Control the color difference of plastic mould products
Color difference is a common defect in injection molding. It is not uncommon for the injection molding machine to be scrapped in batches due to the color difference of accessories. There are many influencing factors of color difference, involving raw material resin, master color (or toner), master color and raw material mixing, injection molding process, injection molding machine, mold, etc., because of the wide range of interference, color difference control technology is also recognized as one of the more difficult to master technology in injection molding. In the actual production process, we generally control the color difference from the following six aspects.
- Eliminate the influence of injection molding machine and mold factors:
To choose the injection molding machine with the capacity of injection products, if the injection molding machine has problems such as material dead Angle, it is best to replace the equipment. The color difference caused by the casting system and exhaust groove of the mold can be solved by the maintenance mold of the corresponding part of the mold. The injection molding machine and die problems must be solved first before production can be organized to reduce the complexity of the problems.
- Eliminate the influence of raw material resin and color master:
(1) Control of raw materials is the key to completely solve the color difference. Therefore, especially in the production of light color products, can not ignore the thermal stability of the raw material resin on the product color fluctuations brought by the obvious influence;
(2) Since most injection molding manufacturers do not produce plastic master or color master themselves, attention can therefore be focused on production management and raw material inspection. That is to strengthen the inspection of raw material warehousing; In the production of the same product as far as possible using the same manufacturer, the same brand of master material, master color production;
(3) for the master, we in the mass production before the sample test color, both with the last proofreading, and in this comparison, if the color difference is not big, can be considered qualified, as the batch master has a slight color difference, the master can be remixed after reuse, in order to reduce the color difference caused by the uneven mixing of the master itself. At the same time, we also need to focus on testing the thermal stability of raw material resin and masterbatch. For those with poor thermal stability, we suggest the manufacturer to replace them.
- Eliminate the influence of uneven mixing of master and master material:
Plastic master material with color master mix is not good will also make the product color change. After the master material and color master are mixed evenly by machine, when the material is fed into the hopper by lower suction, the master material is separated from the master material due to electrostatic action, which is easy to adsorb on the hopper wall, which is bound to cause the change of the master quantity in the injection cycle, resulting in color difference.
In this case, the raw material can be inhaled into the hopper and then manually stirred to solve the problem. For adding toner to produce colored products, the most effective way is not to use the suction machine, and the use of hot air dryer, with artificial feeding method to prevent the color difference caused by the separation of toner and master material.
- Reduce the influence of barrel temperature on color difference:
In the production, it is often encountered that the temperature of the long firing material cylinder changes dramatically due to the damage and failure of a heating ring or the uncontrolled heating control part, resulting in color difference. The color difference caused by this kind of reason is easy to determine. The general heating ring damage and failure will be accompanied by the phenomenon of uneven plasticization at the same time, and the heating control part is often accompanied by the product gas spot, serious discoloration and even coking phenomenon. Therefore, it is necessary to often check the heating part in production, and timely replacement and maintenance when the heating part is found to be damaged or out of control, so as to reduce the probability of this kind of color difference.
- To reduce the impact of injection molding process adjustment:
When the injection molding process parameters need to be adjusted for non-chromatic reasons, the injection molding temperature, back pressure, injection molding cycle and the amount of color master should not be changed as far as possible. At the same time, it is necessary to observe the effect of the change of process parameters on the color. If the color difference is found, it should be adjusted in time.
As far as possible to avoid the use of high injection speed, high back pressure caused by strong shear injection process, to prevent local overheating or thermal decomposition and other factors caused by the color difference. Strictly control the temperature of each heating section of the barrel, especially the nozzle and the heating part adjacent to the nozzle.
- Master the influence of cartridge temperature and master quantity on product color change：
Before the adjustment of color difference, it is necessary to know the trend of product color with temperature and master quantity. With the change of the production temperature or the amount of color master, the product color change law is different. The changing rule can be determined by the process of color testing.
Unless THE master color changes are known, it is not possible to adjust the color difference quickly, especially when using a new master color palette.