When the first change-can double planetary mixer was introduced in the early 1940s by Charles Ross and Son Company, it quickly became a popular machine for batching heavy paste paints. It had two sets of prong-like stirrers that rotated on individual axes while revolving around the vessel interior. All the gearing and other moving parts were enclosed in cast-iron housings and operated in a continuous oil bath. The vessel was fully covered during mixing, but the operator could see the internals from a lamp-lit inspection window. When the batch was complete, a hydraulic lift raised the driving components and stirrers from the mix can. As the finished product was discharged, another vessel could be placed under the mixer to start a new batch.
This was a revolutionary design that improved upon pony-style mixers that were widely being used at that time. The pony mixer’s agitator comprised vertical blades or fingers oriented off-center of the vessel. The blades rotated from a fixed central axis while the vessel (installed on a turntable) turned in the opposite direction of the agitator. The pony mixer was an open system, making it prone to dusting and cross-contamination. The double planetary mixer eliminated these issues and improved operator safety.