How to Control Color Variation in Injection Molded Products
Color variation is a common defect in injection molding. Situations where entire batches of products are scrapped due to color differences with matching components are not uncommon. The factors influencing color variation are numerous, involving raw resin, color concentrates (masterbatch), the mixing of masterbatch with the resin, injection molding processes, the injection molding machine, and the mold. In actual production processes, we generally control color variation by focusing on the following six aspects.
Eliminating the Influence of Injection Molding Machine and Mold Factors
It is necessary to select an injection molding machine with a clamping force appropriate for the main product. If the machine has issues like material dead zones, it’s best to replace the equipment. If the mold’s gating system or venting slots cause color variation, the corresponding parts of the mold can be repaired to resolve the issue. It is essential to first address the problems with the injection molding machine and the mold before organizing production, to reduce the complexity of the issues.
Eliminating the Influence of Raw Resin and Color Concentrate
Controlling raw materials is key to fundamentally solving color variation. Therefore, particularly when producing light-colored parts, the significant impact of differences in the thermal stability of the raw resin on color fluctuation must not be overlooked. Since most injection molding manufacturers do not themselves produce plastic pellets or masterbatch, the focus should be placed on production management and raw material inspection. This means strengthening the inspection of incoming raw materials; during production, using masterbatch and color concentrate from the same manufacturer and with the same grade for the same product whenever possible; and conducting sampling and color testing of the masterbatch before batch production. This involves comparing it with the previous batch and also comparing within the current batch. If the color difference is minimal, it can be considered acceptable. If there is a slight color variation within the same batch of masterbatch, the masterbatch can be remixed and reused to reduce color variation caused by uneven mixing of the masterbatch itself. At the same time, it is crucial to thoroughly test the thermal stability of the raw resin and the color concentrate. If the thermal stability is poor, we recommend that the supplier make a replacement.
Eliminating the Influence of Uneven Mixing Between Masterbatch and Resin
Poor mixing between the resin pellets and the color concentrate can also cause unpredictable changes in product color. After the pellets and masterbatch are mechanically mixed uniformly, when they are vacuum-fed into the hopper, due to static electricity, the masterbatch can separate from the pellets and adhere to the hopper walls. This inevitably changes the amount of masterbatch in the injection cycle, thereby causing color variation.
To address this, a method can be adopted where the raw material is sucked into the hopper first and then manually stirred. Nowadays, many companies use feeders to add masterbatch, which saves significant labor and greatly aids color variation control. However, many companies achieve unsatisfactory results due to improper use.
Under a fixed feeder speed, the amount of masterbatch added depends on the plasticizing time. However, the plasticizing time itself fluctuates, sometimes significantly. Therefore, to ensure a constant feed rate, the feeder’s feeding time must be fixed, and the set time should be less than the minimum plasticizing time. When using a feeder, it is important to note that due to the small outlet of the feeder, after some time of use, accumulated fine particles from the material in the feeder screw might cause inaccurate feeding or even cause the feeder to stop rotating. Therefore, regular cleaning is required.
Reducing the Influence of Barrel Temperature on Color Variation
Production often encounters color variation caused by a failed heating element or malfunctioning temperature control leading to extreme fluctuations in the barrel temperature. This type of color variation is relatively easy to identify. Generally, color variation caused by a failed heating element is accompanied by uneven plasticization. Malfunctioning temperature control leading to prolonged overheating often accompanies phenomena such as gas marks, severe discoloration, or even charring of the product. Therefore, during production, the heating system should be checked frequently. If heating elements are damaged or the control system malfunctions, they should be replaced or repaired promptly to reduce the likelihood of this type of color variation.
Reducing the Influence During Injection Process Adjustments
When adjusting injection molding process parameters for reasons other than color variation, it is best to avoid changing the barrel temperature, back pressure, injection cycle, or the amount of masterbatch added. During adjustments, it is also necessary to observe the impact of the parameter changes on color. If color variation is detected, adjustments should be made promptly. It is best to avoid using injection processes with high injection speeds or high back pressure that cause strong shear effects, to prevent color variation caused by localized overheating or thermal decomposition. Strictly control the temperature of each heating zone of the barrel, especially the nozzle and the heating zone immediately adjacent to it.
