materials

Article A New Method for Estimating the Clamping Force of Shrink Sleeve

Jarosław Szusta 1,2 , Adam Tomczyk 1,2 and Özler Karaka¸s 3,* 1 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.) 2 Masterpress S.A., Kuronia 4 Str, 15-569 Białystok, Poland 3 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 presents an original method for estimating the shrink sleeve 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

1. Introduction Heat shrink films are now widely used in various industries to produce all kinds of labels used on , metal and plastic packaging (Figure1). The labels on 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 methods; • the possibility of covering the entire surface of even with complicated, unusual shapes, for which it would be difficult to adapt self- 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.;

Materials 2018, 11, 2544; doi:10.3390/ma11122544 www.mdpi.com/journal/materials Materials 2018, 11, 2544 2 of 14

• cost-effective low-volume printing in flexographic technology. Materials 2018, 11, x FOR PEER REVIEW 2 of 14

FigureFigure 1. 1.SampleSample packaging packaging with with heat-shrink heat-shrink film film labels. labels. (Adapted from resourcesresourcesof of Masterpress Masterpress S.A.). S.A.).

Based onon these these clear clear advantages, advantages, it is it safe is tosafe predict to predict a significant a significant increase inincrease the scale in ofthe production scale of productionof shrink sleeve of shrink labels sleeve in the labels coming in years.the coming In fact, years. it would In fact, be reasonableit would be to reasonable state that theto state labels that of thisthe labelstype will of this dominate type will the dominate packaging the of pa productsckaging in of the products near future. in the near future. Obtaining the desired visual effect depends on a number of factors, such as the filmfilm shrinkage process parameters, the the shape shape and and size size of of the the packag packaginging or or the the selection selection of ofmaterial material with with respect respect to chemicalto chemical and and strength strength properties. properties. Knowledge Knowledge of ofma materialterial characteristics characteristics in in turn turn allows allows for for the application of appropriate parameters of the film film printing process and its subsequent shrinking on packaging. TheThe selectionselection ofof suitable suitable strength strength parameters parameters is is primarily primarily determined determined by by the the strength strength of the of thefilm film compressive compressive force, force, its corresponding its corresponding deformation, deformation, lack lack of undesirable of undesirable cracks cracks etc. etc. Shrink sleeve films films are of interest to many authors.authors. This has been the since the beginning of the first first applications of this type of filmfilm in 1958, when it was possible to combine styrene with synthetic rubber [1,2].The [1,2].The literature provides a nu numbermber of works on tests of various properties, including mechanical properties of the films.films. An An extensive extensive review review of of various various films films in in terms of strength parameters can be found in work of [[3].3]. St Strengthrength tests of have also been described in studies studies by by AvérousAvérous et et al. al. [4], [4] Matzinos, Matzinos et etal. al.[5], [Mo5], Moet al. et [6], al. Srinivasa [6], Srinivasa and Tharanathan and Tharanathan [7], Tserki [7], Tserkiet al. [8,9], et al. Wang [8,9], Wanget al. [10], et al. Zhang [10], Zhanget al. [11], et al. Zhang [11], Zhanget al. [12], et al. Szusta [12], Szustaet al. [13]. et al. Many [13]. studies Many studiesplaced placedemphasis emphasis on one on of one the of cheapest the cheapest and andmost most commonly commonly available available materials materials used used for for labels, starch-based polymers [[4,10].4,10]. It should also be noted that the method of manufacturingmanufacturing filmsfilms has a significant significant effect on their mechanical properties. This particularly pertains to cross-linked polymers such as PE, PP, acrylate,acrylate, where the cross-linkingcross-linking method has a decisive influenceinfluence on the aforementioned propertiesproperties [ 14[14,15,15].]. Very Very good good mechanical mechanical properties proper areties obtained are obtained for oriented for oriented layered polymerslayered polymers [16]. The [16]. number The number of layers of layers in materials in materials of this of type this usuallytype usually falls withinfalls within the range the range of 2–7. of 2–7.Besides Besides strength strength properties, properties, much much attention attention is also isdevoted also devoted to shaping to shaping optimal optimal optical optical and visual and propertiesvisual properties [17,18]. [17,18]. The quality ofof thethe labellabel shrinking shrinking process process on on the the packaging packaging is assessedis assessed by by visual visual inspection inspection of the of theouter outer container container with with the label the appliedlabel applied at the presentat the present time. The time. adhesion The adhesion of the label of the to the label container to the wallscontainer is checked walls is and checked the compressive and the compressive force is assessed force is by assessed the trial involvingby the trial moving involving or rotating moving the or rotatinglabel on thethe container.label on the Such container. verification Such is verificati not objectiveon is andnot objective does not giveand does unambiguous not give unambiguous results. There 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

