IE45077B1 - Method and apparatus for bonding tread strips to tire bodies - Google Patents

Description

This invention relates to a method and apparatus for bonding precured tread strips to tire bodies.

According to the present invention, there is provided a method of : bonding a precured tread strip to a tire body by means of a self5 vulcanizing bonding material interposed between the tire body and the tread strip, said method comprising: applying to an assembly of tire body and tread strip a flexible impervious cover which extends over the tread strip and is sealed to the tire body, there being an outlet from the space between the said cover and said assembly via which air can be exhausted from such space, arranging the assembly in a vessel with said outlet connected to an exhaust point externally of the vessel, and the interior of the tire and the interior pf the vessel connected to a common fluid supply conduit via a manifold, supplying fluid through a first pressure regulator and said conduit into said manifold, passing fluid from said manifold into the interior of the tire and simultaneously passing fluid from said manifold through a pressure differential check valve and a second pressure regulator into the interior of the vessel to bring the tire inflation and vessel pressures to predetermined values while keeping the tire inflation pressure afc all times greater than the -2«3077 vessel pressure, effecting vulcanisation of said bonding material by heating while continuing to maintain the tire inflation pressure greater than vessel pressure, and thereafter reducing the fluid pressures in the tire and in the vessel.

In this method, the temperature in the vessel may be controlled and recorded automatically over a complete treatment cycle. Pressure reductions can be initiated automatically by a timer device on completion of the bonding operation.

The pressure reductions which follow the vulcanisation stage are preferably controlled to cause complete or partial collapse of the tire to aid in breaking the tire loose from a rim on which it is mounted. Preferably this is achieved by reducing the tire inflation pressure and the vessel pressure simultaneously but at different rates, the tire inflation pressure reduction being the faster. As an alternative, the tire inflation pressure may be reduced first, followed by reduction of the vessel pressure.

In certain methods according to the invention, the tire is left partially inflated while it cools, either within the vessel or outside such vessel.

In preferred embodiments, once the tire pressure and vessel pressure have increased to the predetermined values the differential between such pressure is kept constant until completion of the vulcanisation stage.

The maintenance of the vessel pressure lower chan tire pressure during the period in which tire inflation takes place is ensured automatically by the pressure-differential check valve between the aforesaid fluid supply conduit and the interior of the vessel.

The invention includes a method as hereinbefore defined wherein the said assembly of tire body and tread strip is one of a plurality of like assemblies which are simultaneously and identically subjected to the same -3method in the said vessel, each of the tires being connected to said manifold.. .., . .. . .

. By employing a common fluid pressure control system it is ensured that wide variations in inflation rates for different tires and slow reduction of pressure differentials after bondihg, such as are occasioned when using hand-operated valves for independently controlling pressures in different tires, can be avoided and there is a considerable saving in personnel time. The said method according to the invention makes it much easier to avoid introducing variables into the bonding operations such as may lead to tread and tire distortions and poor air exhaustion. For example, too low a tire pressure may result ih a poor bond along the centre of the tire periphery, whereas insufficient air removal may result in a poor bond along the shoulder.

Supplementary pressure-transmitting means may be disposed between the flexible cover and the tread strip of the or each assembly, for causing or promoting transmission of vessel pressure from said flexible cover to the bottoms of the tread grooves.

In one such embodiment of the invention a special liner is inserted between the tread strip and cover as described and claimed in the specification of our co-pending patent application No. 45076 . The liner in the form of a thin flexible perforated film of stretchable non-stick material, provides lubrication between the cover and the tread strip, and due to its stretch properties it allows the liner and cover to conform to the tread pattern during the pressing and bonding operation. This penetration of the liner and cover to the bottom of the tread grooves aids in exhausting all air from the space between the cover and the assembly of tire body and tread strip because the air in the tread grooves passes outwardly through the perforations in the liner and is then exhausted through the conduit provided for this purpose.

