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Analysis of color instability factors of injection molded products and color difference control methods

Analysis of color instability factors of injection molded products

Injection molding is one of the most common molding methods in plastic product molding. In the face of colorful injection molded parts, we will think about achieving colorful and colorless products. Those engaged in color matching of injection molding production will feel that a product will sometimes change a color in a few days or even a variety of color deviations in a day. So, what factors affect the color stability of injection molded parts?

1:Raw material and its treatment

Raw materials include plastic raw materials, color powder, masterbatches, and so on. Due to the different base colors of each raw material, the color of the same color powder injection molded parts will be different. Sometimes the same supplier and raw material batch numbers are different, and the base color will have some deviation, so for products with high color deviation requirements, we even need to control whether the base color of each batch of incoming material is consistent. In addition to the plastic raw material batch problem, the batch stability of raw materials such as color pigments or masterbatches must also be concerned.

The raw material will have a humidity effect; too much humidity will cause the color powder bonding can not to spread. Each raw material and pigment has its range of baking temperature and time, and if the range is seriously exceeded, the color of the injection molded parts will change. Suppose the material is added more or less often. In that case, the color will be unstable if it is affected by temperatures, such as the color of fluorescent materials or light-colored injection molded parts. Therefore, for temperature-sensitive plastics, it is better not to bake the material or use the bottom temperature for baking the material, then mix the color powder after baking it.

Another point is that in the baking hopper, due to the influence of hot air, the color powder appears to be locally concentrated, resulting in darker and darker colors. The injection molded parts that need more pockmarks will also have too many. We need to add some diffusion oil to make the color powder firmly stick to the raw material.

Production will be used to return to the material, and the use of water mouth back to the amount of change will also affect the color of injection molded parts, especially light-colored injection molded parts. The plastic crystalline type also needs to be considered; colorants will affect the crystallinity, affecting the performance between the outside and the coloring stability.

2:Colorants: color powder, masterbatches, etc.

The colorant is the root of the color, colorant products are very much, and their performance varies. Coloring powder quality is too poor, not heat-resistant, or the wrong coloring powder is not suitable for the raw materials used, making the color unstable. Colorant material is too coarse, and the concentration level of color masterbatch is too low Junhui leads to color instability.

3:Forming conditions


Temperature is the key factor of molding and has a very important influence on color stability. The injection molding machine’s temperature is unstable, sometimes high and sometimes low, and the color will be unstable. Usually, if the temperature difference is about 10℃, the color deviation of the injection molded parts can be seen. When the deviation of the color of the injection molded part is not very big, it should be solved by adjusting the temperature. Adjusting 5 to 15℃ up or down within the normal injection molding temperature of raw materials, you can already receive the obvious effect of improving the color deviation.

Conversely, as the temperature decreases, the color becomes darker, and the white is whiter. However, some unique colors are opposite or elusive; for example, colors with fluorescence are more elusive. The temperature adjustment is difficult to solve if the color deviation is too large. Notifying the color matching staff to adjust the color is not too late, which can reduce the number of color mixing. However, when adjusting the color, remember to adjust the injection molding temperature required for the production of the product; otherwise, the production and coloring temperature will be different, and color deviation will occur.

Since temperature greatly influences color, it is also necessary to control the temperature during production. Once the color is determined, the temperature variation should be kept within plus or minus 5 degrees. For light colors and other materials that are affected by temperature, even the temperature and time of baking should not be too high and too long. Otherwise, it will also have dangerous effects on the color, such as fluorescent colors and purple. Therefore, if it is necessary to bake the material for a long time to solve the water vapor problem, it is better to bake the material after the color matching.

(2)Other molding conditions

In addition to temperature, we should also pay attention to the amount of material, production cycle, etc. The irregular production cycle, on and off, back pressure is too large to cause a temperature run, which will make the color change. The color powder and raw materials must be mixed according to the requirements of the mixing process; otherwise, insufficient mixing time, incorrect way, or order of adding material will lead to the uneven color powder. Molding conditions should ensure that the colorant and plastic can be fully mixed and plasticized while not affecting the stability of the colorant and plastic itself.

