Key Design Points and Molding Notes of Plastic Gear Molds

Plastic gears feature light weight, low noise and self-lubrication, and are widely applied in home appliances, office equipment, power tools and precision transmission assemblies. Gears have strict requirements on tooth profile precision, roundness, runout and dimensional consistency. Tooth defects, uneven shrinkage, deformation, flash and weld lines directly cause transmission jamming, loud noise and accelerated wear. Rational control of mold design and injection molding parameters is critical to guarantee stable transmission performance of plastic gears.

Product Preliminary Structure and Material Selection

Low-shrinkage, wear-resistant plastics with good dimensional stability are preferred for plastic gears. POM, PA and PPS are common materials for precision gears. POM has good fluidity and low friction coefficient, suitable for small and medium module gears. Nylon has good toughness, but water absorption brings dimensional change, so dimension compensation after water absorption must be reserved in the early stage. PPS is high-temperature resistant and used for transmission gears working under high-temperature conditions. Gear wall thickness should remain uniform as far as possible to avoid sink marks caused by local thick rubber. Fillet transition is added at tooth root to eliminate stress concentration and prevent tooth fracture under load. Reasonable draft angle is set in product design. Too small draft angle easily scratches tooth surfaces during demolding, while too large angle affects tooth profile precision. The smaller the module, the stricter the control of draft angle.

Gear Cavity and Mold Insert Design

The processing precision of gear mold inserts determines the tooth quality of finished products. Integral mold inserts are preferred for cavities to reduce parting lines brought by split assembly. Parting lines cannot fall on tooth surfaces to avoid influence on gear meshing. Balanced layout is adopted preferentially for multi-cavity gear molds to ensure consistent filling pressure of each cavity and reduce dimensional differences among cavities. Mold insert steel is selected according to materials. S136 and 2316 are used for common materials such as POM and nylon for wear and corrosion resistance. High-hardness mold steel is chosen for glass fiber reinforced materials, and nitriding treatment can be conducted when necessary to improve tooth surface wear resistance. Polishing is carried out after tooth profile processing. Polishing direction follows the demolding direction to prevent transverse scratches on gear surfaces.

Gating System Design Points

Gate positions must avoid gear meshing zones. Gate traces cannot appear on tooth surfaces to prevent friction noise and wear during meshing. Point gates are commonly used for small-module gears for automatic gate break and small residual marks. Fan gates can be adopted for large gears to reduce melt shear and internal stress. Symmetric balanced layout is applied to runner systems to ensure simultaneous filling of all cavities and uniform pressure distribution, preventing unilateral shrinkage deformation of gears. Cold material wells of sufficient size are set to intercept front cold material and stop cold material entering tooth profile areas and causing short shot and pockmarks on tooth surfaces. Melt flow paths are shortened as much as possible to reduce melt pressure loss and product internal stress.

Exhaust System Design

Poor exhaust of gear cavities easily leads to burning, trapped gas, short shot and weld line defects. Weld lines on gear teeth greatly reduce gear strength. Exhaust grooves are opened at tooth ring ends and melt convergence positions. The depth of exhaust grooves is strictly matched with plastic materials. Excessive depth causes flash while insufficient depth results in poor exhaust. Exhaust channels are reserved on fitting surfaces of inserts and parting surfaces to discharge gas inside cavities smoothly. Each cavity of multi-cavity gear molds has independent exhaust instead of shared exhaust channels, to avoid inconsistent exhaust effect of cavities and fluctuation of product yield.

Cooling System Design

The roundness and radial runout of plastic gears are largely affected by uneven mold temperature. Cooling channels are arranged annularly around gear cavities. The distance between channels and tooth surfaces keeps consistent to realize uniform cooling and prevent elliptical deformation of gears. Baffles or through-core cooling are added at core positions if space permits to control temperature difference between cores and cavities. Mold temperature fluctuation directly changes material shrinkage rate. Channels of precision gear molds need sufficient flow to reduce heat dissipation difference caused by channel blockage and scale. For gears with large wall thickness difference, cooling is strengthened separately for thick-wall zones to control shrinkage difference and stabilize pitch circle dimensions of gears.

Ejection and Demolding Mechanism Design

Ejection marks cannot be left on tooth surfaces of gear products. Tube ejection or annular ejector plates are preferred for ejection mechanisms with uniform stress, avoiding gear tilt, whitening and deformation caused by single-point ejection. Ejection speed remains gentle. Excessive demolding resistance easily scratches tooth surfaces. Mold guiding mechanisms are strengthened to guarantee smooth movement of ejector plates and prevent eccentric friction from damaging mold inserts. Demolding draft angle is strictly controlled. Mold surface polishing is completed. Demolding performance of mold surfaces can be optimized when necessary to reduce tooth scratching during demolding.

Injection Molding Process Control

Raw material pretreatment is the foundation. Hygroscopic materials such as nylon must be fully dried before molding. Water causes bubbles and silver lines on tooth surfaces and reduces gear strength. Barrel temperature is controlled within the recommended range of materials. Too low temperature brings filling difficulty while too high temperature leads to material degradation and brittle products. Injection speed is controlled by sections: slow speed in the early filling stage and proper acceleration when melt enters tooth areas to avoid short shot and reduce gas encapsulation. Packing pressure and packing time are reasonably set to compensate material shrinkage and control pitch circle dimensions. However, excessive packing easily produces flash and increases internal stress. Sufficient cooling time ensures gears are fully set before demolding and prevents elliptical deformation from continuous shrinkage after demolding. Mold temperature is stabilized during production to reduce dimensional fluctuation among batches.

Trial Mold and Dimension Inspection

The first trial mold focuses on checking tooth integrity, flash and weld line positions. Key parameters such as tooth thickness, addendum circle and radial runout are measured. Cavity dimensions are fine-tuned according to measured shrinkage rate. Sampling is conducted for continuous multi-mold production to evaluate dimensional stability. Meshing tests are carried out to check transmission noise. Molding parameters are recorded in the trial mold stage and the process window is locked. Sampling inspection is implemented regularly in mass production to monitor dimensional changes and investigate hidden troubles such as cavity wear and mold temperature fluctuation timely.

In summary, mold design of plastic gears needs to balance tooth profile precision, uniform cooling, balanced feeding and stable demolding. Raw material drying, process stability and dimension monitoring must be guaranteed in molding stages. Overall control from mold structure to production process reduces shrinkage deformation and tooth defects and ensures stable long-term transmission of plastic gears.