AU676379B2

AU676379B2 - Retreading device

Info

Publication number: AU676379B2
Application number: AU76111/94A
Authority: AU
Inventors: Paul Meyer
Original assignee: Az Formen & Maschbau GmbH
Current assignee: A-Z Formen und Maschinenbau GmbH
Priority date: 1993-08-05
Filing date: 1994-08-05
Publication date: 1997-03-06
Anticipated expiration: 2014-08-05
Also published as: WO1995004648A1; CN1111903A; JPH08506292A; JP3851347B2; CN1050323C; AU7611194A

Description

-1 The present invention relates to a retreading device for tires as well as to a method for retreading tires.

Tire retreading is currently carried out in two different ways: A. Mold cure retreading where the tire casing is prepared by applying a blank tread of uncured rubber with cushion gum layer facing the tire casing and is then cured. In this case the tread pattern is impressed in the tread strip and the connection to the tire casing is ensured by the application of pressure and heat in a mold, the mold receiving only one tire at a time. The working temperature is at about 150°C. This method is capital-intensive.

B. Precure retreading where a tread, already cured and patterned in a flat mold or a ring mold and sized to fit S* the tire casing, is bonded to the tire casing by a thin layer of previously uncured binding or cushion gum. This is done at a much reduced temperature (between 95 0 C and 120 0

C)

:20 and pressure in an autoclave, in which usually a number of tires can be treated at the same time. This method is far .less capital-intensive.

Both methods require the preparation of the tire casing (carcass), involving the removal of the worn tread, that is a buffing step to generate a uniform surface, curved in a plane parallel to the axis of the tire (the so-called "buffing radius" and the "buffing contour"). This results in a predetermined degree of roughness to enable the bonding of the new tread.

2 Especially in the retreading of truck tires, which represent much bigger unit values than automobile tires, a separate operation of repairing local damage which becomes apparent on the surface of the tire casing is carried out. This is done by grinding out the damaged area to expose a roughened and nonragged surface. Thereafter the "skives" are filled with heated repair material, which is also mostly cushion gum. This is often done with small extruders in the size of hand-tools, which, however, is a labor-intensive operation requiring much care and attention.

Especially in precure retreading, the binder or cushion gum has historically been prepared by a calendering operation as a wide sheet, with the processing temperatures commonly not exceeding 80°C. The cushion gum was cooled and rolled up with intermediate plastic layers being later cut into widths corresponding to different sizes of tire, and applied as part of the labor-intensive process of "building" a precured tire.

The reason for this method is that the cushion gum thus prepared does due to its compounding, designed to provide a 1'20 reasonable time of curing in the autoclave, begin to "age" in storage. the cushion gum already begins curing to such an extent that later it cannot fullfil its bonding function at all or only incompletely. By the same token, it is liable to "scorching" in an extrusion operation unless extrusion is carried out under very carefully controlled conditions, especially when the extruded sheet has to be as thin or thinner than the calendered sheet.

This involves extrusion dies having the width of a tire casing, approximately between 140 mm and 350 mm, but of a die opening of less than the 1.2 mm or 1 mm thickness for the cushion gum and, depending on the swell-factor, considerably less, down to 0.4 mm. This results in the generation of a high back pressure during the extrusion -3operation, and it is thus virtually impossible to keep the extrusion temperatures in the range considered safe by the rubber suppliers namely that of the calendering operation where exceeding the temperature by 6 to 15 0 C is already considered critical, depending on the compound.

As the guarantee for safe bonding is a liability of the rubber supplier, an extrusion apparatus for direct application of cushion gum to a buffed casing to be used by the individual retreader has not found support from the rubber suppliers.

Thus, although there have been direct application cushion gum extruders on the market for some years and also in use by a few retreaders, there has been no widespread application of this technology as yet.

1..5 Moreover, for the sake of safety of "green" adhesion, i.e.

adhesion before curing, the conventional industrial practice of spraying the prepared tire casing surface with rubber solution has been retained even by those few retreaders.

S..

However, such rubber solution contains volatiles which present a health hazard. Therefore, in some counties of California and S. one state of the Federal Republic of Germany, spraying such a rubber solution has already been prohibited, and retreaders must phase out this method. Most likely, this measure will spread worldwide.

