FR2528020A1 - METHOD AND DEVICE FOR REGULATING PRE-STRETCHING OF A FILM OF PLASTIC MATERIAL, ESPECIALLY FOR THE PACKAGING OF A LOAD - Google Patents

Abstract

METHOD AND DEVICE FOR REGULATING THE DEGREE OF PRE-STRETCHING OF A STRETCH PLASTIC FILM. THE DEVICE INCLUDES AT LEAST TWO SUCCESSIVE STRETCH CYLINDERS C, C OF WHICH AT LEAST ONE ROTATES AT AN ADJUSTABLE SPEED AND, MORE PARTICULARLY, A MEAN R, CT OF CONTINUOUS SENSING OF THE TENSION OF THE FILM IN ITS INCLUDED PART, CONSIDERING THE DIRECTION OF MOVEMENT OF THE FILM, BETWEEN THE LAST STRETCH CYLINDER C AND THE ENTRY POINT ON LOAD P, A SERVO-CONTROL SC PERMANENTLY RECEIVING THE OUTPUT SIGNAL FROM THE FILM TENSION CAPTURE MEANS AND ENSURING ITS COMPARISON WITH A SET VALUE OF WAY TO VARY CORRESPONDENTLY THE SPEED OF AT LEAST ONE OF THE STRETCH CYLINDERS C, C AND OR OF THE LOAD P, SO THAT THE SUM OF THE ELONGATION PRODUCED BY THE PRE-STRETCHING AND THE ELONGATION PRODUCED BY THE TENSION CREATED IN THE FILM BEFORE ITS ARRIVAL ON THE LOAD TO BE PACKED DOES NOT EXCEED THE RUPTURE ELONGATION. APPLICATION TO THE FIELD OF PACKAGING OF PREPACKED OR NON-PRE-PACKAGED LOADS, OR PALETTIZED.

Description

The invention relates to a method and a device for regulating the

  stretching a plastic film

  stretchable, in particular for packaging a load.

  There are already known methods and devices for

  both to wrap a load with a film of stretch plastic material by subjecting the film, prior to arrival on the load, to a pre-stretching to perform the packaging

  the load by applying only a very good force to the load.

  than would otherwise have been necessary to

  At the same time, the stretching of the film is carried out. Thus, in French Patent No. 74 27 653 there is described a device for packaging fillers by means of a film of stretchable plastic which

  includes first and second successive pairs of cylinders

  stretchers between which the film passes, the cylinders a-

  val of the second pair rotating at a peripheral speed greater than that of the upstream cylinders of the first pair,

  the free end of the stretched film being driven by

  yen, placed downstream of the second pair of cylinders

  at a speed less than or equal to the peripheral speed

  the cylinders of this second pair.

  However, most of the plastic films used in

  used for packing loads have their properties of

  improved strength when stretched. In the particular case of low density polyethylene,

  after exceeding the elastic limit corresponding to

  to an elongation of about 40%, an increase of

  increasing tensile strength as the allon-

  It is believed that the breaking of the film occurs for an allon-

  about 400% It is therefore of great interest to make maximum use of

  stretching a film-plastic; not only the

  tilting acquires better resistance properties, but there is also a saving in packaging material which

  as the degree of stretching increases.

  In the known achievements, we sought to ex-

  use this property of the 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 apparatus for obtaining a 2 are described.

  pre-stretching of 300% using sets of rollers

  rors. However, the known packaging machines equipped with a pre-stretching device have the disadvantage of not allowing a perfect exploitation of the stretching range of interest of between 50 and 400%. Indeed, the pre-stretching devices are set at the beginning of the program

  in order to obtain a certain degree of stretching which

  the material of the film, the profile of the load to be packaged, the risk of crushing corners, particularly in the case of fragile cardboard packets and other parameters.

  Consequently, initial measures have to be taken to

  bail so that it does not happen breakage of the film of

  plastic material while the machine is running, these measurements

  res in fact resulting in a sharp reduction in

  upper limit of the aforementioned range.

