Background to the technology When manufacturing valve sacks of paper, a tube is formed of the paper webs and cut into pieces of desired length, the ends are closed to form the bottom by first folding inwards the thinner end flaps and, on top of these, the wider side flaps. In all ends except the valve end, the side flaps are attached to the end flaps. The valve end can be supplemented with a separate paper-formed valve piece. In addition, the valve can comprise a closing flap which after the filling of the sack is folded and attached over the valve mouth to the bottom of the sack.

Moisture-proof valve sacks have been manufactured from paper coated with plastic film or by placing a loose plastic film between paper layers. A preferred plastic has been polyethylene.

A problem with valve sacks is that when the sack is being filled air must be allowed to escape from the sack. This is a problem particularly in the case of sacks equipped with barrier films. Therefore the barrier film of these sacks has usually been perforated, which substantially impairs the barrier properties of the film.

A multilayer sack can be made to be air permeable during the filling also as presented in the application publication FI-A-945831. Then the inner layer is of paper with good air permeability and has channels on its outer surface. During the filling of the sack the

air that gets entrapped between the layers moves through the channels to the ends of the sack where it is allowed to escape. The ends of the sack blank mouth are end to end in this sack. This structure is nevertheless not adequately tight for all applications.

Furthermore, in practise this sack always requires a separate bottom cover sheet.

General description of the invention A valve sack according to claim 1 has now been invented. Preferred embodiments of the invention are presented in the other claims.

A sack according to the invention can utilize a barrier film completely impermeable to air and moisture, yet during the filling of the sack air exhaustion from the sack is efficient. Also the sack bottoms are very tight due to the overlap structure.

Drawings The attached drawings are a part of the detailed description of the invention. In the drawings: Fig. 1 shows a sack blank before the forming of the bottoms, Fig. 2 shows a blank into which the valve piece has been added, Fig. 3 shows a blank with the end flaps folded, Fig. 4 shows a blank onto which the bottom glueings have been formed, Fig. 5 shows a blank where the side flaps to be underneath are folded, and Fig. 6 shows a finished sack.

Detailed description A sack according to the figures is made of a three-layer material. Innermost is an inner layer (1) of paper, outside of which is an air-impermeable barrier layer (2) and outermost an outer layer (3) of paper. On both ends of the flattened tubular blank (Fig.

1), cuts (4) are formed from the ends thus forming the side flaps (5) and the end flaps (6). In the region of the side flaps (5) the blank is slightly longer than in the region of the end flaps (6). The figures also show the transverse side creases (7) and the diagonal end creases (8) of the bottom along which the blank is folded to form a plane bottom, as well as the fold creases (9) along which the bottom is folded against the mantle.

The inner layer (1) and the outer layer (3) can be of sack paper suitable for the purpose (strength e. g. 95 g/m2). The barrier layer (2) can be of suitable plastic film (e. g.

HDPE).

The barrier layer (2) is attached at both ends to the inner layer (1) with a transverse spot-glueing (10) and to the outer layer (3) with a transverse glueing (11) which in this case is also a spot-glueing. The glueings help in the alignment of the layers and they also strengthen the sack since stress is directed more evenly to all layers.

In the region of the side flaps (5) the ends of the layers are graded to be of different length so that at the upper end of the blank on the front side of the mantle the inner layer (1) is the longest and the outer layer (3) the shortest, and on the back side the inner layer is the shortest and the outer layer the longest. At the lower end of the blank on the front side of the mantle the inner layer (1) is the shortest and the outer layer (3) the longest, and on the back side the inner layer is the longest and the outer layer the shortest. The grading of the material enables each material to be attached to each other in the bottom structure.

In the region of the end flaps (6) at the upper end, the inner layer (1) is slightly longer than the barrier film (2) and the outer layer (3). At the lower end, on the other hand, the inner layer (1) is slightly shorter than the barrier layer (2) and the outer layer (3).

In the next stage (Fig. 2) a valve piece (12) of paper is attached to one of the ends of the upper end of the blank. Its lower edge is placed in the region of the side crease (7) and its upper edge reaches slightly above the side flaps (5). The valve piece (12) is glued to the end flap (6) from both sides slightly outside the cut (4) with a longitudinal glue line (13). On the inside of the lower end of the valve piece (12) there is a fold. Its upper edge is at the level of the upper edge of outer layer (3) of the end flap (6). On the valve piece (12), a line (13) is further marked along which the valve piece is folded together with the side flap (5) along the side crease (7).