5 From the U.S. patent 5,162,070 by the present applicant a binding or cushion gum extruder for direct extrusion onto a rotating tire casing is known. With this extruder the layer of cushion gum is basically applied to gauge 1.2 mm thickness or less and is applied to the tire casing with a gentle, elastic force whereby the die radius in the horizontal Splane corresponds to the biggest "buffing-radius" occurring in 4 practice. Outside the more or less wide band of applied pressure, the extruded cushion gum which cools very rapidly shrinks onto the outer casing areas and achieves good adhesion without air inclusion.

This principle works even when, for precured treads having thin "wings" or lateral flanks, the cushion gum has to be applied 20 mm or 30 mm down the sidewalls of the casing. The speeds of extrusion are varied for width adjustment, carried out by mechanically moved side plates, as well as for affecting the thickness of the applied cushion gum layer. For this purpose, the adjustment of the casing rotation speed may also be used within limits. The adjustments may be interconnected and preprogramed by a control unit.

European Patent Application 0 528 683 Al shows a similar 15 apparatus in which a different way of adjusting the width of the extrusion to that of the tire casing is proposed, namely the rotation of the extrusion head about its longitudinal axis out of the horizontal plane, in which it is situated for accommodating the greatest width of the surface of the tire casing.

To ensure conformity of the surface of the tire casing to the radius of the forming die in all angular positions, the die is provided with a spacer lip contacting the casing at a position which, due to the rotation of the tire 25 casing, is coming to rest upstream of the extruded cushion gum. The other lip, at small distance from the surface of the tire casing, shapes the outside of the layer of cushion gum.

The German Patent Application DE 40 25 459 Al teaches the spiral winding of a thin and narrow strip of extruded cushion gum onto a tire casing. The spirally wound strips overlap each other which does not reliably avoid air inclusions. This reference shares this feature with the well-known method of strip-sinding for building up tread blanks for mold curing, disclosed for example in German Patent 17 29 555.

All of these known methods operate the cushion gum extruders with small die slots, i.e. in the mode of high back-pressure 1poduced in the die, and therefore in an extrusion temperature range exceeding the operating temperature of the conventional calendering operation for the cushion gum sheet. Also, none of these methods can effect skive-filling.

The French patent application 23 59 700 deals with the filling of skives and damaged spots on unbuffed tires. In this case a tread strip is applied for mold cure retreading.

In order to achieve this a die head is provided with interchangeable dies, adapted to the desired contour of the 1 5 tire. The die is advanced to the surface of the tire casing, however, without contacting it and spaced at a fixed distance to the surface of the tire casing onto which rubber is to be applied by the extruder. The tread strip layer is then shaped from an agglomeration of rubber formed between the die outlet and the tire casing by rotation of the tire casing against the center of the die lying downstream, the die opening adjusting itself between the tire casing and the downstream curved die surface. The radial distance between these determines the thickness of the tread strip applied on the tire casing.

Such machines have found application in mold cure retreading, although they have, as far as is known, been used only on casings prepared and buffed in the conventional way and in this case have served to a limited degree the purpose of skive-filling.

One reason for this is that the material filling the skives is r tread rubber, compounded for mileage and road-holding, and not -6the soft and sticky cushion gum normally used. A more important reason is that the pressure generated by the extruder and pushing on the excess of plasticized rubber can freely expand and is not sufficient to ensure penetration of rubber into any but shallow skive holes, which must, however, be carefully shaped to allow easy flow of the rubber. In practice, this means that the skiving operator has to exercise judgement as to which skives to fill in the conventional manner and which can be filled by the machine. This introrl"ces a factor of uncertainty in quality control which many retreaders find unacceptable.

In this method, moreover, the joining of the beginning and the end of the rubber after a full rotation of the tire casing is a problem and this joint or so-called "splice" has generally required an additional operation, namely cutting off excess rubber and hence producing potential waste. It is furthermore disadvantageous that the thickness of the tread exhibits grert fluctuations when the carcass is not exactly centered.

Thus, there is clearly a need for a new and improved method and apparatus of retreading tires which overcomes one or more of the above problems or disadvantages in the prior art. That is, a method and apparatus in which the 20 safety in the production of retreaded tires is increased, while the labour and costs involved are reduced. The present invention is directed to providing such a method and apparatus.

According to one broad aspect, the present invention provides a tire retreading device including an extruder for extruding a layer of cushion gum onto a tire casing before application of a tread strip, said extruder having two lips defining an S•extrusion die, wherein one of said lips trails the other relative to a direction of movement of the tire casing and projects closer toward the tire casing than said other lip, and wherein the lip which is closer to the tire casing is formed as an applicator shoe adapted to be pressed against the tire casing under a prestress deforming the tire casing.

