OK, usually when we hear about retread tires it's because there's a tread lying in the road and we do not want to run over it, and someone will say something about those worthless retreaded tires on the CB. The fact is most treads lying in the road are because the casing failed, not because the retread failed.
I had the opportunity to tour a retread plant near my home and here's my take of what I learned. Retreaded tires are called other things besides retreads, like caps, or recaps. For this blog we will call them caps from now on. Understanding tire construction needs to come first before we can understand the retreading process, and a process it is.
We all know what the tread is so we won't spend much time here, just that it comes in many patterns, and depths. So moving on to the sidewall, the name explains it all, it is the side of the tire that has all the pertinent information about the tire, size, make, model, and DOT required information. We can see all this also, so lets go inside and look at the actual construction. Under the tread is what’s called the belt package, this, in most tires, is wound steel cable that goes diagonally across the tread, back and forth from one bead to the other. This belt package is encased in a rubber compound that seals it from moisture damage. On the inside of the tire is an inner liner, bonded to the rubber of the belt package; this is the bladder where the air is held, next to the wheel. The only part of a tire that actually contacts the wheel is the bead; it is shaped to fit the angle of the wheel and sealed to hold air pressure. This is a very simplified overview of tire construction but if you want to learn more Google "tire construction", it will teach you lots.
Back to caps, nearly every part of a tire can be repaired, sidewall damage, severe tread damage, and large holes, I was amazed at the size of damage that can be repaired. Before you dispose of that damaged tire, check with a retreader to see if it can be repaired. As the tires are brought in to the plant, they are first cleaned of any debris and all nail holes and other damaged areas are marked for repair. From here the tire will not touch the ground again until it is finished or rejected. For a repair, sometimes called a section repair, it will go to that area now, inspected, repaired and returned to the shipping dock for return to the owner. The tires that are in for capping go on to a machine that mills or grinds all the extra rubber off the casing, making it truly round, possibly for the first time in it's life, and it will not be touched by bare human hands again until it is finished. There are oils on the skin that can cause the new tread to not adhere to the casing.
The tire is then inspected using an electrostatic device that will find any nail holes or damaged belts in the tread and sidewall, these places are marked for repair at the skiving station. All spots that needed repaired are then drilled or ground out, removing any rust that may be in the casing. Once these spots are removed and cleaned they are filled with raw rubber to fill any air space that might exist under the new tread. Any patches on the inside will now be removed and repatched.
Once the new patches are installed, a final inspection is done. Now a thin layer of new rubber is laid on the casing where the new cap will be placed. There are many tread patterns that can be chosen by the owner of the casing. The new tread is then measured and aligned for placement on the casing. The tread seams are hand fitted and the seam is "glued" with gum rubber. The last part of the process involves placing the casing with the new tread inside an envelope, seating the beads to the envelope; the envelope is then evacuated, putting a vacuum on the casing holding the tread to the casing for the cooking process. This vacuum must remain on the tread/casing assembly throughout the cooking process or the cap will have to be removed (ground off). To monitor the vacuum, there is a gauge on the outside of the cooking chamber connected to each of the 20 tires that the chamber will hold. The tires will be "cooked" for four hours and twenty minutes. We have now completed the retreading process, other than a coat of special tire paint on the sidewall to make the tire look new. The tire is tagged to identify the owner, size, tread model, and date of retreading, then stacked awaiting shipping back to its owner.
In my years as shop foreman I used caps for all wheel positions except steer axles. We didn't have much trouble with caps coming off but we did have blowouts, just like we did with new tires. I always wondered why a new tire would blow when we could cap a casing as many as five times. After doing some research I learned the majority of blown tires blow because they are run low or flat, creating heat and the casing comes apart. A study was done and found that the treads you see on or beside the road are about 50/50 new/retread, and most because the casing failed. If you see steel cords with the tread its because the casing failed. I’ve see caps come off and the casing still had full air pressure in it, bad cap job. The tire industry says the average warranty adjustment for a new tire is just over 1% of production, for retreads its under .5%, meaning a retread is a better built tire. You must have a good casing to make a good retread.
For more info on recapping go to :
Retreads