Question: Can a vibratory sieve shaker be used in place of a rotary and tapping device when running the ARM Test Method for Particle Size of Rotational Molded Powders?
The test method sprecifies devices that impart a rotary motion and tapping action, such as devices from WS Tyler. Would a device like the Gilson SS-10 be acceptable, provided the sieve stack specified in the method is used?
Dr. Nick: When this test method was written (2015), the focus was very much on North American practice. Most rotomolders in North America use a “Rotap” unit to shake the sieve deck and move powder from the top sieve to a full distribution of fractions. As the test method states, the Rotap produces a circular horizontal swirl with a regular downward tap.
Practice in the rest of the world is mostly to use vibratory shakers, like the one you have. I’m not aware of any studies comparing results from the two different machines.
However: I suspect that the results from each machine will be broadly similar. The main function of the movement is to give the original sample enough disturbance that smaller particles can progressively drop through the sieve deck and get caught on whichever sieve has gaps of similar dimension.
My guess is that the main difference would be found if any of the particles were elongated in shape. The vibratory deck might allow these particles to up-end and pass through a sieve gap that would not be possible with the Rotap motion. However, this would cause a relatively minor difference to the final distribution.
For rotomolders, the key features of the particle size distribution (PSD) center around the largest and smallest fractions.
Excessively large particles in a powder may not melt out sufficiently when rotomolded and could cause an imperfection on the inside surface of the part. The photo shows a real-world example of this. For this reason, many powder specifications stipulate zero particles above 600 micron (US Mesh 30).
It is important to have some fine particles in a roto powder (usually defined as particles passing a 150 micron — US Mesh 100 — sieve). Fine particles get picked up first as the mold heats up and will help to create a pinhole-free outside surface. For this reason, many powder specifications stipulate that at least 5% by weight of the sample should pass through the finest sieve.
An excess of fine particles will retard the flow of powder around the mold during rotomolding. For this reason, many powder specifications stipulate that no more than 15% by weight of the sample should pass through the finest sieve.
Health warning: There is no such thing as a “standard” PSD specification. The different powder suppliers all seem to have variants on the theme described above. This is not an exact science.
I’ve recently amended the 2015 version of the ARM procedure, after comments from members that it appeared too prescriptive. My amendments are currently going through our technical committee process for ratification.
Bottom line: You’ve got very little to worry about by using a shaking mechanism that is different from the one specified. I commend you for measuring something — some molders don’t bother. The important thing is to use tests like this to spot major trends away from your normal practice.
Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.
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