  The invention aims to remedy this deficien-

  this of exploiting the aforementioned range of elongation by a

  continuous adaptation of the pre-stretching degree of the material film

  plastic to the voltage generated in this film by the enve-

  development of the load to be packaged, so that the sum of the

  long time produced by pre-stretching and lengthening sup-

  supplemental produced by the tension created in the film before

  his arrival on the load to be packaged does not exceed the allon-

breaking.

  To solve this problem, it is planned

  in accordance with the present invention, a method for regulating

  pre-stretching of a stretchable plastic film which passes over one or more pre-stretching means prior to

  to a film use station, this process consists of

  both to continuously capture the tension in the included film portion, considering the direction of movement of the film,

  between the last pre-stretching device and the user station.

  tion, and to vary, in correspondence with the voltage

  the speed of at least one of said pre-stretching means

  and / or use of the film, so that the sum of the

  produced by pre-stretching and additional elongation

  produced by the tension created in the film before it arrives at the use station does not exceed the elongation

a break.

  According to another feature of the aforementioned method, in the case where the pre-stretching means consist of

  several pre-stretching cylinders, the last one of which

  born by an independent motor, the method consists in continuously capturing the tension in the film in the zone located

  between the last pre-stretching cylinder and the user's station

  tion and react on the speed of this last cylinder of

  pre-stretching in correspondence with the sensed tension.

  In a highly preferred mode of application, the

  method is used for packing a load.

  The invention also relates to a device for

  regulating the degree of pre-stretching of a film of plastic material

  stretchable tick, which can be used in particular in a packaging machine

  1.5 liters of loads, comprising pre-stretching means, in particular

  particular at least two successive drawing rolls

  minus one rotates at an adjustable speed, the peripheral speed

  that 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, characterized in that it comprises a means of continuously collecting the tension of the film in its part, considering the direction of movement of the

  between the last drawing cylinder and the point of

on the load,

  a servo control permanently receiving the signal of

  of the film tension sensing means and ensuring its

  comparison with a setpoint so as to make it

  match the speed of at least one of the cylinders

  stretchers and / or the load so that the sum of the

  lengthily produced by pre-stretching and lengthening

  produced by the tension created in the film a-

  before arrival on the load to be packed does not exceed 1 '-

breaking elongation.

  The device according to the invention may also comprise the following characteristics, considered individually or in all their technically possible combinations: the voltage sensing means comprises a device with 25280 t O 4. at least one return roller for forming a loop

  in the film and a film tension sensor placed in

  the return roller so as to analyze its movement

  cement occurring depending on the film tension; said return roller of the loop forming device is mounted to move by translation;

  said idler roll of the

  cle is mounted so as to move by rotation; the film loop forming device of said tension sensing means comprises, downstream of the first deflection roll, considering the direction of progression of the film, a second deflection roll which is adjustably mounted to allow a modification of the angle of arrival of the film on the load during its packaging; in the case where the last pre-stretching cylinder is used for the regulation of the pre-stretching degree, this

  cylinder is powered by a hydraulic motor which is

  licity continuously by a servo valve incorporated in the

  servo-control and reacting to the signals provided by the

film tension tester.

  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

  It should be noted, moreover, that the invention

  is applicable both when the packaging is made

  by progressive winding of the film around the load

  in the case where the "curtain" method is used,

  that is, the load is pushed against the material film

  plastic so as to be wrapped as a whole.

  Other advantages and characteristics of the invention

  will be highlighted in the rest of the description, giving

  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 6-

  2 is a schematic plan view of a wrapping machine of the progressive wrapping type, equipped with a 5.

  embodiment of the pre-regulation device

  film print according to the invention, and FIG. 3 is a schematic plan view of a wrapping wrapping-type wrapping machine, equipped with another embodiment of the wrapping device.

  pre-stretching film according to the invention.

  Before going on to the description of the project itself

  transferred and the pre-stretching film regulator

  according to the invention, the principal structure will be explained.

