Patent Publication Number: US-2011056627-A1

Title: Step cure for securing a tread to a tire casing

Description:
FIELD OF INVENTION 
     The present invention relates generally to the retreading of tires, and more particularly, to the retreading of tire casings with precured treads. 
     BACKGROUND OF THE INVENTION 
     Conventionally, worn tread on a used tire casing may be buffed off to a profile suitable for mounting a new tread, or retread. Then, a precured and buffed tread is adhered about the periphery of the casing with a laminate of uncured cushion gum cemented between the tread and the casing. An outer annular elastomeric curing envelope having an inwardly-opening U-shaped radial cross-section may be positioned over the new tread and a portion of the outer sidewall of the tire casing. A pair of annular elastomeric sealing rings may be secured about the inner peripheral beads of the tire casing and cooperate with the curing envelope to enclose the outer sidewalls of the tire casing. A vacuum may be drawn through a valve in the curing envelope for stretching the envelope into intimate contact with all surfaces of the tread. The tire may then be placed in an autoclave for several hours at an elevated temperature/pressure to cure the cushion gum and positively bond the tread to the tire casing. The curing envelope and sealing rings may be removed and the retreaded tire is ready for use. Thus, retreading envelopes have been used for years and typically are composed of a compound material that is heat resistant and molded in an annular configuration which covers the peripheral tread and the two opposing sidewalls of the tire casing. 
     More specifically, a precured rubber tread is applied over the crown region of a tire casing in which a layer of vulcanizable rubber-based material, or cushion gum, is interposed between the tread and the tire casing. A flexible airtight member or envelope is then placed over the tread to cover at least a portion of the sidewalls of the tire casing. The envelope may provide pressure to the tread for enhancing the bonding of the tread to the tire casing. The curing envelope may bridge the groove areas of the tread and cause a lower pressure to be applied at the base of the grooves, as opposed to the remainder of the tread. This unequal pressure can cause irregularities, such as reduction of nonskid, tread distortion, etc. 
     Fluid pressure may be applied to the curing envelope at a reduced pressure from that of the chamber curing pressure. A pressure differential of about 15 psi may be maintained between the pressure under the envelope and the chamber, thereby reducing the bridging effects of the envelope. Further, during a cure cycle, the cushion gum may not soften, flow, and cure uniformly across the bond line. The bond line is an interface in cross-section between the tread and the tire casing in the axial direction. Thus, the bond line extends from one axial tread edge to the other axial tread edge. Non-uniformity associated with the bond line may thus result in tread distortion and reduced adhesion of the tread to the tire casing. 
     Additionally, infiltration of air, steam or other contaminants may occur between the tread and the tire casing, thereby affecting the bond therebetween and/or the useful life of the retreaded tire. As a result, a flexible curing envelope may cover the tread and at least a portion of both sidewalls of the tire casing. 
     SUMMARY OF THE INVENTION 
     A tire retreading system in accordance with the present invention includes a chamber for receiving an assembly of a precured tread disposed over a crown portion of a tire casing and a controller for determining a first predetermined temperature in the chamber at a first predetermined amount of time. The controller subsequently determining a second predetermined temperature in the chamber at a second predetermined amount of time. The second predetermined temperature is less than the first predetermined temperature. 
     In accordance with another aspect of the present invention, the tire retreading system further includes a means connected to a manifold for evacuating fluid pressure from the assembly. 
     In accordance with still another aspect of the present invention, the tire retreading system further includes a means for evacuating fluid pressure from the assembly. 
     In accordance with yet another aspect of the present invention, the first predetermined temperature is in the range of 290° F.-330° F. 
     In accordance with still another aspect of the present invention, the second predetermined temperature is in the range of 210° F.-260° F. 
     In accordance with yet another aspect of the present invention, the first predetermined amount of time is in the range of 35 min-80 min. 
     In accordance with still another aspect of the present invention, the second predetermined amount of time is in the range of 50 min-100 min. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings in which like parts may bear like reference numerals and in which: 
         FIG. 1  is a schematic representation of a partial cross-sectional view of a tread/casing/envelope assembly for use with the present invention. 
         FIG. 2  is a schematic representation of a tire retreading system for use with the present invention. 
         FIG. 3  is a schematic representation of an alternate tire retreading for use with the present invention. 
         FIG. 4  is a schematic representation of another alternate tire retreading system for use with the present invention. 
     
    
    
     DEFINITIONS 
     The following definitions are applicable to this specification, including the claims wherein: 
     “Axial” and “axially” are used herein to refer to directions and/or displacements that are parallel to the axis of rotation of a tire.
 