(3) Pouring system

The pouring system of injection molding needs to be reasonably designed according to the shape of the part. The gate’s location impacts the plastic filling situation, causing the color difference in some parts of the part, which should be modified when necessary.

How to control the color difference of injection molded products

The color difference is a common defect in injection molding. It is not uncommon to see batch scrapping of injection molding machines due to the color difference of matched parts. Many factors affect the color difference, including the raw material resin, color masterbatch, mixing of color masterbatch and raw material, injection molding process, injection molding machine, mold, etc. Because of the wide range of factors involved, color difference control technology is also one of the more difficult techniques to master in injection molding. We generally control the color difference from the following six aspects in the production process.

1:Eliminate the influence of injection molding machines and mold factors

To choose an injection molding machine with the same capacity as the main product, if the injection molding machine has problems such as material dead space, it is best to replace the equipment. The mold pouring system, exhaust slot, etc. causing the color difference, can be solved by repairing the mold of the corresponding part. The injection molding machine and mold problems must be solved first before organizing production 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 solving 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. Because most injection molding manufacturers do not produce plastic masterbatch or masterbatch themselves, the focus of attention can be placed on production management and raw material inspection. In other words, we should strengthen the inspection of raw materials in the warehouse; use the same manufacturer and the same brand of masterbatch and masterbatch in the production as far as possible; for the masterbatch, we should carry out sample color test before mass production, not only to check with the last time but also to compare in this time, if the color difference is not big, it can be regarded as qualified, if there is a slight color difference with the same batch of masterbatch, the masterbatch can be re-mixed and used again to reduce the color difference caused by uneven mixing of the masterbatch itself. The uneven mixing of the masterbatch itself causes the color difference. At the same time, we also need to focus on testing the thermal stability of the raw material resin and masterbatch, and we suggest the manufacturer replace those with poor thermal stability.

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

Bad mixing of plastic masterbatch and masterbatch will also make the color of the product change irregularly. After mixing the masterbatch and masterbatch mechanically, the masterbatch will be sent into the hopper through the lower suction material; because of the electrostatic effect, the masterbatch will be separated from the masterbatch and easily adsorbed on the hopper wall, which will inevitably cause the change of the masterbatch amount in the injection cycle, thus producing color difference. This will inevitably cause the amount of masterbatch to change during the injection cycle, thus resulting in color differences. This situation can be solved by taking the raw material into the hopper and then mixing it manually. Nowadays, many companies use the feeder to add color masterbatch, which saves a lot of workforces and provides great help for color difference control. Still, many companies often have unsatisfactory results due to improper use. The amount of masterbatch added to the feeder at a fixed speed depends on the plasticizing time, which itself fluctuates and sometimes even fluctuates greatly, so to ensure a constant amount of feeding, the feeding time of the feeder needs to be fixed and the set time is less than the minimum plasticizing time. In the use of a feeder need to pay attention to because the feeder outlet is small after using a period; this may be due to the feeder screw in the accumulation of raw material powder particles caused by inaccurate feeding or even cause the feeder to stop, so you need to clean up regularly.

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

Production is often encountered due to a heating ring damage failure or heating control parts out of control long burn caused by barrel temperature changes dramatically, resulting in color differences. This kind of reason produces a color difference that 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 the heating control part of the uncontrolled long burning is often accompanied by by-product gas spots, serious discoloration, and even coking phenomenon. Therefore, it is necessary to check the heating part frequently in production and replace it when it is damaged or out of control to reduce the chance of such color differences.

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 the strong shear effect to prevent color differences 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 immediately adjacent to the nozzle.

6:Master the influence of barrel temperature and color masterbatch amount on product color change

Before adjusting the color difference, we must also know the trend of product color change with temperature and masterbatch quantity. 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. Unless the color change pattern of this masterbatch is already known, it is impossible to adjust the color difference quickly, especially when using new masterbatch production for color mixing.

Elimold is an enterprise integrating mold design, mold manufacturing, and batch processing of injection molding products. With a professional design team and rich experience in plastic product design and production, we can provide product design, mold design and manufacturing, mold flow analysis, and other services, focusing on mold manufacturing and batch processing of injection molding products. We can also provide free technical support for mold and product structure optimization and improvement. Provide customers with quality products and services to improve the value of the enterprise.

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