The extreme complexity of a tyre calls for a long, careful manufacturing process prior to curing. That’s what it takes to achieve perfect, constant quality. Originally a manual process requiring a lot of room, production was automated with the advent of new more compact and flexible machines.
Michelin tyre manufacture video
Preparation of the components: a compound for each use
The composition of a compound is closely adapted to the intended use of the tyre and the formulae are prepared according to the specific properties required. At each phase in the process, the manufacturer’s work is particularly painstaking: dimensional checks, weight operations and thermal measurements guarantee constant quality.
The various products involved in the composition of the required rubber are first carefully blended in a powerful 200-litre mixer. Carbon black is incorporated at this stage. The mechanical work involved in dispersing it in this extremely viscous substance leads to a substantial rise in temperature. The compound gradually softens and acquires the required viscosity.
After leaving to settle for a time sulphur and other additives which facilitate vulcanisation are introduced using homogenisation equipment. The compound obtained is presented in sheet form.
Cord plies cut into lengths
These plies are made up exclusively of cords joined together and arranged in parallel, coated with two layers of rubber on a calendering machine. They are then cut into lengths at an angle determined according to their use in the construction of the tyre.
The other components
At the same time, other machines produce profiles, bead heel protectors, sidewall rubber and tread bands (still smooth)... These profiles are produced with precision of around 0.1 mm by extrusion or by calendaring between rollers with recesses providing the required profile.
Build up: each component has its own specific place!
The aim of build-up, also known as confection, is to place the various components with great precision in a flat stack in a determined order on a cylindrical drum.
Their cohesion is ensured by uncured bonding, the natural binding of each component. After each component has been positioned, the rubber products are put together and rolled to exclude any air and form a perfectly uniform composite.
The first product to be put in place is the interior, airtight rubber (tubeless tyre). The carcass ply is then laid across it. The bead wires are positioned on either side of the ply. Finally, the various filler and sidewall protection rubbers are put in place.
Conformation: the future tyre is shaped into a ring
By means of conformation operation, the product is transformed from a cylindrical to and “O”-ring shape, similar to that of the future tyre. To do this, the beads are pulled up toward the centre while the central portion is inflated, stretching the layers of rubber and causing them to bulge along the length of the ply cord. In view of the significant deformation caused by this step, adhesion between the assembled elements must be guaranteed.
The belt plies are then rolled around this “O” shape, as is the tread band which is still smooth. A variety of additional profiles are also put in place. We then obtain what is known as an uncured tyre.
Curing: for perfect cohesion of the tyre as a whole
Up to this point, the tyre has not been heated, in order to preserve the plasticity of the various rubber compounds used in its construction. In this last step, it is placed in a mould heated by superheated water and steam.
The water circulates continuously in a membrane. As it expands, it pushes the tyre against the mould walls to print the tread designs and external markings. This is when cohesion of the whole is achieved by vulcanisation.
By linking the rubber molecules together, the sulphur molecules prevent them from separating. The rubber loses its plasticity and, depending on the degree of vulcanisation, acquires elasticity, solidity and resistance to abrasion. All these phenomena generated by curing ensure perfect cohesion between the rubber compounds and the metal and fabric reinforcements.
These various operations show how all the zones of a tyre are interdependent and prove the extent to which the slightest modification of any parameter in the design stage might affect the tyre as a whole.
The mould gives the tyre its final appearance
An essential part of the production line, the mould is a key element which gives the tyre its geometry and final appearance, while transmitting the heat required for vulcanisation.
Moreover, with the other mechanical components of the curing press, it must be able to withstand the thousands of opening and closing cycles generated by curing cycles. The part of the mould responsible for the tread design is made by various casting or injection processes.
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