Ask Dr. Nick: Ignition Properties of Rotomolding Powders

Question: Has anyone ever tested the roto powders for ignition properties? Will rotomolding powders ignite and if so, under what conditions?

Dr. Nick:

I’m not aware of any formal data on the ignition potential of rotomolding powders available in the public record.  However, there should be no doubt that, under certain conditions, the materials we use could constitute an explosion risk.

Almost any powder, including chemically inert materials, can create an explosion in favorable conditions.  The main risk multiplier is the size of the powder particles; the finer the powder, the greater the explosion risk.  At some point, a stream of air containing fine material will behave like an explosive gas.

In a normal roto powder, “fines” (fractions passing a 100 mesh sieve) are kept low, for process reasons.  This helps mitigate the explosion risk, especially for small quantities of material.

However, there have been documented instances where a large powder spill in an oven has caused an explosion.  The relatively fine fractions of the spilled material become airborne, get sucked into the circulation duct and then explode when the dust cloud encounters an ignition source (in this case, the burner).  In one case I know of, this happened in a rocking oven machine: a large mold was insecurely clamped and fell open on entry, creating the conditions just described.  The oven end panel was blown, the resultant fireball came into contact with the roof of the factory and a major fire occurred.

I’ve seen ovens in other industrial applications that are fitted with explosion panels (see photo).  These are formed from insulation blocks that are supported inside the oven by a large opening steel mesh.  If an explosion occurs, the pressure wave is released by failure of the panel, rather than rupture of the solid walls of the oven.  This could be a useful, relatively low cost, safety addition for rotomolding machines.

I have also heard that, in the past, explosions have happened inside large molds that use steel wool for vent packing.  This material can become sufficiently hot to act as an ignition source.  Anecdotally, I’ve been told that the risks are increased if crosslink PE is being molded.  However, I have not been able to find a definite confirmation of this, backed by contemporaneous information.  Steel wool is a less than optimum vent packing medium, for a number of other reasons.

Another area of potential concern is powder storage silos and powder conveying systems.  Here you have a large quantity of powder which, in some cases, can become airborne.  If this creates a relatively high concentration of fines, the risk of explosion is increased.  In such systems, typical protections will be explosion panels and earthing, to dissipate static accumulations.

A picture containing steel, industry, pipe, clothing

Description automatically generated

Initial explosions in a plant can be followed by more serious, secondary, explosions.  In such cases, the first explosion loosens material that has accumulated on roof trusses, which becomes airborne and is then prone to further ignition.  Often the secondary explosion causes worse damage that the primary one.  Good housekeeping and regular cleaning can mitigate this risk.

Various modules of ARM’s Safety Webinar series cover explosion risks.  This is available in the “Members” section of the ARM website; I recommend that you review this valuable free-to-member information source. Members can access the webinar directly here.

Dr. Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30+ years experience in rotomolding, specializing in the fields of materials development and process control. In 2022 he was inducted in the Rotational Molding Hall of Fame. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.

3 responses to “Ask Dr. Nick: Ignition Properties of Rotomolding Powders”

  1. Jon Ratzlaff has circulated, in years past, a DVD that highlights the explosive nature of powders. The video discussed a case study of a primary powder explosion that then caused a secondary explosion from dust that had collected on flat surfaces throughout the facility (like machines and rafters and such).

  2. The USCSB video is viewable on YouTube
    Combustible Dust: An Insidious Hazard (just under 30 min. long.)
    Worth watching the whole thing but at the 12 min mark they show “fine” polyethylene dust being ignited.

    I used the complete video as part of training on Housekeeping when I ran a plant in Ohio.

Leave a Reply

%d bloggers like this: