Plain-English definitions of the 20 terms buyers and engineers search before specifying a mill.
This glossary explains the wet grinding and coating equipment terms most often searched by buyers, formulators and plant engineers. Every definition below is written in plant language, and each entry links to the related POLYC product page or technical article. If a term you need is missing, or you need a machine sized for your formulation, send the details through our contact form and the engineering team will answer with a recommendation.
Why This Glossary Matters
Wet grinding is a precision process: the wrong mill, media size or tip speed can cost you hours of batch time, contaminate your product, or fail to reach the fineness your formulation requires. Buyers who understand the vocabulary below make better specification decisions, ask sharper questions during supplier calls, and avoid the most common sizing mistakes.
This glossary covers the 20 terms that appear most often in bead mill, basket mill and disperser specifications. Use the Quick Reference Table below for a one-line answer, or scroll to Term Definitions for the full explanation with typical operating ranges.
Quick Reference Table
| Term | One-line definition | Typical use |
|---|---|---|
| Bead Mill | Wet grinding machine using agitated media in a chamber to reduce particle size | Paints, inks, coatings, battery slurry |
| Sand Mill | Media mill using small abrasive grains, historically sand, now glass or ceramic beads | Medium-viscosity coatings, printing inks |
| Basket Mill | Immersion mill lowering a media-filled basket into a standing tank for batch grinding | Small batches, lab work, specialty pigments |
| Wet Grinding | Particle size reduction with the material suspended in liquid | Pigments, coatings, agrochemicals |
| Dry Grinding | Particle size reduction without liquid, using air or impact | Minerals, chemicals, food powders |
| D10/D50/D90 | Particle size percentiles: 10%, 50%, 90% of particles are below this size | Quality control, mill performance |
| Particle Size Distribution | The range and spread of particle sizes in a sample, not just the average | Formulation, QC, regulatory |
| Tip Speed | Linear speed of the rotor/agitator outer edge, in m/s | Mill sizing, energy input |
| Specific Energy | Energy consumed per kg of product, in kWh/kg, correlating with fineness | Scale-up, cost estimation |
| Grinding Media | The balls or beads inside a mill that transfer energy to the product | All media mills |
| Zirconia Beads | High-density, high-wear-resistance ceramic grinding media (YTZ/PZ95) | Fine grinding, high-value products |
| Glass Beads | Lower-cost, lower-density grinding media for coarser grinding | Paints, general coatings |
| High-Shear Mixing | Mixing with high velocity gradients to break agglomerates and emulsify | Premixing, emulsification |
| Rotor-Stator Principle | A rotating rotor passing a stationary stator creates high shear in the gap | High-shear mixers, homogenizers |
| Disperser | High-speed agitator with a disc blade for pigment wetting and premixing | Paint/ink premix before bead mill |
| Pigment Wetting | Displacing air from pigment surfaces with resin/solvent to enable dispersion | First stage of dispersion |
| Viscosity | Resistance to flow, measured in cps/mPa·s, affecting mill throughput | Process control, mill selection |
| Solids Content | Percentage of non-volatile material in a formulation, by weight | Formulation, throughput |
| Scale-Up | Transferring a lab or pilot process to production size while maintaining quality | Process engineering |
| ATEX | EU directive for equipment used in explosive atmospheres (solvent-based coatings) | Safety compliance, solvent plants |
Term Definitions
Bead Mill
A bead mill is a wet grinding machine in which a solid-liquid feed is ground by grinding media held inside a chamber. The shaft and rotor agitate the beads so that collisions and shear forces break pigment, filler or active-material agglomerates. The media are retained by a dynamic or static separator while the finished slurry passes through. Bead mills are the standard choice for paints, inks, coatings, agrochemicals and battery slurry, reaching particle sizes from around 1 micron down to below 100 nm with controlled temperature and contamination.
Typical range: Chamber 0.5-100 L; media 0.1-3.0 mm; tip speed 6-18 m/s; D50 0.1-10 um.
See also: Pin Type Bead Mill · Grinding & Milling Equipment · How a Bead Mill Works
Sand Mill
A sand mill is a horizontal or vertical media mill that uses small abrasive grains or beads as the grinding medium, historically river sand. Modern sand mills replace the sand with glass, ceramic or zirconia beads and operate with a shaft-mounted agitator plus a separator screen. They suit medium-viscosity coatings, printing inks and agrochemical suspensions where a fineness of roughly 5 to 20 microns is acceptable and capital cost must stay low, and they remain common in paint plants that already run batch dispersers.
Typical range: Media 0.8-3 mm glass or ceramic; tip speed 6-14 m/s; D50 3-20 um.
