Plastic Gear Mold Design Points

- Sep 04, 2018-

Due to its light weight, low cost and low transmission noise, plastic gears are widely used in automobiles, instruments, household appliances, toys and various timing devices. They have replaced metal gears in many occasions and can be used for metal. Gears should not be used in many areas.


At present, the research on the molding of plastic gears is mostly for the drawing of the external gears, while the research on the internal gears is less. The small modulus plastic internal gear has obvious advantages in assembling products with small size and complicated transmission structure.


1. Pouring system design


Moldflow software is a professional injection molding CAE software with powerful analysis functions, which is widely used in the optimization design analysis of injection molding. When designing an injection mold for a small modulus plastic internal gear, the Moldflow software was first used for optimal gate position analysis. Surface mesh (fusion) can reduce the computational time of the computer while ensuring the accuracy of the analysis, so the mesh type is selected for fusion. The average side length of the grid is set according to the local minimum size of the plastic part. The software analysis results show that the gate should be placed on the inside or outside of the middle height of the plastic part.


The size of the latent gate should not be too large, otherwise the flow path and the plastic part are not easily disconnected, which in turn affects the normal demoulding of the flow channel aggregate and the plastic part.


2. Molded component design


The tooth profile of the small modulus plastic internal gear is formed by a small modulus metal external gear core. Due to the small tooth shape, the core cannot be processed by a gear cutter; and because the height of the core exceeds 1 mm, the photolithography resin cannot be used. The method of electroforming the metal is processed. Therefore, the most suitable processing method is to use wire EDM. First, the mold alloy steel billet is refined into a plate of a desired thickness, and then a small modulus metal external gear is cut from the plate using a molybdenum wire on a numerical control wire cutting machine to obtain a desired core. The standard ejector rod is used as a core fixing rod, and a thread is machined on the rod portion to cooperate with the nut to fix the core to the upper end of the movable mold insert.


3. Mold overall structural design


When the plunger of the injection molding machine pushes the push rod fixing plate of the mold to move forward, the push rod is driven by the reset rod to realize the ejection movement of the plastic part. Since the cold material hole is arranged on the push plate, if the pull rod is still fixed on the push rod mounting plate as in the conventional design, there will be no relative movement between the pull rod and the push plate, and the flow channel aggregate cannot be separated. Pull rod. To this end, the pulling rod is fixed on the moving plate, and the pushing plate forcibly pushes out the flow channel aggregate from the pulling rod while pushing out the plastic piece. In order to facilitate the removal of the flow channel cement adhered to the push plate, the cold hole on the push plate needs to be designed as a shallow spherical pit.