EP0096635A2 - Method and device for regulating the pre-stretching of a plastic film, especifically in view of the packaging of a load - Google Patents

Abstract

1. Process for controlling the degree fo prestretching of a stretchable plastic film which passes over at least one prestretching roll before arriving at a use station, particularly for the purpose of wrapping loads, especially palletized loads, making it possible to avoid the sum of the elongation produced by the prestretching and of the additional elongation produced by the tension created in the film before it arrives on the load to be wrapped being greater than the breaking elongation, characterized in that it consists in - continuously sensing the tension in the part of the film included, when the direction of travel of the film is considered, between the last prestretching roll and the use station ; and - causing the speed of at least one prestretching roll to vary correspondingly to the sensed tension.

Description

The invention relates to a method and a device for regulating the pre-stretching of a film of stretchable plastic, in particular for the packaging of a load.

Methods and devices are already known which make it possible to wrap a load with a film of stretchable plastic material by subjecting the film, before it arrives on the load, to a pre-stretching making it possible to pack the load in n ' applying to it only a force much lower than that which would have been otherwise necessary to ensure at the same time the stretching of the film. Thus, in French patent n ° 74 27 653, a device for packaging loads is described by means of a film of stretchable plastic material, which comprises first and second successive pairs of stretching cylinders between which the film passes, the downstream cylinders of the second pair rotating at a peripheral speed higher than that of the upstream cylinders of the first pair, the free end of the stretched film being driven by suitable means, placed downstream of the second pair of cylinders, at a lower speed or equal to the peripheral speed of the cylinders of this second pair.

However, most of the plastic films used for the packaging of fillers have their improved resistance properties when they are stretched. Thus in the particular case of low density polyethylene, there occurs, after exceeding the elastic limit which corresponds to an elongation of approximately 40%, a progressive increase in the tensile strength as the elongation increases, the rupture of the film occurring for an elongation of about 400%. We therefore realize that it is extremely interesting to make the most of the stretching capacity of a plastic film; not only does the packaging acquire better resistance properties, but also a saving in packaging material which increases as the degree of stretching increases.

In known embodiments, attempts have been made to exploit this property of plastic films during their pre-stretching before the packaging of the load. Thus in the patent applications FR 80 19 791 and 80 24 814, methods and apparatuses are described making it possible to obtain a 300% pre-stretch using sets of stretch rollers.

However, known packaging machines equipped with a pre-stretching device have the drawback of not allowing perfect exploitation of the advantageous stretching range of between 50 and 400%. Indeed, the pre-stretching devices are adjusted at the start of the program so as to obtain a certain degree of stretching which takes into account the material of the film, the profile of the load to be packaged, the risk of crushing the corners, in particular in the case of fragile cardboard packages and other parameters. We are therefore led to take precautionary measures initially so that the plastic film does not rupture during the machine's operation, these measures actually resulting in a sharp reduction in the upper limit of the aforementioned range.

The object of the invention is to remedy this operating deficiency in the above-mentioned elongation range by continuously adapting the degree of pre-stretching of the plastic film to the tension generated in this film by the wrapping of the load. to be packaged, so that the sum of the elongation produced by the pre-stretching and the additional elongation produced by the tension created in the film before its arrival on the load to be packaged does not exceed the elongation at break.

To solve this problem, there is provided in accordance with the present invention, a method of regulating the degree of pre-stretching of a film of stretchable plastic material, which passes over one or more pre-stretching means before reaching a station. of film use, this process consisting in continuously sensing the tension in the part of film included, considering the direction of movement of the film, between the last pre-stretching means and the station of use, and in making vary, in correspondence with the sensed tension, the speed of at least one of said pre-stretching and / or film use means, so that the sum of the elongation produced by the pre-stretching and the elongation additional produced by the tension created in the film before its arrival at the station of use does not exceed the elongation a break.

According to another particular feature of the aforementioned method, in the case where the pre-stretching means consist of several pre-stretching cylinders, the last of which is driven by an independent motor, the method consists in continuously capturing the tension in the film in the area between the last pre-stretching cylinder and the use station and reacting on the speed of this last pre-stretching cylinder in correspondence with the sensed tension.

In a highly preferred mode of application, the method is implemented for packaging a load.