Ih addition penetration of the liner and cover into the tread pattern -4tends to stabilize the shape and position of the tread strip, thereby avoiding bond discontinuities which would result from deformation of the tread strip during bonding. The liner can be made of a variety of natural or synthetic materials such as natural or synthetic rubber, polyester, or polyolefin's such as polyethylene, polypropylene, polybutenes, polyisoprene and their copolymers. Polypropylene film has been found superior to films of rubber, polyethylene, mylar and other materials because even when it is perforated with many small holes it provides good lubrication without tearing or breaking during entry into the tread pattern and during subsequent removal. That is, it permits the cover to slip easily into the tread grooves without tending to hang up on and distort the tread, when the pressing operation causes the cover to press against the tread. This in turn aids in obtaining uniform pressure on the tread strip. The film also retains its non-stick properties so that it can be removed as one piece after the bonding operation. A particularly useful liner is polypropylene film of about 0.00125 inches thickness provided with spaced apart rows of 1/16 inch fine perforations or score marks. The rows may be spaced apart about 1 3/16 inches, and the fine perforations, or score marks in each row may be spaced apart about 3/16 inch. The spacings may be less or more by about 255!. During use the stresses on the film cause the fine perforations or score marks to open up in a direction transverse to the rows so as to form larger perforations of perhaps 3/16 inch diameter.

Strips or pads of special stretchable wick material may be placed between the flexible cover and the tire body and tread assembly at the location of the air exhaust connection to aid in the release of entrapped air. The strips or pads of about 4 inches in width are placed transversely of the tire body so as to extend from below the bond line on one side of the tire body over the tread strip to a location below the bond line on the other side of the tire. If necessary a similar strip may be placed circumferentially around the tread strip; this is often desirable if the tread strip is slick'', that;is, having no tread grooves. The preferred wick material is a porous flexible fabric having stretch properties in two directions which are at a right angle to each other.

The fabric is preferably woven fabric of 70 denier nylon filling yarn and 280 denier spandex warp yarn or equivalent structure having 160% stretch in the warp direction and 70% stretch in the width direction.

The fabric may be .020 inches thick with a yarn count of about 20 warp yarns per inch and about 60 filling yarns per inch. A pad for use TO as described above may consist of 4 to 6 layers of the fabric treated with bag coat lubricant. The function of the pad cooperates with the function of the perforated polypropylene film, if the latter is present, to release entrapped air and to achieve equal curing pressure on the tread ribs. The pads must be of sufficient width to extend beyond the base dimensions of the exhaust valves on the envelope. The pads are particularly useful in retreading large tires such as are employed on heavy-duty earth moving vehicles (off-the-road vehicles); this type of tire does not have small tread grooves, and normally a perforated lubricating liner is not used in the retreading operation.

In certain embodiments of the invention the problem of unequal pressure distribution in and on the tread strip and bonding medium is at least partially solved by inserting temporary pressure transmitting, members, or support members into the circumferential tread grooves during the pressing and bonding operation. The support members are preferably in the form of elastic rings or endless ribs each of which engages the bottom and sides of its respective groove so as to substantially'fill the groove. It has been found that the resilient material of the support members positively transmits the pressure of the pressing element to the bottom and sides of the grooves to distribute the pressure evenly.. This technique is very useful in -6i <13 0 7 7 retreading tires which have relatively narrow grooves, such as passenger car tires and airplane tires. It may also be useful at the tread splice area when retreading truck tires. The technique is generally unnecessary when the tread voids are sufficiently wide to permit the flexible cover to penetrate to the bottom of the voids.

