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The construction of tires has always been seen as "labor intensive".
The earliest of tires were constructed over a form or "core" whose outer surface would correspond to the inner surface of the vulcanized product. This entailed forcing layers of rubberized fabric to be contoured on a wooden or metal form. The "finished", but un-vulcanized [green] tire was wound with strips of wet cloth and placed in a heated chamber. When the temperature had permeated the tire so that its rubber components had reached the vulcanization point and maintained that temperature for sufficient time, the tire was removed, un-wrapped, and dismounted from its internal core. The flexibility of the clincher bead structure facilitated this procedure.
Advances in the "art" permitted the eventual use of rings, then molds that could replace the wrapping; and curing "bags" to make it possible to retain the "cores" in the tire building area.
The "airbag" was a molded product somewhat like an innertube. The main difference was in the overall shape. Although a torus, like the tube, the airbag had a massive base that defined the inner shape of the lowest extremity of the tire. It developed into a "vee" shaped structure that would help center the uncured tire/ airbag assembly in the two-part curing mold. The composition of the curing bag was a rubber and clay combination that resisted becoming "over-cured", when used repetitively.
Initially, Installing and removing the bag required skilled strong hands. Eventually, purpose built machinery assisted in the work, but it never became totally automated.
The move from woven to bias fabric soon allowed for a drastic change in the shape of the built tire as well as the machinery and tasks used to produce it.
Instead of requiring laborious contouring to assume the shape of the internal form, it could now be built "flat".That is, the principal portion of the structure could be put together on a cylindrical sleeve that could be collapsed so as to facilitate the removal of the tire.
The finished "green" tire no longer bore much resemblance to how the cured product would appear.
It was then placed on a "bagger" which would inflate the carcass to a torus, and insert the bag to keep it in that shape.
The "bagged" tire was placed in the bottom half of a symmetrical mold (split at, or near its circumferential centerline), and the upper mold half was lowered over it. Special "keys" aligned the halves. The tire and mold, along with many others, would then be taken to a vertical curing chamber.The center of this chamber was equipped with an hydraulic piston, upon which the molds were sequentially stacked. The valve on each airbag was connected to a source of internal heat and pressure. Finally a lid was placed on the chamber, and rotated to the locked position. The hydraulic ram, would rise and cause all of the molds to forcibly close on their tires. The massive lid (equipped with pressure seal) resisted the upward pressure. Steam entered in and around the stacked molds, and "curing" took place. As the rubber vulcanized and expanded, rubber was extruded into strategically located vents drilled into each mold.
The "straight sided" tire with its wire bead construction moved smoothly into this scheme of production when it replaced the clincher.
The tire curing room was a hot and somewhat dangerous place to work.
Lubricants that facilitated the installation of curing bags, mixing with the water from condensing steam, made for slippery floors. Substantial amounts of manual labor were needed to extricate tires from their molds, and the bags from their tires.
When tires required more than one bead coil to anchor the plies needed for higher pressures, the construction technique became a compromise between the older "core" method, and the flat method of the single bead tires.
A building form (or drum) was designed with a diameter sufficiently great so as to allow the bead portion to continue to be hand built, but the remainder of the tire could be built flat, on the larger diameter surface that separated the contoured ends.
The plies were pre-assembled into "bands". Bands consisted of two, or four plies, pre-assembled into a cylinder whose diameter was smaller than the flat part of the drum, but larger than the bead portion of the drum. These bands were stretched over the drum, or previous lay-ups, by one of several methods. (Compressed air; spiral wound cloth strips etc.) The band assisted in preserving the angle at which the fabric had been cut, and reduced the "wrinkling" that occurred as the plies were contoured to the smaller diameters needed to pass under and around the beads
Curing followed the same principles used by the smaller tires.
Changes in the curing techniques occurred at a measured pace.
The first was to install molds in dedicated stands. By "jacketing" the mold for improved heat transfer, steam could be used more efficiently. The process continued to use airbags and did not reduce the labor required.
The next (and to date) last change in curing involved the elimination of the curing bag. It was replaced with a curing "bladder", and operated in concert with a programmable curing press that could accept up to two mold cavities. This curing philosophy did lead to automation and reductions in the labor force, but at the expense of flexibility in scheduling. Machinery costs climbed.
The bladder is a relatively thin walled, cylindrical rubber structure that is clamped at its extremities. It is much smaller in diameter than the tire it will "shape" and "cure". It contains the pressurized steam that will quickly inflate the tire to mold shape as its press closes, forcing the mold halves together.
Following "cure", the internal pressure leaves the bladder prior to the opening of the press, and its diameter presents no obstacle to stripping the tire from the mold.