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I

-7- According to another aspect, this invention provides a tire retreading device for applying a tread strip to a tire casing, including a tread feeding device for feeding the tread strip to the tire casing and an extruder for extruding a layerof cushion gL ito the tire casing before application of the tread strip, said extruder having two lips defining an extrusion die, wherein one of said lips trails the other relative to a direction of movement of the tire casing and projects closer toward the tire casing than said other lip, and wherein the lip which is closer to the tire casing is formed as an applicator shoe adapted to be pressed against the tire casing under a prestress deforming the tire casing.

Preferably, the extruder is adapted to heat the cushion gum to a temperature within the calendering temperature range up to about 800C at an extruder temperature of about 850C. The device preferably further includes an alarm sensor with a limiting value of approximately 900C.

According to a furthlr aspect, the invention provides a method of applying a cushion gum layer for retreading tires including the steps of: feeding cushion gum through an extruder die to a rotating tire casing; and an applicator shoe of the extruder die against the tire casing to 20 deform the casing to the curvature of the applicator shoe as the cushion gum is fed through the extruder so that excess cushion gum fills any depressions or skives in the tire casing.

In a preferred method of the invention, rotation of the tire casing is started shortly 25 before activating the extruder, the layer of cushion gum is built up to full thickness from a thin layer sufficient to barely cover the roughness of the surface of the tire casing, and after a complete revolution of the tire casing, extrusion speed is cut so that the amount of excess cushion gum is reduced to zero upon completion of the cushion gum layer.

Surprisingly, the inventive tire retreading device achieves a complete filling of Sholes or skives upon conventional manual S, EP C:WI ORDELENoSPECIetDMo761.al4.uC -8preparation, grinding of the damaged locations. This is favored by a suitable selection of the temperature of the cushion gum whereby the adhesion of the cushion gum to the tire is sufficiently strong at the lower temperature such that a cushion gum layer against the pressing force of the inventive applicator shoe is entrained by the tire casing surface.

Due to the strong pressing forces the formation of air inclusions is prevented even -ore safely. At the same time, the lower temperature ensures that the cushion gum is worked within a definitely safe temperature range so that the specifications of the rubber manufacturers can be maintained as within the calendering process.

After filling of the skives and holes within the tire casing in an automatic manner by the inventive spreading of the warm and tacky--soft cushion gum material, a manual filling even with small repair extruders, is entirely obsolete.

It is especially advantageous in that respect that due to the applied pressing force the penetration of the cushion gum .:ee material into the roughness of the tire casing surface is "T0 ensured. It is only necessary that the buffed casing surface remains clean thus eliminating the spraying of a rubber solution for the "green" adhesion and thereby eliminating environmental pollution and also eliminating an entire work station.

Since the cushion gum layer safely penetrates the holes and skives due to the present excess of cushion gum, for example, *e in the form of a "rolling bank", which is rolled from the rotating tire, it is possible to eliminate the process of manual filling which with respect to quality assurance is labor-intensive and critical to the product quality.

The invention may not only be used for tire retreading with Apre-cured tread strips in which cushion gum that is more i -9expensive than the tread rubber is applied as a layer which is as thin as possible, but is also applicable for retreading with a curing mold where the base gum, especially for truck tires, may be less expensive than tread gum. This may then be applied such that the base gum is thinner in the center of the tire casing and stronger towards the shoulders in order to form a greater portion of the rubber needed for retreading as a base for the tire, as represented in Fig. 9. This results in cost and quality advantages.

It is especially advantageous when an extrusion die with two lips is provided which are adjustable to cover at least the width of the tire casing and at least are spaced apart from one another by a multiple of a median thickness of the cushion gum layer. Due to this, the extruder is advantageously operated at a low die resistance which allows for an extrusion temperature somewhat higher than the temperature of calendering the known cushion gum sheet.

Additionally, the lip which determined by the direction of rotation of the tire casing is downstream of the desired cushion gum flow projects relative to the other lip by a spacing that exceeds by a multiple the medium thickness of the cushion gum layer to be applied and is formed as an applicator shoe which is in contact with the tire casing surface.