“Radial” and “radially” mean directions and/or displacements from the axis of rotation of the tire.
 
“Inner” means directions and/or displacements toward the inside of the tire while “outer” means directions and/or displacements toward the exterior of the tire.
 
     DESCRIPTION OF AN EXAMPLE OF THE INVENTION 
     Referring to  FIG. 1 , there is illustrated a partial cross-sectional view of a conventional tire casing  10  having a precured rubber tread  12  located radially outwardly from a crown portion  14  of the tire casing  10 . Interposed between the tire casing  10  and the tread  12  is a layer of a vulcanizable material, also known as a cushion gum  16 , for adhering the tread  12  to the tire casing  10 . A flexible airtight member, also known as a curing envelope  18 , may be placed radially outward over the tread  12  and extending to enclose at least a portion of sidewalls  20  of the tire casing  10 , thereby enclosing an area  22  under the curing envelope  18 . The curing envelope  18  may be further supplied with an input means  24  connecting the curing envelope with a source of fluid pressure. 
     The curing envelope  18  may be attached by any suitable method, such as locking to the bead regions (not shown) or the sidewalls  20  of the tire casing  10 . The precured tread  12  may be a standard precured tread with one or more splice portions or it could be a one-piece, spliceless precured tread. When the entire assembly is placed in a chamber and the temperature within the tire chamber increases, heat energy may be transmitted to the tread/tire assembly causing a temperature increase therein. The temperature increase within the tread/tire assembly depends primarily on the tread and tire casing thicknesses, the distance from the surface, and the temperature in the chamber. Generally, the temperature rises initially on the outer surfaces and then penetrates inwardly. At the cushion gum bonding line  26 , the interface between the tread  12  and the tire casing  10 , heat may first be received at the tread edges  28  causing a temperature rise, which is subsequently passed axially inwardly therefrom. At a given temperature, the cushion gum  16  may begin to soften and flow. Again, this may occur initially at the tread edges  28  and propagate axially inwardly therefrom. This heating, coupled with inconsistent pressures, may cause the cushion gum  16  to flow unevenly from under the tread  12 , which may then cause tread distortion. 
     As the temperature continues to rise in the chamber, the cushion gum  16  begins to cure, and the flow thereof, stops. This curing, like the original softening and flowing of the cushion gum  16 , may begin initially at the tread edges  28  and progress axially inward therefrom. Therefore, because the temperature is not uniform throughout some areas of the cushion gum  16 , curing may occur in some areas while other areas are just beginning to flow. The result may be a non-uniform thickness of the cushion gum  16 . For example it may be thinner at the tread edges  28  while being thicker under the axially inner tread grooves. 
     Referring to  FIGS. 2 and 3 , there are illustrated schematic diagrams of conventional systems for retreading tires. An interior portion  52  of a pressure chamber or autoclave  50  may receive a plurality of tread/tire casing/curing envelope assemblies  56  therein for retreading. The interior portion  52  of the pressure chamber  50  may be pressurized from a fluid pressure source  54 . The fluid may be either heated before introduction into the chamber  50 , or heated within the chamber by a heater. The fluid may be hot air, steam, hot water, etc. 
     The curing envelopes  18  of each assembly  56  may be connected to a manifold  58  by process piping, conduits, hoses, etc.  60  passing through an outer wall  62  of the chamber  50 . A three position valve  64  may be located along the piping  60  to the manifold  58 . The valve  64  may be closed, open (allowing passage of fluid through the piping  60 ), or the fluid may be vented to the atmosphere. Alternatively, the three position valve  64  may be replaced with two 2-way valves and a tee: one valve for venting and the other valve for opening/closing the piping connecting the manifold  58  to the curing envelopes  18 . 
     Fluid may be supplied to the manifold  58  from a supply  66  through a conduit  68 . The supply  66  may be connected to either an external fluid pressure source  70 , such as that used by the chamber  50 , or may be connected to the chamber to bleed off some of the fluid used within the chamber. The supply  66  may include a solenoid valve  72  and a regulator valve  74 , connected in series. The supply  66  may also include a “variable rate of flow” valve  75  for controlling the rate of flow of fluid. This may be useful if the fluid source is external, such as that supplied within a plant, as opposed to the chamber pressure. 
     A temperature sensor  76  may be provided for sensing the temperature of the chamber  50 . The temperature sensor  76  may be connected to a controller  78  with a timing means. The temperature sensor  76  may include a temperature switch which opens and closes at a specific setting or a thermocouple which provides temperature information to the controller  78 . 
     At the start of a cure cycle, fluid pressure and heat may be supplied to the interior portion  52  of the chamber  50 . The temperature of the chamber  50  may increase until the normal operating temperature, or curing temperature, of the chamber  50  is reached, at which point the temperature may conventionally be maintained until curing of the tread  12  to the tire casing  10  is complete. Until a first temperature point or setting is reached, however, the envelopes  18  of the assemblies  56  may be vented to the atmosphere. 
     The venting to the atmosphere of the envelopes  18  may be accomplished in a number of different ways. One way is to use the three position valves  64  or the combination of two 2-way valves by setting them to the vent position until actuated by the controller  78  to move to either the open or closed position. Another way is to set the valves  64  (3-way or the 2-way combination) to the open position to allow the passage of air from the envelopes  18  to the manifold  58 . A solenoid valve  80  may then be connected to the manifold  58  that opens to vent to the atmosphere upon actuation. 
     The continual process of venting the envelopes  18  to the atmosphere may provide for the maximum pressure to be applied to the tread edges  28 . Once the temperature of the chamber  50  reaches a predetermined temperature, the timing sensor  76  may be activated for a predetermined length of time. This predetermined length of time may assure that the tread edges  28  seal and begin to cure. Alternatively, the length of time may be set to start once a cure cycle begins. However, in either case, both the timing function and the temperature setting must both be satisfied before the next step may be undertaken. As used herein, the sealing of the tread edges  28  means that the cushion gum  16  has softened and begun to cure such that air will not infiltrate into the bond line  26 . It does not necessarily mean that the cushion gum  16  has completely cured at the tread edges  28 . If a leak develops before the chamber  50  reaches full curing pressure, the system may automatically vent to the atmosphere and the retread should not then be affected. 
