A New Method for Estimating the Clamping Force of

materials Article

A New Method for Estimating the Clamping Force of Shrink Sleeve Labels Jarosław Szusta 1,2 , Adam Tomczyk 1,2 and Özler Karaka¸s 3, * 1

2 3

*

Faculty of Mechanical Engineering, Department of Mechanics and Applied Computer Science, Bialystok University of Technology, Wiejska 45C Str., 15-351 Białystok, Poland; [email protected] (J.S.); [email protected] (A.T.) Masterpress S.A., Kuronia 4 Str, 15-569 Białystok, Poland Mechanical Engineering Department, Engineering Faculty, Pamukkale University, 20070 Kinikli, Denizli, Turkey Correspondence: [email protected]; Tel.: +90-258-296-3228

Received: 16 November 2018; Accepted: 10 December 2018; Published: 14 December 2018

Abstract: The paper presents an original method for estimating the shrink sleeve label compressive force on packaging. One of the most popular methods of measuring deformations was used, i.e., the electrical resistance strain gauge measurement. It was assumed that the packaging was a thin-walled axially symmetrical vessel. The packing walls on one side are loaded with internal pressure generated by heating the liquid contained inside the packaging. On the other side, the film shrinking on the packaging generates additional deformation. By measuring the changes in circumferential deformations in the shrinking process at various packaging heights, it is possible to infer the uniformity of the film compressive force. Results of research on changes of these deformations over time with different intensity values of the shrinkage medium were presented. Keywords: shrink sleeve; thin film; principal strains; strain gauge; thin-walled container

1. Introduction Heat shrink films are now widely used in various industries to produce all kinds of labels used on glass, metal and plastic packaging (Figure 1). The labels on bottles that are pleasing to the eye, making the item more attractive and appealing to the customer in the shops. This often determines the marketing success of the product and thus the manufacturer’s income. Along with the relevant developments in the production technology of heat-shrinkable sleeve labels, this method gained distinct advantages over other forms of labeling. Among the many essential advantages, we can distinguish the following:

• • • • • • • • • •

excellent visual effects; greater design options when compared to traditional printing methods; the possibility of covering the entire surface of containers even with complicated, unusual shapes, for which it would be difficult to adapt self-adhesive labels; saving costs compared to using many labels on the container; additional benefits resulting from the possibility of securing the entire container; the possibility of printing also on the inner part of the sleeve; ensuring complete resistance to abrasion of prints by printing the inside of the sleeve; increasing the brands’ impact through excellent presentation at sales points; ideal properties to adapt to marketing campaigns, such as so-called multipacks; possibility of making perforations, breakable parts, coupons, etc.;

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cost-effective low-volume printing in flexographic technology.

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Figure from resources resourcesof ofMasterpress MasterpressS.A.). S.A.). Figure1.1.Sample Samplepackaging packagingwith withheat-shrink heat-shrink film film labels. labels. (Adapted from

advantages, it is itsafe predict a significant increase in the scale production Based on onthese theseclear clear advantages, is to safe to predict a significant increase inofthe scale of of shrink sleeve labelssleeve in the labels coming In fact,years. it would be reasonable state that the labelsthat of this production of shrink inyears. the coming In fact, it would betoreasonable to state the type will dominate the packaging of products in the near future. labels of this type will dominate the packaging of products in the near future. Obtaining the desired visual effect depends on a number of factors, such as the film shrinkage process parameters, parameters, the the shape shapeand andsize sizeofofthe thepackaging packagingororthe theselection selection material with respect of of material with respect to to chemical and strength properties.Knowledge Knowledge materialcharacteristics characteristicsininturn turn allows allows for for the chemical and strength properties. ofofmaterial parameters of the film printing process and its subsequent shrinking on application of appropriate parameters packaging. The selection of suitable is is primarily determined by by thethe strength of the packaging. The selection of suitablestrength strengthparameters parameters primarily determined strength of filmfilm compressive force, its corresponding deformation, lacklack of undesirable cracks etc. etc. the compressive force, its corresponding deformation, of undesirable cracks Shrink sleeve films are of interest to many authors. This has been the case since the beginning of the first applications of this type of film in 1958, when it was possible to combine styrene with provides a number number of works works on tests of various various properties, properties, synthetic rubber [1,2].The literature provides including mechanical properties of the films. An An extensive extensive review review of of various various films films in terms of strength parameters can be found in work of [3]. Strength tests of polymers have also been described studiesby byAvérous Avérousetetal. al.[4], [4],Matzinos Matzinos al.Srinivasa [6], Srinivasa and Tharanathan [7], in studies et et al. al. [5],[5], Mo Mo et al.et[6], and Tharanathan [7], Tserki Tserki et al. [8,9], Wang et al. [10], Zhang et al. [11], Zhang et al. [12], Szusta et al. [13]. Many studies et al. [8,9], Wang et al. [10], Zhang et al. [11], Zhang et al. [12], Szusta et al. [13]. Many studies placed placed emphasis the cheapest most commonly availablematerials materialsused usedfor for labels, labels, emphasis on oneonofone theof cheapest and and most commonly available starch-based polymers [4,10]. [4,10]. It should also be noted that the method of manufacturing polymer films has a significant effect on their mechanical properties. This particularly pertains to cross-linked polymers such as PE, PP, PP, acrylate, acrylate, where the cross-linking cross-linking method has a decisive influence on the aforementioned properties [14,15]. Very good mechanical properties are obtained for oriented layered aforementioned properties [14,15]. Very good mechanical properties are obtained for oriented polymers [16]. The number of layers in materials of thisoftype falls within the range of 2–7. layered polymers [16]. The number of layers in materials this usually type usually falls within the range of Besides strength properties, muchmuch attention is alsoisdevoted to shaping optimal opticaloptical and visual 2–7. Besides strength properties, attention also devoted to shaping optimal and properties [17,18].[17,18]. visual properties the packaging is assessed by by visual inspection of the The quality quality of ofthe thelabel labelshrinking shrinkingprocess processonon the packaging is assessed visual inspection of outer container with with the label at the present time. The adhesion of the label to the container the outer container the applied label applied at the present time. The adhesion of the label to the walls is checked the compressive force is assessed byassessed the trial involving or rotating container walls isand checked and the compressive force is by the trialmoving involving movingthe or label on the Such verification not objective does not give unambiguous results. There rotating the container. label on the container. Suchisverification is and not objective and does not give unambiguous results. There is a lack of more accurate methods to assess the uniformity of the compressive force of the label on the packaging. The main purpose of the manuscript is to present a new, original method, which is quantitative rather than just qualitative, in order to improve shrinkage process assessment. This method can be used to accurately estimate the film clamping force, and above all the uniformity

