Is it a common strategy to use pellets instead of powder for rotomolding? Are there any advantages to using pellets over powder during processing? I want to move away from powder and its potential to become airborne.
Dr. Nick: The main raw material used for rotomolding (polyethylene, PE) is originally produced in the form of pellets; small (average diameter approx. 3mm, 1/8 inch) spheroids, which are generally produced from a large extruder.
For rotomolding, these pellets are pulverized to produce a powder. This powder has a wide size distribution, but the average particle size is usually around 300-350 microns, 50-45 US mesh. Therefore powder represents an average of 10 times the size reduction from the original pellets.
Can you rotomold PE pellets and what will you produce? Yes you can, but you would encounter two main problems.
Firstly, although the pellets would probably fuse together, they are unlikely to fully melt and flow to form a cohesive and contiguous solid layer. Put simply, there would still be a lot of holes and voids in the part you produced. By the time you heated the pellets long enough, to get everything to melt out properly, the PE would probably have started to degrade and break down.
Secondly, you might find that the wall thickness distribution of rotomolded pellets was non-optimum compared to using powder.
Despite these difficulties, the use of pellets is attractive from the point of view you mentioned in the question, ie issues involved in material handling. There is also the potential for cost reduction, by eliminating the whole pulverizing step.
In the late 1990’s and 2000’s, several suppliers experimented with PE micro-pellets. This was a technology borrowed from the precolor (aka masterbatch) industry, where much smaller-sized pellets were produced using a special underwater die system. To begin with, they experimented with PE micropellets of approx. 500-600 micron (25-20 US mesh) diameter, but producing rotomolded parts that were pin-hole free was challenging. Acceptable parts were finally able to be produced when the micropellet size was reduced somewhat (to below 400 micron, 40 US mesh).
Whilst rotomolded parts made with smaller-sized micropellets were acceptable, there was a commercial penalty in most cases. The problem is that installing a micropellet die on a normal commercial extruder (whether twin- or single-screw) will significantly reduce the machine output, usually to the point where the additional cost of extrusion is greater than the running cost of the pulverizer. So micropellet technology has not been widely adopted for the production of PE materials for rotomolding. It may be commercially suitable for producing very small quantities of PE custom colors (<2000 lb runs). In addition, I recently experimented with coarser micropellets and even “mini-pellets” (size approx. 1000 microns) for colored polypropylene (PP); my motivation was fuelled by the relative high cost of pulverizing PP (usually requires expensive cryogenic grinding).
Bottom line: unless you have a major incentive, you’re probably going to be stuck with using PE powder!
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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