• . un moyen de captage continu de la tension du film dans sa partie comprise, en considérant le sens de déplacement du film, entre le dernier cylindre étireur et le point d'arrivée sur la charge,
• . une servo-commande recevant en permanence le signal de sortie du moyen de captage de tension de film et assurant sa comparaison avec une valeur de consigne de façon à faire varier en correspondance la vitesse d'au moins un des cylindres étireurs et/ou de la charge, afin que la somme de l'allongement produit par le pré-étirage et de l'allongement supplémentaire produit par la tension créée dans le film avant son arrivée sur la charge à emballer ne dépasse pas l'- allongement de rupture.

The invention also relates to a device for regulating the degree of pre-stretching of a film of stretchable plastic material, usable in particular in a machine for packaging loads, comprising pre-stretching means, in particular at least two stretching rollers. successive of which at least one rotates at an adjustable speed, the peripheral speed of the downstream cylinder being greater than or equal to the peripheral speed of the upstream cylinder, and means for adjusting the speed of application of the film on the load so that this speed is equal to or less than the peripheral speed of the downstream cylinder, device characterized in that it comprises:

• . a means for continuously capturing the tension of the film in its included part, considering the direction of movement of the film, between the last stretching cylinder and the point of arrival on the load,
• . a servo-control permanently receiving the output signal from the film tension sensing means and ensuring its comparison with a set value so as to vary in correspondence the speed of at least one of the stretching cylinders and / or of the load, so that the sum of the elongation produced by the pre-stretching and the additional elongation produced by the tension created in the film before it reaches the load to be packaged does not exceed the breaking elongation.

• - le moyen de captage de tension comprend un dispositif avec au moins un rouleau de renvoi servant à former une boucle dans le film et un capteur de tension de film placé en regard du rouleau de renvoi de manière à analyser son déplacement se produisant en fonction de la tension du film;
• - ledit rouleau de renvoi du dispositif de formation de boucle est monté de façon à se déplacer par translation;
• - ledit rouleau de renvoi du dispositif de formation de boucle est monté de façon à se déplacer par rotation;
  • - le dispositif de formation de boucle de film dudit moyen de captage de tension comporte, en aval du premier rouleau de renvoi, en considérant le sens de progression du film, un second rouleau de renvoi qui est monté de façon réglable afin de permettre une modification de l'angle d'arrivée du film sur la charge au cours de son emballage;
• - dans le cas où l'on fait intervenir le dernier cylindre de pré-étirage pour la régulation du degré de pré-étirage, ce cylindre est actionné par un moteur hydraulique qui est sollicité de façon continue par une servo-valve incorporée à la servo-commande et réagissant aux signaux fournis par le capteur de tension de film.

The device according to the invention may also include the following characteristics, considered in isolation or in all their technically possible combinations:

• the voltage sensing means comprises a device with at least one deflection roller for forming a loop in the film and a film tension sensor placed opposite the deflection roller so as to analyze its displacement occurring as a function of the tension of the film;
• - Said return roller of the loop forming device is mounted so as to move by translation;
• - Said return roller of the loop forming device is mounted so as to move by rotation;
  • - The film loop forming device of said tension sensing means comprises, downstream of the first deflection roller, considering the direction of film progression, a second deflection roller which is mounted in an adjustable manner in order to allow modification the angle of arrival of the film on the load during its packaging;
• - in the case where the last pre-stretching cylinder is used to regulate the degree of pre-stretching, this cylinder is actuated by a hydraulic motor which is continuously actuated by a servo-valve incorporated in the servo -control and reacting to the signals supplied by the film tension sensor.

The method and the device according to the invention are applicable in particular to the packaging of loads, in particular palletized loads consisting of a certain number of elementary packages. It should also be noted that the invention is applicable both when the packaging is carried out by progressive winding of the film around the load and in the case where it is operated by the so-called "curtain" method. ie where the charge is pushed against the plastic film so as to be wrapped as a whole.

• la fig. 1 est un diagramme donnant la tension du film de matière plastique en fonction de l'angle de rotation de la charge, les différentes positions angulaires de la charge étant mises en évidence,
• la fig. 2 est une vue en plan schématique d'une machine d'emballage du type à enveloppement progressif, équipée d'un exemple de réalisation du dispositif de régulation de pré-étirage de film conforme à l'invention, et
• la fig. 3 est une vue en plan schématique d'une machine d'emballage du type à enveloppement en rideau, équipée d'un autre exemple de réalisation du dispositif de régulation de pré-étirage de film conforme à l'invention.

Other advantages and characteristics of the invention will be highlighted in the following description, given by way of nonlimiting example, with reference to the appended drawings in which:

• fig. 1 is a diagram giving the tension of the plastic film as a function of the angle of rotation of the load, the different angular positions of the load being highlighted,
• fig. 2 is a schematic plan view of a progressive wrapping type packaging machine, equipped with a exemplary embodiment of the film pre-stretch control device according to the invention, and
• fig. 3 is a schematic plan view of a wrapping machine of the curtain wrapping type, equipped with another embodiment of the film pre-stretch control device according to the invention.

