Injection Mold Design for Small Modulus Plastic Internal Gears
Plastic gears are widely used in automotive components, household appliances, toys, timing devices, and precision instruments because of their light weight, low cost, and low operating noise. In many applications, plastic gears have replaced metal gears and are now used in products that require compact structures and quiet transmission performance.
Compared with external gears, the mold design and manufacturing process for small modulus plastic internal gears is more complicated. Internal gears often have smaller tooth profiles, tighter assembly requirements, and more difficult demolding conditions, especially when the product size is very small.
1. Gating System Design
For small modulus plastic internal gears, gate location has a direct influence on filling balance, dimensional accuracy, and gear tooth quality. Injection molding CAE software such as Moldflow is commonly used to optimize gate position and reduce molding defects.
In most cases, the best gate position is located near the inner or outer side of the middle height of the gear part. A properly positioned gate helps improve material flow and reduces the risk of short shots, weld lines, and uneven shrinkage.
The latent gate size should also be controlled carefully. If the gate is too large, the runner and molded part may not separate easily during ejection, which can affect automatic demolding and reduce production efficiency.
2. Molded Component Design
The tooth profile of a small modulus plastic internal gear is typically formed by a precision metal external gear core. Because the gear teeth are extremely small, conventional gear cutting tools are often not suitable for processing the core.
Wire EDM is usually the preferred method for manufacturing the core insert. First, alloy tool steel is machined into a thin plate with the required thickness. Then, a precision wire cutting machine is used to cut the small modulus gear profile and create the metal core.
To secure the core inside the mold, a standard ejector pin can be modified into a fixing rod. Threads are machined onto the rod and used together with a nut to fasten the core to the moving mold insert.
3. Mold Structure Design
The mold structure for small internal gears must allow smooth part ejection while also separating the runner system effectively. During the ejection process, the push rod fixing plate moves forward and drives the ejector system to remove the molded part.
Because the cold slug well is located on the ejector plate, the pull rod cannot remain fixed in the same way as a conventional mold design. Instead, the pull rod is mounted on the moving plate so that the runner can be separated from the molded part during ejection.
To improve removal of the runner material, the cold slug well on the ejector plate is often designed as a shallow spherical cavity. This helps release the runner more easily and improves mold reliability during mass production.
Conclusion
Small modulus plastic internal gears require careful consideration of gate position, core manufacturing, and mold structure design. By combining Moldflow analysis, wire EDM core machining, and optimized ejection systems, manufacturers can improve molding accuracy, reduce defects, and achieve stable production of precision plastic gears.