This is a guest post by Bob Wilcox
Overhead stirrers find wide application in the cosmetic industry. They provide a fast way to develop formulations and processing parameters ranging from benchtop to pilot scale production. One of the chief advantages of these stirrers is the variety of impeller configurations available — each one designed to perform a specific mixing operation.
Overhead stirrers are generally called for when mixing formulations having viscosities or mixing requirements that cannot be handled efficiently by their cousins, the magnetic stirrers.
Overhead cosmetic mixers
Regardless of size, overhead stirrers are comprised of a compact microprocessor-controlled motor that is affixed to a support rod attached to a base that holds the sample beaker. The motor is adjusted on the rod in such a way that the stirring tool — the impeller — is positioned at the proper level in the sample container.
Tips for choosing Overhead Stirrers
When you add an overhead stirrer to your lab equipment there are a few things you should consider when making your selection and purchase. Following are some examples.
Know your viscosity. Have an understanding of the sample viscosities you will be processing. Viscosities are given in millipascal seconds (mPa-s) and indicate the amount of torque energy measured in Newton centimeters (Ncm) that the stirrer motor will have to apply to assure proper mixing. The thicker your formulation the more torque is required. Some formulations may experience a change in viscosity — higher or lower — as stirring proceeds. Take this into account and note that there are models available to handle dynamic viscosity samples. The configuration of the stirring impeller (see below) must also be taken into account when considering viscosity.
What is your sample size — that is the amount of material you’ll be processing? Laboratory overhead mixers are designed to process up to a certain amount, usually specified in liters. Examples: 35, 100, 150 and 200 liters. Since viscosity issues enter the equation in direct proportion to volume both must be considered when deciding on a purchase.
Mixing speed. Stirring speed is another criterion. Select a model that offers a soft start (to avoid splashing contents all over the place) and is steplessly adjustable to the speeds you need to accomplish the mixing. These mixers usually are equipped to provide a soft stop as well. Examples are stirrers operating from 50 to 500 RPM, from 40 to 2,000 RPM and 20 to 700 RPM. Higher speeds to 6,000 RPM are available on request.
Mixer controls. Stirring time can be controlled by the motor on-off button on basic models. More sophisticated models allow researchers to program mixing time from 1 minute to 99 days. Related to stirring time is an automatic shutdown if the stirrer motor overheats or is subjected to an overload due to viscosity increases.
Stirrer control panels can range from basic models designed for small samples and stepless adjustable speeds from 50 to 2,000 rpm. Stirring speeds are shown on the control panel LCD that also has a speed control knob and an on-off switch. Panels on larger pilot plant models provide a wealth of information including programmable torque limits, maximum and minimum stirring speeds and stirring time. Actual and programmed parameters are accessible via the control panel. As an added feature high end overhead stirrers will self-diagnose and present data on why the motor has shut down.
As all researchers know, record keeping is a critical part of good laboratory practices. This is facilitated by PC Connectivity usually provided by an RS 232 interface that also supports programming and monitoring stirrer operation.
A Brief Look at Impeller Designs
Impellers, also called blades and paddles, are the business end of the over head stirrer. Most impellers are permanently attached to a stainless steel rod that is fitted into a chuck. Impellers are specified based on vessel size and sample viscosity, the latter of which applies resistance to the impeller. The effect is most pronounced when using a blade configuration due to its relatively large surface area.
Common configurations are stirrers that resemble boat propellers and windmills, X-shaped cross blade impellers, dissolvers or dispersers, blade impellers and centrifugal stirrers.
Need more info? See the Overhead Stirrer category index for a representative guide to overhead stirrers and impellers.
Bob Wilcox has represented CAT Scientific’s family of 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.