ASTM D1921 Standard Test Methods for Particle Size (Sieve Analysis) of Plastic Materials

Get certified ASTM D1921 testing for reliable plastic particle size data.

ASTM D1921 testing gives manufacturers and buyers a documented particle size distribution for powdered, granular, or pelleted plastic. The laboratory weighs the sample, runs it through a stack of sieves in a mechanical shaker, and reports the percentage retained on each sieve. Buyers and suppliers use the report to check lot-to-lot uniformity and compliance with material specifications. The report also lists the sample weight and any antistat the laboratory used, so reviewers can repeat the measurement.

The scope of ASTM D1921 includes

• Material forms: powdered, granular, and pelleted plastic materials
• Technique: dry sieving, with a lower measurement limit of about 38 µm (No. 400 sieve)
• Test Method A: multiple sieves selected to span the particle size of the material, which determine the mean particle diameter and the particle size distribution
• Test Method B: an abbreviated version of Test Method A with a few specific sieves, which determines percent passing or percent retained on a given sieve
• Method B applicability: materials without a normal particle size distribution, such as pellets and cubes
• Sieves: woven wire cloth that conforms to ASTM E11
• Units: SI values are standard for the dimensions of the wire cloth openings and the diameter of the wires
• International equivalent: none known

What are the uses of ASTM D1921 testing?

ASTM D1921 helps laboratories, manufacturers, and buyers measure the particle size distribution of plastic powders, granules, and pellets. The results show lot-to-lot uniformity and how particle size can affect handling and processing. This standard:

• Determines particle size distribution, which shows lot-to-lot uniformity.
• Gives manufacturers and buyers a consistent way to compare production lots.
• Helps processors judge how particle size affects material handling and, sometimes, processing.
• Lets laboratories report the mean particle diameter through Test Method A.
• Lets laboratories report percent passing or percent retained on a given sieve through Test Method B, including for pellets and cubes.
• Directs users to sedimentation test methods when particles are smaller than about 38 µm.

What materials can be tested under ASTM D1921?

ASTM D1921 applies to plastic materials in powdered, granular, or pelleted form. The mean particle size calculation applies to materials with a normal particle size distribution, while Test Method B suits materials without one, such as pellets and cubes. The material must flow freely, and the laboratory conditions should match the material’s temperature and humidity when possible. Dry sieving does not measure particles smaller than about 38 µm.

Why is ASTM D1921 important?

Particle size affects the handling characteristics of plastic materials and sometimes affects the processing characteristics of some polymers. ASTM D1921 gives laboratories one dry sieving procedure, so they can compare size distributions between lots and check lot-to-lot uniformity.

ASTM D1921 Testing process and data collection

The technician homogenizes the lot where possible and takes a representative sample. The technician cleans and tares the sieves, nests them with the coarsest on top, and weighs the sample onto the top sieve. The mechanical shaker runs for 10 min, and the technician then weighs each sieve with its retained material. The technician subtracts the tare masses to find the net weight on each sieve and calculates the percent retained. For Test Method A, the analyst also calculates the mean particle size.

Common challenges and troubleshooting

Static charge is a common problem because some materials develop a charge during sieving, and the charge biases the result toward coarse particles. Technicians add an antistat for these materials, record which one they used, and keep in mind that the choice of antistat can also shift the result. Too much material on a sieve blinds it and also biases the result toward coarse particles, so technicians choose the sieves and the sample weight to avoid overloading. Sieves with wavy or torn wires no longer conform to ASTM E11, so laboratories do not use them. Technicians clean the sieves with a brush, vacuum cleaner, or air hose before each test, and they wash them periodically with soap and water or a suitable solvent for some materials. Sieving and cleaning release fine plastic particles and antistatic agents into the air, so operators avoid breathing them, and the user must set up appropriate safety, health, and environmental practices.

• The laboratory reports the percentage of material retained on each sieve with its corresponding sieve size.
• Analysts check that the net weights add up to at least 98 % of the sample and repeat the work if they do not.
• Analysts calculate the mean particle size only for materials with a normal distribution, and report the calculation method with the result.
• The laboratory reports the sample weight and the antistat or slip agent used, so others can repeat the measurement.
• Engineers compare the percent retained between production lots to judge lot-to-lot uniformity.
• Engineers use Test Method B results, percent passing or percent retained on a given sieve, for materials such as pellets and cubes that lack a normal distribution.

Link to ASTM D1921