One more round
Retreads give worn-out tires a new lease on life – and save you money.
After fuel, tires are an owner-operator’s highest expense. Retreading lets you reuse the good parts of the tire instead of buying all new, saving about half the cost of new tires.
Like your engine, tires have some parts that wear a lot faster than others. The tread takes the most punishment – both continued rolling and rubbing along the road, and punctures from nails and screws. The casing, when properly cared for, can outlast the tread by two to three times because it’s isolated from the road by the tread.
The goal of retreading is to rebuild the tire, removing the tread from the casing and replacing it, much as a cylinder liner is knocked out and replaced. The new tread is securely bonded to the tire through a process called “vulcanizing.” The original tire was fused together, using heat, out of vulcanized raw rubber in the same way. Vulcanizing makes the rubber stronger and harder, like heat-treating metal, and fuses the parts together, like welding. When the job is done right, the retreaded tire’s structure is virtually identical to that of a new tire.
The three processes
There are two basic methods of retreading, mold cure and pre-cure. In mold cure, raw strip rubber is applied to the casing, and the assembly is then inserted into a heated mold, where the strip rubber is vulcanized and the tread pattern molded into the added rubber. In pre-cure, a cushion gum adhesive layer of uncured, raw rubber is applied to the tire. A strip of pre-cured, molded tread is then applied, and the assembly is cured inside an envelope in a pressurized mold to bond the new tread to the casing. Bandag, Inc. has been well known for the pre-cure process for many years, but Goodyear and others also produce many pre-cured retreads.
A variety of pre-cure is the Goodyear Unicircle process, which produces jointless pre-cured treads formed into a circle of exactly the right diameter for fitting around the casing.
Mold cure is a well-established process, but it requires the retreading plant to provide the tread pattern in the form of mold “segments” that have the tread pattern in reverse cast into them. The segments form a circle inside the mold press when they close around the casing and strip rubber (A). Typically, the number of mold cure tread styles available is limited because of their cost, even though the segments in a mold can be changed in a lengthy operation.
Pre-cure provides a great number of tread styles because the treads are mass-produced in a large, central plant. Pre-cured tread naturally has a seam where the two ends of the pre-cured tread segment meet and are carefully fitted and then vulcanized together. Properly done, the seam is problem-free. But Goodyear’s seamless Unicircle design eliminates any potential trouble with the seam and makes the resulting product a little easier to balance.
We toured the Wingfoot Commercial Tire Systems retreading and repair plant in Westminster, Md., not far from Baltimore. Bob Bohn, production director and Paul Muckle, satellite manager, walked us through the entire retreading process.
- Casing Inspection
Casing maintenance, including keeping the tire properly inflated and doing all repairs properly, are critical to making retreading work. One of the keys to having an effective retreading program in any operation is caring for the casing so you can take a known good one back for retreading and save the cost of purchasing another.
At the retreading plant, the first step is casing inspection. Improved inspection techniques are one of the keys to a great improvement in retreaded tires over the years.
Here, (B) the inspector is getting ready to inspect the casing. He mounts it on a machine that rotates it and allows him to look inside. The machine also tests the tire much the way you’d test electrical insulation. It drapes small chains across the inner tread and energizes them with high voltage (C). Because rubber is an excellent insulator, there is no noise until the chains encounter a small hole in the structure, often containing a nail. The technician then hears a buzzing noise and can locate where the puncture is immediately. He removes the nail and marks the damage for later repair.
He also inspects the casing visually with a bright fluorescent light.
All such repairs are made in a way that seals off the inner liner of the tire, so moisture in the inflation air cannot seep into the casing and rust the cords, or separate the casing’s structural cords with the pressure. Except for a temporary repair just to get you home, it’s vitally important that all tire repairs be done this way – with the tire removed from the wheel and sealed from inside. This is one of the most important keys to preserving the casing for use in retreading.
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