Overwrapping film, film pre-stretching device and overwrapping method

The present invention mainly relates to a wrap film in a pre-stretching device of the film likely to be implemented for pre-stretching of such a film overwrap, and a wrapping method using such a film. Prior stretching advantageously comprises an unwinding stage of the film from a supply reel, a stretching step and a step of winding the stretched film on a mandrel or spool, in awaiting later use.

A helical wrapping technique reserved today for low rate secured simple machines stretching a film directly on a pallet load forming a cuboid. The film was disposed on a vertical axis reel equipped with a brake. is firstly fixed one end of the film on the load, which is then twirled around a vertical axis. The braking coil secured stretch film to the desired value.

Unfortunately, the load is not cylindrical, but generally forming a rectangular parallelepiped, the radial distance to be covered with the film varies with the angle, and a constant angular speed rotation of the load associated with a constant braking force n ' not ensure a uniform elongation of the film. In addition, the plastic film immediately after stretching undergoes relaxation which leads, during a period substantially equal to 48 hours, to a relaxation of the elastic restoring force substantially equal to 50%. Thus, with this type of machine, we were limited to stretching forces may be borne by the load with elastic restoring forces, ensuring cohesion of the load during handling and transportation, substantially less. One was therefore led him to abandon this type of machine for the benefit of wrapping machines with a pre-stretching device comprising a plurality of motorized rollers driven at different peripheral speeds. Output rollers of the pre-stretching device, the film banner was no tension or under low tension preferably on the load to be wrapped, as will be explained with reference to Figure 1.

In Patent Application No. 92 10254 (published under the number FR 2695102), the Applicant has described a wrapping method for a charge with a pre-stretched film, removal of the film involved only after a relaxation time under tension of the film. This patent application indicates, as will be explained with reference to Figure 2, which can thus obtain a gain at the level of the residual elongation after relaxation of the film, thereby reducing the consumption.

In Patent Application No. 92 10254 (published under the number FR 2695102), the Applicant describes the optimum way to use a pre-stretched film with relaxation under a high voltage.

However, films relaxed under a high voltage are not suitable for the realization of any type of packaging. On the one hand, they exert significant constriction force on the film reel, it can, in some cases, up to the destruction of such a coil. Thus, it is necessary to implement coils having a high compressive strength, for example steel. These coils have a significant cost and, in most cases, have to be reused. The return of empty spools induces a significant cost. On the other hand, the film placed around a load exerts thereon, due to its elastic memory, a constriction force whose intensity is substantially equal to half the value of the mechanical tension with which the film s is relaxed. Thus, certain expenses are too fragile or too compressible to withstand the forces exerted by constricting film relaxed under a high voltage. In addition, films relaxed under a high tension are rather rigid, inelastic and often brittle. This type of film has in some cases a significant risk of perforation, in particular at the corners of the pallet.

WO 89/06594 describes a thin film production apparatus making the best use of production capacity extruder. first extruding a relatively thick film having a stretching increases the surface.

In it, we look to release maximum force in the film before relaxation to wind the film around a load with the lowest possible force.

It is therefore an object of the present invention to provide a wrap film exerting a controlled constriction force on its storage reel.

It is also an object of the present invention to provide an outer wrapping film capable of exerting a moderate force on the wrapped load.

It is also an object of the present invention to provide an outer wrapping film having good mechanical properties, especially an elastic film, resistant to abrasion and puncture.

It is also an object of the present invention to provide an outer wrapping film may be deposited around a load with a mechanical voltage lower than the voltage that will later carry the load to ensure cohesion.

It is also an object of the present invention to provide a film adapted to the mechanical characteristics of the products forming the load to be wrapped.

These objects are achieved by a film according to the present invention which has been stretched in advance under a high level of tension, released, and then wound under moderate tension under which the film is relaxed.

In a first alternative embodiment of the film production method according to the present invention, releasing the mechanical tension in the film just after the stretching step. Upon reaching the desired elongation, the released film is wound under moderate tension on a storage reel on which it will relax. In a second variant of implementation of the method for manufacturing a film according to the present invention, the film is maintained under tension for a short time before the release of the mechanical tension in the film and winding on a reel. For example, we followed the stretched film and maintained under tension a path between guide rollers so as to delay the release of the film after stretching. According to the exerted constriction force, the film is wound on a cardboard core, plastic, steel or other.