Specifically it has been found that the presence of the rings during pressing and bonding are very effective to transmit the radial pressure of the pressing element evenly to the tread and bonding medium to avoid the formation of thick and thin areas of bonding medium to stabilize the tread strip against distortion of the tread ribs and the undertread portions under the grooves, and to prevent lateral displacement of the tread strip itself. A further advantage of the rings relates to the splice which is normally present between the two ends of the tread strip. The splice is usually made by placing bonding medium between the two ends of the tread'strip as it is wrapped around the tire casing. During the pressing operation this bonding medium is compressed between the two ends of the tread strip, and when the cover does not extend to the bottom of the tread groove some of this bonding medium may be forced radially outward into the groove where it cures and forms a dam which may completely fill the cross section of the groove at the location of the splice. The rings, in transmitting pressure to the splice area, prevent the formation of dams by forcing any displaced bonding medium against the bottom of the grooves so as to form thin layers.

The rings are particularly useful during bonding of tread strips which have been slit with a large number of closely spaced parallel cuts extending across the tread at an angle to the tread grooves. These slits or cuts penetrate radially into the tread ribs whereby each rib is divided into a large number of small separate tread elements which enhance the gripping power of the tread during road use. These tread elements, and consequently, the undertread portion of the tread strip -7are rather easily deflected or distorted during the pressing and bonding operation, but it has been found that the rings of this.invention are very effective in supporting or stabilizing the elements. The slitting of tire treads to improve performance is, broadly, a well-known /Specification No. technique as disclosed, for example, in United States Patent'3,683,728 (Keserve).

The rings may be inserted into the tread grooves prior to or after the conventional stitching operation. Stitching is the term applied to the step of applying a roller or rollers to the circumference of the assembly of tire casing, bonding medium and tread strip while rotating the assembly in order to provide initial adhesion of the tread strip to the tire casing prior to applying a cover to the assembly.

By. inserting the rings prior to the stitching operation the rings become firmly seated in the tread grooves by the action of the stitching roller, inasmuch as the latter presses against the outer surface of the · tread strip with sufficient force to temporarily deflect the tire casing radially inwardly.

The support members may be employed in any type of bonding operation involving the pressing of a tread strip against the tire casing by a , flexible pressing element such as a cover or by a solid pressing element such as smooth rigid moTd members or a flexible metal band.

The support members have particular advantage in the cover type of operation, because it has been found that it is very seldom possible ι to force the cover completely into the tread grooves due to the thickness of the cover which is required for strength and durability.

The cover may penetrate part way into the grooves, and, therefore, the support members need not always completely fill the grooves. The members need only substantially fill the grooves and by this is meant that the cover will tightly engage the outwardly facing surfaces of the members so that the pressure exerted by the cover will be transmitted -853 07 7 to the bottom of the grooves.

The preferred form of support members are resilient rubber rings capable of substantial elastic extension, each ring being formed from a length of rubber stock by stapling or splicing or otherwise securing the ends of the length together. Preferably the rings have circumferences less than the circumference of the tire and tread assembly so that they may be inserted into the tread grooves by stretching them over the circumference of the assembly and then allowing them to contact so as to fit snugly into the tread grooves, or at least to lie in the upper portion of the grooves so that they can be pressed completely into the grooves. The flexibility of the rings is such that they follow the normal serpentine or zig-zag shape of the grooves.

The cross sectional shape of a ring may be circular or other shape, it being only necessary 'that the flexibility and resilience of the material of the ring is such that the ring generally conforms to the shape of the groove when pressure is applied to the outer surface of the ring. It is important, however, that the ring have a width dimension which will permit easy insertion of the ring into the groove and subsequently provide support for the tread profiles when pressure is being applied. In the relaxed condition the ring may have a width dimension of 802-100% of the groove width. The height dimension of the ring may be 502-100% of the groove depth. As an example, a 0.250 inch diameter ring is appropriate for a passenger tread having a groove width of 0.250 inch to 0.300 inch and a groove depth of 0.4375 inch.

The rings may be of solid or tubular cross section, and if tubular they may be inflatable so that upon introduction of fluid pressure they expand into even tighter engagement with the sides and bottom of the tread grooves. The rings may also take the form of spaced apart ribs projecting from a common thin web which in the operative position of the rings will overlie the outermost surfaces of the tread ribs.

