Molded part guide: gears
Most plastic gear problems are settled before the first shot: which resin, where the gate goes, and how the tooth form was corrected for shrink when the steel was cut. This page covers those three decisions and what to send with your CAD.
Why plastic
Plastic gears run quietly, often need no grease, and let you mold the hub, cam, or pinion into the same part. What they give up is load capacity and heat tolerance.
Printers, dispensers, window actuators, utility meters, small pumps, and toy drivetrains all run on molded gears, because one shot replaces a cut gear, a pressed-on hub, and sometimes a separate cam. The tradeoff lives at the tooth. Plastic flexes more than steel under the same load, holds heat at the mesh instead of shedding it, and changes size with temperature and, in the case of nylon, with humidity.
If the gear carries a light or moderate load at modest speed, the molded version is usually the cheaper and quieter part once volumes are real. If it sits next to a motor that runs hot for hours, or has to hold a tight center distance in a damp environment, the resin and the clearances deserve more attention than the tooth count.
NetProto molds gears in one hardened steel tool that stays with the part for its production life. That matters more for gears than for most parts. The tooth profile you approve on sample gears is the profile cut into that steel, so there is no second tool whose teeth have to be corrected all over again.
Resin
General material behavior, not a NetProto specification. Put load, temperature, and the mating material on the quote so the choice is reviewed for your gear train.
| Resin | Why engineers pick it | Watch for |
|---|---|---|
| Acetal (POM) | Low friction, good fatigue life, and it holds its size in humid air | Poor resistance to strong acids and chlorine; limited options once heat climbs |
| Nylon (PA6, PA66) | Tough, forgiving of shock loads, and a good running partner for acetal | Absorbs moisture and grows, which eats into backlash |
| Glass-filled nylon | Stiffer teeth, less creep, and thermal growth closer to a metal housing | Fibers can wear a softer mating gear, and gate placement shows up in roundness |
| PTFE-lubricated acetal | Runs dry with less wear where grease is not allowed near the product | Lower strength than unfilled acetal and often priced higher |
| PBT | Dimensional stability and electrical properties suited to motor-side gears | Notch sensitive, so sharp corners at the tooth root are a real risk |
| PEEK | Keeps its strength at temperatures that soften acetal and nylon | Priced well above commodity gear resins, hard to justify on light duty |
Pairing
When both gears in a pair are molded, the combination matters as much as either part on its own.
Gating
Plastic shrinks by different amounts along and across the direction it flows. The gate layout sets that flow pattern, and runout follows it.
| Gate layout | What it does to the gear | Where it fits |
|---|---|---|
| Diaphragm gate across the bore | Melt spreads evenly in every direction with no weld line, giving the most uniform shrink | Precision gears, when the bore can be finished after the gate is removed |
| Single gate on the web | Flow wraps around the core pin and meets itself, leaving a weld line and a pull toward an oval gear | Low-precision gears where runout is not the limiting factor |
| Several gates evenly spaced on the web | Rounder than a single gate, but the gear tends to show one high spot per gate | Most production spur gears in unfilled resin |
| Central gate with fiber-filled resin | Avoids the weld lines where fibers line up and leave runout bumps | Glass or carbon filled gears |
The housing
Center distance is set by the bores in the housing, not by the gear. When both are molded, their growth with heat and moisture has to be planned together.
Unreinforced plastics expand several times more than metals when they warm up. Put an acetal gear in a die-cast housing and the mesh tightens as the gearbox heats. Put it in a plastic housing and the housing grows too, but plastic housings conduct heat poorly, so the gears inside run warmer than a bench test on an open frame suggests.
Gear designers check the mesh at both extremes: tight mesh, with the largest gears at the shortest center distance, and open mesh, with the smallest gears at the longest. If the housing and the gear are both on your BOM, send the housing CAD with the gear so bore location is reviewed in the same DFM pass instead of on a separate quote.
Validation
The tooth form of a molded gear belongs to the cavity it came from. Change the tool or the process and the profile changes with it.
Process
What happens to a gear program between the quote and production.
Upload CAD for the gear and, if you have them, the mating gear and the housing. Put the gear data table on the drawing. A STEP file alone does not carry tooth thickness or the quality grade you inspect to.
Review flags gate location, undercut at the tooth root, ejection for helical teeth, and heavy hubs that will sink or pull the teeth out of round.
The cavity is not a scaled copy of the gear. Tip, root, and tooth thickness shrink by different amounts, and helical gears shrink differently along the axis than across the diameter, so the tooth form in the steel is corrected for your resin.
Sample gears go out for approval against the gear data on your drawing. Tell us how your incoming inspection measures them, over pins or on a double-flank roll tester, so both sides read the same numbers.
Once the gears are approved, production runs from that tool. There is no minimum order quantity, and the tool is yours once the tooling charge is paid.
Drawing
Without gear data, a gear gets quoted as a round part with bumps on it. These items let the tooth form be cut correctly the first time.
Number of teeth, module or diametral pitch, and pressure angle
Profile shift, or tooth thickness at the pitch circle, or a measurement over pins
The mating gear and the operating center distance
The quality grade you inspect to, and whether it comes from AGMA or ISO
Operating temperature range, and whether the product lives somewhere humid
Lubricated or dry running, with rough torque and speed
Faces where a gate mark or ejector marks cannot go
Keep reading
Sibling gear pages, the resins that show up most in gear trains, and the design rules behind the tables above.
Tooth form choices for plastic spur gears, including thicker teeth on small pinions.
Spur gearsWorm and wheel pairs, and why ejection drives the mold layout.
Worm gearsMold the gear around a metal hub or shaft when a molded bore cannot carry the torque.
Insert moldingThe default gear resin when humidity and dimensional drift are the worry.
Acetal (POM)For gear trains that have to run dry.
PTFE-lubricated acetalStiffer teeth and less creep, with a roundness tradeoff at the gate.
Glass-filled nylonHow cavities are sized so the cooled part lands on the drawing.
Shrinkage compensationHow gate position shapes flow, weld lines, and warp.
Gate locationEvery part-type guide, grouped by family.
Molded parts hubFAQ
Yes. Insert molding places the metal part in the tool and molds the gear around it, which removes a press fit and carries more torque than a molded bore. Knurls or flats on the insert keep it from spinning loose under load.
It can save tooling, but gears of different sizes fill and cool at different rates, and accuracy suffers when one cavity has to be balanced against another. For gears with a tight quality grade, separate tools or matched cavities are often the better choice. DFM review will tell you which way your set leans.
Nylon picks up water from the air and grows. If the mesh was set with very little backlash on dry parts, it can close up once the product sits in a humid room. More clearance at the design stage, checking samples after moisture conditioning, or moving to acetal usually solves it.
No part-specific gear tolerances are published. Commercial tolerances for general dimensions are listed on the manufacturing standards page. Gear accuracy is set by the gear data on your drawing and confirmed on sample gears.
No. There is no minimum order quantity, so a low-volume gear program runs from the same hardened steel tool as a high-volume one.
Next step
Upload STEP files for the gear and its mate, and attach the drawing with the gear data table. DFM feedback comes back before any steel is cut. NetProto manufacturing is ISO 9001 certified.
