INTRODUCTION
The manufacture of plastic goods involves a wide range of processes and materials. In addition to plastics goods manufacturers per se, an element of plastics manufacturing may also be encountered in other occupancies for the production of ancillary components.
Foamed/cellular plastics are excluded from these notes owing to their special hazards.
PLASTIC MATERIALS AND THE PLASTICS INDUSTRY
Plastic material may be loosely defined as a material of an organic nature, consisting of a high molecular weight substance, which is usually shaped by heat and/or pressure at some stage of its manufacture, during which the material flows into the shape of the mould or die and on release of the heat or pressure retains its new shape at ordinary atmospheric temperatures and pressures.
The industry can be considered in three sections:
Synthesis. The manufacture of the raw plastic material usually involving polymerisation at high temperatures and pressures. This is normally considered as part of the Chemical Industry and is excluded from these notes.
Compounding. The mixing of the plastic material with various fillers, colourants, plasticizers etc. Often done by the original chemical manufacturer or by specialist compounders. There is an increasing trade in the reprocessing of plastics which can include an element of compounding.
Conversion. This comprises the main plastic goods manufacturing industry involving moulding, forming, fabrication and a range of other processes.
Plastics are normally divided into two classes:
Thermoplastics: those which flow repeatedly and can be remoulded almost indefinitely. Examples are PVC, polyethylene, polypropylene and polystyrene.
Thermosetting plastics: those which flow only for the initial moulding and cannot be remoulded. Examples are melamine and formaldehydes.
FIRE PROPERTIES
As with other organic materials, most plastics will burn readily and the majority of plastics ignite at temperatures in the region of 300oC. Most plastics have high calorific values which mean that for a given weight of fuel they give off a high amount of heat compared with other substances. It is this factor which usually makes plastics fires so intense. The degree of combustion and burning rate, however, varies according to the type and composition of the material.
Other properties of plastics are their ability to melt, possibly leading to a flowing fire. This has been a feature in some very large fires in the UK. Also, some plastics give off highly toxic or corrosive fumes which can cause additional damage and difficulties in fire fighting and pose serious environmental issues.
COMMON PROCESSES
Some of the more common manufacturing processes and their related fire hazards are briefly described as follows:
1. Injection Moulding. The most common method of producing thermoplastic mouldings. Granules are fed into a machine hopper from where the required amount is pushed by a feed shoe into an electrically heated barrel. Molten plastic is subsequently forced by a piston or screw through a nozzle into a mould which is normally unheated. The mould opens releasing the finished article and the cycle repeats.
Machines are becoming increasingly larger and more complex with computer control and use of multi-station machines.
Granules may be dried prior to moulding in electric ovens. Minor finishing and packing operations are frequently carried out alongside the machines. Waste is commonly re-granulated.
Potential Hazards:
Electrical
Hydraulic oils
Overheating
Portable LPG torches for cleaning screws or moulds
Combustible packaging
Re-granulating of waste
Newly moulded goods may still be hot
Tool room - spark erosion machines
Unattended operation
2. Compression Moulding. Used almost exclusively for thermosetting plastics. Male and female moulds are used, one half being fixed and the other movable on guides and operated by oil or water hydraulic power. Heat is applied to the moulds in the form of steam, hot water, gas or most commonly by electricity. Extremely high mould pressures are used.
Raw materials are in powder form which may be pre-pelleted and heated prior to moulding. Accumulations of charred flashings or waste are common.
Potential Hazards:
Hydraulic oils
Overheating
Electrical
Dust explosion - flash grinding or buffing.
3. Extrusion Moulding. Mostly of thermoplastics for the manufacture of pipe, rod, strip or like products. Raw materials are hopper fed into a screw in a similar manner to injection moulding. The moulded plastic emerges as a continuous stream and usually passes through a water trough to cool and harden. Some products pass through a naked gas flame (flame finishing) to polish the surface.
