CA1088857A - Closed circuit shrink tunnel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A shrink tunnel for use in shrink packaging of articles with shrinkable films comprises a gaseous return flow, having a working section along which gas passes from an inlet to an outlet in a substantially rectilinear direction, and a return section around which the gas is recirculated for heat and velocity boosting prior to the next cycle through the working section.

Description

T~)e pr,rsent inv,Dntion relates to shrin~
packaging and in particular to a shrin~ tunnel f~r shrin~ing a p~astics envelope enc~osing a product, in oxder t~ for.~ a tightly wrapped package.
~t is ~:nown to use air to impart shrin~age heat to i a the~m,opl2stic film en~elo~e h~ound z product, so as to j achieve shrin};age o~ the ,_nveloping film and thereby to provide a tidier pac~age. However, traoition21 shrink tunnels have consisted olenclosures',hrouoh which the pac~age~ 2re gradually moved at a ~ela.ively ~o~ rate of travel, and across ~:hich enc~os.1res ho'. air is blas'ed s~ , that at various points in the ~assage ol the product through ~.
the tunnel the film enveloplng the product is subjected to shrin~;age heat. However,`because of the relatively high temDerature of the air required in order to achieve high ;¦
heat transmission rates to the bag, conventlonal shrink ~ :
tunnels either need a long dwell time for the shrinka~ge step~ ;
or they are liable to cause burning of certain areas of he bag. .
It is an object of the present invention to pro~-ide~ ~
a hot gas shrin~i tunnel in ~hich hi 5h mass flow rates of 1 :
shrin~age ~cas can be used thereby en2bling high shrin~age~s2sl temperatures to be av~ided, or alternati~rely enabling sho=. ¦
hea~ ing times to be em~loyed, whilst still :ai~Ting a re~a.ivel~ high the~.al .ransmis~ion rate .o~the bag and ,;
hence effecti~9 adequate shrin~age ~ith only a short dwell time.