     Once both the time and temperature conditions have been satisfied, the venting to the atmosphere may be stopped and fluid may be allowed to pass through the series combination of the regulator valve  74  and the solenoid valve  72  to fill the manifold  58  and the envelopes  18  with fluid. The fluid within the envelopes  18  may provide a more uniform force against the bond line  26 . The pressure within the envelopes  18  may not exceed the pressure of the chamber  50 , but could be equal thereto. However, the difference may usually be less than the chamber pressure, such as for example 15 psi less than the chamber pressure. A range may be from about 15 psi to about 3 psi less than the chamber pressure. 
     The pressure regulator valve  74  may supply fluid up to its set point. Therefore, the pressure regulator valve  74  may be set at a value less than the curing pressure of the chamber  50 , as determined above. If a leak develops in one or more envelopes  18 , the pressure within the manifold  58  may begin to increase as fluid is passed from the chamber  50  into the envelopes  18 . A pressure relief valve  82 , attached to the manifold  58 , may relieve this excess pressure. If the leak is large enough that the leaking envelope  18  may be determined, the corresponding three position valve  64  for that assembly  56  may be vented to the atmosphere. A “variable rate of flow” valve  81 , however, may also be provided to the manifold  58  to relieve the pressure causing the pressure relief valve  82  to actuate, but which is not great enough to vent the three position valve  64  associated with the leaking assembly  56  to the atmosphere. 
     Referring specifically to  FIG. 3 , in some cases it may be more preferred to operate the retreading system in at least two stages. In a two-stage system, an additional pressure regulator valve  83  and solenoid valve  84 , in series with one another, may be connected in parallel with the first set of the regulator valve  74  and solenoid valve  72 . The second regulator valve  83  may be set at a pressure which is considerably less than the full chamber pressure, or chamber curing pressure, and less than the setting of the first regulator valve  74 . For example, the second regulator valve  83  may be set at a pressure which is equal to or less than 50% of the chamber curing pressure. In any event, the pressure selected must not cause air infiltration to the tread/casing bond. It further should be noted that the setting should not be greater than the chamber pressure at that given time. This may be prevented by a pressure switch  86  which is set to be equal to or greater than the pressure setting of the second regulator valve  83 . The pressure switch  86  may be connected to the controller  78  to prevent the solenoid valve  84  from allowing fluid to enter an envelope  18 . Thus, the envelopes  18  will not be pressurized more than the chamber pressure. 
     The operation of the conventional cure cycle may begin with the addition of fluid to the chamber  50 . As the pressure and temperature within the chamber  50  increase, the envelopes  18  may be vented to the atmosphere, as above. Once a first temperature setting has been reached, the venting of the envelopes  18  may be stopped and the solenoid valve  84  may be opened, allowing fluid to pass through the second regulator valve  83  to fill the envelopes  18  at the first pressure level set by the regulator valve  83 . After a length of time has passed, which should assure that the tread edges  28  have sealed, the solenoid valve  84  closes and the solenoid valve  72  is opened bringing the envelope pressure to the higher setting of the first regulator valve  74 . At the end of the curing cycle, blow down of the envelopes  18  may be accomplished by the solenoid valve  80 . Pressure may be further vented from the supply  66  by the addition of a three-way solenoid valve  90  along the external fluid pressure source  70  having an open, close, and vent position. 
     The conventional retread system may, for example, have the regulator valve  83  set for 65 psig and the regulator valve  74  set for 82 psig, for a curing pressure of 85 psig. The temperature setting could be from about 235° F.-240° F. for a system having a 260° F. curing temperature and the time period at which the system maintained the 65 psig may be about 15 minutes. 
     Referring to  FIG. 4 , there is illustrated a conventional system having two manifolds  100 ,  102 . In some retread chambers, the temperature within the chamber may not be exactly uniform. In such cases, it may then be better to activate those tire/tire casing assemblies in the hotter regions first, because they will reach the predetermined temperature first, and then activate those in the cooler regions at a later time. For example, in some chambers the central portions  104  of the chamber heats more slowly than the end portions  106 . In such cases, the retreading system may be supplied with a central manifold  100 , supplying retread assemblies in the central region  104 . The assemblies of the end portions can be supplied by a split manifold  102  having a first portion  102   a  and a second portion  102   b . The two portions  102   a ,  102   b  may then be joined by a conduit  108 . Each manifold  100 ,  102  may then be connected to a corresponding supply  110 ,  112  by conduits  114 ,  116 , respectfully. The supply  110 ,  112  may be either the single stage system having a regulator valve  118  and a solenoid valve  120  or the dual stage system as described above having additional regulator and solenoid valves in parallel with the first set  118 ,  120 . 
     In this manner, the tire/tire casing assemblies of one manifold  100  or  102  may be activated independently of the tire/tire casing assemblies of the other  102  or  100 . Each manifold  100 ,  102  may require at least one temperature sensor  122  to activate the corresponding portion  104 ,  106  of the system. More manifolds, supplies, temperature sensors, etc. may be employed as required to reduce the influences of temperature fluctuation within the chamber. 
     A method for retreading tires that reduces cure time would save energy expenditures, as well as increase production of retreaded tires per unit of time. Such a method, in accordance with the present invention, reduces cure time by initiating the cure process at a relatively high temperature range, such as between 290° F.-330° F., or more specifically 300° F. Prior to the cure being complete, the temperature may be stepped down to a lower temperature range, such as between 210° F.-260° F., or more specifically 225° F., to complete the cure. The warm up/cure time at the higher temperature may be in the range of 50 min-100 min, or more specifically 90 min, and the cure time at the lower temperature may be in the range of 35 min-80 min, or more specifically 44 min. This is a unique improvement over a standard chamber cure temperature (270° F.) for an entire cure cycle. 
     The method in accordance with the present invention improves curing efficiency of retreaded tires in a retread cure chamber by getting the tread/tire casing “hotter sooner,” thereby initiating the curing process earlier in the cure cycle and acquiring cure equivalents at a faster rate. This allows for shorter cure times and enables increased productivity and reduced energy usage. 
     Below is an example illustration of the cost and time savings of one example implementation of the step cure method in accordance with the present invention. The cure step temperatures were 300° F. and 225° F. 
     Production Simulation Trial 
     Step Cure Savings 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 Cure Cycle Time Savings 
               
