Holding Gear Tooth Profile Across an 8-Cavity Mold
A home appliance OEM's gear mold drifted 0.06 mm in tooth profile between cavities, causing gear noise at final assembly. DieStrike rebuilt the tool with wire-EDM'd S136 inserts and a trial-shot shrinkage correction loop — holding profile deviation to ±0.02 mm across all eight cavities.
PROJECT OVERVIEW
"A home appliance OEM running a small drive gear program in POM (acetal) was struggling with an 8-cavity gear mold that had been supplied by a previous toolmaker. The gear is a module 0.8, 24-tooth spur gear, roughly 19 mm in pitch diameter, molded at an annual volume of 2 million pieces. Tooth profile deviation measured up to 0.06 mm from cavity to cavity — far outside the 0.02 mm the gear train allowed — and the assembly line rejected finished units on gear noise. Because the noise showed up only after final assembly, the OEM was scrapping complete appliances, not just gears. They came to DieStrike for a rebuilt 8-cavity tool that could hold profile deviation inside ±0.02 mm across every cavity, verified by CMM before the tool left the shop."
PAIN POINTS
- Tooth profile deviation up to 0.06 mm between cavities — three times the 0.02 mm the gear train allowed
- Gear noise only appeared after final assembly, scrapping complete appliances instead of single parts
- Dimensional drift after roughly 20,000 shots as the previous tool's cavity steel wore at the tooth tips
OUR SOLUTION
- Rebuilt the 8-cavity tool in S136 ESR cavity steel hardened to HRC 48-52 — corrosion- and wear-resistant for long POM runs without profile drift
- Gear tooth forms wire-EDM'd into the hardened inserts at ±0.002 mm, with the gear bore jig-ground for true position against the tooth form
- Shrinkage compensation corrected through a trial-shot loop: first articles measured on the CMM at 24 hours, cavity scaling adjusted per measured shrink, then a second trial to lock the geometry
- Full gear inspection — profile, lead, and runout — on every cavity before delivery, with the CMM report shipped with the tool
MEASURED RESULTS
| Tooth profile deviation | 0.06 mm → within ±0.02 mm across all 8 cavities |
| Cavity-to-cavity spread | Cut from 0.06 mm to 0.02 mm — gears interchangeable between cavities |
| Assembly noise rejects | Eliminated — zero gear-noise failures in the first production quarter |
| Dimensional stability | No profile drift after 200,000+ shots; CMM checks every 50,000 shots confirm stable geometry |
WHY IT HELD
Gear molds fail on three things: tooth-form accuracy in the steel, shrinkage compensation that is guessed instead of measured, and cavity wear that shows up as drift. This tool attacked all three — wire-EDM'd forms at ±0.002 mm in hardened S136 ESR, a two-loop trial correction that locked the real POM shrinkage into the cavity scaling, and a cavity steel chosen to hold its geometry past 200,000 shots. The gear inspection report that ships with the tool is the same one the OEM's quality team uses for incoming acceptance.
YOUR APPLICATION MAY DIFFER
"Send us your gear drawing or part file and we will quote the cavity count, steel and shrinkage correction plan for your resin — DFM review is free within 24 hours."