Diameter Needs a Mold. Length Usually Does Not.

What the OEM asked for

An HVAC OEM needed an urgent quotation on four cross-flow impeller models, 120 units of each. Impeller tooling decided how fast we could answer.

The largest ran 109mm in diameter by 830mm long. Meanwhile the set covered three diameters and four lengths between them.

What the spread implied

Four sizes in one enquiry usually means a product family rather than a single machine. Given the long, linear geometry, that points at commercial air curtains or trench heaters.

Their spreadsheet gave exact dimensions for each variant. So the requirement was clear. Consequently the only question was how fast we could quote.

Why impeller tooling decided the timeline

In the end the whole answer turned on one check. It is worth explaining why.

How impeller tooling is actually built

Essentially a cross-flow impeller is molded as blade segments joined to end rings. The mold defines the blade profile, and therefore the diameter.

Length behaves differently. A longer impeller simply uses more segments of the same molded part, joined along a shaft. Consequently changing length does not need new impeller tooling, whereas changing diameter does.

That distinction is the single largest cost lever on a custom impeller, and customers rarely know it going in.

Checking the four against inventory

Usefully the requested diameters were 97mm, 101.5mm and 109mm. All three already exist in our tooling inventory.

So a full NPI cycle, which runs 90 days when new tooling is genuinely needed, was unnecessary. The remaining work was drawings for the specific lengths and shaft configurations.

Aluminum considered for the long ones

Here the 848mm spans raised the usual question about material.

Aluminum is stiffer, and over that length stiffness matters for runout. However, glass-filled AS held the required runout tolerance anyway. Meanwhile it weighs less and costs considerably less.

It also handles humidity well, which suits an air curtain sitting above a doorway.

What it cost

Flexibility on diameter. The customer got a fast quotation precisely because they happened to want diameters we already tool.

Had they asked for 105mm instead of 101.5mm, none of this applies. That is worth knowing before a housing is designed around a diameter nobody has a mold for.

Technical Specifications

Three diameters and four lengths sit below. Read them as a grid, since that is how the tooling works.

Parameter	LWPI-Φ97X621-01	LWPI-Φ101.5X848-01	LWPI-Φ101.5×848-02	LWPI-Φ109X830-01
Diameter	97 mm	101.5 mm	101.5 mm	109 mm
Length	621 mm	848 mm	848 mm	830 mm
Material	AS+GF30% (Acrylonitrile Styrene w/ 30% Glass Fiber)
Shaft Diameter	6 mm
Shaft Length (Exposed)	25 mm
Rotation Direction	CCW (Counter-Clockwise)
Bushing Type	External	Built-In	Built-In	External
Bushing ID	8 mm	10 mm	8 mm	8 mm
Balancing Specification	≤0.3g @ 1200 RPM
Radial Runout	<0.7 mm
Axial Runout	<0.6 mm

Technical Documentation

The following documents were generated to define the solution and enable manufacturing. These are the final, approved deliverables for this project.

LWPI-φ97X621-01.pdf: This drawing provides detailed dimensional and tolerance data for the 97mm diameter, 621mm length impeller, critical for housing design and motor shaft integration.

LWPI-Φ101.5X848-01.pdf: This drawing specifies the 101.5mm diameter x 848mm length impeller with a 10mm ID bushing, ensuring correct assembly for this specific variant.

LWPI-Φ101.5×848-02.pdf: This document details the second 101.5mm diameter x 848mm length impeller, identical except for its 8mm ID bushing.

LWPI-φ109X830-01.pdf: This engineering drawing outlines the largest diameter impeller at 109mm with a length of 830mm and an external bushing.

This is the original requirement sheet provided by the customer, which served as the primary input for the engineering team.

How to keep a custom impeller cheap

Custom impellers are cheap or expensive depending on one decision. These keep you on the cheap side.

• Ask which diameters are already tooled before fixing your housing. Impeller tooling is the dominant cost, and it is priced per diameter. Designing around an existing one costs nothing; designing around a new one costs a mold and 90 days.
• Treat length as free and diameter as expensive. Within limits, length is a matter of how many molded segments go on the shaft. If your design can flex on one dimension, flex on length.
• Send diameters with tolerances, not nominal sizes. A 101.5mm requirement and a 100mm requirement look similar and are not. Stating the tolerance tells the supplier whether an existing tool actually covers you.
• Check runout before choosing material. Long spans need stiffness, and that is where plastic sometimes loses to aluminum. Give the runout figure you need and let the material follow, rather than specifying metal defensively.

See our cross flow impeller range, the cross flow fan section, or fan impellers. Balance grades follow ISO 21940.

Questions about custom impellers

Why does diameter need new tooling but length does not?

Because the mold forms the blade profile in cross-section, and that cross-section is what diameter changes. Length is built by joining more of the same molded segments along a shaft. So a new length is a drawing change, while a new diameter is a new mold.

What does new impeller tooling cost in time?

Our full NPI cycle runs 90 days from spec lock to first samples, and the mold is the long pole in that. Compare it with an existing diameter, where drawings come back in days and samples follow within a couple of weeks.

Is there a length limit before plastic stops working?

It depends on diameter, speed and the runout you need, rather than on length alone. A slender rotor sags and resonates as it gets longer, so the practical limit falls as diameter drops. Send us the length, diameter, speed and runout tolerance together and we will say whether plastic holds.

Can you machine an existing impeller to a different diameter?

No. Unlike a bore, the blade profile is formed by the mold and cannot be reduced afterwards without destroying the aerodynamics. Where an existing diameter is close but not exact, it is usually better to adapt the housing than to modify the impeller.