Materials 2018, 11, 2544 3 of 14 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 ) 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:

• 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. Figure2 shows the production and application process of the shrink-sleeve label on the packaging. Materials 2018, 11, 2544 4 of 14 Materials 2018, 11, x FOR PEER REVIEW 4 of 14

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 can beMost assumed of the that packaging these are on thin-walled which the axiallyshrink sleeve symmetrical labels vesselsare applied loaded are withaxially internal symmetrical. pressure. It Thiscan be pressure assumed is generated that these byare the thin-walled presence axially of fluid symmetrical inside the container. vessels loaded It can with additionally internal pressure. increase itsThis volume pressure due is to generated the temperature by the presence increase insideof fluid the inside shrink the tunnel. container. This It results can additionally in the appearance increase ofits positive volume containerdue to the walltemperature deformations. increaseAt inside the th samee shrink time, tunnel. additional This results deformations in the appearance are usually of generatedpositive container with the oppositewall deformations. sign as a result At the of thesame film time, compression. additional Therefore, deformations we deal are withusually an axiallygenerated symmetrical with the thin-walled opposite sign vessel as loadeda result by of pressure the film evenly compression. distributed Therefore, with respect we todeal the with axis ofan symmetry.axially symmetrical In such cases, thin-walled it can be vessel assumed loaded that by the pres stresssure in evenly the walls distributed of the container with respect corresponds to the axis to theof planesymmetry. stress caseIn such (Figure cases,3). The it can generalized be assumed Hooke’s that laws the expressed stress in in the terms walls of principalof the container stresses havecorresponds the following to the form plane (e.g., stress [19 case]): (Figure 3). The generalized Hooke’s laws expressed in terms of principal stresses have the following form (e.g., [19]): E E = ( + ) = ( + ) σ1 2EEε1 vε2 , σ2 2 ε2 vε1 (1) σεεσεε1 =+− v ()vv, 1 =+− v () (1) 11222111−−vv22 where ε1, ε2—principal strains, σ1, σ2—principal stresses, E—Young’s modulus, ν—Poisson’s ratio. whereWhen ε1, ε circumferential2—principal strains, deformations σ1, σ2—principalε1 are known,stresses, theE—Young’s compressive modulus, force ν of—Poisson’s the film and ratio. the uniformityWhen of circumferential the compression deformations can be estimated. ε1 are known, the compressive force of the film and the uniformityWith the of stress the compression value given bycan the be provided estimated. Equation (1), it is possible to propose the determination of theWith circumferential the stress clamping value given force asby a forcethe provided acting in aEquation narrow band (1), ofit theis container.possible to Assuming propose that the thisdetermination band is subjected of the to circumferential compression in clamping the circumferential force as direction,a force acting this F 1inforce a narrow can be determinedband of the bycontainer. multiplying Assuming the stresses that thisσ1 by band the is cross-sectional subjected to compression area S of the band:in the circumferential direction, this F1 force can be determined by multiplying the stresses σ1 by the cross-sectional area S of the band: F = σ S (2) 1 =1σ F11S (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.

