GB2242857A - Paper sacks - Google Patents

Abstract

A paper sack having at least three piles is manufactured such that the outermost ply 40 and innermost ply 44 have stepped ends, the or each ply 42 intermediate the said outermost and innermost piles having substantially straight ends. Thus the conventional capping strip may be dispensed with. <IMAGE>

Description

PAPER SACKS Field of the Invention The present invention relates to multi-ply paper sacks and to a method for their manufacture.

Background to the Invention Sacks suitable for the packaging of such pulverulent materials as cement are commonly made from two or more plies of paper. In a typical process for the manufacture of such sacks, a first continuous web of paper, corresponding to the outermost ply of the sack, is fed through printing apparatus which applies to the intended outer surface such printed matter as the relevant product details, trade marks and the like, and is then fed into a perforating station which applies perforations at regular intervals across the width of the web. The perforations are produced by means of an arrangement of serrated knife blades mounted lengthwise on a rotating shaft. One or more further webs of paper are fed to the perforating station and are also perforated in a similar manner by means of respective blade-and-shaft assemblies.

The plies are then transported to a gluing station, where dabs of glue (known as cross paste) are applied across the webs in the region of the perforations (for tacking the plies together) and then to a second gluing station where glue (known as seam paste) is applied to an edge of each ply. The plies are then superimposed, with the perforated regions in register, and are passed to a tube-forming station wherein one edge of the superimposed plies is bought into contact with the opposing edge and sealed thereto by means of the previously applied seam paste. The tube is flattened and is then passed to a separating station which applies sufficient impact at the perforated regions to effect separation of the continuous plies into individual "blanks" for the sacks.

The blanks are then passed to an end-opening station wherein suction cups are applied to either side of each blank in order to open the ends thereof (the previously applied transverse dabs of cross paste ensuring that each end is opened with equal numbers of plies to either side).

The separated ends are flattened in readiness for presentation to the so-called bottomer end pasters which apply adhesive so that the ends of the sack may be sealed.

At one corner a "valve", in the form of an insert-of stiff paper is positioned, this providing an aperture for the reception of a tube through which the appropriate product may be fed into the sack. Once the sack has been filled, it is detached from the feeder tube and the valve selfseals.

There are four corners to the sack at which the valve could be situated and, accordingly, it is conventional practice to include, immediately after the separating station, a station in which the sack blanks may be turned over and/or turned round if desired.

At one time, it was conventional to produce "flush cut" sacks, wherein all the plies in each sack are cut with straight ends (i.e. the perforating station is arrange to import a straight line of perforations across the width of each ply). However, such sacks have been superseded, to a certain extent, by "stepped end" sacks, wherein the plies within a sack have been cut to varying lengths, the cut edges being stepped, rather than rectilinear. Compared with flush cut sacks, stepped end sacks are advantageous in that they provide a more economical and efficient use of paper for sacks of any given size. Machinery for cutting paper webs to form stepped end sacks is commercially available, for example from Messrs. Windmöller & Hölscher, West Germany.

It will be appreciated, of course, that the precise configuration of the stepped arrangement of blades for the production of stepped end sacks must be selected carefully, since the perforations imparted by a blade-andshaft assembly will determine not only the rear end of each blank but also the leading edge of the corresponding ply in the next blank.

In the production of both flush cut sacks and stepped end sacks, the blade-and-shaft cutting assembly for each ply is provided not only with the requisite blades along its length but also with one or more so-called quadrant blades at right-angles thereto, which quadrant blades impart slits in the paper web that are aligned with the machine direction. The purpose of such slits is to ensure the correct presentation of the various parts of the sack ends to the subsequent folding, pasting and sealing stations; in the stepped end bags the slits also serve to connect the stepped lines of perforations so that successive sacks may be parted at the separating station.

However, the slits are a possible source of weakness in the resultant sacks and, for this reason, it is conventional practice to adhere a reinforcing strip of paper (usually referred to as a "capping strip") to the sealed ends of the sacks.

Thus, there is a need in the art for a method of producing a paper sack which retains the advantages of the conventional stepped end constructions yet which has sufficient strength to obviate the need for a capping strip, with the consequential savings in material costs and simplification of the manufacturing process.

Summarv of the Invention The present invention now provides a process for the manufacture of a paper sack having at least 3 plies, wherein the outermost ply and innermost ply are so cut as to have stepped ends, characterised in that the or each ply intermediate the said outermost and innermost plies is so cut as to have substantially straight ends. (The term "cut" is to be construed broadly, to include for example the production of the plies by the subdivision of a paper web along lines of perforations.Furthermore, the expression "straight ends" will include ends having minor roughnesses due for example to detachment along a perforated line.) The present invention also provides a paper sack having at least 3 plies, wherein the outermost ply and innermost ply have stepped ends, characterised in that the or each ply intermediate the said outermost and innermost plies has substantially straight ends.