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s~ -According to the present invention there i8 provided a shrink tunnel for use in shrink packaging articles, comprising:
means defin~ng a work section with an inlet end and an outlet end; :
means defining a return section communicating with said inlet end and sa~d outlet end;
velocity boosting means located ~ithin said means for - .
defining said retuxn secti.on for boost~.ng the velocity of air .
traveling through.said return section;
.~ . . . . .
heating means for heat~ng said air located withi.n said .~ -means defining said return section; and control means res.ponsive to air temperature fox controlling said velocity boosting means. : ~
Pre~erably the shrinkage ~as i~ air. More preferably ~. ;
the shrinkage air ;~s urged around the return fl~w tunnel by . ~
. : , means of a fan wh~ch induces air flo~ across a heater, to maintain the temperature of the circulati.ng air in the tunnel despite the transmiss~on of heat to a package being shrunk.
Deæirably the fan I:s driven at a Yaria~le speed and~the fan speed i5 controlled, together ~ith thé energy supply to the : :~
heater, by a control system responsive to air temperature in ,:~
the tunneI. .
Advantageously a support mem~er is.mounted for .;
movement ~nto and out of the ~orking section of the shrink i. ~ :
tunnel ~or bring~ng a closed, heat-æhrinka~le bag:into the path : -of hot shrink ai.r passing through.the~orking section of the ..
tunnel.:
In a particularl~ conVenient embodiment of the tunnel .~:.
of the present invention, the working section may be constructed as a vacuum chamber with means for alternatel~ diverting the hot shrink air around the vacuum cham~er or : :
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..3 pa~sing it throl~g}) the vacuum ch~ber l.,~e cor)ven;ently the vacuum chamber may ir)c~ude means for sa1.hering the neck of A bag and for closing the neck under vac~um, ~or e~ample by the appl;cation of a metal c1~su~e clip. Suitably the means for a~tel-nately diverting the air around the chamber, Gr p2ssing it thro~gh the ch~mber, may C~mDrise a pair of shutters ~hich in a first position guioe a1l the hot shrin~
air to Dne side of a baffle and past the loca~ion where a filled bag ~ill, in use, ~e positioned, and in a second p~sition define, t~gether with the ba~lle, a v~cuum chamber p~st the Dutside DL ~hich the air is causeo tD flow.
~ne present invention 21s~ rclate~ to a shrun};
pac~aoe IDrmed using the shrink tunnel defirled above.
In orcer that the present invention may be more readily understoDd the fo1lowing description is given, merely by way of example, reference ~eing made~to the accompanylng ~l~
drawing in which~
Pigure ~1 is a schc~atic sice ele~ational view ~a first emD~diment of ~oas shrln~lng apparatus in accor~ance ~th the present in~ention:
Pigure-2 is a schematic si~e eleva.ional vie~ of an alternati~e ~b~diment; ~
-~ ~ioure 3 is a cross-sectional vie~ through the ~Gr~ing section of the tunnel and sho~in~ in this case a vertlcal~ return circuit~ the ~section De~ng ,a~;en on line ~
III of Pigure 4; and :
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~3 Figure 4 is a side elevational, partly sch~matic, view showing the vertical return circuit tunnel of Figure 3.
Referring now to Figure 1, there can be seen, in 'sch~matic' representation, a closed circuit shrink tunnel having its walls lined with a thel~al insulation, and in which the working section 1 has a package P consisting of a bagged prod~ct positioned therein by being mounted on a support member 2 which consists of a closure plate 3 for the side wall of the working section of the tunnel and a grid-type package support 4 spacing the bottom of the package Pfrom the edges of the wor~ing section of the tunnel thereby ensuring that the hot shrinking gas is able to pass all around the pac~age periphery and the wall of the working section 1.
The direct-on of flow of shrinking gas, in this case~
air, througn the tunnel is illustrated by double headed ~
arrow 5 and follows an anti-clockwise path as viewed in :
Figure~l around the first bend 6 in the tunnel and then to the circulator fan 7 driven by motor 8. ~ .
Although not illustrated in detail in the schematic view of Figure 1, the drive from motor 8 to the fan 7 is by , :
way of a variable speed pulley mechanism schematically ~ -~ : .
depicted at 9, this enabling the rate of gas ~low around the tunnel, induced by the fan 7, to be controlled~
In the return section 10 of the tunnel is a heater 11, in this case an electrical resistance heater, the supply to :

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10~8~3~7 which is controlled by a control device 12 which is also connected to a temperature sensing transducer 13 in the return circuit but downstream of the heaters.
From the temperature transducer 13 the heated gas through the tunnel passes into the final bend and thence back tD the upstream end of the wor~ing section 1.
The fan 7 sweeps substantially the entire cross-sectional area of th~e working circuit just downstream of the first bend 6 and thereby ensures that the cross-section of the moving body of gas wlthin the tunnel is as extensive as:
possible, leaving only a thin boundary la~er on the tunnel walls. The speed of operation of the fan 7 is also such that .
relatively high gas flow velocities (for example of the order of 600 to 1300 metres/minute) can be obtained. If desired, 1~ the speed of movement of the air can~be~increased to any desired value above 1300 metres/minute, it belng;understood~
that the rate of travel of air over the surf;ace ~of the ~;
package will be dictatèd by th~e shrlh~ing heat~requlrements of the film enveloping the product in the package P, and by the temperature of the gas flow.
Although, throughout the present specification, the :
term "gas flow" is referred to,.it will be~understood that in ~; ~ :~
practice the use of air will ma~e for a cheap shrin~ing gas.
In the embodiment schematically shown in Figure 1, the pacXage~support member 2 is sho~n as being vertlcally ~ .
movable, between the solid line position in which it holds j !