The invention mainly relates to a wrap film having subjected to stretching under a tension of between 10 and 20 N / mm² initial section of width, to winding, and to relaxation on a mandrel or a receiving coil under tension between 10 and 90% of the stretching tension.

The invention also relates to a film, characterized in that it has undergone a winding and relaxation under tension lying between 10 and 50% of the stretching tension.

The invention also relates to a film, characterized in that after the stretching operation and before release, it has been subjected to partial relaxation at the stretching tension for a period between 0.1 and 10 s .

The invention also relates to a reel of film, characterized in that it comprises a card hub wound on a film of the invention.

The invention also relates to a reel of film, characterized in that it comprises wound on a hub of plastic film according to the invention.

The invention also relates to a film stretching machine including stretching means to stretch film, film guide means and the film winding means stretched on a storage reel, characterized in that it comprises means for partially releasing the tension in the film after stretching and before it is wound on the storage reel.

The invention also relates to a machine, characterized in that the stretching means and the means for partially releasing the tension comprise motorized rollers and in that the variation of the peripheral speeds of the output rollers of the stretching means relative to that of the roller means for partially releasing the tension in the film is between 10 and 90%, preferably between 10% and 50%.

The invention also relates to a machine, characterized in that the stretching means and the means for partially releasing the tension comprise motorized rollers and in that the variation of the peripheral speeds of the output rollers of the stretching means relative to that of the roller means for partially releasing the tension in the film is between 10 and 50%, preferably between 10% and 30%.

The invention also relates to a machine, characterized in that it comprises means for varying the ratio of the peripheral speeds of the outlet roller of the drawing means relative to that of the roller means for partially releasing the tension in the movie.

The invention also relates to a wrapping method, characterized in that it comprises a step of depositing a film according to the invention around a load, in particular palletized.

The invention also relates to a method, characterized in that the film is deposited on the load under a tension of less than half the stretching tension of the film.

The invention will be better understood from the following description and the appended figures given as non-limiting examples and in which:<ul><li>Figure 1 is a graph explaining a behavior of a stretched film simultaneously with the removal of a load (prior art);</li><li>Figure 2 is a graph explaining the behavior of a stretched film with relaxation under a high voltage (prior art);</li><li>Figure 3 is a graph explaining the behavior of a film according to the invention having undergone stretching under tension, a moderate release of the tension, a winding and relaxation under this moderate tension;</li><li>Figure 4 is a graph explaining the behavior of a film according to the invention that has been stretching under tension, a relaxation of greater tension than that of Figure 3 with a relaxation and recovery force, such a coil at a lower tension than that illustrated in Figure 3;</li><li>Figure 5 is a graph explaining the behavior of a film according to the invention having undergone stretching under tension, a lower release of the mechanical tension as that illustrated in Figure 3, a winding and a relaxation forcefully recovery , under greater tension than that illustrated in Figure 3;</li><li>Figure 6 is a graph explaining the behavior of a film according to the present invention that has been stretching under tension, to partial relaxation under tension stretching, relaxation of the tension and a complete or substantially complete relaxation with recovery force under low tension; </li><li>Figure 7 is a schematic plan view of the film path in a first embodiment of a pre-stretching machine according to the present invention;</li><li>Figure 8 is a schematic plan view of the film path in a second embodiment of a pre-stretching machine according to the present invention.</li></ul>

In the non limiting examples of Figures 1 to 6 there is shown the behavior of a film having a thickness of 20 microns, 0.5 m of cloth, linear low density polyethylene manufactured on flat or tubular die.

Figures 7 and 8, the same reference numerals for designating the same elements have been used.

The corresponding areas of the curves of Figures 1 to 6 are designated by the same reference numerals followed by a dot and the number of the figure.

