How to specify powder particle-size distribution
Agree the measurement method, reporting basis and dispersion conditions before setting particle-size limits. Check the full distribution and coarse fraction on representative batch samples, then confirm performance in your intended process. Cambridge China Bridge can coordinate factory sampling and buyer-approved checks through its staff in China.
Define what the size result must describe
Start with the process problem: slow dissolution, blocked filters, poor mixing, settling or an uneven finish. Decide whether the test should describe individual particles after deagglomeration or the clusters present during use. Aggressive laboratory dispersion may remove clusters that your production process cannot break.
Write a particle-size annex to your purchase specification. Name the size units, reporting basis, selected percentiles and acceptable ranges, and require the full distribution curve. A median alone leaves the tails undefined. Use our product specification guide for the wider order document and chemical intermediates guide for chemical quality documents.
Agree a method that both laboratories can reproduce
Ask the factory and your laboratory to agree the measurement technique before comparing results. Laser diffraction reports a volume-based distribution; sieving measures material passing or retained at an aperture. Treat these as different measurements. For needles, plates or irregular particles, ask whether microscopy or image analysis is needed to interpret the reported size.
For laser diffraction, record the instrument, dispersion accessory, software version, optical model and optical inputs. Specify whether the distribution is reported by volume, mass or particle count. Agree background checks, measurement duration and repeat preparations. Compare split samples at the factory and buyer laboratories before using their reports interchangeably.
Fix dispersion conditions before testing batches
For wet testing, record the liquid, additives, sample concentration, stirring, circulation and any ultrasound settings and duration. Ask the laboratory to check that the liquid does not dissolve or alter the powder. Investigate whether results change during soaking or measurement, and whether coarse particles remain suspended.
For dry testing, record the feeder settings and dispersion pressure. During method development, vary dispersion energy and examine whether changes reflect cluster separation or particle breakage. Select conditions suited to the agreed measurement purpose, then freeze the procedure. Do not let a supplier increase dispersion energy simply to obtain a passing result.
Specify the coarse fraction separately
If oversize particles cause blockages or visible defects, specify an agreed size cut-off and maximum fraction above it. State whether that fraction is measured by volume or by mass. An upper percentile is not a maximum particle size. Where useful, add a sieve-residue check with the aperture, preparation, sieving procedure and residue acceptance limit defined.
Agree where batch samples come from, how increments are combined and how the laboratory portion is divided without losing coarse particles. A convenient scoop from a bag surface may miss segregation. Keep sealed retained samples and link every result to its batch and sampling record; our laboratory sample chain-of-custody guide covers that handover.
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Approve process performance and control changes
Trial a production-representative sample in the intended formulation and equipment. Record loading, mixing conditions, temperature, residence time and the relevant outputs, such as dissolution, filtration, settling or surface finish. Agree pass criteria before the trial. A laboratory size result is evidence about the powder; the trial establishes whether it works in your process.
Use successful trials to set justified distribution and coarse-fraction limits. Keep the approved sample and its method record as a comparison. Require batch-specific results and agree independent checks and a dispute laboratory. Ask for notice of changes to milling, classification, drying or packing, and agree when repeat process trials are needed before accepting changed material.
Frequently asked questions
Is median particle size enough for a powder specification?
A median leaves fine and coarse tails undefined. Ask for the full distribution, agreed percentile ranges and a separate oversize limit where coarse particles affect your process.
Why do factory and UK laboratory particle-size results differ?
Check sampling, reporting basis, instrument settings and dispersion conditions. Compare split samples using an agreed procedure before deciding that the batch has changed.
Should powder particle size be tested wet or dry?
Choose according to the powder and what you need to measure. Wet testing needs a compatible liquid; dry testing needs controlled dispersion pressure. Check that preparation does not alter the particles.
Can a powder pass particle-size testing but fail in production?
Yes. Laboratory dispersion may break clusters that remain in production, and particle size does not establish every performance property. Approve a representative sample in the intended process.