Radiant Shrink Tunnel

Abstract

An open-ended tunnel-shaped heating chamber for shrink wrapping of large quantities of merchandise in pallet loads, the chamber being open at each end for erection on a conveyor system without the use of doors, and having electrical radiant heating units arranged in three separate zones consisting of forward, rear and intermediate zones, along the tunnel, the forward and rear zones being arranged and directed to progressively heat and shrink different portions of the shrink film draped around the merchandise on the loaded pallet and the intermediate zone being arranged to heat the film-draped around the lower region of the pallet and shrink the same around and underneath the pallet, the heating units being provided with parabolic reflectors to focus and direct the heat in narrow intense bands so as to procure progressive heating of small areas of the films, and the tunnel being provided with heat reflective baffle means to trap any stray radiant heat which may otherwise escape from the tunnel through its open ends, and the heating being provided with separate zone controls whereby the operator can vary the heat applied by each of the zones, thereby enabling the construction of a shrink tunnel without doors for incorporation in a continuous flow production line.

Description

The present invention relates to a heating chamber for use in the heating of shrinkable thermoplastic film material for packaging and the like, generally referred to as a "shrink tunnel".

The use of shrink films to enclose entire pallets loaded with merchandise is well known. Various different proprietary makes of tunnel heating equipment are available on the market, which all more or less adequately perform the task of heating a relatively large portion of shrink material around a pallet load of goods. For example, recent developments in this field are discussed in an article entitled "New concept in Unitized shipping" by S. Zelnick, Modern Packaging Magazine, Apr. 1969, pages 193 to 198. In general, tunnel heating equipment, such as is described in the article and other such equipment available at the present time for the same purpose is relatively inefficient, and unnecessarily expensive. For example, most of this type of equipment incorporates some means for heating the air within the tunnel to a high temperature, the hot air being circulated by fans, thereby heating the shrink film material by convection.

A relatively complex system of fans, ducts, and sensing devices are employed in this type of equipment so as to maintain a uniform temperature throughout various areas in the tunnel.

In addition, the heat loss from such systems is very great. Obviously if large volumes of air are to be heated, in order to treat each pallet load, then large volumes of hot air will escape from the tunnel each time the pallet is removed, overheating the surrounding plant, and causing very substantial wastage of fuel. In order to overcome this, the tunnels have been made still larger, and automatic doors are employed at each end of the tunnel since there is such a substantial volume of heated air being circulated within the tunnel that the opening and closing of the doors must be carefully timed and regulated. However, in spite of these precautions, it is customarily accepted as a standard in the engineering of this type of system, that shrink packaging of pallet loads of goods requires power ratings ranging from 100 to 600 kilowatts.

These disadvantages and wasteful procedures have obviously increased the cost of the shrink packaging equipment to the point where it was only economically feasible for high volume users, and in addition, the equipment had become so large and unwieldy that it required an excessive amount of space, all of which disadvantages have tended to restrict the wider applications of shrink packaging.

The present invention is directed to the construction of shrink tunnel equipment which is smaller and more compact than that presently available, while yet being able to accommodate full-sized pallet loads, which is much more economical in fuel consumption, which does not cause excessive heat loss to the surrounding plant thereby causing discomfort in the building, and in which the use of doors and the like can be dispensed with, thereby greatly simplifying the installation of such equipment in a continuous packaging line, and in which the requirement for air circulation equipment such as fans, motors, ducts and the like is completely dispensed with, thereby achieving very significant cost reductions, enabling wider use of shrink packaging.

In general terms, the invention sets out to achieve these various advantages by the use of a shrink tunnel preferably although not essentially open at each end whereby to permit installation on a continuously moving packaging line, and in which the heating is achieved by means of radiant heating elements preferably electrical radiant heating elements arranged in parabolic reflectors and focused and preset in a particular manner so as to heat the various side surfaces of the film envelope which is draped over the pallet load in sequence, rather than heating them all together, the tunnel additionally incorporating reflector baffles to catch any stray radiant heat emanating from the tunnel and retain it within the interior, thereby avoiding unpleasant overheating of the surrounding atmosphere in the plant, and permitting operators to work closely adjacent to the shrink tunnel without discomfort, additionally, the various radiant heating means are preferably controlled in separate zones, corresponding to the separate sequential areas of the pallet which are heated by the radiants, thereby providing a greater degree of process control. It will be understood that this is essentially a generalized summary of the invention, and that variations can and will be made, such as come within the scope of the appended claims, without departing from the invention.

