Plastic gears are widely used in automobiles, instruments, household appliances, toys and various timing devices due to their light weight, low price and low transmission noise. They have not only replaced metal gears in many occasions, but also can be used in metal gears. There are many areas where gears should not be used. At present, most of the researches on the forming of plastic gears are aimed at the forming of the external gears, while the research on the internal gears is less. Small modulus plastic internal gears have obvious advantages when assembling products with small size and complex transmission structure. The author studies the design of the injection mold for small-modulus plastic internal gears with a modulus of 0.2 mm, and verifies the molding effect of the injection mold through the injection molding test.

1. Analysis of the characteristics of injection molding of small modulus plastic internal gears

    Figure 1 is a schematic diagram of the size of the small modulus plastic internal gear to be injection molded. Its tooth shape parameters are: module 0.2mm, number of teeth 36, displacement coefficient 1.6, indexing circle pressure angle 20 degrees; internal dimensions are: tip circle diameter 7.5mm, tooth root circle diameter 8.2mm; external dimensions are: The outer diameter is 10mm, and the height is 4mm.

Figure 1: Schematic diagram of the dimensions of the plastic internal gear

    Since the plastic internal gear will be tightly wrapped on the core due to shrinkage after injection molding, and the number of teeth of the molded plastic internal gear is large, the required demoulding force is relatively large. In addition, since the wall thickness of the tooth root of the small modulus plastic internal gear is only 0.9mm, if a push rod is used to eject the plastic parts, the diameter of the push rod should be about 0.5mm, which makes the plastic parts bear the maximum impact on the local area. If the output pressure is too large, it is very likely that the plastic parts will be pushed through, causing the plastic parts to be damaged and unable to be demolded, so it is necessary to use a push tube or a push plate for ejection. If the push tube is used for ejection, not only the axial dimension of the core is too large, but also the positioning accuracy of the push tube is very high. Based on the above analysis, the author chooses to use the push plate to eject the plastic parts.

    Polyoxymethylene (POM) has the characteristics of high strength, fatigue resistance, creep resistance and good dimensional stability. It is an ideal plastic gear material. Therefore, POM is used for injection molding of small modulus plastic internal gears.

2. Analysis of key points of mold design