See also: Static Disc Sand / Bead Mill · Sand Mill vs Bead Mill
Basket Mill
A basket mill, also called an immersion or basket bead mill, lowers a perforated basket containing grinding media into a standing tank of paint or slurry. The basket rotor agitates the beads in place, so the machine never has to be drained between batches. This design gives very fast colour changes and simple cleaning, so it is popular for small batches, laboratory work, specialty pigments and offset inks, and it avoids the media separation problems of through-flow horizontal mills.
Typical range: Basket 5-100 L; media 1.0-3.0 mm; typical cycle 15-40 min per batch.
See also: Basket Mill · Laboratory Basket Mill
Wet Grinding
Wet grinding reduces particle size while the material is suspended in a liquid carrier, usually water, solvent or resin solution. The liquid controls temperature, prevents dust, and allows the product to be pumped through the mill continuously. Almost all coating, ink and pigment production uses wet grinding because it gives finer, more consistent results than dry grinding and avoids the explosion risk of fine dry pigment dust.
Typical range: Solids 30-75%; viscosity 100-10,000 cps; D50 0.05-20 um.
See also: Grinding & Milling Equipment · CHINACOAT Recap: Wet Grinding Trends
Dry Grinding
Dry grinding reduces particle size without a liquid carrier, using impact, compression or attrition in air or inert gas. It is common for minerals, chemicals and food powders where the final product must be dry, but it is rarely used for pigments or coatings because fine dry pigment dust is explosive and difficult to handle. Dry grinding generally cannot reach the sub-micron fineness that wet grinding achieves consistently.
Typical range: D50 5-200 um; moisture <1%; requires dust extraction and ATEX.
See also: Wet vs Dry Grinding and Reading d10, d50, d90
D10/D50/D90
D10, D50 and D90 are particle size percentiles. D50 (the median) means 50% of particles are smaller than this value; D10 and D90 mean 10% and 90% respectively are smaller. Together they describe not just the average size but the width of the distribution, which matters because a narrow distribution gives consistent colour strength, gloss and viscosity. Buyers should always ask for D90, not just D50, because the coarse tail determines whether the product will pass a fineness gauge.
Typical range: D50 0.1-20 um for coatings; D90/D50 ratio 1.5-3 for good distributions.
See also: Wet vs Dry Grinding and Reading d10, d50, d90 · Dynamic Disc Horizontal Bead Mill
Particle Size Distribution
Particle size distribution (PSD) describes the full range of particle sizes in a sample, not just the average. A narrow PSD means most particles are close to the same size, giving consistent colour, transparency and viscosity; a broad PSD means a mix of fine and coarse particles, which can cause settling, poor gloss and variable strength. PSD is usually measured by laser diffraction and reported as D10/D50/D90 plus a span value.
Typical range: Span (D90-D10)/D50 0.5-2 for good dispersions; measured by laser diffraction.
See also: How a Bead Mill Works · Grinding & Milling Equipment
Tip Speed
Tip speed is the linear speed of the outer edge of the mill rotor or agitator, measured in m/s. It is the single most important parameter for grinding energy input: higher tip speed means more collision energy between beads, finer particle size, but also more heat and more media wear. Different products need different tip speeds, and a mill that runs too fast for a sensitive product can cause shear degradation or temperature rise.
Typical range: 6-18 m/s for bead mills; 4-12 m/s for basket mills; 15-25 m/s for high-speed dispersers.
See also: Pin Type Bead Mill · Platform High Speed Disperser
Specific Energy
Specific energy is the amount of energy consumed per kilogram of product, measured in kWh/kg. It is the most reliable scale-up parameter because it correlates directly with the fineness achieved: if a lab batch needs 0.3 kWh/kg to reach D50 1 um, a production mill running at the same specific energy should reach the same fineness. Specific energy also determines operating cost, so buyers should compare mills on kWh/kg at target fineness, not just motor power.
Typical range: 0.05-0.5 kWh/kg for coatings; 0.2-2 kWh/kg for fine grinding to sub-micron.
See also: How a Bead Mill Works · Grinding & Milling Equipment
Grinding Media
Grinding media are the balls or beads inside a media mill that transfer kinetic energy from the rotor to the product. When the rotor agitates the media, bead-to-bead collisions and shear break the agglomerates. Media choice affects grinding efficiency, product contamination, wear cost and final fineness. The main types are zirconia (high density, high wear resistance, premium cost), glass (lower density, lower cost, for coarser grinding) and steel (very high density, but contamination risk limits use).
Typical range: 0.1-3.0 mm diameter; charge volume 70-85% of chamber; density 2.5-6.0 g/cm3.