In Figure 1, we can see the evolution in time of the curve 1.1 expressing the restoring force expressed in daN exerted by a stretchable polyethylene film according to its elongation expressed in% at a type prestretch substantially simultaneous with the known wrapping a load. The elongation zero corresponds to the film before stretching, whereas an elongation of 100% corresponds to the doubling of the length of the film. Avoids tearing of the film on the sharp edges of the load and the crash load CARDBOARD REINFORCEMENT by a relaxation of the film during wrapping, at the cost of a decrease in elongation and therefore an increase in the consumption of movie. In a conventional wrapping machine with motorized pre-stretching means with control of the force of removal is decreased preferably substantially 50% the mechanical tension in the film at the output of pre-stretching rollers, this reduced tension being immediately applied to the load during the wrapping. Substantially all of the relaxation is performed in the films banded around the load with a low tension is equivalent to the residual tension after relaxation. This voltage after relaxation and relaxation substantially corresponds to half of the tension required to obtain the optimal elongation. In the example illustrated in Figure 1, the simplified curve 1.1 comprises firstly a substantially straight area 2.1 followed by a rounded zone 3.1, corresponding to a plastic flow threshold and approaches the horizontal in a 4.1 area corresponding, for example, an elongation between 100 and 200%, and a tensile of 12 daN. We stop pulling in 5.1 corresponding to 12 daN and an elongation of 200%. An additional elongation illustrated dotted in 6.1 lead to breakage point 7.1 corresponding substantially to a 500% elongation and to a force of 15 to 20 daN. With advanced conventional machines, the film is released in 8 with an elastic return of the film corresponding to a decrease in tension and elongation until reaching point 9 corresponding substantially to 6 daN force and at an elongation of 170%. The prestretched film is charged at a slightly higher speed than the banding and relaxation occurs in this halved mechanical tension, mainly in the portion of the film located between the prestretching device and the load. This reduced return force corresponding to the residual spring force after relaxation the film. Thus one avoids subjecting the load to be wrapped to a tensile force greater than that actually ensure its cohesion during subsequent handling.

In 10.1, is illustrated in dotted relaxation that would occur in the film if it is fully relaxed the tension. In such a case, the residual elongation obtained in 11.1 is substantially equal to 120%, that is to say that the length of film would have been multiplied by a factor equal to 2.2. The tension in the film deposited around the load back to 14.1 to a value substantially equal to 3 daN.

The curve 1.2 of Figure 2, corresponding to a pre-stretching with relaxation under a high voltage coil, includes 2.2, 3.2 areas 4.2 and 5.2 equivalent to 2.1 areas, 3.1, 4.1 and 5.1 respectively, of Figure 1 . Then, relaxation, corresponding to a vertical descent of the curve 13.2 is obtained for example by maintaining power to the pre-stretched film on a mandrel. The implementation of a common polyethylene stretch film is extremely easy to the extent that this film is slightly sticky, which avoids having to take special precautions when handling. After relaxation 13.2, it is at point 15.2 corresponding to a restoring force of 6 daN, only half the pre-stretching force equal, in the example shown at 12 daN for an elongation of 200%, higher than the residual elongation obtained at point 9 of Figure 1 leading to a film economy already described in Patent Application No. 92 10254.

1.3 curve of Figure 3, corresponding to a pre-stretching with relaxation under moderate tension on coil has 2.3 areas, 3.3, 4.3 and 5.3 equivalent to 2.1 areas, 3.1, 4.1 and 5.1 respectively, of Figure 1. then, 17.3, releasing the tension preferably 18.3 to the point corresponding substantially to half the tensile force ensuring the elongation of the film upon stretching (6 daN in the example shown). In 19.3, shown dotted residual elasticity would happen in the movie if we totally relaxed the tension when laying on the load. In such a case, the residual elongation obtained in 20.3 is substantially equal to 175%. Advantageously, the film is applied to a load, for example palletised with mechanical tension less than or equal to the voltage at which the film was released (item 18.3) to avoid a re-stretching. In the exemplary embodiment, the laying of the film around the load is performed at a very low tension, for example between 0.5 and 1 daN. However, it is understood that the laying of the film according to this invention on a load to be wrapped under greater tension is not beyond the scope of the present invention.

To release a substantially equal to half the stretching force (tension on coil equal to half the stretching force), the tension in the film and elongation of the film does not vary during Phase relaxation which is symbolized by the point 18.3 in Figure 3. After relaxation of at least a few tens of seconds (knowing that complete relaxation requires several hours), it can perform the wrapping a palletized load with preferably the film according to the invention. In the example shown in Figure 3, the wrapping is done at the point 20.3 with an extremely low voltage, the release of tension causes a contraction of 19.3 film loses some of elongation. In the example illustrated in section 20.3, to substantially zero tension, the films retain substantially equal to 175% elongation. After laying the film around the pallet, the film takes in 21.3 an important cohesive force of up to 22.3 substantially half of the tension with which it was wound before relaxation (item 18.3) and thus one quarter of the voltage stretching (5.3). The line connecting the substantially straight 23.3 22.3 point 5.3 is the response curve of the film in particular by the stresses in the load during handling and transport. A near vertical reaction curve decreases unwanted deformations of the wrapped load subjected to stresses during handling and transport (tilt, braking, acceleration, shock, etc ...).

