China Precision Plastic Gear Mold Dimensional Tolerance Control Standard

Precision plastic gears are core transmission components widely used in precision reducers, office automation equipment, home appliance drive systems and automotive small transmission structures. Different from metal gears, plastic gears are affected by material shrinkage, mold temperature expansion, molding stress and processing errors, making their dimensional stability and tooth profile accuracy more dependent on high-precision mold control. To ensure stable meshing, low noise, low wear and long service life of mass-produced plastic gears, it is necessary to formulate complete mold tolerance standards and systematic control specifications for processing, assembly and trial production.

1. Basic dimensional tolerance specification of gear mold cavity

The mold cavity size must compensate the shrinkage rate of plastic materials in advance, and set hierarchical tolerance standards according to different precision levels. For general precision plastic gears, the addendum circle and dedendum circle of the mold cavity adopt IT6 tolerance standard, while high-precision transmission gears implement IT5 tolerance. The tooth thickness tolerance is strictly controlled within ±0.003mm to ±0.005mm, and the over-pin distance, the key assembly dimension, is controlled within ±0.004mm. For glass fiber reinforced materials with obvious anisotropic shrinkage, radial and axial shrinkage deviations need targeted profile compensation instead of unified shrinkage parameters. For multi-cavity gear molds, the dimensional difference of tooth thickness and tooth pitch among all cavities is controlled within 0.005mm to guarantee product interchangeability in batch production.

2. Tooth profile and tooth direction precision control

Referring to GB/T 10095 and DIN 3962 international gear precision standards, high-precision plastic gear molds adopt grade 5 tooth profile accuracy. The tooth profile deviation is controlled within 0.003mm to ensure smooth meshing and avoid local tooth surface friction. The tooth direction deviation is strictly limited to prevent axial inclination, which may cause unilateral stress and transmission noise during gear operation. The root transition fillet must be smooth without tool marks and knife lines, because tiny processing defects will be completely copied to plastic gears, resulting in stress concentration and tooth breakage under long-term cyclic load.

3. Mold assembly geometric tolerance standard

Coaxiality is the core index restricting gear operation accuracy. The coaxiality of gear core, cavity and mold base guide mechanism is controlled within 0.002mm, and the core end face runout is not more than 0.002mm. Excessive runout will directly lead to large radial runout of finished gears, causing jitter and abnormal noise during operation. The parallelism of cavity inserts is controlled within 0.003mm, and the splicing gap is less than 0.002mm to eliminate flash and tooth surface distortion caused by assembly gaps.

4. Shrinkage compensation and temperature tolerance control

Uniform shrinkage compensation is adopted for non-fiber reinforced plastics with a reserved tolerance of ±0.002mm. For PA6, POM and other fiber-containing materials with anisotropic shrinkage, asymmetric tooth profile compensation is required according to actual test data. Meanwhile, thermal expansion tolerance of the mold under high-temperature production is reserved to prevent dimensional deviation caused by core expansion. The first trial product is tested after full dimensional stabilization to correct the shrinkage compensation value accurately.

5. Processing detection and mass production monitoring

The precision of the processing electrode must be one level higher than the cavity precision to ensure the foundation of tooth profile processing. After cavity finishing, full tooth profile scanning detection is carried out in a constant temperature environment of 20℃. Polishing only removes tool marks without changing tooth profile dimensions. In mass production, the wear degree of the mold tooth surface is regularly monitored, and early warning and repair are carried out when the dimensional deviation exceeds the standard, so as to maintain long-term and stable precision of plastic gears.