In the next stage (Fig. 3) the side flaps (5) of the front side are folded open along the fold crease (9) at the same time folding the end flaps (6) inwards along the end creases (8). In addition, the sides of the valve piece (12) are folded inwards in the region of the side creases (7) and glue lines (15) are formed on top of the folded sides from the edge of the fold inwards. In the region of the double end edge of the valve piece (12) the glue line (15) is slightly wider. After this, the side edges of the valve piece (12) are folded open again, and they adhere tightly to the end flap (6) to the inner layer of the side flaps (5).

In the next stage (Fig. 4) glue lines are formed with which the bottoms are closed. On the side flap (5.1) to be underneath, where the inner layer (1) is the longest, end glue lines (16.1) are formed from the outer surface of the end flap (6) from the side crease (7) to the inner surface of the side flap reaching to its end. On the side flap (5.2) to be on top, where the inner layer (1) is the shortest, similar but shorter end glue lines (16.2) are formed. In addition, the end glue lines (16.2) are united along the edge of the inner layer (1) with a thinner inner side glue line (17). On the end of the outer layer (3), a thin outer side glue line (18) reaching from one end to the other is formed on the inner side. In the region of the valve piece (12) the end glue lines (16.1) and (16.2) and the inner side glue line (17) are naturally formed on top of the valve piece.

120 In the next stage (Fig. 5) the side flaps (5.1) to be underneath are folded against the bottom as a result of which the inner surface of their inner layer (1), with the help of the end glue lines (16.1), adheres tightly to the end flaps (6), and on the valve end correspondingly to the valve piece (12). When finally the side flaps (5.2) to be above 125 are folded against the bottom, its outer layer (3) adheres to the outer layer of the lower side flap (5.1) with the help of the outer side glue line (18) and the inner layer (1) adheres to the end flap (6), in the valve corner to the valve piece (12), and to the inner layer of the lower side flap with the help of the end glue lines (16.2) and the inner side glue line (17).

130 Thus in the bottom structure each paper layer (1) and (3) is glued longitudinally to the sack against themselves (grading) with glue seams (17) and (18). The graded barrier layer (2) is not longitudinally glued to itself but it is cross glued with spot-glueing to the paper layers. Thus the loose unglued film structure enables air exhaustion from the ends of the bottom, but since overlapped forms a moisture barrier to the bottom 135 structure. The barrier layer can be glued by a cheap process glueing; no expensive and staining special glues are required.

The glueing of the bottom corners with seams (16.1) and (16.2) closes and tightens the structure but allows the escape of dust free air that comes between the layer (1) and (2), 140 from the ends of the bottom.

The bottom structure is completely tight towards the inside of the sack except for the valve which has a fill opening between the outer layer of the end flap (6) and the valve piece. When the sack has been filled, the material inside presses the end flap (6) firmly 145 against the valve piece (12). If desired, the valve mouth can also be further closed by a

separate closing strip attached over the mouth against the bottom. Due to the overlap structure of the materials, the bottom is completely covered with an intact barrier film (2).

During the filling of the sack, air moves from the sack to the area between the inner layer (1) and the barrier layer (2) from where it flows out through their ends in the bottoms of the sack. In the bottoms the air moves between the side flaps (5.1) and (5.2) to the ends where it is allowed to escape (Fig. 5). To facilitate the travel of the air, the outer surface of the inner layer (1) can be fluted, but a smooth-surface paper will also work.

Air exhaustion is very efficient when the air flows on both sides of the sack to both ends of the sack.

No bottom cover sheet is required to form the exhaust air channel or to increase sack strength.

The sacks can be manufactured with tube and bottom machines of conventional sack production lines, with additions to enable the necessary cuts, overlappings and glueings. No extra material is needed.

The sack is particularly suitable for use with materials that need to be protected from the moisture of outside air as well as possible (cocoa, for example) or with materials the leaking of which from the sack must be prevented as well as possible (chemicals harmful to the environment, for example).