An air heating apparatus for packaging materials, provided with means for recirculation of the heated air

Abstract: Heating apparatuses for packaging materials operating with hot air are often employed for heating thermoplastic packaging materials in connection with heat-sealing, forming or the like. The heating apparatuses are energy-intensive and suffer from a relatively low degree of efficiency, which primarily relates to a considerable waste flow of heated air. A heating apparatus in which these drawbacks have been obviated displays a suction box provided with apertures, the box being located adjacent a nozzle for the hot air and connected, by the intermediary of a recirculation duct, to the intake side of means generating an air current.

Description: The present invention relates to a heating apparatus for packaging materials, comprising an air duct with a nozzle, a heating device and means for generating an air current, according to the preamble of claim 1. Packages or cartons of the single-use disposable type for liquid contents such as milk, juice or the like are often manufactured by reforming and sealing of laminated packaging material. The packaging material consists wholly or partly of thermoplastic material which, during the reforming process into cartons, is heated to its softening or melting temperature point in order to make possible reforming and/or sealing of the material. One generally employed working operation is to heat the edge regions of a material web until the outer thermoplastic layer of the packaging laminate melts, whereafter the edge regions are compressed together and allowed to cool so that a liquid-tight seal is created. The prior art heating apparatuses employed for carrying out the above-mentioned heating operations are of varying types but operate remarkably often with hot air which is led to those surfaces of the material which are to be heated. Hot air heating is preferred in many cases, since a large number of prior art heating apparatuses are available on the market at a relatively reasonable cost, and such apparatuses share the common feature that hot air heating makes it possible to restrict the heating to a defined, optionally shaped region at the same time as the heating level may readily be regulated. Such heating apparatuses normally include an air nozzle which is aimed towards that region of the packaging material which is to be heated. Air is supplied to the nozzle by means of a fan that sucks in ambient air which, before reaching the nozzle, passes a heating device, normally some form of electric resistor element, so that the air, on departing from the nozzle, is at a desired temperature, generally between 200 and 400°C. The disadvantages inherent in prior art heating apparatuses of the hot air type as described in e.g. DE-A-2 405 943 are primarily that they are extremely energy-intensive, since the heated air, after having come into contact with the packaging material and given off but a minor fraction of its heat, flows out into the ambient atmosphere, which is not only a disadvantage from the energy viewpoint, but also entails an unnecessary and, in many cases, hazardous warming of the atmosphere. Another disadvantage which contributes to poor heat economy is the sluggishness of response shown by heating apparatuses of the resistor type to adjustment. In order to ensure that a sufficient volume of air at the desired temperature is available on those occasions when packaging materials are to be heated, the heating device proper normally operates continuously and the generated hot air is channelled off into the atmosphere so that it does not impinge on the packaging material when this is not desirable, for instance in the intermittent heating of successively advanced, sheet-formed packaging material blanks. Thus, the low degree of efficiency of the prior art constructions is not only profligate with energy but also constitutes a hazardous warming of the atmosphere and it is, therefore, a general desire within this art to devise a heating apparatus of the hot air type which does not suffer from the above-mentioned drawbacks. Therefore, it is known by US-A-4 511 426 to reduce the waste of hot air by using a heating apparatus according to the opening part of claim 1 in order to heat the bottom parts of bottles before they are closed by heat sealing. The hot air is directed into said bottom part while a gas collector is disposed above said bottom part. It is an object of the present invention to enable the apparatus to heat also web like material which is not yet formed into the form of a container e.g. a rectangular container or a bottle but also to avoid the waste of energy very much. Yet a further object of the present invention is to also restrict total thermal emissions, so that a high degree of efficiency is achieved. Still a further object of the present invention is to devise a heating apparatus of the hot air type which is of simple and reliable design and construction and which may be designed as a compact insulated unit. Finally, yet a further object of the present invention is to devise a heating apparatus of the hot air type which is eminently suited for heating of the edge regions of thermoplastic-coated packaging materials. These and other objects are attained according to the present invention by a heating apparatus as claimed in claim 1. Preferred embodiments of the apparatus according to the present invention are claimed in sub-claims. By providing the heating apparatus with a recirculation duct and by a suction box, it will be possible to take care of a very large proportion of the hot air flowing out towards the packaging material and recycle it, which first considerably reduces loss into the atmosphere and secondly raises the degree of efficiency of the apparatus manifestly in comparison with the otherwise necessary heating of new ambient air. The design of the heating apparatus as a compact unit disposed in a thermally insulated box makes for a further improvement of the degree of efficiency of the apparatus as a whole. One preferred embodiment of the apparatus according to the present invention will now be described in greater detail with particular reference to the accompanying schematic drawing which shows only details indispensible for an understanding of the invention. In the accompanying Drawing: Fig. 1 is a side elevation of the apparatus according to the present invention, partly in section; and Fig. 2 is a cross section through the apparatus of Fig. 1. The heating apparatus according to the invention is intended to be placed in a packaging machine for manufacture of packages or cartons from a packaging laminate which comprises outer layers of thermoplastic material which are utilised for realising a liquid-tight seal of the carton. The apparatus is intended to heat an edge region of the packaging material to the softening temperature of the thermoplastic layer prior to sealing. If necessary, the apparatus may of course also be employed separately, for example in the prefabrication of sleeve-shaped package blanks, or in the reforming of a packaging material web into hose or tube form by joint sealing of both of the longitudinal edges thereof. The preferred embodiment of the heating apparatus illustrated in the Figures is of compact design and enclosed in a casing 1 which is of substantially rectangular configuration and includes a lid 2 and a bottom portion 3. Both the lid 2 and the bottom portion 3 consist of walls with an inner insulation 4. The bottom wall of the bottom portion 3 displays a centrally disposed longitudinal channel 5 for the packaging material and has corresponding recesses in the end walls. Via the channel 5, the packaging material (not shown) reaches up into a treatment chamber 6 which is laterally defined by two identical mutually facing nozzles 7 and a suction box 8 disposed above the nozzles, these parts together defining the treatment chamber 6 so that it is only open downwardly. Both of the nozzles 7 have mutually facing rows of outflow ports 9, via which hot air may flow out and realise a substantially linear heating of packaging material (not shown) located in the treatment chamber 6. The upper defining wall of the treatment chamber 6 formed by the suction box 8 displays a number of apertures 10 in the form of holes of varying diameter. To ensure a uniform air flow, the hole located furthest to the right in Fig. 1 is of the largest diameter, while the size of the holes progressively diminishes in a direction to the left, i.e. in the direction of flow of the air (see arrows). At its left-hand end, the suction box 8 connects to a recirculation duct 11 which, via a return bend, leads to air current generating means in the form of an ejector 12. The opposite end of the ejector 12 is connected to a heating device 13 which includes a cassette 14 with a number of electric resistor elements 15 of per se known type. At the opposite end of the cassette 14 in relation to the ejector 12, there is disposed an air duct 16 which, via a return bend, leads the air to the two nozzles 7 located in parallel. At the end of the one nozzle 7 (seen in the direction of flow), there is disposed a thermocouple 17 which is electrically connected to a conventional control unit (not shown) which regulates the current supply to the resistor elements 15. The ejector 12 is connected, via a pipe 18, to a compressed air source (not shown) from which compressed air for driving the ejector may be obtained. The compressed air source should be of such capacity that the ejector can be fed with compressed air at a pressure of between 0.6 and 0.8 bar at a flow rate of 50-100 litres/min. In such instance, the ejector (which is of conventional type) gives an air flow rate of 200-300 litres/min. When the apparatus according to the invention is made operational, the pipe 18 of the ejector 12 is placed in communication with the compressed air source, the ejector sucking air via the apertures 10 of the suction box 8 so that an air flow into the cassette 14 of the heating device 13 is created. The resistor elements 15 are connected to a power source which makes it possible to heat up the passing air to a temperature of between 300 and 500°C. The thus heated air is fed, via the bent air duct 16, to a branch pipe which distributes the air to the two parallel nozzles 7. The air flows out through the linearly disposed ports 9 of the nozzles at high speed in a direction towards the edge of the packaging material which is to be heated (not shown) located in the treatment chamber 6. When the air reaches the surface of the packaging material, it is led off, but thanks to the suction box 8 located directly above the two nozzles 7, this air is immediately taken care of and sucked into the suction box in order once again to reach the ejector 12 via the recirculation duct 11 and be recirculated (clock-wise) via the heating device 13 to the two nozzles 7. In such instance, the requisite heating of the air is governed by the thermocouple 17 which senses the temperature of the air and ensures that the elements 15 are regulated so that the desired temperature increase is achieved when the air flows past. The use of the recirculated air which, after passage through the suction box, is still at a high temperature (approximately 200-300°C) is made possible thanks to the utilisation of an ejector as the means generating an air current instead of, for instance, an electric fan which would not withstand the high prevailing temperatures. The design of the apertures 10 of the suction box with progressively diminishing diameters seen in the direction of flow has proved to make it possible to achieve a relatively uniform extraction of the return air throughout the entire length of the nozzles 7, which is a guarantee of uniform heating of the packaging material. The compressed air to the ejector 12 may possibly be pre-heated by leading the pipe 18 through a longer or shorter distance within the casing 1. In practical operation of the apparatus according to the present invention, It has proved possible to reduce energy requirements most dramatically on heating of a longitudinal edge of a packaging material, at the same time as working environment conditions are greatly enhanced as a result of the reduction in thermal emissions from the apparatus.