S*

It is especially advantageous when the applicator shoe has a 25 curvature, which is not required to be entirely circular, but .ooo must partially have a radius that corresponds at least to the greatest possible buffing radius of any tire to be retreaded up to the extreme situation with the radius infinite, i.e. a straight line, such that under a great pressing force of the extruder the tire casing can be deformed in correspondence to this curvature. In the case of an infinite radius an even front surface of the applicator shoe will accordingly result.

10 According to a further preferred embodiment the applicator shoe has a chamfer having a depth that varies along this curvature.

During operation the cushion gum is thus extruded to form an agglomeration adjacent to this applicator shoe and is hydrodynamically entrained into the chamfer between the applicator shoe and the rotating tire casing and entrained into the gap. The agglomeration thus acts in the manner of a "rolling bank" of a mill or a calender, from which "rolling bank" cushion gum is forced with the pressing force of the extruder into the depressions of the buffed surface of the tire casing and into the holes or skives present. The great pressing force of the extruder deforms the surface of the tire casing in conformity to the curvature of the applicator shoe which makes obsolete the requirement of exchanging extrusion dies for different buffing radii of the tire casings.

A chamfer which is deep in the center and which is reduced to almost zero towards the sidewalls generates the greatest pressure for the cushion gum flowing in the center in order to 70 counteract a pressure peak within the applicator shoe which deforms the surface of the tire casing there against the stiffness of the steel belt. When the dimensions are chosen properly this will lead to the surprising result that a S.substantially even thickness of cushion gum across the surface of the tire casing is generated, as required for retreading by means of pre-cured tread strips.

III.

Contrary to this, a chamfer which is shallow in the center and becomes deeper towards the shoulders results equally surprisingly in a thickening of the cushion gum towards the shoulders, as may be required for the "base gum" in mold curing.

,Sh The chamfer angle relative to the surface of the tire casing 11 can be adjusted in a broad range to the requirements. In a preferred embodiment the angle is, for example, 150, however, substantially steeper or substantially lower angles or rounded recesses or rounded projections are possible if desired. The length of the chamfer relative to the entire height of the applicator shoe, in the tangential direction of the tire casing, may, for example, be two thirds whereby in this case also substantial variations are possible. Also, the length of the projection between the applicator shoe and the oppositely arranged lip of the extruder, for an average thickness of the cushion gum layer, can be, for example, ten fold, but other, substantially varying values may also be possible. The same holds true also for the distance between the applicator shoe and the oppositely arranged lip whereby this distance is preferably somewhat smaller than the aforementioned length of the projection.

The axis of extrusion preferably extends radially with respect to the tire casing.

According to an advantageous embodiment movable lateral forming projections for adjusting the width of the extrusion to the different width of the tire casing surfaces are provided. These projections may be provided with different curvatures in three dimensions in order to generate alternative cross-sectional surfaces for the sidewalls or lateral edges of the cushion gum, including a slight local thickening with a sharp edge or a tipering downward along the sidewalls of the tire casings, or combinations of these effects.

The applicator shoe may also be provided with a separate heating element, preferably operated electrically, which serves to raise the temperature of the shoe somewhat above that of the extruded cushion .gum to facilitate an "ironing" 7action. If the entire cycle of tire building takes about at

I

12 least 3 minutes, the application of the cushion gum should take one minute or less, and this heating element may be switched to operate only to assist during this ironing action, and allowing the applicator shoe to return to the lower temperature of the surrounding metal mass of the extrusion head during the rest of the cycle.

Further details, advantages and features will appear more clearly from the following description of several embodiments in conjunction with the drawing, in which: Fig. 1 shows a cushion gum extruder according to the invention of the type CTC (Cushion-to-Casing) by means of which the cushion gum is applied on the tire casing, and which according to the invention can be used in connection with an already existing retreading device of any manufacturer; Fig. 2 shows an inventive embodiment of a cushion gum extruder which is an integrated unit, which also applies tread strips, this being again of the type CTC of the extruder and a so-called Swing-Builder 20 according to the DE-OS 42 03 027, which was not prepublished and which is included herein by reference, .is used; Fig. 3 shows a further embodiment of a retreading device with a cushion gum extruder according to Fig. 1 employed in 25 a known tread extruder for mold cure retreading; Fig. 4 is a sectional, diagrammatic view of a detail of Figs.

