1.  Field of the Invention
This invention relates generally to apparatus for retreading tires, and in particular to a retread curing ring that is mounted on a tire retread assembly for effectuating a seal of the retread assembly during the curing process. 
2.  Description of the Prior Art
In general, there are two methods commonly used in the retread industry for retreading tires.  The first method involves buffing the surface of a tire carcass to be retreaded and covering it with a layer of uncured rubber.  This assembly is then placed in a standard tire retread mold.  The rubber is shaped to the desired pattern by applying pressure inside the tire and then heating the entire assembly until the uncured rubber is cured and bonded to the tire carcass. 
The second method involves retreading without the use of a standard retread mold.  In this method, a pre-cured tread strip is applied to the prepared carcass together with an intermediate thin layer of uncured rubber.  This assembly is then placed into a flexible elastomeric envelope which encases a portion of the carcass and the pre-cured tread strip.  Normally, the tire carcass is mounted on a rim prior to placing the assembly within the flexible envelope.  In addition to the rim, an inner tube is normally positioned within the tire carcass.  Finally, the entire assembly is positioned within an autoclave to cure the intermediate layer, thus bonding the pre-cured tread strip to the tire carcass. 
A third method recently developed in the retread industry uses a pair of retread curing rings that eliminate the tire rim and inner tube, thereby reducing the time required for curing the tire assembly.  Retread curing rings of this nature can normally reduce the curing time by at least one third, thereby significantly increasing the rate of production in a tire retreading operation. 
In particular, U.S. Pat.  No.  4,579,619, issued to Symmes et al.  on Apr. 1, 1986, discloses a retread curing ring that includes a body member that defines a circular platform.  A plurality of clamps, which are movable between an open and closed position, are mounted at circumferentially spaced locations on the body member.  The retread curing ring is designed for use with a tire retread assembly that includes a tire carcass, a tread strip, and a flexible elastomeric envelope that fits over the tire carcass and tread strip.  The tire carcass includes a retread area upon which the tread strip is placed and a pair of integral side walls that extend radially inwardly until they end forming a circumferential bead.  The circumferential bead is generally considered to include two portions: an exterior edge and an inner edge.  The exterior edge is that portion of the bead which faces outside the tire carcass and the inner edge is that portion of the bead between the exterior edge and the interior of the sidewall.  Each tire carcass has two openings so that the carcass can be mounted upon a rim.  The inner edge of the bead essentially defines the circumference of one opening within the tire carcass.  Also, a bead angle is formed by the inner edge between the exterior edge of the bead and the interior of the sidewall.  Many tires that are subject to retreading have a bead angle of between about 14.degree.  and 15.degree..
The retread curing ring of Symmes et al.  is designed so that an upper surface of the circular body member fits adjacent the sidewall of the bead.  The clamps mounted on the circular body member define a vertical guide surface against which the inner edge of the bead rests when the ring is placed within an opening of the tire carcass.  The function of the vertical guide surface is to assure that the tire carcass is concentrically aligned with the retread curing ring during operation.  When the tire carcass and envelope are in position, the end portions of the envelope are sandwiched between the sidewall near the exterior edge of the bead and the upper surface of the circular body member With the tire assembly mounted upon the retread curing ring in this manner, the clamps are moved to their closed position and pressure is exerted against the interior of the sidewalls so that the envelope is sealed against the exterior of the sidewall near the exterior edge of the bead. 
It has been discovered that the retread curing ring of Symmes et al.  does not always provide sufficient pressure to seal the envelope of the tire assembly against the exterior edge of the bead and sidewall.  Thus, gas leakage occurs during the curing process.  A failure rate of approximately 10 to 50 per cent has been experienced by some segments of the retread industry using the Symmes ring. 
An improvement was made to the Symmes ring in an effort to remedy this problem in U.S. Pat.  No.  4,850,834, issued to Hinson et al on July 25, 1989.  The Hinson patent discloses a retread curing ring similar to the ring of Symmes et al.,  but which also includes a circular elastomeric gasket against which the exterior edge of the bead and sidewall engage.  In a manner similar to Symmes et al.. the clamping arms engage the interior edge of the bead and sidewall at an angle of approximately 90.degree.. Again, the seal is primarily effectuated along the outer wall and exterior edge of the bead.  The curing ring of Hinson also provides a guide surface against which the inner edge of the bead and envelope rest for proper alignment of the tire assembly with the ring during operation.  The Hinson ring also had a relatively high percentage of retread tires which would not seal with it satisfactorily.