GB2033296A

GB2033296A - Welding die for packaging machines

Info

Publication number: GB2033296A
Application number: GB7936075A
Authority: GB
Original assignee: Rovema Verpackungsmaschinen GmbH and Co KG; Rovema GmbH
Current assignee: Rovema Verpackungsmaschinen GmbH and Co KG; Rovema GmbH
Priority date: 1978-10-18
Filing date: 1979-10-17
Publication date: 1980-05-21
Also published as: DE2845271A1; IT7922904V0; IT1125502B; JPS5555915A; IT7926614A0; FR2439133A1

Abstract

A welding die (2 or 200) comprises a carrier strip (3) supporting a plurality of die segments (4), each having a thrust face (11) urged a helical compression spring (15) carried round a thrust bolt (14) against the material to be seamed. The thrust faces of adjacent segments are contiguous or nearly so at their ends so that a continuous seam is produced. This independent resilient mounting of a plurality of segments compensates for a different number of layers over the length of the seam line. A polytetrafluorethylene cover may extend over the thrust faces of all of the segments. <IMAGE>

Description

SPECIFICATION Welding die for packaging machines The invention relates to a welding die for packaging machines for producing seams on packaging films, especially transverse seams on tubular films with inserted lateral folds; the die having a die body, a heating mechanism and a welding strip.

By means of such welding dies, tubular films are acted on to form bags, in that, transverse seams are produced on the tubular film. These transverse seams are formed by welding layers of film lying one on top of the other, the required welding heat being transmitted to the film by the welding dies. In the case of multi-layered films, only certain layers of the film can ever be welded.

In this case sealing is effected. The same dies as for weldings are used therefor. To produce a durable seam, the temperatures of the welding dies and the duration of their action upon the film must be chosen accurately. In addition to the material of the film, these parameters also depend substantially upon the thickness of the film and the number of film layers.

When the same number of film layers lie one on top of the other over the entire seam length, e.g.

two film layers, the correct values can easily be determined. The number of film layers, however, is not identical over the entire seam length in all cases. This applies especially to bags with inserted lateral folds. In the region of the lateral folds there are in fact four film layers, whilst in the region passing from lateral fold to lateral fold there are only two film layers. If conventional welding dies are used to produce such seams, then the seam in the lateral fold region must be squeezed so much that its thickness is less than two film thickness, so that welding takes place where there are only two film layers.This is especially disadvantageous in the case of thick bag films, i.e. when processing films in the thickness range of approx. 120 to approx. 280y. The intense squeezing of the seam in the lateral fold region also produces a weakening (of the film) and there is therefore the risk of ripping.

Because of these problems, it has already been proposed (German Offenlegungsschrift 25 19 253) to effect a separate welding initially in the region of the lateral fold, i.e. where four film layers lie one above the other, and to produce in a later working process the region of the seam where only two film layers lie one above the other.

This presents an increase in plant expenditure, however, since an additional welding station has to be provided. It is also disadvantageous to heat twice a welding point of the film material in the seam region.

An object of the invention is to provide a welding die which enables seams to be produced in one working process, in the region of which seams the number of film layers changes, whereby an excessive squeezing of the seam is to be avoided in the region having the larger number of film layers.

According to the invention, this object is achieved in that the welding strip is divided into segments which are mounted in an elastically resilient manner on the dle body so that each segment is compressible independently of adjacent segments by the pressure applied during welding.

A welding die of such a design may be adapted to the various seam sections in that the segments are compressed to a greater or lesser extent counter to the elastic restoring force. In the region of the larger number of film layers, the segments are compressed more than in the region of the fewer film layers. It is thus no longer necessary to press the seam in the region of larger number of film layers so flat that welding takes place in the region of fewer film layers. Weakened places are thus avoided. The seam is produced in one working process so that it is not necessary to heat the seam region twice, and thus any associated damage to the film material is avoided. The total plant expenditure for a packaging machine is practically no more than when conventional dies are used, since no additional drives or the like are required.Compared with the total cost of a packaging machine, the low additional cost for producing the dies according to the invention is insignificant. Two welding dies according to the invention are generally used to produce one seam.

It is also possible, however, to allow a die according to the invention to co-operate with a rigid fixed welding die, since it is also still possible then to effect an adaptation to the various seam sections.

Each segment may be stepped on one or both of its lateral faces and abut a bearing surface or surfaces as the case may be of the die body with its stepped surface or surfaces. The advantage of this is that the front end position of the segments and thus their alignment one with the other are achieved by particularly simple means. Other means, however, could also be provided therefor, e.g. transverse pins in the segments which abut ends of elongated slots. The segments may be supported on the die body by helical compression springs received in the segments, and each spring may be mounted around a thrust bolt guided in the segment, and supported on the segment at one end and on a shoulder of a belt at the other end with the bolt protruding beyond the rear end of the segment each bolt being supported on the die body.The advantage in using helical compression springs in this manner is that a relatively large spring assembly can be accommodated in a small space. Other elastic means could, however, also be used, e.g. leaf springs or even rubber springs which have adequate heat resistance.

