US3777446A

US3777446A - Method and apparatus for wrapping a load with heat-shrinkable film

Info

Publication number: US3777446A
Application number: US00223997A
Authority: US
Inventors: O Graver
Original assignee: FERNHOLT AND GIERSTEN AS
Current assignee: FERNHOLT AND GIERSTEN AS
Priority date: 1971-02-10
Filing date: 1972-02-07
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11
Also published as: NO127044B; NL7201708A; GB1311828A; SE389851B; DE2205975A1; NO127044C; IT948469B

Abstract

Method and apparatus for wrapping a load with heat-shrinkable film comprising the steps of initially providing the load with an open-bottomed envelope of heat-shrinkable film, positioning the load in a shrink tunnel, withdrawing air from the lower open end of the envelope in order to create a sub-pressure therein and simultaneously withdrawal of air from the tunnel space passing the lower side edges of the load, heat the air withdrawn from the tunnel space and the inside of the envelope, re-circulate the air into the tunnel space towards the top side of the load where the air currents will turn around and move downwards along the outside of the film envelope to the lower side edges of the load from where the air a-new is withdrawn from the tunnel space and fulfills a closed circulation loop.

Description

United States Patent [191 11 3,777,446 Graver 1 Dec. 11, 1973 [5 METHOD AND APPARATUS FOR 3,668,817 6/1972 Bell 53/184 x WRAPPING A LOAD WITH I-IEAT-SHRINKABLE FILM Inventor:

Assignee:

Filed:

Appl. N0.: 223,997

Odd Graver, Haslum, Norway A/S Ingeniorfirmaet Fernholt & Giersten, Sandvika, Sweden Feb. 7, 1972 Foreign Application Priority Data Feb. 10, 1971 Norway 493 References Cited UNITED STATES PATENTS 11/1971 Grocke 53/184 X 9/1968 Howe 53/30 X Primary Examiner-Robert L. Spruill Att0rneyS. Delvalle Goldsmith et al.

[57] ABSTRACT Method and apparatus for wrapping a load with heatshrinkable film comprising the steps of initially providing the load with an open-bottomed envelope of heatshrinkable film, positioning the load in a shrink tunnel, withdrawing air from the lower open end of the envelope in order to create a sub-pressure therein and simultaneously withdrawal of air from the tunnel space passing the lower side edges of the load, heat the air withdrawn from the tunnel space and the inside of the envelope, re-circulate the air into the tunnel space towards the top side of the load where the air currents will turn around and move downwards along the outside of the film envelope to the lower side edges of the load from where the air a-new is withdrawn from the tunnel space and fulfills a closed circulation loop.

4 Claims, 2 Drawing Figures METHOD AND APPARATUS FOR WRAPPING A LOAD WITH HEAT-SHRINKABLE FILM The present invention relates to a new method and apparatus for wrapping loads, especially pallet loads, with heat-shrinkable plastic film, in the technique today usually called shrink film packing.

. This technique is based on using thermoplastic film material which during the making has been incorporated'with tensions which are releasable upon heating of the film simultaneously as the film shrinks. When positioning such film material around a load which shall be wrapped thereby and thereafter subjects the film to heat, the film shrinks and may after cooling form a tight-fitting envelope around the load. The load may constitute a separate piece of good or a number of pieces of good which shall be wrapped as a unit load, for instance a number of boxes placed on a pallet. A pallet may constitute part of the load when wrapped with film or may be used only for positioning and stacking purposes.

The technique of wrapping with shrinkable film has developed rapidly during the past years. Such wrapping is today usually carried out by initially wrapping or enveloping the load in the plastic film, whereafter this unit is positioned in a so-called shrink tunnel wherein the film is heated to a suitable temperature either by means of currents of hot air, and/or radiation heat from infra-red rays or the like. Currents of hot air are mostly used as heat source, since this method is cheapest and most effective. The plastic film is supplied in the shape of web which is wrapped around the sides of the load, preferably such that the topand bottom edges of the film overlap the edges of the load at the top and at the bottom. In shrink film packing one does, as a rule, not obtain a completely closed wrapping enclosure.

Particularly for larger loads, such as pallet loads, it is customary to use film envelopes in the shape of a preformed bag which is inverted and threaded over the load. The present invention is primarily but not restrictively based upon the use of bags of heatshrinkable film.