  The invention concerns a machine equipped with a pre-stretching film device. FIG. 2 represents a machine operating by progressive enveloping of a load P which is

  placed on a turntable T This load may be

  of several unit packets which are maintained in

  this on a pallet in the form of an assembly that must be wrapped with a plastic film This film, designated FP in Fig 2 is unwound from a coil B and it passes first on a return roller R 1 then on means of

  pre-stretching, such as C 1 and C 2, which are constituted in the e-

  xemple represented by cylinders coated with a substance,

  rubberized or otherwise, exerting a great force of friction

  on the plastic film The film suffers as well

  a first pre-stretching on the cylinder C 1 and a second pre-stretching

  stretching on the cylinder C 2 which rotates at a higher speed than

  In the known productions, the film reaches

  following directly on charge P to ensure its co-operation

  Such a structure corresponds to what is called a

  progressive wrapping machine, which rolls up gradually

  the FP film around the load P, the turntable can

  before performing a vertical translation movement.

  There is also another machine structure of

  The invention relates to a wrapping machine of the wraparound wrapping type according to which the load P is wrapped in being thrust and is shown in FIG.

  in the direction F against the film FP 1, FP 2 unwound simultaneously

  It should be noted that initially the two rolled film portions of the coils BI, B 2 are welded before the start of the transfer movement of the load 6 in the direction F so as to form in front of the two coils BI, B 2. this load, which is called a "curtain" After completion of the transfer movement of the charge P in the direction F, the two film strands F P1, FP 2 are brought together by means of a device, not shown, so as to completely envelop the load and the two strands F Pl, FP 2 are welded together behind the load to complete its packaging thus a machine of this kind also provides a pre-stretching of the two strands

  F Pl, FP 2 of the plastic film using means of

  appropriate stretching and known, which have materialized on the

  fig 3 by the two pairs of cylinders Cil C 2 and C'J, C'2.

  In the known machine, the film strands Ffl and FP 2 arrive

  then directly on the load to be packaged.

  As stated in the preamble,

  plastics used to form a packaging film, in particular

  low-density polyethylene may be subject to

  400% rage up to failure while exhibiting, after exceeding the elastic limit which corresponds approximately to an elongation of 40 to 50%, a significant improvement of their properties, in particular the tensile strength The pre-stretching a plastic film before his arrival

  the load to be packaged is therefore extremely

  the fact that not only are the properties of the film improved but in addition significant savings are achieved

  of material since it is possible in 1: lengthening considerably

  in the range indicated above, to pack a lot

  more products with the same original film length Ce-

  during, if one pushes too far 1 pre-stretching by getting closer

  singing excessively of the limit of 400 'ras it is likely to occur a rupture of film Indeed i-es loads to pack

  often consist of sets of unitary packets

  It has parallelepipedal or cubic shape and, during the rotational or translational movement of the load to be packaged, a variation of the voltage generated in the plastic film is demonstrated. This is shown in FIG. 1, which is a graphical representation giving the

  tension in the plastic film according to 1 '-

  angle O t of rotation of the load

  graph, a sinusoidal curve,

  7. laughing between a maximum and a minimum and that represents the

  tension in the plastic film during the pass-

  ge of the point S in the different positions designated by Sl, 52, 53, 54 and 55 in FIG. 1 If this tension of the film is not taken into account, it is possible to approach

  excessively of the 400% elongation limit for

  what the film is broken Or we do not take into account the tension of the film in the known machines equipped with

  pre-stretching devices and the result is that the users

  take precautionary measures resulting in

  limiting the degree of pre-stretching to a value well below

  than that which could be adopted in optimal conditions. The method and the device according to the invention are intended to remedy the deficiencies of the embodiments

  known at the same time taking into account the degree of pre-stretching

  ge.of plastic film and generated voltage

  in this one on arrival on the load to be packaged.

  Thus, in its most general aspect, the method according to the invention for regulating the degree of pre-stretching of a

  stretching plastic film which passes, during packaging

  a charge, on one or more pre-stretching means before reaching the charge, said method of continuously sensing the tension in the film portion

  including the direction of motion of the film,

  the last pre-stretching means and the load to be packaged, and to vary, in correspondence with the sensed tension, the speed of at least one of said prestretching means and / or the load, so that the sum the elongation produced by the pre-stretching and the additional elongation produced by the tension created in the film before it arrives on the load at

  Packing does not exceed the breaking elongation.

