Agitator in Painting: What It Does and How to Select One
Agitator in Painting: What It Does and How to Select One
The phrase "agitator in painting" is asked by two different people. One is standing in a paint shop holding a paddle mixer over a 20 liter can. The other is looking at a 5,000 liter dispersion tank with a shaft coming down through the lid. The first answer is a hand tool. The second answer is a piece of process equipment whose geometry decides whether the pigment is dispersed at all, and that is what this page covers.
In production, the agitator is not the machine. It is the shaft and impeller inside the machine. The frame, the hydraulic lift, the motor and the control cabinet position it and spin it, but the actual work on the paint happens at the impeller, and only at the impeller. Almost every dispersion problem that gets blamed on the machine turns out to be a geometry problem: the disc is too small for the tank, the speed is wrong for the disc, or the impeller is the wrong type for the viscosity the batch reaches at the end of the cycle.
An agitator in painting is the shaft-and-impeller assembly inside a paint mixing vessel. It turns the whole batch over, wets powder into the resin and holds solids and temperature uniform. Selection is set by three numbers: the viscosity the batch reaches at the end of dispersion, the tip speed needed at the impeller edge, and the impeller-to-tank diameter ratio.
What an Agitator Does in Paint: Three Jobs
People talk about "mixing paint" as one activity. Inside a vessel it is three separate jobs, and an impeller can be good at one and poor at the others. Keeping them apart is the fastest way to work out what is wrong with a batch.
1. Bulk turnover. The whole batch has to pass through the working zone, repeatedly. Titanium dioxide and dense fillers will settle if any region of the tank is left undisturbed, and in a tall vessel a small fast disc creates a nice vortex at the top while leaving a still layer at the bottom. Turnover is a flow property, so it comes from impeller diameter and pumping capacity more than from rpm.
2. Powder wetting and deagglomeration. This is the only job that needs genuine high shear. Dry pigment arrives as agglomerates held together by surface forces; wetting the outside does not break them. The sawtooth disc does it by pulling material through a narrow high-velocity zone at the tooth edge, where the shear rate is high enough to separate particles rather than just push them around. It also does it by generating the vortex that pulls powder down into the liquid in the first place.
3. Holding uniformity and temperature. After let-down the batch is thinner, the pigment concentration is lower, and the risk changes from agglomeration to stratification and skinning. Now the requirement is gentle, continuous movement and heat removal, not shear. Many lines use two different agitators for the two stages, or a concentric shaft carrying a disperser disc and a slow scraper frame at the same time.
| What you see in the batch | Which job failed | What actually changes it |
|---|---|---|
| Pigment settles into a hard cake overnight | Bulk turnover | Disc too small relative to tank diameter, or a fast disc that only moves the top layer. Move to a larger, slower impeller or add a second level. |
| Lumps and seeds survive a full dispersion cycle | Powder wetting | Powder fed faster than the vortex can wet it. Slow the feed and keep a stable vortex; check disc-to-tank ratio before blaming the speed. |
| Gloss or color strength drops batch to batch | Deagglomeration | Tip speed below the working range for that pigment, or a worn disc. Measure the actual disc edge and the actual rpm, not the nameplate. |
| Color drifts between batches of the same formula | Uniformity | Not enough movement after let-down. The batch is stratified rather than mixed, and a sample taken near the surface is not the batch. |
| Temperature climbs and the batch skins in the tank | Heat from shear | Overall shear energy is right but there is no wall cooling and no way to shorten the cycle. Reduce hold time, cool the jacket, or split the batch. |
The Four Impeller Types Used in Paint Production
Four geometries cover almost everything in a paint plant. The names are not standardized across suppliers, so the shape matters more than the label: what distinguishes them is whether the blade pushes paint radially outward, downward, or around the wall.
Sawtooth disc. A flat disc with teeth bent alternately up and down. It is the workhorse of paint dispersion because it does two things at once: the pumping faces drive the batch outward and up the wall, while the tooth edges create the high-shear zone where agglomerates break. It is a high-speed, low-viscosity device — pushed into a stiff paste it simply stops moving material and starts heating it.
Cup or turbine impellers. Shaped cups or vanes on a shaft, sized to move a large volume at relatively low shear. They are the right choice when the batch is already dispersed and what is needed is keeping solids in suspension without adding heat or air. Using them for the dispersion stage is a common cause of "we cannot get the grind down" — the geometry has no high-shear zone to break agglomerates in.
Flat paddle, single or multi-level. Blades that sweep the batch at low speed. The multi-level arrangement exists because a single paddle in a tall tank leaves dead zones above and below it; stacking levels reduces that without raising the speed. It is used where shear is unwanted — filled systems, high solids, and stages where the batch must be kept moving without being worked further.
