1. Two temperature conditions that are not conducive to solving the shrinkage problem
- Too low mold temperature is not conducive to solving the shrinkage problem
The shrinkage problem of hard plastic parts (surface shrinkage and internal shrinkage cavities) is caused by the fact that when the melt cools and shrinks, the space left by the concentrated shrinkage cannot be fully replenished by the melt coming from the direction of the water inlet. Therefore, factors that are not conducive to replenishment will affect our ability to solve the shrinkage problem.
Most people know that too high a mold temperature can easily cause shrinkage problems, and they usually like to lower the mold temperature to solve the problem. But sometimes if the mold temperature is too low, it is not conducive to solving the shrinkage problem, which many people don’t pay much attention to.
The mold temperature is too low and the melt cools too quickly. The slightly thicker glue position far away from the water inlet is blocked because the middle part cools too fast and the feeding channel is blocked. The melt is not fully absorbed in the distance. In addition, the shrinkage problem is more difficult to solve, and the shrinkage problem of thick injection molded parts is particularly prominent.
Furthermore, if the mold temperature is too low, it is not conducive to increasing the overall shrinkage of the injection molded parts, which will increase the concentrated shrinkage and make the shrinkage problem more serious and obvious.
Therefore, when solving the more difficult problem of shrinkage, it is beneficial to remember to check the mold temperature. Experienced technicians usually touch the surface of the mold cavity with their hands. Each raw material has its suitable mold temperature. For example, the shrinkage problem of PC materials will be better improved if the holes are properly adjusted, but if the mold temperature is too high, the injection molded parts will shrink again.
- Too low melt temperature is not conducive to solving the shrinkage problem
Most people also know that if the melt temperature is too high, injection molded parts are prone to shrinkage problems. If the temperature is appropriately lowered by 10~20°C, the shrinkage problem will be improved.
However, if the injection molded part shrinks in a relatively thick part, and the melt temperature is adjusted too low, for example, when it is close to the lower limit of the injection melt temperature, it will not be conducive to solving the shrinkage problem, and it will even become more serious. The thicker the piece, the more obvious this is.
The reason is similar to that the mold temperature is too low. The melt condenses too quickly. A large temperature difference that is conducive to feeding cannot be formed from the shrink position to the nozzle. The feeding channel at the shrink position will be blocked prematurely. Solution to the problem It becomes more difficult. It can also be seen that the faster the melt condensation speed is, the less conducive it is to solving the shrinkage problem. PC material is a raw material that condenses very quickly, so its shrinkage problem can be said to be a big problem for injection molding.
In addition, too low a melt temperature is not conducive to increasing the overall shrinkage, leading to an increase in concentrated shrinkage, thus exacerbating the shrinkage problem.
Therefore, when adjusting the machine to solve a difficult shrinkage problem, it is extremely important to check whether the melt temperature is too low. In addition to looking at the thermometer, it is more intuitive to use air injection to check the temperature and fluidity of the melt.
2. Injecting glue too fast is not conducive to solving the problem of serious shrinkage
To solve the shrinkage problem, the first thing that comes to mind is to increase the injection pressure and extend the injection time. But if the injection speed has been adjusted very fast, it will not be conducive to solving the shrinkage problem. Therefore, sometimes when shrinkage is difficult to eliminate, it should be solved by reducing the injection speed.
Reducing the glue injection speed can create a large temperature difference between the melt in front and the water inlet, which is beneficial to the solidification and feeding of the melt in sequence from far to near, and is also beneficial to the shrinkage position far away from the water inlet. Getting a higher pressure supplement will go a long way in solving the problem.
As the injection speed is reduced, the temperature of the melt in front is lower and the speed has been slowed down. The injection molded parts are less likely to produce batch fronts. The injection pressure and time can be increased and lengthened, which is more beneficial. Solve the problem of severe shrinkage.
In addition, if the last stage of terminal filling with slower speed, higher pressure and longer time and the pressure holding method of gradually slowing down and increasing pressure are adopted, the effect will be more obvious. Therefore, when it is impossible to shoot at a slower speed from the beginning, it is also a good remedy to use this method from the later stage of the injection.
However, it is worth reminding that charging too slowly will not be conducive to solving the shrinkage problem. Because by the time the mold cavity is filled, the melt has been completely frozen, just like the temperature of the melt is too low, and there is no ability to compensate for shrinkage in the distance.