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If ~sired, a multiple pac~age support can be used s~ch that while one packc,ge is being shrunk in the working section of the tunnel one or rnore other package supports can be accessible for removal of the shrunk pacXage and replacement by a fresh one to be shrunk~ In this way, steps can be taken to ensure that the hlghest possible utilisation I ;
of the working time of the tunnel can be devoted to shrinking ..
operations.
If desired, it may also be possible for the wor~ing~
section of the tunnel to include a by-pass unit so that while i5 a package support 2 with package P thereon;is being moved ~;
into or out of the working section of the~tunnel the air flow can be diverted through the b~-pass~section in order to , ~ :
maintain the uniformity of temperature and air flow ~ -conditions in the return circuit of the tunnel so that when next the by-pass circuit is switched out and air is directed~
along the main workin~ section there will be no delay before maximum shrink heat-imparting effect can be obtained from the; I ~ `
circulating gas stream. ¦
A first alternative embodiment of the shrink ~ ¦
tunnel of the present inventlon is shown in Figure 2, again in the~form of ;a~return flow~tunnel. The fan 7, motor 8, .:
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pulley drive 9, retul~ circuit 6, 10, heater ll, control device 12 and transducer 13 are all identical to the corresponding comp~nents shown in Fig~re 1. The main difference between the Figure l and Figure 2 e~bodiments S lies in the working section where, in the case of Figure 2, :
there is a vacuum chamher provided in the working section of the tunnel.
In Figure 2, the working section la has a raised ceil;ng 14 which cooperates with a pair Gf movdble shutters ~ , :
15 so that when the shutters are in their br~};en line ¦
positions 15' all the circulating hot gas passes beneath a ¦
fixed baffle 16 whereas when the shutters 15 are in their solid line positions they define, together with a package :
support tray 180 a closed vacuum chamber C and will divert i5 the hot gas around the outside of this vacuum chamber C
through the gap remaining between fixed baffle 16 and the raised ceiling 14 of the worhing sect1on la.
The vacuum chamber C is adapted to be evacuated by means of a vacuum pump 19 which communicates with an aperture .
20 in the support tray l8 by way of a vacuum line 21 and -the~eby pulls air from the chamber C along vacuum line 21 and discharges it through discharge port 22. :
As can also be seen in Figure 2, the neck of the ~ag 23 of package P which envelops the product is arranged in : 25 con3unction with an "in-chamber" gathering and cllpping l ~:
device 24 which is capable of effecting clipping after the : . :
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" 10~8~7 vacuum chamber C has been closed.
An "in-chamber" bag neck gathering and clipping system is disclosed and claimed in our British Patent No.
1,353,1~7 and this form of clipping syst~m could readily be adapted by the expert in the art for use in the chamber C
illustrated in Figure 2.
With the Figure 2 embodiment, it will therefore be possible to carry out the various operations of evacuating ~the bag, gathering the bag nec~ closing the bag neck and shrinking the pac~age without removing the package from the support tray 18 and it will thereby be possible to cut down the time taken to convert a loaded bag into an evacuated, shrink-tidied package. ~ ~ ~
Although not specifically lllustrated in Flgure 2, . :
1~ it will of course be possible for the package P to be positioned on a grid similar to that illustrated at~ 4 in : :
~igure 1, tXereby ensuring that the maximum~possible area of ~ ¦
the bag 23 is exposed to the~passing alr flow during shrinking.
By way of example, the:operation of the two illustrated e.~b3diments of gas shrink tunnel will now be described.
~ eferring firstly to the ~igure 1 embodiment, the motor 8 is energised to drive the fan 7 at the speed required ¦
to provide a given rate of air flow along the worXing secti~n 1.
The selected fan speed will be~monitored b~ the control devlce :! :~
12 which also ensures that the temperature sensed:by ¦

