Introduction: In the molding of plastic products, injection molding is the most common molding method. Color difference is the most common defect in injection molding, and it is very difficult to grasp because of the many factors involved. So what factors will cause color difference? Is there any way to reduce chromatic aberration? This article shares the main factors that affect the color difference and the practical technology to control the color difference. It provides some good experiences and methods for reducing the problem of color difference. The content is for the friends of the excellent packaging material system for reference:

Color difference

Color difference is one of the common defects in injection molding. It is not uncommon for injection molding parts to be scrapped in batches due to the color difference of the matching parts. There are many factors affecting color difference, including raw resin, color masterbatch, mixing of color masterbatch with raw materials, injection molding process, injection molding machine, mold, etc., because of the wide range of involved, color difference control technology is also recognized as one of the more difficult technologies in plastics. To master color difference control technology, we must fully understand the main factors that cause color difference.

Main factors affecting color difference
Effect of raw resin
The raw material resin itself will affect the color difference, and many people engaged in injection molding may be surprised, but the fact does exist, especially when the resin itself has a certain basic color. Several types of ABS of different models were compared under the same injection molding conditions, and it was found that most of the products have a certain degree of color difference. Even if the same model is used but different production batches, the product may sometimes have color differences. Tests have shown that the degree of product color difference is approximately proportional to the resin base color, that is, the greater the difference in resin base color, the greater the product color difference. In addition, the stability of the resin itself also has a significant impact on the color difference. For resins with poor stability, the production is more susceptible to fluctuations in external conditions, and the possibility of color difference will be greatly increased.

The influence of masterbatch
Undoubtedly, the color masterbatch is the most important factor affecting the color difference. The color quality of plastic products depends directly on the quality of the color masterbatch. The thermal stability, dispersion, and hiding power of the masterbatch are not only indicators of the quality of the masterbatch, but also directly related to the color difference. The color masterbatch is mainly composed of colorants, dispersants, and plastic materials. Among them, colorants have the greatest influence on color, and slight changes in the proportion of colorants will directly affect the color of the product. The stability of colorants of different colors is different, showing that the decomposition rate is different at different temperatures, and the decomposition rate is also different at the same temperature. These will be directly expressed in production at different production temperatures, or the same production temperature but different residence time, and the color of the produced products will be different. Therefore, any process changes that can cause changes in material temperature or injection cycle will cause changes in product color. It is recommended to use high-quality color masterbatch as far as possible to produce products with high color requirements, in order to reduce the color difference caused by temperature and cycle changes and improve the overall appearance of injection products.

Influence of compatibility of masterbatch and raw material resin
The color of an object is the optical impression of the coloring material when it is radiated, transmitted, and absorbed by the outside light. The change in the amount of color masterbatch will change the ratio of light emission, transmission, and absorption, resulting in a visual color change. If the color masterbatch is mixed unevenly with the raw material resin, it will cause a difference in the amount of color masterbatch added during the injection cycle, resulting in color difference. The compatibility between the raw resin and the masterbatch will affect the color of the product by affecting the uniformity of the colorant dispersion. If the compatibility between the two is not good, it will also cause color difference.

Impact of injection molding process
Changing the injection temperature can directly affect the temperature of the material and cause the color of the product to change. Taking the camel PP masterbatch as an example (the colorants are mainly titanium dioxide, phthalocyanine blue, red, etc.), the color of the injection molded product is obviously red when the barrel temperature is 160 ℃. When the temperature rises to 190 ℃, the product color is white, when the barrel temperature rises above 220 ℃, the product color is obviously black. Changing the injection back pressure can not only directly affect the temperature of the material, but also affect the residence time of the material in the barrel, so it will have a greater impact on the color difference. The injection speed can affect the temperature of the material by changing the shear rate of the flow, especially when high-speed injection, sometimes the product will become discolored or even partially scorched to yellow. But generally speaking, the low and medium injection speed has little effect on the color of the product.

Influence of injection molding machine and mold
The nozzle size of the injection molding machine can affect the color of the product by affecting the shear rate of the material, and the dead angle of the material of the injection molding machine not only affects the color of the product, but also causes phenomena such as gas spots and coking. In addition, when choosing an injection molding machine, it is necessary to pay attention to the capacity of the injection molding machine and the weight of the product. Excessive or too tight will cause the color of the manufactured product to be unstable. If the mold runner and gate are too small, it will produce harmful strong shearing and discolor the plastic, causing local color difference, especially for plastics with high thermal sensitivity such as PVC; if the gate is too wide, then The effect of turbulent flow is poor, and the temperature rise is not high. It is possible to bring uneven ribbon into the mold cavity, which will also cause color difference. When the mold is exhausted poorly, the plastic is adiabatically compressed and reacts violently with oxygen under high temperature and pressure, which can also cause local discoloration.

