Key Points of Color Difference Control and Process Adjustment for China Injection Molding Parts
2026-06-09 11:41:49
Plastic Molds
Color difference is one of the most common appearance defects in China injection molding production. It undermines the aesthetic effect of finished products and may even lead to large-scale product rejection. In China’s injection molding industry, the emergence of color difference is related to raw materials, equipment, molds, process parameters and on-site management. To maintain consistent color of products, manufacturers need to carry out full-process control and conduct targeted process adjustment based on actual working conditions. The detailed control methods and adjustment essentials are elaborated as follows.
Raw Material Control PointsRaw materials are the primary cause of color difference, so classified management and pre-production inspection are essential for China injection molding. Raw materials of the same model from different batches or suppliers shall not be mixed for direct use. All incoming plastic pellets will be sampled for color inspection and compared with standard color swatches to confirm color uniformity. Color masterbatches, color powders and other colorants shall be sourced from fixed suppliers with constant mixing ratios. They need to be sealed and stored away from light to prevent color change due to moisture and oxidation.
In China’s conventional production mode, the proportion of new materials and recycled sprue materials must be kept stable. Only clean recycled materials of the same color are allowed, while foreign colored impurities and carbonized blocks are strictly forbidden. Raw material drying shall be set with proper temperature and duration. Excessive moisture will make products look hazy with lighter color, while over-drying will cause material aging and yellowing. When switching materials of different colors in domestic injection workshops, special purging compounds must be used to thoroughly clean the equipment and remove residual color materials inside barrels and screws, so as to prevent cross-color issues.
Equipment and Mold Control PointsAbnormal equipment operation will trigger batch color difference, so regular maintenance and cleaning are required in China’s injection molding workshops. Carbon deposits and color stains attached to the inner walls of barrels, screws and nozzles shall be removed on a regular basis. Residual materials will be carbonized under sustained high temperature and continuously affect the color of subsequent products. The temperature control system needs periodic calibration to ensure the actual temperature is consistent with the set value. Temperature deviation will result in inconsistent melting status of raw materials and obvious color variation. Parameters including injection speed, back pressure and screw rotation speed shall remain stable, and the fluctuation of hydraulic and pneumatic pressure shall be controlled within a reasonable range. Frequent parameter changes will reduce the uniformity of color mixing.
The surface condition of molds also exerts an impact on product color. Oil stains, water scales, rust and scratches on mold cavities will lead to uneven light reflection and form visual color difference. Operators in domestic factories will wipe mold cavities regularly to keep the surface clean and smooth. The overall temperature of molds shall be uniform with minor temperature difference across different areas. Uneven mold temperature will cause different cooling speed and inconsistent surface color. For multi-cavity molds widely adopted in China’s mass production, each cavity shall maintain the same state to avoid color deviation of products from individual cavities.
Injection Molding Process Adjustment PointsTemperature is the core parameter for color difference control in China injection molding, and the production principle of low temperature and stable operation is generally followed. If the overall barrel temperature is too high, plastic materials and colorants will decompose, making products look pale and dark or even generating color spots. If the temperature is too low, raw materials cannot be fully melted and colorants will disperse unevenly, resulting in striped color difference. The nozzle temperature shall be adjusted separately to avoid material blockage and discoloration caused by local high temperature and material retention. Mold temperature shall be fixed according to color requirements. Higher mold temperature brings better surface gloss and deeper color, while lower mold temperature leads to lighter color. All temperature adjustments shall be made slightly instead of drastic changes.
Back pressure and screw speed directly determine the mixing uniformity of colorants. Insufficient back pressure will cause incomplete fusion of color masterbatches and base materials, giving rise to uneven color and striped defects. Properly increasing back pressure can improve the mixing effect, yet excessive back pressure will produce extra friction heat and trigger decomposition and fading of colorants. In most Chinese injection workshops, the screw runs at a low and stable speed to guarantee uniform color mixing without extra heat generation.
Injection parameters, holding pressure and cooling process also need coordinated adjustment. Segmented injection speed is applied. Increase the speed for thin-wall parts and reduce the speed for thick-wall parts to avoid local heat accumulation and subsequent color difference. The injection pressure shall stay stable, as sudden pressure change will alter material flow and affect surface color presentation. Constant holding pressure and holding time are required. Insufficient holding pressure will form surface dents and visual color difference due to light variation, while excessive holding pressure will cause uneven internal stress and gradual discoloration of some plastic products. The cooling time is determined by product thickness. Uneven cooling will lead to inconsistent shrinkage, gloss and color tone of finished parts.
Production Process and Abnormal Handling PointsProduction environment will indirectly affect product color in China’s injection molding industry. The light, temperature and humidity of workshops shall be kept stable. Standard light sources are equipped at color checking stations to prevent wrong color judgment caused by varying light conditions. It is not allowed to replace raw materials, colorant brands or adjust mixing ratios arbitrarily during production. After material or batch replacement, several trial products shall be manufactured and compared with standard color swatches. Mass production can only start after the color is confirmed stable.
Routine inspection runs through the whole production process. Samples are taken at regular intervals to compare with standard color plates, and targeted solutions are adopted for overall color deviation, local color difference and batch color difference. When the overall color is too dark or too light, priority shall be given to fine-tuning barrel temperature, back pressure and colorant proportion. If local stripes or color spots appear, inspect the screw operation status, injection speed and carbon deposits inside equipment. Finished products shall not be exposed to strong sunlight or high temperature for a long time to prevent oxidative discoloration.
When restarting production after shutdown, empty the high-temperature retained materials inside barrels first. Materials staying under high temperature for a long time have deteriorated and will cause abnormal color. For restart after long-term shutdown, conduct comprehensive inspection on equipment, molds and raw materials in accordance with China’s workshop operating norms, and carry out trial production step by step to minimize color difference and ensure stable color quality of batch products.
