What is shrinkage in plastic molding?
The effect of injection temperature and speed on product shrinkage
1. Two unfavorable temperature conditions to solve the shrinkage problem
(1) The mold temperature is too low is not conducive to solving the shrinkage problem
Hard plastic parts shrinkage problems (surface shrinkage concave and internal shrinkage) are due to the melt cooling shrinkage, concentrated shrinkage left space not from the direction of the melt into the mouth of the water to fully replenish the defects caused by.
Therefore, the factors that are not conducive to shrinkage replenishment will affect us to solve the shrinkage problem.
Most people know that the injection molding temperature is too high to produce shrinkage problems, usually like to reduce the injection mold temperature to solve the problem. However, sometimes if the injection mold temperature is too low, it is also not conducive to solving the problem of shrinkage, which is not very much noticed by many people.
If the injection mold temperature is too low, the melt glue will be cooled too fast, and the thicker glue position far away from the water inlet will be blocked by the middle part of the cooling too fast, so the distant part will not be fully replenished by the melt glue, which will make the shrinkage problem more difficult to solve, and the shrinkage problem of thick and large injection molded parts is especially prominent.
Moreover, the injection mold temperature is too low, which is not conducive to increasing the overall shrinkage of the injection molded parts so that the number of concentrated shrinkage increases and the shrinkage problem becomes more serious and obvious.
Therefore, when solving the more difficult shrinkage problem, it would be beneficial to remember to check the injection mold temperature. Experienced technicians will usually touch the surface of the mold cavity by hand. Each raw material has its own appropriate injection mold temperature.
(2) Melt temperature is too low is also not conducive to solving the problem of shrinkage
Again, most people know that the melt temperature is too high, the injection molded parts are easy to produce shrinkage problems, appropriate to reduce the temperature of 10 ~ 20 ℃, the shrinkage problem will be improved.
However, if the shrinkage occurs in a thicker part of the injection molded part, and then the melt temperature is adjusted too low, such as close to the lower limit of the melt temperature, but not conducive to solving the shrinkage problem, and even more serious, the thicker the injection molded part, the more obvious the situation.
The reason is similar to the mold temperature is too low, the melt condenses too quickly, and the temperature difference between the shrinkage position and the water outlet cannot be formed to facilitate the shrinkage, the shrinkage channel will be closed prematurely, and the problem becomes more difficult to solve.
It can also be seen that the faster the melt glue condensation rate is not conducive to solving the problem of shrinkage, PC material is a fast condensing raw material, so its shrinkage problem can be said to be a big problem of injection molding.
In addition, the melt temperature is too low is also not conducive to increasing the overall amount of shrinkage, resulting in an increase in the amount of concentrated shrinkage, thus exacerbating the problem of shrinkage.
Therefore, when adjusting the machine to solve the more difficult shrinkage problem, you should also check whether the melt temperature is adjusted too low is extremely important, in addition to looking at the temperature table, using the empty shot method to check the temperature and fluidity of the melt is more intuitive.
2. Too fast injection speed is not conducive to solving the problem of severe shrinkage
To solve the problem of shrinkage, the first thing that comes to mind is to increase the injection pressure and extend the injection time.
However, if the injection speed has been adjusted very fast, it is not conducive to solving the problem of shrinkage. Therefore, when shrinkage is difficult to eliminate, it should be solved by lowering the injection molding speed.
Reduce the injection molding speed, so that the melt in front of the melt and the water inlet forms a larger temperature difference, and therefore conducive to the melt from far to near the order of solidification and shrinkage, but also conducive to the water inlet far from the shrinkage position to obtain a higher pressure supplement, the solution to the problem will be of great help.
As a result of reducing the injection molding speed, the melt in front of the lower temperature, the speed has been slowed down, the injection molding parts will not be easy to produce batch front, injection pressure and time can be raised and put longer, which is also more conducive to solve the problem of severe shrinkage.
In addition, if the last level of end-filling and pressure-holding method with slower speed, higher pressure, and longer time is adopted, the effect will be more obvious.
Therefore, when it is not possible to shoot at a slower speed from the beginning, it is a good remedy to start using this method at a later stage.
However, it is worth reminding that too slow a filling is not conducive to solving the shrinkage problem. Because by the time the cavity is filled, the melt is completely frozen, just like the melt temperature is too low, there is no ability to make up for the shrinkage in the distance.
How to solve the injection-molded parts surface shrinkage?
- Increase the injection pressure
- Adjust the mold temperature.
(1) When the mold temperature is high, the surface is prone to water locking.
(2) When the mold temperature is low, the internal vacuum bubble of the plastic part is easy to occurs. Wall thickness of molded parts, you can respond by increasing the mold temperature, to be completely sealed gluing cold solid approach to the gate.
- Increase the holding time.
- Reduce the resin temperature and injection speed.
- Increase the injection margin.
- Confirm whether there is a backflow of resin.
- Check the mold structure to make the cooling speed of each part of the product consistent.
- Review the injection compression.
- Make design changes to the product.