Potential Hazards:
Electrical
Overheating
Heating elements left on when screw is not turning
Flame finishing
Friction
4. Blown Film Extrusion. Process basically as extrusion except that the machine is vertical. The plastic is extruded as a tube with compressed air blown through a hole in the die to expand the tube into a large bubble. The plastic cools and then passes through nip rolls and taken up on a roller. Normally for the manufacture of polythene bags, which may be subsequently heat sealed and printed, or slit to produce sheet.
Potential Hazards:
As per extrusion
Flammable inks
Static electricity
5. General Fabrication. The manufacture of products from semi-finished stock in the form of sheet, tube etc., predominantly involving thermoplastic materials. Various processes include sawing, drilling, turning, grinding and polishing, hot bending, welding by hot air torch, high frequency, ultrasonic; friction or electrical heaters. Highly flammable adhesives and cleaning solvents may be used.
Potential Hazards:
Flammable liquids
Welding and heating
Friction
Dust explosion - grinding and polishing
6. Vacuum and other Thermoforming. A post-forming operation of thermoplastic sheet or film. The sheet is softened by heat, usually by infra-red electric heaters or in an oven, and pressed over a mould by means of vacuum or compressed air. Shrink wrapping where the plastic is moulded directly over the article being covered, or blister packing, are common in many packing operations.
Potential Hazards:
Electrical
Overheating
Printing with flammable inks
7. Blow Moulding. Thermoplastic material is extrusion or injection moulded. A split cold mould is then clamped around and compressed air injected which forces the wall of the tube to expand to the shape of the mould. After cooling the moulding is ejected and excess material removed. Very common in the manufacture of the familiar PET (polyethylene terephthalate) bottles.
Potential Hazards:
As for extrusion
Printing with flammable inks
8. Reprocessing. Re-granulating of waste thermoplastics is a common ancillary feature in many plastics risks. There are however specialist firms which deal in recycling of scrap material. Materials are normally sorted into types, possibly cleaned, and then granulated or shredded. May be followed by mixing with various additives and re-compounded to produce fresh granules for re-moulding.
This is a highly unattractive part of the industry with a poor loss record.
Potential Hazards:
Dust explosion
Friction and spark ignition from tramp metal
Storage of waste plastics in the open
Housekeeping issues
9. Fibreglass (GRP) Moulding. A common process used in boat building and the manufacture of plastic furniture etc. It usually involves the hand laying up of glass fibre matting as a reinforcing material and polyester resin on a metal, timber, plaster or fibreglass mould, which has been initially covered with a release agent.
The polyester resin is mixed with a catalyst (normally an organic peroxide) to promote polymerisation. This is an exothermic reaction which can lead to spontaneous combustion if not done under carefully controlled conditions. Mixed resins if left unused, or spillages not cleared up, may ignite. Normally resins are delivered pre-accelerated but occasionally a separate accelerator is used. Accelerators are usually metallic salts (typically cobalt naphthalene) and if mixed directly with catalysts will give rise to a serious fire or even an explosion.
Moulding is usually carried out at ambient temperatures but in some cases artificial heating is used to promote curing.
Copious quantities of acetone (very low flash point FP -18⁰C) are commonly used for cleaning, often from buckets or other open containers. Finishing operations may include trimming, grinding or sanding, which will produce dust.
In some instances, a spray method is used. Glass rovings are fed through a spray gun where they are chopped and mixed with the resin and catalyst.
Fibreglass moulding is an extremely hazardous process.
Potential Hazards:
This is a particularly messy process and effective cleaning arrangements are essential.
Flammable liquids.
Use of portable heaters to promote curing.
Unsuitable methods of space heating.
Spontaneous combustion.
Dust explosion
RISK ASSESSMENT
The following features, amongst others, need to be identified and evaluated as part of an effective risk assessment:
Management. Key features include:
Good organisation with separate storage areas for raw materials, finished stock, packaging and any other combustibles. In larger risks separation should be by compartment walls of at least 120min fire resistance. Finished mouldings should not be allowed to accumulate around machines.
