Facts About Tire Manufacturing Defects
Manufacturing defects contributing to tread/belt separations are
common place. During the manufacturing process, it is not uncommon for
contaminants, such as moisture, grease, oil, dirt and water to
accumulate on the surfaces of the components, including the steel
belts. If a tire is manufactured with an area of contamination on the
belt surface, the proper chemical reactions will not occur during the
vulcanization process and tire will leave the plant with an area of
poor or non-adhesion.
It is essential that a tire be manufactured so that areas of
trapped air do not exist within the tire. Trapped air is considered a
contaminant that interferes with the adhesion between the adjacent
rubber components, including the two steel belts.
While there are cases where wood chips or plastic are present in
the tire, in the vast majority of cases, due to the manner in which the
tire is vulcanized in a curing press at high temperatures, one will
usually be unable to identify the contaminants.
After the steel belts are calendared by coating the steel belt
cables with rubber known as belt skim stock, the belts are rolled and
taken to an area of the plant where they cool. The steel belts are
rolled around a cotton or plastic liner that leaves impressions in the
rubber coating of the steel belts. If the steel belts (and other rubber
components) are used before they become overaged and no contaminants
are present on the belt surface when the tire is vulcanized, the
adjacent rubber surfaces will flow together eliminating any the
“pattern marks” or “liner imprints” from the rubber surfaces.
If, however, the steel belts or other rubber components are
overaged (have lost their “tack”) or contaminated, it is not uncommon
to see pattern marks on the failure surface of the accident tire.
Occasionally these pattern marks will appear as diamond shaped as
illustrated in the photograph below (see Figure 3). More commonly,
these pattern marks will appear as dimple shaped as shown in the
photograph below (see Figure 4).
Snaked, Wavy or Off Centered Steel Belts
Tire companies design and manufacture steel belted radial passenger
and light truck tires with two steel belts. The lower, or number one,
steel belt is wider than the top, or number two, steel belt. This
creates a step off at the belt edges, and is known as a stepped or
pyramid belt construction. The step off reduces the stresses at the
belt edges. Unfortunately, it is not uncommon for the tire builder to
improperly position the top belts so that at various locations there is
no step off. This is a clear manufacturing defect that increases the
stresses at the belt edge and the development of separations. An
illustration of snaked or wavy belts appears below (see Figure 5).
Because steel does not adhere well to rubber, the thin wires
forming the steel cables used in the steel belts are coated with brass.
Spools of these cables are stored in a humidity controlled area of the
plant known as the creel room. Cables from these spools are fed through
a small opening in the creel room to the calendars where the cables are
coated with belt skim stock rubber to form the steel belts. If a tire
manufacturer does not follow proper quality control procedures, during
this process contaminants, including moisture, can attach to the belt
cables. These contaminants can and do prevent adequate adhesion between
the cables and the rubber in the belt skim rubber (poor wire to rubber
adhesion). As a result, certain tread/belt separations occur because of
the lack of adhesion between the belt skim stock rubber and the belt
cables. This appears in an accident tire in the form of bare cables.
These cables may or may not be yellow or brassy in color.