Helical ribbon or butterfly frame, with wall scrapers. A frame that follows the wall and bottom of the vessel, turning slowly. It handles the viscous end of the range where a disc cannot move material at all, and the scrapers matter as much as the ribbon: in a thixotropic or heat-sensitive paint the layer against the jacket is the one that overheats or gels. Note that a ribbon alone does not disperse pigment — where both are needed the slow frame and a high-speed disc are carried on concentric shafts, and the disc does the dispersion while the frame handles turnover and heat transfer.
| Geometry | Flow pattern | Duty in a paint line | Where it stops working |
|---|---|---|---|
| Sawtooth disc | Radial out, then up the wall | Powder wetting and pigment deagglomeration | Once the batch is too stiff to form a vortex — it heats the paint instead of moving it |
| Cup / turbine | High volume, low shear, axial plus radial | Let-down, tinting, keeping solids suspended in storage | When asked to reduce the grind — there is no high-shear zone |
| Multi-level paddle | Broad sweeping, nearly no shear | Filled and high-solids systems, gentle homogenization | In a tall tank with one level only — dead zones above and below the blade |
| Ribbon / butterfly frame | Full-batch circulation along the wall and bottom | Pastes and thixotropic systems, heat transfer through the jacket | When real dispersion is needed — pigment still has to be broken down by a disc |
Tip Speed, Not RPM: the Number That Decides Dispersion
Operators talk in rpm because rpm is what the panel shows. But rpm on its own says nothing about how fast the paint is moving. The number that matters is the speed at the outer edge of the impeller, where the shear actually happens:
Two consequences follow, and both catch people out. First, doubling the rpm doubles the tip speed but roughly quadruples the power drawn — a disc that sounds fine at 800 rpm can be overloaded at 1,200. Second, and more importantly for scale-up: a bigger disc reaches the same tip speed at lower rpm. A 600 mm disc at 1,200 rpm is running faster at the edge than a 300 mm disc at the same 1,200 rpm, because the edge has twice as far to travel in the same time.
Pigment dispersion with a sawtooth disc is commonly run in the 18 to 25 m/s tip-speed range, with the exact figure set by the pigment hardness, the resin viscosity and the temperature limit of the batch. Below the range, agglomerates survive and the grind gauge reading stalls; above it, the batch heats faster than the jacket can remove the heat and heat-sensitive resins start to shift color.
The table below is not a claim about performance. It takes the published disc diameter and top speed of each POLYC PDM model and calculates the resulting tip speed, so you can see the range the same product family actually spans.
| Model | Power (kW) | Disc dia. (mm) | Top speed (rpm) | Tip speed at top rpm (m/s) | Batch range (L) |
|---|---|---|---|---|---|
| PDM 1.5/2.2 | 1.5 / 2.2 | 150 | 1,450 | 11.4 | 50 - 100 |
| PDM 4.5/5.5 | 4.5 / 5.5 | 200 | 1,450 | 15.2 | 100 - 200 |
| PDM 7.5/11/15 | 7.5 / 11 / 15 | 250 / 300 | 1,450 | 19.0 / 22.8 | 400 - 600 |
| PDM 18.5/22/30 | 18.5 / 22 / 30 | 350 / 400 | 1,450 | 26.6 / 30.4 | 600 - 2,000 |
| PDM 37/45 | 37 / 45 | 450 | 1,200 | 28.3 | 1,000 - 2,500 |
| PDM 55/75/90 | 55 / 75 / 90 | 500 / 550 / 600 | 1,200 | 31.4 / 34.6 / 37.7 | 5,000 |
Read the right-hand column against the left and a practical point appears: the small machines sit below the usual dispersion range at full speed, and the large ones sit above it. The lab disperser at 11.4 m/s cannot reproduce what the 600 mm disc does at 37.7 m/s, which is exactly why lab results so often fail to scale — and why the fix is usually to change the disc or the speed on the production unit rather than to run the lab machine longer.
Where the Agitator Sits in a Paint Line
The agitator does not finish the job on its own. In a conventional paint line it is one stage of three, and confusing the stages is behind a lot of wasted effort — running a disperser for another hour will not do what a mill does.
Dispersion wets the pigment and breaks the agglomerates down to roughly the size of the primary particles still clustered together. A high-speed disperser with a sawtooth agitator gets a typical decorative paint to a workable grind. Milling then reduces the particle size itself and narrows the distribution; this is where a bead mill or a three-roll mill earns its place, and it is the only way to reach fine or narrow specifications. Let-down and finishing dilutes, tints and holds the batch uniform, and that stage needs flow, not shear.
So when a batch will not come down to grind on a disperser alone, the honest answer is usually that the target requires milling, not that the agitator is weak. Equally, when a mill is being fed a poorly pre-dispersed paste, the mill does more passes than it should — which is the single most common reason a bead mill underperforms against its rated throughput.