In most cases the increase of the constrictive force exerted by the film of the present invention after removal of the film on a load present many advantages. First, to the extent that the load can withstand such force, it ensures cohesion during handling and transport. In addition, it increases the engine speed to implement packaging machines with low power or even the easy manual removal of the film on the load. Moreover, removal under low power or low, for example between 0.1 and 2 daN minimizes the risk of breaking the film, especially on sharp corners of the load. By cons, some loads, eg, empty plastic bottles, are too fragile, or toilet paper, paper towel rolls and other too compressible cellulose based household products to safely withstand a force of shame significant constriction. In such a case, it is advantageous to use the film whose behavior is illustrated in Figure 4 which has undergone a relaxation at a tension less than that of the film whose behavior is illustrated in Figure 3.

In Figure 4, we can see the behavior of a film according to the present invention, the stretching tension was released after 1.2 daN in point 18.4. During relaxation in 54.4, the voltage rises to 3.5 daN. After the film was placed around a load in 20.4 under very low tension, the tension exerted by the film on the load back after laying in 21.4 to a value substantially equal to 1.5 daN. The release of tension during installation causes a film length of recovery that the residual elongation is 150%. This type of strength is compatible with loads easy to be wrapped. In addition, the reaction curve 23.4 has a lower slope than the reaction curve 23.3 in Figure 3, but higher than that of the reaction curve 23.1 in Figure 1.

It is understood that the present invention is not limited to a release of tension of the film after elongation whose value is greater than or equal to half the value of the stretching force.

In the example illustrated in FIG 5, the film is released at a voltage substantially equal to 8 daN 18.5. During relaxation in 24.5, the elongation does not vary (the film is still wound on a reel) but the voltage drops. The example in Figure 5 shows an intermediate case between the known type of film shown in Figure 2 and advantageous embodiment of Figures 3 and 4.

In Figure 6, we can see the behavior of a film having been subjected to partial relaxation 17.6 a fraction of a second to a few seconds under mechanical tension stretching, followed by 18.6 by releasing the tension and complete relaxation or substantially complete at a voltage less than half the value of the stretching tension. During relaxation in 54.6 to 1.2 daN, the voltage rises to 3.5 daN. In 20.6, the film is placed around a load at a very low voltage, which then rises to 21.6 to 1.5 daN for example.

The embodiment of Figure 6 allows, thanks to a prior partial relaxation under drawing tension, to limit the contraction of the film upon release of the voltage and thus to a law film savings of 10 to 15%. Indeed, the residual stretching of 21.6 (Figure 6) is 160 to 165% instead of 150% at 21.4 (Figure 4).

In Figure 7, show the preferred embodiment of a machine 30 for prior stretching with relaxation film of the present invention. The machine 30 comprises means 31 for receiving a roll of film 32 to be stretched, a pair of rollers 33, 34 of film stretching, a roll 35 ensuring the partial release of tension in the film 36, means 37 receiving a spool 38 of stretched and relaxed film, guide rollers 40, 41, 57 and 42 and at least a motor 43. Advantageously, the roller 35 drives the receiving coil 38. motion transmission means 44 comprising chains, toothed belts, sprockets and / or gearbox allows the drive rollers 33, 34 and 35 by the motor 43. for example, the transmission means 44 comprise a chain or a belt 61 driven by the motor shaft 43 and driving the roller shafts 34 and 42. a second belt or chain 63 connects the axis of the roller 42 to the roller shaft 35. means, symbolized by the arrow 45 to ensure the application of the roller 35 of the coil 38 of the stretched and relaxed film. Roller 35 deviates from the axis of the spool 38 along the arrow 46 gradually as the diameter of the coil 38 increases by winding more of the film on the spool. Advantageously, the rollers 33, 34 and / or 35 are grooved, knurled and / or are provided with a surface having good adhesion to the film. Advantageously, household between the rollers 34 and 35 a film path having a length greater than 0.5 m, for example equal to 0.8 m, to allow transverse elastic return of the film before winding 36, so as to release the transverse stresses in the film caused by the longitudinal stretching tension. This facilitates the film on the winding coil 38 and its subsequent unwinding.

