Color deviation is a common defect in injection molding, often leading to mass rejection due to differences in component colors. The factors affecting color deviation are diverse, involving raw material resin, colorants, their mixture, injection molding processes, machines, molds, and more. Due to the wide scope involved, mastering color deviation control technology is recognized as a challenging skill in the plastics industry. To master color deviation control, a comprehensive understanding of the main contributing factors is necessary.

1. Key Factors Influencing Color Deviation
Impact of Raw Material Resin

Surprisingly, the raw material resin itself can influence color deviation, particularly when the resin has a certain base color. Comparative tests between different ABS types under the same injection conditions revealed varying degrees of color deviation in the products, even with the same type but different production batches. The extent of color deviation in products is directly proportional to the difference in resin base colors. Additionally, the stability of the resin itself significantly affects color deviation. Poorly stable resin is more susceptible to external fluctuations, resulting in increased possibilities of color deviation.

Influence of Colorants

Colorants are undoubtedly the primary factor influencing color deviation, as the quality of the plastic product’s color depends directly on the colorant’s base color. The thermal stability, dispersion, and covering ability of colorants are indicators of quality and directly linked to color deviation. Colorants primarily comprise coloring agents, dispersants, and plastic material blends. Among these, coloring agents have the most significant impact on color. Even slight changes in the proportion of coloring agents directly affect the product’s color. Different coloring agents have varying stabilities, showing differences in decomposition rates at different temperatures. These variations result in product color differences at different production temperatures or even the same temperature but varying dwell times.

Impact of Colorant Compatibility with Raw Material Resin

The color of an object results from light interaction with pigments in a manner that emits, transmits, or absorbs light. Altering the amount of colorant added changes the ratio of light emission, transmission, and absorption, leading to visual color changes. Uneven mixing of colorants with the raw material resin can cause differences in the amount of colorant added during the injection cycle, resulting in color deviation. The compatibility between raw material resin and colorants affects the uniform dispersion of coloring agents, ultimately impacting the product’s color. Poor compatibility between the two also leads to color deviation.

Impact of Injection Molding Process

Modifying the injection temperature directly affects material temperature, leading to changes in product color. For instance, camel-colored PP colorants under different molding temperatures cause observable color changes: products are notably reddish at 160°C, whitish at 190°C, and significantly darker at temperatures above 220°C. Changes in injection back pressure not only influence material temperature but also affect material dwell time in the barrel, thus having a more substantial effect on color deviation. Injection speed affects material temperature by altering material flow shear rate, especially at high injection speeds, sometimes resulting in discoloration or even localized charring. However, in general, low to medium injection speeds have a smaller impact on product color.

Impact of Injection Molding Machine and Mold

The nozzle size of an injection molding machine affects material shear rate, thereby impacting product color. Dead spots in the machine not only affect product color but also cause defects such as gas marks or charring. When selecting an injection molding machine, it’s crucial to match the machine’s capacity with the product’s weight; oversizing or undersizing can cause unstable product colors. If the mold’s runner and gate are too small, they create harmful strong shear forces that cause plastic discoloration, leading to localized color deviations, especially in highly thermally sensitive plastics like PVC. If the gate is too wide, inefficient turbulence occurs, and uneven color bands may enter the mold cavity, causing color deviation.

Effects of Storage Time and Other Factors

After plastic products are molded, the internal high polymer material still undergoes complex physical and chemical reactions, resulting in color changes. For matching components, prolonged storage times between production may cause batch rejection due to significant color differences during assembly. To minimize this, apart from selecting components with similar production times during assembly, production planning must consider the impact of storage time on product color. There are many other factors influencing color deviation, such as impurities in raw materials (moisture, scrap, etc.) and additives, which, although not major influencing factors, should also be considered.

