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Sep 26.2022

Injection Molding | How to avoid dimensional instability of injection molded products?

In the mass production of products, dimensional instability is a process situation that is more likely to occur. If it can be avoided in advance, it can save a lot of costs and avoid unnecessary waste.

1. Definition of dimensional instability

Dimensional instability refers to the change of product size in each batch of molded products under the same injection molding machine and the same molding process conditions. In layman's terms, it means that the dimensional tolerance fluctuates up and down.

2. the factors that lead to the change of product size

The variation of product size is caused by the abnormal control of equipment, unreasonable injection conditions, poor product design or changes in material properties, etc.

(1). Inconsistent molding conditions or improper operation

When injection molding, the parameters of 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, which requires the usual production process, the need to do a good job of recording the parameters, do a good job of filing, for the next production reference, in principle, can not be casually changed. If the injection pressure is too low, the holding time is too short, the mold temperature control is unreasonably unstable, the temperature at the barrel or nozzle is too high, and the cooling time of the plastic part is not enough, it will lead to the unstable size of the product.

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

However, if the shape size of the molded part is larger than the size required by the tolerance, the injection pressure and melt temperature should be appropriately reduced, the mold temperature should be increased, the filling time should be shortened, and the gate cross-sectional area should be reduced, so as to improve the shrinkage of the molded part. If the size of the molded part is smaller than the size required by the tolerance, the molding process measures should be taken to the contrary.

(2). Molding raw materials lead to

The shrinkage rate of molding raw materials has a great influence on the dimensional accuracy of plastic parts. If the precision of molding equipment and mold is very high, but the shrinkage rate of molding raw materials is very large, it is difficult to ensure the dimensional accuracy of plastic parts. In general, the greater the shrinkage rate of the molding raw materials, the more difficult to ensure the dimensional accuracy of the plastic parts. Therefore, when choosing plastic particles, make sure that the shrinkage is the same as the mold development and design. In the early stage of mold design, after determining the shrinkage, do not change it at will in the later production.

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

3. Factors caused by the mold

If there is hard filler or glass fiber reinforced material in the molding raw material, which leads to serious wear of the mold cavity, or when using a mold with multiple cavities, there are errors between the cavities and gates, runners and other errors and poor balance of the inlet and other reasons to produce inconsistent mold filling, will also cause size fluctuations.

In the molding process, if the rigidity of the mold is not enough or the molding pressure in the mold cavity is too high, so that the mold deformation, it is easy to cause the molding size of the plastic parts is not stable.

If the fitting clearance between the guide pillar and guide bush of the mold is poor due to poor manufacturing accuracy or too much wear and tear, it will also make the molding dimensional accuracy of the plastic parts decrease.

Therefore, in the design of the mold, should design enough mold strength and rigidity, strict control of processing accuracy, mold cavity material should use wear-resistant materials, cavity surface prohibited words for heat treatment and cold hardening treatment.

In addition to the causes of dimensional deviation there are mold design a mold cavity, so in the early design, should fully assess the reasonableness of the number of open cavities, for high precision requirements of the product, but also mold flow analysis, can greatly avoid the above situation.

In addition to the above three points, there are also equipment reasons, test methods and conditions are inconsistent and other reasons to cause. If you find that there are other causes, welcome to contact us to communicate with each other and grow together!