Introduction: In the molding of plastic products, injection molding is the most common molding method. Chromatic aberration is the most common defect in injection molding, because it involves many factors, it is very difficult to grasp. So what are the factors that can cause chromatic aberration? Is there any way to reduce chromatic aberration? This article shares the main factors affecting the color difference and the practical technology to control the color difference, and provides some good experiences and methods for reducing the color difference problem. The content is for your reference:
Chromatic aberration is one of the common defects in injection molding. It is not uncommon for injection molded parts to be scrapped in batches due to the color difference of matching parts. There are many factors affecting chromatic aberration, 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 involvement, color aberration control technology is also recognized as one of the difficult technologies in plastics. To master the chromatic aberration control technology, it is necessary to fully understand the main factors that cause chromatic aberration
The main factors affecting chromatic aberration
Influence of raw resin
Many people engaged in injection molding may be surprised that the raw material resin itself affects the color difference, but the fact does exist, especially when the resin itself has a certain base color. Several different types of ABS are tested under the same injection molding conditions, and it is found that most of the products have a certain degree of color difference. Even if the same type but different production batches are used, the products may sometimes have color difference. Tests show that the degree of product color difference is approximately proportional to the resin base color, that is, the greater the resin base color difference, the greater the product color difference. In addition, the stability of the resin itself also has a significant impact on the chromatic aberration. For resins with poor stability, the production is more susceptible to fluctuations in external conditions, and the possibility of chromatic aberration will greatly increase.
The influence of color masterbatch
Undoubtedly, color masterbatch is the most important factor affecting color difference, and the color quality of plastic products directly depends on the quality of the color masterbatch. The thermal stability, dispersion, and covering ability of the color masterbatch are not only indicators of the quality of the color masterbatch, but also have a direct relationship with the color difference. Color masterbatches are mainly made by mixing colorants, dispersants, and plastic raw materials. Among them, colorants have the greatest impact 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 at the same production temperature but with different residence times, and the colors of the products produced will have certain differences. Therefore, any process change that can cause changes in material temperature or injection cycle will cause changes in product color. It is recommended to use higher-quality color masterbatches as far as possible when producing products with higher color requirements to reduce the color difference caused by temperature and cycle changes and improve the overall appearance of injection molded products.
The influence of color masterbatch and raw material resin compatibility
The color of an object is the optical impression that the colored material presents after radiating, transmitting, and absorbing the light input from the outside. 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 and the raw resin are mixed unevenly, the amount of color masterbatch added in the injection cycle will be different, resulting in color difference. The compatibility of the raw resin and the masterbatch will affect the color of the product by affecting the uniformity of the colorant dispersion. If the compatibility of the two is not good, it will also cause color difference.
Influence 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. Take PP masterbatch as an example (the colorants are mainly titanium dioxide, phthalocyanine blue, red, etc.). When the barrel temperature is 160℃, the color of the injection product is obviously reddish. When the temperature rises to 190°C, the color of the product becomes white, and when the temperature of the barrel rises above 220°C, the color of the product becomes 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 material flow, especially during high-speed injection, sometimes it will cause the product to become discolored or even partially coked and yellow. But generally speaking, low and medium injection speeds have little effect on the color of products.
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. 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 air spots and coking. In addition, when choosing an injection molding machine, the capacity of the injection molding machine should be compatible with the weight of the product. Too much or too tight will cause the color of the product to be unstable. If the mold runner and gate are too small, it will produce harmful strong shearing effect and discolor the plastic, causing local chromatic aberration, especially for those highly heat-sensitive plastics such as PVC; if the gate is too wide, then The turbulence effect is poor, and the temperature rise is not high, which may bring uneven color bands into the mold cavity, which will also cause color difference. When the mold is poorly exhausted, the plastic is adiabaticly compressed and reacts violently with oxygen under high temperature and high pressure, which will also cause local discoloration.
