EP0351308A1 - Temperature control process for an open-ended tunnel furnace, and device for carrying it out - Google Patents

Abstract

Temperature control in a tunnel oven (1) comprising heating or cooling means (4) arranged along its side walls and means (5) for blowing a gaseous fluid at a given temperature in an essentially perpendicular direction to the conveying direction of the objects (3) passing through said tunnel oven. The temperature prevailing in an area affected by the blowing of the gaseous fluid is detected, and the movement of the blowing means (5) is organized parallel to the conveying direction of the objects (3) using a servo-control so that the blowing means move automatically when the sensed temperature deviates from a predetermined temperature. Application in particular to the retraction of a heat-shrinkable sleeve for the decoration and / or protection of bottles, and to the cooling of glass containers.

Description

The present invention relates to the field of heat treatments of objects passing through a tunnel oven open at its two ends, and more precisely the temperature control in such tunnel ovens.

The tunnel ovens concerned comprise means for conveying objects, heating or cooling means arranged along their side walls and means for blowing a gaseous fluid at a given temperature in a direction essentially perpendicular to the direction of conveying of the objects.

The blowing of gaseous fluid, for example air, can be used in such tunnel ovens as well for cooling as for heating of the conveyed objects, and it is in this broad acceptance that the term "tunnel oven "should be understood in the context of the present invention.

Many fields of application may in fact be concerned, and there may be mentioned, for cooling, the case of glass bottles which should be cooled in a controlled manner after baking mainly for their lightening, and for the heating in the case of the application of a section of heat-shrinkable sheath or sleeve on an object, case in which the sleeve, loosely threaded on the object to be decorated and / or protected, is heated to a temperature higher than that of softening of the constituent film for its retraction on said object.

The invention relates more specifically to the latter case, but it will be understood that this is only a particular application of the temperature control method which is the subject of the said invention, and that the latter is in no way limited to such application.

It is now well known to use certain plastic films, to print them or not, and to form them in a tube by sealing the two folded edges one over the other of the strip, in order to be able to decorate and / or protect an object, or more particularly the packaging of a product.

This is how we are increasingly providing containers such as bottles, bombs, aerosols, flasks, cans and other packaging objects, with a protective sleeve or sheath, or even with a protective ring. tamper-evident, made of heat-shrinkable plastic. This sleeve is placed around the container and, after external heating to a temperature higher than that of softening, it must conform with the minimum of deformation to the outline of the container. To produce such retractable sleeves, plastic films (generally made of polyvinyl chloride) are used to which a memory is imparted during their manufacture, and in general qualified as retractable. These films are generally stretched essentially in the direction of the perimeter of the objects to be coated, so that they acquire a memory in the stretching direction (or percentage of shrinkage) of up to 70% (in fact the commonly used films have a percentage between 50 and 60%); the memory in the longitudinal direction, that is to say that corresponding to the height of the sheath section is only of the order of 3 to 7%.

We know that to give a memory to a plastic film (polyvinyl chloride, polystyrene or polypropylene for example), it must be heated to a very precise temperature, generally chosen lower than the glass transition point of the plastic, while subjecting it to transverse and / or longitudinal traction. By heating the film, it causes its softening, allowing the creep of the molecules and thus making it possible to increase the initial dimensions of the film, but in return with a reduction in the initial thickness of said film.

Such films, which generally are printed and / or decorated to serve as labels on the object to be coated, are called "mono-oriented" or "predominantly mono-oriented". If crystal polyvinyl chloride films are used, printing can be done backwards, resulting in an external gloss combined with protection of the printing against the risk of erasure. In addition to the decorative aspect, it may be a question of protection, not only for the inviolability of a container, but also as a barrier, for example to reduce the loss of perfume with a polypropylene packaging, or the loss of carbon dioxide for carbonated drinks in polypropylene terephthalate packaging.

Such an application has found a large outlet in the sale of products offered to the general public, since it allows in particular a great wealth of decoration, with possible reproduction of photographic photographs, and a use for objects of very varied contours.

If the techniques for manufacturing shrink films, their printing and putting them into tubes, as well as the fitting of sleeves around objects or packaging are now practically mastered, the same cannot be said for the operation of shrinking said films. sleeves, all the more so since the object or the packaging has an irregular section of triangular, square or rectangular shape, with faces having convex and / or concave zones.

