Method and apparatus for manufacturing sacks

Abstract: A method and apparatus are described for manufacturing cushion-shaped and/or bellows-shaped sacks, made of polyethylene or other heat-sealable material, obtained by superposing two films or sheets which are unwound from respective rolls and are submitted to a pair of longitudinal sealings to form the tube to be filled, and to a pair of transverse seals and an intermediate cut for closing, at the front and rear ends, the sacks obtained from said tube. According to the invention, the edges of at least one of said sheets are formed with at least one respective re-entrant fold (P2-P2′) whereby said edges will overlap, with a portion of their outer faces, corresponding portions of the inner face of the other sheet, or vice versa. When only one of said sheets is folded at the edges, the sealing of the overlapped end portions is effected by a single respective sealing bar (15-15′) acting on the outer face of the edges to be joined and having an opposite cold and possibly stationary contrasting member (16-16′). In order to concentrate the sealing heat only on the edge portions to be secured to each other, and to prevent said heat from reaching the sheet whose edges (P2-P2′) have been folded and acted upon by said stationary contrasting member (16-16′), preferably a strip of any suitable material, sufficiently flexible and of such a length as to co-extend with said sheets over the entire extent of the sealing bars, operates between the parts of said edge portions. As an alternative, both sheets are folded at their edges, and in this instance a double sealing can be effected on the superposed edges by pairs of sealing bars (15-115-15′-115′).

Description: Methods and apparatuses are known for manufacturing - on a machine for packaging loose materials, as shown, for example, in the diagrammatic side elevation of Figure 1 of the accompanying drawings, to which reference is made - sacks made of polyethylene or other heat-weldable material, starting from two films or sheets N1-N2 of said material, which are unwound from respective rolls, which are superposed on each other and are conveyed with a longitudinal movement, along a vertical path of travel along which they are held at their longitudinal edges by pairs of superposed chains C1-C2 which are synchronized with each other and are powered as shown diagrammatically at M, said chains transporting downwards the tube B formed by the two sheets. G1 and G2 indicate stationary guides controlling the active runs of chains C1-C2 to oblige them to firmly clamp the edges of said sheets. Usually, said chains are advanced intermittently, and during the dwell period thereof the superposed portions of the two sheets, which are disposed outside of said chains, will be engaged by pairs of rectilinear, parallel and opposite welders (not shown) which effect longitudinal and continuous heat-welds on said sheets to sealingly join them. Therefore, a continuous tube B exits from the lower portion of the pairs of chains and is then closed by a dual unit for transverse welding and intermediate cut indicated at S1-T-S2. After the bottom has been closed, the tube is advanced as far as to engage a conveyor R, whereafter said chains are stopped and while the longitudinal welders are operated and the unit S1-T-S2 is opened, a hopper (not shown) discharges a suitable amount of product into the tube, whereafter the unit S1-T-S2 is operated to close the filled sack, to separate the latter from the tube thereabove and to close the bottom of the latter. The filled sack is then moved away by said conveyor R and the cycle is repeated. The known art teaches also how to pleat longitudinally, with symmetrical configuration, the longitudinal edges of at least one of the sheets constituting the tube, whereby the sacks made therefrom will have a bellows-shaped or folding cross section, this condition giving the sack a shape which is very similar to that of a parallelepiped when filled with a product, closed and lying horizontally lengthwise. This configuration renders the sack better stackable and palletizable. Document IT-A-1 156 700 (in the name of the same proprietor) discloses an apparatus with rotating discs, effecting the longitudinal pleating of the edges of one of the two sheets and which is adjustable to match sheets of different dimensions and to form pleats of different width. The welding carried out longitudinally according to the conventional art requires that the two thermoplastic sheets N1-N2 are caused to adhere to each other due to local melting in the condition of Figure 2, by pairs of opposite welding bars 15-115. When the wall of a sack is subjected to traction, as from Figure 3, a weld effected as described above often leads to the rupture of said wall even with traction forces much lower than those which would have been supported by a not welded sheet, whereby sheets of comparatively high thickness must be used in order to obtain on them particularly strong welded joints. The higher thickness of the sheets, obviously, involves higher costs, also as a result of the longer time required for the