Particle size distribution (PSD) testing determines the range and relative proportions of particle sizes in plastic pellets, granules, powders, and recycled materials. By applying internationally recognized methods such as sieve analysis and laser diffraction, accredited third-party laboratories quantify key parameters including D10, D50, D90, and fines content to support quality control, process optimization, and compliance with ISO and ASTM standards.
Technical Overview
Particle size distribution (PSD) is one of the most important physical characteristics of plastic materials in pellet, granule, or powder form. It describes the relative amounts of particles present according to size, typically expressed as a frequency histogram or cumulative percentage curve. For plastics processing, PSD directly influences bulk density, hopper flow behavior, feeding consistency in extruders and injection molding machines, melting uniformity, and the dispersion of pigments and additives.
The determination of particle size distribution for plastic particles is performed using several complementary techniques, including mechanical sieve analysis, laser diffraction, and image analysis. The most commonly referenced standards include ASTM D1921 (particle size of plastic materials by sieve analysis), ISO 13320 (laser diffraction methods), ISO 13322-1 and ISO 13322-2 (static and dynamic image analysis), ISO 2591-1 (sieve analysis), and relevant GB/T and JIS national standards. Depending on the material and particle size range, results are reported as volume-based, mass-based, or number-based distributions.
Testing Samples
Virgin Thermoplastic Resin Pellets
Standard commodity resins such as polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS, PVC, and EVA are routinely tested to verify pellet dimension consistency, pellet length distribution, and the presence of oversized pellets or fines.
Engineering Plastic Granules
High-performance polymers including PA6, PA66, PC, POM, PET, PBT, PPS, and PEEK are tested for granule geometry, dust and fines content, and flow-related size uniformity before compounding or precision molding.
Plastic Powders
Rotational molding PE powders, PVC paste and dry-blend resins, PTFE fine powders, and spray-coating or sintering powders require accurate PSD measurement, as powder fineness and distribution span govern sintering behavior and surface finish.
Masterbatch and Compound Pellets
Color and additive masterbatches, as well as filled and reinforced compounds, are checked for pellet size uniformity to ensure consistent dosing and downstream processing stability.
Recycled Plastics and Regrind
Recycled flakes, regrind, and granules from post-industrial or post-consumer sources are characterized for flake dimension distribution, fines level, and contamination-related oversize fractions.
Plastic Microspheres
Expandable and solid polymer microspheres used in foams, cosmetics, and specialty applications are measured for median diameter and distribution width in the micrometer range.
Testing Items
Typical parameters determined in a particle size distribution test include:
- D10, D50 (median particle size), and D90 — the diameters at which 10%, 50%, and 90% of the population is finer;
- D[4,3] volume (moment) mean diameter and D[3,2] surface (Sauter) mean diameter;
- Distribution width indicators such as span, uniformity coefficient, and distribution modulus;
- Cumulative undersize and oversize percentages at specified sieve or measurement apertures;
- Fines and dust content, typically the fraction below a defined threshold such as 500 µm or 63 µm;
- Oversize fraction, agglomerates, and long or malformed pellets ("tails");
- Pellet length, diameter, aspect ratio, and shape factors measured by image analysis;
- Frequency (histogram) and cumulative distribution curves with full tabulated data.
Testing Methods
Dry Sieve Analysis
Following ASTM D1921 and ISO 2591-1, a representative sample is placed on a stack of calibrated test sieves conforming to ASTM E11 or GB/T 6003 and mechanically shaken for a defined time. The retained mass on each sieve is weighed, yielding a mass-based distribution. This method is best suited to pellets, granules, and powders above approximately 38 µm and is widely used for lot release testing of resins.
Wet Sieve Analysis
For fine powders prone to agglomeration or static charging, such as PVC paste resins and certain PTFE grades, wet sieving with a suitable liquid and dispersant improves accuracy by breaking up soft agglomerates and preventing screen blinding.