Materials 2018,, 11,, 2544x FOR PEER REVIEW 55 of of 14

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 measurementThe most (e.g., popular [20]). method If the values of measuring of the principal deformatio strainns directionsis the electrical are not resistance known, strain strain rosettes gauge aremeasurement usually used. (e.g., However, [20]). If the in thevalues analyzed of the caseprinci ofpal the strain axially directions symmetrical are not vessels known, it can strain be assumed rosettes thatare usually these directions used. However, are known in the (Figure analyzed3). case Therefore, of the axially linear strainsymmetrical gauges vessels were used.it can be They assumed were appliedthat these only directions along the are circumferential known (Figure direction 3). Therefore, 1. Direction linear 2 wasstrain omitted gauges in measurementswere used. They because were theapplied film only shrinks along only the in circumferential the circumferential direction direction. 1. Direction In the 2 was vertical omitted direction, in measurements the film shrinkage because isthe negligible. film shrinks only in the circumferential direction. In the vertical direction, the film shrinkage is negligible.The probability of gluing the strain gauge ideally along the principal direction is low. However, this isThe not probability a problem, of since gluing the the deviation strain gauge of the ideally strain gaugealong axisthe principal from this direction direction is by low. up However, to several degreesthis is not will a problem, not significantly since the change deviation the of measurement the strain gauge results axis in from the casethis concerned.direction by Strain up to gaugesseveral weredegrees installed will not on significantly the outer wall change of the the container measuremen at differentt results heights in the along case theconcerned. main direction Strain gauges 1. This allowedwere installed us to estimateon the outer the strength wall of the of thecontainer film compressive at different force heights at different along the heights main direction of the container, 1. This andallowed to determine us to estimate the possible the strength non-uniformity of the film ofcomp theressive compression. force at different heights of the container, and toBy determine studying thethe deformationspossible non-uniformity for various configurationsof the compression. of nozzles, e.g., steam nozzles and their variousBy shapes,studying it isthe possible deformations to determine for various the most config optimalurations solution of nozzles, where e.g., the filmsteam compression nozzles and at differenttheir various levels shapes, will be similar.it is possible Testing usingto determ the presentedine the methodmost optimal allowed solution for continuous where recordingthe film andcompression analysis ofat deformationsdifferent levels (and will hence be similar. stresses). Testing Thus, using changes the presented could be estimatedmethod allowed in the film for compressivecontinuous recording force from and the momentanalysis of thedeformations steam blow (and until hence cooling. stresses). Thus, changes could be 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) (Figure4), where strain gauges (2) are installed to analyzeThe proposed the deformations method inuses the plastica reference parts. containe The numberr (1) of(Figure strain gauges4), where depends strain on gauges the container (2) are height.installed The to strainanalyze gauges the deformations were located in in the plastic height directionparts. The of number the container of strain along gauges the helical depends line on to coverthe container the entire height. circumference The strain of gauges the tested were packaging. located in Strainthe height gauges direction (2) were of installedthe container along along the first the principalhelical line direction to cover in the the entire half-bridge circumference system, i.e.,of the each tested of the packaging. strain gauges Strain had gauges a compensator (2) were installed (3) that wasalong glued the ontofirst principal a piece of direction unloaded in material the half-bridge (4) identical system, to the containeri.e., each material.of the strain Such gauges arrangement had a allowscompensator for the (3) compensation that was glued of elevated onto a temperature piece of un generatedloaded material during (4) the identical steam blow, to the e.g., container steam in thematerial. tunnel. Such Additionally, arrangement after allows installing for the all compensa strain gaugestion of and elevated necessary temperature electrical wires,generated the gaugesduring themselvesthe steam blow, and the e.g., exposed steam electrical in the connectorstunnel. Additionally, were protected after with installing a special all flexible strain polyurethanegauges and coatingnecessary against electrical the effects wires, of the moisture gauges generated themselv insidees and the tunnel.the exposed electrical connectors were protectedThe location with a ofspecial strain gaugesflexible must polyurethane be sufficiently distant against from placesthe effects where of the moisture stress distributions generated ininside the containerthe tunnel. walls may be inhomogeneous. The areas in the immediate vicinity of the bottom of the container,The location its threaded of outlet,strain asgauges well asmust the minimal be suffic thicknessiently distant of thewall from resulting places fromwhere the the process stress of injectingdistributions the liquidin the materialcontainer into walls the may split be mold inhomo shouldgeneous. be avoided. The areas in the immediate vicinity of 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.