Good housekeeping standards are necessary to prevent accumulations of trade waste and other litter. Cleanliness around machines is vital. Fibreglass moulders in particular have problems with spilt resins and cardboard or plastic sheets are frequently used to protect the floors. This procedure is acceptable providing the sheets are changed frequently and disposed of in a safe manner. Spilt resin or catalyst should be cleared up quickly because of the risk of spontaneous combustion and mixed resins surplus to requirements are to be removed from buildings prior to closure.
Strict adherence to smoke-free regulations.
Planned maintenance systems are desirable in all cases but are essential where machines are run 24 hours. This is particularly common in the injection moulding industry.
2. Overheating. Most processes involve some form of heating and to maintain correct operating temperatures, cooling is often necessary. Overheating may be caused by:
Failure of cooling circuits. Absence or failure of interlocks.
Faulty heating arrangements or thermostats. Incorrect setting of thermostats, commonly encountered when changing over from one type of plastic to another with a lower moulding temperature.
Prolonged heating, even at correct temperature. Common causes are operator failure, failure of timing mechanisms or cut outs and jamming of ejector apparatus.
Stoppage of reel fed material through a heat source. Interlocks may be incorporated providing automatic shutdown of heating in the event of stoppages, but these may fail.
3. Friction. Fires may arise from the following causes:
Failure of cooling circuit and/or failure or absence of interlocks.
Incorrect mix of the raw materials or the absence of lubricants may give rise to excessive frictional heat. Foreign bodies in the mix might have a similar effect especially in the regrinding of waste plastic.
Restriction or blockage of outlet from machine or roughening of working surfaces due to abrasion or corrosion.
Excess material.
Common external causes such as inadequate lubrication and worn machine parts.
4. Spontaneous Combustion. Some manufacturing processes involve an exothermic reaction accompanied by polymerisation, and fires caused by poor mixing or incorrect proportions are not uncommon. This particularly applies to the fibreglass and resin casting processes (Technical Bulletin 41 – Oxidising Agents refers).
5. Flammable Liquids. Frequently present in the form of cleaning solvents, adhesives or in finishing processes. Fibreglass moulding processes use large quantities of flammable resins and cleaning solvents and good ventilation is essential. Where large quantities of resin are used, they should be piped in from segregated storage areas.
6. Dust Explosions. As an organic material, most plastic dusts may present an explosion hazard. Examples of operations which may give rise to this hazard are handling or transferring fine powders, machining and polishing, pelleting and regrinding of waste material.
7. Hydraulic Fluids. Injection and compression moulding machines commonly use mineral hydraulic fluids under considerable pressure, usually in conjunction with flexible hoses. Unless a high degree of maintenance is in operation leaks will be a regular problem. Regular inspection of hydraulic hoses, pipes and couplings should be carried out and documented. Bursts are likely to give rise to fine mists or sprays of oil which can easily be ignited, even with relatively high flash point oils. Low oil level sensors and interlocks in the form of an oil pump interlock should be installed on larger oil reservoirs/accumulators to enable automatic hydraulic system shutdown in the event of a sudden loss. Mineral absorbents should be used to soak up leakages and drip trays provided if necessary (Technical Bulletin 14 refers).
8. Static Electricity. Most plastics are good insulators and during processing or handling considerable static charges may accumulate. This becomes a hazard when flammable vapours or explosive dusts are present for which special protection considerations apply (Technical Bulletin 35 refers).
9. Unattended Operations. With the development of automation many machines are now capable of running for long periods of time without human supervision. Combined with the large capital sums invested in these machines it is becoming more common for them to be run unattended overnight. This particularly applies to injection moulding. As a result, the provision of sprinkler protection for such risks may need to be considered. (Technical Bulletin 15 refers)
These notes are intended to provide general guidance to Consultants who may not be totally familiar with the full range of plastics goods manufacturing processes. For more information and advice reading a specific case, Consultants should contact the RSS Technical Helpline.
