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Oct 19, 2023

Cellular Plastics Processes

Some processes involving cellular plastics (CP) may increase fire risk. Below are precautions that can be used to reduce the risks for some of those processes. When a naked flame is passed over PU

Some processes involving cellular plastics (CP) may increase fire risk. Below are precautions that can be used to reduce the risks for some of those processes.

When a naked flame is passed over PU foam for bonding it can be overheated and become a fire risk. To reduce the risks of fire from the flame bonding process it should (where practicable) be separated from other operations and be contained within a fire-resisting enclosure.

The gas burner should:

Escape of fume into the workroom should be prevented by enclosing processes and providing local exhaust ventilation (LEV). LEV systems should:

Newly-bonded materials should be transferred to a well-ventilated area and kept there until they stop fuming.

When applying adhesive on at the coating head of the machine to bond laminates there is an increased risk of fire because of the flammable solvent that is used.

There are non-flammable adhesives available for most applications that should wherever possible be used. If it is not possible to use non-solvent adhesives then additional precautions are needed. Fire separation between adhesive lamination and other processes is recommended.

The bonding machine should:

If drying ovens are used:

More advice on storing highly flammable liquids can be found in the fire and explosion pages.

Fabrication is the process of manually bonding together sections of PU foam and/or other materials to make an article or component. This is normally done by an adhesive. There can be a fire hazard from large amounts of vapour given off when large areas of freshly-applied adhesive are exposed. The vapour can be worse if the adhesive is spray applied.

To reduce the risks:

If spraying solvent-based adhesives then controls are needed to ensure a flammable atmosphere does not build up. Spraying should be carried out, as far as is reasonably practicable, inside a spray booth or enclosure constructed of sheet steel and provided with mechanical LEV.

Where it is not reasonably practicable and spraying is done at a fixed location eg a work bench then mechanical exhaust ventilation should be used as near as possible to the source of vapour emission. There should be a minimum air flow rate of 0.7 m/s (approximately 140 ft/min) at collecting points, which should be positioned to draw vapours away from the operator.

Shaping sections of flexible PU foam can be carried out mechanically by grinding or routing. This generates lots of dust and granular material. Dust lying on surfaces can be a fire hazard and fine airborne dust can be an explosion hazard.

Dust particles are often electrostatically charged and can be strongly attracted to certain surfaces, If dust on surfaces is ignited the fire can travel quickly along it. Fine dust can also penetrate motor casings and bearings and can cause electrical equipment to become overheated increasing the risk of fire.

Dust and fine particles produced during grinding and routing operations should be controlled. An enclosure with provision of mechanical exhaust ventilation as near as possible to where dust is being generated should be provided. A dust collector such as a dry filter and/or a suitable cyclone should be provided in accordance with the recommendations contained in the booklet HS(G)103 Safe handling of combustible dusts.

Where dust penetration of electrical equipment is a problem dust-tight casings to the correct electrical rating should be provided. If there is the risk of a build up of electrostatic charges then steps should be taken to prevent this. More information on electrical safety in the workplace, including information on electrical safety in potentially explosive atmospheres can be found in the electricity pages.

Regular cleaning to reduce dust levels should also be a part of your controls for managing the fire risks.

When using heated resistance wires to process blocks of flexible PU foam normally the risk of ignition is low. However, fires may result if the temperature of the wires is set too high or if the wires short-circuit during use. Ignition may also occur if the PU feed to the machine stops so the foam stays in contact with the hot wire. In each of these situations foam may be heated to temperatures above 300-400oC causing toxic fumes and flammable vapours to be given off.

To reduce the fire risks from hot-wire cutting:

Crumbing is the process of reducing down the size of plastics eg PU foam or polystyrene for use as a cushion filler or for recycling. Most commonly this is done by a granulator. Modified rag-pulling machines, which tear PU off-cuts to fine shreds and hammer mills, which grind the material can also be used.

Unless strict controls are in place for producing and using crumb there can be a significant fire hazard. If not contained, the crumb is likely to spread and lye on floors and other surfaces increasing the risk of spread of fire in a workroom. Dust and fine crumb produced in some plant can lead to a dust explosion.

To reduce the risks:

More information on electrical safety in the workplace, including information on electrical safety in potentially explosive atmospheres can be found in the electricity pages.

Cutting processes include horizontal, vertical or inclined cutting on band knives, peeling, press cutting and rotary cutting. PU foam is not likely to be ignited during cutting processes but sparks from grinding wheels used to sharpen blades can ignite dust accumulations inside guards. In addition, cutting lubricants may contain highly flammable liquids (HFLs) that give off vapour when used. The vapour could be ignited eg by the discharge of static electricity and so could be a fire risk.

To reduce the risks:

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