It is indeed imperative that the retraction is carried out uniformly around the object or the packaging, that is to say without crease, crimp or crater of the sheath, and without deformation of the impressions produced. on the film, which, in addition to the purely aesthetic aspect, also has an effect on direct use (reading of bar codes and / or instructions for example).

The difficulties encountered in mastering the retraction operation come largely from problems of an essentially thermal order: in fact, each oriented plastic film has, depending on its nature and formulation, its own softening point and its temperature for rearranging the zones crystalline and amorphous. Knowledge of the rearrangement temperature is of primary importance for the shrinkage of a film, because the film restores all of its memory as long as its temperature remains below that of rearrangement, chosen as the stretching temperature, the memory acquired being at least partially lost if this temperature is exceeded.

It therefore appears essential to be able to control the temperature with great precision in the tunnel oven used.

More especially in the context of this particular application, it is very advantageous to be able to choose a shrinking temperature as low as possible so that the printed decoration is influenced as little as possible by the action of the longitudinal shrinking.

Indeed, a very accentuated mono-orientation (50 to 60% in the transverse direction, and 5 to 7% only in the longitudinal direction) induces an instability of the film which is stronger in the transverse direction, and a beginning of retraction in the transverse direction operating at a lower temperature than that of the retraction in the longitudinal direction (it is generally called 20 to 25% of the retraction in the transverse direction before the longitudinal retraction begins). This makes it impossible to know precisely the variations in the position of a given area of the decoration, and means that the decoration cannot be correctly amplified in both directions when it is printed to take account of the shrinkage of the film, in order to remove printing distortions.

Other difficulties in obtaining a perfect envelope, free from any defect and any deformation, on the object to be coated, arise from the fact that the sleeve is in contact with the object during the softening phase, before the actual retraction, this contact inevitably resulting in absorption of calories as long as the thermal equilibrium between film and wall of the object is not reached. This is practically inevitable insofar as, before laying, the sleeve is in the form of a flat sheath section, it must be opened to be threaded onto the object to be coated. The fold is never completely erased. The opening section of the sleeve is thus never a circle, but rather an ellipse very flattened at its ends: therefore, this section does not correspond to the section of the object, a fortiori if this section is triangular or quadrangular. The calories absorbed by the film to shrink are then dispersed on the surface of the object on the film-object contact areas. However, the surface of the areas in contact tends to increase at the same time as the film softens, the latter collapsing against the object to be coated; in addition, the film area near the fold is always out of contact with the object to be coated due to its spring effect which moves it away. It is understandable, under these conditions, that it will not be possible to obtain a homogeneous retraction between the surfaces in contact with the object and those which are not. The temperature thus varies on the periphery of the film according to the zones, leading to different percentages of retraction when this is desired to be homogeneous: certain parts of the film will have their retraction stopped as soon as they come into contact with the object, while the zones of the fold (s) will retract too much in relation to what was originally planned.

In an attempt to resolve some of the difficulties mentioned above, various means have been recommended, such as: the use of tunnel-ovens with multiple preheating-shrinking zones with flexible air-blowing tubes (European Patent N ° 0 058 602), or the production of re-entrant folds in contact with the surface of the objects on the perimeter of the sleeve to be constricted. We can also cite the French Patent N ° 75 30 896.

More recently, the applicant has proposed to inject a gaseous fluid between the object and the loose sleeve to be shrunk in order to inflate the sleeve and keep it out of contact with the object to be coated, the temperature of the gaseous fluid being chosen to be significantly lower than that of the softening of the film constituting the sleeve, which allows the temperatures of the inner and outer faces of the sleeve to be progressively balanced, and the thermal gradient in the film to be controlled in order to achieve film-object contact at the desired instant (see this effect the French Patent N ° 85 15 717).

This solution is interesting, but does not solve the problem of controlling the temperature in the tunnel oven, in the zone concerned by the means for blowing the gaseous fluid.

However, this temperature control is crucial in order to achieve effective shrinkage of the sleeve by simultaneously bringing the inner and outer faces of said sleeve to the same predetermined temperature, which is precisely that corresponding to the separation between the elastic zone and the amorphous zone. of the film.

Such control in tunnel ovens is very difficult to carry out because of the numerous external disturbances constantly upsetting the thermal level of the environment in the zone concerned by the blowing means.