welding operations, both in the heating step and in the successive cooling step. The poor resistance of the longitudinal welds carried out according to the conventional art is mainly due to the fact that the force exerted by the sacked product on the welded zone is perpendicular to said zone which, therefore, is subjected exclusively to traction. In a sack manufactured according to the conventional art, the welded edges protrude considerably from said sack and dispose themselves perpendicularly thereto, thus originating problems of space requirements, safety and outer appearance. Document DE-A-3 721 303 describes a bag forming process using one sheet of film which is folded in half, pleated to form bellows at the folded sides and at the open sides and then welded to itself. One edge of the film is given a further longitudinal fold such that the outer face of this edge will overlap a corresponding portion of the inner face of the other edge. Document US-A-2 440 664 describes a method of welding the edges of one or two sheets of thermoplastic film by interposing between the sheets which are not required to be welded an insert barrier sheet of thermal insulating material. According to the invention, there is provided an apparatus for manufacturing sacks by effecting longitudinal welds on two superposed sheets of heat-weldable material, to form a tube from which sacks are obtained by successive transverse welding and cutting operations, comprising two pairs of sheet-transporting chain, means for folding the longitudinal edges of at least one of two opposing sheets constituting the sack-forming tube, so that the outer faces of said edges are directed towards the inner faces of the edges of the opposing sheet when said sheets are superposed on each other between said two pairs of chains which clamp said sheets and advance said sheets longitudinally, welding bars acting on non-folded edges of one of the two sheets, while contrasting members act in opposition to said bars. The apparatus is characterized by the fact that insert strips are provided supported at the upper ends thereof by suitable stationary supporting means arranged upstream of the zone where the two sheets are superposed on each other to form said tube, said insert strips being inserted in the folded edges so as to prevent said folded edges from being welded to the sheet from which they are originated. Said insert strips are flexible to permit a uniformly-distributed compression of the sheet portions interposed between the sealing bars and the respective contrasting members. Moreover, said insert strips are made of a material which does not adhere to the welded edges of said two sheets. Further characteristics of the invention and the advantages resulting therefrom will become apparent from the following description of a preferred embodiment thereof, shown merely as a non-limiting example in the Figures of the accompanying drawings, in which: Figure 1 is a side elevational view of the machine in which the apparatus of the invention has been incorporated; Figures 2 and 3 diagrammatically show the execution of longitudinal wedls according to the conventional art and with the disadvantages mentioned above; Figure 4 is a perspective view of one of the units for folding the longitudinal edges of one of the sack-forming sheets; Figure 5 is a front elevational view of the unit of Figure 4; Figure 6 is a diagrammatic sectional view on the line IV-IV of Figure 1, showing the configuration and mutual arrangement of the two sack-forming sheets; Figure 7 is a perspective view of a possible embodiment of one of the heat-insulating strips inserted in the longitudinal edges of one of the two sack-forming sheets; Figures 8 and 9 show, according to the sectional view of Figure 6, as many possible and different configurations of the longitudinal edges of the sack-forming sheets; Figures 10, 11 and 12 show, according to the sectional views of Figures 6, 8 and 9, as many possible and different embodiments. In the apparatus according to the invention, there is required to fold over in a re-entrant manner the longitudinal edges of at least one of the two sack-forming sheets, so that these edges will contact the inner face of the other sheet, with a portion of their outer face. According to the invention, said folding is effected, for example, on the sheet N1, preferably at the point indicated by the arrow K in Figure 1, before said sheet comes within the reach of the pairs of transporting chains C1-C2 and upstream of the turnround roller H. Secured at 1 on the side frames of the machine, there are the plates 2-102 supporting, crosswise to the sheet N1 and at a suitable distance therefrom, a pair of rods 3-103 having a cross member 202 secured intermediately thereof. Mounted on said rods, at both sides of the intermediate cross member 202, there are pairs of slides 4-5 and 104-105 whose positioning may be adjusted by means of respective screws 6-7 and 106-107 rotatably supported by said plates and cross member, parallelly to the rods 2-102, co-operating with nut members arranged in the respective slides, passing freely through a