1, 2 or 3, showing a part of the tire casing being flattened by the applicator shoe of the inventive extruder and having a cushion gum layer applied and I skives filled in the manner of the invention; 13

S.

a

S

Fig. 5 similarly to Fig. 4, shows a view of the curvature of the tire casing ring surface, being flattened by the contour of the applicator shoe, with Fig. indicating the distribution of radial forces in this action; Fig. 6 is a front-view of an extrusion die according to the invention with the chamfer shaped so as to produce a layer of cushion gum of an even thickness over substantially the entire width of the buffed crown of the casing, with Fig. 7 indicating this effect on a section of a tire casing; Fig. 8 is a front-view of an inventive die with the chamfer shaped so as to produce a thickening of the layer of i cushion gum towards the sidewalls; Fig. 9 indicates this effect on a section of a tire casing, showing additionally a tread rubber blank on top of the cushion gum, as it may appear in tire building in the embodiment according to Fig. 3; and Fig. 10 shows an embodiment of the applicator shoe which is 20 modified relative to the illustration in Fig. In the embodiment according to Fig. 1, a cushion gum extruder is according to the invention provided with an extrusion head 11 and an extrusion die 12, which will be described in detail in Fig. 4 and the following ones. The tire casing 13 is mounted on a tire builder 14 of conventional construction, a central drive for rotating the casing being provided. The extruder 10 is slideable in the directions to and from the casing, by means of four ball bearing boxes 16, two of which 7 are shown and which allow for easy gliding of the extruder y \STa

OF

I L 14 on rails 17. A working cylinder 18 moves the extruder relative to the tire builder 14, preferably pneumatically, which additionally provides for an easily adjustable approach force for the applicator shoe of the die 12 for deforming the casing 13, as will be described infra.

The tire builder 14 additionally incorporates a stitching device 19 for the precured tread after having been applied to the cushion-gummed surfaces 15 of the casing in a known manner.

Fig. 2 shows an integrated unit of a cushion gum extruder and tire builder, like reference numerals indicating here as well as in the further figures like parts as in Fig. 1. Tn place of a mechanism providing for horizontal displacement of the extruder 10 relative to the casing 13, here the tire builder features a swing arm 20 movable about a pivot 21 by a linear screw drive 22 powered by an electric motor 23.

.This arrangement provides for the tire casing 13, in the positi-' shown, to move in a circular arc which approximates to the horizontal and to be pressed substantially radially 20 against the inventive extruder die 12, an approach force being monitored by the drive motor 23 of the linear screw drive 22 S" acting on the swing arm In movements away from the extruder 10, the swing arm 20 can :o serve to allow for easy mounting of the casing 13 from the 25 floor as well as pressing the casing 13 into the unit for the application of the tread strip and for stitching 24, which can be pivoted towards the casing for guidance of the precured tread from the roller table 25 and for applying at least the stitching force against the casing by means of the roller 26.

This embodiment produces a particularly effective and fast /integration of all the steps of tire building for a precure 15 process, details of which may be taken from German patent application DE-A1-42 03 027, which was not pre-published and which is incorporated herein by reference.

In both units shown, the various movements of the parts relative to one another as well as the extrusion speed can be coordinated by a process control unit, allowing for various actions thereof to be initiated and, where needed, defined by an operator via a control-panel 27 shown in Fig. 2, but equally applicable to Fig. 1.

This applies particularly to the coordination of the rotation of the casing, of the relative displacement and pressing force between the die and the casing, and the timing of the starting and stopping of rotation of the casing and of the extruder screw, which will be more particularly described with reference to figures explained infra.

999*9 0 0 Fig. 3 shows an embodiment similar to Fig. 1, wherein a tire 9 0 builder with a tread extruder for mold curing is employed.

The embodiment comprises a tread extruder 30 feeding the p ot extruded tread blank 31 into a 2-loop accumulator 32 which is controlled by dancer potentiometer 33 in a known manner. The accumulator loop control 32, operating in a known manner, stops the extruder when the loops are full and restarts it after withdrawal of the tread for tire building. However, the speed of the tread extruder 30 is preferably set to simply provide f. r increases and decreases of the loops without coo.

interruption so that a stable tread extrusion is provided.

An operator positioned near the control panel 27 will load and unload the casing and feed the tread, cutting it by a knife.

He will also form the tread splice and will operate the stitching device 19 in a known manner.

The action of the cushion gum extruder is automated as 16 described in the following. Automation may also be used for the tread application, cutting, splice formation, and stitching, however, the simplest version of tread handling here shown suffices to illustrate the present invention. As far as the cushion gum extruder in this and in the further examples is concerned, it operates fully automatically to such an extent that it will never interfere with the operation of tire building essentially to be performed by one man.

