Dongying (Suzhou Dongying Electronic Technology Co., Ltd.) is a mold maker and injection molder in Suzhou, China. We build precision molds and run injection molding for overseas teams, including gearbox components and plastic gear components. Projects use mold handover, or Dongying holds the mold and supplies parts. Responsibility for gears, purchased elements and assembly is agreed separately for each project.
Why the housing and gears should be molded as a set
Short answer: the housing decides where each gear sits, so center distance, shaft alignment and backlash come from housing and gear tolerances added together, not from the gears alone.
Gear references define backlash as the clearance between mating tooth flanks and tie it directly to center distance: a larger center distance means more backlash, a smaller one means less (KHK, Q&A about Gears). In a plastic gearbox the housing's bores or shaft bosses fix that distance. If the housing is tooled by one team and the gears by another, each side can be inside its own drawing and the assembled mesh can still bind or rattle.
Reviewing both together lets the mold design treat the bores, shaft holes and locating features as part of the gear train. It also makes clear who owns the stack-up: the customer's gear design team, the molder, or both.
Interfaces that link the housing to the gear train
Short answer: bore positions, shaft or pin seats, bearing seats, axial stops and the housing split line are the features that directly affect how the gears run.
| Interface | What it controls | What to state on the drawing |
|---|---|---|
| Bore or boss positions | Center distance between each gear pair | Position from a shared datum, both housing halves |
| Shaft, pin or bearing seats | Gear alignment and radial play | Fit type, mating part (molded, metal pin, purchased bearing) |
| Axial faces and thrust stops | End float and face contact | Which faces are functional, how they are checked |
| Housing split and fastening | Whether bores stay aligned after closing | Screw, snap or weld method; locating pins between halves |
| Seals, vents and grease pockets | Lubricant retention, contamination | Whether grease is part of the assembly scope |
Datum choice matters as much as the numbers. If each housing half is measured from a different reference, two bores can each pass and still sit out of line when the halves close. Agree one datum scheme that both halves and the gear inspection use.
Material pairing and moisture
Short answer: choose the housing and gear materials as a pair, because each family expands differently with heat and humidity, and the mesh has to work at every service condition.
Plastic gears are light and quiet and can be molded in volume, but they show larger dimensional change, including backlash, than metal gears, depending on temperature, moisture uptake and chemical exposure (KHK, Plastic Gears). Polyamides absorb moisture after molding, which enlarges parts and lowers stiffness; the change is slow and reversible (DuPont Minlon/Zytel design information). POM absorbs far less water and is valued for dimensional stability and sliding behavior (Ensinger, POM).
| Common pairing (general trend) | Watch-out |
|---|---|
| POM gears in a glass-filled PA or PBT housing | Gears stay stable; check housing size after conditioning if PA is used |
| PA gears in a PA housing | Both grow with humidity; agree dry vs conditioned inspection |
| Plastic gears on metal pins in a plastic housing | Pin fit and housing creep around the pin |
| Plastic gears with purchased bearings | Housing bore fit to the bearing outer ring |
These are trends only. Compare datasheets for the actual grades, and keep the material choice for each part in the same RFQ package. For a deeper material comparison on one gear type, see POM vs PA for plastic worm gears.
How shrinkage and mold design affect the mesh
Short answer: each resin shrinks differently and often unevenly, so gate location, wall layout and cooling on the housing matter as much as tooth cutting on the gear cavities.
A housing with deep bosses, ribs and a long flat wall can warp, moving a bore away from its partner. A gear with a heavy hub and thin rim can shrink out of round. Practical points include:
- Keep housing walls around the bores as even as the design allows, and support bosses with ribs rather than solid mass.
- Place gates so weld lines avoid bore walls and gear rims where possible.
- Balance cooling around bore cores so they do not pull the bore off position.
- Confirm the shrinkage basis for each material before cutting cavity steel; a late resin change can mean steel rework on both bores and teeth.
- Run mold-flow with the real grades when the housing is large or the bore pattern is long.
For tooling scope and handover, see mold making and tooling and supply scope.