Inventory time and other factors
After the plastic product is formed, the internal polymer material is still undergoing complex physical and chemical reactions. Although the process is slow, it will inevitably change the color of the product. For accessory parts, if the inventory time interval is too long, batches of products may be scrapped due to large differences in the color of the accessory parts during product assembly. In order to reduce the occurrence of this kind of situation, in addition to the assembly of parts with similar production time should be selected as much as possible in the assembly, the impact of inventory time on the color of the product should be fully considered when arranging the production plan. There are many factors that affect the color difference, such as impurities (moisture, crumbs, etc.) and additives in the raw materials. Although these are not the main factors, they need to be paid attention to.
Control chromatic aberration
Eliminate the influence of injection molding machine and mold factors
It is necessary 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. The color difference caused by the mold casting system, exhaust groove and other problems can be solved by repairing the corresponding parts of the mold. The problem of injection molding machine and mold must be solved first 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 chromatic aberration. Can not ignore the obvious impact of the thermal stability of the raw resin on the color fluctuation of the product, especially when producing light-colored products. In view of the fact that 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 the same brand as much as possible Production of masterbatch and color masterbatch; for color masterbatch, sampling test color should be carried out before mass production. It should be compared with the previous time and this time. If the color difference is not large, it can be considered qualified. For the same batch of color If there is a slight color difference in the masterbatch, you can remix the masterbatch before using it to reduce the color difference caused by the uneven mixing of the masterbatch itself. The thermal stability of the raw material resin and color masterbatches needs to be inspected. For those with poor thermal stability, it is recommended that the manufacturer replace them.
Eliminate the influence of uneven mixing of masterbatch and masterbatch
Poor mixing of plastic masterbatch and color masterbatch will also make the product color change impermanent. When the masterbatch and color masterbatch are mechanically mixed and sent into the hopper through the lower suction, the color masterbatch is separated from the masterbatch due to static electricity and is easily adsorbed on the hopper wall. This will inevitably cause the change in the amount of color masterbatch during the injection cycle. Chromatic aberration. In this case, the raw materials can be sucked into the hopper and then manually stirred. Many companies now use feeders to add color masterbatches, which saves a lot of manpower and provides a great help for color difference control. However, many companies are often unsatisfied with the results due to improper use. The amount of color masterbatch added to the feeder at a fixed speed depends on the plasticizing time, and the plasticizing time itself fluctuates, sometimes even 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. Pay attention when using the feeder. Because the feeder has a small outlet, after a period of use, the feeding may be inaccurate due to the raw material powder accumulated in the feeder screw, and the feeder may even stop rotating, so it needs to be regular Clean up.
Reduce the influence of barrel temperature on chromatic aberration
In production, it is often encountered that the temperature of the barrel changes drastically due to the failure of a certain heating ring or the out of control of the heating control part, resulting in chromatic aberration. The chromatic aberration caused by this type of reason is easy to determine. Generally, the chromatic aberration caused by the failure of the heating ring will be accompanied by uneven plasticization, and the out-of-control heating of the heating control part is often accompanied by product air spots, severe discoloration and even coking. Therefore, it is necessary to check the heating part frequently during production, and replace and repair it in time when the heating part is found to be damaged or out of control, so as to reduce the chance of this kind of color difference.
Reduce the impact of injection molding process adjustment
When the injection molding process parameters need to be adjusted for non-chromatic aberration reasons, try not to change the injection temperature, back pressure, injection cycle, and the amount of masterbatch added. At the same time, the adjustment also needs to observe the effect of the process parameter changes on the color. If the color aberration is found, it should be adjusted in time. Try to avoid the use of injection molding processes that cause strong shearing effects such as high injection speed and high back pressure to prevent color difference 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 influence of barrel temperature and color masterbatch volume on product color changes
Before adjusting the color difference, it is necessary to know the trend of the product color with the temperature and the amount of color masterbatch. With the change of the production temperature or the amount of color masterbatch, the color change law of the product is different. The color test process can be used to determine its changing law. Unless the color change law of this masterbatch is known, 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 eliminated before production, such as color difference found in production, in most cases, it is only necessary to judge whether it is the mixing problem of the color masterbatch and the raw material resin or the injection temperature problem. If the mixing problem is the mixing problem, the mixing problem should be solved. Temperature problems can be adjusted by changing the temperature or back pressure. This is extremely easy to do when the color change trend is known.