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is a lack of more accurate methods to assess the uniformity of the compressive force of the label on the packaging. The main purpose of the manuscript is to present a new, original method, which is quantitative rather than just qualitative, in order to improve shrinkage process assessment. This method can be used to accurately estimate the film clamping force, and above all the uniformity of the force. In addition, using this method of preliminary assessment, the optimal parameters of the shrinking process such as temperature, the intensity of the working fluid stream or the distribution of the working fluid sources inside the shrink tunnel can be determined. One of the essential elements affecting the energy intensity of the label’s shrinkage process is the shape and size of the packaging on which the heat shrink film is applied. It is reasonable that one should strive for a shape that will facilitate uniform distribution of the film compressive force on the packaging (e.g., a bottle) in the direction of the package axis i.e., in the vertical direction, and will have an appropriate value. The film can be properly adhered to the packaging walls by the appropriate selection of these two parameters, i.e., the value of the shrinkage force and uniformity of shrinkage in the vertical direction. This prevents deformation of the film itself as well as the packaging, for example due to too tight compression of the film. Deformations primarily result in various distortions of graphics that are already on the shrunk film. Consequently, incorrectly compressed film can make the final product visually unattractive to the prospective buyer. 2. Materials and Methods 2.1. Production Process of a Heat-Shrink Label The production process of the heat-shrink label begins with the selection of the type of film on which the imprint will be applied. The most popular materials used for the production of the sleeve are: polyolefin, PP, PE, PVC, OPS and PET. PET heat-shrinkable film is ideal for general-purpose packaging due to its properties such as:

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low price and availability worldwide; high gloss and transparency; high strength and elasticity—the shape of the packaged products do not warp and provides good formability; high shrinkage at low temperatures in a wide range of shrinkage ratios; cold resistance, making it suitable for labeling products stored at low temperatures.

Heat-shrinkable films are usually printed using the flexographic printing technique with appropriate paints. According to general trends in flexographic printing, the most commonly used paints are curable paint fixations and, more often, radical-fixed paints. Their advantages, such as high gloss, elasticity, as well as resistance to low and high temperatures, make them particularly well suited for printing heat-shrinkable sleeves. The most important feature that determines the type of paint used is its ability to shrink with the substrate. The transverse shrinkage often required for atypical shapes can reach 70%. The technological process of packaging production in shrink sleeve technology is multi-stage. First, the foil web is printed. In the next stage, after gluing the edges of the web, a sleeve is formed which is then wound up so that a role is created. Afterwards the sleeve is cut on the so-called “utilities”, i.e., individual labels, which are then manually or machine-applied to the packaging. The last procedure in the production process is shrink sleeve packaging process. This is usually done in a special tunnel where, using heated air or steam, the labels shrink the packaging thoroughly. Figure 2 shows the production and application process of the shrink-sleeve label on the packaging.

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Figure 2. The production and application process of the heat-shrinkable label. (Adapted from resources Figure 2. The production and application process of the heat-shrinkable label. (Adapted from of Masterpress S.A.). resources of Masterpress S.A.).

