Tire manufacturing

Tire manufacturing is a complex process,starting with the raw materials which pass through several processes to arrive at the finished tire. The scope of this article is to include both current materials and processes together with a tire manufacturing history.

Tire Construction

• Beads made of high tensile steel wire. They anchor the tire to the rim.

• Fillers are triangular strips of hard rubber, applied above the beads, to give stiffness to the lower sidewall.

• Liner made of halobutyl rubber. This replaces the inner tube.

• Plies, made of rubberised textile fabric. This gives the tire its strength.

Small tires have a single ply, larger tires have two plies. The radial tire construction gives more strength than the old cross ply, so that one radial ply is the equivalent of two crossed plies. The cords cross the carcass at 90 degrees, i.e. "Radial".

• Steel belts made of rubberised steel cord. They provide reinforcement to the tread. The cords cross at approx. 15 degrees.

• Nylon belts applied above the steel belts to improve the rigidity of the structure. The nylon cord is applied at zero degrees.

• Tread made of rubber compounds designed to give high mileage, together with good traction on wet and dry roads.

Since there is a conflict between these requirements, tread compounding uses a combination of rubbers and blacks designed to give the best compromise.

• Sidewalls protect the tire carcass and uses compounds which give resistance to cutting and cracking.

• Chafers protect the tire beads against the rim. They can be made of rubberised fabric, or hard rubber compound.

Automobile manufacturers assemble cars and trucks, made to their designs and specifications, using components supplied from a number of specialised manufacturing industries. The tire is one such component.

Tire manufacturers, by contrast, carry out the entire manufacturing process, in house, from many different raw materials.

The Manufacturing Sequence

The sequential manufacturing sequence consists of :

1. Raw Materials and Semi-Manufacturing.
2. Mixing the Rubber Compounds.
3. Calendering and Extruding.
4. Assembly (Tire Building).
5. Vulcanization (Curing).

Raw Materials and Semi-Manufacturing

Rubber is the best known material in tires. The term rubber describes the physical properties of the material, rather than its chemical composition.

Before the Second World War, the only elastomer used in tires on an industrial scale was natural rubber. After Pearl Harbor, the United States of America found itself cut off from the Far East, where by far the greatest amount of natural rubber was produced. Within a few years the U.S. was able to produce a million tons per annum of synthetic rubber. Synthetic rubbers can be "tailor made" to provide the required properties.

Today, natural rubber is still used in large quantities, mainly because of its low heat build-up properties.

• styrene/butadiene co-polymer is the most common synthetic rubber, usually abberviated to SBR. In the 1970's, oil extended SBR was used for the tread compounds, taking advantage of the better wet grip provided. Its lower cost was a big factor in its popularity. The trade-off was that this type of rubber gave lower mileage.

• Polybutadiene is used in combination with other rubbers because of its low heat build up properties.

• Halobutyl rubber is used for the tubeless inner liner compounds. The halogen atoms provide a bond with the carcass compounds which are mainly natural rubber. This inner liner replaces the butyl inner tube.

• Ingredients must be added to achieve the desired properties in the cured rubber compounds.

• Carbon black, forms a high percentage of the rubber compound. This gives reinforcement and abrasion resistance.

• Sulphur cross-links the rubber molecules in the vulcanization process.

• Accelerators are complex organic compounds which speed up the vulcanization.

• Activators assist the vulcanisation. The main one is zinc oxide.

• Antioxidants prevent sidewall cracking, due to the action of sunlight.

• Textile fabric reinforces the carcass of the tire.

Early tires were made of layers of square woven cotton fabric, coated with rubber. Hence the expression that the tread was "worn down to the canvas".

In 1918 a new type of cord fabric was introduced, where the cross threads, or "weft" were replaced by a few light strands of cotton called "picks". These were just there to hold the fabric together until it could be stabalised with a rubber coating in the Calendering Process.

By 1938, cotton was being replaced by rayon, although its use continued until the 1960s on high speed cross ply tires. This was because the cotton fibres provided a better bond to the rubber, and avoided ply separations at high speed.

By 1958 rayon was being replaced by nylon. This had cost advantages. Because of nylon's greater strength it was possible to reduce the number of plies in most tires from four to two. This brought about the introduction of the term "ply rating" , for example "4 PR". Modern radial tires are normally monoply. This takes advantage of the fact that the 90 degree cord path is more efficient.

• Polyester has been in and out of favour since the 1980s. It has a better strength/cost factor than nylon, but in the early stages there were problems with bonding polyester to the rubber compound in the cured tire. Catastrophic failures could occur if the tire overheated and the bond could then break down and leave the cords on the failed tyre to be totally exposed like white spaghetti.

• Steel cord fabric is used in truck tires for the single ply of the steel cord carcass.

Also the belts under the tread of radial tyres is made of steel cord. This steel cord consists of many fine filaments of brass coated steel wire, twisted together to give a cord like structure. The brass coating serves not only to protect the steel from rusting, but also assists the rubber to metal bonding in the curing process. Here the copper in the brass cross-links with the sulphur in the rubber, to combine as copper sulphide.

Mixing the Rubber Compounds

The raw rubber, whether natural or synthetic is called the polymer. The process of combining this with the other ingredients is called mixing. Within the industry, the terms mixture and compound are synonymous. This may seem strange, but the mixing and subseguent curing of the rubber and its ingredients is analogous to baking a cake.

