The present invention relates to a liquid-tight cardboard barrel having a plastic film on the inner sur¬ face of the wall portion and bottom plate, whereby the plastic film on the wall portion forms a coating faste¬ ned substantially over the entire area of said wall portion. The invention also relates to a method and a device for manufacturing such a cardboard barrel.

A plurality of methods are previously known for making a cardboard barrel liquid-tight. One method is to provide the inner surface of the Wall and bottom plate of the barrel with a plastic coating before joining the bottom plate to the wall portion. The bottom plate is hereafter mechanically joined to the cylindrical wall portion. This solution suffers from the disadvantage of an unreliable liquid-tightness at the joint between the bottom plate and the wall because if the barrel during transport is subjected to bumps, a slit may be formed in the joint through which liquid is able to flow out of the barrel. The strength of the joint can be improved by welding the coatings the each other at the joint but because the thin plastic film is easily torn in connec- tion with the deformation of the joint, is this sealing method neither very reliable. In this connection it is also to be noted that the upper and lower edge of the barrel are in general mostly subjected to bumps.

In addition to the above described barrel, barrels are known inside which a plastic bag is arranged which is secured to the barrel at its mouth only. The liquid- tightness of such a barrel in general remains satisfac¬ tory inspite of bumps because the deformations of the barrel do not directly affect the bag. However, barrels provided with a loose inner bag suffer from a plurality of disadvantages, wherefore they are not very commonly

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used. They are particularly difficult to fill and empty. The object of the present invention is to provide a liquid-tight cardboard barrel, which is not sensitive to bumps. The cardboard barrel according to the inven- tion is characterized in that the plastic film located at the bottom plate and abutting against it is loose with respect to the bottom plate and joins the plastic coating of the wall portion. Because the plastic film at the bottom plate is loose with respect to said plate, deformations at the lower edge of the barrel and rela¬ tive movements between the wall and the bottom do not directly affect the sealing film, which thus in all Tikejyood will remain intact when the barrel is subjected to bumps. However, because the sealing plastic film is fastened to the wall of the barrel, the same problems do not arise in use of the barrel as in connection with barrels comprising a loose inner bag. In addition, it is to be noted that the connection point between the loose plastic film and the wall coating can be located inside the junction between the wall portion and the bottom plate at a short distance from said junction, due to which the tendency of the film to break is further re¬ duced.

The invention also relates to a method for manu- facturing a liquid-tight cardboard barrel, according to which method the inner surface of the wall portion and the bottom plate of the barrel is provided with a con¬ tinuous plastic film. The method according to the inven¬ tion is characterized in that, in order to provide the inner surface of the bottom plate with the plastic film, a plastic film disc having a diameter somewhat larger than the inner diameter of the barrel is pressed against the inner surface of the bottom plate, whereafter the edge of the disc is secured .to the plastic coating of the wall portion of the barrel by welding. In a card¬ board barrel so manufactured the film does not extend

to the joint between the wall and the bottom plate, which is easily subjected to bumps, so that deforma¬ tions of the lower edge of the barrel are not able to break it. The strength ofthe liquid-tight film is im- proved by the fact that the plastic film disc is not secured to the inner surface of the bottom plate so that the liquid-tight film corresponds, as far as the barrel bottom is concerned, to a loose inner bag the tightness of which does not suffer from deformations of the bottom. According to the invention, a liquid- tight joint is obtained between the inner coating of the barrel wall and the creased edge of the plastic film disc by welding the plastic parts to each other, for example, by means of high-frequency radio wave radiation.

In order to position the plastic film disc even¬ ly against the bottom plate, the plastic film disc is preferably pressed against the bottom plate with a planar plunger which is somewhat smaller in diameter than the bottom plate.

The invention also relates to a device for manufacturing a liquid-tight cardboard barrel in which a continuous plastic film is provided on the surface of the wall and the bottom plate. The device according to the invention is characterized in that the device com¬ prises a plunger for pressing a plastic film disc of somewhat larger diameter than the inner diameter of the barrel against the bottom plate, and welding means for securing the edge of the disc disposed against the wall of the barrel to the plastic coating of the wall.

One preferred embodiment of the invention will be described in more detail in the following with reference to the accompanying drawing, in which

Figure 1 is a schematical longitudinal section of a cardboard barrel,

Figure 2 illustrates the situation before forcing

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the plastic film disc to the bottom of the barrel. Figure 3 illustrates the welding step, and Figure 4 shows on an enlarged scale one part of a barrel manufactured by means of the method according to the invention.

Figure 1 shows a barrel made of cardboard, a so- called fibre barrel, in which to one end of a cylind¬ rical wall portion 1 is secured a circular bottom plate 2. Instead of cardboard, the bottom plate can be made of, for example, plastic or metal. In the embodiment shown in the Figure, the wall portion and the bottom plate are joined to each other so that the edge of the bottom plate is bent downwards and the edge of the wall portion is bent around the edge of the bottom plate. Also other conventional joining methods are conceivable. To the inner surface of the wall portion 1 of the barrel is applied a coating 3 (Figure 4) of plastic, for examp¬ le, polyvinyl chloride or polyethylene, which is secured to the wall portion by means of any conventional techni- que. Thus, the barrel is made liquid-tight as far as the wall is concerned before starting the method accor¬ ding to the invention.

Figure 2 illustrates schematically how a circular plastic disc 4 having a diameter somewhat larger than the inner diameter of the barrel is positioned at the open end of the barrel and pressed into the barrel down to the bottom by means of a plunger 5. The plunger has a planar underside, due to which the plastic film disc will be evenly disposed against the inner surface of the bottom plate. Because the disc 4 which is preferably of thin, flexible PVC is larger than the cross-sectional area of the barrel, the edge of the disc will be posi¬ tioned in the vertical position shown by Figures 3 and 4 against the inner surface of the barrel wall. The plunger 5 is so thick in the vertical direction that its edge keeps the edge 6 of the plastic film disc in this

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position. As the edge 6 of the disc is bent into the vertical position, it is curtailed so that it is partly pressed against the wall of the barrel and partly against the edge of the plunger. When the plastic film disc 4 is positioned in place according to Figure 3, its edge 6 is welded to the coating 3 of the wall portion of the barrel. For this purpose, the lower end of the barrel is surrounded by an outer welding electrode 7, while an inner elec- trode is arranged in the plunger 5. In order that the edge 6 of the disc, inspite of its creases, should be tightly secured to the coating 3, the electrode 7 is made in the shape of a hoop the diameter of which is adjustable so that the wall of the barrel, by means of it, can be somewhat compressed in order to clamp the edge 6 of the disc between the coating 3 and the edge of the plunger 5. The welding is carried out by means of radio waves having a frequency of about 20 to 30 MHz. This welding method causes no changes in the cardboard of the wall portion 1.

It appears from the foregoing that the plastic film disc does not adhere to the bottom plate 2. As a result, no stresses are applied on the plastic film disc even though the wall portion and the bottom plate would be displaced with respect to each other, for example, due to a blow applied on the barrel during transport. Thus, the barrel remains liquid-tight also in the event that its lower edge would be considerably deformed. The liquid-tight barrel according to the inven¬ tion can be also produced by fastening a plastic bag of a suitable size to the wall portion of the barrel but by leaving it loose at the bottom plate. In addition, it is obvious that the barrel can have another cross- sectional shape than circular, for example, square. The tightening hoop can be dispensed with if an expanding

plunger is used.

Instead of the whole hoop 7, a part of the hoop or a wheel rotating around the outer periphery of the barrel can be used as outer electrode.