An investment mixer is a small bench-top vacuum mixer used in jewellery casting workshops to combine investment powder with water and remove entrained air before the slurry is poured into a casting flask. The machine is simple, but its job is essential: any air bubbles that survive the mixing stage end up against the surface of the wax pattern and produce raised pimples on the cast metal.

Construction

A typical investment mixer has a transparent or stainless bowl, a stirrer paddle driven by an electric motor, and a vacuum bell that fits over the bowl with a hose connected to a small vacuum pump. The bowl holds the water and powder mixture; the paddle stirs at low speed; the vacuum pulls air out through a regulator. Many machines also vibrate the bowl during vacuum to encourage bubbles to migrate to the surface. The user mixes powder and water in carefully measured proportions according to the investment manufacturer's instructions.

Use

The procedure is to add measured water to the bowl, add measured investment powder, mix briefly under air to wet all the powder, then close the vacuum bell and continue mixing under vacuum until the slurry is smooth and bubble-free, typically two to three minutes. The slurry is then poured into the prepared flask, which is itself placed on a vibrating table under vacuum to draw any final bubbles to the surface. Larger casting houses use industrial mixers that handle multiple flasks simultaneously, but the bench-top investment mixer remains the standard for small studios and prototype shops.