Tire manufacturing is a complex process, but can be broken down into three main processes: tire preparation, tire production and tire building. Preparation includes fabric cord manufacturing, bead and belt steel cord manufacturing and Banbury mixers. Tire production includes fabric cord calendering, fabric ply cutting, steel belt calendering, steel belt cutting, bead assembly, inner liner calendaring and tread and sidewall extruding. Tire building includes tire curing, visual inspections, x-ray inspections, balance inspections and force and movement inspections.
Preparation includes the measuring and mixing of raw rubber, process oils, silica, chemical additives, carbon black, bulk fillers, and rubber chemicals in pre-determined proportions. The finished mixture is known as rubber compound.
Before being added to the mixer the, raw rubber may need to be cut into small pieces on a bale cutter or guillotine. Ingredients are mixed together and fed into an enclosed mixing chamber via a feed hopper and mixed by the shearing action of two winged rotors and the walls of the mixing chamber (e.g., Banbury mixers). The final process of Banbury mixers is extrusion that involves forcing uncured rubber through a die under pressure to form a shaped profile or sheet. Rotating knives (or die face cutters) can then convert extruded material into pellets or slugs for further processing. Rubber can be dumped direct from an internal mixer into an extruder below it. The major hazards with Banbury mixers and extrusion are the use of ignitable liquids or powder (e.g., organic peroxides, sulphurs, azodicarbonamide, calcium and zinc stearates), dust explosion hazards in powder handling, hydraulic oil hazards and mechanical breakdown of extruders.
Tire production begins with calender machines that have several horizontal rolls heated or unheated through which rubber is passed under extreme pressure. Rubber sheets are created as per required thickness, and then receive an additional coat layer of liner or ply onto the rubber compounds. The liners or ply are treated with solvents to achieve a good bond with rubber in addition to drying in a hot air oven prior to coating. The major hazards associated with calendaring is the use of flammable solvents, risk of static charges during the rolling states, and heated ovens.
Carcass assembly and belt assembly occur concurrently. Carcass assembly involves the inner liner (thin airtight rubber sheet) coated in a ply textile rubber compound. Next beads are added via steel wire covered in rubber compound wound into a ring. Ply layers are then turned up around the beads and rubber compounds forming the sidewalls, then sides are turn down. Belt assembly and carcasses are pushed into each other in the last step and are joined together using compressed air. At this point, tire production is completed, and tires are called “green tires.”
Tire assembly involves placing new formed green tires in a mold and vulcanizing them at a temperature of over 340°F. The tire leaves the vulcanized station as a patterned, finished tire. After the mold is closed, a bladder is inflated inside the green tire which presses it against the mould from either high-pressure steam or hot water. Depending on the tire type and size, vulcanization takes approximately 10 min. at temperatures between 340 to 390°F, at a pressure of about 320 psi (22 bar).
Tire composition traditionally includes nine layers, from inside to outside – inner liner, carcass ply, lower bead area, beads, sidewall, casing ply, belts and tread.
- Inner Liner: An airtight layer of synthetic rubber (the modern equivalent of an inner tube).
- Carcass Ply: The layer above the inner liner, consisting of thin textile fibre cords (or cables) bonded into the rubber. These cables largely determine the strength of the tire and help it resist pressure. Standard tires contain about 1,400 cords, each one of which can resist a force of 33 lbs. (15 kg.).
- Lower Bead Area: This is where the rubber tire grips the metal rim. The power from the engine and braking effort is transmitted from the rim of the tire to the contact area with the road’s surface.
- Beads: They clamp firmly against the tire’s rim to ensure an airtight fit and keep the tire properly seated on the rim. Each wire can take a load of up to 4,000 lbs. (1,800 kg.) without risk of breaking.
- Sidewall: Sidewalls protect the side of the tire from impact with curbs and the road. Important details about the tire are written on the sidewall, such as tire size, pressure, and speed rating.
- Casing Ply: Casing largely determines the strength of the tire. Casings are made up of very fine, resistant steel cords bonded into the rubber. This means the tire can resist the strains of turning and doesn’t expand due to the rotation of the tire. Casings are also flexible enough to absorb deformations caused by bumps, potholes and other obstacles in the road.