The present invention also includes apparatus for use in bonding . a precured tread strip.to a' tire body by means of self-vulcanising· bonding material interposed between the tread strip and the tire body, said apparatus comprising a vessel having means whereby it can be 5 heated and having releasable closure means for permitting entry and removal of a tire assembly and for sealing the vessel to permit its interior to be pressurized with fluid pressure, said vessel further having an outlet connectable to a space between a said tread strip and a member applied thereover to allow exhaust of air from between said strip and member, a fluid supply conduit having an inlet for receiving pressurized fluid from a source thereof, said conduit including a first pressure regulator and a manifold connected to said first pressure regulator to receive fluid pressure therefrom, an inflating conduit connected at one end to said manifold and connectable at the other end to the interior of the tire in said vessel for inflating the tire, means •connecting the manifold with the interior of the vessel, said connecting means including in sequence a pressure differential check valve which maintains the pressure to the vessel at a lower value than the pressure in-the inflating conduit when pressurized fluid is supplied to said manifold and a pressure regulator which maintains the pressure in the vessel at a lower value than the pressure in the inflating conduit after the tire has been pressurized to the pressure existing in the said manifold.

Preferably means is· provided for exhausting the tire and the vessel to atmosphere, said means operating to exhaust the tire before or at a faster rate than the vessel whereby vessel pressure will exceed tire pressure during exhaustion.

Further optional and advantageous apparatus features which can be adopted are defined in claims 13 to 17 of this specification..

Particular importance is attached to apparatus having a plurality -10S Ο 7 7 of inflating conduits connected to said manifold and connectable to different tires so that two or more tire and tread'assemblies can be simultaneously treated in like manner in the vessel. It is of great benefit to avoid the need for manipulation of hand-operated valves to control differential pressure during inflation and deflation of the tires as this is liable to result in a wide variation in inflation rates for different tires in the pressure vessel and in slow reduction of the pressure differential after bonding. Such manual processes moreover require considerable personnel time, and in addition the variables introduced into the bonding operation sometimes lead to tread and tire distortions and poor air exhaustion. For example, if tire pressure is too low a poor bond might result along the center of the periphery of the tire whereas if insufficient air is removed a poor bond along the shoulder might result.

A multiple tire treatment apparatus according to the invention can operate to apply a uniform heat and pressure treatment to all tires in the vessel. The employment of the manifold connected to each tire individually and to the interior of the vessel enables inflation of the tires and pressurisation of the vessel to proceed simultaneously, while by means of the pressure differential check-valve and pressureregulating devices, it can be ensured that the tires are always pressurized to a higher pressure than the vessel. The vessel temperature can be controlled and recorded automatically throughout an operating cycle. On completion of the bonding operation a timer and special exhaust valve arrangement can permit the tires to exhaust more rapidly than the vessel pressure so that the vessel pressure causes partial or complete collapse of the tires thereby rendering them loose on their rims. Alternatively a degree of tire pressure may be maintained so that the tires are still partially inflated when they are removed from the vessel. This is desirable when the tires are to be allowed -11to cure for an additional period of time, in an inflated condition • ' While cooling outside the vessel. Certain embodiments of the invention, selected by way of example, will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a pressure chamber for carrying out a bonding operation on a plurality of tires; Figure 2 is a schematic end view of the pressure chamber of Fi gure 1; Figure 3 is a schematic sectional view of a tire and tread 10 assembly being bonded together by a prior art method and illustrating in exaggerated form some of the distortion problems which may arise; Figure 4 is a schematic fragmentary view of a tire and tread assembly being bonded together using one embodiment of the method and apparatus of the present invention, in which support rings are provided in the tread grooves; Figure 5 is a view similar to Figure 4 illustrating the use of a porous wick at the location of the air exhaust conduit; Figure 6 is a view similar to Figure 4 illustrating the use of a perforated lubricating film between the tread and the cover.