See also: Grinding Media Zirconia Beads · Grinding Media Category
Zirconia Beads
Zirconia beads are high-density ceramic grinding media made from yttria-stabilized zirconia (YTZ) or similar formulations. Their high density (6.0 g/cm3) gives high collision energy, so they grind finer and faster than glass beads, and their extremely low wear rate means minimal product contamination and long media life. They are the standard choice for fine grinding of high-value products such as automotive coatings, electronic materials, battery slurry and pharmaceutical APIs, where contamination must be near zero.
Typical range: 0.05-2.0 mm; density 5.8-6.1 g/cm3; hardness 1200-1400 HV; wear rate <5 ppm/ton.
See also: Grinding Media Zirconia Beads · Zirconia Grinding Media Guide
Glass Beads
Glass beads are lower-cost grinding media made from soda-lime or borosilicate glass. Their lower density (2.5 g/cm3) means lower collision energy than zirconia, so they are best for coarser grinding of medium-value products where capital cost matters more than ultimate fineness. Glass beads wear faster than zirconia and can add silica contamination, so they are not suitable for products sensitive to silica or for sub-micron grinding. They remain common in general industrial paints and primers.
Typical range: 0.5-3.0 mm; density 2.4-2.6 g/cm3; hardness 500-600 HV; lower cost than zirconia.
See also: Grinding Media Category · Zirconia vs Glass vs Steel Media
High-Shear Mixing
High-shear mixing uses high velocity gradients to break agglomerates, create emulsions and disperse solids into liquid. It is usually done with a rotor-stator head or a high-speed disperser disc, and it is the first stage in most coating production: wetting the pigment and breaking down the largest agglomerates before the bead mill. High-shear mixers do not reach the fine fineness of a bead mill, but they are essential for preparing the feed so the bead mill can work efficiently.
Typical range: Tip speed 10-30 m/s; rotor-stator gap 0.5-2 mm; used for premix and emulsification.
See also: High Shearing Emulsifier · High Shear Emulsifier Selection Guide
Rotor-Stator Principle
The rotor-stator principle uses a rotating rotor passing very close to a stationary stator to create extreme shear in the narrow gap between them. As the rotor teeth pass the stator slots, the product is accelerated, sheared and expelled through the stator openings, creating high turbulence and breaking agglomerates or droplets. This is the working principle behind high-shear mixers, homogenizers and some inline dispersers, and it is extremely effective for emulsification and pigment wetting.
Typical range: Gap 0.3-2 mm; rotor tip speed 10-30 m/s; generates shear rates up to 100,000 s-1.
See also: High Shearing Emulsifier · High Shear Emulsifier Selection Guide
Disperser
A disperser (also called a high-speed disperser or dissolver) is a high-speed agitator with a toothed disc blade that creates high shear and turbulence at the blade tip. It is the standard premixing machine in paint and ink plants: it wets the pigment, breaks down the largest agglomerates, and prepares the feed for the bead mill. Dispersers do not reach fine fineness by themselves, but they are essential because a bead mill fed with poorly wetted pigment will run slowly and never reach target fineness.
Typical range: Tip speed 15-25 m/s; disc diameter 100-500 mm; batch 50-5000 L; used for premix.
See also: Single Shaft High Speed Disperser · Platform High Speed Disperser · Disperser vs Bead Mill
Pigment Wetting
Pigment wetting is the process of displacing air from pigment particle surfaces and replacing it with resin solution or solvent. It is the first and most critical stage of dispersion: if the pigment is not properly wetted, no amount of bead milling will break up the agglomerates, because the liquid cannot penetrate inside. Wetting is done by the disperser during premix, and it depends on resin chemistry, solvent choice, dispersant additives and sufficient mixing time and shear.
Typical range: Premix time 15-60 min; disperser tip speed 15-25 m/s; measured by fineness gauge after premix.
See also: Pigment Dispersion Fundamentals · Platform High Speed Disperser
Viscosity
Viscosity is a fluid's resistance to flow, measured in centipoise (cps) or millipascal-seconds (mPa·s). It is one of the most important parameters in mill selection because it determines how easily the product flows through the mill, how much energy the mill can transfer, and what type of pump is needed. Very high viscosity pastes need special feed systems or basket mills; very low viscosity products may need higher media retention pressure. Most coating pastes run between 500 and 10,000 cps during milling.
Typical range: 100-50,000 cps for coatings; measured by Brookfield or cone-and-plate viscometer.
See also: Horizontal Plough Shear Mixer · Ink Production Line
Solids Content
Solids content is the percentage of non-volatile material (pigment + resin + additives) in a formulation, measured by weight after drying. It affects mill throughput, viscosity and final product cost: higher solids means less solvent to evaporate, higher throughput, but also higher viscosity and more difficult milling. Modern high-solids coatings push solids as high as possible to reduce VOC emissions and energy use, but this requires mills that can handle high viscosity without overheating.