The segments may be substantially wedgeshaped whereby the inclined surfaces thereof run generally transversely of the longitudinal direction of the welding die, and the segments are tapered towards the rear, with the thrust faces being positioned without any substantial separation between adjacent ones thereof.

The advantage of this is that it is also possible to incline the segments when their front ends are adjacent to each other without a join. Such an inclined position permits an even better adaptation to the places of various thicknesses along the seam to be welded. Embodiments also fall within the invention wherein the segments can only be pushed back in a straight line.

In one embodiment the die body comprises a base part and lateral plates which are detachably fixed to this base part and between which the segments are disposed, the segments abutting the lateral plates with large areas of contact. This permits the segments to be heated without heating elements being disposed in the segments themselves. The heating elements could therefore be disposed in the lateral plates which extend over the entire length of the welding die or even in the base part of the die body.

A heat resistant cover film may be used to lie between the die thrust faces and the material to be seamed. This renders the thrust faces of the welding die completely smooth since the cover film covers the regions where the segments abut each other. Markings could in fact be formed on the seam at these regions of abutment.

One embodiment of the invention is shown in the drawing.

Fig. 1 is a cross-section through a tubular film with inserted lateral folds, Fig. 2 is a partial plan view of two welding dies according to the invention, Fig. 3 is a section along line Ill-Ill in Fig. 2, and Fig. 4 is a lateral view of a welding die provided with cover film.

The tubular film which is shown in Fig. 1 and is denoted by S as a whole has lateral folds 1 inserted in the regions I and Ill, so that there are four film layers there. In the region II between the lateral folds 1, there are only two film layers. Since bag films are relatively thick, e.g. have a thickness of 250#, a substantially greater overall thickness (e.g. a thickness of 1 mm) is produced in the regions I and Ill than in the region II where the overall thickness is only half as great, i.e. for example 0.5mm. As was explained initially, when rigid mounted welding dies are used, the seam would have to be compressed in the regions I and Ill by appropriate pressure to such a small thickness that the pressure in the region 11 effects welding.

The welding die according to the invention has (see Figs. 2 and 3) a die body denoted by 2 as a whole and a welding strip which is denoted by 3 as a whole and is composed of many, individual segments 4.

As can be seen from Fig. 3, each segment 4 has lateral faces 5 and 6 which each have a step 7, so that each segment has an upper wide part 8 and a lower narrower part 9. Sloping faces 10, which verge into a thrust face 1 1, are located at the narrower part 9.

A stepped bore with sections 12 and 13 is formed in the segment. A bolt 14, which comprises a shaft 1 4a and a head 1 4b, fits in this bore. The shaft 14a fits slidingly into the bore section 13, and the head 14b which has a spherical cap 1 4c at the top, also fits slidingly into the bore section 12. A helical compression spring 15 is disposed between the bolt section 1 4a and the wall of the bore section 12 and is supported with its one end at the shoulder 16 between the bore sections 12 and 13 and with its upper end at the shoulder 17 which is located between the bolt sections 14a and 1 4b. The head 1 4b protrudes beyond the end face 1 8 of the segment.

The helical compression spring 1 5 is under a certain amount of initial tension in the state which is illustrated.

As can be seen from Fig.2, the lateral faces 19 and 20, at which adjacent segments about one another, incline relative to each other so that the segments taper rearwardly, i.e. towards the end faces 18.

The die body 2 has a base part 21 and lateral plates 22 and 23. Located in the base part 21 are a large bore 24 for accommodating an electrical heating cartridge and a smaller bore 25 for accommodating a temperature sensor.

The lateral plates 22 and 23 engage in recesses 26 and 27 on the base part 21. so that the external faces of the lateral parts are in alignment with the external faces of the base part 21. There is, however, a large area of contact between the lateral parts and the base part, above all in the region of the strip-shaped projection 21 a of the base part 21. The lateral parts are screwed down to these strips by means of screws 28 which pass through the lateral plate 23, engage the strip 21a and are screwed into a threaded hole in the lateral plate 22. The width of the segments 4 is dimensioned so that they can move slidingly between the internal faces of the lateral plates 22 and 23. Shoulders are also located on the lateral parts for supporting the stepped surfaces 7 of the segments.The lateral parts also have sloping faces 22a and 23a by means of which the welding die is tapered forwardly.