hole formed in the slide which is not concerned, and selectively controlled by means of knobs 8 and 9. The positioning of said slides with respect to the intermediate longitudinal axis of the sheet N1, which is identifiable with the cross member 202, may be ascertained by means of graduated scales 30 fixed to the supporting members 2-102-202, parallel to the rods 3-103 and co-operating with reference pointers 31 (see Figure 4) on said slides. It is to be understood that the actuation of the adjusting screws 6-7-106-107, as an alternative, may be remotely controlled through servo-controls, encoders and programming units, in a manner which may be easily conceived and constructed by those skilled in the art. Rotatably mounted on the slides there are respective shafts 10-11, 110-111 extending downwards perpendicularly to the sheet N1 and having mounted thereon discs 12-13-14 and 112-113-114 acting on the edges of said sheet to suitably fold them. The discs 13-14 and 113-114 effect the S-shaped folds at the edges of the sheet, indicated by P1 and P1' on the edges of the sheets, which are necessary for the bellows-shaping of the sack-forming tube. The discs 12-13, 112-113 and the shafts 10-110 form the channels required to form the folds P2-P2' which make the sheet N1 overlap the inner face of the sheet N2, through portions of its outer face. During the vertical path of travel when the two sheets are held by the pair of chains C1-C2 and C1'-C2', the two sheets are, in fact, in the condition shown in Figure 6. Otherwise, the folds P1-P1' may be such as to overlap, with a portion thereof, the portions P2-P2', as shown for example in Figure 8, so that the edges of the sack-forming tube will have no projections, contrarily to the example of Figure 6. This condition may be obtained easily by acting on the adjusting screws for the idle folding rollers acting on the sheet N1, the whole in a manner which may be easily conceived and carried out by those skilled in the art. It appears from Figures 1 and 4 that, thanks to the longitudinal tensioning exerted on both sheets by the transporting chains and by the tensioning means which control them after being unwound from the respective rolls, and thanks to the supporting action of the roller H and the fastening action of the transverse bottom welding of the tube effected by the unit S1, the sheet N1 will be maintained steadily in the folded condition to which it is brought by the device of Figure 4. As mentioned in the introductory part of the present specification, the longitudinal welds of the sheets N1 and N2 are effected only by the welding bars 15-15' which act by directly engaging the edges of the sheet that is not provided with the folds P2-P2', while the opposite bars 16-16' are not heated and may be even secured to the frame of the machine, the whole with obvious simplification and economy in the manufacture of said machine. The bars 16-16' are now used only as contrasting means for the welding bars 15-15' which are the only members which are heated, as indicated at 17-17', and to be moved with a rectilinear, horizontal, reciprocating movement, as indicated by the arrows 18-18' in the same Figure 6. The contrasting members 16-16' may not be coated with teflon as the welding bars are, because they are now acting on the sheets, if desired, for dissipating the heat which is emitted by the bars 15-15'. For this purpose, it is not to be excluded that said contrasting members may be suitably cooled by appropriate means. By using co-extruded thermoplastics or other composite sheets, as shown in the example of Figure 12 where N1 and N2 indicate high-density layers and N1'-N2' indicate low-density layers which due to said folds P2-P2' overlap at the sealing areas, it is not to be excluded that the simple solution disclosed above can permit to achieve the objective of limiting the welding only to the overlapped edges of the two sheets, avoiding that it may involve also the superposed portions of the same sheet N1. According to the invention, at the point of the edges of the sheet N1, indicated by the arrow K1 in Figure 1, when said edges are not yet coupled with those of the sheet N2, there may be provided supports 19-19' of the type which is shown, for example, in Figure 7, which are secured at one end to the frame of the machine and which overhangigly support the upper portion of respective flexible strips 20-20' which are accommodated in the folds P2-P2' and which are of such a length as to reach the longitudinal welding elements 15-15' and, preferably, to extend beyond them for a suitable length, however, without interfering with the transverse sealing and cutting unit S1-S2-T. Said strips are intended to be interposed between the pair of superposed sheet edges to be welded together, in order to concentrate onto them the heat from the bars 15-15' and to prevent said heat from reaching the outer portion of the fold P2 with obvious consequences. The strips 20-20' are trapped between the outer edge of the respective folds P2-P2' and the pairs of sheet-transporting chains, whereby they are maintained suitably in their correct position