The foregoing and other advantages will become apparent from the following description of the preferred embodiment of the invention which is given here by way of example only with reference to the following drawings, in which like reference devices refer to like parts thereof throughout the various views and diagrams and in which:

FIG. 1 is a sectional side elevation of a shrink tunnel according to the invention;

FIG. 2 is a sectional top plan view of the shrink tunnel shown in figure 1, and

FIG. 3 is a schematic circuit diagram showing the separate zone controls of the various radiant heating elements shown in figures 1 and 2.

Referring now to figures 1 and 2, the shrink tunnel according to the invention will be seen to comprise a generally rectangular box-shaped housing defined by sidewalls 10, a first end wall 11 and a second end wall 12 and a top wall 13. End wall 11 is provided with a generally rectangular-shaped entrance opening indicated as 14, and end wall 12 is provided with a generally rectangular-shaped exit opening 15.

In order to move loaded pallets through the tunnel, any suitable conveyor system is provided such as the conveyor belt system 16 driven by electric motor 17 in any known manner. Obviously, bearings, tensioners, and reduction gears, are included in such conveyor systems according to the well known practice in this art, these features being omitted for the sake of clarity.

As stated, according to the present invention, heat is applied to the synthetic film to shrink it by means of electrical radiant elements. At first sight, it would appear that this form of heating was particularly unsuitable for use in a shrink tunnel. Obviously, in any shrink packaging or wrapping operation, great care must be taken to avoid overheating of the merchandise being wrapped. In the case of conventional shrink-wrapping systems using hot air and heating by means of convection, this presents little or no problem, since convection heating of the shrink film will result in the shrink film being brought up to the appropriate temperature before any substantial heating of the merchandise itself has taken place. In addition, any heating of the merchandise that does take place, according to such conventional techniques, will take place only as a result of conduction of heat through the plastic medium. Since most plastics, particularly those used in shrink-wrapping such as polyvinylchloride and polyethylene are both relatively inefficient heat conductors, the heating of the merchandise will be kept to a minimum. However, in the case of radiant heating, quite different considerations apply. In this case, very great care must be taken to ensure that the merchandise on the pallet is not raised above an acceptable maximum temperature during the shrink-wrapping operation. According to the invention, this particular objective is achieved by heating the shrink film in predetermined areas at separate times, and during the heating of any particular area, a considerable excess of intensive radiant heat is applied to the area being treated in order to bring it rapidly to the temperature at which shrinking takes place, the duration of such intensive heating being carefully controlled by suitable control circuits, and being cut off just as soon as the predetermined shrink temperature is reached, after which further areas of the shrink film are then treated in the same way. In this way, the shrink film itself is brought to the desired temperature in the shortest possible space of time, while keeping the heating of the merchandise itself to a minimum. In addition, the use of zone controlled heating in this way restricts the total power consumption of the entire installation at any given moment. At the same time, these factors make it possible to treat full sized pallet loads in a relatively small compact installation on a continuously moving basis.

In order to achieve these various objectives, the invention provides three separate groups of radiant heating units. The first such group, which is designed and arranged to heat the front and two side panels and top panel, of the shrink material on the pallet, comprises the pair of front ceiling radiant heater units 20 and two front side heater units 21, all of which are oriented and directed to focus radiant heat rearwardly into the tunnel, thereby heating the front surface of the advancing pallet. Each of elements 20 and 21, consists of the parabolic reflector unit 22, 23 and the electrical radiant heater elements 24, 25. In order to heat the rear panel of the pallet, and also apply additional heat to the side panels thereof, and the top panel, there is provided the second group of elements comprising a pair of rear ceiling units 26 and the two identical rear side units 27, which are of substantially identical construction to the units 20 and 21 already described, and incorporate parabolic reflectors 28, 29 and electrical radiant heating elements 30, 31. Units 26, 26 and 27, 27 are so arranged and directed and focused as to direct heat onto the side panels of a pallet and onto the top panel and also onto the rear panel of the pallet sequentially as the same is moving through the tunnel.

