The invention concerns a sealed package of a punched corrugated cardboard blank for use as a transport or storage package for fresh food as well as a method of making it. According to the invention, a waterproof plas¬ tics sheet is adhered to both sides of the corrugated cardboard blank, and the sheet is welded together outside the periphery of the blank so that the corrugated card¬ board blank and the two layers of sheet form a unit which is waterproof and which can form the desired package when folded.

Several methods are known today which aim at improving the good properties of the corrugated cardboard, that is in particular the low weight in relation to the strength, by moreover making the corrugated cardboard resistant to water and moisture.

The known methods of protecting corrugated cardboard for use as a package comprise sealing the liner (i.e. its sur¬ face layer) of the corrugated cardboard by applying to the liner a material (e.g. plastics sheet, coating with plas¬ tics, rubber or wax) which makes the surface of the card¬ board water-repellent, or adding to the paper pulp impreg¬ nating agents to reduce the ability of the paper fibres to absorb moisture. The known sealing or impregnation methods are performed prior to punching or cutting of the corru¬ gated cardboard blank. Punching or cutting of the corru¬ gated cardboard blank results in a punching or cutting edge which exposes the paper material and thereby makes the package vulnerable to entering moisture.

A method of sealing paper materials, such as e.g. card¬ board, by coating with a plastics sheet on both sides of a punched blank with subsequent adhesion or welding along the rim of the blank is already known, e.g. from ring- files, covers for books, magazine holders, etc. However, this seal rarely serves to make the sealed material water¬ proof, but rather to improve the wear strength, or the purpose is of a purely cosmetic nature. The known ring- files, etc. do not use corrugated cardboard, but cardboard (compressed paper) as a core, just as this product is not used for transport or storage packages for fresh food.

The object of the invention is to provide a method of mak¬ ing a waterproof corrugated cardboard package from rela- tively inexpensive corrugated cardboard.

This object is achieved, as stated in claim 1, by applying a waterproof sheet on both sides of the punched corrugated cardboard blank and then welding the two layers of sheet outside the periphery of the blank.

As starting material may be used a non-waterproof corru¬ gated cardboard which just has the necessary mechanical properties, among which in particular the stack strength of finished packages should be mentioned.

When, as stated in claim 2, punched holes are covered with a sheet which may be transparent, these holes serve as windows through which the contents of the package are vi- sible, without direct access thereto, e.g. for touching or contamination.

When completing a package by folding a flat corrugated cardboard blank covered with sheet, the sheet over the punched holes may expediently be used for welding, as stated in claim 3.

Claims 4-6 concern a package obtained by the method ac¬ cording to the invention.

The waterproof package thus provided is useful for trans- port and storage of fresh food such as fruit and vege¬ tables, fresh meat and fish as well as dairy products.

The package can replace a large number of known plastics trays and can be produced in cheaper paper pulps, since no great demands are made on the liner of the corrugated cardboard either because the adhered plastics sheet can be coloured in colours which are normally not used on corru¬ gated cardboard packages.

The invention will be explained more fully below with re¬ ference to the drawing, in which

fig. 1 shows a cross-section through a rim area of a corrugated cardboard package according to the invention, and

fig. 2 shows a portion of a corrugated cardboard package prior to completion.

Fig. 1 shows in cross-section a portion of a punched corrugated cardboard blank with its liner 1 and fluting 2 as well as the cut edge 3 of the blank. Both sides of the liner have applied thereto a plastics sheet 4, which is welded together outside the edge of the blank with a weld 5 which thus seals the blank completely along its entire edge.

Fig. 2 shows a corner portion of a corrugated cardboard blank before it is folded to a finished package. A slit 6 is punched in the blank, said slit having a width permit¬ ting formation of a weld 5 of plastics sheet 4 which is

applied on both sides of the blank. A punched hole 7 is covered with the plastics sheet 4. When the shown blank portion is assembled to a right angled corner of a package box, it is folded along folding lines 8, so that the hole 7 and the superposed sheets 4 are placed over the section 7' indicated in dotted line, where the sheets 4 thus over¬ lapping are welded together and provide a stable corner.