US 20060075724 A1
To be able to weld together layers of film (23, 24) lying one on top of the other with improved sealing seam quality, a distance (a) between sealing surfaces (30) of welding jaws (30) moved toward each other or between a sealing surface and a resistance is prescribed within a sealing duration by a control device (27) of a drive (8) configured as an electromagnetic linear drive (linear motor), the value for the distance (a) always being less than or equal to the value for the thickness (d) for the two layers of film (23, 24) lying one on top of the other.
1. Method for positioning a welding jaw during a welding operation, in particular a welding jaw of a transverse sealing station of a vertical tubular bag machine, the welding jaw being moved in a circulating manner along a closed path or back and forth along a linear path, to be set against a resistance, in particular a second, oppositely running welding jaw, two layers of film lying one on top of the other, with a total thickness (d) for both layers of film together, of a film tube being pressed together between the resistance and the sealing surface of the welding jaw, in particular between the sealing surfaces of two welding jaws, in order to weld the film tube by means of heat introduced into the film tube via the sealing surface of the welding jaw or the sealing surfaces of the welding jaws, with an exerted pressure in a sealing plane (S) under a sealing force within a sealing time, and a drive being provided for the purpose of moving the welding jaw in a direction normal to a surface of the resistance or an oppositely running welding jaw, in order in this way to carry out the welding operation in a fixed location or along a sealing path within the sealing time, wherein a distance (a) between the sealing surface and the resistance or between the two sealing surfaces of the welding jaws during the welding operation is prescribed by a control device of the drive, and in that the value for the distance (a) is always less than or equal to the value for the thickness (d) for the two layers of film lying one on top of the other.
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The proposed innovation relates to packaging machine construction and in respect thereof to a method and an apparatus for positioning a welding jaw during a welding operation, in particular a welding jaw of a transverse sealing station of a vertical tubular bag machine.
It is known from DE 44 25 207 A to move a welding jaw of a transverse sealing station of a vertical tubular bag machine along a closed path. In this case, the path has a straight portion, which is provided for making contact with a film tube pressed together between the welding jaw and a second, oppositely running welding jaw. In this case, the film tube and the welding jaw are moved further at the same speed in the direction of the straight portion, in order to weld the film tube by means of heat introduced into the film tube via the sealing surfaces of the welding jaws, with an exerted pressure in a sealing plane under a sealing force within a sealing time. In this case, a drive is provided for the purpose of moving the welding jaw in a direction normal to a surface of the resistance, in order in this way to carry out the welding operation in a fixed location or along an effective sealing path within the sealing time.
In the case of another vertical tubular bag machine, only one drive is provided for the purpose of moving two welding jaws, which can only move in a horizontal direction toward each other and away from each other, along a linear path in each case, in order to weld a film tube, with the film at a standstill in each case, transversely to its transporting direction.
The known methods and apparatuses have the disadvantage that the welding jaw generally presses the layers of film that are to be welded to one another together too strongly, so that melted polymer is forced out of the sealing seam, which leads to a weakening of the sealing seam. The reason for this is that, as a result of a prescribed sealing force, the welding jaw is pressed against the layers of film only by the mechanical resistance of the latter themselves. The sealing force, or the sealing pressure obtained as a product of the sealing surface and the sealing force, is set by a prestressing of a spring force. This force, preselected by a spring excursion (known as overstretch) is run through during a sealing time. In this case, the sealing force is abruptly produced, with increasing spring compression and subsequent spring relaxation made to exceed a maximum value, and then abruptly removed by separation of the welding jaws. The functional relationship between the sealing force and the time elapsing within the interval for a sealing time is technically prescribed. It can only be changed by an assembly operation or manual setting. And even after such a change, the sealing force profile mentioned is in itself retained, it is just that different values are reached.
Optimum guidance of the welding jaw for gentle, ideal film welding, with which the welding jaw is not set too much against the film tube or into a layer of film, in order not to damage it, is not possible in this case. Moreover, in the case of the known apparatus, the sealing force is relatively high at the end of the sealing time, so that the undesired expulsion of flowable polymer from the sealing seam can occur as a result of the welding jaw pressing into the layers of film too strongly.
This disadvantage is also observed in the case of packaging machine units with which a welding operation is performed only by means of a single welding jaw, in that the welding jaw is moved against a film tube to be welded or a wrapped-around film web, in order to press the film against a fixed resistance and weld it in this way.
The invention is based on the object of developing a method according to the precharacterizing clause of Claim 1 in such a way that the disadvantages mentioned can be overcome.
The object is achieved according to the characterizing clause of Claim 1. According to this, a distance between the sealing surface and the resistance or between the two sealing surfaces of the welding jaws is prescribed during the welding operation by a control device of the drive, and the value for the distance is always less than or equal to the value for the thickness for the two layers of film lying one on top of the other.
The proposed method has the advantage that, during a sealing time, i.e. at the beginning, during and toward the end of the sealing time, which describes a sealing duration, the distance is always chosen such that optimum welding of a film tube by means of transverse or longitudinal seams or optimum welding of a horizontally aligned, folded film web takes place. It is immaterial here whether two welding jaws are moved one against the other or a single welding jaw is moved against a mechanical resistance. It is also immaterial whether a drive producing the normal component, and with it the sealing force, drives only one welding jaw or simultaneously two welding jaws that can be moved one against the other. The distance is prescribed by the operation of the drive precisely in terms of time and location, so that the sealing surface of a welding jaw does not press the layers of film together too much, and consequently cannot force melted polymer out of the sealing seam too much.