It will be appreciated that in the heating of the portions of shrink film arranged around the lower regions of the pallet and its load, particular care must be taken to ensure that such plastic is fully heated to the predetermined shrink temperature since in the first place, the heat within the tunnel will, if anything, tend to rise, thereby causing somewhat unequal heating.

In addition it is desirable to heat and shrink the lowest portion of the film first, before heating the remainder. The film is in the form of a tubular bag and is cut to provide an excess of 3 inches or so hanging below the pallet. This lower dependent portion is heated first and shrunk onto the pallet itself and clamps around under the edges of the pallet thereby holding the film bag firmly in position during subsequent heating of the sides and top of the film, which might otherwise cause the bag to shrink upwardly and leave the pallet and lower merchandise uncovered. Accordingly, such initial heating of the lower portions of the shrink film is provided by a third group of heating units consisting of the short intermediate side heating units 32 which are arranged on either side of the tunnel, substantially midway between front and rear side units 21 and 27, and the longitudinal lower side heating units 33, which are arranged along either side of the tunnel, just above the level of the conveyor 16 whereby to provide continuous additional heat in this region throughout the passage of the pallet through the tunnel. The short vertical units 32 comprise the reflectors 34 and the electrical radiant heating elements 35 which are arranged and erected more or less at right angles to the path of movement of a pallet through the tunnel. The longitudinal units 33 consist of the reflectors 36 and the electrical radiant heating elements 37.

Such lower heating units 32 and 33 also compensate for any tendency towards cooling of the lower portions of film due to upward flow of unheated air.

The orientation of the reflectors 22 and 23 of the front heating units, is so arranged that radiant heat from upper front units 20, is focused at a predetermined area in the plane of conveyor 16 more or less intermediate the positions of rear side units 27, and short side units 32, whereby to apply heat directly to the lower region of the front panel of a pallet advancing along conveyors 16 through the tunnel, and to apply heat to the front panel in an upward wiping manner, after which the heat from units 20 will then be directed onto the top panel of the pallet again in substantially a wiping manner. The reflectors 23 of units 21 are focused and directed so as to direct heat rearwardly on converging axes, meeting substantially at the central vertical axis of the tunnel, i.e. an imaginary vertical axis extending upwardly normal to conveyor 16 intermediate the short side heating units 32. In this way, again, heat is first directed to the front panel of the advancing pallet, after which it is applied in a wiping manner along the side panels thereof.

Similarly, mirrors 28 of rear upper units 26 are so arranged and focused as to direct and focus heat therefrom forwardly onto an area of the conveyor 16 intermediate heating units 21 and 32, the path of such heat therefore intersecting the path of heat coming from front heating unit 20 at a point somewhere between 1 foot and 2 feet above the surface of the conveyor 16. Unit 26 will therefore apply heat to the top panel and subsequently to the rear panel of the loaded pallet moving on conveyor 16. The mirrors 29 of the two rear side heating units 27 are arranged and focused so as to direct and focus heat therefrom forwardly on converging axes meeting substantially at the central vertical axis of the tunnel as defined above, i.e. more or less at the same point as the heat focused and directed by mirrors 23 of the front side unit 21. In this way, heat from such rear side units 27 is first directed to the side panels of the advancing pallet load and afterwards are applied in a wiping manner across the rear panel thereof.

The mirrors 34 of the short side units 32 are arranged and focused to direct heat directly across the path of conveyor 16 i.e. directly onto the lower side surfaces of the advancing pallet, and in the same manner, the mirrors 36 of the side units 33 are also arranged to direct heat in the same manner.