Shrink tunnels are made both as stationary and as moveable units. In both cases the tunnel is made as a box-shaped heat-insulated housing provided with one or two doors closeable with a curtain or the like in order to trap the heat therein. If the tunnel comprises a stationary unit, the load is charged into the tunnel on a trolley, on a belt conveyor or the like, for instance such that the load is feeded in on one side and is discharged on the opposite side of the tunnel.

ln known shrink tunnels where the heating means constitutes currents of hot air, the hot air aggregate is usually mounted on the outside of the tunnel, and it comprises usually electrical heating elements and a fan mounted in an air duct system, and the tunnel is provided with a number of ducts having ports directed into the tunnel space such that hot air currents can be blown directly against all sides of the load with the cover of plastic film in order to heat and shrink same. In order to obtain the right shrinking procedure it is important to obtain even and simultaneous heating of the film. A well known special difficulty in connection with the heating of the film, particularly when currents of hot air are used as heating means, is that the film attains a propensity to flap and billow, particularly at the moment when the supply of hot air currents is initiated.

This phenomena is primarily due to the expansion of the air behind or inside the bag which is heated or is partly trapped between the film and the load. A such flapping or billowing of the film results easily in an irregular film envelope with areas Pat. thin films and with areas with thicker films and also in unwanted shrinks and folds. The billowing of the film may also cause the film to rupture, making the wrapping a complete failure. One has discovered that such ruptures in the film frequently occur due to the fact that the film by the billowing during the heating, sticks to the inside walls of the tunnel and/or to the outside of the load.

The difficulties in connection with the flapping and billowing of the film during the heating of same are partly solved by creating a partial vacuum in the film enclosure around the load, which vacuum is provided by withdrawing air from the enclosure from the lower open end of same underneath the load, such as for instance shown in (1.8. pat. No. 3,508,375. In accordance with the invention described in the before-mentioned patent, air is withdrawn from the inverted film bag simultaneously as the exterior of the bag is heated. However, the described method and the apparatus used therefore offer several shortcomings of which shall be mentioned that the simultaneous heating of the film and withdrawal of air from the inside of the film envelope give no assurance for that the billowing and flapping is avoided in a satisfactory manner. Furthermore, the method and the apparatus described in the patent necessitate separate air withdrawal means and the hot air current supply means.

SUMMARY OF THE INVENTION The principal object of the invention is to provide an improved method and apparatus for wrapping a load with a heat-shrinkable film.

A particular object of the invention is to provide a method which renders itself especially well for wrapping a load on a pallet with a heat-shrinkable film.

A further object of the invention is to provide a method which eliminates the problems in connection with the flapping and billowing of the film and thereby opens possibilities to utilize the available interior space of the tunnel in a more effective manner without causing a risk for that the film shall stick to the inside wall of the tunnel.

A still further object of the invention is to provide a method which makes it possible to design and construct shrink tunnels at a lower price and which still are more efficient than known apparatus of this kind.

The method in accordance with the invention comprises the steps of initially providing the load with an open-bottomed envelope of heat-shrinkable film, positioning the load in a shrink tunnel, withdrawing air from the lower open end of the envelope in order to create a sub-pressure therein and simultaneously withdrawing air from the tunnel space passing the lower side edges of the load, heating the air withdrawn from the tunnel space and the inside envelope, recirculating same into the tunnel space in the form of upwardly directed air currents along the inside walls of the tunnel towards the top side of the load where the air currents will turn around and move downwards along the outside of the film envelope to the lower side edges of the load from where the air a-new is withdrawn from the tunnel space and fulfills a closed circulation loop.

It will be realized that through this method the withdrawal of air from the film envelope and the air in the tunnel constitutes part of a closed air circulation system and the sub-pressure in the enclosure of film around the load will be maintained until the film is sufficiently heated and shrunk.

The method is in the preferred mode of practice carried out such that the withdrawal of air in the film envelope will take place somewhat prior to that the heating and shrinking of the film takes place thus insuring that flapping and billowing of the film is avoided.

An improved embodiment of an apparatus to practice the invention comprises:a shrink tunnel, a support for a load therein, said support leaving lateral air openings there-between, an air conduit system beneath said support, said air conduit system including an upwardly open central shaft, initially laterally directed air conveying means leading from said shaft and ending in spaced port openings in the shrink tunnel, a fan means and an air-heating means positioned in said air conduct system able to simultaneously withdraw the air from the film envelope and from the shrink tunnel space through said lateral openings between the support and the lower side edges of the load, operative to move the air as upward air currents out through said port openings into the shrink tunnel space and down again into the shafts etc., establishing a closed air circulation system.

BRIEF DESCRIPTION OF THE DRAWING The invention shall be described further with reference to the attached diagrammatic drawing, wherein:

FIG. 1 is a sectional view through a shrink wrapping apparatus in accordance with the invention, and

FIG. 2 is a plan view along the plane II-II shown in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT In the FIGURES the reference number 2 designates the shrink tunnel housing, the hot air and fan assembly are generally designated with the number 4, the load with the number 6 and the floor in the tunnel space is designated with the number 8.

