Molded part guide: spur gears
A spur gear looks simple because the teeth are straight. In molding, straight teeth still shrink unevenly in the plane of the gear, and a small pinion still needs profile shift before the steel is cut.
Straight teeth
Most molded gear trains start with spur pairs because the tool can pull straight along the bore.
A spur gear ejects like any round part: the mold opens, the part stays on the core or the cavity, and it comes off in a straight line. That is why spur pairs dominate printers, meters, small actuators, and appliance drives. The tooth form still has to be corrected for plastic shrink, which is not the same as scaling the whole disk evenly.
Involute teeth shrink differently along the flow than across it. If the cavity is cut from a metal gear drawing with only a single shrink percentage, the molded teeth often come out too thin at the tips and too heavy at the roots, and the effective pressure angle climbs. Mesh feels tight, noise goes up, and wear concentrates at the tips.
Tooling practice is to enlarge the cavity module using the resin shrink rate, then inspect sample gears and adjust the tooth profile before the production cavity is finalized. Send the gear data listed below with your CAD so DFM review starts from numbers, not guesses.
Geometry
Typical design levers on spur pinions. Final values come from your load, center distance, and resin.
| Parameter | Why it matters in molding | Common adjustment |
|---|---|---|
| Tooth count on the pinion | Very small pinions risk undercut at standard pressure angle | Positive profile shift, or a higher pressure angle |
| Module or diametral pitch | Sets tooth size and how much plastic sits between teeth | Smaller module fills faster but leaves less material at the root |
| Pressure angle | Steeper angles resist undercut but are less forgiving of center distance error | A commonly used pressure angle is the usual starting point on plastic pairs |
| Profile shift | Moves material from tip to root without changing the ratio | Positive shift on the pinion is the usual fix for undercut |
| Tooth form standard | Root fillet and tip relief change bending stress and contact | Modified forms with full fillet roots are common on molded plastic gears |
Service tiers
Motion part focus: Straight teeth: profile shift, shrink-corrected cavities, and gating. Upload CAD with module, pressure angle, and tooth count in notes when the model does not show them.
| Tier | Environment | What matters most | Typical molded route | Quote path |
|---|---|---|---|---|
| 1 Module prototype | First article spur pair | Profile shift and shrink | POM spur on hub | Instant quote; field POM |
| 2 Enclosed gearbox | Lubricated or greased box | Wear and heat from friction | PA or PBT spur | Instant quote; note lube type in comments |
| 3 Small pinion | Few teeth on a small pinion | Fill and vent at root | POM high flow family in notes | Upload CAD; expect thin tooth DFM |
| 4 Wide face width | Wide face on a spur pair | Hub stiffness and warp | GF PA hub with POM teeth if split | Full RFQ if split materials |
| 5 Quiet running | Noise-sensitive drive | Tooth profile and concentricity | Stable POM; gate off pitch line | Upload CAD; note noise goal qualitatively |
Materials
Pick one primary family per molded part line. Put commercial grade or wear filters in comments.
| Family | Typical use on this part type | Notes for instant quote |
|---|---|---|
| POM (acetal) | Precision gears and low friction | Field POM; note moisture if long service |
| PA (nylon) | Loaded gears and wear resistance | Field Nylon; note GF for stiffness |
| PP | Low-load gears and cost-sensitive drives | Field PP; watch creep under load |
| PC/ABS | Housings with integrated gear features | Field PC/ABS; separate line if distinct part |
| PBT | Gears near moderate heat and humidity | Field PBT; note glass fill in comments |
Gating
The gate location sets flow direction, roundness, and where the weld line lands.
Validation
The tooth form of a molded spur gear belongs to the cavity it came from.
Avoid these
Two choices that are hard to undo after the steel is cut.
Cutting the cavity from the gear drawing with only one shrink percentage. Involute teeth shrink differently along the flow than across it. Tips then come out thin, roots come out heavy, and the effective pressure angle climbs.
Gating into a tooth flank that meshes with the mate. Put the gate in the hub, a thick web, or the parting line land. The working flank should not carry the vestige.
Drawing
A STEP file alone rarely carries the tooth specification the mold needs.
Module or diametral pitch, tooth count, pressure angle, and profile shift on each gear
Center distance and backlash target in the assembly
Mating gear material, whether metal or plastic
Resin grade and any glass fill
Accuracy class or inspection method you plan to use on samples
Hub features molded in the same shot: bore, key, cam, or snap features
Keep reading
Parent gear topics and supporting design rules.
Resin pairing, general gating, and gear train context.
Molded gearsHelical teeth and rotational demolding.
Worm gearsCommon spur gear material pairings.
Acetal vs nylon wearLow friction spur and pinion resin.
Acetal (POM)Stiffer teeth and flow direction effects.
Glass-filled nylonKeeping knit lines off loaded teeth.
Weld line strengthCalling runout and bore on gear drawings.
GD&T for molded partsWhen a thin web warps and opens backlash.
WarpageEvery part-type guide, grouped by family.
Molded parts libraryFAQ
Yes. Pinion in plastic and gear in metal is common. Send both parts so center distance and backlash are checked against the actual metal tooth form, not a generic catalog.
Cavity details depend on the shop process. What matters for you is that the involute profile is shrink corrected for your resin and verified on molded samples before production release.
Often yes, with profile shift or a steeper pressure angle. DFM review flags undercut risk and suggests geometry changes before tooling is cut.
Yes, when the insert suits the resin and the tool can load it each cycle. Send the insert drawing with the gear CAD.
There is no single tooth count. It depends on the pressure angle and any profile shift. Very small pinions at a standard pressure angle are the ones to check. Positive profile shift is the usual fix, and it belongs on the drawing before the steel is cut.
When the tooth count is low enough that a standard pressure angle would undercut the root, or when a fixed center distance does not fit the standard tooth form. Put tooth count, pressure angle, and any shift on the drawing. DFM review flags undercut risk before tooling is cut.
Next step
Upload CAD plus module, tooth count, pressure angle, and mating gear details. DFM feedback covers undercut, gate location, and shrink correction.
