Plastic hub bore
- One part, nothing to assemble
- Torque carried by a key, a flat, or a spline
- Grip from interference alone fades over time
Molded part guide: impellers
Injection Molded Impellers
An impeller spins fast enough that anything uneven about it becomes a vibration. It also has to grip a shaft without slipping, and its vanes have to survive the pressure they create. Most of the engineering is about symmetry: every vane the same, every part of the disc the same weight, and the flow reaching them all at once.
Symmetry
Anything uneven becomes a vibration
Centrifugal impellers, closed and open impellers, agitator rotors, and blower wheels.
Balance comes from mass being spread evenly around the axis. On a molded impeller that means the melt has to reach every vane at the same moment and pack each one the same amount. A feed point on the center of the hub does exactly that: the melt spreads outward in all directions and every vane fills from the same distance, so no vane is heavier or denser than its neighbors.
A feed point anywhere else breaks the symmetry. Vanes near the feed pack harder and shrink less, vanes on the far side arrive last and pack least, and the finished wheel has more mass on one side. At low speed nobody notices. At running speed the wheel shakes, the bearings take the load, and the noise gives it away.
Every vane also creates a joint. As melt spreads out from the center it splits around the root of each vane and closes again behind it, and along that closing line the plastic has less strength than the material around it. Those lines sit in a predictable place, so the design puts a generous fillet where each vane meets the disc and keeps the closing line out of the highest stress.
The hub is the last piece. A plastic hub pressed onto a metal shaft grips by interference, and plastic under constant strain slowly relaxes, so the grip fades. A key, a flat, a splined bore, or a metal insert molded into the hub carries the torque instead of relying on friction alone.
Features
Impeller areas and what each one needs
Balance, grip, and strength each live in a different part of the wheel.
| Feature | Purpose | Design note |
|---|---|---|
| Center feed point | Even fill to every vane | Melt spreads outward the same distance all round |
| Vanes | Move the fluid | Same shape and same section on every one |
| Vane roots | Carry the load into the disc | Generous fillets, joints kept out of peak stress |
| Hub bore | Grips the shaft | Key, flat, spline, or a metal insert |
| Shroud | Closes the vane passages | Even section so the wheel stays flat |
Service tiers
Impeller service tiers and quote path
Part focus: Center gating for balance, hub grip, vane knit lines. Upload CAD with critical fits and environment in notes when not modeled.
| Tier | Environment | What matters most | Typical molded route | Quote path |
|---|---|---|---|---|
| 1 Center feed | Even mass around the axis | Every vane fills from the same distance | Gate on the hub center | Upload CAD; note the balance target |
| 2 Off-center feed | Breaks the symmetry | Near vanes pack harder than far vanes | Move the feed to the center | Instant quote |
| 3 Vane roots | A closing line behind each vane | Less strength than the wall around it | Generous fillet, joint kept off peak stress | Upload CAD |
| 4 Hub bore | Has to carry the torque | A press fit relaxes and the grip fades | Key, flat, spline, or a metal insert | Instant quote; note the shaft |
| 5 Shroud | Closes the vane passages | An uneven section lets the wheel go out of flat | Even section | Upload CAD; say if it is shrouded |
Materials
Resin families: quick reference for quote notes
One primary family per line item. Put commercial grade, fillers, and environment filters in comments.
| Family | Typical use on this part type | Notes for instant quote |
|---|---|---|
| Drawing resin | The wheel and the hub | Field the grade on the drawing; name the fluid |
Balance
Feeding from the center keeps the wheel balanced
Where the melt enters decides whether the finished wheel is symmetric.
- A feed point on the hub center reaches every vane at the same moment
- An off-center feed packs the near vanes harder than the far ones
- Uneven packing leaves more mass on one side, which shows as vibration at speed
- Plastic relaxes over time, so a press fit on the shaft loses grip
Hub
Plastic hub bore or molded-in metal hub
The torque and the service life decide which one the impeller needs.
Molded-in metal hub
- Metal grips the shaft and keeps its size
- Carries the torque in the metal insert
- Adds an insert and a loading step at the press
Avoid these
Common design mistakes on molded impellers
Two choices that show up as vibration, or a wheel that slips on the shaft.
A feed point away from the hub center. Vanes near the feed pack harder than vanes on the far side, so one side of the wheel is heavier. Feed from the center so every vane fills from the same distance.
Relying on a plastic press fit alone to grip the shaft. The plastic relaxes under constant strain and the grip fades. Use a key, a flat, a spline, or a metal insert to carry the torque.
Drawing
What to send with an impeller
Speed, fluid, and shaft interface drive the design.
Impeller CAD with vane profiles
Running speed and duty cycle
Shaft size, torque, and how the wheel is retained
Fluid, temperature, and anything abrasive in it
Balance target, if you have one
Expected annual volume
Keep reading
Related guides
Fans, casings, and the joints behind every vane.
Fan blades
Axial blades that move air.
Fan bladesPump housings
The casing around the wheel.
Pump housingsGears
Another part that grips a shaft.
GearsMotor housings
What drives the wheel.
Motor housingsWeld lines
Where flow closes behind a vane.
Weld linesWeld line strength
How much strength is lost.
Weld line strengthGlass filled nylon
A stiff impeller resin.
Glass filled nylonIndustrial and transportation
How we support industrial programs.
Industrial and transportationMolded parts library
Every part-type guide, grouped by family.
Molded parts libraryHVAC
Blower wheels in moving air.
HVACFAQ
Questions about molded impellers
Why does our impeller vibrate at speed?
Mass is not spread evenly around the axis. The usual cause is a feed point away from the hub center, which packs one side harder than the other. Feeding from the center is the first thing to change. Checking balance on the first molded wheels, before the tool is finished, leaves room to move the feed point.
Why do vanes break at the root?
The joint where flow closes behind the vane usually sits there, and a sharp corner makes it worse. A generous fillet at the root and a feed position that moves the joint away from peak stress solve most of it.
Why does the impeller slip on the shaft after a while?
A press fit in plastic relaxes under constant strain, so the grip fades. A key, a flat, a splined bore, or a metal insert carries the torque instead of friction.
Can a molded impeller be balanced after molding?
Small corrections are possible by removing material at the rim, but it adds a step to every part. Getting the symmetry right in the tool avoids that step.
Why a fillet where the vane meets the disc?
Melt splits around the vane root and closes behind it. That closing line has less strength than the material around it. A generous fillet keeps the line out of the highest stress.
What does an even shroud section do?
It closes the vane passages and helps the wheel stay flat. An uneven section lets the disc move out of flat, which puts the mass off the axis.
Next step
Quote an impeller with its duty
Upload the impeller and tell us the speed and fluid. DFM feedback covers feed position for balance, vane joints, and the shaft interface.