In the mold cure process, uncured, untreaded rubber is applied to the casing in much the same way as in the precure process.
Since tires typically represent a fleet’s biggest maintenance expense, and since retreads now can perform as well as new tires at about half to two-thirds the price, they’re an indispensable tool for containing operating costs.
But first, let’s get the Big Myth out of the way. Retreaded tires are not responsible for rubber on the road, alligators or whatever you like to call those shards of rubber on our highways. Studies by the Technology & Maintenance Council (TMC) and the Tire Retread Information Bureau (TRIB) have shown that roughly half the rubber fragments found on roadways come from tires that have never been retreaded.
Savvy fleet executives know that what causes any tire to disintegrate is underinflation. That’s because the reinforcing belts in a tire are meant to flex a certain amount while rolling under load, which generates a manageable amount of heat. When a tire is underinflated significantly, however, the belts flex to a much greater degree and generate enough heat to break down the tire’s integrity – just like rapidly bending a coat hanger or paper clip back and forth will burn your fingers.
Often, the casing – belts and all – comes apart. If you see a rubber fragment on the road with wires sticking out of it, that’s not just tread. It’s casing, and it has nothing to do with whether or not the tire has been retreaded. So don’t tread on retreads, OK?
Better casings make better retreads
Myths aside, experts agree that the quality and performance of retreads definitely have improved over the past several years. “It’s more like a quantum leap,” says Harvey Brodsky, TRIB’s managing director. “Non-destructive casing inspection technology is the single biggest reason.”
“Identifying casing problems is the first critical step in producing a high-quality retread product,” agrees Agustin Baez, general manager of Oncor Operations for Bridgestone/ Firestone North America. “Much advancement has taken place in the industry over the last 10 years that has substantially improved the performance of retreaded tires.”
Needless to say, if a casing’s integrity is questionable – due to road damage or abuse, for example – it isn’t a good candidate for retreading. But such damage often is not obvious, even with careful visual inspection. “Even the most conscientious inspector isn’t Clark Kent,” quips Brodsky.
An often-used, nondestructive testing technology is the X-ray. Another uses ultrasound to form electronic images of anomalies. Still another uses electrical current, which is passed through or into a casing as it rotates, while resistance is measured. Substantial variations in resistance indicate inconsistencies or damage in the tire.
A fourth method that’s becoming more common is called shearography, or laser shearography. In this process, images are created of a casing’s interior, while vacuum is applied to a chamber housing the casing and camera. Under vacuum, a slight deformation occurs wherever there is a belt separation, poor adhesion, casing fatigue or other deviations from normal, and the deformations are recorded for evaluation. “Our shearographic Casing Integrity Analyzer is used in combination with proprietary software that ensures the operator will interpret the results correctly,” says Tom Brennan, president of Michelin Retread Technologies.
“Laser shearography at initial inspection has added another tool for the trained inspector,” says Eugene Johnston, manager of process development, for Bandag’s Innovation Core Process. “This technology allows retreaders to locate internal tire damage created during the previous tread life. Fleets can have greater confidence that their retreadable and repairable tires are separated from those that should be scrapped.”
Such testing does more than just weed out bad casings, however. By determining the nature and severity of imperfections, retreaders are better able to determine what application will be most suitable for a usable casing, once retreaded. Also, this type of testing can show a casing to be structurally sound despite a rough appearance – which, again, means safe, inexpensive, extra miles. “Our operators tell us they’re retreading casings that, before this process, might have been scrapped,” says Brennan.
If a damaged casing can be repaired, fleet operators can rest assured that it will be. Tire repair techniques have improved by leaps and bounds, and have been better defined – thanks, in large part, to TMC, which recently published the latest version of its Radial Tire Conditions Analysis Guide. If the technology exists to repair a tire that previously would have been destined for the scrap heap, that easily could mean a hundred thousand or so safe, extra miles.
Computer-controlled buffing technology has been another significant casing preparation development, according to Bandag’s Johnston. “Automation in the process of removing the remaining tread has resulted in more precise roundness and consistency from one tire to the next,” he says.
Michelin’s Brennan agrees that buffing to precise dimensions is important. “We use ultrasound to ensure that buffing depth is consistent around the casing,” he says.
“Our buffers are computer-interfaced,” adds Richard Kirk, Goodyear’s retread product manager. “Sending data directly from a computer to the buffer takes the human element – and the opportunity for error – out of the process.”
Processes, materials & performance
Retreading processes fall into two major categories. The cold cap process, more correctly called precure, uses tread rubber that already has a tread pattern and is partially cured. The tread rubber is applied over a layer of adhesive “cushion gum,” in one full-width strip, cut to match the width and diameter of the buffed casing. The ends of the tread are spliced together. Exceptions to this application method are the Goodyear UniCircle and Marangoni Ringtread processes, where the tread, supplied in a seamless circle, is stretched around the casing by a special machine. Seamed or seamless, the precured assembly then is vulcanized under pressure and heat.
In the hot cap – or mold cure – process, uncured rubber without any tread is applied to the casing in much the same way as in the precure process. The tire is cured under pressure and heat – but generally at a higher temperature – in a mold where a tread pattern is imparted to the rubber. As in the UniCircle precure process, there is no splice.
Which is better? “Different customers have different needs,” says Marc Laferriere, vice president of Michelin Americas Truck Tires. “A mold curing operation requires a high volume of casings that are roughly the same size. The finished products look more like new tires than precured products, but their performance is equal.”
How about splice vs. no splice in precured retreads? “There used to be some splice issues,” says Michelin’s Brennan. “But, for us, automated manufacturing systems have eliminated splice stretching and related problems.”
However, some fleet operators favor spliceless precured retreads. “The Goodyear UniCircle wears evenly,” says Rick Cleveland, vice president of transportation for Alexandria, La.-based Petron. “It performs like a new tire.”
But no matter what the process, it’s almost certainly care and craftsmanship that makes for a trouble-free product. “The differences among retreading techniques are far less significant than how well any given supplier follows the recommended procedures for the systems he is using,” says Marvin Bozarth, expert witness and former TRIB executive, truck driver and fleet manager. “Any retread will only be as good as the care and quality of material used to make it.”
And, apparently, care and material quality have been on the rise. “Tread materials and patterns are extremely complex,” says TRIB’s Brodsky. “I don’t care what brand you’re talking about