even if they are only anchored at one end from the supports 19-19' and even if they are relatively flexible. A sufficient flexibility, conversely, is desirable and necessary to uniformly distribute the pressure exerted by the welding units on the sheet portions which are to be welded together. Good results have been obtained by using strips 20-20' made of stainless steel, with suitably rounded edges and a thickness of a few millimetres. However, it is to be understood that said strips may be made of any other suitable material or a plurality of suitable materials, for example, partly of teflon, of silicone-rubber or other materials which are suitable for the purposes mentioned above. For example, it is not to be excluded that said strips may be formed by a very flat U-shaped metallic body the hollow portion of which is filled with a strip of silicone rubber which is directed towards the welding bar and which is held in place by a teflon wrapping. Figure 9 shows that according to a modification in the execution of the welding method disclosed above, the fold P2-P2' may concern the edges of the sheet N2, while the edges of the sheet N1 are only provided with the bellows-shaped folds P1-P1'. In this instance, the sealing bars 15-15' will act against the edges of the sheet Nl, while the contrasting members 16-16' will concern the sheet N2. It is also to be understood that the preceding considerations concerning the solution of Figure 8 apply also to the modification of Figure 9. The modification of Figure 9 is of some importance because it permits the use of sheets N1-N2 having the same width, with the obvious advantages deriving from this condition. According to a modification in the execution of the method disclosed above, shown in Figure 10, the formation of the folds P2-P2' may be effected on the edges of one of the two sheets, e.g. the sheet N2, when said sheets are already clamped between the pairs of transporting chains and are superposed to each other. The edges to be folded of the sheet N2 would protrude beyond the other sheet N1 and would be folded thereover by any suitable stationary or movable means. In the present case, said strips 20,20' would be placed between two edges to be welded together and the sheet N2 from which said folds P2-P2' are originated. The welding bars 15-15', in this instance, would operate on the folded over edges, while the contrasting members 16-16' would act on the sheet N2 whose edges have been folded over. The means to carry out this modification of the method are not shown in the drawings, since they may be easily conceived and constructed by those skilled in the art only on the basis of said disclosure of the modification. Figure 11 shows a modification utilizing the combination of the solutions of Figures 6 and 10. The sheet N1 is provided with the above mentioned folds P2-P2', effected upstream of the transporting chains, while the edges of the sheet N2 are folded over P2-P2', as indicated at P3-P3', when the sheets have already been clamped by said transporting chains. In this instance, the superposed edges of the two sheets may be submitted to pairs of opposite welds by corresponding pairs of welding bars 15-15' and 115-115'. The double welding should ensure a junction even more resistant of the sheets N1 and N2.

Claims: An apparatus for manufacturing sacks by effecting longitudinal welds on two superposed sheets (N1-N2) of heat-weldable material, to form a tube from which sacks are obtained by successive transverse welding and cutting operations, comprising two pairs of sheet-transporting chains (C1-C2, C1'-C2'), means (12-13-14, 112-113-114) for folding the longitudinal edges (P2-P2') of at least one (N1) of the two opposing sheets constituting the sack-forming tube, so that the outer faces of said edges are directed towards the inner faces of the edges of the opposing sheet (N2) when said sheets are superposed-on each other between said two pairs of chains which clamp said sheets and advance said sheets longitudinally, welding bars (15-15') acting on non-folded edges (P2-P2') of one of the two sheets, while contrasting members (16-16') act in opposition to said bars, characterized by the fact that insert strips (20-20') are provided supported at the upper ends thereof by suitable stationary supporting means (19-19') arranged upstream of the zone where the two sheets are superposed on each other to form said tube, said insert strips being inserted in the folded edges (P2-P2') so as to prevent said folded edges from being welded to the sheet from which they are originated, said insert strips (20-20') are flexible to permit a uniformly-distributed compression of the sheet portions interposed between the sealing bars (15-15') and the respective contrasting members (16-16'). An apparatus according to claim 1, which said insert strips (20-20') are made of a material which does not adhere to the welded edges of said two sheets (N1-N2).

Inventors: VETTORATO NATALE, VETTORATO, NATALE
Applicants: BL MACCHINE AUTOMATICHE, B.L. MACCHINE AUTOMATICHE S.P.A.
Application year: 1990
Main classification: B31B23
IPC codes: B31B23, B31B19