            
           
           
               
               
               
               
               
            
               
                 Cure Method 
                 Warm Up 
                 Cure Time 
                 Blow Down 
                 Total 
               
               
                   
               
               
                 Standard Care 
                 40 
                 150 
                 5 
                 195 min 
               
               
                 Step Cure 
                 90 (Step 1) 
                 65 (Step 2) 
                 5 
                 160 min 
               
               
                 Cycle Time Savings 
                   
                   
                   
                  35 min 
               
               
                   
               
            
           
           
               
            
               
                 Energy Savings 
               
            
           
           
               
               
               
            
               
                   
                   
                 Cost of Elect/Cure 
               
               
                 Cure Method 
                 Steam Cond/Cure 
                 $.061/KWH) 
               
               
                   
               
               
                 Standard Cure Method 
                 374 lbs 
                 $0.823 
               
               
                 Step Cure Method 
                 332 lbs 
                 $0.670 
               
               
                 Energy Savings 
                  42 lbs of steam 
                 $0.153 
               
               
                   
               
               
                 Cure Cycle Time Savings - 17.9% 
               
               
                 Steam Savings - 11.2% reduction in steam usage 
               
               
                 Electricity Savings - 18.6% cost savings 
               
            
           
         
       
     
     While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the scope of the invention and the following claims.