The core of material selection for plastic gears is matching the load, temperature, and wear resistance requirements of the application scenario. They are mainly divided into three categories: general-purpose plastics, engineering plastics, and high-temperature specialty plastics. General-purpose plastics such as nylon (PA) and polypropylene (PP) are low-cost and easy to mold, but their strength and wear resistance are generally average. They are suitable for low-load, short-life applications, such as toy gears and auxiliary transmission gears in simple household appliances. Engineering plastics such as polyoxymethylene (POM), polycarbonate (PC), and ABS resin have high strength, superior wear resistance, and temperature resistance. They are the mainstream materials for plastic gears and are suitable for conventional transmission scenarios requiring longevity and stability, such as printers, automotive interiors, and core transmission gears in household appliances. High-temperature specialty plastics such as polyetheretherketone (PEEK), polyphenylene sulfide (PPS), and polyimide (PI) have high strength and high temperature resistance, capable of withstanding medium to high loads and harsh environments. However, they are expensive and difficult to process, making them suitable for applications with special requirements, such as automotive engine components, industrial equipment, and medical equipment.
The design steps for plastic gear trains include: understanding the working task and environmental conditions of the train; drafting a preliminary design scheme and defining the main parameters of the gear teeth; performing design calculations for the train parameters; selecting the precision level of the plastic gear train; performing calculations to avoid root undercut and tip sharpening; adjusting the center distance to meet the minimum backlash requirements; checking the contact ratio of the train; evaluating the load-bearing capacity of the train; creating a gear train parameter table; and finally, drawing the gear product drawing according to the DFM injection molding part design guidelines.
Key design considerations for plastic gear structures include uniform wall thickness, avoiding sharp corners, appropriately setting reinforcing ribs, and flange-hub structure design. Generally, the wall thickness of plastic parts is controlled at around 3mm. The variation range for wall thickness should be less than 25% for low-shrinkage materials and less than 15% for high-shrinkage materials. Sharp corners should be avoided at the junction of two walls; rounded corners should be provided. The rounded corner size is generally 0.25-0.75 times the wall thickness, but preferably greater than 0.5mm. Reinforcing ribs enhance gear rigidity. Their height is typically 2.5-3 times the wall thickness, and their thickness is 0.5-0.75 times the main wall thickness. The distance between ribs should be greater than twice the main wall thickness. When the gear has a plate-like structure with a thickness greater than 4.5 mm, it should be designed as a web and flange-hub structure. The thickness of the flange and web should be 1.25-3 times the tooth thickness. The web thickness can be slightly greater than the edge thickness, and care should be taken not to drill holes in the web.
The mold design for plastic gears needs to consider several key points to ensure molding accuracy. The cavity and core should be precisely positioned using cylindrical tapered inserts. The gate should be a point gate, positioned close to the center of the gear. Using an odd number of gates (3, 5, 7, etc.) can improve coaxiality. Ejector pins should be evenly distributed during ejection. For bearing areas, a straight push tube ejection is recommended. In addition, it is necessary to set a suitable draft angle, correctly place vents at the final resin filling location, and control the number of cavities, commonly 1, 2, or 4 cavities. Too many cavities may lead to accuracy deviations.
Regarding accuracy, due to the influence of various factors such as material shrinkage and injection molding process, the accuracy of injection-molded plastic gears is relatively low, generally at or below grade 9-10 of the national standard, while the accuracy of hobbing-cut plastic gears is at or below grade 7-8 of the national standard. Reference standards include the AGMA PT (Plastic Gear Tooth Profile) standard introduced by ANSI/AGMA 1106-A97.