It is indeed easy to inject a gaseous fluid at a predetermined constant temperature, but it is however practically impossible to freeze the temperature in a given zone of the tunnel oven, in particular by setting itself to a temperature corresponding to that chosen for the retraction of a sleeve.

External disturbances are of various origins.

The interior of the tunnel oven is first of all subjected along its length to convection currents which permanently shift the temperature zones (due in particular to the temperature and to the number of objects passing through the tunnel oven and / or to an air draw in said tunnel oven). The thermal inertia of the retraction system and that of the system for conveying objects must also be mentioned.

The state of the art is also illustrated by English patent N ° 1,062,349. This document describes a tunnel oven comprising hot air sources adjustable in height and / or in a transverse direction (this is not, however, that of a preliminary adjustment, carried out in relation to the external shape of the conveyed objects, so that the positioning of the sources then remains unchanged during operation), and a thermostat sensing the temperature of the air at the outlet of the external housing of heating by resistors: such a tunnel oven has no means allowing precise temperature control.

Finally, we can cite the French patent N ° 2 233 234 describing the use of a current of hot air to effect the smoothing of a heat-shrinkable capsule.

Finally, to date, given an imperfect and random retraction of the sleeve with regard to the desired results, the use of heat-shrinkable films in this application has very significant brakes.

The object of the invention is to propose a method and an implementation device for precisely controlling the temperature in a tunnel oven open at its two ends, in order to resolve the aforementioned difficulties.

Another object of the invention is to propose a method and a device which are both simple and reliable, and which in particular make it possible to ensure a perfectly homogeneous retraction for a heat-shrinkable sleeve, without deformation of the impressions, or formation of folds, crimps or craters, regardless of the shape and size of the object to be decorated and / or protected.

Alternatively, in the context of the aforementioned particular application, another object of the invention is to be able to operate this controlled shrinkage at average temperatures at the surface of the film, for example 100 ° C., which avoids the many aforementioned drawbacks of random retractions under temperatures conventionally reaching 180 to 250 ° C., and furthermore reduces the energy consumption as well as the length of the retraction tunnel oven used.

It is more particularly a process for controlling the temperature in a tunnel oven open at its two ends and in which objects move by the action of conveying means, said tunnel oven comprising heating means or cooling devices arranged along its side walls and means for blowing a gaseous fluid at a given temperature in a direction essentially perpendicular to the direction of conveying of the objects, characterized in that it consists in capturing the temperature prevailing in an area of the tunnel furnace concerned by the blowing of the gaseous fluid, and in organizing the displacement of the blowing means in a direction essentially parallel to the direction of conveying of the objects by means of a servo-control ensuring that said means d blowing moves automatically when the sensed temperature deviates from a predetermined temperature, which has the effect of ensure a constant temperature environment in this area of the tunnel furnace for the object or objects concerned.

Preferably, the temperature is sensed in said zone of the tunnel oven in the vicinity of the passage of the objects, as far as possible from the heating or cooling means, and as close as possible to the conveying means. The enslavement is then particularly effective, because the temperature is captured where the disturbances are generally the strongest.

Advantageously, the displacement of the blowing means is organized in such a way that its value varies linearly as a function of the difference detected between the sensed temperature and the predetermined temperature.

Preferably, the temperature is sensed using a thermo-probe moving in synchronism with the blowing means; in particular, the thermo-probe is previously positioned so that the sensed temperature corresponds to the predetermined temperature, after which the automatic movement of the blowing means is authorized from the corresponding initial position of said blowing means .

In the context of a particular application of the method, aimed at applying a section of heat-shrinkable sheath or sleeve to an object, and in which the sleeve, loosely threaded on the object, is heated for its retraction on said object, the predetermined temperature of the gaseous fluid will be chosen to produce a homogeneous retraction of the sleeve after a prior swelling of said sleeve when the object coated with its sleeve enters the zone of the tunnel furnace concerned by the blowing of said gaseous fluid.

In particular, throughout their movement, the blowing means direct a flow of gaseous fluid through the top of the object, in a flow of generally vertical direction.