Figures 1 and 2 illustrate a cylindrical pressure vessel 10 of sufficient size to receive a plurality of assemblies 12 of tire casings and tread Strips. One end of the vessel 10 is provided with a door 14 so that each assembly 12 mounted on a rack 16, which is movable along a track 18, may be placed in and withdrawn from the vessel 10.

The typical assembly 12 is illustrated in Figure 2. For each assembly 12 the pressure vessel 10 is provided with a flexible vent line 20 having an inner end 22 adapted to be releasably connected to a vent element 24 of the respective assembly 12. The -12Ί3077 outer end of the vent line 20 communicates with the atmosphere by way of a shutoff valve 28. For inflating the tire of each assembly 12 the vessel 10 is provided with a flexible line 30 having an inner end fitting 32 adapted to be connected to the tire inflating element 34 of the respective assembly 12. The fitting 32 includes a check valve which prevents flow of pressure fluid from the line 30 when the line 30 is not connected to the assembly 12.

The system for supplying fluid pressure to the interior of the vessel 10 and for inflating the tire of each assembly 12 includes IQ a manifold 44 whicn receives fluid pressure from an assembly provided with an inlet fitting 43 adapted to be connected to a pressure source such as compressed air. From the fitting 43 the fluid passes through a filter 52, a check valve 50, a solenoid valve 48 and a pressure regulator 45 into the manifold 44. Each of the several tire-inflation lines 30 is connected to the manifold 44 by a line 41 which includes a solenoid valve 62 and a manual shut-off valve 56. The assembly which transmits fluid from the manifold 44 to the interior of the vessel includes a differential check valve 42, a pressure regulator 40 and a pipe 38 connected to the vessel by a fitting 36. With appropriate valving changes pressure fluids other than air can be used.

The manifold 44 and the vessel 10 are provided with pressure relief valves 58 and with solenoid-operated exhaust valves 59 which exhaust to atmosphere.

Resistance heating elements 65 are located within the vessel for heating the interior of the vessel 10 to appropriate vulcanising temperatures of 190°F to 212°F. The heating elements 65 are protected from damage by perforated metal grids 66.

Operation of the vessel 10 is as follows: Several tire assemblies 12 are moved through the door 14 into -13the vessel 10 by sliding the respective supporting rack 16 along the fixed track 18. The vent conduits 20'and the pressure conduits’ are connected to the fittings 24 and 34, respectively. The valves 48 and 62 are open. Fluid under pressure, for example air at 110 psi (pounds per square inch), is then supplied to the fittings 43 so as to pressurize the manifold 44 and to begin inflating the tires and pressurising the vessel 10. The pressure in the vessel 10, and hence the pressure on the exterior of the assemblies 12, is maintained 15 to 20 psi less than the inflation pressure of the assemblies by means of the differential check valve 42. This valve 42 remains closed if the pressure differential across its valve seat is less than about 15-20 psi, and it opens if the differential is greater than about 15-20 pSr. Thus, this valve opens and allows fluid to enter the vessel 10 after a preset pressure has been obtained in the manifold 44. If the pressure in the vessel 10 increases over the preset differential pressure, the valve 42 closes by spring pressure and the tire assemblies 12 continue to inflate until the differential pressure is again below that at which the valve 42 is set to open. The valve 42 then opens and again allows fluid to pass into the vessel 10.

At the same time the interior of the chamber TO is heated to 190°F to 212°F by the electric resistance elements 65 which are automatically controlled by a thermostat (not shown).

Subsequent to the attainment of the required tire inflation pressure the differential pressure is maintained during the ensuing bonding operation by the pressure regulators 40 and 46. Tha pressure differential is necessary to maintain the corr’ect tire configuration and to effect a seal of the cover 122 (Figures.3-6) to the tire 110.

The pressure on the cover 122 forces all the air out from between the cover 122 and the tire 110 and this air flows to the atmosphere through the conduit 20. Simultaneously, the tread strip 116 is pressed -144 3 0 7 7 against the tire 110 and the bonding medium 118 vulcanises.

