Part Ⅲ

4. Shrinkage problems in precision injection molding

There are four factors that affect shrinkage: thermal shrinkage, phase change shrinkage, orientation shrinkage and compression shrinkage.

4.1 Thermal shrinkage

It’s the inherent thermal physical properties of molding materials and mold materials. If the mold temperature is high, the product temperature will be high and the actual shrinkage rate will increase. So the mold temperature of precision injection should not be too high.

4.2 Phase change shrinkage

The shrinkage results in the decrease of specific volume along with the crystallization of polymer during the orientation process of crystalline resin. The high mold temperature and the high crystallinity will cause the high shrinkage rate. But on the other hand, the increase in the crystallinity will increase the product density, reduce the linear expansion coefficient. And finally reduce the shrinkage rate. Therefore, the actual shrinkage rate is determined by the combined effect of the two.

4.3 Orientation shrinkage

Due to the forcible stretching of the molecular chain in the flow direction, the macromolecules have a tendency to re-curl and recover during cooling, and shrink in the orientation direction. The degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But the main thing is the injection speed.

4.4 Compression shrinkage and elastic reset

General plastics are compressible. That is, the specific volume changes significantly under high pressure. Under normal temperature, increasing the specific volume of pressure-molded products will decrease. Moreover, the density will increase, the expansion coefficient will decrease, and the shrinkage rate will decrease significantly. Corresponding to the compressibility, the molding material has an elastic reset function to reduce the shrinkage of the product. The factors that affect the molding shrinkage of products are related to the molding conditions and operating conditions.

5. Precision injection mold

5.1 The accuracy of the mold mainly depends on the following 3 conditions:

  • whether the dimensional accuracy of the mold cavity is high.
  • whether the positioning of the cavity is accurate.
  • Orwhether the accuracy of the parting surface meets the requirements.

The dimensional tolerance of general precision injection molds should be controlled below 1/3 of the product dimensional tolerance.

5.2 Machinability and rigidity

In the mold structure design, the number of cavities should not be too much. While the bottom plate, support plate, and cavity wall should be thicker to avoid severe elastic deformation of parts under high temperature and high pressure.

5.3 Product demoldability

For product demoldability, the mold should use as few cavities as possible. Also, fewer and short runners, and a higher finish than ordinary molds. And that is conducive to demolding.

5.4 Precision mold materials

For the material of precision molds, select alloy steel with high mechanical strength. The materials used to make the cavity and runner must undergo rigorous heat treatment. And select materials with high hardness (molded parts must reach about HRC52), good wear resistance and strong corrosion resistance.