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(3 ~3 ~08~b~S7 transducer 13 is at a gi~en value corresponding to the optimum temperature for efficient shrin~ing of the envelope o~ the pacXage P onto the prod~ct in the working section.
For complete automation of control of the shrinking parameters, a control connection 25 between the control device 12 and the motor 8 is illustrated in broken lines for each embodime~t.
Once the temperature of the air has been brought up to the desired value and the rate of air circulation through the return flow tunnel is as required, the shrin~ing operation can be comnienced by removing the pacXage support 2 from the worXing s ction of the tunnel, placing a pacXage P
:
consisting of a closed, loaded en~elope (in this case a plastics bag secured by a metal fastening clip) on the pacXage support bars 4 and then~reinserting the package ~ `
support member 2 into position in the wor~lng section and~
holding it there for the desired shrinkage time. ~
In practice it has been found that with single- and ;
multl-layer polyvinylidene chloride b~sed bags having a film thickness of 3 mils and with a gas flow rate of 590 metres ~`
per minute ~corresponding to 35.5 m3/minute) and a temperature of 220C., good shrinkage (in terms o~ ~ tidy package) was obtained after a mere 2 seconds shrinkage time.
Dropping the temperature to 210C. at an alr speed of 590 metres/minute could still give good shrin~age results if the shrinXage time is increased to 4 seconds.~ Reducing the bag thickness will enable a reduction in the shrinking time.

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.,................ '`^~ 11~ i At the end of the predeter~rlined exposure time, for example 2 seconds at 590 metres/minute and 220C., the product support 2 is withdrawn from the working section into its broken line p~sition as viewed in Figure 1 to 3110w the shrun~ package P to be rernoved and replaced by a fresh clipped, evacuated bag so that the package support 2 can then be re-inserted in the working section for shrinXage of the ne~t package.
After the last package has been shrunk the heater 11 will be de-energised, for e~ample, manually, and the motor 8 allowed to run down so as to maintain cooling air flow over the heater 11 in the return section of the tunnel while the heater cools off.
~he Figure 2 embodiment operates in much the same way except that the cycle of operations for loading the working section is rather more complex. -Assu~ing that the air flow has been set up at the correct flow rate and temperature,~ then wiih the shutters 15 in their solid llne positions the pac~age support tray 18 is then lo~ered and a bag 23, already containing a product, (for example a block of cheese, or a cut of meat, or other food article) is placed on the tray 18 with the bag neck in register with the clipping device 24. , '~he support tray 18 is then raised into the Figure 2 ; 25 posltion and ~-acuum is applied through the line~2l by ~ay ~ I of the vacuum pump 19. If desired, the application of vacu~

, ~ :r ~-`` 108B857 to the tray 18 may be achieved by providing a closure valve in the aperture 20 and opening this valve when it is desired to evacuate the ch~ber C.
Once the chamber C has ~een evacuated, and the residual air pockets within the bag 23 have been withdrawn by the application of vacuum to escape through th e neck of the bag, the clipping device 24 is operated to clip, and thereby completely close and seal off, the neck of the bag 23.
At this stage t~e application of vacuum through aperture 20 ceases (and if desired the chamber C may be vented before opening, by means of any suitable venting means, not .
shown).
The shutters 15 are then moved into the broken line positions 15' and this will allow hot, fast~moving air to : ~ :
pass over the package P conslsting of now evacuated bag 23 and the enclosed product to effect shrinkage.~ When the~
desired shrinkage time has~elapsed (say 2~seconds:) the shutters~
I5 are once again moved to their solid llne positions to :`
deflect the hot air flow through the by-pass section de~ined by the gap between the baffle 16 and the ceiling 14 of the ,, wor~ing section to enable the pacXage support tray 18 to be .

lowered for release of the vacuum sXin package P, and to allow replacement by a fresh loaded bag.
In:~accordance wlth~our Britlsh~Patent~No, 1,353,157 it may;~be possible for automatic loading o~ the tray 18 to b~
: effected~if the product and~the bag 23 are moved, mouth : ~

