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How to control the color difference of injection products?

Core Tip: Color difference is a common defect in injection moulding. It is not uncommon for the injection moulding machine to be scrapped in batches due to the color difference of matching parts. There are many factors affecting the color difference, including raw resin, Color Masterbatch (or color powder).

Color difference is a common defect in injection moulding. It is not uncommon for injection moulding machines to be scrapped in batches because of the color difference of matching parts. There are many factors affecting the color difference, such as raw resin, Color Masterbatch (or color powder), mixture of color masterbatch and raw material, injection molding process, injection moulding machine, mould and so on. Because it involves a wide range of aspects, the color difference control technology is also recognized as one of the more difficult technologies to master in injection moulding. In the actual production process, we generally control the color difference from the following six aspects.

1. Eliminate the influence of injection moulding machine and mould factors

To choose the injection machine with the same capacity as the injection product, if the injection machine has the problem of material dead angle, it is better to replace the equipment. For the color difference caused by the casting system and the exhaust groove of the die, it can be solved by the maintenance die of the corresponding part of the die. It is necessary to solve the problems of injection moulding machines and moulds in order to organize production and reduce the complexity of the problem.

2. Eliminate the influence of raw resin and colour masterbatch

Control of raw materials is the key to completely solve the problem of chromatic aberration. Therefore, especially in the production of light-colored products, we can not ignore the obvious influence of different thermal stability of raw resin on the color fluctuation of products.

In view of the fact that most injection moulding manufacturers do not produce plastic masterbatches or color masterbatches themselves, attention can be focused on production management and raw material inspection. That is, to strengthen the inspection of raw materials into storage; to produce the same product, the same manufacturer, the same brand masterbatch and color masterbatch should be adopted as far as possible;

For masterbatches, we have to carry out spot check and test before mass production, not only with the last proofreading, but also in this comparison, if the color difference is not big, we can consider qualified, just as the batch masterbatches have slight color difference, we can re-mix the masterbatches and then use them to reduce the color difference caused by the uneven mixing of the masterbatches themselves. At the same time, we also need to focus on testing the thermal stability of raw resins and masterbatches. For those with poor thermal stability, we suggest that manufacturers change.

3. Eliminate the influence of uneven mixing of masterbatch and Masterbatch

Bad mixing of plastics masterbatches with masterbatches can also make the color of products changeable. When the masterbatch and masterbatch are mixed evenly and fed into the hopper through downdraft, the masterbatch is easily adsorbed to the hopper wall due to static electricity, which will inevitably cause the change of the masterbatch amount in the injection molding cycle, thus resulting in color difference.

This situation can be solved by inhaling raw materials into hoppers and then mixing them manually. For the production of non-ferrous products by adding colour powder, the most effective way is not to use suction machine, but to use hot air dryer and manual feeding method to prevent colour difference caused by separation of colour powder and masterbatch.

4. The effect of reducing barrel temperature on chromatic aberration

In production, it is often encountered that the temperature of barrel changes dramatically due to the failure of a heating ring or the uncontrolled long burning of the heating control part, resulting in chromatic aberration. The chromatic aberration caused by this kind of reason is easy to judge. Generally, the chromatic aberration caused by the failure of heating ring is accompanied by the phenomenon of non-uniform plasticization, while the uncontrolled long firing of heating control part is often accompanied by product gas spot, serious discoloration and even coking. Therefore, it is necessary to inspect the heating part frequently in production, and replace and repair it in time when the heating part is damaged or out of control, so as to reduce the probability of this kind of chromatic aberration.

5. Reduce the influence of injection process adjustment

When adjusting the injection process parameters for Non-chromatic reasons, the injection temperature, back pressure, injection cycle and the amount of colour masterbatch should not be changed as far as possible. At the same time, the influence of the change of process parameters on the colour should be observed. If the colour difference is found, it should be adjusted in time.

As far as possible, avoid using injection moulding technology with high injection speed, high back pressure and other factors that cause strong shear effect, and prevent color difference caused by local overheating or thermal decomposition. Strictly control the temperature of each heating section of the barrel, especially the heating part of the nozzle and the adjacent nozzle.

6. Master the influence of barrel temperature and masterbatch quantity on product color change

Before adjusting the color difference, it is necessary to know the trend of product color changing with temperature and masterbatch. Different masterbatches have different color changing rules with the change of temperature or quantity of masterbatches. The changing rule can be determined by the process of color test.

It is impossible to adjust the color difference quickly unless the changing rule of the color of the masterbatch is known, especially when the new masterbatch is used.