At the completion of the bonding operation a timer 60 is energized and allows the vessel 10 and the manifold 44 to exhaust.

Each assembly 12 also exhausts through the respective conduit 30 and the respective valve 55 to the manifold 44. Due to the relative size of the exhaust ports the assemblies 12 become exhausted before the vessel 10. This allows the pressure in the vessel 10 to collapse the tire 110 of each assembly and to thereby aid in unseating the tire from its rim 112.

By providing a solenoid valve 62 the tires 110 may be reinflated automatically at the end of the bonding process to a preset pressure and allowed to cool. Another alternative is to stop the exhausting of the tires at a preset pressure. An alternative is to reinflate the collapsed tires 110 outside the vessel 10 to a desired pressure and allow them to cool.

The reason for these alternatives is to provide inflation after bonding (post cure inflation) in order to reduce or eliminate shirnkage during the time the tire 110 is cooling. This a a particularly useful technique for passenger car tires and light truck tires.

Figure 3 illustrates in schematic form a bonding operation being carried out by the technique disclosed in United States Patent No. 3,236,709 and showing in exaggerated form some of the problems which can arise as a result of non-uniform distribution of pressure on the tread strip. There is shown a tire body 110 mounted on a rim 112 so as to be inflatable with air or other fluid through a suitable valve 34. A prevulcanised tread strip 116 has been wrapped around the circumference of the tire casing 110 with a bonding medium 118 interposed between the two. The bonding medium 118 may be any suitable rubber base composition which is capable of self-vulcanisation at elevated temperature and capable of forming a bond between the tread strip 115 and the tire casing 110. Preferably the bonding medium 118 is vulcanisable at temperatures between 190°F and 250°F.

The tread strip 115 has been prevulcanised under high pressure and as shown includes conventional grooves 120 and ribs each of which extends continuously around the circumference.

Overlying the tread strip 116 and the side walls of the tire casing 110 is a pressing element in the form of a flexible rubber annular cover 122 which extends around the circumference of the tire and tread assembly. The edges of the cover 122 are temporarily sealed to the side walls of the tire casing 110, as by being clamped at 124 between the side walls and the rim 112. The entire assembly of rim 112, tire body 110, tread strip 116, bonding medium 118 and cover 122 is disposed in a vessel such as an autoclave. In order to press the tread strip 116 against the circumference of the tire body 110 and .vulcanise the bonding medium 118, heated fluid such as a mixture of steam and air is injected into the vessel, a suitable connection being provided in the wall of the vessel for this purpose.

Alternatively, the pressurizing fluid may be air and heat may be supplied by other means such as the resistance heating elements ι of Figure 2. The tire will always be inflated above the pressure in the vessel.

The space between the inside surface of the cover 122 and the assembly of tire body and tread strip is in communication with the atmosphere outside the vessel by means of a conduit connected to the cover 122. The fluid pressure in the vessel thereby presses the cover 122 into tight contact with the assembly and forces the tread strip 116 against the circumference of the tire body 110, the air in the cover 122 being forced out through the conduit. -16-53077 The thickness and flexibility of the cover 122 and the width of the tread grooves 120 are such that normally the cover 122 cannot penetrate to the bottom of the grooves, as shown in Figure 3, even though this would be desirable. Instead, the cover 122 will normally penetrate only part way into the grooves 120 with the results that air will be trapped in the grooves and that there is less force at the bottom of the grooves 120 and less pressing of those portions of the tread strip 116 against the tire body. This alone may produce a weaker bond under the grooves 120. At the same time, there is maximum force being applied on the outer surfaces of the tread ribs 121 without there being any support for the sides of the ribs 121. The overall result of this state of non-uniform pressure being exerted on the tread strip 116 is that several forms of distortion of the latter and of the bonding medium 118 may occur. Qne form of distortion is that the undertread portions of the tread strip may become humped, as shown at 132 in exaggerated form. Also, the individual tread ribs 121 may pitch or tilt. Further, the bonding medium 118 may become distorted into thick and tnin portions 136 and 138 from movement of the individual ribs 121 or from lateral movement of the entire tread strip 116. Still further, as explained above but not shown in the drawing, bonding material from the spliced ends of the tread strip 116 may form a dam across one or more of the grooves 120, Figure 4 illustrates how these defects are avoided by the presence of support or stabilizing members in the grooves 120 during the pressing and bonding operation. In the illustrated embodiment, the support members are resilient flexible elastic 0-rings 140 which have been inserted into the grooves 120. The material of the rings 140 may be neoprene or other age resisting rubber having as original properties a tensile strength of at least 100 psi, 270% elongation and 60+3 Shore A Durometer and having aged properties after 8 days at -17230°F of 70 psi tensile strength, 70% elongation and 72+Shore A Durqmeter, In a relaxed condition, the rings 140 should have a circumference of about 75% of the / circumference of the assembly of tire body and tread strip.