Before going on to the actual description of the process and of the device for regulating the pre-stretching of film according to the invention, we will explain the basic structure of a packaging machine equipped with a pre-stretching device. of movie. Thus fig. 2 represents a machine operating by progressive wrapping of a load P which is placed on a rotary table T. This load can be formed of several unitary packages which are held in place on a pallet in the form of an assembly which must be wrapped a plastic film. This film, designated by FP in fig. 2 is unwound from a reel B and it passes first on a deflection roller B ₁ then on pre-stretching means, such as CI and C _Z , which are constituted in the example represented by cylinders coated with 'a substance, rubberized or otherwise, exerting a great friction force on the plastic film. The film thus undergoes a first pre-stretching on the cylinder CI and a second pre-stretching on the cylinder C ₂ which rotates at a speed greater than C ₁ . In known embodiments, the film then reaches the load P directly to ensure its envelopment. Such a structure corresponds to what is called a progressive wrapping machine, which gradually wraps the film FP around the load P, the rotary table being able to execute a movement of vertical translation.

There is also another structure of a packaging machine, to which the invention can be applied and which is shown in FIG. 3. We are dealing in this case with a wrapping machine of the curtain wrapping type, according to which the load P is wrapped by being pushed in the direction F against the film FP1, FP2 unwound simultaneously from two reels B1, B2. It should be noted that initially the two parts of film unwound from the reels Bl, B2 are welded before the start of the load transfer movement in the direction F so as to form in front of this load what is called a "curtain". After completion of the load transfer movement P in the direction F, the two film strands FP1, FP2 are brought together using a device, not shown, so as to completely envelop the load and the two strands FP1, FP2 are welded together behind the load to complete its packaging. A machine of this type also provides a prestretching of the two strands FP1, FP2 of the plastic film using d'- suitable stretching means known, which have been evidenced to _f ig. 3 by the two pairs of cylinders C ₁ , C ₂ and C ' ₁ , C' _{2 '} In the known machine, the film strands FP1 and FP2 then arrive directly on the load to be packaged.

As specified in the preamble, the plastics used to form a packaging film, in particular low density polyethylene, can undergo a stretching of 400% until breaking while having, after exceeding the limit elastic which roughly corresponds to an elongation of 40 to 50%, a significant improvement in their properties, in particular the tensile strength. The pre-stretching of a plastic film before it arrives at the load to be packaged is therefore extremely advantageous since not only does it improve the properties of the film but also makes significant savings in material since It is possible, by lengthening it considerably within the range indicated above, to pack many more products with the same initial length of film. However, if the pre-stretch is pushed too far, approaching the 400% limit excessively, there is a risk of a film rupture. In fact, the loads to be packaged often consist of sets of unitary packages of parallelepipedic or cubic shape and there occurs, during the rotation or translation movement of the load to be packaged, a variation in the tension generated in the film of material plastic. This has been highlighted in fig. l, which is a graphic representation giving the tension in the plastic film as a function of the angle α of rotation of the load. We have indicated at the top of the graph a sinusoidal curve, going laughing between a maximum and a minimum and which represents the tension in the plastic film during the passage of the point S in the different positions designated by Sl, S2, S3, S4 and S5 in fig. 1. If this film tension is not taken into account, there is a risk of getting too close to the elongation limit of 400% for which the film rbmpt. However, the tension of the film is absolutely not taken into account in known machines equipped with pre-stretching devices and it follows that users take precautionary measures resulting in a limitation of the degree of pre-stretching to a good value. lower than that which could be adopted under optimal conditions.

The method and the device in accordance with the invention aim to remedy the shortcomings of known embodiments by simultaneously taking into account the degree of pre-stretching of the plastic film and the tension generated in it when it arrives on the load. to pack.

Thus, in its most general aspect, the method according to the invention for regulating the degree of pre-stretching of a film of stretchable plastic material which passes, during the packaging of a load, on one or more means pre-stretching before reaching the load, said method consisting in continuously sensing the tension in the part of the film included, considering the direction of movement of the film, between the last pre-stretching means and the load to be packaged , and to vary, in correspondence to the sensed tension, the speed of at least one of said pre-stretching means and / or of the load, so that the sum of the elongation produced by the pre-stretching and of the The additional elongation produced by the tension created in the film before it reaches the load to be packaged does not exceed the elongation at break.

