One more round
Patches (D) of different sizes are vulcanized to the inner liner of the tire in order to seal it effectively.
Many retreaders have even more sophisticated inspection systems. Bandag touts its 7400 Insight system that provides a “shearographic” inspection. This uses laser technology and varying vacuum pressures to stress the casing structure and make sure it’s still strong and defect free.
Bohn says Goodyear has its own entirely automated system now in use in many locations.
Bad casings are discarded. Once a reusable casing is found to be in ideal condition, the next step is to remove the old tread. This is accomplished by using an abrasive wheel to cut the tire down to size, called “buffing.” At this facility, the process is completely automated.
The tire type is input into the computer (E), which has data on the exact dimension the tire must be buffed down to in order to reach the outside of the casing. This removes the old tread to within 3/32 of the tire belts(F). The surface is then textured with a grinding wheel. The type of abrasive wheel used produces exactly the right texture, termed RM83, ideal for the new tread vulcanizing process. Technicians can check the texture by comparing it with various examples(G).
A special machine then applies layers of cushion gum to the casing, if the tire is to get pre-cured tread (H). This is raw rubber which, when subjected to heat, fuses to the inside of the pre-cured tread and the outside of the casing.
If the tire will be mold cured, it receives raw strip rubber that is carefully wound around the casing to produce an even layer of sufficient thickness to form a tread (I).
In pre-cure, the tread is then carefully located on the outside of the casing. (J) Here, a Unicircle pre-cured tread is being slightly expanded and then located onto the casing. Laser light is used to center the tread so it’s right on the money.
Another type of machine applies the standard pre-cured tread (K).
Mold curing is done at 300 degrees F. for one hour, with 200 psi pressure in the casing in order to force the tire outward and into the mold segments.
Pre-cured tires are cured 23 at a time inside this curing chamber (L). They are placed inside rubber envelopes. Pressure inside each envelope is held at 70 psi with a separate air line to each envelope. This is called EPS or “Envelope Pressurization System.” There is 85 psi in the chamber. The chamber is held at exactly 260 degrees F., using high-pressure steam inside pipes and an air circulating blower to make sure the temperature is even.
Technicians can watch the pressure inside each envelope to make sure it is correct. The temperature inside the chamber is recorded throughout the curing period of three hours to make sure the tires are perfectly cured (M).
After all this, the finished product receives a careful look-see to make sure the tread is properly aligned and defect-free. The sidewall, inner liner and the appearance of the cushion gum layer will be checked. An inspector can see right away if the curing envelope leaked because the gum will be wrinkled or have bubbles in it. It should be smooth.
The precision with which this seam was formed shows what kind of work a quality retread plant can produce (N). The final product gets a label verifying the work done and helps guarantee that it will be returned to the proper customer (O).
Typical tread and rotation patterns
Steer tires are not normally retreaded, and running retreaded steers is prohibited in most jurisdictions. Worn steers receive either a lug tread and become drives, or a simple grooved tread for trailer tires. The lugs allow the drive tires to produce greater forward traction, while the grooved trailer tread has less rolling resistance.