Mesh, D50 and Top Cut — Reading a Mineral Filler Particle Size Spec
Direct answer
Mesh is a sieve number, D50 is the median particle diameter in microns, and the top cut (D97 or D98) is the coarse tail. For plastics and coatings the D50 tells you how the filler behaves on average, but the top cut is what decides whether you get film breaks, screen-pack pressure, surface defects or gloss loss. A supplier who quotes only mesh, or only D50, has not told you the thing most likely to cost you a shift.
Why mesh alone is not enough
A mesh figure describes what passed through a screen, so it is effectively a statement about the coarse end of the distribution and nothing else. Two 1250 mesh calcium carbonates can have very different medians and very different fines content depending on how they were milled and classified. Mesh is still useful as a commercial shorthand between buyers and mills, but it is not a specification you can design a formulation around.
What D50 controls
The median diameter drives the property trade you are buying:
- Coarser fillers (D50 roughly 3–10 μm) are cheap per kilogram, easy to disperse, and good where the job is bulk and stiffness — pipe, profile, woven sacks, crates.
- Finer fillers (D50 roughly 0.8–2 μm) cost more per kilogram and are harder to disperse, but they preserve impact strength and surface finish at the same loading, which is why they appear in thin film, sheet and visible parts.
- Below roughly 1 μm the surface area rises steeply, oil and coating demand rises with it, and agglomeration becomes the dominant processing problem. You are no longer buying a filler, you are buying a dispersion challenge.
Why the top cut decides the failure mode
In a 25 μm blown film, a 40 μm agglomerate or a single oversize particle is a hole. In a coating, it is a visible defect and a gloss reduction. In an injection-moulded part it is a stress concentrator. The average particle size cannot express that risk; only the top cut can. Ask for D97 or D98 in microns and, for thin-gauge film, ask whether the grade is screened or classified after milling.
The questions that make a filler spec quotable
- D50 in microns, plus D97 or D98 — not mesh alone.
- The measurement method (laser diffraction versus sedimentation give different numbers for the same powder; comparing across methods is not meaningful).
- Residue on a stated sieve, for grades destined for thin film.
- Surface treatment and treatment level, since coating changes the effective dispersed size as well as the wetting behaviour.
- Moisture content, because damp fine mineral agglomerates in the feeder and vents as voids in the melt.
A practical rule
Specify the finest grade the defect you are trying to avoid actually requires, and no finer. Fineness is bought twice — once at the invoice and once in dispersion energy, dosing accuracy and screen changes. Where a coarser grade passes the part test, it is usually the cheaper total cost even when the price per tonne is close.