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trailing, onto the support tray 1~ by any suitable conveyor mechanism so as to pass between two opposed gathering arms of the clipping mechanism 24. The gathering arms can then cooperate to effect gathering of the neck of the bag 23 after the tray 18 has been raised into its Figure 2 position and the vacuum has been applied to the chamber C by way of line 21.
Alternatively the introduction of bags into the .

tunnel, and removal therefrom, may be performed using a conveyor, for example a belt conveyor, a rol]er conveyor or a rod conveyor, to advance a stream of loaaed bags across the tunnel at the working or shrinking station.
Such an arrangement is illustrated in Figures 3 ~ and 4;where the return circuit 6 of the tunnel is in this case in the vertical plane. The entire tunnel is lined with thermally insulating material over the floor, ceiling and side walls.
~ he working section has a central portion of square cross-section having a ceiling 31 and an inlet ante-chamber 32 a on one side and an outlet ante-chamber 32 b on the other side. The two ante-chambers are closed off from atmosphere by respective yieldable curtains 33 a and 33 b whlch, althouah not completely air-tight, provide a sufficiently thorough obstruction to air inlet~(through curtain 33 a) and outlet (through curtain 33 b) as to avoid undue energy losses by escape of~the~hot shrink air through the curtains. The side , ~ ,, , : .

walls 34 of the tunnel are cut away at ~}~e worklng section to allow pac~ageS P to be passed transversely through the working section, in either continuous or intermittent manner, on a belt conveyor 35 which cornprises one continuous belt 36 passing around two main outrider auide rollers 37 and various smaller guide rol]ers shown in Figure 3.
As shown in Figure 4, a vertical duct 38, again of thermally insulating material, extends vertically upwardly from the fan region adjacent the downstream end 41 of the 0 working section in the lower tunnel portion. Alr passing around the return circuit 6, the whole o~ which is also themally insulated, then passes over a h~ating element 11 (Figure 3~ before passing back to the inlet end 96 of the working section of the tunnel to pass over the package P now 1`5 on the conveyor belt passing through the working section.
As in the embodiments of Figures 1 and 2, the direction of air flow through the workino section is coaxial between inlet 46 and outlet 41 consequently there will be a substantialIy square cross-section air jet passino throuah the working section between the ante-chamber 32 a and 32 _ and ,.
having a cross-sectional area corresponding to the area bounded by the ceiling 31, the floor 31 a, and the two side walls 34 of the working section. There ~ill be little or no movement of air in the ante-chambers 32 a and 32 b and it will 25 therefore be possible to ~pen the inlet curtain 33 a for admlssion of a fresh package P to be shrunk or the outlet : -14- ; :