Laser Diffraction
In accordance with ISO 13320, laser diffraction measures the angular scattering pattern of particles illuminated by a laser beam and calculates a volume-based size distribution, typically covering approximately 0.1 µm to 3500 µm. Both dry powder dispersion (air stream) and wet suspension (with appropriate liquid and surfactant) configurations are available. Accurate optical properties, particularly the refractive index and absorption of the polymer, are required for reliable results.
Dynamic Image Analysis
Based on ISO 13322-2, particles are streamed past a camera and analyzed individually in motion. This technique simultaneously delivers size and shape information, making it ideal for plastic pellets and granules where pellet length, width, and aspect ratio cannot be fully described by a single equivalent diameter.
Static Image Analysis and Optical Microscopy
In accordance with ISO 13322-1, dispersed particles are imaged and counted on a substrate. The method provides number-based distributions and detailed morphological data, and is well suited to microspheres and quality investigations.
Electrical Sensing Zone (Coulter Principle)
Per ISO 13319, particles suspended in an electrolyte are counted and sized as they pass through a small aperture. This high-resolution, number-based method is applicable to fine polymer powders and microspheres in the sub-micrometer to tens-of-micrometers range.
Good analytical practice requires representative sampling using riffle splitters or cone-and-quartering, sample conditioning to reduce static and agglomeration, method validation with certified reference materials, and repeat measurements to confirm reproducibility.
Applications
Particle size distribution data for plastic materials serve a broad range of industrial purposes:
- Incoming resin inspection and supplier qualification for converters and compounders;
- Lot release testing and statistical process control at polymerization, compounding, and pelletizing plants;
- Optimization of extrusion, injection molding, and rotational molding processes where feeding and melting behavior depend on pellet or powder fineness;
- Quality evaluation of color and additive masterbatches for consistent let-down ratio dosing;
- Characterization of recycled flakes and regrind for compatibility with washing, extrusion, and repelletizing lines;
- Foam and specialty applications where microsphere expansion behavior correlates with particle size;
- Failure analysis, customer complaint investigation, and specification dispute resolution;
- Regulatory and contractual documentation requiring test reports from accredited third-party laboratories.
FAQ
What does D50 mean in a particle size distribution report?
D50 is the median particle diameter — the size at which 50% of the particles (by volume, mass, or number, depending on the method) are smaller and 50% are larger. Together with D10 and D90, it summarizes the central tendency and spread of the distribution.
Which method should I choose for plastic pellets versus plastic powders?
Pellets and granules with dimensions of roughly 1–10 mm are usually best measured by sieve analysis or dynamic image analysis. Powders finer than about 1 mm, especially in the micrometer range, are more accurately characterized by laser diffraction, wet sieving, or the electrical sensing zone method. The appropriate method also depends on whether mass-, volume-, or number-based results are required.
How much sample is needed for testing?
Sieve analysis typically requires 50–200 g of pellets or powder depending on particle size, while laser diffraction and image analysis can work with smaller quantities, often a few grams. Regardless of the method, the submitted sample must be representative of the lot, so proper sampling and sample splitting before submission are strongly recommended.
Why do sieve and laser diffraction results sometimes differ?
Sieving reports a mass-based distribution of the minimum cross-sectional dimension, whereas laser diffraction reports a volume-based equivalent spherical diameter. Differences in measurement principle, dispersion conditions, and particle shape mean that small deviations between the two methods are normal, which is why a single agreed method should be referenced in specifications.
How should samples be prepared and submitted?
Samples should be sealed in clean, moisture-tight containers, clearly labeled with material designation and lot number, and accompanied by the applicable standard or specification limits. Hygroscopic materials such as polyamide granules should be protected from moisture pickup during transport and storage.
What is included in a typical test report?
A professional report includes the test method and standard applied, measurement conditions (sieve apertures or dispersion medium, refractive index settings), tabulated distribution data, D10/D50/D90, mean diameters, span, frequency and cumulative curves, sample photographs where applicable, and a clear statement of results against specified acceptance limits.