The shrink tunnel housing 2 can be of in per se known type. In the shown embodiment for the invention the hot air current assembly 4 is designated as a stationary unit which is mounted in a recess in the floor 8 as shown in FIG. 1. The shrink tunnel housing 2 may be moveable and comprises walls and a closed ceiling 7. As shown in FIG. 2 two opposed walls in the tunnel are provided with doors or ports 9 which may be closed I by means of curtains 14 or the like.

The hot air current assembly comprises a separate unit with a supporting structure 10 and the air duct system comprises a centrally located duct or shaft 12 which as shown is open upwards but closed downwards. From the lowermost part of the shaft extends a number of laterally and upwardly directed air ducts 16, which end in slit-shaped discharge ports 18 which, as best shown in FIG. 2, are positioned such that they are located just inside the wall 20 of the shrink tunnel housing 2.

In the central shaft 12 is mounted a downwardly sucking fan 22 having a drive spindle 24 which is rotated by means of an electrical motor 26 which is mounted in the supporting structure 10 of the assembly. In the shaft 12 is furthermore mounted electrical heating elements 28, preferably in connection with a heat exchange unit in the shape of plate ribs 29 or the like.

The load 6 which shall be wrapped is positioned in the shrink tunnel housing on top of the duct system etc. as above described, preferably positioned upon a support pallet 34 or the like, such that there exists laterally directed air passages 36 between the underside of the load and the support or top surface of the hot air assembly.

The shrink wrapping method in accordance with the invention will, in the shown apparatus, take place as follows:

One assumes that the load 6 is positioned in the shrink tunnel 2 on top of the hot air assembly 4. The load is provided or covered with a bag-like enclosure 50 of heat-shrinkable film such that the film covers the load but for the bottom side. The electrical heating means is preferably switched on one hour or two before the apparatus shall be utilized such that the hot air system is heated up and the fan 22 may during the preheating time period be put into operation. When the fan now is switched on, air is sucked down through the shaft 12 from the upper opening with the result that a sub-pressure is created in the inverted bag enveloping the load. Furthermore, air is simultaneously sucked in through the spacing or openings 36 between the bottom edge 21 of the load and the film and the top side of the support, i.e., the top side of the hot air assembly. The air currents flow through the heating means and therefrom down the shaft and thereafter laterally out through the ducts 16 and therefrom upwards through the ports 18 and up into the shrink tunnel along the inside walls of same. In the top space of the tunnel the hot air currents turn around and flow downwards generally along the outside of the bag such that the film is heated from the top and downwards. As previously described the air is thereafter sucked in below the bottom edges of the film and a-new arrives into the shaft 12 and thereby closes the air current circuit. The air current flow pattern is illustrated with arrows in FIG. 1. In a shrink packing apparatus in accordance with the invention the film is thus generally subjected to a suction force simultaneously as it is heated.

It will be understood that the method in accordance with the invention can be accomplished in several fashions. In some cases it may thus be desirable to carry out the withdrawal of air from the inverted bag prior to the time that the hot air currents are supplied such that the certain sub-pressure is established in the bag prior to the shrinking process. In other cases the opposite may be preferred, and can easily be arranged with implements such as baffles or the like in the air duct system.

A substantial further advantage of the method in accordance with the invention is that the hot air fan as sembly and the shrink tunnel can be constructed as independent units. This fact simplifies the fabrication as well as the operation of a shrink packing plant in accordance with the invention. This implies further that the hot air assembly, since it is made independent of the shrink tunnel, can be given the particular design which is found most purposely in order to be adapted to various needs.

With respect of the realization of the bag-like enclosure of plastic film enveloping the load it will be understood that the bag can be realized in different fasions. The shape and form of the bag will among others depend upon what kind of goods or loads which shall be wrapped, and the dimensions of the loads. In most cases it will be most suitable to utilize plastic bags which are preformed and threaded over each separate load successively during the packing, which bag may be available readily made or one can utilize film in the shape of hoselike, web-shaped blanks. It shall, however, be understood that in order to practice the invention it is not necessary to utilize bags as such since one may with about the same result utilize web-shaped plas tic film which is wrapped around the load, preferably from above such that the film is folded downwards the side faces of the load. When utilizing an initially flat film problems may frequently arise in that folds are created in the film making the shrinked enclosure loose and not tight-fitting, but this can to a certain extent be compensated by exposing the film to a stronger heating along the areas which include the folds, whereby overlapping. film sticks together.

A further substantial advantage by the method in accordance with the invention arising from the fact that the difficulties with the flapping and billowing of the film are eliminated, is that the shrink tunnel to a larger degree than hitherto can be filled with goods since the minimum distance from the outside of the goods or the load to the inside wall of the shrinking tunnel can be reduced radically. Thus the clearance from the load to the wall may when utilizing the method in accordance with the invention, be reduced from 30-50 cm which is usually in shrink tunnels today, to approximately 5-10 cm. This involves that a shrink packing apparatus having a rated capacity and which operates on basis of the method in accordance with the invention will have substantially larger capacity and is able to wrap larger units, for instance -30 percent larger units compared with a conventional apparatus with otherwise comparable dimensions. This condition together with the fact that the entire hot air current and fan assembly is positioned below the floor in the tunnel results in a reduced requirement to space.

