China Precision Gear Injection Mold Tooth Profile Accuracy Control

Plastic gears are widely adopted in reduction motors, smart home devices, office equipment and automotive electronics for their lightweight structure, low noise and self-lubricating features. Gear transmission has strict requirements on tooth profile, tooth lead, tooth pitch and radial runout. Mold machining, molding shrinkage, cooling deformation and venting conditions will directly affect the final gear precision. Control measures on the mold side are the core to stably produce qualified plastic gears.

1. Machining Datum and Electrode Control of Gear Cavity

Most precision gear cavities are formed by EDM. The selection of machining datum should be consistent with the product assembly datum. The datum hole and positioning surface are finished in one clamping setup to reduce cumulative errors caused by repeated re-clamping. Copper or tungsten copper is commonly used for gear electrodes. Tungsten copper has lower electrode loss, which suits gears with small module and high precision grade. After electrode machining, tooth profile and pitch deviation must be inspected on a gear tester. Tooth profile modification is preset on the electrode to compensate tooth profile change brought by plastic cooling shrinkage. For multi-cavity molds, all electrodes need batch inspection. Inconsistent electrodes will lead to uneven precision among gears from different cavities. Discharge parameters should be controlled during EDM to reduce the recast layer on the cavity surface. A thick recast layer increases polishing difficulty and easily generates debris in mass production, harming tooth surface finish and meshing stability.

2. Shrinkage Compensation and Pre-deformation Design for Tooth Profile

Plastic gears shrink during cooling. Crystalline materials such as POM and PA show much larger shrinkage than amorphous PC and ABS. Shrinkage differs at tooth tip, tooth root and hub due to wall thickness variation. Simply scaling the cavity by average shrinkage rate will result in tooth profile distortion and tilted tooth lead. Besides basic shrinkage compensation, deformation direction should be predicted with mold flow analysis for pre-compensation. Thick hub cools slowly, and subsequent shrinkage pulls the gear ring inward to reduce tooth thickness. Thin-spoke gears tend to deform into an oval shape with excessive radial runout. For POM gears with wide shrinkage fluctuation, stable material grade is preferred, and cavity tooth profile can be revised a second time based on early trial molding data.

3. Layout Design of Mold Cooling System

Uniform cooling is the most effective way to restrain deformation. Cooling channels are arranged close to the gear cavity and distributed annularly along the gear ring to avoid local hot spots. For gears with thick hubs, straight or baffle cooling channels are built inside inserts to narrow the temperature gap between hub and gear ring. Stable mold temperature is critical for tooth precision. Precision gear molds are equipped with independent mold temperature controllers with temperature accuracy within ±1℃. Uneven cooling causes asynchronous shrinkage. Residual deformation continues after ejection, leading to drifting tooth pitch and runout during storage. PA gears will deform secondarily after absorbing water, so ambient temperature and humidity influence shall be evaluated in advance.

4. Selection of Gate, Vent and Parting Line

Gate position changes melt flow direction and further affect filling and stress distribution. Annular gate or point gate at the hub is recommended for gears to prevent melt from impacting tooth surface directly and avoid flow lines and weld lines on tooth profile. Weld lines falling on meshing tooth surface cause local strength reduction and collapsed contour. Venting at tooth groove is difficult, and trapped air leads to short shot, burning and incomplete tooth outline. Vent depth must match plastic material. Too deep vents on POM cause flash while shallow vents block exhaust. The parting line is arranged at addendum circle instead of meshing tooth profile. Misaligned parting and burrs change tooth thickness and trigger abnormal noise and wear during meshing.

5. Demolding Structure Protection for Tooth Profile

Ring sleeve ejection is preferred for precision gears to spread ejection force. Single small ejector pins on gear ring are not recommended, as concentrated force twists the gear ring and tilts tooth lead. The fit clearance of ejection components is strictly controlled. Asynchronous ejection inclines gears and creates plastic deformation. Sliders and lifters should stay away from tooth profile. If intersection is unavoidable, tighter tolerance is required to prevent scratching tooth surface. Demolding draft angle is kept small for high-precision gears. Fine EDM finishing and polishing reduce demolding resistance and scraping.

6. Molding Monitoring and Precision Re-inspection

After mold completion, first articles and trial samples cannot be measured only by calipers. A gear measuring center is required to test tooth profile deviation, tooth lead deviation, single pitch, cumulative pitch and radial runout. Continuous samples from the same cavity are collected to record precision changes under different packing, mold and barrel temperatures for locking a stable process window. Excessive packing compresses tooth shape while insufficient packing brings large shrinkage. Regular sampling inspection is implemented in mass production to distinguish precision drift caused by mold wear and process fluctuation. POM and PA gears should be stored for stress release before inspection.

7. Mold Wear Maintenance and Lifespan Control

Thin sharp corners at tooth grooves wear easily under long-term plastic friction, which gradually increases tooth thickness and abnormal meshing clearance. Tooth thickness trend is monitored in production. Cavities are cleaned regularly to eliminate carbon deposit and plastic precipitation. Proper surface treatment such as nitriding improves wear resistance, yet nitride layer thickness must be strictly controlled, as over-thick coating changes original tooth dimension. Precision data of each cavity in multi-cavity molds are recorded separately. If one cavity exceeds tolerance, the corresponding insert is replaced instead of remaking the whole cavity to cut modification cost and lead time.