In a particular aspect, in the case where the pre-stretching means are constituted by stretching cylinders actuated by independent motors, the method consists in continuously sensing the tension in the film in the area between the last pre-stretching cylinder. -drawing and the load to react on the speed of this last pre-stretching cylinder in correspondence with the sensed tension.

Thus the method according to the invention makes it possible to continuously adapt the operating conditions of the machine to the parameters governing the packaging of a determined load, in particular the shape of this load, its fragility in the corners, its speed of entrainment, nature and strength of the plastic film, etc.

It should also be noted that the method according to the invention is absolutely not limited to the field of packaging and that it is possible to envisage its application in all cases where it is advantageous, for reasons of improvement of the film's resistance and profitability properties, of ensuring its pre-stretching before its implementation.

Figures 2 and 3 highlight the implementation of the method according to the invention in the two types of packaging machines which have been defined above.

We will first consider the embodiment of FIG. 2 which corresponds to the progressive wrapping packaging machine. After the pre-stretching of the film by the cylinders C1, C2, it is necessary to measure the tension of this film in the strand between the point S and the point of tangency to the cylinder C2 so as to be able to react on the conditions of pre stretching the film to prevent it from breaking while making the most of the stretching range available.

• a; un mécanisme de formation de boucle dans le film FP, qui est placé entre le dernier moyen d'étirage constitué par le cylindre étireur C2 et la charge à emballer P; ce mécanisme est constitué, dans l'exemple représenté, par un rouleau de renvoi R3 qui est monté élastiquement sur un support, désigné par SP sur la fig. 2 et qui, suivant sa conception de montage, peut exécuter un mouvement rectiligne ou bien un mouvement circulaire; la fig. 2 met en évidence un autre rouleau de renvoi R2 dont la fonction sera précisée dans la suite;
• b) un capteur de tension de film CT, placé en regard du rouleau R3 de manière à capter le déplacement rectiligne ou circulaire dudit rouleau, qui est fonction de la tension engendrée dans le film de matière plastique FP au cours de l'- enveloppement de la charge P.

In the embodiment shown in FIG. 2. the device for regulating the degree of pre-stretching of the plastic film comprises a film tension sensing means comprising the following elements:

• at; a loop forming mechanism in the film FP, which is placed between the last stretching means constituted by the stretching cylinder C2 and the load to be packaged P; this mechanism is constituted, in the example shown, by a deflection roller R3 which is resiliently mounted on a support, designated by SP in FIG. 2 and which, depending on its mounting design, can execute a rectilinear movement or a circular movement; fig. 2 highlights another deflection roller R2 whose function will be explained below;
• b) a film tension sensor CT, placed opposite the roller R3 so as to pick up the rectilinear or circular movement of said roller, which is a function of the tension in gendré in the plastic film FP during the wrapping of the load P.

The regulating device further comprises a servo control SC which is connected to the tension sensor so as to permanently receive signals representing the film tension; furthermore the servo-control SC is connected by conductors L1, L2. L3 respectively to the motor Ml for actuating the stretching cylinder Cl, to the motor M2 for actuating the stretching cylinder C2 and to the motor M3 for driving the rotary table T on which the load P is placed. It also includes means for analyzing instantly the actual tension and pre-stretching conditions of the film to modify in correspondence, as indicated by the links L1, L2, L3, the speed of one or more of the motors Ml, M2, M3 so that the sum of the elongation caused by pre-stretching and elongation caused by tension in the final film strand remains below the allowable limit.

According to an advantageous embodiment of the invention, the LC servo-control reacts, as a function of the signal supplied by the film tension sensor, on the last pre-stretching cylinder C2 before the load in order to thus modify the degree film pre-stretch. In this case, it is advantageous to ensure the control of the last pre-stretching cylinder C2 by a hydraulic motor M2 and there is then provided in the servo-control SC a servo-valve which thus ensures very precise control of this M2 hydraulic motor.

