Analysis and elimination of the reasons for the instability of injection molding size- Taizhou Huangyan Anye Industry and Trade Co., Ltd.

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

First, the definition of dimensional instability

Dimensional instability means that under the same injection molding machine and the same molding process conditions, the size of the product changes in each batch of molded products. In layman's terms, it means that the upper and lower tolerances of the dimensions are floating.

2. Factors leading to changes in product size

The change in product size is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design or changes in material properties.

1. Inconsistent molding conditions or improper operation

During injection molding, various process parameters such as temperature, pressure and time must be controlled strictly in accordance with the process requirements, especially the molding cycle of each plastic part must be consistent, which requires a good record of parameters in the usual production process. Do a good job of archiving and use it as a reference for the next production. In principle, you cannot change it casually. 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 barrel or nozzle is too high, and the cooling time of the plastic part is insufficient, all of which will lead to the instability of the product size.

Under normal circumstances, using higher injection pressure and injection speed, appropriately extending the filling and pressure holding time, and increasing the mold temperature and material temperature can effectively overcome the situation of dimensional instability.

However, if the external dimension of the plastic 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 cross-sectional area of the gate should be reduced, thereby increasing the shrinkage of the plastic part. If the size of the plastic part after molding is smaller than the size required by the tolerance, the opposite molding process measures should be taken.

2. Caused by molding raw materials.

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 molds is high, but the shrinkage rate of molding raw materials is large, it is difficult to ensure the dimensional accuracy of plastic parts. In general, the larger the shrinkage rate of the molding material, the more difficult it is to guarantee the dimensional accuracy of the plastic part. Therefore, when selecting plastic particles, it is necessary to ensure that the shrinkage is the same as that of the mold development and design. In the early stage of mold design, after the shrinkage is determined, do not arbitrarily change it in the later production.

In addition, if 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 not uniform, and the properties of each batch of raw materials are different, which will also cause fluctuations in the molding size of the plastic parts.

3. Factors caused by mold

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

During the molding process, if the rigidity of the mold is insufficient or the molding pressure in the mold cavity is too high, the mold is deformed, and the molding size of the plastic part is likely to be unstable.

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 wear, the molding dimensional accuracy of the plastic part will also be reduced.

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

In addition, the cause of the dimensional deviation is that the mold is designed with one mold and multiple cavities. Therefore, in the early stage of design, the rationality of the number of mold cavities should be fully evaluated. For products with high precision requirements, mold flow analysis should be performed, which can greatly avoid the above situation arises.

Revolving Logistics Storage Box Injection Mould