Figs. 4, 5 and 6 show the downstream lip of the /extrusion die 12 which is according to the invention in contact with the crown of the casing 13, the extrusion injection head being shaped in the form of an applicator shoe This applicator shoe 40 has a vertical distance to the lower lip 41, between which the depth of the die or of the injection head is formed and it also projects past the lower lip towards i the casing by another distance, these distances always being a multiple of the median thickness of the layei 15 of the cushion gum to be applied.

S

These features effect that the extruder can operate against a low die resistance with an extrusion temperature, which is closer to that of conventional calendering than to that of 0 known extruders, and that adjacent to the applicator shoe an agglomeration of rubber is formed.

fees*: ooA chamfer of the applicator shoe causes rubber to be drawn by the rotation of the tire under the contact surface of the buffed crown. Moreover, the chamfer makes the agglomeration of rubber act like a rolling bank of a mill or a calender, from which the layer of cushion gum is formed with a thickness determined by the size of the rolling bank and the magnitude of the approach pressure of the extruder as well as by the deformation resistance of the crown of the casing.

17 The size of the cushion gum rolling bank, on the other hand, will determine what size of holes or skives can reliably be filled with respect to the pressing force of the extruder to ensure in-depth penetration, and a bigger cushion gum rolling bank can fill a group of holes or skives occurring adjacent to one another, without interrupting the intervening thin layer of cushion gum.

An electrical heating element 43 may be provided for heating the applicator shoe to a temperature above that of the rubber mass. The heating element 43 may be switched to operate only during the cushion gumming operation, corresponding to, for example, one fourth of the entire tire building cycle, making it possible that the shoe returns almost immediately to the temperature of the surrounding metal mass during the remaining A5 steps of the tire building cycle. In Fig. 4, reference numeral 48 indicates some filled skives and 49 indicates still open skives in the surface of the carcass.

Fig. 5 provides an illustration of the deformation of the casing, Fig. SA indicating a pattern of pressure variation across the crown or width of the tire casing due to the varying resistance to deformation of the steel belt of the casing.

0* Figs. 5 and 6 show also side pieces 45, which are slideably adjustable by a suitable mechanism, not shown, to work different crown widths.

Fig. 5 shows two versions of such side pieces which differ by having different forward projections: Projection 46, see also Fig. 6, is designed to limit the width of the cushion gum exactly to the width of the ring surface of the tire casing.

Projection 47 allows for the agglomeration of rubber to reach to a certain length down the flank of the casing to provide a ,ecovering of cushion gum there also.

18 Fig. 6 ,shows the applicator shoe 40 with the chamfer 42 being deeper in the center and becoming shallower towards the flanks, as shown diagrammatically in Fig. 6A.

Fig. 7 shows a portion of a casing 13 provided with a thin and substantially uniform layer of cushion gum by the shoe chamfered as in Fig. 6, with the side pieces 46.

Fig. 8 shows an applicator shoe 50 with a chamfer 51 which is shallower in the center and becomes deeper towards the flanks, as shown diagrammatically in Fig. 8A.

Fig. 9 shows the cushion gum layer 52 resulting from this, which becomes thicker towards the flanks. Using also a side piece 47, the cushion gum reaches to a certain length down the flanks of the casing. 53 shows the corresponding tread blank as it would appear after complete tire building according to the embodiment of Fig. 3.

In a preferred mode of operation of the inventive retreading device, the rotation of the casing will be started a predetermined small period of time before the activation of the extruder, so that the thickness of the cushion gum builds t" ':20 up from a thin film barely covering the roughnesses of the buffed surface until the extruder has reached its preset speed. If the casing has completed substantially a full rotation, the extruder speed is reduced to zero over substantially the same distance that was used to build up the full thickness of the cushion gum layer. Over this distance, the cushion gum rolling bank is reduced to zero so that a smooth "splice" is formed. In this way, there is no waste, and for the next tire a newly formed cushion gum rolling bank can be used.

SSince, as mentioned, the size of the cushion gum rolling bank y/P 5-T and the approach pressure of the extruder determine what size 19 of skives can be filled, it should be noted as a surprising result that the skive-filling with the inventive apparatus is more reliable than that performed by an operator, who always would have to take due care and pay close attention in this task. This effect may be aided by cushion gum material.more specifically formed for this purpose and also for the purpose of adhesion without rubber solution spraying, although good results have been obtained with normal cushion gum materials presently available.

