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03/07/2014
FROM TREES TO ROAD

The creation of a tire

 


The path leading from raw materials to a finished tire is long and complex requiring very advanced procedures



Massimo Clarke

As with all products with a high technological content, even tires need modern manufacturing processes and make use of a large number of sophisticated controls. At the production plant raw materials arrive in the form of "bales" of natural rubber obtained from the Hevea brasiliensis, a plant from which the starting latex is extracted. Then there are synthetic elastomers, among which the styrene-butadiene copolymer, and reinforcing fillers, i.e. carbon black (of which there are many different types), and for several years now, fused silica. The picture, as far as the production of the compound is concerned, is completed by sulfur, necessary for the vulcanization process, accelerators, from a small but important amount of zinc oxide (activator) and a series of additives (antioxidants , oils, resins...). The casing’s starting materials are various types of textile fibers, such as rayon, nylon , Kevlar and steel wires.

Different production lines are used to produce casings, beads and the parts in "rubber", tread and sidewalls. These elements are then joined to form the "raw" tire. Vulcanization is the final stage.

The natural rubber, synthetic elastomers and fillers, plus the substances necessary for vulcanization and additives are introduced in special banbury mixers. The materials are then mixed several times, in order to obtain a complete and uniform compound. Since in most modern tires different types of compounds are used, each of them is produced separately.

From the banbury mixer the compound is pressed to obtain layers of the material which is then hot-extruded; thinner layers of "rubber" ( of the various compounds) are produced ready to be cut with precision so as to form the various parts that will make up the sidewalls and the tread.

 

Along another production line the textile materials, used in making the casings, are woven. Here they are spun creating the cords that will form the ply (consisting of parallel cords, whose spacing is critical). The plies are then rubber-coated and passed between two steel rollers in a special calender; this allows the rubber to penetrate and gives the plies the proper thickness. Strips of rubber-coated fabrics are so produced and must then be cut so as to obtain pieces having the expected shape. It is a delicate operation, which requires great accuracy: it is fundamental that these pieces have the correct width and lip angle. The pieces obtained are then joined to form a continuous strip having the right length, ready for assembly.

To realize the tire beads a separate production line is used, in which steel wires are rubber-coated, cut to size and wrapped in rings.

 

All the parts from the different production lines are then appropriately arranged on a rotating drum. Here the plies are wound, the ends of which are joined together, the beads are positioned and joined to the structure of the casing. The edges of the latter are then folded around the beads so as to obtain a stable and sturdy bond: this area, in fact, is interested by a number of forces affecting the tire during the operation of the vehicle. At this point the "rubber" parts are assembled, that is the tread and sidewalls, then carefully joining their edges to form a single structure. At the end of these operations a "green tire" is produced, ready to be cured. After this the tire goes through a series of accurate checks for size and geometry.

 

The cure is a process that transforms the rubber from a soft, sticky and prone to deform substance in one that is more consistent, rigid, has much higher resistance and is very elastic. Elastomers, both natural and synthetic, are made up of long molecular chains. The cure forms bonds between these chains and between them and the tire’s reinforcements; this is a heated process, thanks to the presence of sulfur. The cross-linked structure that is obtained, while allowing the movement of the polymer chains on the one hand, acts as "brakes" on the other, preventing irreversible sliding from taking place (which would cause permanent plastic deformation). The density of this structure is crucial to the elasticity of the compound and is directly linked to the percentage of sulfur used.

 

The green tire is placed inside metal mould and is therefore "inflated" with steam. The outer part of the tire adheres perfectly to the inner surface of the mould. After closing the mould the cure starts, which is carried out at a temperature between, depending on the case, 130 to 200 ° C; this process can last from 10 minutes to more than an hour.
Once extracted from the mould, the tire is cooled and is then subjected to a series of controls, both static and dynamic (for the latter a special rim is used). The structure of the tire is also tested using x rays.

 

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