What causes oven fires and how to respond

ARM Technical Director Nick Henwood

A rotomolder member of ARM asked about recommended actions in relation to fires in rotomolding ovens. These fires are typically caused by powder spilling out of a mold during the cooking process.  This can be due to:

  • Incorrectly closed clamps, or clamps that fail, causing the mold to open as it rotates
  • A missing vent pipe, either because the operator forgot to insert it or because it was inadequately fixed and fell out during mold rotation
  • Missing vent fill media, either because the operator forgot to insert it or because it was incorrectly inserted and fell out during mold rotation
  • A damaged parting line that is leaking powder

Once the powder is released from the mold, most of it will fall to the oven floor where, as a minimum, it will melt and causes a mess.  With prolonged heating, it may catch fire spontaneously and create smoke.  Some of the powder (the finer sized fractions) may be drawn into the burner duct by the circulation fan.  This finely divided, combustible, material will then come into contact with the burner flame and may create an explosion.  Alternatively, repeated spillages may coat the sides of the burner duct and create an on-going fire hazard.

Good procedures for mold filling and mold closing should eliminate the occurrence of accidental spillages.  If a spillage does occur, you should get it cleaned up as soon as is practicable.  However, no-one should ever enter the “machine” envelope unless it is safe to do so.  This will involve machine isolation, allowing hot surfaces to cool and wearing the correct PPE.

Have your procedures written down and immediately available to consult, in case a fire happens.  Make sure that there is a designated, named, individual responsible for taking charge of the situation and liaising with first responders.  Written fire procedures should be posted in full view.

Clearly, the very best approach to fire safety is to prevent fires from happening in the first place! 

Emergency response if a fire should occur

Responses to fire and explosion will depend on the precise circumstances prevailing.  However, there are some general guidelines that should be followed, in the absence of other extenuating circumstances.

If a powder explosion occurs, this will be evident from sound and, potentially, damage visible external to the machine.  If there is a fire without an explosion, the first obvious signs are likely to be smell and visible smoke.

The temptation to open the oven doors should be resisted in most cases.  There is a strong possibility that opening doors or hatches will make things worse, by allowing air to rush into the oven space and accelerate any combustion.  The best strategy will usually be to allow the fire to burn down, before opening the doors.

It makes sense to try to make a visual assessment without opening doors.  In many machines, it may be possible to see into the oven through the slot provided for arm travel, or through other openings.

In most cases, it will be advisable to stop both the duct circulation fan and the oven exhaust fan.  This will minimize the throughput of air, which supplies fresh oxygen to the fire.  You should be aware that, in other oven industrial situations, it is sometimes recommended not to stop air circulation, in case the fire has been caused by accumulated flammable vapors inside the oven.  This is not likely to be an issue in a rotomolding context.

Isolation of gas and electric services to the machine should be initiated as soon as possible.  This will interrupt the action of other arms, in the case of a shuttle or carousel machine.

When safe to do so, use a dry powder extinguisher, if available.  Be mindful that, on larger fires, dry powder may only damp down the fire and solid residue may continue to smolder and then possibly re-flash.

Spraying water or water-extinguishants directly on to hot metal components is likely to lead to excessive mechanical damage and could precipitate a steam explosion.  If water is used, it should be applied in short bursts of fog, allowing steam to fill the oven and saturate the contents.

If external assistance, eg the local Fire Department, has been called, make sure that someone who fully understands the machine set-up is on hand to answer any of their immediate questions.  Don’t assume that external first responders will immediately understand all the ramifications of your process; they may never have seen a rotomolding machine before.  Without direction, they are likely to use excessive water and may cause unnecessary damage and production downtime.

The oven exhaust fan should only be used to clear smoke once it is certain that the fire is completely extinguished.

You may choose (hopefully in collaboration with Fire Department personnel) to adopt a wait-and-see strategy and allow the fire to burn out.

If the fire has spread to building components, it will be necessary to employ more general evacuation and firefighting procedures.

In all cases, prioritize human safety over machinery and building damage.

Once the incident and its after-effects have been tackled, instigate through check of equipment, including ductwork and chimneys, before attempting to re-start the machine.

Any significant incident should be fully investigated, in order as to learn lessons for the future and, if necessary, to modify procedures.  Such investigations are likely to be most productive if a “blame culture” can be avoided.  Invariably, safety incidents have several causes, including faults or failures in systems and procedures.

Hopefully the above comments will be useful, to help you formulate robust procedures for your operation.  As previously stated, these are general guidelines and may be varied according to specific set-ups and circumstances.

ARM developed an eight-part Health & Safety Webinar Series as a free-to-member service.  Several Modules of the series discuss fire and explosion risks, so this is an extra resource of useful information.

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.

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