The process of retreading a tire is exceedingly more complex and sophisticated than most people imagine. Retreading is not simply a matter of removing the old tread and "gluing" on new tread.
One of the most important elements in the retreading process is the first step: the thorough rigorous inspection of worn tires, called casings, to determine whether they can be repaired and retreaded or should be rejected and scrapped.
A tire's retreadability is not merely a function of its age. Other considerations include the application in which the tire was used, the type of equipment it was and will be used on, how well the tire was maintained, the number of times it has been retreaded, the number and types of repairs and the customer's specifications.
The initial inspection of casings is done by highly trained inspectors who, under bright lighting, carefully examine each casing for cuts, bruises, punctures, wrinkles, splits or cracks. These are marked and the inspectors make the determination as to whether repairs can be made or the casing is deemed unsuitable and is rejected.
The visual inspection is typically followed by the use of various high-tech, non-destructive inspection equipment to further ensure that a casing is a prime candidate for a full and useful additional life. This equipment is used to see inside (between the layers of rubber and steel belts) a casing to uncover invisible damage, such as manufacturing defects, signs of impact that might have broken the casing, belt separations and whether a tire has been run underinflated to the point it should not be retreaded. Internal damage is a frequent cause of tire failure.
There are a number of non-destructive inspection methods available. These include:
Shearography: One of the most popular methods, it can be a very effective tool in locating certain types of anomalies, known in the retread industry as casing separations. Most casing separations are considered detrimental to the durability of the casing.
The output from a shearography scan is visually displayed and offers a two dimensional graphic representation of the change in elevation of a three dimensional surface. The inspector makes a subjective judgment on the casing based on his interpretation of the visual display.
Since there is a continuum of "gray area" between the extremes of a perfectly sound casing and an obviously unacceptable casing, the resulting interpretation can vary from inspector to inspector, and even from one shearography equipment manufacturer to another.
Although an extremely important non-destructive testing tool, shearography is not effective in finding separations associated with open penetrations, cuts, snags, potential sidewall ruptures (also known as zipper failures) and similar in-service damage.
Electrical Impulses: The most popular machine using electrical impulses is the NTD Tire Tester, developed by Hawkinson. By sending electrical impulses through the casing, the machine detects penetrations and other imperfections—nails, nail holes, cuts, tears and bad repairs—that may not be seen by the human eye.
X-rays: X-ray machines enable inspectors to have yet another inside view of the casing and can show broken steel cords and other defects that are invisible to the human eye. The x-rays machines that are made especially for use in retread plants use low doses of radiation well within government-established guidelines.
Ultrasonic: This technology sends sound waves through a casing to a receiver. The strength of the sound waves is measured as they pass through the casing, which alerts inspectors to potential trouble spots. Problem areas are outlined with patterns of colored dots. The inspectors can then examine the dot pattern to determine the shape, size and location of the irregularity and make a decision as to whether the casing should continue through the retread process.