Factors affecting color instability in injection molding production
Color instability in injection molding is a common problem that affects product appearance consistency. This manifests as noticeable color variations between molded parts within or across batches. This can negatively impact product aesthetics, or even lead to customer returns and financial losses. Numerous factors contribute to color instability, encompassing raw materials, equipment, processes, and the environment. Identifying the root causes requires systematic analysis. For example, a cosmetics packaging factory encountered color instability in its PP bottle cap production, resulting in customer rejections and discrepancies between batches. This resulted in direct losses exceeding 500,000 yuan.
Raw material factors are the primary cause of color instability. Unstable masterbatch or color powder quality, such as insufficient color powder purity and uneven particle size distribution, can lead to poor pigment dispersion in the melt, resulting in localized color deviations. Poor compatibility between the masterbatch and the base resin can also lead to uneven color distribution. Furthermore, raw material batch variations are a significant factor. Different batches of base resin may have varying adsorption capacities for pigments due to differences in molecular weight distribution and additive content, which in turn affects the final color. When a pipe factory used the same brand but different batches of PE raw material to produce colored pipes, it discovered significant differences in the color depth of the pipes. Testing revealed that differences in the density of the different batches of PE led to different pigment dispersions. By using raw materials from the same batch and premixing, the color difference issue was alleviated.
Equipment factors also have a significant impact on color stability. If the residual material in the injection molding machine barrel is not cleaned out, it will mix with the new material, resulting in color contamination. For example, if the barrel is not thoroughly cleaned after the previous batch of red plastic parts, the residual red material will cause pink spots to appear on the next batch of white plastic parts. Uneven barrel temperature distribution will lead to inconsistent pigment dispersion. If the local temperature of the barrel is too high, the pigment in the masterbatch may decompose, causing the color to lighten. When an electronics factory was producing ABS black housings, the local temperature was low due to damage to the barrel heating coil. As a result, the masterbatch in this area was not fully dispersed, and gray-white stripes appeared on the surface of the housing. After replacing the heating coil and calibrating the temperature, the color returned to uniformity.
Fluctuations in process parameters are a key factor contributing to color instability. Excessively high injection temperatures can cause pigment decomposition or discoloration (for example, organic pigments are susceptible to decomposition at high temperatures); too low temperatures can lead to poor pigment dispersion, resulting in the appearance of color spots or streaks. Variations in injection speed and pressure can affect melt mixing and cooling rates, which in turn impact color uniformity. Excessively long holding times can cause the surface of the plastic part to darken due to excessive compression. When producing beige PP door panels, an automotive interior manufacturer experienced inconsistent color depth due to injection speed fluctuations (±10%). By stabilizing the injection speed and keeping speed fluctuations within ±3%, color consistency was significantly improved.
Environmental factors and operating procedures can also affect color stability. Drastic fluctuations in temperature and humidity in the production environment can cause raw materials to absorb moisture or masterbatch to clump, impacting pigment dispersion. For example, in high-humidity environments, PA raw materials are prone to hygroscopicity, which can lead to uneven dispersion of the masterbatch in the melt and cause color deviation. Operator irregularities, such as inaccurate masterbatch addition and insufficient mixing time, can also cause color instability. At a daily necessities factory, an operator failed to weigh masterbatch according to regulations, resulting in an addition deviation of more than 5%, causing noticeable color variations in plastic basins. By introducing an automated metering system and providing operator training, the color instability issue was effectively controlled. Furthermore, mold temperature fluctuations can affect the cooling rate of plastic parts, leading to color variations, necessitating a constant temperature control system to maintain a stable mold temperature.
