Method and apparatus for positioning a welding jaw

Description

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:

FIG. 1 shows a side view of a vertical tubular bag machine with welding jaws of a transverse sealing station that can be moved toward each other;

FIG. 2 shows a view from above of the transverse sealing station for the tubular bag machine of FIG. 1, in which a linear motor with a directly acting gear mechanism having a cam plate is used for moving the welding jaws against the film tube, in order to weld the latter transversely to its transporting direction;

FIG. 3 shows a schematic representation of the circulation of two welding jaws, which can be moved toward each other and weld the film tube of FIG. 1 along a straight portion of a respectively circulating path and during the welding operation are at a distance a from each other which has been prescribed for the linear motor (electromagnetic linear drive) by a control device;

FIG. 4 shows a schematic representation of two welding jaws that can be moved toward each other along a linear path, which are at a distance a in the closed position of the jaws;

FIG. 5 shows a sectional representation of the welding of two layers of film lying one against the other, having a total thickness d, a minimal distance a having been set between the sealing surfaces of two welding jaws, and

FIG. 6 shows a sectional representation of the subject of FIG. 5, but with a product inclusion between the layers of film to be welded to each other, the product inclusion being detected from the additional sealing force that is necessary to arrive at a distance a_set from a distance a_act (without product inclusion).

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 (FIG. 1). The film tube 7 is welded in the transporting direction 19 by means of a longitudinal sealing device 18 and filled by a filling pipe 20. Two circulating welding jaws 1, 6, which can be moved toward each other, serve the purpose of welding the film tube 7 transversely to the transporting direction 19 by means of transverse seams 10. A cutting device 11 in one welding jaw 1 serves the purpose of in each case severing a tubular bag 12 with a specific bag length from the 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 (FIG. 3). For this purpose, both welding jaws 1, 6 are moved in each case with a point of movement A along a closed path 4. Each path 4 has a straight portion 5, which is provided for making contact with the film tube 7 pressed together between the welding jaws 1, 6. In this case, the film tube 7 and the welding jaws 1, 6 are moved further at a constant speed in the direction of the straight portion 5, in order to weld the film tube 7 by exposure to the heat of the welding jaws 1, 6 within a sealing time.

In another exemplary embodiment (FIG. 4), a film tube is pressed together and welded in a sealing plane S by welding jaws 1, 6 moved toward each other. The welding jaws are not moved in a y direction. Their point of movement A follows a linear path 29. This corresponds to the so-called cyclical or discontinuous film transport. By contrast with a method according to FIG. 3, in this configuration no drive is required to superpose a movement in the y direction. A single linear drive acting in the horizontal x direction, operating on the suspension railway principle, is provided in both exemplary embodiments (FIG. 3 and FIG. 4) for the movement of the welding jaws 1, 6. The transverse welding of the film tube takes place by means of the two welding jaws 1, 6 clamping the film tube 7 between them (FIG. 2). Two jaw holders 44, 45 serve for respectively holding a welding jaw 1, 6 and a jaw carrier 46, 47 respectively serves for carrying a jaw holder 44, 45, and consequently the welding jaws 1, 6. The jaw carriers 46, 47 are connected to the jaw holders 44, 45 by means of in each case two rigid connections 58 guided in guides 57. In this case, two connections 58 are fitted in a sliding way in two clearances in the jaw carrier 46. Serving as a drive for the jaw movement is a linear drive 8 with a part 22 which is linearly movable along a line of effect 49 and a fixed part 21. The movable part 22 is connected to the gear mechanism 52. Similarly, the gear mechanism 52 is connected to the jaw carriers 46, 47, in order to produce an opposing movement of the jaw carriers 46, 47, and consequently of the welding jaws 1, 6, and in order to be able to move the welding jaws 1, 6 toward each other and away from each other.

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 (FIG. 4) or along a straight portion 5 for a sealing path (FIG. 3) within the sealing time. A distance a between the two sealing surfaces 30 of the welding jaws 1, 6 is prescribed during the welding operation by a control device 27 of the drive 8. The value for the distance a is always less than the value for the thickness d for the two layers of film 23, 24 (FIG. 5) lying one on top of the other, in order not to force the heated, flowable polymer of the heated layers of film 23, 24 out of the transverse seam 10 to an increased extent toward the end of the welding operation. For this purpose, the drive 8, which has a driving part 21 and a part 22 which is movable in relation to the latter, moves the moving part 22 into a specific position along the fixed part 21 in such a way that the distance a is set (FIG. 3 and FIG. 4). During the entire duration of the sealing time, the distance a is kept constant by the drive 8, a specific sealing force, resulting from the distance a, being applied by the drive 8. In this case, the prescribed values for the distance a are constantly compared with a sealing force required for this value. If a sealing force is too high for the prescribed distance a_set, it is concluded that there is a product inclusion 9 between the sealing surfaces 30 (FIG. 6).

In the case of the exemplary embodiment of FIG. 6, the sealing force otherwise only achieved with a distance a_set is already achieved with a distance a_act. The product inclusion 9 is detected from the then increased expenditure of force, measured by the power consumption of the drive 9, for achieving the distance a_set.