2.2. Measurement Method 2.2. Measurement Method 2.2.1. Axially Symmetrical Thin-Walled Vessels 2.2.1. Axially Symmetrical Thin-Walled Vessels Most of the packaging on which the shrink sleeve labels are applied are axially symmetrical. It of thethat packaging which the axially shrink sleeve labels vessels are applied arewith axially symmetrical. can beMost assumed these areonthin-walled symmetrical loaded internal pressure.It can be assumed that these are thin-walled axially symmetrical vessels loaded with internal This pressure is generated by the presence of fluid inside the container. It can additionally pressure. increase This pressure by the presence of fluidthe inside thetunnel. container. can additionally increase its volume dueistogenerated the temperature increase inside shrink ThisItresults in the appearance its volume due to the temperature increase inside the shrink tunnel. This results in the appearance of of positive container wall deformations. At the same time, additional deformations are usually positive container wall deformations. At the same time, additional deformations are usually generated with the opposite sign as a result of the film compression. Therefore, we deal with an generated with thethin-walled opposite sign as loaded a resultbyofpressure the filmevenly compression. Therefore, we to deal an axially symmetrical vessel distributed with respect thewith axis of axially symmetrical thin-walled vessel loaded by pressure evenly distributed with respect to the axis symmetry. In such cases, it can be assumed that the stress in the walls of the container corresponds to of plane symmetry. In such cases, it can be assumed thatlaws theexpressed stress in inthe walls of the container the stress case (Figure 3). The generalized Hooke’s terms of principal stresses corresponds to theform plane(e.g., stress case (Figure 3). The generalized Hooke’s laws expressed in terms of have the following [19]): principal stresses have the following form (e.g., [19]): E E σ2 = (1) σ1 = (ε + vε 2 ), E2 (ε 2 + vε 1 ) 2E 1 1 − v 1 − σ1 = σ 2 = v 2 ( ε 2 + vε 1 ) (1) ( ε 1 + vε 2 ) , 2 1− v 1− v where ε1 , ε2 —principal strains, σ1 , σ2 —principal stresses, E—Young’s modulus, ν—Poisson’s ratio. where ε1, εcircumferential 2—principal strains, σ1, σ2—principal stresses, the E—Young’s modulus, ratio. When deformations ε1 are known, compressive force ν—Poisson’s of the film and the Whenofcircumferential deformations ε1 are known, the compressive force of the film and the uniformity the compression can be estimated. uniformity the compression can beprovided estimated. With theofstress value given by the Equation (1), it is possible to propose the determination the stressclamping value given bya force the provided (1), of it the is container. possible to proposethat the of the With circumferential force as acting in aEquation narrow band Assuming determination of thetocircumferential force as direction, a force acting a narrow band of the this band is subjected compression in clamping the circumferential this F1inforce can be determined container. Assuming that this band subjected to compression in the circumferential direction, this by multiplying the stresses σ1 by the is cross-sectional area S of the band: F1 force can be determined by multiplying the stresses σ1 by the cross-sectional area S of the band: F1 = σ1 S (2) F1 = σ 1 S (2) However, it should be noted that, in fact this force is not the force of the film clamping, but However, it should be noted that, in fact this force is not the force of the film clamping, but the the force generated in the wall of the packaging due to the clamping of the film. But of course, force generated in the wall of the packaging due to the clamping of the film. But of course, the nature the nature of changes in this force is the same as the nature of changes in the clamping force of the of changes in this force is the same as the nature of changes in the clamping force of the film. On the film. On the other hand, the force in the film has relatively higher value. other hand, the force in the film has relatively higher value.

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Figure 3. Schematic representation of principal strains in the walls of a sample axially symmetrical Figure 3. Schematic representation of principal strains in the walls of a sample axially symmetrical thin-walled vessel. thin-walled vessel.

2.2.2. Electrical Resistance Strain Gauge Measurement 2.2.2. Electrical Resistance Strain Gauge Measurement The most popular method of measuring deformations is the electrical resistance strain gauge The most(e.g., popular of measuring deformations is the electrical strainrosettes gauge measurement [20]).method If the values of the principal strain directions are notresistance known, strain measurement (e.g., [20]). If the values of the principal strain directions are not known, strain rosettes are usually used. However, in the analyzed case of the axially symmetrical vessels it can be assumed are usually used. However, in the(Figure analyzed of the axially vessels can be assumed that these directions are known 3). case Therefore, linear symmetrical strain gauges were it used. They were that these directions known (Figure 3). Therefore, linear strain gauges were used. They were applied only along theare circumferential direction 1. Direction 2 was omitted in measurements because applied only along the circumferential direction 1. Direction 2 was omitted in measurements because the film shrinks only in the circumferential direction. In the vertical direction, the film shrinkage thenegligible. film shrinks only in the circumferential direction. In the vertical direction, the film shrinkage is is negligible. The probability of gluing the strain gauge ideally along the principal direction is low. However, gluing strain gauge along axis the principal low. this isThe notprobability a problem,of since thethe deviation of the ideally strain gauge from thisdirection directionisby up However, to several this is not a problem, since thechange deviation the strain gauge axisinfrom this concerned. direction byStrain up to gauges several degrees will not significantly the of measurement results the case degrees will not significantly change the measurement results in the case concerned. Strain were installed on the outer wall of the container at different heights along the main direction gauges 1. This were installed on the outer wall of the container at differentforce heights along the main direction 1. This allowed us to estimate the strength of the film compressive at different heights of the container, allowed us to estimate the strength of the film of compressive force at different heights of the container, and to determine the possible non-uniformity the compression. and to determine the possible non-uniformity of the compression. By studying the deformations for various configurations of nozzles, e.g., steam nozzles and their By shapes, studying thepossible deformations for various configurations of nozzles, steam nozzles and various it is to determine the most optimal solution where e.g., the film compression at their various shapes, it is possible to determine the most optimal solution where the film different levels will be similar. Testing using the presented method allowed for continuous recording compression at deformations different levels willhence be similar. Testing the presented method allowed for and analysis of (and stresses). Thus,using changes could be estimated in the film continuous recording analysis of of the deformations stresses). Thus, changes could be compressive force fromand the moment steam blow(and untilhence cooling. estimated in the film compressive force from the moment of the steam blow until cooling. 2.2.3. Test Stand 2.2.3. Test Stand The proposed method uses a reference container (1) (Figure 4), where strain gauges (2) are installed The proposed methodinuses a reference (1) of (Figure 4), where strainon gauges (2) are to analyze the deformations the plastic parts.container The number strain gauges depends the container installedThe to strain analyze the deformations parts. Theofnumber of strain gauges depends height. gauges were located in in the plastic height direction the container along the helical line on to the container height. The strainofgauges werepackaging. located in Strain the height direction of installed the container the cover the entire circumference the tested gauges (2) were alongalong the first helical line to coverinthe circumference of the tested packaging. Strain gauges (2) were installed principal direction theentire half-bridge system, i.e., each of the strain gauges had a compensator (3) that along the onto first principal the half-bridge system, i.e., each material. of the strain had a was glued a piece of direction unloaded in material (4) identical to the container Suchgauges arrangement compensator that was glued onto atemperature piece of unloaded material to the allows for the (3) compensation of elevated generated during (4) theidentical steam blow, e.g.,container steam in material. Such arrangement allows for the elevated temperature generated during the tunnel. Additionally, after installing allcompensation strain gauges of and necessary electrical wires, the gauges the steam blow, steamelectrical in the connectors tunnel. Additionally, afterwith installing strainpolyurethane gauges and themselves and thee.g., exposed were protected a specialallflexible necessary electrical wires, gauges themselves the exposed electrical connectors were coating against the effects of the moisture generated insideand the tunnel. protected with aofspecial flexiblemust polyurethane coating against the effects generated The location strain gauges be sufficiently distant from places whereofthemoisture stress distributions inside the tunnel. in the container walls may be inhomogeneous. The areas in the immediate vicinity of the bottom of the The location of outlet, strain as gauges be sufficiently from places from where stress container, its threaded well asmust the minimal thicknessdistant of the wall resulting the the process of distributions in thematerial container walls inhomogeneous. The areas in the immediate vicinity of injecting the liquid into the may split be mold should be avoided. the bottom of the container, its threaded outlet, as well as the minimal thickness of the wall resulting from the process of injecting the liquid material into the split mold should be avoided.

