In this article, we will continue the analysis on the reasons for the unstable sizes of plastic parts. And the corresponding solutions will be put forward. We will think about the issue from 3 aspects. They respectively are:

  1. Mold failure

The structural design and manufacturing accuracy of the mold directly affect the dimensional accuracy of the plastic parts. During the molding process, if the rigidity of the mold is insufficient or the molding pressure in the cavity is too high, the mold will be deformed. And this will cause instability of the molding size of the plastic parts.

If the matching gap between the guide post and the guide sleeve of the mold is out of tolerance due to poor manufacturing accuracy or too much wear, it will also reduce the molding dimension accuracy of the plastic part.

The fluctuation of size can be caused by the problems below:

  • There are hard fillers or glass fiber reinforced materials in the molding raw materials that cause severe wear of the mold cavity.
  • When amulti-cavity mold is adopted, there are errors in gates and flow channel between the mold cavities.
  • Poor balance of the feed port, causing inconsistent mold filling.

# solutions

Therefore, when designing the mold, should ensure sufficient mold strength and rigidity, and strictly control the processing accuracy. The cavity material of the mold should be wear-resistant materials. In addition, the surface of the cavity should be heat treated and cold hardened.

When the dimensional accuracy of the plastic parts is very high, it is best not to use a multi-cavity structure. Otherwise, in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices must be set on the mold. And that will result in the high production cost of the mold.

When the plastic part has a partial thickness error, it is often caused by mold failure. The wall thickness of the plastic part is too thick under the condition of one mold and one cavity. And this is generally the deviation of relative position of the mold cavity and the core for the installation error and poor positioning of the mold. At this time, for those plastic parts having very precise wall thickness requirements, they cannot be positioned solely by the guide post and guide sleeve. The other positioning devices must be added.


If it is a thickness error under the condition of a multi-cavity mold, in general, the error is small at the beginning of molding. But the error gradually increases after continuous operation, which mainly results from the error between the mold cavity and the core. In particular, this phenomenon is most likely to occur when using hot runner molding.


Set a dual cooling circuit with a small temperature difference in the mold.

If it is a thin-walled round container, use a floating core, but the core and cavity must be concentric.

In addition, when making molds, to facilitate the mold repair, generally we make the cavity smaller than the required size. While the core is made larger than the required size, leaving a certain amount of mold repair.

When the inner diameter of the molded hole of the plastic part is much smaller than the outer diameter, should make the core pin larger. This is because the shrinkage of the plastic part at the molded hole is always greater than other parts. Besides, it shrinks toward the center of the hole. Conversely, if the inner diameter is close to the outer, the core pin can be made smaller.

       4. Equipment failure

  • The factors that makes the molding dimensions of plastic partsunstable:
  • The plasticizing capacity of the molding equipment is insufficient
  • The feeding of the feeding system is unstable
  • The rotation speed of the screw is unstable
  • The stop function is abnormal
  • The check valve of the hydraulic system fails
  • The thermocouple in the temperature control system burns out
  • The heater is broken, etc.

As long as these faults are detected, targeted measures can be taken to eliminate them.

  1.  Inconsistent test methods or conditions

If the method, time and temperature when measuring the size of plastic parts are different, the measured size will be very different. Among them, the temperature has the greatest influence on the test. Because the thermal expansion coefficient of plastic is 10 times larger than that of metal industry.

Therefore, we must adopt the standard method and temperature conditions to test the structural dimensions of the plastic parts. Moreover, we must measure after the plastic parts are fully cooled and shaped. Generally, the plastic parts have a large dimensional change within 10 hours in the off-mode. And they are basically set in 24 hours.