Brief Description of the Drawinas Figure 1 is a schematic plan view of part of a platen which serves as a support for the blades used to perforate a paper web in the perforating station.

Figure 2 is a schematic section along the line Z-Z of the part shown in Figure 1.

Figure 3 is a pattern for the knife arrangement for the production of conventional 3-ply sacks.

Figure 4 is a pattern for the knife arrangement for the production of a 3-ply sack according to the present invention.

Figure 5 is a another pattern for the knife arrangement for the production of a 3-ply sack according to the present invention.

Figures 3A, 4A and 5A illustrate the possible positions of the valve in sacks produced by the use of knife arrangements as shown, respectively, in Figures 3, 4 and 5.

Figure 6 illustrates the possible positions of the valve in sealed sacks produced according to this invention (this Figure giving more detail than Figures 4A and 5A).

Figure 7 is a schematic illustration of sacks, corresponding to those shown in Figure 6 at a stage in their manufacture after the ends have been opened and flattened but prior to pasting.

Figure 8 is a diagrammatic illustration of the bottomer paste parts suitable for use with sacks of the type shown in Figure 7.

Figure 9 illustrates a bottomer paste part of special configuration required for the pasting of certain priorart sacks.

In the drawings, like parts are designated by like numerals.

Description of Preferred Embodiments The platen 2 illustrated in Figures 1 and 2 is an elongate device having a cross-section generally in the form of a segment of a circle. The base 4 of the platen is adapted for mounting on a rotatable shaft in the perforating station of a sack-making apparatus whereas the curved surface 6 of the platen 2 is provided with three parallel, longitudinal grooves 8 having a dovetail crosssection. (The disposition of the grooves 8 is indicated, for convenience, by dotted lines in Figure 1.) The function of the platen 2 is to carry knife blades for the perforation of a paper web. The blades are of two types. The first type comprises longitudinal blades 10 which are mounted on blade-carriers 12 by means, for example, of screws 14. The blade-carriers 12 are provided with locating flanges 16 which are slidably received within a respective groove 8.The blade-carriers may be releasably fixed in position by means of set screws 18 or the like. It will be seen that the longitudinal blades 10, when the platen is mounted for operation in the perforating station, run in a direction transverse to the machine direction (the latter being the direction in which the paper web passes through the said station). Thus, these blades 10, which have serrated cutting edges 11, each provide a line of perforations extending across at least part of the width of the paper web. Each longitudinal blade 10, which will be of preselected length, may be stepped from its neighbouring blade or blades 10 by one or two increments, each increment being determined by the distance between adjacent grooves 8.

Typically, the increment is about 20 mm.

Between each pair of longitudinal blades 10 there is provided a blade 20, termed a "quadrant blade", arranged at right-angles to the longitudinal blades 10, which quadrant blades 20 may be supported by means (not shown) similar to the blade carriers 12. It will be seen that the quadrant blades 20 will provide slits and/or a line of perforations (depending upon the configuration of the edge 21 of the quadrant blade) in the machine direction, such slits and/or lines of perforations serving, as mentioned above, at least in part to connect the lines of perforations produced by the stepped longitudinal blades 10.

A shaft, with its respective platen and knife arrangement, will be provided in the perforating station for each ply used in the manufacture of the sacks. The arrangement of the longitudinal and quadrant blades as seen in plan view may be represented by a pattern as shown in Figures 3, 4 and 5. It will be seen that the arrangement of blades shown in Figure 1 corresponds to the left-hand end of the uppermost arrangement depicted in Figure 3, this Figure showing a known arrangement for producing 3-ply sacks (although the knife arrangements used in processes of the present invention may be constructed in an analogous manner).

The chain-dotted line in Figures 3, 4 and 5 represents the running centre line of the machine. The knives 30, 32 and 34 are displaced progressively to the left in order to match the paper plies which are themselves offset with respect to one another so as to allow seam paste to be applied to the edge of each individual paper ply by respective paste wheels.

Comparison of Figure 4 with Figure 3, will show that the arrangement according to the present invention is distinguished from the known arrangement by the use of a blade configuration 32 for the intermediate or sandwich ply - marked 42 in Figure 7B - giving a rectilinear (i.e.

unstepped) line of perforations across the ply width.

This can be achieved by a single long blade 10 or by a series of aligned, contiguous shorter blades 10.