The rings 140 are inserted in the grooves 120 by stretching them over the circumference of the tire body and tread assembly and pressing them into the grooves 120 prior to enclosing the assembly in the cover 122. It is preferred that the insertion step be carried out simultaneously with stitching the tread strip 116 to the tire body 110 because this ensures that each ring 140 will be properly seated in the bottom of its groove before the cover 122 is applied.

As shown in Figure 4, each ring 140 should fit snugly in its groove 120 so as to be in contact with the bottom and at least a substantial portion of the side walls of the groove 120. Under the pressure in the vessel the cover 122 contacts the outer surface of the rings 140, so that vessel pressure is transmitted to the undertread portions of the tread 116 directly below the grooves T20.

At the same time, the tread ribs are supported against lateral distortion. The result is that all portions of tread strip are pressed against the circumference of tire body 110 at essentially the same pressure, and there is no distortion of the bonding medium 118, the undertread, or the tread ribs 121.

Figure 5 illustrates the use of a pad 150 of porous flexible wick material, as described previously, disposed between the cover 122 and the tread strip 116. In this embodiment the tread grooves 120 are rather wide. The pad 150 and the cover 122 penetrate to the bottom of grooves 120 and therefore there is no need to provide supporting rings in the grooves 120.

Fugure 6 illustrates the use of a stretchable lubricating film 152 of perforated polypropylene between the cover 122 and the tread strip 116. The flexibility and lubricating properties of the film 152 -18permit the cover 122 to penetrate to the bottom of the grooves 120 to thereby press out all air and achieve uniform pressure on the tread strip 116 without deformation of the latter. Without the presence of the film 152 the conditions illustrated in Figure 3 may occur.

Claims (19)

1. A method of bonding a precured tread strip to a tire body by means of a self-vulcanising bonding material interposed between the tire body and the tread strip, said method comprising: applying to an assembly of tire body and tread strip a flexible impervious cover which extends over the tread strip and is sealed to the tire body, there being an outlet from the space between the said cover and said assembly via which air can be exhausted from such space, arranging the assembly in a vessel with said outlet connected to an exhaust point externally of the vessel, and the interior of the tire and the interior of the vessel connected to a common fluid supply conduit via a manifold, supplying fluid through a first pressure regulator and said conduit into said manifold, passing fluid from said manifold into the interior of the tire and simultaneously .passing fluid·from said manifold through a pressure differential check valve and a second pressure regulator into the interior of the vessel to bring the tire inflation and vessel pressures to predetermined values while keeping the tire inflation pressure at all times greater than the vessel pressure, effecting vulcanisation of said bonding material by heating while continuing to maintain the tire inflation pressure greater than vessel pressure, and thereafter reducing the fluid pressures in the tire and in the vessel.

2. A method according to CTaim 1, wherein the tire inflation pressure is than reduced at a faster rate/or before vessel pressure to cause collapse or partial collapse of the tire.