I claim:

1. A method of wrapping a load with heat-shrinkable film that comprises the steps of initially providing the load with an open-bottom envelope of heat-shrinkable film, positioning the load in a shrink tunnel, withdrawing air from the lower open end of the envelope in order to create a sub-pressure therein and simultaneously withdrawing air from the tunnel space past the lower side edges of the load, heating the air withdrawn from the tunnel space and the inside of the envelope, re-circulating same into the tunnel space in the form of upwardly directed air currents along the inside walls of the tunnel toward the top side of the load, causing the air currents to turn around and move downwards along the outside of the film envelope to the lower side edges of the load to provide a closed circulation loop.

2. An apparatus for wrapping a load with heatshrinkable film which comprises: a shrink tunnel in the form of a closed top open bottom insulated box, a support for a load therein, said support providing lateral air openings between said support and the lower side edges of the load, an air conduit system beneath said support, said air conduit system including an upwardly open central shaft, initially laterally directed air conveying means leading from said shaft and ending in spaced port openings at the open bottom of the shrink tunnel, a fan means and an air-heating means positioned in said air conduit system, said fan means being arranged to simultaneously withdraw air from the film envelope and from the shrink tunnel space through said lateral openings between the support and the lower side edges of the load to move the air as upward air currents out through said port openings and along the inside walls of the shrink tunnel and back over the outside of the load to establish a closed air circulation system, wherein said support including said port openings, said air conduit system, said fan means and said heating means are built as an integral self-contained unit structurally independent of said shrink tunnel.

3 An apparatus according to claim 2 wherein the laterally directed air conveying means from said shaft comprises a plurality of air ducts ending in air port openings circumferentially arranged in the support adjacent the inside walls of the shrink tunnel.

4. An apparatus according to claim 2 wherein said fan means and said air-heating means are located in said shaft.

11mm ame ATE T 01mm QERTIHQATE OF QORREQTHGN Patent No 3,7773%6 Dated December 11, 1973 0d 1" Inventofls) d G aver It is certified that error appears in the above-identified patent and that said Lettete Patent are hexeby correctei'as shown below:

On the introductory page the address of the assignee [ICIREPAT 73] should be "Sandvika, Norway".

Signed and sealed this is; day of October 1974,

(SEAL) Attest:

Go MARSHALL DANN MCCOY Mo GIBSON JRa Commissioner of Patents Attesting Officer UNHTEE 1.. s PATENT owners CERTIMQATE' @F ,QQRREQTHO N Patent, No. 3,777 Dated December 11, 1973 n Odd Graver It: is certified that error appears in the above-identified patent and that said Lettets Patent are hezreby correctecl as shown below:

, Signed and sealed this 'lszday of October 1974,

(SEAL) Attest:

MCCOY Ma GIBSON JR.a C;o MARSHALL DANN Attesting Officer Commissioner of Patents

Claims

1. A method of wrapping a load with heat-shrinkable film that comprises the steps of initially providing the load with an openbottom envelope of heat-shrinkable film, positioning the load in a shrink tunnel, withdrawing air from the lower open end of the envelope in order to create a sub-pressure therein and simultaneously withdrawing air from the tunnel space past the lower side edges of the load, heating the air withdrawn from the tunnel space and the inside of the envelope, re-circulating same into the tunnel space in the form of upwardly directed air currents along the inside walls of the tunnel toward the top side of the load, causing the air currents to turn around and move downwards along the outside of the film envelope to the lower side edges of the load to provide a closed circulation loop.

2. An apparatus for wrapping a load with heat-shrinkable film which comprises: a shrink tunnel in the form of a closed top open bottom insulated box, a support for a load therein, said support providing lateral air openings between said support and the lower side edges of the load, an air conduit system beneath said support, said air conduit system including an upwardly open central shaft, initially laterally directed air conveying means leading from said shaft and ending in spaced port openings at the open bottom of the shrink tunnel, a fan means and an air-heating means positioned in said air conduit system, said fan means being arranged to simultaneously withdraw air from the film envelope and from the shrink tunnel space through said lateral openings between the support and the lower side edges of the load to move the air as upward air currents out through said port openings and along the inside walls of the shrink tunnel and back over the outside of the load to establish a closed air circulation system, wherein said support including said port openings, said air conduit system, said fan means and said heating means are built as an integral self-contained unit structurally independent of said shrink tunnel. 3 An apparatus according to claim 2 wherein the laterally directed air conveying means from said shaft comprises a plurality of air ducts ending in air port openings circumferentially arranged in the support adjacent the inside walls of the shrink tunnel.