This analysis is divided into two parts. And in this part, we will analyze the following two reasons which causing the unstable dimensions of plastic parts:

  1. Inconsistent molding conditions or improper operation

  2. Improper selection of molding materials

Dimensional instability means that with the same injection molding machine and molding process , the size of the plastic part changes between each batch of molded products or between each cavity molding products produced by each mold. The change in product size is caused by abnormal equipment control, unreasonable injection conditions, poor product design, and changes in material properties.

  1. Inconsistent molding conditions or improper operation

During injection molding, we must strictly control the various process parameters like temperature, pressure and time according to the process requirements.Especially the molding cycle of each plastic part must be consistent and cannot be changed optionally. The following situations can cause the unstable shapes and dimenshions of the plastic parts.

  • The injection pressure is too low
  • The pressure holding time is too short
  • The mold temperature is too low or uneven
  • The temperature at the barrel and the nozzle is too high
  • The cooling of the plastic parts is insufficient

Generally, the solutions are:

  • Higher injection pressure and injection speed
  • Appropriate extension of mold filling and holding time
  • Increase mold temperature and material temperature

If the appearance size of the plastic parts is bigger than the required size, we should reduce the injection pressure and melt temperature appropriately. Then, increase the mold temperature, shorten the filling time and reduce the gate cross-sectional area. So, it can increase the shrinking percentage of the plastic parts. If the size is smaller than the required size, vice versa.

It’s worth noting that the change of the ambient temperature also has a certain effect on the fluctuation of the molding size of plastic parts. The process temperature of the equipment and the mold should be adjusted in time according to the change of the external environment.

unstable dimensions
  1. Improper selection of molding materials

The shrinking percentage of molding materials has a great influence on the dimensional accuracy of plastic parts. If the precision of the molding equipment and the mold is high, but with large shrinkage rate of the molding material, it is difficult to ensure the dimensional accuracy of the plastic parts. In general, the greater the shrinkage rate of material, the more difficult it is to ensure the dimensional precision of the plastic parts.

Therefore, when selecting molding resins, we should fully consider the impact of shrinkage of the raw materials after molding on the dimensional precision of the plastic parts. For the selected raw materials, the variation range of the shrinkage rate cannot exceed the requirements of the dimensional accuracy.

The shrinkage of various resins varies greatly. Analyze acccording to the crystallinity of the resin. Generally, the shrinkage of crystalline and semi-crystalline resins is larger than that of non-crystalline resins. Besides, the range of shrinkage changes is relatively large. And the fluctuation of shrinkage of the corresponding plastic parts after molding is relatively large.

Crystalline resins has the features of high crystallinity, reduced molecular volume, and large shrinkage of plastic parts. The size of the resin spherulites also affects the shrinkage rate. And the resin spherulites have the following features:

small spherulites, small gap between the molecules, and small shrinkage of plastic parts, relatively high impact strength.

In addition, with the following problems, the molding size of the plastic parts may fluctuate.

  • The particle size of the molding raw material is uneven
  • The drying is poor
  • The mixing of the recycled material and the new material is uneven
  • The performance of each batch of raw materials is different