  In a particular aspect, in the case where the means of

  pre-stretching are constituted by drawing rolls

  With independent motors, the method consists in continuously sensing the tension in the film in the area between the last pre-stretching roll and the load to be reacted on the speed of the latter pre-drawing roll.

  in correspondence with the sensed voltage.

  8. 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 specific load,

  also the shape of this load, its fragility in the corners, its driving speed, the nature and strength of the plastic film, etc.

  It should also be noted that the process according to

  This invention is by no means limited to the field of packaging and it is possible to envisage its application in any case where it is advantageous for reasons of improvement.

  film strength properties and cost-effectiveness, as well as

  to pre-stretch before it is used.

  FIGS. 2 and 3 highlight the practice of the method according to the invention in both types of

  packaging machines that have been defined above.

  We will first consider the embodiment of

  FIG. 2, which corresponds to the wrapping machine

  gradually after the pre-stretching of the film by the cylinders

  dras Cl, C 2, it is necessary to measure the voltage of this

  film in the strand between the point S and the point of tan-

  C 2 cylinder in order to react on

  pre-stretching the film to prevent its breaking while

  by making maximum use of the available stretching range.

  In the embodiment shown in FIG. 2, the pre-stretching degree control device of the plastic film comprises a film tension sensing means comprising the following elements: a) a loop-forming mechanism in the FP film, which is placed between the last stretching means constituted by the drawing roll C 2 and the load to be packaged P; this mechanism is constituted 4, in the example shown, by a return roller R 3 which is resiliently mounted on a support, designated

  by SP in fig 2 and which, according to its conception of

  ge, can execute a rectilinear motion or a movement

  circular; fig 2 shows another roll of

  see R 2, the function of which will be specified later; b) a film tension sensor CT, placed opposite the roller R 3 so as to capture the rectilinear motion or

  circular circuit of said roller, which is a function of the voltage

9.

  found in the plastic film FP during 1 '-

  wrapping of the load P. The regulating device further comprises a

  SC servo-control which is connected to the voltage sensor of

  constantly receiving signals representing the film tension; furthermore the servo-control SC is connected by

  conductors LI, L 2, L 3 respectively to the motor Ml of ac-

  the drawing cylinder C1, the motor M 2 actuator

  the drawing roll C 2 and the driving motor M 3 of the rotary table T on which the load P is placed.

  It also includes means for instantaneous analysis

  the actual tension conditions and pre-stretching of the film to change correspondingly, as indicated by the links LI, L 2, L 3, the speed of one or more of the motors Ml, M 2, X 3 so that the sum of 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, depending on the signal

  nor by the film tension sensor, on the last pre-stretching roll C 2 before loading so as to modify the degree of pre-stretching of the film. In this case, it is advantageous to ensure the control of the last pre-stretching roll. C 2 drawing by a hydraulic motor M 2 and it is then provided in the servo-control SC a servo-valve that ensures and

  a very precise control of this hydraulic motor M 2.

  In the case of the embodiment shown on

  FIG. 3, which corresponds to the wrapping mode of a car-

  "curtain", the application of the method according to the invention is similar to what has been shown in FIG 2; thus, for each of the film strands F P 1 and FP 2, a loop forming mechanism is provided which has been represented in a form

  simplified, that is to say, it is simply provided a rou-

  the return water R 1, R'1 having the function of forming the voltage sensing loop between the last corresponding pre-stretching cylinder C 2, C'2 and the point SI, 52 of arrival of the strand of

  corresponding film S Pl, SP 2 on the load P The roll cor-

  respondent R 3, R'3 is mounted on a SPI support, SP'l can perform a rectilinear or circular motion; it is planned 10.

  facing the corresponding return roller R 3, R '3 a cap

  CT 1, CT 2, whose function is to measure the voltage in the associated film strand F Pl, FP 2 by displacement analysis

  rectilinear or circular of the corresponding roller R 3, R'3.