With respect to the extrusion temperature a direct comparison with the aforementioned US patent 5,162,070 can be drawn: In order to achieve a cushion gum application time of 1 minute for a truck tire of the size 12-R-22.5, an extruder speed of 100 rpm against the die resistance of a slot of 0.4 mm width :15 had to be employed, resulting in a cushion gum layer of 1 mm thickness at an extrusion temperature of approximately 90 0

C.

With an inventive extruder die of a slot width of 2.8 mm the extruder only had to be operated at 40 rpm resulting in an extrudate temperature of 80 0

C.

:20 The embodiment of the applicator shoe 40 illustrated in Fig.

10 shows an applicator shoe with a straight ironing edge. The radius of curvature of the applicator shoe 40 is in this embodiment in so far equal to infinite. When applying the cushion gum layer the applicator shoe deforms the tire casing such that a shallow contact over the entire width of the casing is produced although the casing is slightly arched in a transverse direction. The pressure distribution in so far corresponds to that of Fig. 5A the center portion, however, being more stressed.

It is particularly advantageous that for the forming gum s rolling bank the roughened surface of the tire casing acts l ike a kind of feed pump for solid matter, which entrains a 20 gum layer in spite of the high approach pressure at the applicator shoe 40. All the skives to be repaired are filled, the filling being effected more reliably than by manual repairing with a hand extruder. Due to the considerable pressure applied all the fine scores, grooves and other depressions of the casing will also be filled evenly, which also serves to improve the fixing of the cushion gum layer in the casing, and also venting is already prepared which otherwise has to take place while autoclaving.

The thickness of the cusion gum layer can be adapted to the requirements to the desired extent, adapting the evenness of the cushion gum layer over the width of the tire being easily possible due to the different possible shapes of the applicator shoe.

It is further particularly advantageous that a splice virtually is not produced when applying the cusion gum layer.

S By starting the rotation of the casing simultaneously with the beginning of the extrusion the cushion gum rolling bank will first build up slowly so that the cusion gum layer will also first increase slowly to the preset thickness. When the extruder is correspondingly switched off in due time before :the desired thickness is obtained, a kind of overlapping between the beginning and the end of the cusion gum layer is obtained since the cusion gum rolling bank will then dissassemble evenly with the end of the rotation. By a corresponding coasting and removal of the casing the splice will then be spread off completely.

A further particular advantage is that due to the strong pressure applied additional rubber solution spraying of the casing will become obsolete if the application takes place within a few hours after roughening of the casing. This is preferred for various reasons.

21 It is also easily possible to apply so-called wings on the treads by applying a cushion gum sheet of about 100 mm width manually in a known manner. This is also possible for treads with a cambered lower surface, so-called contour treads, which taper off to very short wings, wherein a further possibility for such treads is forming side pieces 46, 47 correspondingly so that up to a depth of 30 mm spreading will be effected reliably down the flanks.

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Claims

1. A tire retreading device including an extruder for extruding a layer of cushion gum onto a tire casing before application of a tread strip, said extruder having two lips defining an extrusion die, wherein one of said lips trails the other relative to a direction of movement of the tire casing and projects closer toward the tire casing than said other lip, and wherein the lip which is closer to the tire casing is formed as an applicator shoe adapted to be pressed against the tire casing under a prestress deforming the tire casing.

2. A tire retreading device for applying a tread strip to a tire casing, including a tread feeding device for feeding the tread strip to the tire casing and an extruder for extruding a layer of cushion gum onto the tire casing before application of the tread strip, said extruder having two lips defining an extrusion die, wherein one of said lips trails the other relative to a direction of movement of the tire casing and projects closer toward the tire casing than said other lip, and wherein the lip which is closer to the tire casing is formed as an applicator shoe adapted to be pressed against the tire casing under a prestress deforming the tire casing. S. 20

3. A tire retreading device according to claim 1 or claim 2, wherein said applicator shoe has a chamfer that extends at an angle in the range of 20 to S. and preferably between 100 and 300, to the surface of the tire casing. S* S. i

4. A tire retreading device according to any one of the preceeding claims, 25 wherein said applicator shoe projects by a length in the range of three to fifteen *d times, and preferably about ten times, a thickness of the cushion gum layer o° relative to said other lip of the extruder. A tire retreading device according to any one of the preceeding claims, wherein spacing provided between said applicator shoe and said other lip is in the range of two to twelve times, and preferably about eight times, a thickness of the cushion gum layer.

