Modern tires are engineering marvels capable of carrying thousands of pounds hundreds of thousands of miles over scorching desert black top or frozen steel chain links on mountain ice. Considering the work that they do, and the quantity that exist on this planet, tires may be the most durable asset man creates.
Across America entrepreneurial small businesses and businesses that used to be small but have grown into large, state-of-the-art, 1000 tire/day production facilities, replace the wear component of the truck tire, or tread-area. The most common kind of modern retreading is pre-mold, or “cold cure” retreading. In pre-mold retreading, a new product is manufactured by curing a layer of uncured rubber between two layers of cured rubber.
The cured rubber base layer of the retread is the truck tire casing. A truck tire, like an automobile, is made of different parts, and each part of the whole, is made of different materials and does different jobs.
The interior of a tubeless casing is wrapped in a balloon like layer of additive-rich rubber called the liner. The liner keeps compressed air from escaping a tubeless tire and wheel assembly. When a retreader selects a casing for retreading they must verify that the liner does not need repair and will last the life of the tread that they’ll be investing.
Just outside of the liner, new tire manufacturers use a special mixture of rubber, additives and steel to create radial body plies. If the liner is like a balloon, small steel wires in the radial ply can be viewed as a strong basket that contains the force of the compressed air inside the tire. The most common 14 ply truck tires contain 110 pounds of air in a single square inch.
The terminology; “ply rating” shows the history of tires. Although modern 14 or 16 ply tires have a single layer of steel strands running at a 90 degree angle to the beads of the tire, around the late 1800’s ply rating indicated the layers of cotton weave that were inside the tire. As the layers of cotton weave increased, the casing was able to contain more air. As the air pressure that the tire was able to contain increased, so did the tire’s load-carrying ability. Around WWII tire body ply construction began to be nylon as opposed to cotton. And the thickness and strength of the newer material allowed the same job to be done with less volume and weight. A tire that used to need 16 layers of cotton weave running from bead to bead at a 45 degree angle now only needed about 8 layers of nylon running from bead to bead at a 45 degree angle to do the same job, or contain the same amount of compressed air, or carry the same load.
The retreader must discover and repair any damage to the body ply as well so that the truck tire casing will last the life of the tread, Nails and rocks and all other kinds of debris that could damage the body ply of modern of truck tires are deflected by multiple layers of steel cords wove in the tread area of the tire. Some types of new tires or new retreads have over one inch of rubber on top of that, and this helps spare the body ply of damage too.
The plies of steel between the tread and the body ply are covered with a layer of rubber and additives that is specially formed to withstand degradation from the heat that carrying loads at speed can create. The tread of the tire has groves, and the material between the deepest groove and highest tread ply is the rubbery “undertread” of the casing. The undertread layer of the casing is the layer that retreaders cut into with CNC lathes called “buffers”. Buffers remove old tread, true casings for roundness, clean and texturize the casing, and create the base for the next layer in the retread process.
When a new tire is created, it is laid out in sheets and then blown into a tire-shaped object called a “green casing” on a new tire building machine. If contamination occurs at any point in the process, or if additives that help the layers adhere during curing are missing, it is possible for the resulting tire to have a separated layer within the finished product. Tires that have separations in the material are like feet on a hiker with untreated blisters. As the tire rotates, it gets stress, and a separation allows movement which allows friction which causes heat which leads to disintegration. Retreaders use inspection technology that sees inside the material of the casing to verify that a truck tire’s previous service has not actualized latent anomalies resulting in separation. In addition, prepared surfaces with in a “cold-cure” retread shop are decontaminated and are not to be touched.
Although tread plies protect the body ply of the casing, many imperfections in the casing after it is buffed must be refined to accept the next layer without trapping contaminants inside. Rust, oil (either from human hands or tools) and air are all contaminates that retreaders remove with hand tools. Preparing these superficial imperfections is called skiving. Damage that fully penetrates the body ply is repaired using reinforced patches and curing cement.
With all blemishes perfected and penetrations patched, a layer of uncured rubber is applied to the truck tire casing. Extruding machines heat a gum-like rubber into semi-solid ooze and spread it across the prepared casing like black peanut butter.
The third layer of the retread is then applied. Retread tread rubber can be much tougher material than new tire tread rubber because of the way that the retread tread rubber is made. Retread tread rubber is cured in 50 foot-long strips, in presses that are six stories tall, applying thousands of tons of pressure. Because of this process, retread tread rubber is extremely dense, durable, and tough. The tread layer of a new tire as well as all the other layers of the green casing are cured all at once in a mold. The material fills the mold while heat is applied, and a combination of time temperature and pressure converts the uncured rubber to cured rubber in a new tire.
Although the science as to why precure tread can last longer than new tire tread may not here be sufficiently discussed, old timers in the tire business and maintaining trucks know that this is fact: you get more miles per 32nd of wear from a retread than you do from a new tire.
Since precured tread rubber is so long, it must be cut to fit the casing, and then the tread ends are spliced together. Retread building operators must be careful to adequately join the ends of the tread while not touching and thereby contaminating the bonding surfaces. The tread layer is then stitched to the freshly wrapped casing.
The uncured built retread is placed in an air-tight assembly that allows the rubber layer to cure inside a curing chamber. As the chemicals in the cushion layer become cured, process byproducts are evacuated through exhaust lines plumbed to the assemblies, while air pressure and heat generated by the curing chamber are applied to the outside of the assembly. In about three and a half hours, the layer of cushion gum is cured and that new product, or retread, has been made.
The final steps are quality control and beautification. Modern retreads can be indistinguishable from new tires to the untrained eye. Thanks for reading.