Influence of inventory time and other factors
After the plastic product is molded, the internal polymer material still undergoes complex physical and chemical reactions. Although the process is slow, it will inevitably cause the color of the product to change. For matching parts, if the inventory time interval is too long, a large number of products may be scrapped due to the large difference in color of the matching parts during product assembly. In order to reduce the occurrence of such situations, in addition to the use of matching parts with similar production time as far as possible in assembly, the production plan must fully consider the impact of inventory time on the color of the product. There are many factors that affect the color difference, such as impurities (moisture, crumbs, etc.) in the raw materials, additives, etc. Although these are not the main influencing factors, but also need to pay due attention.

Control chromatic aberration
Eliminate the influence of injection molding machine and mold factors
To choose an injection molding machine with the same capacity as the injection molded product, if the injection molding machine has problems such as material dead ends, it is best to replace the equipment. For the color difference of the mold pouring system and the exhaust groove, it can be solved by repairing the corresponding part of the mold. The problem of injection molding machines and molds must be resolved before production can be organized to reduce the complexity of the problem.

Eliminate the influence of raw resin and color masterbatch
Controlling raw materials is the key to completely solving the problem of color difference. The obvious influence of the thermal stability of the raw resin on the color fluctuation of the product cannot be ignored, especially when producing light-colored products. Since most injection molding manufacturers do not produce plastic masterbatch or color masterbatch themselves, they can focus on production management and raw material inspection, that is, strengthen the inspection of raw material storage; the same product in the production should use the same manufacturer and brand Masterbatch and color masterbatch production; for color masterbatch, sample color test should be conducted before mass production. It should be compared with the last time and this time. If the color difference is not large, it can be considered qualified. For the same batch of color If the masterbatch has a slight color difference, the masterbatch can be remixed before use to reduce the color difference caused by the uneven mixing of the masterbatch itself. It is important to check the thermal stability of the raw resin and color masterbatch. For those with poor thermal stability, it is recommended that the manufacturer replace it.

Eliminate the effect of uneven mixing of masterbatch and masterbatch
Poor mixing of plastic masterbatch and masterbatch can also make the color of the product volatile. When the masterbatch and the masterbatch are mixed mechanically and evenly fed into the hopper through the lower suction, the masterbatch is separated from the masterbatch due to static electricity, and is easily adsorbed on the hopper wall. This will inevitably cause the change of the amount of the masterbatch in the injection cycle, resulting in Color difference. This situation can be solved by taking the raw materials into the hopper and then manually mixing them. Many companies now use feeders to add color masterbatches, save a lot of manpower, and provide a lot of help for color difference control, but many companies are often unsatisfactory due to improper use. The amount of masterbatch added to the feeder at a fixed speed depends on the plasticizing time, and the plasticizing time itself fluctuates, and sometimes the fluctuation is still relatively large. Therefore, to ensure a constant feeding amount, the feeding time of the feeder needs to be fixed , And the set time is less than the minimum plasticizing time. When using the feeder, please note that because the feeder outlet is small, after using for a period of time, the material powder in the feeder screw may cause inaccurate cutting, or the feeder may stop, so it needs to be regular Clean up.

Reduce the effect of barrel temperature on color difference
During the production, it is often encountered that the temperature of the barrel changes drastically due to the failure of a heating ring or the loss of control of the heating control part, resulting in color difference. The color difference caused by this kind of reason is easy to determine. Generally, the color difference caused by the failure of the heating ring is accompanied by uneven plasticization, and the uncontrolled long burning of the heating control part is often accompanied by product gas spots, severe discoloration, and even coking. Therefore, it is necessary to check the heating part frequently during production, and replace and repair the heating part in time when it is found to be damaged or out of control, so as to reduce the probability of such color difference.

Reduce the impact of injection molding process adjustment
When the injection molding process parameters need to be adjusted for non-color difference reasons, the injection temperature, back pressure, injection cycle, and the amount of color masterbatch should not be changed as much as possible. At the same time, the effect of the process parameter changes on the color should also be observed. If the color difference is found, it should be adjusted in time. As far as possible, avoid the use of high injection speed, high back pressure and other injection molding processes that cause strong shearing to prevent color differences caused by local overheating or thermal decomposition. Strictly control the temperature of each heating section of the barrel, especially the nozzle and the heating part close to the nozzle.

Grasp the effect of barrel temperature and color masterbatch on product color changes
Before adjusting the color difference, you must also know the trend of the product color with temperature and the amount of color masterbatch. Different color masterbatch varies with the production temperature or the amount of color masterbatch. The color change process can be determined through the color test process. Unless you already know the color change law of this masterbatch, it is impossible to adjust the color difference quickly, especially when using new masterbatch to produce color.

Color difference adjustment
Because raw materials, injection molding machines, molds and other factors have been excluded before production, if color difference is found in production, in most cases, it is only necessary to judge whether the color masterbatch is mixed with the raw material resin or the injection temperature problem. If the mixing problem is to solve the mixing problem, yes Temperature problems can be adjusted by changing the temperature or back pressure. This is extremely easy to do when knowing the color change trend.