Hot gas sealing device

A device for hot air welding, comprising a nozzle for activation of a sealing area around an opening at the inside of a hollow body. The nozzle has a slot device for directing a flow of air out through the opening for activating the sealing area when the nozzle is in the operative position thereof. Means are arranged for cooling the hollow body externally around the sealing area. There are also provided means for recirculation of the gas.

FIELD OF THE INVENTION 
The present invention relates to a device for welding, preferably sealing 
of hollow bodies, for instance plastic tubes or laminate tubes. More 
precisely, the invention relates to an arrangement for welding/sealing by 
using a hot gas, for instance hot air. 
BACKGROUND OF THE INVENTION 
The problem behind the invention is to find a cheaper and more useful 
method than conventional high frequency welding contact heat jaw welding 
and radiation heat welding and which is useful for both plastic and metal 
foil laminates. The problem behind the invention also is to find an 
arrangement for internal welding of hollow bodies, preferably for sealing 
of openings in flexible packaging containers, for instance for transverse 
sealing of tubes, i.e. forming a seam in a direction crosswise the 
longitudinal axes of the tube. 
Of the conventional methods, the contact heat welding is an indirect method 
where heat is supplied from the exterior by heated jaws. When radiation 
heat welding is applied on tubes, radiation elements provide a weld joint 
by melting the tube end at the same time as the tube is compressed by some 
kind of tool. High frequency welding means activation of the welding area 
by heat production directly in such area whereby transfer losses are 
eliminated. However, it has turned out that some times high frequency 
welding implies practical problems and, additionally, that such method, 
like the other two methods, is rather expensive. 
Therefore, the object of the invention is to find a more advantageous 
alternative in the type of application mentioned. 
SUMMARY OF THE INVENTION 
The invention provides a device for hot gas welding, preferably hot air 
sealing. The device comprises: a nozzle for activating a sealing area 
around an opening at the inside of a hollow body, for instance a tube; a 
slot device in said nozzle arranged for directing a flow of gas out 
through the opening for activating said sealing area when the nozzle is in 
the operative position thereof; means for cooling the hollow body 
externally around the sealing area; means at the said opening of the 
hollow body for recirculation of the gas stream; and a gas guiding element 
of a heat insulating material, acting as a heat shield for the goods 
within the hollow body. Preferably, said means for cooling the hollow body 
externally comprises a loop containing a cooling medium. 
In order to achieve a uniform activation of the welding area and allow a 
simple construction, the slot of the nozzle preferably is formed as one or 
several circumferential slots. 
The technique according to the invention especially is useful where the 
hollow body is a tube to be sealed in the transversal direction at the 
region of said opening. The nozzle co-operates with press jaws, in the 
operation direction of the machine preferably located after the nozzle in 
a filling/sealing machine equipped with at least one set of nozzles/press 
jaws. 
The slot device for directing the gas stream may of course vary as to form. 
The device may for instance comprise a set of holes along the 
circumference of the nozzle.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The nozzle generally indicated by the numeral 10 in FIG. 1 comprises a 
lower generally cylindric end 11 having a circumferential slot 12 acting 
as an outlet for hot air from a supply conduit 13. The slot 12 is directed 
obliquely upwards for striking with hot air a well defined area of the end 
of a tube 14 to be sealed in the transversal direction. The end 11 and 
tube 14 are dimensioned such that a circumferential slot having a constant 
width of approximately some millimetres, possibly centimetres at a 
maximum, is formed and allows a flow of air in the direction of the arrows 
15. 
The attack area of the flow of air at the inside of the end of the tube 
body 14 to be heat sealed is determined by the slot 12 and the insertion 
depth into the tube. The well defined area of attack is followed, in the 
flow direction of the air, by a collector housing 16 for the air flowing 
out from the tube. Via a conduit 17 the air is fed into a recirculation 
system having an appropriate arrangement (not shown) for the return flow 
of air at an appropriate temperature level through the supply conduit 13. 
In FIG. 1 the tube 14 is filled with a product 18 and preferably is 
supported displaceable in the vertical direction in a filling/sealing 
machine, as indicated by the double arrows 6 in FIG. 1. Such a positioning 
determines the insertion depth of the end of the nozzle 11. In order to 
avoid heat transfer to the goods, the end 11 preferably is provided with a 
heat shield 19 formed for guiding a gas stream and possibly comprising a 
heat insulating material and forming the portion of the hot air slot 12 
facing the goods. 
Alternatively, the relative vertical displacement may be obtained by making 
the nozzle movable in the vertical direction. 
A centering sleeve 20 surrounds the nozzle 10 in the area of the end 11 and 
as shown in FIG. 1 the sleeve may be provided with a loop 21 for a cooling 
medium. The loop forms an essential part of the system and provides for a 
well controlled environment at the external side of the welding area. From 
an esthetic point of view such temperature controlled environment is 
highly interesting. The cooling during heat activation of the inside of 
the tube also contributes to a well defined sealing area. 
The nozzle 10 which is intended for a machine of the type schematically 
shown in FIG. 3 co-operates with press jaws 22 following directly after 
the nozzle 10 in the operation direction of the machine. The jaws provide 
the necessary sealing pressure and are stationary in the machine stand 23 
as is the nozzle 10 in the actual embodiment. In one and the same machine, 
preferably, there are several sets of nozzles/press jaws 10, 22 operating 
in parallel on tubes forwarded in the machine by a conveyor system. 
In the embodiment according to FIGS. 3 and 4 tubes 14 are transported 
intermittently by an endless conveyor 24 to the intended position by using 
appropriate guiding devices (not shown). The displacement of the tubes in 
the vertical direction for positioning of the tube end relative the nozzle 
is obtained by stationary guides (not shown) which operate displaceable 
brackets 25 forming part of the conveyor 24. 
In FIGS. 5 and 6 there is shown an alternative slot embodiment. The slot 20 
is a circumferential double slot 20 having an axial distance between 
individual slots. Each one of the slots communicates with the supply 
conduit 13 via a hole 20'. 
As a further alternative to the basic embodiment it is possible to replace 
the slot by an appropriate set of holes. 
The positioning of the nozzle slot/set of holes for directing the air flow 
out through the opening implies--it has turned out--an advantageous 
temperature distribution in the tube. The venturi effect arising due to 
the outflowing hot air implies that the air flow sucks out also some of 
the air at the interface of the goods in the tube. This means that the 
goods is protected against a high temperature due to the venturi effect.