Laboratory Dispersion Equipment: Choosing by Sample Volume

Why Volume Comes First, and Motor Power Last

Motor power describes the drive. It says how much work the machine can do per hour, not how well it disperses. Two machines rated at 1.1 kW can behave completely differently in the same beaker if one carries a 100 mm disc and the other a 150 mm disc, because the shear is generated at the rim of the disc and not at the shaft.

Volume enters before either of those, because it sets the geometry that physically fits. A 150 mm disc cannot be immersed in a 250 mL beaker: the vessel wall is too close to the rim, the vortex climbs the side and the batch is thrown out rather than circulated. The practical rule used on the shop floor is that the disc diameter should be roughly one third of the vessel diameter, which is why a small sample forces a small head and a small head caps the shear you can reach.

The same logic applies at the other end. In a 60 L batch a 100 mm disc is too small to turn the whole volume over, so the top layer sits still while the bottom is over-sheared. Neither case shows up on a specification sheet, and both waste a week of trials.

The Three Bench Machines and What Each One Actually Does

Three machine families cover almost all bench work, and they are not interchangeable. What separates them is the mechanism that puts energy into the product.

Rotor-stator homogenizer: droplet size and dissolution

A rotor turns inside a fixed stator screen at a small clearance, drawing product in axially and throwing it out through the slots. Shear in the gap plus repeated passes through the screen reduce droplet size and break down solid clusters. This is the right tool for emulsions, for wetting powders into a vehicle, and for dissolving polymer granules. In the POLYC range the PSL / PSF series covers 10 mL to 10 L with interchangeable stators from 10 to 70 mm.

Cowles disc disperser: wetting out, at pilot volume

A saw-tooth disc turns fast enough to pull a vortex that carries powder down from the surface into the shear zone at the teeth. It does not reduce particle size; it separates agglomerates into primary particles and wets them. This is the workhorse of paint and ink formulation, and the BPF-H series covers the 5 to 80 L band with 100 to 150 mm discs on a counterbalanced column.

Basket mill: actual particle size reduction

The basket holds a charge of small beads behind a retention screen, and an impeller inside the basket circulates the batch up through the media bed. Because the same liquid passes through the bed many times, grinding time becomes the control variable and a sub-litre batch can still be brought to a fineness figure. The PZM-L covers 0.5 to 2.5 L with 1.6 to 2.2 mm zirconia media.

A laboratory that only buys one machine usually buys the disperser, because it is the least expensive and the most forgiving. A laboratory that compares formulations usually ends up with a disperser and a mill, because comparing two pigment pastes on dispersion quality alone does not tell you which one will pass a fineness specification.

Tip Speed Is the Number That Transfers Between Machines

Rpm on its own means nothing, because the same rpm on a different diameter gives a different rim speed. The number that carries across machines and across scales is tip speed, calculated from diameter and speed:

v = π × D × n / 60, where D is the disc or stator diameter in metres and n is the shaft speed in rpm, giving v in metres per second.

Applied to the published speeds and diameters of the three families above, the differences are larger than the motor ratings suggest. These figures are calculated from the catalogue speed and diameter, not measured:

Calculated from catalogue diameters and speed ranges. Actual working tip speed is set by the product and the vessel, not by the maximum the drive can reach.

The consequence for a trial report is simple. Recording “ran at 1,800 rpm” is not a specification, because it does not say which machine or which disc. Recording the tip speed, or the rpm together with the disc diameter, is something a production engineer can act on.

Matching the Volume Band to the Machine

The table below is the decision most laboratories actually face: which machine covers the volumes that arrive on the bench. Where two machines overlap, the tie-break is the output you need, not the volume.

Volumes and diameters from the published specifications of the PSL / PSF, PZM-L and BPF-H pages.

Where a Bench Machine Stops Working

Every geometry has a ceiling, and the ceilings are not the same. Knowing them in advance is what stops a project from buying a second machine three months later.

• Viscosity ceiling, the laboratory basket mill is rated to roughly 5,000 cP. Once circulation through the media bed falls away, grinding effectively stops even though the shaft is still turning.
• Vortex collapse, a Cowles disc needs to turn fast enough to pull powder down from the surface. In a thick paste the vortex does not form, and adding speed only adds heat and air.
• Shaft length on a floor machine, a benchtop disperser has a short, stiff shaft. Scaling the same design to a 600 L vessel means a longer shaft, more runout and a natural frequency the drive has to avoid — which is why large machines use different frame designs rather than longer columns.
• Media retention, smaller beads give a finer grind, but the retention screen has to be able to hold them. Bead size and screen are one decision, not two.

Above these limits the correct answer is a different machine family rather than a larger version of the same one: a planetary or dual-shaft mixer for high-viscosity products, and a horizontal or vertical bead mill for continuous fine grinding at production volume.

The Bead Question: When a Disperser Is Not Enough

The distinction that decides most equipment budgets is between dispersion and grinding, and it is routinely blurred by both suppliers and buyers.

A disperser separates agglomerates into the primary particles they already contain. If the pigment arrives as 20 µm clusters of 0.2 µm crystals, a disperser can free the crystals and stabilize them, and the resulting fineness is the fineness of the raw material. A media mill goes further: it reduces the primary particle itself, which is the only way to reach a target that the raw material does not already have.

In practice the two steps are sequential. Pre-dispersion puts the powder into the vehicle and removes the air; milling then reduces the particle size. The energy spent on the first step is not wasted, because poorly wetted powder cannot be ground efficiently — the media contacts slide past agglomerates that are still coated in air rather than breaking them.

On a bench mill, bead size is the main lever on the result and the retention screen is the constraint. A 1.6 to 1.8 mm charge in the smaller model gives more contact points and a finer distribution; 1.8 to 2.2 mm in the larger model carries more energy per contact and suits a coarser feed. Neither is a substitute for running the batch long enough, and time is the variable that has to be recorded if the result is going to be reproduced.

Four Assumptions That Cost a Week of Trials

• That more rpm means better dispersion, above the speed that forms a proper vortex, the extra energy mostly becomes heat, air entrainment and wear. Tip speed at the rim is the useful figure.
• That a smooth-looking batch is a dispersed batch, a batch can look uniform and still contain agglomerates that will show up as a grind reading, a gloss difference or a filtration blockage later. The check is a fineness measurement, not a visual one.
• That a bench result transfers at the same rpm, a larger disc at the same rpm runs a higher tip speed, and the same power spread over a larger volume is less energy per litre. Transfer on tip speed and the disc-to-tank ratio, then expect the batch time to change.
• That any bead size works if you run long enough, the retention screen sets the floor on bead size, and media that is too fine for a coarse feed cannot break the feed at all. Bead size follows the target and the screen, not the other way round.

Recording three numbers for every trial — volume, tip speed and time — costs nothing and removes most of the ambiguity when the result has to be repeated or transferred.

Where the Bench Machines Are Built

The laboratory range is made in the same plant as the production machines it scales up to, which is the reason the diameters, speeds and media sizes on a bench unit correspond to real figures on a production unit rather than to a separate small-machine design.

Frequently Asked Questions