Now, with no airbag to support the tire as it cools, it is sometimes necessary to place the tire on a machine that will pressurize it and thus provide for enhanced dimensional control as the tire cools.
Before any of the above may occur, a myriad of operations will have had to take place in order to present the tire construction area with the components required.
Fabrics are deserving of being described. They are custom made of fibers that are wound into cables, and these, in turn, wound into larger cables by twisting one or more together. Different spacings of the parallel cords result in further specializations. The cords are held in a loose confederation by widely spaced, small diameter, cotton filler cords that are woven into the width of cord lay-ups. These rolls of fabric must be carefully processed to function as intended.
Moisture must be excluded from the storage environment prior to immersing the cords in a chemical bath that will enhance rubber adhesion. As drying takes place, the cords are uniformly stretched prior to being coated with a rubber compound. That compound will have been tailored for the function expected of the final component.
A process called calendering is usually selected as the method for rubber application. This involves passing the fabric between large, smooth rolls and presenting it with uniform, thin sheets of rubber. These are pressed onto first one cord surface, and then the other, (and the interstices between).
The coated fabric, protected by a film of cloth, or plastic, is rolled for storage.
Fabric is rarely used in the as-calendered condition. It is most often needed in a state where the cords will lie at an angle to the edges. This necessitates use of machinery called bias cutters.
These usually utilize rotary knives (saws) that are guided precisely across the array of parallel cords at high speed. Individual lengths of cuts are rotated so that they can be spliced along the cord line into continuous strips of material. These are protected from cohering to one another, and re-rolled for further processing. (These would include band construction, or in some cases single ply preparation).
Calendering is also called upon in the preparation of the innerliner. In this case it may be desirable to build up a laminate of thin sheets of a compound that is resistant to permeation by air or nitrogen. The use of discrete layers lessens the chance that small imperfections will align so as to provide a leakage path in the finished product
Extrusion is another significant manufacturing technique used in the manufacture of tires.
This involves forcing hot rubber to pass through a contoured orifice to produce a desired shape.
The tread area represents one of the most significant zones where this is employed. The die through which that rubber is extruded must be designed to anticipate the final needs of the mold cavity, and the three dimensional changes that will occur during shaping.
Extrusion is also used in the bead area. The wires that make up the bead employ a chemical coating that will ensure rubber adhesion. These wires are passed through an extruder, either singly, or in parallel alignments of specific numbers, and are wound on collapsible forms to provide coils of overlapping plies at a closely controlled inner diameter.
Extrusion also is used to make uniquely shaped components that may be necessary to fill areas within the tire where changes in thickness occur.
Radial tires have presented new challenges to construction and curing.The uncured radial tire body is a somewhat fragile structure. It requires few layers of rubberized cord to perform its duties. The nearly circumferential belt/tread portion is preferably constructed as close to the molded diameter as possible. These attributes have led to the design, construction, and use of dedicated tooling, sometimes uniquely tailored for individual applications.
This approach may have reduced the manpower required in some instances, but it also has increased the levels of precision, and quality control needed in the production of components.
The curing process for radial tires can also cause problems.
It may not be desirable to put shear forces on the hot tread as it leaves the mold. In fact, some of the curing process may still be taking place as the mold opens. Since those portions of the mold structure that produce the tread design may still be imbedded in the tread below the surface, any lateral force applied to the tread, such as the opening of the mold could cause a shearing action.
As a result, some molds are segmented so that the circumference is moved radially out of the tire at the finish of cure. (Having been moved radially into the tire at the beginning of cure).
Obviously, this requires a more sophisticated mechanism within the mold, and more maintenance for its moving parts than are required for a two piece mold.
Not all radial constructions require all of these refinements.
Unique rubber recipes are produced to meet the needs of certain areas of the tire: the tread is compounded for wear resistance and traction; the sidewalls for flexibility and resistance to ozone and sunlight; the liner to be gas tight; the bead areas to resist chafing and slippage; the mixture that encoats the cords is in need of tenacity in its adherence to the fabric, and its neighboring layer. Each material needs to be as "cool running" as possible and contribute to the overall success of the tire. All of the materials need to retain their utility at extreme temperatures, and of course be capable of being produced routinely, while cooperating with the construction processes.
Many expensive considerations go into the planning for any new size of tire.
If it is to be an aircraft tire, those are only the beginning.
At least, however, it is unlikely, (although not impossible), that anyone will be designing a new "core".
The need to maintain and protect the torus of high pressure gas that encircles our wheels continues to foster new approaches.
The dustbin of history is littered with short-lived ventures that promised much, but fell short of permanence.
Until some practical replacement emerges that can better perform all of the tasks that the tire routinely does for us, the principle that first graced early bicycles will be sticking around.