Rule of thumb worth remembering: pre-dispersion quality decides how much work the mill has to do. A stable, well-wetted premix can cut the passes a bead mill needs — and therefore its energy and wear per tonne — more than any change to the mill itself.
Selection: Six Variables That Actually Decide It
Most selection conversations start with batch size and stop there. Batch size sets the frame and the motor, but it is not what decides whether the agitator works. Six variables do, and they should be settled in this order.
Not the viscosity at charging. A mill base thickens as pigment loads in, often by an order of magnitude, and the impeller has to still be moving material at the end.
A dispersion target and a milling target need different equipment. Agree the fineness figure in microns or Hegman before choosing a geometry.
Set by pigment hardness. This plus the batch diameter fixes the disc diameter; the disc diameter and the tip speed then fix the rpm.
Too small and the batch bypasses the shear zone; too large and the vortex collapses and the disc runs in a well it has dug for itself.
All shaft power ends up as heat in the batch. If the jacket cannot remove it at the tip speed you need, the cycle is limited by temperature, not by dispersion.
Hard fillers and coarse pigment wear the disc edge, and a worn edge quietly loses tip speed. Specify the disc material against the filler, and check it periodically.
How the Agitator Is Built and Tested
The shaft, the disc and the coupling are where a disperser is won or lost. POLYC machines are built in our own 9,000 m² plant in Wuxi rather than assembled from bought-in sets, which is what allows the disc diameter, shaft length and lift stroke to be set to a customer's tank instead of to a catalogue page.
A disc that does not run true loses tip speed on one side and wears unevenly. Run-out is measured on the assembled shaft, not on the component.
Hard-facing or a heavier section where calcium carbonate or coarse pigment is abrasive, so the edge geometry lasts rather than rounding off.
The lift has to raise the shaft out of a full tank, not an empty one. Hydraulic lift stroke is specified against the customer's tank height.
Solvent-based paints need a hazardous-area assessment. Explosion-proof configurations are available upon request, confirmed against your area classification.
Every unit carries a 3-Year Global Warranty. Custom disc diameters, shaft lengths and tank-matched lift strokes are normal orders rather than exceptions, and the standard delivery window is 30-45 days.
Send Us the Formulation and the Viscosity Curve
Selection is quick when the three numbers are known: the viscosity at the end of dispersion, the target grind, and the batch size. Send those with the pigment system and we will come back with a disc diameter, a tip speed and a model rather than a catalogue page. Sample testing at our Wuxi facility is available before an order is placed.
Engineering reply within 24 hours · 3-Year Global Warranty · 30-45 days delivery
Direct email: export@polycmill.com
Frequently Asked Questions
What is an agitator in painting?
In paint manufacturing, an agitator is the shaft and impeller assembly that turns the batch inside a mixing vessel. The vessel, frame and drive are the machine; the agitator is the part that actually moves the paint. In a paint shop the same word is used for a hand-held stirrer, which is a different scale of equipment.
What is the use of an agitator in painting?
It does three jobs: turning the whole batch over so pigment and filler do not settle, wetting powder into the resin so agglomerates break down, and keeping temperature and solids uniform through the batch. Settlement, seed and color drift are the three problems it exists to prevent.
What does an agitator do in paint?
It converts shaft power into fluid motion. A sawtooth disc at the right tip speed creates a high-shear zone at its edge where pigment agglomerates are broken apart, and a bulk roll-over pattern that carries the whole batch through that zone repeatedly.
How do you use an agitator in painting?
Charge the liquid phase, start the agitator at low speed, add powder into the vortex at a rate the disc can wet out, then raise to dispersion tip speed and hold until the grind gauge reading stops falling. Adding powder faster than the vortex can wet it is the most common cause of lumps.
What is the difference between an agitator and a disperser?
The agitator is the shaft and blade. The disperser is the machine around it: the frame, hydraulic lift, drive motor and controls that position and spin that agitator. A high-speed disperser uses a sawtooth agitator; a planetary mixer uses a different agitator on the same kind of frame.
What tip speed should a paint agitator run at?
Pigment dispersion with a sawtooth disc is normally run in the 18 to 25 m/s tip-speed range, with the exact figure set by the pigment, the resin viscosity and the batch temperature limit. Tip speed is calculated as pi times disc diameter times rpm divided by 60, so a larger disc reaches the same tip speed at lower rpm.
Explore Related Resources
1.5-90 kW, disc 150-600 mm, batch 50-5,000 L. Full specification table.
Sawtooth disc geometry, tip speed versus rpm, and how price drivers change with size.
Anchor scraper plus two dispersing discs on concentric shafts, 100-2,000 L, to -0.098 MPa.
Slow scraper frame with a fast dispersing shaft, for high-viscosity paste and sealant.