The rubber polymer used to be mixed with the other ingredients on a series of open mills. However, modern tyre factories use large internal mixers. The Banbury Internal Mixer manufactured by Farrel is typical of these.

Mill mixing comprised a series of machines with pairs of large steel rollers, rotating with a small gap between them, called a "nip". As the rubber was forced through the nip it provided shearing forces to masticate and work the rubber. Natural rubber required the input of a lot of energy to break it down, so it was first fed into a mill with corrugated grooves cut into it. This is called a "cracker mill". Carbon black for mill mixing was normally obtained as a masterbatch (pre-mixed with rubber) from an external source, to avoid the handling problems of the fine powder. The curing ingredients are added as late as possible, to avoid the compound curing up.

With Banbury mixing, the process is be better controlled. The mixing is done in three or four stages to incorporate the ingredients in a logical order.

The carbon blacks are delivered to the factory in tote bins and are stored in hoppers above the Banbury tower.

The finished rubber compounds are sheeted off and stacked on pallets awaiting subsequent processing.

Calendering and Extruding

The fabrics and rubber compounds undergo a number of semi-manufacturing processes, before being assembled together on the building macnine.

Calendering

The raw carcass materials made of textile or steel are coated with rubber. This is done by passing them through a machine called a calender which has a series of rollers in a stack, normally four. The rollers are called boles. The fabric passes between the middle rollers and rubber is fed from above and below. The thickness of the rubberised fabric is controlled by the gap between the rollers. This gap is called the nip.

Extruding

The profiled slabs of rubber which go to make up the tread and sidewall of the tire are produced using an extruder. This machine forces the green rubber through a die to give the required shape.

Assembly

In the days of cross ply tires, all the components were assembled together on a tire building machine. The assembly was done on a building drum. The carcass plies were applied with the cords running at an angle, each layer in the opposite direction. Hence the term cross ply.

The change from cross ply to radial tires happened more slowly in the USA than in Europe, Since French Michelin and Italian Pirelli held the patents on Radial tires; Michelin for Steel belted and Pirelli for Textile belted. For a while, Dunlop made steel belted tires called Duraband, under licence to Michelin. Later, together with most other manufacturers, they made textile belted radial tires under licence to Pirelli. Once the Michelin patent lapsed, the entire market moved to steel belted tires, under pressure from the car manufacturers. This change of manufacturing process was known in the industry as metalisation.

For a limited period, the U.S. concentrated on bias-ply tyres. These were made with one very wide diagonal ply which, after the steel beads had been applied, was folded over itself to give the effect of two plies in the sidewall region, but three plies under the tread. This permitted the use of existing tire building machinary.

With radial, tires the assembly is carried out in at least two stages.

• First stage building is done on a flat collapsable steel building drum.

• The tubeless liner is applied, then the body ply which is turned down at the edges of the drum. The steel beads are applied and the liner/ply is turned up.

• The chafer and sidewall are combined at the extruder. They are applied together as an assembly. The drum collapses and the tyre is ready for second stage.

• Second stage building is done on an inflatable bladder mounted on steel rings. The green first stage cover is fitted over the rings and the bladder inflates it, up to a belt guide assembly.

• The steel belts are applied with their cords crossing at a low angle.

Often a nylon zero degree belt is applied above the steel belts, to make the structure work more efficiently.

• The tread rubber is then applied. The tread assembly is rolled to consolidate it to the belts and the green cover is detached from the machine.

For popular sizes, the tire building process has been automated. Each component is applied separately along a number of assembly points. Such process has a high initial investment, but gives good quality and productivity.

Curing and Finishing

The term curing is prefered to vulcanization in tire manufacturing. The meaning is the same.

The green covered are moulded in a curing machine also known as a vulcanizer, or a press.

The cover is placed over a butyl curing bladder which expands the cover to meet the mould surfaces as the press closes. Here the tread pattern is moulded onto the tire. Originally, tire moulds were in two halves. With radial tires, the tread elements are split into segments to avoid distortion to the steel belts as the mould is closing. The sidewall plates are engraved to provide the inscriptions of size etc. on the cured tire.

The tire is cured with high pressure hot water inside the butyl bladder. This has replaced steam as the curing medium, since pure steam at the pressures required would give too high a temperature for an even cure. The mould sidewall assembly is fixed to steel plates which are heated by steam.

Ater curing the tires pass to the finishing department, where they are inspected for defects. After which they are run on balance machines and the heavy spot marked. They then undergo other uniformity controls on a force variation machine. This measures the tires for radial run out, lateral run out and conicity.

Some tire manufacturing companies include:

• Bridgestone
  • Firestone
• Continental
• Cooper
• Dunlop
• Goodyear
• Kumho
• Michelin
  • B.F. Goodrich
• Nokian
• Pirelli
• Toyo tires
• Yokohama
• Doublestar Huaqing
• Double Coin
• Long March
• Yellow Sea
• Chengshan
• Triangle

Currently, there are over 20 brands in China wich are using models deprecated by the big companies and the price has dropped to the bottom due to the low cost of labor in China.

See also

• Used tires and waste
• tire

Other external links

• The Tire Society
• The Wheel and Tyre Bible