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Figure Schematic representation representation of of aaa container container with with the the attached attached strain strain gauges gauges on stand: Figure 4. 4. Schematic Schematic representation of container with the attached strain gauges on aa stand: 3—compensating strain gauges, 4—unloaded material, 5—base, 1—container, 2—strain gauges, 3—compensating 4—unloaded material, 5—base, 1—container, 2—strain gauges, 3—compensating strain gauges, 4—unloaded material, 5—base, 6—mandrel, 6—mandrel, 7—thermocouples, 7—thermocouples, 8—magnet, 8—magnet, 9—wiring. 9—wiring.

In In order order to to ensure ensure the the stability stability of of the the container container moving moving inside inside the the heat heat tunnel tunnel with with the the gauges gauges installed, and to allow the temperature to be recorded in the vicinity of each strain gauge, the container the installed, and and to to allow allow the the temperature temperature to to be be recorded recorded in in the the vicinity vicinity of of each each strain strain gauge, gauge, the was mounted on a stand (Figure 4) consisting of the base (5), the mandrel (6) and the installed container was mounted on a stand (Figure 4) consisting of the base (5), the mandrel (6) and the container was mounted on a stand (Figure 4) consisting of the base (5), the mandrel (6) and the measuring thermocouples (7). installed thermocouples (7). installed measuring measuring thermocouples (7). base weight should provide container position when moving in the shrink tunnel (10) The base weight should provide container position when moving in shrink tunnel The base weight should providea astable a stable stable container position when moving in the the shrink tunnel (Figure 5) between the steam nozzle systems (11). The container (1) is mounted on the base of the stand (10) (Figure 5) between the steam nozzle systems (11). The container (1) is mounted on the base (10) (Figure 5) between the steam nozzle systems (11). The container (1) is mounted on the base of of by of magnet (Figure (8) 4) its bottom. wiring (9) attached mandrel the stand of aa magnet (Figure 4) inside its The (9) attached to themeans stand by byameans means of(8) magnet (8)located (Figureinside 4) located located inside The its bottom. bottom. Theiswiring wiring (9)tois isthe attached to (6). solution wiring movements between the stand pin andthe the stand container, the mandrel (6). This eliminates wiring movements between and the the This mandrel (6). eliminates This solution solution eliminates wiring movements between the stand pin pin and andthus the eliminates the impact of these movements on the deformation of the container walls. The signal from container, and thus eliminates the impact of these movements on the deformation of the container container, and thus eliminates the impact of these movements on the deformation of the container the strain gauges transmitted to a special HBM Quantum MX840A (Darmstadt, Germany) walls. The signal the gauges is to 8-channel HBM walls. The signalisfrom from the strain strain gauges8-channel is transmitted transmitted to aa special special 8-channel HBM Quantum Quantum measuring amplifier (12) (Figure 5). Data is logged via a portable computer (13) with specialised MX840A (Darmstadt, Germany) measuring amplifier (12) (Figure 5). Data is logged via a MX840A (Darmstadt, Germany) measuring amplifier (12) (Figure 5). Data is logged via a portable portable software for operating the amplifier. computer (13) specialised software (Catman DAQ) computer(Catman (13) with withDAQ) specialised software (Catman DAQ) for for operating operating the the amplifier. amplifier.

1—shrink wrapped film film tested, tested, 10—shrink tunnel, Figure Figure 5. 5. Schematic Schematic view view of of the the measuring measuring stand: stand: 1—shrink 1—shrink wrapped wrapped film tested, 10—shrink 10—shrink tunnel, tunnel, 11—nozzle layout fed from a steam generator, 12—measuring amplifier, 13—portable computer. generator, 12—measuring amplifier, 13—portable computer. 11—nozzle layout fed from a steam generator, 12—measuring amplifier, 13—portable computer.

