A New Bore Does Not Need a New Mold, Only an Insert

What the OEM specified

An OEM needed a 406mm axial impeller as a standalone component, with their own motor already chosen. A mold insert is what made it quick.

The requirement that mattered was the mechanical interface: a D-shaped bore, 12mm outer diameter with a 10mm flat.

Why a D-bore rather than a round one

A flat on the shaft transmits torque directly through the shape of the joint. Consequently there is no grub screw to work loose and no keyway to machine.

On a component that must not slip under load, that is a deliberate mechanical choice rather than a preference.

• Four blades, 96mm total height, 406mm diameter.
• Next, ABS with 20% glass fiber, for blade rigidity under load.
• Meanwhile a weight target of 455g plus or minus 15g.
• Residual unbalance no greater than 0.3g at 1000 RPM.
• Finally, RoHS compliance for their target markets.

That balance figure points at a condenser unit or heat pump, where vibration and noise reach the people living beside the equipment.

Why a mold insert was enough

Here the first question was whether an existing tool covered any of this. It covered most of it.

How an impeller mold is actually divided

A 406mm four-blade axial impeller mold already existed. That tool forms the blade profile, the diameter and the overall geometry.

The hub at the center is a separate part of the same tool. So changing the bore means a new mold insert, not a new mold.

That distinction is why this ran as a quotation rather than a development program. A mold insert costs a small fraction of a full new mold, and the schedule difference is larger still.

The tolerances that make a press fit work

Meanwhile the drawing specifies +0.05mm on the bore diameter and +0.1mm on the flat.

Those are tight because the impeller is pressed onto the motor shaft. Too loose and it runs slightly off-center, which becomes vibration no amount of balancing will remove. Too tight and it cracks a glass-filled hub during assembly.

Meeting the balance figure

Under 0.3g at 1000 RPM is a production standard rather than a marketing number, and it is checked on every unit rather than sampled.

The glass-filled ABS matters here too. It holds shape across a 406mm span, and a rigid blade is what makes a balance figure hold in service rather than only at the moment of test.

What it cost

Flexibility on everything except the bore. The customer had to accept the existing blade profile, diameter and height exactly as they are.

That was fine here because the aerodynamics suited them. Had they wanted a different blade count or a 380mm diameter, no insert helps and the answer becomes a full mold with the schedule that implies.

Technical Specifications

Here the bore row below is the only thing that was custom. Everything else came from an existing tool.

Technical Documentation

LW406x96-4 Engineering Drawing (PDF)
The engineering drawing provides exact dimensions, material specifications, and quality tolerances for the LW406x96-4 fan blade, essential for OEM design integration and quality assurance.

Here the product image shows the final 4-blade axial impeller, highlighting the hub and blade profile developed for the customer’s specific application.

If you need a non-standard bore

A custom impeller is priced by which part of the tool has to change, and often that is only a mold insert.

• Ask whether your change is a mold insert or a new mold. Bore shape, bore diameter and hub details usually sit in an insert. Blade profile, blade count and impeller diameter sit in the main tool. The cost and schedule difference between those two answers is very large.
• Give the bore tolerance, not just the nominal. A press fit onto a motor shaft lives or dies on tolerance. State what you need and what the shaft actually measures, since the pairing is what matters rather than either number alone.
• Consider a D-bore where torque is high. It transmits through geometry rather than friction, so nothing loosens over thermal cycles. The cost is that the shaft must be machined to match, which is worth confirming with your motor supplier first.
• Pair the balance figure with a speed. Residual unbalance produces force proportional to the square of speed, so 0.3g means very different things at 1000 and 3000 RPM. Always state both.

See our fan impeller range, the fan blade section, or axial fans. Balance grades follow ISO 21940.

Questions about custom bores

What can a mold insert change and what cannot it?

An insert typically covers the hub: bore diameter, bore shape, keyway or flat, and sometimes hub depth. It cannot change the blade profile, the blade count or the impeller diameter, because those are formed by the main tool. If your change is in the first list, expect days; if the second, expect a mold program.

Why is the bore tolerance one-sided?

Because a press fit is directional. A bore allowed to run undersize would crack the hub during assembly, so the tolerance is specified from nominal upward. That gives the assembler a predictable interference rather than a range that could go either way.

Is a D-bore better than a keyway?

For this size and torque, generally yes. It transmits through the shape of the joint with no separate key to shear or shift, and nothing to loosen through thermal cycling. A keyway suits larger shafts and higher torque, where the flat area alone would not carry the load.

Does glass-filled ABS affect the balance figure?

Indirectly, and it matters. A stiffer blade deflects less under aerodynamic load, so the mass distribution at speed stays closer to what was measured at balancing. An unfilled plastic blade of this diameter would flex, and the effective balance in service would be worse than the test figure suggests.