In the case of the embodiment shown in FIG. 3, which corresponds to the mode of wrapping a load "curtain", the application of the method according to the invention is similar to what has been shown in FIG. 2; and is provided for each of FP1 and FP2 strands film loop forming mechanism which has been shown in a simplified form, that is to say it is simply provided a Ro- _the return water R _1, R ⁱ ₁ having the function of forming the tension sensing loop between the last corresponding pre-stretching cylinder C2, C'2 and the point S1, S2 of arrival of the corresponding film strand SF1. SP2 on the load P. The corresponding roller R3, R'3 is mounted on a support SP1, SP'l which can execute a rectilinear or circular movement; it is provided opposite the corresponding return roller R3, R'3 a sensor CT ₁ , CT _{2 '} which has the function of measuring the tension in the associated film strand FP1, FP2 by analysis of the rectilinear or circular displacement of the corresponding roller R3, R '3. The signals from the two sensors CT ₁ , CT ₂ are applied to a servo-control SC which analyzes them and which reacts accordingly on one or more of the motors Ml, M2, M'l, M'2 for driving the pre cylinders. -drawing Cl, C2, C'l, C'2 and / or on the motor M3 ensuring the translation of the load P.

Again considering fig. 2, it should be noted that the deflection roller R ₂ placed immediately before the load P is intended to modify the angle of arrival of the film strand FP on the load in order to adapt this angle to the packaging conditions of the charge; in fact, when the load is fragile, for example when it is formed from thin cardboard packages whose corners can collapse, it is recommended to reduce this angle to the minimum while in other applications such a parameter may have less important.

As indicated above, the method and the device according to the invention are applicable to the particular field of packaging, in particular of charges formed of unitary packages and placed on pallets, but also more generally in all cases. where it is necessary to control the degree of pre-stretching of a plastic film in order to make the most of these strength properties and also to obtain maximum production profitability.

Of course, the invention is not limited to the embodiments described and shown above, from which other modes and other embodiments can be provided, without thereby departing from the scope of the invention.

Claims (8)

1. Method for regulating the degree of pre-stretching of a film of stretchable plastic material which passes over one or more pre-stretching means before reaching a station of use, a method characterized in that it consists in capturing continuously the tension in the part of film included, considering the direction of movement of the film, between the last pre-stretching means and the station of use, and to vary, in correspondence with the sensed tension, the speed at least one of said pre-stretching and / or film use means, so that the sum of the elongation produced by the pre-stretching and the additional elongation produced by the tension created in the film before his arrival at the workstation does not exceed the breaking elongation 2. Method according to claim 1, in which the pre-stretching means consist of several pre-stretching cylinders, the last of which is driven by an independent motor, characterized in that it consists in continuously capturing the tension in the film in the area between the last pre-stretching cylinder and the station of use on the speed of this last pre-stretching cylinder in correspondence with the sensed tension. 3. Device for implementing the method according to one of claims 1 and 2, in particular for a load packaging machine, comprising pre-stretching means, in particular at least two successive stretching cylinders (Cl, C ₂ ) at least one of which rotates at an adjustable speed, the peripheral speed of the downstream cylinder (C ₂ ) being greater than or equal to the peripheral speed of the upstream cylinder (C ₁ ), and means for adjusting the speed of application of the film to the load so that this speed is equal to or less than the peripheral speed of the downstream cylinder, device characterized in that it comprises: a means (A ₃ , CT) of continuous capture of the tension of the film (FP) in its included part, considering the direction of movement of the film (FP), between the last stretching cylinder (C _Z ) and the point d 'arrival on the load (P), - a servo-control (SC) permanently receiving the signal output of the film tension capture means (CT) and ensuring its comparison with a set value so as to vary in correspondence the speed of at least one of the stretching cylinders (C ₁ , C ₂ ) and / or of the load (P), so that the sum of the elongation produced by the pre-stretching and the additional elongation produced by the tension created in the film before its arrival on the load to be packaged does not exceed the elongation at break. 4. Device according to claim 3, characterized in that the tension sensing means comprises a device with at least one deflection roller (R ₃ ) serving to form a loop in the film and a film tension sensor (CT) placed opposite the deflection roller (R ₃ ) so as to analyze its displacement occurring as a function of the tension of the film. 5. Device according to one of claims 3 or 4, characterized in that said deflection roller (R ₃ ) of the loop forming device is mounted on a support (SP) so as to move by translation. 6. Device according to any one of claims 3 to 5, characterized in that said deflection roller (R ₃ ) of the loop forming device is mounted on a support (SP) so as to move by rotation. 7.A device according to any one of claims 3 to 6, caraetérisé in that in the case where it involves the last cylinder (C ₂₎ for pre-stretching the re _--regulation of the degree of pre-stretching, this cylinder is actuated by a hydraulic motor (M ₂ ) which is continuously actuated by a servo-valve incorporated in the servo-control (SC) and reacting to the signals supplied by the film tension sensor (CT). 8.Application of the method according to one of claims 1 and 2 and of the device according to any one of claims 3 to 7 to the packaging of loads, in particular palletized loads.

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