We shall now explain the operation of the machine 30 according to the present invention.

First, we determine the constriction force that we want the film 36 exerts on the charges to be wrapped and the resistance that the film should have friction and puncturing according to palletized products and level severity of transport conditions. Are adjusted means 44 so as to obtain the draw ratio and release before winding the film. The stretch ratio is determined by the ratio of the peripheral speed of the roller 33 to the peripheral speed of the roller 34. The relaxation ratio is determined by the ratio of the roll peripheral speed 34 and that of the roller 35. The setting this report is performed for example by the choice of gears operated by the movement transmission means 44. However, the implementation of other adjustment means of the reports as a machine having a draw ratio and / or fixed release is not beyond the scope of this invention.

We want to make a film 36 coming from the reel 32, successively on the rollers 40, 33, 41, 34, 42, and 35 to bring about the coil 38. It activates the motor 43 which drives at least the rollers 33, 34 and 35. the film 35 is stretched between the rollers 33 and 34. between the rollers 34 and 35, the film is released in the mechanical release voltage is then wound on the bobbin 38. When stretched and released the desired amount of film is removed the reel 38 of the receiving means 37 which is replaced with a new mandrel. If necessary, one can also change the roll of film 32 to be stretched.

An alternative embodiment of a machine 30 according to the present invention illustrated in Figure 8 further comprises two additional rollers 67, 69, whose peripheral speed is equal to that of roller 34, arranged between the roller 42 and the roller 35, so as to unwind the film 36 over a predetermined distance, for example between 0.5 and 4 m, preferably between 0.6 and 2 m. During the journey time, the film 36 undergoes partial relaxation under tension for stretching.

Advantageously, use is made of a polyethylene film having a thickness of between 10 and 35 .mu.m, preferably between 15 and 23 .mu.m.

The speed of unwinding of the film is advantageously between 1 and 10 m / s, preferably between 3 and 6 m / s.

The elongation of the film is advantageously between 50 and 500%, preferably between 200 and 300%.

The stretching tension is advantageously between 10 and 35 N / mm², Preferably between 10 and 20 N / mm².

The width of film is between 0.2 and 2 m, preferably between 0.2 m and 1 m.

Depending on the stretched film prior to the desired, you can of course implement any combination of the aforementioned value.

In the selection of values, one must consider the fact that the thickness, and hence the frictional resistance and puncture of the first stretched film decreases with elongation and that, for a given film, the resulting elongation depends on the applied voltage

Examples of inventive films have been satisfactory:<ul><li>film for wrapping pressurized beer barrels pallet (load very resistant to compression, large mass): width 0.5 m polyethylene film having a thickness of 23 .mu.m, a 300% elongation under a tension of 20 daN coiled at a voltage between 15 and 18 daN, tension after relaxation equal to 10 daN.</li><li>film for wrapping pallets containing cans (bit fragile load, good resistance to compression, large mass): width 0.5 m, polyethylene film, having a thickness of 19 microns, longer 250% under a voltage of 15 daN wound to 8 daN voltage after relaxation equal to 7.5 daN.</li><li>film for wrapping a pallet of food products in cardboard packaging (moderately fragile, average compressive strength, significant mass): width 0.5 m, polyethylene film, having a thickness of 19 microns, an elongation of 250% under a tension of 15 daN to 3 daN wound, tension after relaxation equal to 3.5 daN.</li><li>film for wrapping a pallet of toilet paper (not fragile, low compression strength, low mass): width 0.5 m, polyethylene film, having a thickness of 16 microns, a 200% elongation under a tension of 10 daN wound at 1 daN, tension after relaxation equal to 1.5 daN.</li></ul>

The present invention applies to the realization of wrapping film on palletized loads or not for manual or mechanized installation. One can for example implement a manual laying apparatus of film described in Patent Application No. 95 05645, a spiral banding machine load carrier turntable or a fixed charge machine and rotary ring.