Typical range: 30-75% for solvent-based coatings; 40-65% for water-based; measured by oven drying.
See also: Dyes & Pigments Production Line · Laboratory Basket Mill
Scale-Up
Scale-up is the process of transferring a lab or pilot mill process to production size while maintaining the same product quality, fineness and throughput. The most reliable scale-up parameter is specific energy (kWh/kg): if the lab mill uses 0.3 kWh/kg to reach target fineness, the production mill should be sized to deliver the same specific energy. Other important parameters are tip speed, media size and charge volume, residence time, and cooling capacity. Poor scale-up is the most common cause of production mills failing to match lab results.
Typical range: Scale factor 10-100x from lab to production; maintain specific energy and tip speed; increase cooling proportionally.
See also: Production Lines · All Products · Request a Quote
ATEX
ATEX is the European Union directive governing equipment used in potentially explosive atmospheres. Solvent-based coating plants are classified as ATEX zones because solvent vapours can form explosive mixtures with air, so all electrical equipment in the production area must be ATEX-certified: motors, control panels, sensors, pumps and mills. ATEX mills have sealed motors, static dissipative components, and certified temperature limits to prevent ignition. Buyers in solvent-based coatings should always specify ATEX-rated equipment, and buyers exporting to the EU must meet ATEX requirements.
Typical range: Zone 1/2 for solvent areas; temperature class T3/T4; certified motors and controls; required for solvent plants.
See also: Ink Production Line · Grinding & Milling Equipment
How to Choose the Right Grinding Equipment
Choosing the right mill depends on four parameters: target fineness, product viscosity, batch size, and solvent vs water-based. Use this quick guide to narrow your selection, then send your formulation details to POLYC for a precise sizing recommendation.
- Need D50 below 1 um, high-value product? Choose a horizontal bead mill with zirconia media (0.3-0.8 mm) for fine grinding with minimal contamination.
- Need D50 5-20 um, general coatings? A sand mill or entry-level bead mill with glass or ceramic media gives good results at lower capital cost.
- Small batches, frequent colour changes? A basket mill is the best choice: no draining between batches, fast cleaning, simple operation.
- High viscosity pastes (above 10,000 cps)? Use a basket mill or a bead mill with a special high-viscosity feed pump; standard inline pumps will not work.
- Solvent-based production? Always specify ATEX-rated motors and controls for safety compliance.
- Premix before milling? Every bead mill needs a high-speed disperser for pigment wetting; do not skip the premix stage.
For a precise machine sizing, send your target fineness, viscosity, solids content, batch size and annual volume through our contact form. The engineering team will recommend the exact mill size, media type and operating parameters for your formulation.
Frequently Asked Questions
What is the difference between a bead mill and a basket mill?
What size grinding media should I use?
Zirconia beads or glass beads?
What tip speed should I run my bead mill at?
Do I need a disperser if I already have a bead mill?
What is ATEX and do I need it?
Recommended POLYC MACHINE Equipment
POLYC MACHINE manufactures the full range of wet grinding and dispersion equipment, from laboratory basket mills to complete production lines. These are the machines most commonly specified based on the terms in this glossary:
- Pin Type Bead Mill — Horizontal bead mill for fine grinding of paints, inks and coatings; chamber 5-100 L; tip speed 8-16 m/s.
- Dynamic Disc Horizontal Bead Mill — High-efficiency bead mill with dynamic disc agitator for medium to fine grinding.
- Static Disc Sand / Bead Mill — Entry-level media mill for general coatings and printing inks; lower capital cost.
- Basket Mill — Immersion basket mill for small batches, lab work and specialty pigments; fast colour changes.
- Laboratory Basket Mill — Compact lab mill for formulation development and small batch testing.
- Platform High Speed Disperser — High-speed disperser for pigment wetting and premix; batch 100-5000 L.
- High Shearing Emulsifier — Rotor-stator high-shear mixer for emulsification and pigment dispersion.
- Zirconia Grinding Media — PZ95 high-purity zirconia beads for fine grinding; 0.1-3.0 mm.
Next Steps
If you are sizing a mill for a new formulation or upgrading an existing production line, the POLYC engineering team can help you select the right machine, media size and operating parameters. Send your formulation details (target fineness, viscosity, solids content, batch size, annual volume) through our contact form, and you will receive a detailed technical proposal with equipment recommendation, operating parameters and pricing within 24-48 hours.
You can also browse the full grinding and milling range or read how a bead mill works for more technical background.