Preferably, before reaching the zone of the tunnel oven concerned by the blowing of the gaseous fluid, the object coated with its sleeve undergoes preheating by the action of the only means of heating of said tunnel oven, to reach a temperature very close to the sleeve retraction point. Preferably also, after the zone of the tunnel oven concerned by the blowing of the gaseous fluid, the object on which the sleeve is retracted undergoes heating by the action of the heating means of said tunnel oven and of a means of additional blowing, to be brought for a short time to a high temperature, significantly higher than the predetermined temperature, which corresponds to a finishing or smoothing phase.

The invention also relates to a device for implementing the method, and intended to equip a tunnel oven comprising means for conveying objects, heating or cooling means arranged along its side walls, and means for blowing gaseous fluid towards said objects at a given temperature, characterized in that it comprises a temperature sensing member disposed in the zone of the tunnel furnace concerned by blowing the gaseous fluid, a carriage supporting the nozzle (s) blowing means, said carriage being movable in translation parallel to the conveying direction of the objects, and motorized drive means for automatically moving said carriage when the temperature sensed by said member deviates from said predetermined temperature.

Preferably, the temperature sensing member is a thermo-probe connected to a temperature regulator and integral with the carriage supporting the nozzle or nozzles of the blowing means.

It is advantageous for the carriage supporting the nozzle (s) of the blowing means to move on rails provided in the upper part of the tunnel oven, the said nozzle (s) passing through the said carriage in order to direct a flow of gaseous fluid from above. objects, in a generally vertical directional flow; in particular, the mounting of the nozzle (s) on the carriage allows height adjustment of the position thereof, in an essentially vertical direction.

According to an embodiment which is particularly advantageous for its simplicity and its reliability, the motorized drive means are essentially constituted by a motor-reduction unit, and a coupling member between said unit and the carriage supporting the nozzle or nozzles of the means of blowing. In particular, the gear motor assembly includes an electric motor, the control of which is connected to the thermo-probe via the associated temperature regulator, and a reduction gear at the outlet of which is provided an endless screw cooperating with a nut. secured to the carriage.

Preferably then, the connection of the gear motor assembly with the worm and with the temperature regulator of the thermo-probe is such that the movement of the carriage varies linearly in function of the temperature difference detected by said thermo-probe, for example one mm per degree Celsius. Advantageously then, the electric motor and the associated reduction gear are mounted on a mobile support which can move in the upper part of the tunnel oven; in particular, the mobile support comprises locking means allowing it to be locked in a determined position and / or said mobile support has two slides serving as rails for the carriage.

It may finally prove advantageous to provide that the carriage supporting the nozzle or nozzles of the blowing means also supports, downstream of said nozzles in the direction of movement of the objects, an additional blowing member, for example of the fan-extractor type. , allowing a possible final overheating of the objects.

• - la figure 1 est une vue schématique en perspective d'un dispositif conforme à l'invention, les moyens d'insufflage du fluide gazeux étant ici seulement représentés par deux buses montées sur un chariot mobile longitudinalement,
• - la figure 2 est une vue en coupe longitudinale du four-tunnel de la figure 1, sur laquelle les moyens d'insufflage ont été illustrés,
• - la figure 3 est une vue de dessus de ce même four-tunnel, dont les canalisations d'amenée du fluide gazeux n'ont pas été représentées pour mieux distinguer la structure du dispositif de contrôle objet de l'invention.

The implementation of the method of the invention can naturally be done according to various variants, but we will now describe a particular embodiment of a device that can be envisaged, with reference to the figures of the appended drawing where:

• FIG. 1 is a schematic perspective view of a device according to the invention, the means for blowing the gaseous fluid being here only represented by two nozzles mounted on a carriage movable longitudinally,
• FIG. 2 is a view in longitudinal section of the tunnel oven of FIG. 1, in which the blowing means have been illustrated,
• - Figure 3 is a top view of the same tunnel furnace, the supply lines of the gaseous fluid have not been shown to better distinguish the structure of the control device of the invention.

The temperature control device according to the invention here equips a tunnel oven 1 comprising means 2 for conveying objects 3, heating means 4 arranged along its side walls, and means 5 for blowing towards said objects a gaseous fluid at a given temperature. In the case of cooling, the means 4 will be replaced by any means, such as forced air, making it possible to cool the atmosphere.

