How to Select Overhead Stirrer Mixing Tools

By Bob Wilcox

In an earlier article on cosmetic mixing with overhead stirrers I outlined how these lab tools are used to provide a fast way to develop formulations and processing parameters ranging from benchtop to pilot scale production. The article also touched on the variety of impeller configurations available — each one designed to perform a specific mixing operation.

Impeller configurations — A.K.A overhead stirrer mixing tools — deserve more attention because of their impact on stirring efficiencies and the capability of the overhead stirrer drive motor to deal with sample viscosity while delivering the required mixing speed. Please refer to the earlier article for a refresher on variables relating primarily to the properties of the samples being mixed and the desired results. These include sample size, viscosity, stirring speed, stirring time, overhead stirrer control options, and record- keeping capabilities.

In a nutshell: Before we get into the nitty-gritty of impeller design overhead stirrer mixing tools must accommodate sample volume and viscosity to provide satisfactory mixing action taking into account the tools’ surface areas. Stirrer motors should deliver the power needed to drive mixing tools at the set speed without overheating.

Overhead Stirrer Mixing Tool Configurations

This equipment has several names including impellers, paddles and blades. Whatever moniker you assign they are fabricated and welded or otherwise attached to a stainless steel rod. The rod must be long enough to immerse the tool into the sample. Somewhat like an electric drill bit the rod is inserted into and tightened in the mixer motor chuck.

Rod diameter and length should be considered when selecting mixing tools. Wider diameters are able to accommodate higher viscosity samples and are offered in 6, 8 and 10mm in size. Rod lengths are selected to place the mixing tool at the correct level in the beaker. Lengths, including the blade, range from 300 to 600 mm. Note that the stirrer drive motors are equipped with a support rod that can be clamped to a mounting stand to permit adjusting the depth of the mixing tool in the sample container.

Here’s a brief explanation of common mixing tool configurations:

• Samples with light or average viscosities can be processed by X-shaped configurations when viewed on end.
• Select a manifold shaped configuration to dissolve or disperse samples in the beaker. These are also produced in a round configuration with alternating up and down tabs.
• Medium and high viscosity samples requiring higher mixing speeds call for propeller-type mixing tools that resemble 4-bladed fans or 3-bladed motorboat propellers.
• Spatula-shaped blade mixers are characterized by a large surface area, often with holes, and are more influenced by viscosity.
• Use a centrifugal mixing tool for processing samples at an average speed. They are also called straight blade mixers and have two flat vertical blades.

As you work with an increasing number of cosmetic formulations you may accumulate a variety of overhead stirrer mixing tools. Protect your investment by cleaning them thoroughly after each use so samples do not harden in cracks and crevices.

Bob Wilcox has represented CAT Scientific’s family of overhead stirrers, homogenizers, magnetic stirrers,
liquid metering and related laboratory equipment since 2002. In addition to heading the sales function in
the US he is chief technician for the CAT equipment service organization.