The essence essence of the the study is is to set set up aa stand stand (including aa container container with strain strain gauges on on which The The essence of of the study study is to to set up up a stand (including (including a container with with strain gauges gauges on which which heat shrink film is applied) on a belt conveyor before entering the shrink tunnel and starting the heat heat shrink shrink film film is is applied) applied) on on aa belt belt conveyor conveyor before before entering entering the the shrink shrink tunnel tunnel and and starting starting the the conveyor and recording equipment. As the conveyor moves, the tooled container moves inside the conveyor and recording equipment. As the conveyor moves, the tooled container moves inside the conveyor and recording equipment. As the conveyor moves, the tooled container moves inside the tunnel, passing passing throughheating heating sections.AsAs a result of the appropriate sequenceheat of heat impacts, tunnel, tunnel, passing through through heating sections. sections. As aa result result of of the the appropriate appropriate sequence sequence of of heat impacts, impacts, the the the film compressed on the container exerts pressure on its walls. Film compression is recorded by film compressed on the container exerts pressure on its walls. Film compression is recorded by strain film compressed on the container exerts pressure on its walls. Film compression is recorded by strain strain gauges as circumferential deformations at various levels. During the testing and calibration gauges gauges as as circumferential circumferential deformations deformations at at various various levels. levels. During During the the testing testing and and calibration calibration of of the the of the test stand, several dozen tests were performed. However, the results of seven of them were test test stand, stand, several several dozen dozen tests tests were were performed. performed. However, However, the the results results of of seven seven of of them them were were presented in the manuscript. presented presented in in the the manuscript. manuscript.

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shouldbe benoted notedthat thatthe thespecific specificdesign designand andshape shapeof ofthe thesteam steamnozzles nozzlesin inthe theshrink shrinktunnel tunnelisis ItItshould appropriate for the packaging of a particular shape and size. With the use of presented measurement appropriate for the packaging of a particular shape and size. With the use of presented measurement methodcontainers containersof ofdifferent differentshapes shapesand andsizes sizescan can be be analyzed. analyzed.At Atthe thesame sametime, time,testing testingthe thesame same method packaging with different configurations of the nozzle system and different nozzle shapes will optimize packaging with different configurations of the nozzle system and different nozzle shapes will this layout. optimize this layout. The primary test bottle of of 500500 mLmL capacity withwith dedicated heat shrink film. Strain The primary test piece piecewas wasa aPET PET bottle capacity dedicated heat shrink film. gauges were installed on the walls of the bottle. The tests were carried out with the use of special strain Strain gauges were installed on the walls of the bottle. The tests were carried out with the use of gaugesstrain 1-LY18-3/120A manufacturedmanufactured by HBM (Figure intended for6), analyzing in special gauges 1-LY18-3/120A by 6), HBM (Figure intendedthe fordeformations analyzing the plastic elements. Strain gauges were glued using special, elastic Z 70 adhesive manufactured by HBM. deformations in plastic elements. Strain gauges were glued using special, elastic Z 70 adhesive Exposed wiringby harnesses were covered withharnesses two layerswere of PUcovered 140 polyurethane to manufactured HBM. Exposed wiring with twoprotective layers ofcoating PU 140 protect them from exposure to moisture. Strain gauges installed in a half-bridge polyurethane protective coating to protect them from were exposure to moisture. Strain configuration, gauges were as shown in Figure 7. installed in a half-bridge configuration, as shown in Figure 7. shouldbe betaken takeninto intoconsideration considerationthat thatthe theinfluence influenceof ofconnecting connectingwires wireson onthe themeasurement measurement ItItshould resultscannot cannotbe beexcluded. excluded.Determining Determiningthe theamount amountof ofthe theinfluence influenceof ofthese thesewiring wiringwould wouldrequire require results measuring the strain with other methods, preferably non-contact, which in the conditions of the steam measuring the strain with other methods, preferably non-contact, which in the conditions of the tunneltunnel wouldwould be extremely difficult or can or even impossible. However, it is also to note steam be extremely difficult canbe even be impossible. However, it important is also important that all that tests all were carried with out the same wiring configuration. Thus, the possible error was always to note tests wereout carried with the same wiring configuration. Thus, the possible error the same and negligible. was always the same and negligible.

(b)

(a)

(c)

Figure 6. Configurations strain gauges on test container: (a) a (a) bottle with strain Figure Configurations ofofthe theattached attached strain gauges on test container: a bottle with gauges strain glued on three heights;heights; (b) a single compensating strain gauges. gauges glued ondifferent three different (b) astrain singlegauge; strain (c) gauge; (c) compensating strain gauges.


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Figure 7. Diagram of connection of strain gauges in a half-bridge configuration. Figure 7. Diagram of connection of strain gauges in a half-bridge configuration. Figure 7. Diagram of connection of strain gauges in a half-bridge configuration.

A single bottle is fitted with three strain gauges mounted at three different heights (Figure 4). A single bottle is fitted with three strain gauges mounted at three different heights (Figure 4). single bottle is fitted withare three strain gauges mounted three different heights (Figure 4). Further inAthis paper these gauges numbered, starting fromatthe bottom of the bottle: the bottom Further in this paper these gauges are numbered, startingfrom fromthe the bottom of the bottle: the bottom Further in this paper these gauges are numbered, starting bottom of the bottle: the bottom gauge, the middle gauge and the top gauge. It is worth mentioning that, as in mass production, the gauge, the middle gauge and the It worthmentioning mentioning that, as mass in mass production, the thewith middle gauge and thetop topgauge. gauge. It is is worth that, as in bottlegauge, is filled liquid (water) which prevents permanent deformation of the production, material inthe contact bottlebottle is filled with liquid (water) which prevents permanent deformation of the material in contact is filled with liquid (water) which permanent the material in three contactsteam with hot steam. The study was carried outprevents in a shrink tunneldeformation (Figure 8a)of equipped with with with hot steam. TheThe study was carried tunnel(Figure (Figure equipped hot steam. study was carriedout outin in aa shrink shrink tunnel 8a)8a) equipped withwith threethree steamsteam sections with each section composed of four systems of steam nozzles in an adjustable position for a sections section composedofoffour four systems systems of in in an an adjustable position for a for a sections withwith eacheach section composed ofsteam steamnozzles nozzles adjustable position total of twelve steam sections. Each of valves allowed independent control over the amount of steam of twelve steam sections. Eachofofvalves valves allowed allowed independent control over the the amount of steam total total of twelve steam sections. Each independent control over amount of steam generated and and the the position of of each section from the control panel(Figure (Figure 8b). generated position each section from the control panel 8b). generated and the position of each section from the control panel (Figure 8b).