Preferably the or each, intermediate, rectilinear knife blade 32 should have no quadrant blades associated with it.

The blade configuration 30 for perforating the outermost ply (the one - marked 40 in Figures 6 and 7B which will usually carry printed matter), as shown in Figure 4, differs from the corresponding configuration shown in Figure 3 by certain preferred features. First, the said configuration 30 in Figure 4 has longitudinal blades that occupy only two stepped positions (albeit 2 increments apart), whereas that in Figure 3 has at least one longitudinal blade in each of three stepped positions.

As a result, the edges formed by separation along the perforations have fewer angles in the embodiment of Figure 4 than in the known arrangement according to Figure 3 and may be expected, therefore, to be less susceptible to inadvertent tearing. Also, with fewer steps vulnerable to air intake the tube-forming and bottomer stations can be run at higher-than-usual speeds in the process of the present invention. Second, one of the edges formed by separation along the perforations according to Figure 3 will have three slits, whereas the corresponding edge according to Figure 4 will have only two and will therefore again have fewer areas of weakness. The embodiment of Figure 5 has similar preferred features to those discussed above for the arrangement shown in Figure 4.

The blade configuration 34 for the innermost ply which ply is marked 44 in Figure 7B - is substantially the same in Figures 4 and 5 as in Figure 3.

Overall, the improved blade arrangements of the type shown in Figures 4 and 5 can result in a saving of up to about 5 in paper requirement, compared with the prior-art shown in Figure 3, for a given size of sack.

Apart from the knife arrangements in the perforating station, the manufacturing process according to the present invention is generally similar to the conventional prior-art process discussed above. When a reverse feed back-end valve is required in a process using the arrangement of Figure 4, or when a reverse feed front-end valve is required in a process using an arrangement according to Figure 5, the turn-over and turn-round stations will be put into operation. The possible types of bag, according to the position of the filling valve, are illustrated in Figures 6 and 7 showing, respectively, sacks in a sealed condition ready for filling and sacks at an earlier stage of manufacture at which the ends have been opened and flattened but not yet pasted and sealed.

The designations A, B, C and D correspond to the designations used in Figures 4A and 5A. The vertical line 46 running along the back of types A and C as shown in Figures 6 and 7 represents the seam formed when the original plies are folded and joined to form a tube. The valve will generally be constituted by a folded strip of stiff paper, as is known in the art; valved bags are disclosed, for example, in US-A-2,275,505.

The bottomer paste parts that may be used to apply paste or other adhesive to the sacks (at the stage of manufacture represented in Figure 7) are shown diagrammatically in Figure 8. It is an advantage of the present invention that, for any given width of sack, the same bottomer paste parts may be used irrespective of whether the sack is of type A, B, C or D. To adjust the bottomer paste parts for a different width of sack, all that is needed is to move the triangular pieces 80 to the left or right (as shown in Figure 8) as appropriate, and to replace the part 82 so as to shorten or lengthen the dimension marked "X". In contrast, sacks manufactured by conventional means commonly require a special shaped part replacing, and of comparable size to, one of the triangular pieces shown in Figure 8 but having the configuration shown in Figure 9. The position of the shaped part according to Figure 9 will vary according to the position of the valve in the sack.

4-ply sacks according to the present invention may be manufactured by means of a modification of the knife arrangements shown in Figure 4 or 5 wherein an additional rectilinear knife is introduced between knives 30 and 34 in order to perforate the additional intermediate ply.

The term "paper" herein is to be construed broadly to include not only wet-laid materials consisting essentially of cellulose fibres but also other nonwovens and like materials having properties comparable to those of conventional papers. One or more of the plies may be coated with a plastics film to improve such properties as, for example, resistance to moisture penetration.

Typically the sacks disclosed herein are manufactured of Kraft paper of appropriate basis weight, e.g. 90 g/m2.

It will of course be understood that the present invention has been described above purely by way of example and that modifications of detail can be made within the scope of the invention.

Claims (4)

1. A process for the manufacture of a paper sack having at least 3 plies, wherein the outermost ply and innermost ply are so cut as to have stepped ends, characterised in that the or each ply intermediate the said outermost and innermost plies is so cut as to have substantially straight ends.

2. A paper sack having at least 3 plies, wherein the outermost ply and innermost ply have stepped ends, characterised in that the or each ply intermediate the said outermost and innermost plies has substantially straight ends.

3. A process for the manufacture of paper sacks, being a process substantially as hereinbefore described with reference to the accompanying drawings.

4. A paper sack substantially as hereinbefore described and as illustrated in the accompanying drawings.