Color difference is a common defect in injection molding, and it is not uncommon for injection molding machines to be scrapped in batches due to color differences in matching parts. There are many factors that affect color difference, including raw material resins, colorants (or color powders), mixing of colorants with raw materials, injection molding processes, injection molding machines, molds, etc. Because of the wide range of involvement, color difference control technology is also recognized as one of the difficult technologies to master in injection molding. In the actual production process, we generally control color difference from the following six aspects.

1. Eliminating the influence of injection molding machines and mold factors:

To choose an injection molding machine with a capacity equivalent to that of the injection molded product, if there are material dead corners or other issues with the injection molding machine, it is advisable to replace the equipment. For the color difference caused by the mold pouring system, exhaust groove, etc., it can be solved by repairing the corresponding parts of the mold. It is necessary to first solve the problems of injection molding machines and molds before organizing production to reduce the complexity of the problem.

2. Eliminate the influence of raw resin and colorants:

(1) Controlling raw materials is the key to completely solving color differences. Therefore, especially when producing light colored products, the significant impact of the different thermal stability of the raw resin on the color fluctuation of the products cannot be ignored;

(2) Given that most injection molding manufacturers do not produce plastic masterbatch or colorants themselves, the focus can be on production management and raw material inspection. Strengthen the inspection of raw material storage; In production, the same product should be produced using the same manufacturer, same brand of masterbatch, and color masterbatch as much as possible;

(3) For color masterbatches, we need to conduct spot checks and color tests before mass production. We need to compare them with the previous calibration and compare them in this batch. If the color difference is not significant, it can be considered qualified, just like if there is a slight color difference in the batch of color masterbatches. The color masterbatches can be re mixed before use to reduce the color difference caused by uneven mixing of the color masterbatches themselves. At the same time, we also need to focus on inspecting the thermal stability of the raw material resin and color masterbatch. For those with poor thermal stability, we suggest that the manufacturer replace them.

3. Eliminate the influence of uneven mixing of colorants and masterbatch:

Poor mixing of plastic masterbatch with colorants can also cause the color of the product to change unpredictably. After mechanically mixing the masterbatch and color masterbatch evenly, when they are fed into the hopper through suction, the color masterbatch separates from the masterbatch due to static electricity and is easily adsorbed on the hopper wall. This will inevitably cause changes in the amount of color masterbatch during the injection molding cycle, resulting in color difference.

This situation can be solved by manually stirring the raw materials after they are sucked into the hopper. The effective method for producing colored products by adding color powder is not to use a suction machine, but to use a hot air dryer and manually add materials to prevent color difference caused by the separation of color powder and masterbatch.

4. Reduce the impact of barrel temperature on color difference:

In production, it is common to encounter color differences caused by severe temperature changes in the material barrel due to damage or failure of a heating coil or uncontrolled burning of the heating control part. The color difference caused by such reasons is easy to determine. Generally, when the heating coil is damaged or fails, the color difference is accompanied by uneven plasticization. However, when the heating control part loses control and burns for a long time, it is often accompanied by product gas spots, severe discoloration, and even coking. Therefore, it is necessary to regularly inspect the heating part during production, and replace and repair it in a timely manner if it is found to be damaged or out of control, in order to reduce the probability of such color differences.

5. Reduce the impact of injection molding process adjustments:

When adjusting the injection molding process parameters for non color difference reasons, try not to change the injection molding temperature, back pressure, injection cycle, and amount of color masterbatch as much as possible. At the same time, observe the impact of changes in process parameters on color. If color difference is found, it should be adjusted in a timely manner.

Try to avoid using injection molding processes that cause strong shear effects such as high injection speed and high back pressure as much as possible, in order to prevent color differences caused by local overheating or thermal decomposition. Strictly control the temperature of each heating section of the material cylinder, especially the nozzle and the heating part adjacent to the nozzle.

6. Master the influence of barrel temperature and masterbatch content on product color changes:

Before adjusting the color difference, it is also necessary to know the trend of the product color changing with temperature and the amount of color masterbatch. The color change pattern of different colorants varies with the temperature or amount of colorants produced. The change pattern can be determined through the color testing process.

It is impossible to quickly adjust the color difference, especially when using a new color masterbatch for color matching, unless the changing pattern of this color masterbatch is already known.