The variation of the distance over time within the duration of a sealing time can be fixed on the basis of a profile prescribed by the control device. These profiles may be changed as desired within the framework of reliable film welding, stored for specific double film layer thicknesses and selected from a memory. A linear drive operating on the suspension railway principle (Claim 8) is suitable for performing this method. This is used for example on a vertical tubular bag machine for prescribing precisely in terms of location the distance when welding two layers of film with a total thickness, i.e. thickness of the two layers of film taken together. In this case, the welding jaw is connected to a movable part of this drive. The drive has a part and a part that is movable in relation to this part. The movable part can perform a movement changing the distance. In this case, the respective value of the distance is prescribed by the respective position of the movable part in relation to the part.
With the proposed innovation, the film can also be safeguarded against mechanical damage, in that a welding jaw at a distance from another welding jaw is set against the film relatively gently at a point of impact, in order to begin a welding operation. In order after that to achieve good welding of two layers of film lying one on top of the other, the distance is then maintained or changed slightly, preferably dependent on the polymer. Then the distance can be increased, in order not to force the molten polymer out of the welding seam in a disadvantageous way. Finally, the sealing force is reduced to zero and the welding jaw is removed from the film tube.
Advantageous refinements of the proposed method and of the proposed apparatus are described in Claims 2 to 7, as well as 9 and 10.
If the drive is connected to the welding jaw directly, preferably without any gear mechanism (Claim 9), a direct conversion of the locational coordinates moved to by the drive precisely in terms of time is possible without any gear mechanism such as a crank, toothed rack or belt and without gear transmission, to achieve in each case an exact distance along a locationally exact effective sealing path (may also be zero) and thereby achieve a specific sealing force, provided by the drive, during the sealing time. A mechanism merely deflecting the direction of movement and/or extending a path, for example camways for converting a drive movement into an opposite movement of two welding jaws, is in this case still regarded as direct. A suitable cam mechanism is described in Claim 10. In this case, the movable part is directly connected to a cam plate, which has a cam line or two cam lines for changing the location of a cam connected to a jaw carrier of a welding jaw.
The proposed method may be used in the case of cyclical film transport, if the welding jaw is moved along the linear path, and, with the film at a standstill and the welding jaw or welding jaws lying against the film tube during the sealing time, values for the distance are prescribed by the control device (Claim 2), the drive converting this distance or these distances. In the case of continuous film transport, i.e. film transport without any standstill, according to Claim 3, the welding jaw is moved along a closed path, which has a straight portion, along which, while the film tube is being moved, the welding jaw moved with it welds the film tube. During the sealing time, values for the distance are prescribed by the control device and converted by the drive.
Very good seam results are achieved if the distance is kept constant during the duration of the sealing time (Claim 4). However, these results can still be optimized if, according to Claim 5, the distance is changed during the duration of the sealing time. Once the sealing operation has been virtually completed, it is of advantage, for continued avoidance of polymer expulsion from the then very hot welding seam, if the distance is increased, in order to perform the remaining film welding with an increased distance.
If, according to Claim 6, given an appropriate type of film, before the beginning of the welding operation, a distance that is greater than or equal to the thickness of the sum of the thickness of the two layers of film lying one on top of the other is prescribed, layers of film can be initially preheated without exerting a sealing pressure or any pressure by a sealing surface or two sealing surfaces, before the welding operation is subsequently performed while exerting the sealing force with a reduced distance.
Furthermore, the proposed innovation may be used to detect even very small products that have got between the layers of film, and which would cause an improperly sealed seam if they were enclosed during the welding. This is possible, since even locational deviations in the range of about 0.01 mm can be produced and/or detected by means of an electromagnetic linear drive. In the case of film thicknesses of from 0.03 to 0.1 mm, that is to say values for the double film thickness d of from 0.06 to 0.2 mm, a product inclusion even of extremely small product parts can be correspondingly detected. In this case, according to Claim 7, prescribed values for the distance are compared with a sealing force to be achieved for these values or this value. If there is excessive sealing force for a specific distance, it is concluded that there is a product inclusion between the resistance and a sealing surface or between the sealing surfaces. An acoustic or optical signal of the packaging machine or ejection of a bag produced with a defective transverse or longitudinal seam are appropriate alternatives to follow the detection of a product inclusion.
The proposed method and a vertical tubular bag machine that can be operated by the proposed method are described in more detail below on the basis of figures representing exemplary embodiments, in which:
In the case of a vertical tubular bag machine 3, a film web 14 is drawn off from a supply roll 16 by means of a takeoff 15, drawn over a shaping shoulder 17 and thereby shaped into a film tube 7 (
The welding jaws 1, 6 are operated in such a way that they circulate in opposite directions, in order to carry out the welding of the film tube 7, which is moved downward at the same speed as the welding jaws 1, 6, along a straight portion (
In another exemplary embodiment (
The gear mechanism 52 is locationally accurate and direct and contains a cam plate 53 connected to the movable part 21. The cam plate 53 has two separate, continuous cam lines 54. A cam 55, respectively connected to the jaw carrier 46, 47, is guided along a cam line 54. The cams 55 are configured as rollers and roll along the cam lines 54. The cam lines 54 are formed mirror-symmetrically in relation to each other with the line of effect 49 as the axis of symmetry and in this way lead to a uniform movement of the welding jaws 1, 6 toward each other or away from each other. Transverse welding of the film tube 7 takes place in between. The cam plate 53 is exchangeable, so that a modified jaw movement could be achieved with identical operation of the linear drive 8.
In order to weld the film tube 7 with heat introduced and with an exerted pressure in a sealing plane S under a sealing force within a sealing time, the drive 8 is provided for the purpose of moving the welding jaws 1, 6 respectively in a direction normal to a surface of the oppositely running welding jaw 1, 6, in order in this way to carry out the welding operation in a fixed location (
In the case of the exemplary embodiment of