(a)

(b)

Figure 8. Photographs the shrinktunnel tunnel in which carried out;out; (b) the panelpanel Figure 8. Photographs of: of: (a)(a) the shrink whichthe thetest testwas was carried (b) control the (b) control (a) of the steam flow rate in a sample section of the tunnel. of the steam flow rate in a sample section of the tunnel. Figure 8. Photographs of: (a) the shrink tunnel in which the test was carried out; (b) the control panel solution allows adjusting the steam flow to packages of varying heights shapes. This solution allows adjusting the steam flow to packages of varying heights andand shapes. During of theThis steam flow rate in a sample section of the tunnel. During the tests, the steam nozzle systems were arranged at equal distances along the length of the the tests, the steam nozzle systems were arranged at equal distances along the length of the tunnel. tunnel.solution However, the vertical position of steam the nozzles was to the of the tested object. It This allows adjusting the to adjusted packages of shape varying heights andIt shapes. However, the vertical position of the nozzles wasflow adjusted to the shape of the tested object. should should be acknowledged that tunnels with different numbers of steam sections are used in industry. During the tests, the nozzle arranged at equal distances along the length of the be acknowledged thatsteam tunnels withsystems differentwere numbers of steam sections are used in industry. The “0” value set on the valve of each section corresponds to the total steam cut off, while “10” tunnel. However, the position of thesection nozzles was adjusted to shape of the tested object. It The “0” value setvertical on the valve of each tovarious thethe total steam cut off,settings while “10” indicates the maximum stream flow. The tests werecorresponds carried out for vapor flow rate should be acknowledged that tunnels with different numbers of steam sections are used in industry. indicates the maximum stream flow. The tests were carried out for various vapor flow rate settings in in each section, however in line with the tunnel operator’s long-term experience, which means that “0” however value setthe oncontrol the with valve of were each close section corresponds the totalused steam off, while each The section, in line the tunnel operator’s long-term experience, which that“10” the the values set on panel to the values thattohad been so cut far means with good

indicates maximum streamwere flow. Thetotests outbeen for various flow rate settings values set the on the control panel close the were valuescarried that had used so vapor far with good shrinkage in each section, however in line with the tunnel operator’s long-term experience, which means that the values set on the control panel were close to the values that had been used so far with good

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quality. The values of settings used for the tests are shown in Table 1. Cut sleeve film with a diameter of 68 Materials mm was used for testing. 2018, 11, x FOR PEER REVIEW 9 of 14 1. Valve settings individual steam subsequent measurements. shrinkageTable quality. The values offor settings used for the sections tests arein shown in Table 1. Cut sleeve film with a diameter of 68 mm was used for testing. Valve Settings for Individual Steam Sections Measure Series Table 1. Valve settings for 1 2individual 3 4steam5sections 6 in7 subsequent 8 9 measurements. 10 11 12 1 Measure Series 2 3 1 4 2 5 3 6 4 7 5

3. Results

6 7

Valve Steam Sections 2.5 Settings for Individual 2.5 2.5 1 5 2 3 4 5 6 75 8 9 10 11 125 7 2.5 7 2.5 7 2.5 0 5 2.5 5 2.5 5 0 7 0 7 2.5 7 2 0 7 2.5 4 2.5 7 0 2 2.5 4 0 2 4 7 7 4 2

3. Results The tests consisted in continuous recording of deformation changes in three strain gauges affixed to the container walls whileinthe containerrecording was moving inside thechanges shrink tunnel. the gauges same time, The tests consisted continuous of deformation in three At strain affixed the container while the container was moving of inside shrink tunnel.the At tests the same it should betonoted that thewalls maximum ambient temperature the the gauges during (during time, blow) it should be noted the maximum ambient of the gauges during the tests the steam recorded bythat thermocouples varied by temperature 2–3 ◦ C for subsequent repetitions and was in ◦ (during the steam blow) recorded by thermocouples varied by 2–3 °C for subsequent repetitions and the range of 70–90 C. Therefore, no additional results of this temperature analysis are presented in was in Itthe range of 70–90 °C. Therefore, additional resultstunnel, of thisthe temperature are to a this report. is worth emphasizing that afterno leaving the steam containeranalysis was cooled presented in this report. It◦is worth emphasizing that after leaving the steam tunnel, the container temperature of approx. 30–31 C. During the cooling stage, the compressed film was removed from the was cooled to a temperature of approx. 30–31 °C. During the cooling stage, the compressed film was container with great caution so as not to damage the strain gauges. The view of the shrink-wrapped removed from the container with great caution so as not to damage the strain gauges. The view of container at the tunnel outlet andatafter cooling is shown in cooling Figure 9. the shrink-wrapped container the tunnel outlet and after is shown in Figure 9.

