causing visible differences. Recycled materials need fixed proportion, crushing size, and cleanliness to avoid graying or darkening. Color masterbatch must match the base resin in dispersion and heat resistance. Materials are mixed with a mixer for 3–5 minutes to ensure uniformity. Color masterbatch is stored in a dry, dark, sealed environment to prevent moisture absorption and pigment precipitation; expired or damp masterbatch is discarded.
Injection Process Parameter Adjustment: Key to Stable ColorFluctuating process parameters directly cause color variation. Barrel temperature is critical: excessive temperature decomposes pigments (yellowing), while insufficient temperature causes uneven dispersion (color streaks). Once qualified parameters are confirmed, no arbitrary changes are allowed. Properly increasing back pressure enhances mixing and color uniformity but avoids overheating. Medium injection speed prevents shear overheating or insufficient filling. Stable holding pressure (40–60% of injection pressure) and time (5–10 seconds) avoid density differences and gloss unevenness.
Mold System Optimization: Eliminate Structural Color VariationUnreasonable mold design causes unadjustable color variation. Short, thick, smooth runners reduce pressure loss and overheating. Proper gate size and position avoid shear-induced color anomalies. 0.02–0.03 mm deep venting grooves at gas trap positions prevent dark spots. Mold temperature must be stable, with cavity-core temperature difference ≤ 5°C. Regular polishing and cleaning of mold surfaces prevent uneven gloss caused by wear or carbonization; glass fiber reinforced PET production requires wear-resistant steel molds with nitriding treatment.
Equipment and Environment Control: Reduce External InterferenceWorn screws or check rings cause uneven mixing and periodic color variation, requiring regular inspection and replacement. Clean dryers and feeders avoid cross-contamination. Stable workshop temperature and humidity prevent color changes in light-colored or transparent products. Raw materials, masterbatch, and products are protected from direct sunlight; color comparison is done under standard light sources to avoid misjudgment.
Common Defect TroubleshootingWhen color variation occurs, first check raw materials and color masterbatch for quality issues. If materials are qualified, adjust process parameters (prioritize temperature and back pressure). If problems persist, inspect mold condition (venting, surface, cooling). For periodic color variation, check equipment wear parts. Systematic troubleshooting avoids blind adjustments and reduces downtime.
ConclusionSolving color variation relies on stability and precision—stable raw materials, parameters, and equipment, plus precise defect inspection. Establishing standardized processes, pre-production inspection, and on-site monitoring reduces batch defects. Operators must master factor impacts and avoid blind adjustments to improve color consistency, reduce costs, and ensure stable quality.
