When producing products in bulk, dimensional instability is a relatively common process situation. If avoided in advance, it can save a lot of costs and avoid unnecessary waste.

1、 Definition of dimensional instability
Dimensional instability refers to changes in the size of each batch of molded products under the same injection molding machine and molding process conditions. Generally speaking, it refers to the fluctuation of the upper and lower tolerance of dimensions.

2、 Factors that cause changes in product size
The changes in product size are caused by abnormal equipment control, unreasonable injection molding conditions, poor product design, or changes in material performance.

1. Inconsistent forming conditions or improper operation
During injection molding, the parameters of various processes such as temperature, pressure, and time must be strictly controlled according to the process requirements, especially the molding cycle of each type of plastic part must be consistent. This requires that parameter records be kept and archived during the production process, which can serve as a reference for the next production. In principle, changes cannot be made arbitrarily. If the injection pressure is too low, the holding time is too short, the mold temperature control is unreasonable and unstable, the temperature at the material barrel or nozzle is too high, and the cooling time of the plastic part is insufficient, all of these can lead to the instability of the product size.

In general, using higher injection pressure and speed, appropriately extending the filling and holding time, and increasing the mold temperature and material temperature can effectively overcome dimensional instability.

However, if the external dimensions of the molded plastic parts exceed the tolerance requirements, the injection pressure and melt temperature should be appropriately reduced, the mold temperature should be increased, the filling time should be shortened, the gate cross-sectional area should be reduced, and the shrinkage of the plastic parts should be increased., If the size of the formed plastic part is less than the tolerance requirement, the reverse forming process measures should be taken.

2. Caused by molding materials.
The shrinkage rate of forming raw materials has a significant impact on the dimensional accuracy of plastic parts. If the precision of the forming equipment and molds is high, but the shrinkage rate of the forming raw materials is large, it is difficult to ensure the dimensional accuracy of the plastic parts. In general, the larger the shrinkage rate of the molding material, the harder it is to ensure the dimensional accuracy of the plastic part. Therefore, when selecting plastic particles, it is necessary to ensure that the shrinkage is the same as during mold development and design. After determining the shrinkage in the early stage of mold design, do not make arbitrary changes during later production.

In addition, if the particle size of the forming raw materials is uneven, the drying is poor, the mixture of recycled materials and new materials is uneven, and the performance of each batch of raw materials is different, it can also cause fluctuations in the forming size of the plastic parts.

3. Factors caused by molds
If there are hard fillers or glass fiber reinforced materials in the forming raw materials, which lead to serious wear of the mold cavity, or when the first mock examination is used for multi cavity forming, there are errors between the cavities, such as gates, runners, and other errors, as well as the poor balance of the feed port, which lead to inconsistent filling of the mold, size fluctuations will also be caused.

During the molding process, if the rigidity of the mold is insufficient or the molding pressure inside the mold cavity is too high, causing deformation of the mold, it is easy to cause instability in the molding size of the plastic part.

If the fit gap between the guide pillar and guide sleeve of the mold exceeds the tolerance due to poor manufacturing accuracy or excessive wear, it will also cause a decrease in the forming dimensional accuracy of the plastic part.

Therefore, when designing molds, sufficient mold strength and rigidity should be designed, and machining accuracy should be strictly controlled. The cavity material of the mold should use wear-resistant materials, and the surface of the cavity should be subjected to heat treatment and cold hardening treatment in violation of regulations. In addition, the reason for dimensional deviation is also that the first mock examination has multiple cavities. Therefore, in the early stage of design, the rationality of the number of cavities should be fully evaluated. For products with high precision requirements, mold flow analysis should also be carried out, which can greatly avoid the above situation. In addition to the above three points, there are also reasons such as equipment reasons, inconsistent testing methods and conditions. If you find that there are other reasons for this, please contact us, communicate with each other, and grow together!