(a)

(b)

Figure 9. The stand with bottleand andshrunk shrunk film: film: (a) tunnel outlet; (b) after cooling. Figure 9. The stand with thethe bottle (a)atatthe thesteam steam tunnel outlet; (b) after cooling.

The first tests were intendedtotocheck check the the entire entire measuring system forfor correct function and and The first fewfew tests were intended measuring system correct function made it possible to obtain an overview of the function of the individual shrink sections of the tunnel. made it possible to obtain an overview of the function of the individual shrink sections of the tunnel. It should be emphasized that due to the violent impact of steam at elevated temperatures, water It should be emphasized that due to the violent impact of steam at elevated temperatures, water contained in a sealed container increased its volume thus causing an increase in the pressure inside contained in a sealed container increased its volume thus causing an increase in the pressure inside the container. This entailed an immediate strain gauge response, which is clearly indicated by a the container. This entailed an immediate strain gauge response, which is clearly indicated by a sharp sharp increase in deformations. At the same time, due to the increased temperature, the material of increase in deformations. At the same due to the increased temperature, the material the container (PET) shrank, which wastime, also recorded by strain gauges. In order to isolate the effectsof the container (PET) shrank, was also recorded by strainmaterial gauges.heat In order to isolate theeffects effects of of increased pressure which inside the container and the container shrinkage from the

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increased pressure inside the container and the container material heat shrinkage from the effects of film compression, series measurements was carried out for the container with no film Materials 2018, 11, xaFOR PEERof REVIEW 10 and of 14 with the shrinking film on it. Each time the container was cooled down to a temperature of approximately film compression, a series of measurements was out forbottle the container with nomeasurements film and 30–31of◦ C, which prevented the continuous heating of carried water in the in subsequent with the the repeatability shrinking film Each time the Each container cooled of down to a temperaturethree of for and thus of on theit.measurements. serieswas consisted six measurements: approximately 30–31 °C, which prevented the continuous heating of water in the bottle in tests a container with no film and three for a container with shrink film on it. The results of these subsequent measurements and thus the repeatability of the measurements. Each series consisted of were averaged with respect to the strain gauges and are shown in Figure 10. The charts shown in six measurements: three for a container with no film and three for a container with shrink film on it. Figure 10a,c,e,g,i,k,m refer to subsequent measurements of deformation of the container walls with no The results of these tests were averaged with respect to the strain gauges and are shown in Figure 10. compressing film. The other charts, i.e., Figure 10b,d,f,h,j,l,n pertain to measurements with the film on. The charts

(a)

(b)

(c)

(d)

(e)

(f)

Figure 10. Cont.


11 of 14 11 of 14

(g)

(h)

(i)

(j)

(k)

(l) Figure 10. Cont.


(m)

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(n)

Figure 10. Averaged Averaged results results obtained obtained for for different valve settings controlling the individual steam 6; 6; (m,n) series 7. sections: (a,b) series 1; (c,d) (c,d) series series 2; 2;(e,f) (e,f)series series3;3;(g,h) (g,h)series series4;4;(i,j) (i,j)series series5;5;(k,l) (k,l)series series (m,n) series

7.

4. Discussion

4. Discussion The results of all series of tests show that the steam impact on the container passing through the first caused a sharp increase insteam deformation at all levelspassing analyzed. In most The active resultssection of all series of tests show that the impact on thethree container through the cases, the maximum deformation level at three different heights is similar. However, the level of these first active section caused a sharp increase in deformation at all three levels analyzed. In most cases, deformations different forlevel different series. A sharp increase in deformation is due an of increase the maximumisdeformation at three different heights is similar. However, the to level these in the pressure inside the container as a result of water being heated inside the container. After deformations is different for different series. A sharp increase in deformation is due to an increasethe in increase, a sharp decrease in deformation occurs. This should be explained by the heat shrinkage of the the pressure inside the container as a result of water being heated inside the container. After the material of whichdecrease the container (PET) was occurs. made. However, thisbe shrinkage takes place a moment after increase, a sharp in deformation This should explained by the heat shrinkage of the pressure increase due to the heating of water inside the container and is clearly dependent on the the material of which the container (PET) was made. However, this shrinkage takes place a moment valvethe settings of the individual sections. Any further in deformation more gradual after pressure increase due to the heating of waterdecrease inside the container andisismuch clearly dependent due to the persistence of the elevated temperature at a relatively constant level. Yet, the oscillations on the valve settings of the individual sections. Any further decrease in deformation is much more of the deformation waveforms of over can be seen clearly the container is passing gradual due to the persistence thetime elevated temperature at as a relatively constant level. through Yet, the subsequent steam sections. This involves a slight decrease in temperature in the areas between the oscillations of the deformation waveforms over time can be seen clearly as the container is passing steam nozzle outlets of the individual sections. This is particularly evident in Figure 10a,c,e,g where through subsequent steam sections. This involves a slight decrease in temperature in the areas each of the to the vapor impact in each section. each case strain between thepeaks steamcorresponds nozzle outlets of the individual sections. This is Eventually, particularlyinevident in Figure gauges always confirmed the presence of compression in the container walls. The degree of 10a,c,e,g where each of the peaks corresponds to the vapor impact in each section. Eventually,this in compression depended such factors as the flowof rate. each case strain gauges on always confirmed thesteam presence compression in the container walls. The Forofidentical settings of all steamon sections (Figure a steam degree this compression depended such factors as10a–f), the steam flowblow rate. is clearly marked at the height of each section. Such clear regularity cannot be observed in the otherisfigures. the For identical settings of all steam sections (Figure 10a–f), a steam blow clearly Increasing marked at the settingsoffrom to 5 and 7 caused a decreasecannot in the maximum value at other the first rapid Increasing load variation. height each2.5 section. Such clear regularity be observed in the figures. the It can be explained by the fact that a higher temperature caused a faster container material response, settings from 2.5 to 5 and 7 caused a decrease in the maximum value at the first rapid load variation. which compensated faster pressure due to water being heated.material At the same time, It can be explained byfor thethe fact that increase a higher in temperature caused a faster container response, for the compensated same first three the value of the at the end ofAt thethe measurement which forseries, the faster increase in compressive pressure duedeformation to water being heated. same time, increased as the settings increased, both for the container with and without the shrink for the same first three series, the value of the compressive deformation at the film endapplied. of the This is mainlyincreased due to higher of the container at higher However, measurement as theshrinkage settings increased, both formaterial the container withtemperatures. and without the shrink the effect of film compression is clearly noticeable here. Compression values for a container with the film applied. This is mainly due to higher shrinkage of the container material at higher film applied are always higher than for film-free containers. The biggest differences are always for temperatures. However, the effect of film compression is clearly noticeable here. Compression the upper while with the smallest are for thealways bottomhigher gauge.than An increase in thecontainers. settings caused values for gauge, a container the filmones applied are for film-free The higher divergence between the gauges located at three different heights. biggest differences are always for the upper gauge, while the smallest ones are for the bottom gauge. The charts shown in Figure correspond tobetween the situation of shortening thethree steam tunnel An increase in the settings caused10g–j higher divergence the gauges located at different by four sections (Figure 10g,h) and by eight sections (Figure 10i,j), while maintaining a low steam heights. flowThe rate.charts For only fourinmiddle in operation, there was aofsharp increase deformation at shown Figure sections 10g–j correspond to the situation shortening theinsteam tunnel by four sections (Figure 10g,h) and by eight sections (Figure 10i,j), while maintaining a low steam flow rate. For only four middle sections in operation, there was a sharp increase in deformation at three