8~3s7 curtain 33 b for removal of a packa~e just shrun~., without causing any serious perturbation to the flow through the working section.
In contrast to the embodiment of Figures 1 and 2, the embodiment of Figure 3 uses a centrifugal f an 39 driven by a 15 horse po~.~er electric motor ~0 capable of pumping 10,000 cubic metres per hour under a pressure head of 250 mm.
water gauge. In this case there is no reduction gearbox and the centrifugal fan 39 is driven by direct drive rom the motor 40.
As air leaves the downstream end ~1 of the lower run 42 of the tunnel, it enters a diffuser section 43 and then into a convergent-diveraent throat nozzle 4a to be discharged in the form of a slightly divergent jet directed towards the centre of the centrifugal fan 39 which then impels the air vertically upwardly along the vertlcal duct 38 and into the return circuit 6 as depicted by arrow 45.
The influence of the nozzle ,~4 discharging the outlet air flow from the lower tunnel section into the low pressure region near the centre of the centrifugal ran 39 is such as to maintain the flow of air bet~leen the inlet 46 and l~ ' the outlet 41 of the lower section 42 of the tunnel in substantially coaxial configuration therebv reducing the tendency for loss of ener9y in hot air passing through the inlet and~ outlet curtains 33 a and 33 k, respectively.
In the pxeferred case, the heating element 11 has ,~
an~output of 30 kw. to generate an air temperature o~ 250C., ' ~
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~0~85'7 and the conveyor belt 36 is moving at a speed of 12 metres per minute giving a dwell tirne of appro~imately 2 seconds between (a) arrival of the .irst portion of the package P
leaviny the inlet ante-chamber 32 a and e~tering the stream of hot air through the working section 30, and (b) departure of that same portion of the package P from the hot air stream as the package enters the outlet ante-chamber 32 _. In this case the working section 30 is approximately 0 4 metres in both width and height.
The enhanced efficiency of shrinkage using air, rather than much higher thermal capacity media such as waterl is due to the relatively high mass flow rates of air avallable with a return flow shrink tunnel and also the conservation of heat by both thermally insulatina the tunnel walls and ensuring that all the air from the worklng section is~returned over the heater 11' to replinish the heat guantity carried by the air before it next contacts a package P.
By driving the shrinkage air axially along the ;
., ~
~orking section of the tunnel and having the directions of entry of the hot alr into the working section and withdrawal of the spent air f~om the working~section in a coaxial, or substantially coaxial, arrangement, as shown in Figure 1, Figure 2, and Figure 3 (see outlet nozzle 44), it is ensured that high mass flow rates of air can be achieved and consequently a high effective thermal capacity of the :: : :
~shrinkage flow can be realised. ~
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. ~3 1U~8857 l~ith this el~odiment also a thermal transducer can be installed in the air flow, preferably just after the heater, to give an indication of the temperature of the air passing from the heater. The output from this thermal transducer can be used to control the speed of the fan and/or the energisation of the heater whereby operation of the shrink tunnel can be properly controlled to give reliable operation in use thereof.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A shrink tunnel for use in shrink packaging articles, comprising:
means defining a work section with an inlet end and an outlet end;
means defining a return section communicating with said inlet end and said outlet end;
velocity boosting means located within said means for defining said return section for boosting the velocity of air traveling through said return section;
heating means for heating said air located within said means defining said return section; and control means responsive to air temperature for controlling said velocity boosting means.

2. A shrink tunnel according to claim 1 and including an air heater element and a fan which both carries out said velocity boosting and induces air flow across said heater.

3. A shrink tunnel according to claim 2, wherein said control means is effective to control the energy supply to said heating means.

4. A shrink tunnel according to claim 1, wherein the working section includes a support member mounted for movement into and out of the working section for bringing a closed, heat-shrinkable bag into the path of hot shrink air passing through the working section.

5. A shrink tunnel according to claim 1, wherein said working section communicates with evacuation means, and includes means for alternately diverting the hot shrink air around the evacuated section of said working section while the package is being evacuated and closed or passing it through the vacuum chamber to shrink an already evacuated and closed package.

6. A shrink tunnel according to claim 5, wherein said closing means comprises bag clipping means.

7. A shrink tunnel according to claim 5, wherein said means for alternately diverting comprise a pair of shutters having a first position in which they guide all the hot shrink air to one side of a baffle and through the location where a filled bag will, in use be positioned, and a second position in which they define together with the baffle a vacuum chamber past the outside of which the air is caused to flow.

8. A shrink tunnel according to claim 1, further including a conveyor for carrying packages transversely through the working section, an inlet ante-chamber through which said conveyor passes the packages before entering the working section, and an outlet ante-chamber through which the conveyor passes the packages after leaving the working section, and means for closing said inlet and outlet ante-chambers from atmosphere.

9. A shrink tunnel according to claim 8, wherein said means for closing said inlet and outlet ante-chambers to atmosphere comprise flexible curtains provided with slits defin-ing parallel fingers which yield to allow a package to pass through the curtain while normally resisting passage of air through the certain.

10. A shrink tunnel according to claim 7 or 8, wherein said return section is in the vertical plane and said conveyor belt moves the packages in a horizontal direction.