It will thus be seen that all of the heating units incorporated in the radiant heating tunnel according to the invention are arranged to direct substantially all the heat radiating therefrom along predetermined paths within the tunnel. However, since the various electrical radiant heating elements are all of conventional loop construction, none of them constitute a point source of heat, and the parabolic mirrors will be unable to prevent a certain amount of scattering of radiant heat. Accordingly by the practice of the present invention, there are provided at each end of the tunnel, both on the sides and on the ceiling, inwardly angled mirror baffle members, comprising the forward ceiling baffle 38, and rearward ceiling baffle 39, and the front side baffles 40, and the rear side baffles 41. As stated, all of baffles 38, 39, 40 and 41 are so angled and oriented as to catch substantially all scattered radiation emerging from the interior of the tunnel, and return such radiation thereby increasing the overall heat within the interior of the tunnel and avoiding heat loss to atmosphere within the plant. In addition, in order to further intensify the heat applied to the shrink film on the pallet load as it passes through the tunnel, there are provided interior light reflector mirrors both on the ceiling and on the sides of the tunnel comprising the ceiling reflector 42, and the side reflectors 43-43.

Referring now to figure 3, there is provided a zone control switching system for regulating and controlling the heat of the three zones of heating referred to above. As shown in the circuit diagram Figure 3, the forward ceiling unit 20 and the forward side units 21 comprise the heaters of zone one, the rearward ceiling unit 26 and the two rear side units 27 comprise the heaters of zone two and the short side heater units 32 and the long side heater units 33 comprise zone three. It will be noted that for each of zones, one, two and three, there are provided percentage time lapse output control switches indicated at t.sub.1, t.sub.2, and t.sub.3 respectively. These switches are of known design in which the contacts are opened and closed in a cyclical manner thereby alternately heating and cooling the heating units, variation of the opening and closing cycle varying the heat output. An operator may thus set up a predetermined heat output for each of the three zones. Preferably, the three controls t.sub.1, t.sub.2, and t.sub.3 are arranged to operate in conjunction with the movement of conveyor 16, the controls being set up in such a manner that the operator can set the dial (not shown) to a predetermined percentage of the total heat output balanced against the speed of the loaded pallet.

Additionally, there may be provided further controls in the form of switches indicated as c, for separate on/off control of the ceiling heating units 20 and 26 in zones one and two respectively.

To prevent air currents within the tunnel, hinged upper end flaps 44 are provided, preferably terminating at or close to the upper limits of a typical pallet load, and having reflective inner surfaces 45.

As shown substantially in FIG. 3, the front and rear ceiling units 20 and 26 are arranged in pairs and each of the units in such pair are mounted spaced apart from one another in end to end relation whereby to direct maximum heat onto the upper corners of the film enclosing a loaded pallet. This ensures maximum heating and shrinking of the film in this region, which is necessary to procure a good tight wrapping of the merchandise.

In operation, a pallet loaded with merchandise (not shown) is enclosed within an oversize loose tube of shrink film material (not shown) in a conventional manner, with a portion of the film hanging downwardly around the pallet. The conveyor system then moves the pallet into the tunnel, through the opening 14 in end wall 11. The operator has previously set up the time control t.sub.1, t.sub.2, and t .sub.3 in a manner to apply sufficient localized heat within the duration of time for which the pallet remains in the tunnel. As the loaded pallet comes into registration with the two lower side heater units 33, the dependent portions of the shrink film material are rapidly heated and shrunk around the pallet, and almost simultaneously, the front panel of material on the pallet comes into the focused bands of radiant heat from the two front side heater units 21, the heat from which gradually moves across the front panel as the pallet moves further into the tunnel, shortly thereafter, heat directed from the front ceiling unit 20 strikes the lowermost portion of the front panel of the pallet and thereafter moves gradually upwardly. As the pallet moves further into the tunnel, heat from the two rear side units will strike the side panels of the material on the pallet, moving gradually thereacross in a wiping manner, after which heat from the two front side units 21 will also strike the side panel and move slowly thereacross, the two bands of heat from the front and rear units 21 and 27 being separate from one another and constituting separate essentially localized areas of high intensity heat moving across the shrink material one after the other. Similarly, heat from the rear ceiling unit will first strike the leading edge of the top panel of the pallet and move gradually thereacross after which heat from the front ceiling unit 20 will then follow the same path. Similarly, on the rear panel of material on the pallet heat from the two rear side units 27 will strike the rear panel on opposite sides thereof and move slowly across in a wiping action, heat from the one unit moving from left to right and from the other unit moving from right to left. Thus, a rectangular shaped body of shrink film material is heated to its shrinking temperature by means of radiant heat arranged in localized narrow bands, striking the various panels of the rectangular shape in a sequential manner, each of the panels being covered by the heat from at least two such focused bands, and in addition, the material being preshrunk around its lower periphery to prevent upward shrinking of the material and exposure of the load.