3. A method according to Claim 2, wherein after the vulcanisation stage the tire pressure is'reduced but not to zero, and the still partially inflated tire is cooled in that partially inflated condition.

4. A method according to any preceding claim, wherein after the tire pressure and vessel pressure have increased to said predetermined values, the differential between such pressures is kept constant until completion of the vulcanisation stage.

5. A method according to any preceding claim, wherein during tire inflation -204 ο Ο 7 7 and pressurising of said vessel the maintenance of vessel pressure lower than tire pressure is ensured automatically by the pressure-differential check valve between said manifold and the interior of the vessel.

6. A method according to any preceding claim, wherein said assembly is one of a plurality of like assemblies which are simultaneously and identically subjected to the same method in the said vessel, eaeh of the tires being connected to said manifold.

7. A method according to any preceding claim, wherein secondary pressuretransmitting means is disposed between said flexible cover and said tread for causing or promoting transmission of vessel pressure from said flexible cover to the bottoms of the tread grooves.

8. A method according to Claim 7, wherein said secondary pressure-transmitting means comprises a lubricating perforated and stretchable film or sheet.

9. A method according to Claim 7, wherein said secondary pressuretransmitting means comprises local pressure-transmitting elements located in the tread grooves.

10. A method according to any preceding claim including mounting the tire on a rim.

11. Apparatus for use in bonding a precured tread strip to a tire body Sy means of self-vulcanising bonding material interposed between the tread strip and the tire body, said apparatus comprising a vessel having means whereby it can be heated and having releasable closure means for permitting entry and removal of a tire assembly and for sealing the vessel to permit its interior to be pressurized with fluid pressure, said vessel further having an outlet connectable to a space between a said tread-strip and a member applied thereover to allow exhaust of air from between said strip and member, a fluid supply conduit having an inlet for receiving pressurized fluid from a source thereof, said conduit including a first pressure regulator and a manifold connected to said first pressure regulator to receive fluid pressure therefrom, an inflating conduit connected at one end to said manifold and connectable at the other end to the interior of the tire in said vessel for inflating the tire, means connecting the manifold with the interior of the -214 .¾ 0 vessel, said connecting means including in sequence a pressure differential check valve Vihich maintains the pressure to the vessel at a lower value than the pressure in the inflating conduit when pressurized fluid is supplied to said manifold and a pressure regulator which maintains the pressure in the vessel at a lower value than the pressure in the inflating conduit after the tire has been pressurized to the pressure existing in the said manifold.

12. Apparatus according to Claim ll, including means for exhausting the tire and the vessel to atmosphere, said means operating to exhaust the tire before or at a faster rate than the vessel whereby vessel pressure will exceed tire pressure during exhaustion to cause collapse or partial collapse of the tire.

13. Apparatus according to Claim 11 or 12 and including a venting conduit via which a sealed space between a said assembly and a cover member applied over the tread strip can be connected to atmosphere outside the vessel.

14. Apparatus according to Claim 13 wherein said venting conduit is connected to a flexible impervious member adapted to be positioned on a said assembly to form between such member and the assembly a space v/hich vents to atmosphere via said venting conduit.

15. Apparatus according to any of Claims 11 to 14 wherein there is a timer which at the end of a pre-set vulcanisation period automatically initiates pressure reduction in the tire and vessel.

16. Apparatus according to any of Claims 11 to 15 wherein said inflating conduit is one of a plurality of inflating conduits which are connected to said manifold and are connectable to different tires so that two or more tire body and tread assemblies can be simultaneously treated in like manner in said vessel.

17. Apparatus according to Claim 16 wherein the vessel has a rack on which a said plurality of assemblies can be suspended within the vessel.

18. A method of bonding a precured tread strip to a tire, such method being according to Claim 1 and substantially as herein described’w’tf) 5 reference to/the accompanying drawings. -22

19. Apparatus for use in bonding a precured tread strip to a tire, substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.