Injection molding color mixing and solutions
Color mixing during injection molding refers to defects such as uneven color, streaks, and spots on the surface or interior of a molded part, resulting in a product appearance that does not meet design requirements. This defect is common during the production of colored or multi-color composite parts. This defect can occur in plastics such as ABS, PP, and PE. For example, a red PP toy developed white streaks due to uneven color mixing, causing the pass rate to drop from 92% to 75%, resulting in batch rework. Color mixing during injection molding is essentially caused by insufficient mixing of the masterbatch and base resin, or by poor pigment dispersion during the molding process due to factors such as temperature and shear. This involves multiple factors, including raw material ratios, equipment parameters, and mold structure. Addressing color mixing requires optimizing the mixing process, adjusting molding parameters, and improving equipment structure to achieve uniform pigment dispersion and ensure consistent color in the molded parts (color difference ΔE ≤ 1.5).
Raw material ratios and mixing processes are fundamental factors influencing color mixing. An improper ratio or uneven mixing of masterbatch to base resin can directly lead to color defects. A low masterbatch addition ratio (e.g., <1%) results in insufficient pigment concentration, easily resulting in a light color shift. A high ratio (e.g., >5%) can lead to dark specks due to poor dispersion. For example, when a company was producing blue ABS plastic parts, increasing the masterbatch addition from 3% to 6% resulted in numerous blue specks. Adjusting the addition ratio to 4% eliminated the defects. Poor compatibility between the masterbatch and base resin is also a major cause of poor color mixing. For example, using a non-polar PE masterbatch with a polar PVC resin can result in delamination and streaking due to poor compatibility. Therefore, specialized masterbatches of the same type should be used. Insufficient mixing time or uneven stirring can lead to localized aggregation of the masterbatch. For example, mixing time of less than 3 minutes in a high-speed mixer can reduce the uniformity of the PP and masterbatch mix by 40%. It is recommended to control the mixing time to 5-10 minutes at a speed of 800-1200 rpm to ensure initial dispersion of the pigment.
The plasticizing capacity and screw structure of injection molding equipment significantly impact color mixing. Uneven plasticization can lead to poor pigment dispersion. An insufficient screw aspect ratio (<20:1) shortens the material's residence time in the barrel, resulting in incomplete mixing. For example, a screw with an aspect ratio of 18:1 is more likely to produce streaks when producing mixed-color PP parts than a screw with a 25:1 ratio. Therefore, a screw specifically designed for color mixing with an aspect ratio of 22:1 or higher should be selected. An inappropriate screw compression ratio (<2.5:1) results in insufficient shear force, preventing pigment particles from being fully broken down and dispersed. A compression ratio of 3:1-4:1 is recommended to enhance plasticization. A worn check ring or excessive clearance (>0.2mm) can cause material backflow, resulting in localized uneven mixing. Check the check ring clearance regularly and replace it if it exceeds 0.1mm. Dead corners in the barrel (such as below the feed port and at the screw head) are prone to accumulation of unmixed material, forming discolored spots. Dead corners can be reduced by increasing the barrel temperature or using a barrier screw. For example, a barrier screw can improve color mixing uniformity by more than 30%.
Improper process parameter settings can exacerbate color mixing defects. Temperature, screw speed, and back pressure are key control factors. A melt temperature that is too low prevents the masterbatch from fully melting and dispersing. For example, when the melt temperature of PP is below 180°C, the pigment particles in the masterbatch have difficulty opening, resulting in color spots. However, temperatures that are too high (e.g., above 230°C for PP) can cause pigment decomposition, resulting in discoloration streaks. The temperature should be controlled within the upper and middle limits of the plastic processing range (190-210°C for PP, 230-250°C for ABS). A screw speed that is too low (<50 rpm) results in insufficient shear force and poor mixing. A screw speed that is too high (>150 rpm) can cause pigment decomposition due to shear overheating. A speed of 80-120 rpm is recommended to ensure uniform shearing. Insufficient back pressure (<5 bar) will result in low material density and incomplete mixing; excessive back pressure (>20 bar) will increase energy consumption and barrel temperature. A back pressure of 5-15 bar is recommended to improve the mixing effect by increasing the compaction of the material. For example, after increasing the back pressure from 3 bar to 10 bar for a certain PE mixed-color plastic part, the color streaks were significantly reduced.
Mold structure and molding aids can positively impact color mixing. Proper runner design and venting systems can reduce color unevenness. A runner diameter that’s too small (less than 6mm) can lead to uneven melt shearing. A runner diameter of 8-12mm is recommended, with a circular cross-section to reduce flow resistance and enhance pigment dispersion within the runner. Improper gate placement can lead to uneven melt flow within the cavity, resulting in color variations. For example, using a single gate for large parts can easily produce color differences between the edge and center. Switching to multiple gates (spacing ≤ 200mm) significantly improves color consistency . Cavity surface roughness (Ra) greater than 1.6μm can easily trap unmixed material, resulting in color spots. Polishing to a Ra of less than 0.8μm is recommended. Furthermore, using a static mixer (installed between the nozzle and the mold) can force material separation and reorganization, improving color uniformity by over 50%, making it particularly suitable for demanding color-mixed parts. Regularly cleaning the accumulated materials in the barrel and runner (once every 1,000 molds) to avoid cross-contamination of materials of different colors is also an important measure to prevent color mixing defects.
Addressing color mixing during injection molding requires a comprehensive approach encompassing raw material control, equipment optimization, and process adjustments. For raw materials, select compatible masterbatches, control the addition ratio (1%-5%), and ensure uniform mixing (mixing time 5-10 minutes). For equipment, use a mixing screw with an aspect ratio ≥ 22:1, regularly inspect the check ring and barrel dead space, and install a static mixer if necessary. For process control, maintain melt temperature within the upper and lower limits, screw speed at 80-120 r/min, and back pressure at 5-15 bar. This promotes pigment dispersion through stable shear and plasticization. For molds, optimize runner and gate design to improve cavity finish and ensure uniform melt flow. Through these measures, one company reduced the color difference ΔE of its PP mixed-color molded parts from 3.2 to below 1.0, increased the pass rate to 98%, and significantly enhanced production stability. Minor color mixing defects can be remedied by adjusting the masterbatch ratio or reworking and painting, but severe defects require scrapping. Therefore, the key to preventing color mixing defects lies in early system optimization.