The aforementioned means are conventional for such tunnel ovens, and their structure will therefore be briefly recalled. The tunnel oven 1 has two side walls 6 made in the form of a box, the length and height of which depend on the object on which the heat-shrinkable sleeve is to be retracted, and on the required rate. On the inner faces of each box 6, a series of heating elements 4 have been arranged which can be emitters with infrared radiation. These transmitters are distributed over the height of the objects concerned, according to one or more series of superimposed elements. In known manner, these elements are connected to a temperature regulation device (not shown) either individually or in groups, with a thermo-probe incorporated in the element or group of elements concerned: this is 'A simple conventional regulation of the heating means of such tunnel ovens. It goes without saying that these infrared radiation emitters can be replaced by any equivalent means, and may be supplemented by flaps for distributing the flow of hot air. The conveying means 2 are illustrated here in the form of an endless conveyor, but it is naturally possible to use as a variant a chain provided with devices allowing the gripping of objects and animated with a continuous movement or step by step, all by keeping the objects in a fixed position or by rotating the said objects on themselves. Thus, the objects to be coated 3 circulate on the endless conveyor 2 after removal of a loose sleeve 7 around each object by a machine known per se, this removal being carried out before the object enters the tunnel oven 1.

The blowing means 5 for blowing a gaseous fluid, in particular air, are only partially shown in Figure 1, by the end of one or more supply lines, each connected to a nozzle associated blowing. The known techniques already used such blowing means, but their longitudinal positioning was either fixed or mobile in synchronization with the movement of the objects on the endless conveyor, in order to accompany them during their conveyance in the oven. tunnel.

In accordance with a fundamental aspect of the invention, there is provided a temperature sensing member 8 arranged in the zone of the tunnel furnace concerned by the blowing of the gaseous fluid, a carriage 9 supporting the nozzle or nozzles of the blowing means. 5, said carriage being movable in translation parallel to the conveying direction of the objects 3, and motorized drive means 10 for automatically moving said carriage when the temperature sensed by said member 8 deviates from a predetermined temperature.

According to the aforementioned fundamental principle, when the collecting member 8 registers in the hot chamber a temperature different from that chosen for the retraction, the information is communicated to the motorized drive means 10 so as to move the mobile carriage 9 to position said carriage and therefore the means for blowing into the temperature zone which is that required for the retraction and displayed on an associated temperature regulator (not shown). Therefore, the instability of this zone due to external disturbances causes a continuous movement of advance or retreat of the movable carriage 9, and therefore of the blowing nozzles supported by said carriage, this movement of advance and retreat preferably around a position determined at the start during the initial adjustment of the retraction system.

Thus, it is easy to implement the essential characteristic of the process of the invention, according to which the temperature prevailing in a zone of the tunnel furnace affected by the blowing of the gaseous fluid is captured, and the movement of the means of movement is organized. blowing in a direction essentially parallel to the conveying direction of the objects using a servo control such that said blowing means move automatically when the sensed temperature deviates from the predetermined temperature, which has for effect of permanently ensuring for the object or objects concerned a constant temperature environment in this zone of the tunnel oven.

The temperature sensing member will preferably be produced in the form of a thermo-probe 8 connected to a temperature regulator (not shown) and integral with the carriage 9 supporting the nozzle or nozzles of the blowing means 5. The temperature is preferably picked up in the vicinity of the passage of the objects 3, as far as possible from the heating means 4 and as close as possible to the conveying means 2: in this way it is possible to confer maximum efficiency on the control of the movement of the means according to the temperature differences detected by the thermo-probe. It may be advantageous to coat the thermo-probe 8 in an insulating sheath, and to provide a lower end cut in a bevel, with the inclined facet facing the hot atmosphere, which allows the end to be better preserved. of capture of the thermo-probe vis-à-vis the heat flux sent by the adjacent emitters with infrared radiation.

The carriage 9 supporting the nozzle or nozzles of the blowing means 5 moves on rails provided in the upper part of the tunnel oven 1, these rails being able to be arranged in the upper part of the side walls 6 of the tunnel oven, or, as illustrated here, on the own support of the motorized drive means 10. The motorized drive means 10 are indeed mounted on a mobile support 11, capable of moving in the upper part of the tunnel oven 1 parallel to the conveying direction objects, and to be locked in a predetermined position by associated locking means (the longitudinal slots 12 schematize here this degree of freedom before locking by any known means such as bolting). The mobile support 11 thus has two slides 13 constituting the support rails of the carriage 9.