  The signals of the two sensors CT 1, CT 2 are applied to a servocontrol SC which analyzes them and reacts accordingly.

  on one or more of the motors Ml, M 2, M'l, M'2 for driving

  pre-stretching rolls C1, C 2, C'1, C'2 and / or on the motor M 3 ensuring the translation of the load P. Referring again to FIG. 2, it should be noted

  that the return roller R 2 placed immediately before the loading

  ge P is provided to change the angle of arrival of the film strand FP on the load in order to adapt this angle to the packing conditions of the load; indeed, when the load is fragile, for example when it is formed of packets in

  cardboard whose corners can crash, it is recommended

  to reduce this angle to a minimum, while in others

  applications such a parameter may have less importance.

  As indicated above, the process and the

  tif according to the invention are applicable to the particular field

  packaging, including charges formed of single packets

  pallets, but also in a more general way in all cases where it is necessary to

  check the degree of pre-stretching of a film of plastic material

  that in order to maximize these resistance properties

  and also to obtain maximum profitability of production.

  Of course, the invention is not limited to the examples

  embodiments described above and shown, from which we can provide other modes and other forms

  embodiment, without departing from the scope of the invention.

11.

Claims (8)

CLAIMS I   A method of regulating the degree of pre-stretching of a plastic stretch film which passes over one or more pre-stretching means before reaching a use station, characterized in that it consists in capturing way continues the tension in the film part   including the direction of motion of the film,   the last pre-stretching device and the position of use, and to vary, in correspondence with the sensed tension, the   speed of at least one of said pre-stretching means and / or   use of the film so that the sum of the elongation pro-   duit by pre-stretching and the extra elongation produced by the tension created in the film before his arrival   at the use station does not exceed the elongation of   re. The process according to claim 1, wherein   the pre-stretching means consist of several cycles   pre-drawing lndres, the last of which is driven by an independent motor, characterized in that it consists in continuously sensing the tension in the film in the zone situated   between the last pre-stretching cylinder and the user's station   tion and react on the speed of this last cylinder of   for 6-drawing in correspondence with the sensed tension.   3 Device for implementing the method se-   lon one of claims 1 and 2, especially for machine   for packing a load, comprising pre-stretching means, in particular at least two successive drawing rolls (C 1, C 2) of which at least one rotates at an adjustable speed, the peripheral speed of the downstream roll (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 on the load so that this speed is equal to or lower   at the peripheral speed of the cylinder downstream device characteristic   characterized in that it comprises: means (R 3, CT ') for continuously sensing the film tension   (FP) in its part, considering the meaning of   placement of the film (FP) between the last drawing roll (C 2) and the point of arrival on the load (P), a servo-control (SC) receiving permanently the signal of 12 2528020   output of the film tension sensing means (CT) and   comparing it with a set value so as to vary the speed of at least one of the draw rolls (C 1, C 2) and / or 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 it arrives on the load to be packaged does not   not exceed the elongation of rupture.   4 Device according to Claim 3, characterized   in that the voltage sensing means comprises a device   tif with at least one return roller (R 3) for forming a loop in the film and a film tension sensor (CT)   placed opposite the return roller (R 3) so as to analyze   its displacement occurring according to the voltage of the film.   Device according to one of claims 3 or   4, characterized in that said return roller (R 3) of the dis-   positive loop formation is mounted on a support (SP)   in order to move by translation.   6 Device according to any one of the 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 Device according to any one of the claims   3 to 5, characterized in that the film-loop-forming device of said tension-sensing means comprises, downstream of the first deflection roll, considering the direction of progression of the film, a second deflection roll (R 2 ) who   is mounted in an adjustable manner to allow for a   angle of arrival of the film on the load (P) during of its packaging.   8 Device according to any one of the claims   3 to 7, characterized in that in the case where the last pre-stretching cylinder (C 9) is used for controlling the degree of pre-stretching, this cylinder is actuated   by a hydraulic motor (;, which is   driven by a servo valve incorporated in the servo control (SC)   and responding to the signals provided by the sensor (CT) of film.   9 Application of the process according to one of the claims   1 and 2 and the device according to any one of the   3 to 8 to the packaging of loads, including palletised loads.