2. Controlling Color Deviation
Eliminating the Impact of Injection Molding Machine and Mold Factors

Select an injection molding machine that matches the product’s capacity. If the machine has dead spots or other issues affecting material flow, consider replacing the equipment. Issues in molds such as gating systems or exhaust slots causing color deviation can be rectified through mold repairs. Addressing injection machine and mold problems is the first step to reduce the complexity of issues during production.

Reducing the Impact of Raw Material Resin and Colorants

Strict control of raw materials is key to completely resolving

color deviation issues. The thermal stability of the raw material resin significantly affects color fluctuation, especially in the production of light-colored products. Since most injection molding manufacturers do not produce plastic raw materials or colorants themselves, the focus should be on production management and raw material inspection. Emphasize thorough inspections of incoming raw materials, and use the same brand and grade of raw materials and colorants for the same product in production. Conduct sample color tests before mass production; if the color difference is minor, consider it acceptable. In case of slight color deviation within the same batch of colorants, re-mixing before use reduces uneven colorant mixing-induced color deviations. Pay special attention to inspecting the thermal stability of raw materials and colorants. For those with poor thermal stability, it’s advisable for manufacturers to consider replacements.

Minimizing the Impact of Poor Mixing of Colorants and Raw Materials

Poor mixing of plastic raw materials with colorants can cause unpredictable changes in product color. When mechanically blending masterbatch and colorants and then introducing them into the hopper, static electricity may cause the colorants to separate from the raw materials, leading to uneven colorant dispersion and differences in colorant additions during the injection cycle. To address this, after material suction into the hopper, manually stir it before use. Many companies now use feeder machines for colorant addition, saving labor and assisting in color deviation control. However, improper usage often leads to unsatisfactory results. Under a fixed speed, the amount of colorant added by the feeder machine depends on the plasticizing time. Since plasticizing time itself fluctuates and sometimes fluctuates significantly, to maintain a constant feed rate, fix the feeding time and set it shorter than the minimum plasticizing time. Regular cleaning is required as the feeder machine’s outlet may become inaccurate due to accumulated material residues.

Reducing the Impact of Barrel Temperature on Color Deviation

Frequent issues arise in production due to heating ring malfunctions or uncontrolled heating control parts, causing drastic changes in barrel temperature and consequently, color deviation. Such deviations are easily noticeable: heating ring malfunctions causing color deviation are often accompanied by uneven plasticization, while uncontrollable heating control parts lead to product gas marks, severe discoloration, or even charring. Therefore, regular inspections of heating components are necessary. If any damages or loss of control are identified in the heating components, immediate repair or replacement is necessary to reduce the likelihood of such color deviations.

Minimizing the Effects of Process Adjustments on Color Deviation

When adjusting process parameters, try to avoid altering injection temperature, back pressure, injection cycle, or colorant addition as much as possible. If necessary, monitor the effects of process parameter changes on color; if any color deviation is noticed, promptly readjust. High injection speed and back pressure, leading to strong shear effects, should be avoided to prevent color deviation caused by local overheating or thermal decomposition factors. Control barrel heating segment temperatures strictly, especially the nozzle and adjacent heating parts.

Understanding the Influence of Barrel Temperature and Colorant Quantity on Product Color Changes

Before adjusting color deviation, it is crucial to understand the trends in product color changes with varying temperatures and colorant quantities. Different colorants have distinct changes in product color with temperature or colorant quantity variations during production. Determine these changing trends through color testing. Unless familiar with these colorant color change trends, rapid color deviation adjustments, especially when using new colorants for production, may be challenging.

3. Color Deviation Adjustment
Identifying the Cause of Color Deviation

If color deviation occurs during production despite eliminating factors related to raw materials, injection molding machines, and molds, it’s essential to distinguish whether the issue stems from poor colorant and raw material resin mixing or temperature-related factors. In case of mixing issues, resolve them; if temperature-related, adjust by altering temperature or back pressure. With knowledge of color changes with temperature and colorant quantity, this becomes relatively easy to accomplish.