It is to be noted that in some cases, where other modifications are made in the packaging system, for example, it is unnecessary to heat the top of the loaded pallet, and accordingly, the operator may cut out the ceiling heater units by means of the switches C.

The foregoing is a description of a preferred embodiment of the invention, and is given here by way of example only.

Claims

I claim:

1. Radiant heat shrink tunnel apparatus for use in association with a continuous conveyor system in the shrink wrapping of bulk merchandise on pallets and the like in shrinkable plastic film material and comprising:

an open ended tunnel-shaped heating chamber having a top, side walls and front and rear end walls, and openings formed in said front and rear end walls for use in conjunction with a conveyor system adapted to move unit loads of merchandise for packaging through said tunnel-shaped chamber from said rear end wall to said front end wall;

A front group of radiant heating units arranged in elongated bands and located within said chamber towards said front end wall thereof said units incorporating radiant heat focusing means arranged and oriented to focus and direct a major proportion of radiant heat therefrom rearwardly into the interior of said chamber for heating the front, sides and top of shrink film on a loaded pallet as the same advances into the chamber;

a rear group of radiant heating units arranged in elongated bands and located in said chamber spaced apart from said front group of heating units, and arranged towards said rear end of said chamber, said units incorporating radiant heat focusing means arranged and oriented to focus and direct a major proportion of radiant heat therefrom forwardly into said chamber to heat the sides, top, and rear of the film on a loaded pallet moving therethrough;

inwardly directed radiant heat mirror baffle members mounted adjacent to said front and rear groups of heating units, and extending substantially therebetween and angled to capture at least some radiant heat scattered therefrom and redirect the same into said chamber;

a lower group of intermediate heating units arranged on each side of said chamber, and located more or less along the plane of said conveyor, in order to heat the lower side portions of said film as said pallet advances through said chamber, and,

heating control means for at least two of said groups of heating units for controlling the heat output thereof.

2. Radiant heat shrink tunnel apparatus as claimed in claim 1, wherein said front group of heating units is arranged partly on the ceiling of said chamber, and partly on each side thereof, and wherein the said rear group of heating units is similarly arranged, partly on the ceiling of said chamber and partly on each side thereof, said front and rear group of heating units being focused and oriented to establish beams of relatively narrow elongated bands of radiant heat having a more or less rectangular shape in cross section, said beams being angled and directed more or less towards the center of said chamber.

3. Radiant heat shrink tunnel apparatus as claimed in claim 1, including flexible draft excluding flap members attached to said front and rear ends respectively along their upper edges, and heat reflective means on the inwardly directed surfaces of said draft excluding members.

4. Radiant heat shrink tunnel apparatus as claimed in claim 1, wherein each of the heating elements in said groups of elements in said groups of elements comprises an elongated radiant heat source, and a parabolic reflector.

5. Radiant heat shrink tunnel apparatus as claimed in claim 1, wherein said lower group of heating units include at least two horizontal heating units arranged lengthwise in the lower portion of said heating chamber on either side of said conveyor system and substantially in the plane thereof, said horizontal heating units being located so as to begin heating of said lower side portions of said film just prior to direct application of radiant heat by said front and rear groups of heating units.

6. Radiant heat shrink tunnel apparatus as claimed in claim 1, wherein said front and rear groups of heating units each include at least two long vertical heating units arranged one on each side of said chamber, and at least two horizontal heating units arranged end to end and supported in the top of said chamber, spaced apart a predetermined distance whereby to direct heat downwardly onto the upper corners of the film on said loaded pallet.

7. Radiant heat shrink tunnel apparatus as claimed in claim 1 wherein said lower group of heating units include at least two short vertical heating units arranged on either side of said chamber more or less equidistant between said front and rear walls thereof, focused and oriented to direct heat substantially directly across said chamber.