Understanding the key points of designing injection moulds
Plastic injection mold is the most widely used mold among all plastic molds, which can form complex and high-precision plastic products. This article is just a rough introduction.
Since it is a chain like structure, the shrinkage direction of the plastic during processing is also related to the orientation and cooling shrinkage of the polymer molecular chain under stress. The shrinkage in the flow direction is more than that in the vertical direction.
Product shrinkage is also related to product shape, gate, thermal expansion and cold shrinkage, temperature, pressure holding time, internal stress and other factors. Generally, the book provides a wide range of shrinkage. In practical application, the wall thickness and structure of the product, as well as the size and orientation of the temperature and pressure when determining the injection molding are considered. If there is no core support for general products, the shrinkage should be larger. Plastic injection mold is basically divided into static mold and dynamic mold.
One side of the injection head of the injection molding machine with the sprue sleeve is a static mold. The static mold is generally composed of the sprue sleeve, backup plate and template. The simple mold (especially the mold without the core of the static mold) can also not use the backup plate, just use a thicker template. The sprue sleeve is generally standard, and it is not recommended to cancel unless for special reasons.
The use of the sprue sleeve is conducive to the installation of the mold, convenient replacement, without polishing. Some special mold sprue sleeve can be drilled out or cut with taper line. When some molds must be demoulded by static mold, the demoulding mechanism of static mold must be added. The structure of the moving die is generally the moving die, the moving die backing plate, the demoulding mechanism, the die foot and the fixed plate of the installation machine.
Gate is directly related to the beauty of plastic parts. If the gate design is not good, it is easy to produce defects. It is easy to produce serpentine flow without any obstruction. For products with high requirements, overflow and exhaust shall also be designed. Top bar can be used at overflow position. Do not leave overflow flash on the formwork, so as not to affect the service life of the mold.
In the demoulding mechanism, in addition to the stripper bar, there is also a return bar. Some molds also need to add springs to realize functions such as automatic demoulding. There are also guide pillars, cooling water holes, flow channels, etc. are also indispensable to the basic structure of the mold. Of course, there are also inclined guide box, inclined guide pillar and so on.
When designing a mold for a product, we should first set the basic structure size of the mold to prepare materials, so as to accelerate the speed of mold manufacturing. For complex products, the product drawing should be drawn first, and then the mold size should be determined. At present, the mold basically needs heat treatment to increase the hardness of the mold and improve the service life of the mold.
Before heat treatment, the template shall be preliminarily processed: drill the guide column hole, return hole (dynamic mold), cavity hole, screw hole, gate sleeve hole (static mold), pulling hole (dynamic mold), cooling water hole, etc., and mill the flow channel, cavity, and some molds shall also mill the inclined guide box, etc.