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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 remoulding.

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 combust


RISK ASSESSMENT

The following features, amongst others, need to be identified and evaluated as part

of an effective risk assessment:


1. 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.

The Plastics Industry

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