Design Essentials of Molds for Cars' Precision Plastic Parts (Part Two)

The injection process is a process that transforms solid plastic powder or granules into molten plastic liquid and molten plastic liquid into the solid plastic product. From granular plastic to molten plastic liquid and then from molten plastic liquid to the finished plastic product, the plastic materials need to be under influences of temperature fields, density fields, flow fields and density fields; different plastics have different thermoset or thermoplasticity, crystallinity or non-crystallinity, enhancement types or non enhancement types, and they have different polymer structural forms and rheological properties under influences of these fields. Any factor that affects the above fields will affect physical and mechanical properties, sizes, shapes, accuracy and apparent quality of precision plastic products. Therefore, internal relations between technical factors and properties of polymers, structural forms or the plastic products will be manifested by the plastic products.

Injection pressure and injection rates of precision injection moldings and ordinary injection moldings are different; precision injection molding often adopts high pressure, ultra high pressure injection or high speed injection to obtain the smaller molding shrinkage rate. In addition to the above factors, we not only need to take account of the regular designs of the molds, but also need to consider the following factors when we design precision injection molds.

① Adopt appropriate size tolerance of molds. 
② Prevent the formation of the molding shrinkage rate's error. 
③ Avoid deformation of injection molding. 
④ Prevent deformation of demolding. 
⑤ Minimize the error of the mold manufacturing. 
⑥ Prevent the error of the mold's accuracy.  
⑦ Maintain the accuracy of the mold.

Preventing the error of the molding shrinkage rate
The shrinkage rate will change due to the injection pressure. Therefore, for molds with the single cavity, pressure in the mold cavities should be kept consistent; for molds with multiple cavities, different mold cavities should have small differences in pressure. In the case of the single cavity with multiple pouring gates or multiple cavities with multiple pouring gates, we need to use the same injection pressure to make the mold cavity's pressure consistent. Thus, it is necessary to ensure that the position of the pouring gate is balanced. We had better keep pressure of pouring gates' entrances consistent in order to make pressure of mold cavities consistent. Whether the pressure at the pouring gate part is even or not is related to flow resistance in the flow channel. Therefore, before the pouring gate's pressure becomes even, we should keep the flow uniform. When we design mold cavities of precision injection moldings, we need to pay attention to arrangements of mold cavities so as to make sure of molding conditions because of influences of melt temperatures and mold temperatures on actual shrinkage rates.