It goes without saying that positional adjustments may be provided both for the thermo-probe 8 and for the nozzles of the blowing means 5, relative to the support carriage 9: it is in particular advantageous to provide that the nozzles passing through the carriage 9, and serving to direct a flow of gaseous fluid over the objects in a generally vertical directional flow, are mounted with a possible height adjustment in an essentially vertical direction, which makes it possible to take account of the height of the objects concerned.

The motorized drive means 10 are preferably essentially constituted by a geared motor assembly, and a coupling member between said assembly and the carriage 9 supporting the nozzle or nozzles of the blowing means 5. In this case, the motor-reducer assembly comprises an electric motor 14, the control of which is connected to the thermo-probe 8 via an associated temperature regulator, and a reducer 15 at the outlet of which is provided an endless screw 16 cooperating with a nut 17 secured to the carriage 9.

The connection of the geared motor assembly 14, 15 with the worm 16 and with the temperature regulator of the thermo-probe 8 is preferably chosen so that the movement of the carriage 9 varies linearly as a function of the temperature difference detected by said thermo-probe. In particular, in order to illustrate the possible precision of the device of the invention, it will be advantageous to achieve a displacement of 1 mm of the carriage 9 per detected degree Celsius deviating from the predetermined temperature. To achieve this, one could for example use a small electric motor rotating at 860 revolutions per minute, with an associated reducer producing a reduction of 1/100, and at the outlet of which is connected an endless screw having a pitch of 3 mm. The blowing means 5 are thus adapted to deliver a gaseous fluid at a predetermined temperature with maximum precision.

If reference is made to FIGS. 2 and 3, there is a watertight box 18, which can be mounted on the external face of one of the side boxes 6 of the tunnel oven, or in any other place, said waterproof box containing a series of electric resistors 19. At one end of this box, there is fixed a fan 20 which sucks in ambient air outside and pulses it on the electric resistors 19 inside the box. The heated air is then discharged from the box through one or more orifices (here two) each connected to an associated corrugated metal pipe 21 connected to a nozzle or associated rigid tube 22 mounted on the carriage 9. As an indication, a watertight box about two meters long, and outlet openings with a diameter of around 60 mm.

In one and / or the other of the pipes 22, there is provided a thermo-probe (not shown) connected to a temperature regulator, making it possible, depending on the display, to obtain air at the outlet of the pipe at the desired temperature . According to experience, the fan 20 must allow the air to be pulsed at a speed of the order of 6 m / sec. at the outlet of the tubing. However, as shown in FIG. 2, it is possible to provide closing butterflies 25 on the delivery orifices 24 of the box 18 allowing the volume and the speed of the air leaving the rigid pipes 22 to be varied as desired.

When the method of the invention is applied to the case of the shrinkage of a heat-shrinkable sleeve 7 on an object 3, the predetermined temperature will naturally be chosen to produce a homogeneous shrinkage of the sleeve 7 after prior swelling of said sleeve when the object coated with its sleeve enters the zone of the tunnel furnace concerned by the blowing of said gaseous fluid. The gaseous fluid blown in by the means 5 fulfills a double function in this case, on the one hand swelling the sleeve to separate it from the object, and on the other hand sweeping the hot zone concerned, on the one hand and on the other side of the wall of said sleeve.

In practice, the thermo-probe 8 is previously positioned so that the sensed temperature corresponds to the predetermined temperature, after which the automatic movement of the blowing means 5 is authorized from the corresponding initial position of said means blowing. The thermo-probe 8 then remains located in the zone where the optimum temperature for shrinking the sleeve has been obtained, a temperature known from the characteristics of the plastic film used, and obtained by adjusting the various heating elements 4 by means of temperature regulators. associated. Due to the connection of the gear motor assembly to the thermo-probe, via the associated temperature regulator, the gear motor assembly automatically moves the rigid air supply pipes forwards or towards the rear (relative to the conveying direction of the objects) depending on whether the sensed temperature deviates less or more from the temperature selected for retraction and displayed on the regulator.

