Avoid color deviation caused by local overheating.
Mold temperature indirectly changes the color of injection molded parts by affecting the crystallization state and cooling rate of the plastic. For crystalline plastics such as PP and PE, too low a mold temperature will lead to excessively rapid crystallization and uneven grain size, resulting in a frosty appearance on the surface of the plastic part and affecting the color saturation. Properly increasing the mold temperature to 40-60°C can promote uniform grain growth and make the color more vivid. For non-crystalline plastics such as PS and PC, mold temperature mainly affects the cooling rate. Excessively high temperatures will prolong the cooling time, causing flow marks on the surface of the plastic part and affecting color uniformity. When controlled at 20-40°C, the cooling rate is moderate and the pigment distribution is more even. In addition, mold temperature uniformity is crucial. If there is a temperature difference of more than 5°C between different areas of the cavity, the crystallinity or density of different parts of the plastic part will be different, resulting in obvious differences in color depth. Therefore, it is necessary to optimize the cooling water circuit design to ensure that the mold temperature fluctuation is controlled within ±2°C.
The stability of nozzle temperature significantly affects the color near the gate and requires strict control to avoid color deviation. If the nozzle temperature is too low, the melt tends to cool and solidify at the nozzle, forming cold slugs. This cold slug enters the mold cavity, causing white or light-colored spots near the gate and disrupting color consistency. Therefore, the nozzle temperature is typically 5-10°C higher than the barrel homogenization section to ensure good melt fluidity at the nozzle. However, the nozzle temperature should not be too high, as this can cause the melt to linger excessively at the nozzle and degrade. This is especially true for melts containing masterbatch, where degradation can result in a grayish-black appearance and contaminate the part’s color. For example, when producing red PVC parts, the nozzle temperature should be controlled between 170-180°C. Excessively high temperatures can cause the PVC to decompose, producing hydrogen chloride gas, which reacts with the pigment, resulting in a lighter color. Furthermore, the contact area between the nozzle and the mold must be properly insulated to prevent the mold’s low temperature from absorbing heat from the nozzle and causing a localized temperature drop.
Dynamic temperature control strategies are key to ensuring color stability during mass production. Before production, mold trials are conducted to determine optimal temperature parameters. Color data at different temperatures is recorded, and a temperature-color correlation model is established. During production, a closed-loop temperature control system monitors the barrel, mold, and nozzle temperatures in real time. If the temperature deviates by ±3°C from the set point, the system automatically adjusts the heating power or cooling water flow to ensure temperature stability. In production workshops susceptible to ambient temperature fluctuations, constant temperature air conditioning is installed to maintain a constant temperature of 20-25°C to minimize external interference with equipment temperature. Furthermore, regular cleaning of residual material in the barrel and nozzle is essential to prevent decomposition and discoloration of old material at high temperatures, which could contaminate new material. By improving temperature control accuracy to within ±2°C, the color difference ΔE value of injection molded parts can be kept below 1.0, meeting the color requirements of high-end products.
