Apr 30, 2020 Viewed: 405 Tag: injection  

Causes of the decline in the molding dimensional accuracy of plastic parts

The structural design and manufacturing accuracy of the mold directly affect the dimensional accuracy of the plastic part. During the molding process, if the mold's rigidity is insufficient or the molding pressure in the cavity is too high, the mold will be deformed, which will cause the plastic part's molding size to change. stable.

If the fitting gap between the guide pillar of the mold and the guide sleeve is too poor due to poor manufacturing accuracy or too much wear, it will also reduce the precision of the molding dimensions of the plastic part.

If there are hard fillers or glass fiber reinforced materials in the molding raw material that cause severe wear of the cavity, or when using a multi-cavity mold, there will be errors between the cavities, errors such as gates, runners, and poor balance of the inlet. Inconsistent mold filling will also cause size 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 use abrasion-resistant materials. The cavity surface should preferably be heat treated and cold hardened. When the dimensional accuracy requirements of plastic parts are very high, it is best not to use a multi-cavity structure. Otherwise, in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices to ensure the accuracy of the mold must be set up, resulting in the production of the mold High cost.

When plastic parts have thicker errors, they are often caused by mold failure. If the thickness of the plastic part has a thicker thickness error under the condition of one mold and one cavity, the relative position of the mold cavity and the core is generally shifted due to mold installation errors and poor positioning. At this time, for those plastic parts with very precise wall thickness dimensions, it is not possible to rely solely on the guide post and guide sleeve for positioning, and other positioning devices must be added.

If it is an over-thickness error in a mold with multiple cavities, in general, the error at the beginning of molding is small, but the error gradually increases after continuous operation. This is mainly due to the error between the cavity and the core. In particular, this phenomenon is most likely to occur when hot runner molding is used. To this end, a dual cooling circuit with a small temperature difference can be installed in the mold. If it is a thin-walled container, a floating core can be used, but the core and the cavity must be concentric.

In addition, when making a mold, in order to facilitate mold repair, it is generally used to make the cavity smaller than the required size, and the core is made larger than the required size, leaving a certain amount of mold repair margin. When the inner diameter of the molding hole of the plastic part is much smaller than the outer diameter, the core pin should be made larger. This is because the shrinkage of the plastic part at the molding hole is always larger than other parts and shrinks toward the center of the hole. Conversely, if the inner diameter of the molding hole of the plastic part is close to the outer diameter, the core pin can be made smaller.

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