Advantageously, before reaching the zone of the tunnel oven concerned by the blowing of the gaseous fluid, the object 3 coated with its sleeve 7 undergoes preheating by the action of the only heating means 4 of said tunnel oven, to reach a temperature very close to the point of retraction of the sleeve 7. The adjustment of this temperature is carried out as previously via the temperature regulators associated with the heating elements. In practice, it will be advisable to adjust the temperature in the upstream zone concerned of the tunnel furnace 1 according to a progressive increase, in order to bring the sleeve 7 to a temperature close to its softening point as it moves between the two walls of said tunnel oven. In this preliminary preheating phase, only the outer face of the film sees its temperature rise; however, the conditions for good retraction have not yet been met, since the internal face of the film is still partially or completely in contact with the walls of the object. This state is naturally modified when the object coated with its sleeve enters the zone concerned by the blowing means.

Preferably also, after the zone of the tunnel oven concerned by the blowing of the gaseous fluid, the object on which the sleeve 7 is retracted undergoes heating by the action of the heating means 4 of said tunnel oven and a additional blowing means 23, to be brought for a short time to a high temperature, significantly higher than the predetermined temperature, which corresponds to a finishing or smoothing phase. This makes it possible to complete the retraction and to give the film an optimal stretch: in practice, this phase will last a few tenths of a second and at most one second, and the temperature concerned will preferably be of the order of 200 to 400 ° C. In fact, the choice of these operating conditions will be essentially dictated by the speed of circulation of the object. The additional means 23 will for example be of the extractor fan type, and may advantageously be mounted on the carriage 9 supporting the nozzles of the blowing means 5. It may also prove advantageous to have a deflector 26 (visible only in the figure 2) at the outlet of the fan-extractor 23, so that the pulsed air does not come to disturb the zone concerned by the blowing means 5.

The method of the invention, and the device for implementing said method, thus makes it possible to operate a perfectly homogeneous retraction of a heat-shrinkable sleeve, by virtue of the constant and precise control of the temperature in the zone concerned of the furnace. tunnel.

This avoids any blowing of air at an excessive temperature, which would have the consequence of making inhomogeneous the shrinkage due to the temperature differences between the inner and outer faces of the heat-shrinkable sleeve, and in addition shrinking the top of the sleeve before the bottom thereof, which would block the passage of air and hinder the retraction of the lower part of the sleeve. It also avoids blowing air at an insufficient temperature, which would have the consequence of making the shrinkage of the sleeve inhomogeneous, and of sealing the lower part of the sleeve due to premature shrinkage of this lower zone.

The blowing means illustrated here comprise two pipes, which naturally constitutes only an exemplary embodiment. However, the presence of a double tube makes it possible to maintain an air pressure for a longer time on the top of the objects. One could also alternatively provide two separate boxes each connected to an associated tubing, so as to be able to blow air at different temperatures. However, in practice, the number of tubes will not be very high, because the length of the retraction zone remains relatively small due to the narrowness of the temperature range concerned.

The invention thus makes it possible to solve in a particularly simple and effective manner the problem of temperature uniformity for an area of a tunnel oven open at its two ends, by eliminating all external disturbances.

The invention is not limited to the embodiment which has just been described, but on the contrary encompasses any variant incorporating, with equivalent means, the essential characteristics appearing in the claims.

Claims (20)