- Cap Ply (or “zero degree” belt): Cap is an important safety layer that reduces friction heating and helps maintain the shape of the tire when driving fast. To prevent centrifugal stretching of the tire, reinforced nylon-based cords are embedded in a layer of rubber and placed around the circumference of the tire.
- Crown Plies (or belts): The belt of the tire provides the rigid base for the tread.
- Tread: Tire tread provides traction and turning grip for the tire and is designed to resist wear, abrasion, and heat.
The main hazards involved with tire manufacturing includes the use of ignitable liquids in the compounding stage, storage of finished tires and combustible dust (from re-treading operations). Ignitable liquids utilized in tire manufacturing are highly hazardous and industry recommended best practices and guidelines should be followed for the handling, transfer, storage of flammable liquids. For storage areas, storage limitations should be followed as recommended in the standards considering storage type, pile dimensions aisle width and clearance to ceiling. In addition, an effective emergency response plan facilitating early discovery, prompt employee action coupled with adequate sprinkler protection will make it possible to control fires at the early stages.
Adequate protection of tire storage is critical to overall site fire protection. Fire in a rubber object tends to burrow into the material. The tire’s hollow, toroidal (doughnut-shaped) form allows flames to grow on its inner surface while at the same time shielding flames from sprinkler spray. The “Chimney Effect” is common when a fire starts in a row of tires, the air inside the stack becomes warmer and less dense than the outside air and begins to rise. As it rises cool air from the room is drawn in feeding the fire. Tires should be stored either on side, on tread or laced. These storage methods minimise the amount of space needed for storage and minimizes the amount of surface area exposed in a fire. Tires are also more stable as they are interlocked in one of the three storage patterns.
Automatic fire protection for the tire storage greatly depends on storage height and height of the building. NFPA recognizes tire manufacturing as an Ordinary Hazard Group 2 occupancy (rubber processing). Tire storage is very demanding. Additional factors that should be noted during the installation or inspection of automatic sprinkler protection is the use of permanent or portable racks. Additionally, both NFPA and FM Global require that protection be provided for steel columns within storage areas. NFPA 13, Standard for the Installation of Sprinkler Systems, states that storage between 15 to 20 ft., one sidewall sprinkler directed to one side of the column at the 15 ft. level is required. Likewise, if storage height exceeds 20 ft. that two sidewall sprinklers, one at the top and the other at the 15 ft. level are required.
Overall superior administration of management programs is critical to prevent large losses. Risk Logic can recommend and help develop preventive maintenance and property loss control programs at your facility. Please contact us to schedule a property survey at your facility by one of our engineering specialists.
References include but are not limited to:
|FM Global||FM DS 1-44: Damage-Limiting Construction |
FM DS 2-0: Installation Guidelines for Automatic Sprinklers
FM DS 3-26: Fire Protection Water Demand for Non-storage Sprinklered Properties
FM DS 5-1: Electrical Equipment in Hazardous (Classified) Locations
FM DS 5-8: Static Electricity
FM DS 6-9: Industrial Ovens and Dryers
FM DS 7-17: Explosion Protection Systems
FM DS 7-29: Ignitable Liquid Storage in Portable Containers
FM DS 7-32: Ignitable Liquid Operations
FM DS 7-73: Dust Collectors and Collection Systems
FM DS 7-76: Prevention and Mitigation of Combustible Dust Explosion and Fire FM DS 7-98: Hydraulic Fluids
FM DS 8-3: Rubber Tire Storage
|NFPA||NFPA 13: Standard for the Installation of Sprinkler Systems|
NFPA 30: Flammable and Combustible Liquids Code
NFPA 68: Standard on Explosion Protection by Deflagration Venting
NFPA 69: Standard on Explosion Prevention Systems
NFPA 70: National Electrical Code
NFPA 77: Recommended Practice on Static Electricity
NFPA 652: Standard on the Fundamentals of Combustible Dust