Design and manufacturing process of injection mold cavity for precision plastic gear

1 Introduction Because plastic gears have the advantages of stable operation, low noise, low inertia, self-lubricating, corrosion-resistant, and low cost, they have been widely used in electronics, watches, precision machinery, office automation and other equipment. The process is divided into cutting processing and non-cutting processing. Non-cutting processing also includes injection molding and compression molding. Precision plastic gears mainly adopt injection molding process. 2 Isometric enlargement method and its defects In mold design, the curve is equidistant The enlargement function is very popular. Generally speaking, for plastic gears, taking into account the shrinkage of the plastic parts, the involute tooth profile of the gear can be enlarged to get the cavity size of the mold, as shown in Figure 1. This method is simple and easy to implement, but it is only suitable for some gears with low precision requirements. The main reasons are:

In actual production, shrinkage will cause tooth shape deformation as shown by the dashed line in Figure 2. The tooth top becomes thinner, the tooth root becomes thicker, and the tooth profile is no longer involute shape after deformation, which will directly affect the gear transmission. ·In order to solve this problem, the method of empirical correction of the tooth profile has been used to improve the meshing conditions of the tooth tip and the tooth root. However, the empirical correction method is subject to certain theoretical limitations, the calculation is cumbersome, and cannot be universally applied. Therefore, in engineering Need a simple, accurate and practical method, this is the variable modulus method, as shown in Figure 3 3 variable modulus method

3.1 Basic Principles The molding shrinkage of plastic parts is directional, but for plastic gears, as long as the appropriate gate position and gate form are selected (usually symmetrical point gates or disc gates), appropriate forming processes are selected to ensure Mold temperature and good cooling conditions can effectively reduce the directionality of shrinkage, so it can be ideally considered that the shrinkage of plastic gears is uniform