Claims: Heating apparatus for packaging materials, comprising a heating device (13), means for generating a gas current, at least one gas duct (7) comprising outflow ports (9) for guiding said hot gas into a treatment chamber (6), a gas collector comprising apertures (10) for receiving said gas, and a circulating system for reintroducing said gas into said heating device (13), characterized in that walls of said treatment chamber (6) are formed at least partially by outer walls of said gas duct (7) and by outer walls of said collector which is formed as a suction box (8). Apparatus as claimed in claim 1, characterized in that two mutually facing gas ducts (7), together with the suction box (8), form three walls of an elongate treatment chamber (6). Apparatus as claimed in claim 1 or 2, characterized in that the apertures (10) of the suction box (8) are of progressively diminishing surface area seen in the direction of flow of the gas current. Apparatus as claimed in one of the preceding claims, characterized in that said outflow ports (9) are small openings forming a line of closely spaced ports (9) while said apertures (10) of said suction box (8) are holes of much larger diameter. Apparatus as claimed in any one of the preceding claims, characterized in that an ejector (12) is connected to a compressed air source by the intermediary of an air preheater device which ejector (12) forms said means for generating a gas current. Apparatus as claimed in claim 5, characterized in that a thermocouple (17) for controlling the heating device (13) is located upstream of the ejector (12) seen in the direction of flow of the air current. Apparatus as claimed in any one of the preceding claims, characterized in that the heating device (13) includes electric resistor elements (15) which are disposed in a cassette (14) constituting a part of the flow path of the gas. Apparatus as claimed in claim 7, characterized in that the cassette (14) is located in parallel with the gas duct (7) and the suction box (8) and is surrounded by a thermally insulating wall.

Inventors: ANDERSSON ROLAND, SVENSSON LARS, ANDERSSON, ROLAND, SVENSSON, LARS
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE S.A.
Application year: 1990
Main classification: B65B51
IPC codes: B65B51, F24H3, B29C65