Two Factors Affecting Injection Molding Shrinkage and Solution

There are 2 main factors affecting the injection molding shrinkage:

  • One is two temperature conditions including the too low temperature and the too high temperature.
  • Second is the excessive injection speed.

Now, let’s analyze the reason and find out the solutions.

1. Too low or too high mold temperature

(1) Overly low mold temperature may cause the injection molding shrinkage

The reason for the shrinkage, including surface shrinkage and internal shrinkage cavity, is that when the melt cools and shrinks, the space left by the concentrated shrinkage cannot be fully replenished by the melt from the direction of the runer gate . Therefore, factors that are not conducive to feeding will affect the solving of shrinkage problem.

As most people know, if the mold temperature is too high, it is easy to cause shrinkage problems. And they usually solve the problem by lowering the mold temperature.

But sometimes if the mold temperature is too low, it may also cause shrinkage. And seldom people can notice this.

The mold temperature is too low and the melt cools too fast. Then the slightly thicker part‘s position farther from the runner gate is blocked due to the rapid cooling of the middle part, and the distance cannot be fully replenished with melt plastic. This makes the shrinkage problem more difficult to solve, especially for thick injection molded parts.

Furthermore, too low mold temperature is not conducive to increasing the overall shrinkage of injection molded parts. Instead, it makes the concentrated shrinkage increase and the shrinkage problem more serious and obvious.

Solutions

Therefore, when solving the difficult shrinkage problems, it is best to check the mold temperature. Experienced technicians usually touch the surface of the mold cavity with their hands.

Each plastic has its suitable mold temperature. For example, for PC materials,

  • if the mold temperature is appropriate, the shrinkage cavity will be better improved.
  • But if the mold temperature is too high, the injection molded parts will shrink again.
Injection molding shrinkage (2)

(2) Too low melt temperature is not conducive to solving the injection molding shrinkage

If the melt temperature is too high, the injection molded parts are prone to shrinkage. Properly lowering the temperature by 10~20°C will improve the shrinkage problem.

However, if the injection molded part shrinks in a relatively thick part, then the melt temperature is adjusted too low like when it is close to the lower limit of the injection melt temperature, it is not conducive to solving the shrinkage problem. And it will even be more serious. The thicker the piece the more obvious the situation.

Reason analysis

The reason is similar to as the overly low mold temperature one.

If the melt condenses too quickly, a large temperature difference that is beneficial to feeding cannot be formed from the shrinkage position to the nozzle, and the feeding channel at the shrinkage position is blocked too early, making it more difficult to solve the problem. 

So we can know from this that the faster the melt condensation speed is, the more difficult it is to solve the shrinkage problem

And PC material is a raw material that condenses quite quickly. Therefore, its shrinkage cavity problem can be said to be a big problem in injection molding.

In addition, too low melt temperature is also not conducive to increasing the amount of overall shrinkage. This results in an increased amount of concentrated shrinkage, exacerbating the shrinkage problem.

Solutions

Therefore, when adjusting the machine to solve the difficult shrinkage problem, it is extremely important to check whether the melt temperature is adjusted too low. 

In addition to looking at the thermometer, it is more intuitive to check the temperature and fluidity of the melt from the nozzle without touching the mold.

Injection molding shrinkage (1)

2. Excessive injection speed

To solve the shrinkage problem, the first thing that comes to mind is to increase the injection pressure and prolong the injection time.

But if the injection speed has been adjusted very fast, it is not conducive to solving the shrinkage problem. Therefore, sometimes it should be solved by reducing the injection speed.

Decreasing the injection speed can cause a larger temperature difference between the melt in front and the water inlet. Therefore, it is conducive to the solidification and shrinkage of the melt from far to near, and also conducive to the higher pressure replenishment at the shrinkage position farther from the nozzle. This will help a lot in solving the problem.

Due to the reduction of the injection speed, the temperature of the melt in the front is lower, and the speed has slowed down. So the injection molded parts are not easy to produce batch fronts. At this time, the injection pressure and time can be increased and extended, which is more conducive to solving the problem of serious shrinkage.

Besides, if the last stage of injecting 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 not possible to use a slower speed injection at the beginning, it is also good to use this method from the later stage of injection.

However, it should be reminded that if the inyection is too slow, it will not be conducive to solving the problem of shrinkage. Because when the cavity is filled, the melt has been completely frozen, just like the temperature of the melt is too low. There is no ability to feed the shrinkage in the distance.

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