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three levels to a value of approximately 800 µm/m for a film-free container. For a container with the film applied, this increase was significantly lower due to the compressing effect of the film. In these cases, despite the low steam flow rate, the highest compressive force of the film was obtained on the packaging, especially at its top. The last four drawings (Figure 10k–n) show the effects of a gradual increase in the shrinking medium flow rate in subsequent sections (Figure 10k,l) and its gradual decrease (Figure 10m,n). In the first case after the standard first spike, we can observe another sharp spike in the direction of the compressive strain, which should be attributed to a strong steam stroke in the last sections. In the latter case, the first rapid increase in deformation caused by water being heated inside the container was limited by equally rapid heat shrinkage of the material. It is worth noting that the high deformation value persisted for a few seconds here (Figure 10m,n), as opposed to the former (Figure 10c). Due to the novelty of the method, currently it is difficult to provide typical statistical processing of experimental data such as the average of values and standard deviation alongside Figure 10 as the results are too little so far. However, in the future, the authors intend to do more repetitions of each test and present comprehensive results of statistical analysis. 5. Conclusions To conclude, it should be stated that the highest film compression values were obtained for the cases of the “shortened” steam tunnel. In this case, however, the divergence between the compressive forces at three different levels was highest. The lowest divergence was obtained for identical settings in all steam sections. Divergences in these values are lowered as the shrinking medium flow rate stream decreased. However, the film compression was the smallest in these cases. What plays a significant role in the film shrinking process is the liquid filling the packaging. As a result of a sudden steam blast, the volume of liquid inside the closed container is increased. This causes a sharp increase in deformation of the circumferential walls of the packaging as evident by the first peaks on the shrinking graphs. The pressure inside decreases as the temperature drops. After that, the dominant load is observed to be generated by the shrinking foil. The method presented herein makes it possible to determine the influence of many factors on the quality of the shrinking process and its energy consumption. It has been shown that it can be used to select the optimum values of the working medium flow rate. Thus, it makes it possible to optimally select the design of the shrink tunnel itself. In particular it is applicable to individual steam sections. The developed method allows, for example, for the appropriate distribution of steam nozzles in individual sections, the selection of the appropriate shape of these nozzles and the selection of the number of steam sections. All of these factors are closely linked to the reliability of the shrink sleeve labelling process while minimizing production costs. In industrial practice, a situation should be sought where the clamping force of the label is comparable at each level of the packaging. The force in question is the final force, i.e., after cooling and drying the package. However, it should be emphasized that tests should be carried out for each shape and size of the packaging. This is, of course, cost-effective only in the case of mass production. Out of the results presented in Figure 10, the results from Figure 10a–c,e,g are most advantageous. The differences in the clamping force of the film on three different levels are the smallest here. Author Contributions: The present research articles is the work of authors stated below according to the their individual contributions; conceptualization, J.S. and A.T.; methodology, J.S. and A.T.; software, J.S.; validation, J.S., A.T. and Ö.K.; formal analysis, J.S.; investigation, A.T.; resources, J.S.; data curation, J.S. and A.T.; writing—Original draft preparation, J.S., A.T. and Ö.K.; writing—Review and editing, Ö.K.; visualization, A.T. and Ö.K.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. Funding: The present paper is financially supported by European Union Smart Growth Programme, project No POIR.01.01.01-00-0341/15. Acknowledgments: The present paper is made possible with the contributions and kind support of Masterpress. Conflicts of Interest: The authors declare no conflict of interest.

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