Materials and manufacturing

You can make a shape by pouring jelly into a mould, by carving it from a block, or by building it up layer by layer like a Lego model.

Impellers are made in all three ways. The choice depends on how many you need, how precise they must be, and what they are made of.

Most metal impellers are cast. In sand casting, molten metal is poured into a sand mould; sand “cores” shape the curved channels inside. Cheap and good for iron and bronze.

Investment casting (lost wax) gives smoother, more precise channels — popular for stainless steel.

Compressor wheels that spin at 100,000 RPM are CNC-machined from a solid block of aluminium or titanium on a 5-axis mill, so every vane is exact.

Millions of small pump impellers are injection-moulded in plastic. 3D printing — in plastic or even metal — is used for prototypes and special one-offs. (You will print one yourself in Module 7.)

Pull the impeller apart and imagine each part cast in a different metal. The geometry is the same; only the material changes with the liquid.

Cast iron: cheap, clean water. Bronze: sea water. Stainless steel: food, drinking water, chemicals. High-chrome iron or rubber: sand and slurry. Plastics: small pumps and aggressive chemicals.

The enemies are corrosion (chemical attack), erosion (grit sanding the metal away) and cavitation. The right material resists whichever one the liquid brings.

A cast impeller is machined where it must be exact (the bore, the wear-ring surfaces, the outer diameter), cleaned up in the channels, and finally balanced so it spins smoothly — the subject of the last lesson in this module.