Bearings, pins and inserts in the housing
Short answer: decide early whether shafts run in molded bores, on metal pins or in purchased bearings, because each choice changes the bore design and who is responsible for the fit.
Engineering-plastic bearing handbooks note that sliding bearings are usually press-fit into the housing and that running clearance must be checked in the mounted state (NTN, Engineering Plastics Handbook). If pins or bushings are molded in, the housing becomes an insert-molding job and the insert drawing, material and supplier need to be in the same brief. See insert molding inputs for what to provide.
What to put in the RFQ
Short answer: send the housing and all gears as one package with the assembly model, material per part, datum scheme, mating purchased parts and the agreed inspection plan.
Include the 3D assembly and part drawings with current revision; gear data (module, tooth count, pressure angle, center distances); housing datums and bore positions; material shortlist for each part; pins, bearings or inserts and who supplies them; lubricant intent; service temperature and humidity; annual volume; mold handover or part supply; and whether Dongying supplies loose parts or an assembly. Our guides on drawings and revisions, inspection and approval and plastic gears, gear sets and gearboxes: drawings and fits cover the detail. Gear strength, noise and endurance remain customer design and validation items.
FAQ
Should the gearbox housing and gears come from the same molder?
It often helps, because one team can review center distance, bore positions and gear tolerances as a single stack-up and tune both molds against the same assembly. It is not mandatory. If parts come from different suppliers, share one assembly model, one datum scheme and one inspection plan so each side measures the mesh-critical features the same way.
What housing features most affect gear backlash?
The positions of the bores, bosses or pin seats that set center distance have the biggest effect, followed by bearing or pin fit and alignment between the two housing halves. Warpage of a long housing wall can also shift a bore. Mark these as functional features on the drawing and measure them from the same datum used for gear inspection.
Can the housing and gears use different plastics?
Yes, and it is common. Gears are often POM or PA, while housings may use glass-filled PA, PBT or other structural grades. Each family expands differently with heat and moisture, so check the mesh at the hottest, coldest, driest and most humid service conditions. Freeze each part's material before cavity steel is cut, because shrinkage differs by grade.
How do you check that the housing and gears fit before production?
Agree an inspection plan that covers housing bore positions, gear tooth data and an assembled check such as a fit or running test defined by the customer. State whether parts are measured dry-as-molded or after conditioning, especially for PA. Sample quantities, methods and acceptance criteria should be written into the approval plan before samples are molded.
Can metal pins or bushings be molded into the housing?
Yes. Pins, bushings and threaded inserts can be insert-molded into the housing, which changes the mold design and the inputs needed. Provide the insert drawing, material, surface condition and who supplies it, plus how the insert is located in the mold. Metal processing, plating and assembly responsibilities are agreed separately from the molding scope.
What does Dongying need to quote a gearbox housing and gear set?
Send the assembly model, housing and gear drawings with revisions, material per part, gear data and center distances, purchased bearings or pins, lubricant intent, service environment, annual volume and whether you want mold handover, molded parts or an assembly. We review moldability and the housing-to-gear interfaces with you; gear performance targets remain your validation scope.
Related pages
Sources
- KHK Gears, Q&A about Gears (center distance and backlash): https://khkgears.net/new/gear_knowledge/q_and_a_about_gears.html
- KHK Gears, Gear Backlash (technical reference): https://khkgears.net/new/gear_knowledge/gear_technical_reference/gear_backlash.html
- KHK Gears, Plastic Gears: https://khkgears.net/new/plastic_gears.html
- DuPont, Minlon and Zytel Design Information, Module II (PDF hosted by Distrupol): https://www.distrupol.com/Minlon_Zytel_Design__Info_Module_2.pdf
- Ensinger, POM overview: https://www.ensingerplastics.com/en-us/thermoplastic-materials/pom-acetal
- NTN, Engineering Plastics Handbook (PDF): https://www.ntnglobal.com/en/products/catalog/pdf/9014E.pdf
- Wikipedia, Backlash (engineering): https://en.wikipedia.org/wiki/Backlash_(engineering)