Why the dimension of molded plastic parts is always instable？

Dimension instability means that under the same injection molding machine and molding process conditions, the dimension of plastic parts change between each batch of molded products or between each cavity molded product of each mold.

 

The dimension change is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design and changes in material performance.

 

 

1. Inconsistent molding conditions or improper operation

 

During injection molding, various process parameters such as temperature, pressure and time must be controlled in strict accordance with the process requirements, especially the molding cycle of each plastic part must be consistent and cannot be changed at will. Under normal conditons, using higher injection pressure and injection speed, appropriately extending the mold filling and pressure holding time, and increasing the mold temperature and material temperature , which are beneficial to overcome the dimensionl instability.

 

The change of the environment temperature also has a certain influence on the fluctuation of the molding size of the plastic part, and the process temperature of the equipment and the mold should be adjusted in time according to the change of the external environment.

 

2. Improper selection of molding materials

 

The shrinkage rate of molding raw materials has a great influence on the dimension accuracy of plastic parts. In general, the greater the shrinkage rate of the molding material, the more difficult it is to ensure the dimension accuracy of the plastic parts.

 

Therefore, when selecting a molding resin, the influence of the shrinkage rate of the raw material after molding on the dimension accuracy of the plastic part must be fully considered. For the selected raw materials, the variation range of the shrinkage rate cannot be greater than the requirements of the dimension accuracy of the plastic parts.

  

In addition, if the particle size of the molding raw material is uneven, poor drying, the mixing of the recycled material and the new material is not uniform, and the performances of each batch of raw materials are different, which will also cause fluctuations in the molding size of the plastic parts.

 

3. Malfunction on mold

 

The design on mold structure and manufacturing accuracy directly affect the dimension accuracy of the plastic parts.

 

If the fitting gap between the guide post and the guide sleeve of the mold is too poor due to poor manufacturing accuracy or too much abrasion, the molding dimension accuracy of the plastic part will decrease.

 

If there are hard fillers or glass fiber reinforced materials in the molding material, the mold cavity is seriously worn, or when one mold is used for multi-cavity molding, there are errors between the various cavities, gate, runner and other errors, and poor balance of the feed port. Inconsistency in mold filling will also cause dimensional fluctuations.

 

Therefore, when designing the mold, sufficient mold strength and rigidity should be designed, and the processing accuracy should be strictly controlled. The cavity material of the mold should be made of wear-resistant materials, and it should be better to get heat treated and cold hardened on the surface of cavity.

 

When the thickness error of the plastic part occurs, it is often caused by the malfuction of the mold. If the wall thickness of the plastic part produces a partial thickness error under the condition of one mold and one cavity, it is generally due to the installation error and poor positioning of the mold, which causes the relative position deviation of the mold cavity and the core. If it is a partial thickness error generated under the condition of one mold and multiple cavities, in general, the error is small at the beginning of molding, but the error gradually becomes larger after continuous operation, which is mainly caused by the error between the mold cavity and the core. This phenomenon is most likely to occur when using hot runner molds in particular. For this, double cooling circuits with very small temperature differences can be provided in the mold.

 

4. malfuction on equipment

 

If the molding capacity of the molding equipment is insufficient, 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 of the temperature control system is burned out, the heater is open circuit, etc. The molding size of plastic parts is unstable. As long as these faults are detected, targeted measures can be taken to eliminate them.

 

 

5. Inconsistent test methods or conditions

 

 If the method, time and temperature for measuring the dimensions of plastic parts are different, the measured dimensions will be very different, and the temperature conditions have the greatest impact on the test.