Worm and wheel
- Short axial length and high ratio in one mesh
- Can be self-locking when friction and lead angle align
- Rotational demolding and higher tool complexity
Molded part guide: worm gears
Injection Molded Worm Gears
Every tooth on a worm is a helix, so a straight pull from the mold would shear the plastic. The tool has to unscrew the part, or unscrew the cavity, along the same spiral the gear was molded with.
Helical teeth
The mold motion has to match the tooth spiral
Worm gears trade tooling complexity for a compact reduction in one mesh.
A worm and wheel set gives a large ratio in a short package, which is why it shows up in seat adjusters, small winches, valve drives, and toy mechanisms. The worm is usually steel or bronze in heavy loads, but many light-duty worms and wheels are both molded, especially when noise and grease-free running matter.
Helical teeth block linear ejection. After the part cools onto the core, the helix angle locks the teeth in place. Releasing the part means rotating it while it moves off the mold, at a speed slow enough that the plastic does not strip. Tooling uses rotating cavity inserts, rack-and-pinion unscrewing, or other mechanisms that turn ejector motion into spiral travel.
That mechanism adds cost and cycle time compared with a spur gear of similar ratio. The design question is whether the envelope savings and self-locking behavior justify it. If a two-stage spur train fits, it is often the simpler mold.
Pair design
Worm versus wheel: what each part needs from the mold
General split of roles. Your torque and self-locking requirement set the details.
| Part | Molding note | Typical resin |
|---|---|---|
| Worm (screw thread form) | Long core with helical lands; demolding follows the lead angle | Acetal or glass-filled PBT for wear and stiffness |
| Worm wheel (throated or cylindrical) | Helical teeth around the rim; throat depth sets contact | Acetal or nylon; glass fill when the wheel carries the load |
| Combined worm on a motor shaft | Often insert molded or assembled; worm thread may be a separate molded piece | Match resin to bore fit and running temperature |
Process
Rotational demolding changes cycle and inspection
Features that differ from straight spur molding.
- Ejection is slower than on spur gears so helical flanks do not chip
- Cooling time is often longer because the core is slender and holds heat
- Cavity shrink correction must follow the helix, not only the pitch diameter
- First articles should check lead error and backlash across the full wheel rotation
Architecture
One worm pair or a two-stage spur train
Both hit similar ratios. Tooling and noise differ.
Two spur stages
- Straight-pull molds on each gear
- More parts and center distances to stack in the housing
- Often quieter at high speed because contact is rolling, not sliding
Drawing
What to send with a worm gear set
Include both members and how they sit in the assembly.
Module, tooth count on the wheel, worm starts, lead angle, and center distance
Whether the worm is throated or cylindrical against the wheel
Target ratio, backlash, and whether self-locking is required
Resin for each member and any lubrication allowed
Bore, bearing seats, and any metal worm you supply separately
Running speed and torque, so wear pairing can be reviewed
Keep reading
Related guides
Straight teeth, materials, and demolding concepts.
Spur gears
Straight teeth and shrink-corrected cavities.
Spur gearsMolded gears
Resin pairing and general gating.
Molded gearsAcetal vs nylon wear
Typical worm wheel pairings.
Acetal vs nylon wearLubricated POM
Dry-running worm contacts.
Lubricated POMUndercuts
Side actions and lifters beyond helical demolding.
UndercutsSide actions
When the wheel hub needs slides.
Side actionsMotor housings
When the worm sits on a motor shaft in plastic.
Motor housingsRollers
Other round parts where runout matters.
RollersMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded worm gears
Can the worm be metal and the wheel plastic?
Yes, and it is common when the worm sees most of the wear. Send the metal worm drawing or sample so the wheel throat and backlash are designed against the real thread.
Why did our molded worm strip on ejection?
Usually ejection was too fast, the part was too hot, or the helix angle and demolding rotation did not match. Slowing ejection and checking cavity rotation alignment are the first fixes on the tool.
Is self-locking guaranteed with a plastic worm?
No. Self-locking depends on lead angle, friction, load direction, and wear. Prototype the pair in your assembly and test the hold condition you care about.
Can you mold double-start worms?
Yes, when the demolding mechanism can follow the lead. Send the thread specification and the wheel drawing together.
Next step
Quote the worm and wheel together
Upload both CAD models with lead angle and center distance. DFM feedback covers demolding, shrink on the helix, and resin pairing.