1. manufacturing tolerances for molded parts

Therefore, the precision of the molding dimension should be higher than the relative accuracy of each dimension of the product. Generally, the mold manufacturing error takes one third or one quarter of the dimensional tolerance of the plastic parts.

2. effect of molding shrinkage

It includes the difference between the calculated shrinkage rate and the actual shrinkage rate selected by the design mold, and the fluctuation of the shrinkage rate caused by the fluctuation of the process conditions and the change of the batch number of the materials when molding the plastic parts. The former causes the systematic error of the size of the plastic parts, and the latter causes the accidental error of the size of the plastic parts.

3. wear of molded parts

Plastic flows in the cavity at high speed and washes the wall of the cavity. When the mold is released, the plastic part is rubbed with the mold cavity and the core. Due to the wear and tear in the forming process, the size of the die becomes larger and larger, and the core size becomes smaller and smaller. Assuming that the circumferential direction of the core is uniform wear, it is considered that the center distance is basically the same.

4. installation error

In the process of forming, parts with no action requirements are usually fitted with transition fit. Parts requiring action, such as cores, require clearance fitting and installation, which will bring errors to the dimension of the product, and the position error of the clamping die will occur when the moving die and the fixed die are used.

5. horizontal edge thickness fluctuation

When overflow compression molding is used, the horizontal overflow thickness often fluctuates with the process conditions and other factors, so that the height and size error of the product increases. Therefore, the size of the compression molding part is regarded as the size affected by the mold part.

To sum up, the maximum error produced by a product is the combination of all the above errors.

Delta = Delta z+ Delta c+ Delta s+ Delta j+ f

Delta Z -- manufacturing error of molding parts

Delta C - total wear volume during cavity operation

Delta s -- size variation of plastic parts caused by fluctuation of plastic shrinkage rate

Delta J - the change in the size of plastic parts due to the change of clearance.

Delta f - height variation of plastic parts caused by thickness fluctuation of pressed parts.

The error values accumulated after various errors should be less than or equal to the dimension difference of the plastic parts, that is, delta less than.