1. A method for controlling the temperature in a tunnel oven open at its two ends and in which objects move by the action of conveying means, said tunnel oven comprising heating or cooling means arranged along its side walls and means for blowing a gaseous fluid at a given temperature in a direction essentially perpendicular to the direction of conveying of the objects, characterized in that it consists in capturing the temperature prevailing in a zone of the tunnel oven concerned by the blowing the gaseous fluid, and organizing the movement of the blowing means (5) in a direction essentially parallel to the conveying direction of the objects (3) by means of a servo control such that said means blowing move automatically when the sensed temperature deviates from a predetermined temperature, which has the effect of ensuring permanently for the o u the objects concerned a constant temperature environment in this area of the tunnel furnace. 2. Method according to claim 1, characterized in that the temperature is sensed in said zone of the tunnel oven in the vicinity of the passage of objects (3), as far as possible from the heating or cooling means (4) and also as close as possible to the conveying means (2). 3. Method according to one of claims 1 and 2, characterized in that the displacement of the blowing means (5) is organized so that its value varies linearly according to the difference detected between the sensed temperature and the predetermined temperature. 4. Method according to one of claims 1 to 3, characterized in that the temperature is sensed using a thermo-probe (8) moving in synchronism with the blowing means (5). 5. Method according to claim 4, characterized in that the thermo-probe (8) is previously positioned so that the sensed temperature corresponds to the predetermined temperature, as a result of which the automatic displacement of the blowing means ( 5) is authorized from the corresponding initial position of said blowing means. 6. Method according to one of claims 1 to 5, intended to apply a section of heat-shrinkable sheath or sleeve (7) on an object (3), and in which the sleeve (7), loosely threaded on the object (3), is heated for its retraction on said object, characterized in that the predetermined temperature is chosen to produce a homogeneous retraction of the sleeve (7) after prior swelling of said sleeve when the object coated with its sleeve enters the zone of the tunnel furnace concerned by the blowing of said gaseous fluid. 7. Method according to claim 6, characterized in that, throughout their displacement, the blowing means (5) direct a flow of gaseous fluid through the top of the object (3), in a flow of substantially vertical general direction. 8. Method according to one of claims 6 and 7, characterized in that before reaching the zone of the tunnel furnace concerned by the blowing of the gaseous fluid, the object (3) coated with its sleeve (7 ) undergoes preheating by the action of the only heating means (4) of said tunnel oven, to reach a temperature very close to the point of retraction of the sleeve. 9. Method according to one of claims 6 to 8, characterized in that after the zone of the tunnel furnace concerned by the blowing of the gaseous fluid, the object on which the sleeve (7) is retracted undergoes heating by the action of the heating means (4) of said tunnel oven and of an additional blowing means (23), to be brought for a short time to a high temperature, markedly higher than the predetermined temperature, which corresponds to a finishing or smoothing phase. 10. Device for implementing the method according to one of claims 1 to 9, intended to equip a tunnel oven comprising means (2) for conveying objects, means (4) for heating or cooling arranged on along its side walls, and means (5) for blowing a gaseous fluid at said temperature to said objects, characterized in that it comprises a temperature sensing member (8) disposed in the area of the tunnel oven concerned with the blowing of the gaseous fluid, a carriage (9) supporting the nozzle or nozzles of the blowing means (5), said carriage being movable in translation parallel to the conveying direction of objects, and motorized drive means (10) for automatically moving said carriage when the temperature sensed by said member (8) deviates from said predetermined temperature. 11. Device according to claim 10, characterized in that the temperature sensing member is a thermo-probe (8) connected to a temperature regulator and integral with the carriage (9) supporting the nozzle or nozzles of the means blowing (5). 12. Device according to one of claims 10 and 11, characterized in that the carriage (9) supporting the nozzle or nozzles of the blowing means (5) moves on rails (13) provided in the upper part of the furnace -tunnel (1), said nozzle or nozzles passing through said carriage in order to direct a flow of gaseous fluid through the top of the objects, in a generally vertical directional flow. 13. Device according to claim 12, characterized in that the mounting of the nozzle (s) on the carriage (9) allows a height adjustment of the position thereof, in an essentially vertical direction. 14. Device according to one of claims 10 to 13, characterized in that the motorized drive means (10) essentially consist of a geared motor assembly (14, 1 5), and a coupling member between said assembly and the carriage (9) supporting the nozzle or nozzles of the blowing means (5). 15. Device according to claim 14, characterized in that the geared motor assembly comprises an electric motor (14), the control of which is connected to the thermo-probe (8) via the associated temperature regulator, and a reduction gear (15) at the outlet of which is provided an endless screw (16) cooperating with a nut (17) integral with the carriage (9). 16. Device according to claim 15, characterized in that the connection of the geared motor assembly (14, 15) with the worm (16) and with the temperature regulator of the thermo-probe (8) is such that the movement of the carriage (9) varies linearly as a function of the temperature difference detected by said thermo-probe, for example of one mm per degree Celsius. 17. Device according to one of claims 15 and 16, characterized in that the electric motor (14) and the associated reduction gear (15) are mounted on a mobile support (11) which can move in the upper part of the tunnel oven . 18. Device according to claim 17, characterized in that the mobile support (11) comprises locking means allowing its blocking in a determined position. 19. Device according to one of claims 17 and 18, characterized in that the movable support (11) has two slides (13) serving as rails for the carriage (9). 20. Device according to one of claims 10 to 19, characterized in that the carriage (9) supporting the nozzle or nozzles of the blowing means (5) also supports, downstream of said nozzles in the direction of movement of the objects , an additional blowing member (23), for example of the fan-extractor type, allowing possible final overheating of the objects.

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