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6. A tire retreading device according to any one of the preceeding claims, wherein said applicator shoe has a radius of curvature that is equal to or greater than the greatest radius of the surface of the tire casing or is essentially straight over its entire width.

7. A tire retreading device according to any one of the preceeding claims, wherein the extruder is adapted to heat the cushion gum to a temperature within the calendering temperature range up to about 80 0 C at an extruder temperature of about 85 0 C, and wherein the device further includes an alarm sensor with a limiting value of approximately 90 0 C.

8. A tire retreading device according to any one of the preceeding claims, wherein the die of the extruder has a chamfer which varies over the width of the tire casing, said applicator shoe having a positive or negative curvature in the die to provide maximum chamfer either in the center or at the sidewalls of the tire, respectively. i 9. A tire retreading device according to any one of the preceeding claims, :wherein the cushion gum has an adhesion to the tire casing and a viscosity such 20 that the cushion gum is conveyed against a pressing force of the applicator shoe into a gap f'rmed by the cushion gum layer between the tire casing and the applicator shoe.

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10. A tire retreading device according to any one of the preceeding claims, 25 wherein supply of the cushion gum through the extruder is controlled electronically, a cushion gum rolling bank which in use forms opposite said applicator shoe acting as an indicator of cushion gum consumption, said cushion gum rolling bank being maintained by supplying sufficient new cushion gum for depressions and damaged areas on the surface of the tire casing to be completely filled. EP C:\WINWORD\ELLEN\SPECIDDM7611 i-94.DOC I -24-

11. A tire retreading device according to any one of the preceeding claims, further including a heating device in the area of the die of the extruder for heating said applicator shoe during application of the cushion gum layer.

12. A tire retreading device according to any one of the preceeding claims, including forming projections disposed laterally adjacent to the applicator shoe for guiding lateral portions of the extruded cushion gum, the distance between said forming projections being adjustable via an adjusting device.

13. A method of applying a cushion gum layer for retreading tires including the steps of: feeding cushion gum through a die of an extruder to a rotating tire casing; and pressing an applicator shoe of the extruder die against the tire casing to deform the casing to the curvature of the applicator shoe as the cushion gum is fed through the extruder so that excess cushion gum fills any depressions or skives in the tire casing.

14. A method of applying a cushion gum layer according to claim 13, wherein: 20 rotation of the tire casing is started shortly before activating the extruder; the layer of cushion gum is built up to full thickness from a thin layer sufficient to barely cover the roughness of the surface of the tire casing; and after a complete revolution of the tire casing, extrusion speed is cut so that a o the amount of excess cushion gum is reduced to zero upon completion of the 25 cushion gum layer.

A method of applying a cushion gum layer according to claim 13 or claim 14 wherein feeding of the cushion gum is controlled electronically; the method further including the steps of: forming a cushion gum rolling bank opposite said applicator shoe to act P C;\WINWORDIELLEN SPECIDOM7 •111 4.DOC '~cArro I I-M as an indicator of cushion gum consumption; and feeding sufficient cushion gum to the rolling bank for depressions and damaged areas on the surface of the tire casing to be completely filled.

16. A method of retreading tires including the steps of: applying a cushion gum layer according to any one of claims 13 to 15; and feeding and applying a tread strip to the tire casing.

17. A tire retreading device substantially as herein described with reference to the accompanying drawings.

18. A method of retreading tires substantially as herein described with reference to the accompanying drawings. DATED: 18 December, 1996 PHILLPS ORMONDE FITZPATRICK S. 20 Attorneys for: AZ FORMEN-UND MASCHINENBAU GMBH *o EP C:'WlNWORD\EILENZSPECIDDM7111i94.DOC now Abstract With a tire retreading device a tread strip is applicable on a tire casing and it has a tread feeding device for feeding the tread to the tire casing and an extruder for extruding a cushion gum layer. The apparatus has two lips defining an extrusion die, in the direction of movement of the tire casing the one lip behind projecting relative to the other lip toward the tire casing. The lip which is closer to the tire casing (13) is formed as an applicator shoe (40) and it is pressed against the tire casing (13) under a prestress deforming the latter one. (Fig. 4)