The thrust faces 11 (denoted by 11' in Fig. 4) are covered by a heat-resistant film 29 as indicated in Figure 4). Polytetrafluoroethylene is suitable therefor. The film 29 is wound on a supply coil 30 and drawn therefrom by means of a winding coil 31 which is progressively rotated in the machine phase. The film 29 is drawn over a stretching frame 32 which comprises an aperture 33 through which the welding die, denoted by 2' as a whole, can pass.

The welding die according to the invention operates as follows.

The tubular film, on which a transverse seam is to be welded, is drawn between the welding die 2 and an identically designed welding die 200 by means of mechanism which are not shown. During the drawing-off process, the welding dies are spaced one from the other. After termination of the drawing-off movement, the welding dies 2 and 200 are moved towards each other. This may be effected either by a movable die being moved relative to a stationary die or even by both dies being moved towards each other. In this case, the thrust faces come indirectly into contact with the tubular film, since the heat-resistant film 27 is still in fact disposed between the thrust faces 11 or 11' respectively and the tubular film.The segments are now compressed with the compression of the compression springs 1 5, whereby the stepped surfaces 7 are removed from the bearing shoulders at the lateral plates 22 and 23. Before the segments come into contact with the film, the compression springs 15 press the segments against the bearing surfaces of the lateral parts and are thus accurately aligned one with the other.

As can be seen from Fig. 2, the outer segments which co-operate with the region I (see Fig. 1) of the tubular film are pressed further than the segments located in the region II. The fourth segment from the left in Fig. 2, of each die is located in the transition zone between the regions I and II and is therefore slightly inclined. This inclination is possible because of the inclination of lateral faces 1 9 and 20, although the thrust faces 11 are joined together without any space inbetween.

By using welding dies according to the invention, therefore, a too intense squeezing of the seam in the regions I and lil is avoided. The characteristic of the compression springs 1 5 is selected so that the resistance of the film to a compression by the springs cannot be overcome, so that an undesirable flattening of the seam occurs in the regions I and III.

The required welding heat is transmitted to the segments by thermal conduction. The heat initially enters the base part 21 from the cartridge located in the bore 24. The heat is introduced into the lateral plates 22 and 23 by way of the large areas of contact with these lateral plates. The heat is introduced into the segments by way of the large areas of contact between the lateral plates and the segments and guided in the segment bodies to the thrust faces 11. A temperature sensor 28 ensures a constant temperature at the thrust faces 11.

After termination of the welding process, the dies are moved apart. Subsequent to the welding process, the film 29 is conveyed further by the winding roller 31 being rotated on by a small angular amount.

Claims

1. A welding die for packaging machines for producing seams on packaging films the die having a die body, a heating mechanism and a welding strip, wherein the die comprises segments which are mounted in an elastically resilient manner on the die body so that each segment is compressible independently of adjacent segments by the pressure applied during welding.

2. A welding die as claimed in claim 1, wherein each segment is stepped on one or both of its lateral faces and abuts a bearing surface or surfaces as the case may be of the die body with its stepped surface or surfaces.

3. A welding die as claimed in either of the preceding claims, wherein the segments are supported on the die body by way of helical compression springs, the helical compression springs being disposed in the segments.

4. A welding die as claimed in claim 3, wherein each helical compression spring surrounds a thrust bolt guided in the segment, and is supported on the segment at one end and on a shoulder of the bolt at the other end, and the bolt protrudes beyond the rear end of the segment and is supported on the die body.

5. A welding die as claimed in any of the preceding claims, wherein the segments are substantially wedge-shaped, whereby the inclined surfaces thereof run generally transversely of the longitudinal direction of the welding die and the segments are tapered towards the rear, whilst the thrust faces of the segments are positioned without any substantial space inbetween adjacent ones.

6. A welding die as claimed in any of the preceding claims, wherein the die body comprises a base part and lateral plates which are detachably fixed to this base part and between which the segments are disposed, the segments abutting the lateral plates with large areas of contact.

7. A welding die as claimed in claim 6, wherein the base part of the die body contains a heating mechanism and the lateral plates abut the base part over large areas of contact.

8. Apparatus comprising a welding die as claimed in any of the preceding claims, and a cover film made of heat-resistant material, which is extended over all the thrust faces of the segments.

9. Apparatus as claimed in claim 8, wherein the cover film is stretched between a supply coil and a winding coil and to be advanced between successive welding operations.

10. Apparatus as claimed in claim 9, wherein the cover film is stretched over a stretching frame through which the welding die can move.

11. Apparatus as claimed in claim 8, 9 or 10, wherein the cover film is polytetrafluoroethylene.

12. A welding die substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

13. Welding die apparatus substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.