Color difference is a common defect in injection molding, and it is not uncommon to see batch scrapping of injection molding machine due to color difference of matching parts. There are many factors affecting color difference, including raw material resin, color masterbatch (or color powder), mixing of color masterbatch with raw material, injection molding process, injection molding machine, mold, etc. Because of the wide range of factors involved, color difference control technology is also recognized as one of the more difficult techniques to master in injection molding. In the actual production process, we generally control the color difference from the following six aspects.

1, eliminate the influence of injection molding machine and mold factors
To choose the injection molding machine with the same capacity as the injection molding products, if the injection molding machine has problems such as material dead space, it is best to replace the equipment. For the mold pouring system, exhaust slot, etc. caused by color difference, can be solved by the corresponding part of the mold maintenance mold. The injection molding machine and mold problems must be solved first before production can be organized to cut the complexity of the problem.

2、Eliminate the influence of raw material resin and color masterbatch
Control of raw materials is the key to completely solve the color difference. Therefore, especially when producing light-colored products, we cannot ignore the obvious impact of different thermal stability of raw material resin on the color fluctuation of products.

In view of the fact that most injection molding manufacturers do not produce plastic masterbatch or masterbatch themselves, so that the focus of attention can be placed on production management and raw material inspection. That is, to strengthen the inspection of raw materials into the warehouse; production of the same product as far as possible using the same manufacturer, the same brand of masterbatch, masterbatch production.

For color masterbatch, we need to carry out sample color test before mass production, both with the last calibration and comparison in this time, if the color difference is not big, it can be considered qualified, such as the same batch of masterbatch has a slight color difference, the color masterbatch can be mixed again and then used, in order to reduce the color difference caused by uneven mixing of the masterbatch itself. At the same time, we also need to focus on testing the thermal stability of the raw material resin and masterbatch. For those with poor thermal stability, it is suggested that the manufacturer should be exchanged.

3. Eliminate the influence of uneven mixing of masterbatch and masterbatch
Bad mixing of plastic masterbatch and color masterbatch will also make the product color change. After mixing the masterbatch and masterbatch mechanically, when feeding into the hopper through the lower suction material, the masterbatch will be separated from the masterbatch due to the electrostatic effect and easily adsorbed on the hopper wall, which will inevitably cause the change of masterbatch amount in the injection cycle and thus produce color difference.

For this situation, we can adopt the method of manual stirring after the raw material is sucked into the hopper to solve the problem. For adding color powder to produce colored products, there is the most effective way is not to use the suction machine, and the use of hot air dryer, with the manual addition of materials to prevent color powder and masterbatch separation caused by the color difference.

4、Reduce the influence of barrel temperature on color difference
In the production, we often encounter that the barrel temperature changes drastically due to a damaged heating ring or uncontrolled heating control part of the long burn, resulting in color difference. This kind of reason produces the color difference is easy to determine, generally the heating ring damage failure produces color difference at the same time will be accompanied by uneven plasticization phenomenon, and heating control part of the uncontrolled long burn often accompanied by product gas spot, serious discoloration and even coking phenomenon. Therefore, the production needs to frequently check the heating part, found when the heating part is damaged or out of control, timely replacement and maintenance to reduce the chances of such color difference.


5、Reduce the impact of the adjustment of the injection molding process
When adjusting the injection process parameters for non-color difference reasons, do not change the injection temperature, back pressure, injection cycle and the amount of color masterbatch added as much as possible, and at the same time, observe the impact of the change of process parameters on the color, and adjust the color difference in time if found.

As far as possible, avoid using high injection speed, high back pressure and other injection molding processes that cause strong shearing effect to prevent color difference caused by local overheating or thermal decomposition and other factors. Strictly control the temperature of each heating section of the barrel, especially the nozzle and the heating section close to the nozzle.

6、master the barrel temperature, the amount of color masterbatch on the impact of product color change
Before adjusting the color difference, we must also know the trend of product color change with temperature and masterbatch amount. Different color masterbatch with the change of production temperature or the amount of masterbatch, its product color change pattern is different. The color change pattern can be determined through the color test process.