Materials 2018, 11, 2544 6 of 14 MaterialsMaterials 20182018,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 1414

FigureFigure 4.4. SchematicSchematicSchematic representationrepresentation representation ofof of aa a containercontainer container withwith with thethe the attachedattached strainstrain gaugesgauges onon aa stand: stand: 1—container,1—container, 2—strain2—strain gauges,gauges, 3—compensating 3—compensating st strainstrainrain gauges, gauges, 4—unloaded 4—unloaded material, material, 5—base, 5—base, 6—mandrel,6—mandrel, 7—thermocouple 7—thermocouples,7—thermocouples,s, 8—magnet,8—magnet, 9—wiring.9—wiring.

InIn order order toto ensureensure thethe stabilitystability ofof thethe containercontainer movingmoving insideinside the the heat heat tunneltunnel with with the the gaugesgauges installed,installed, andandand to toto allow allowallow the thethe temperature temperaturetemperature to be toto recorded bebe recorecordedrded in the inin vicinity thethe vicinityvicinity of each ofof strain eacheach gauge, strainstrain the gauge,gauge, container thethe containerwascontainer mounted waswas onmountedmounted a stand onon (Figure aa standstand4) (Figure consisting(Figure 4)4) consis ofconsis thetingting base ofof (5), thethe the basebase mandrel (5),(5), thethe (6) mandrelmandrel and the (6)(6) installed andand thethe installedmeasuringinstalled measuringmeasuring thermocouples thermocouplesthermocouples (7). (7).(7). TheThe basebase weightweightweight should shouldshould provide provideprovide a a stablea stablestable container containecontaine positionrr positionposition when whenwhen moving movingmoving in the inin thethe shrink shrinkshrink tunnel tunneltunnel (10) (10)(Figure(10) (Figure(Figure5) between 5)5) betweenbetween the steam thethe steam nozzlesteam nozzlenozzle systems systemssystems (11). The (1(1 container1).1). TheThe containercontainer (1) is mounted (1)(1) isis mountedmounted on the base onon of thethe the basebase stand ofof thebythe means standstand by ofby a meansmeans magnet ofof (8) aa magnetmagnet (Figure 4(8)(8)) located (Figure(Figure inside 4)4) locatelocate its bottom.dd insideinside The itsits bottom.bottom. wiring (9) TheThe is wiring attachedwiring (9)(9) to isis the attachedattached mandrel toto the(6).the Thismandrelmandrel solution (6).(6). eliminatesThisThis solutionsolution wiring eliminateseliminates movements wiwiringring between movementsmovements the stand betweenbetween pin and thethe the stand container,stand pinpin andandand thus thethe container,eliminatescontainer, and theand impact thusthus eliminateseliminates of these movements thethe impactimpact onofof theththeseese deformation movementsmovements of onon the thethe container deformationdeformation walls. ofof The thethe signal containercontainer from walls.thewalls. strain TheThe gauges signalsignal is fromfrom transmitted thethe strainstrain to a specialgaugesgauges 8-channel isis transmittedtransmitted HBM Quantumtoto aa specialspecial MX840A 8-channel8-channel (Darmstadt, HBMHBM Quantum Germany)Quantum MX840AmeasuringMX840A (Darmstadt,(Darmstadt, amplifier (12) Germany)Germany) (Figure measuring5measuring). Data is amplifieramplifier logged via (12)(12) a portable (Figure(Figure 5).5). computer DataData isis logged (13)logged with viavia specialised aa portableportable computersoftwarecomputer (Catman (13)(13) withwith DAQ) specialisedspecialised for operating softwaresoftware the (Catman(Catman amplifier. DAQ)DAQ) forfor operatingoperating thethe amplifier.amplifier.

FigureFigure 5. 5. SchematicSchematic view view of of the the measuring measuring stand: stand: 1—shr 1—shrink1—shrinkink wrappedwrapped filmfilm film tested, tested, 10—shrink 10—shrink tunnel,tunnel, 11—nozzle11—nozzle layoutlayout fedfed fromfrom aa steamsteam generator,generator, 12—measuring 12—measuring12—measuring amplifier, amplifier,amplifier, 13—portable 13—portable13—portable computer. computer.computer. The essence of the study is to set up a stand (including a container with strain gauges on which TheThe essenceessence ofof thethe studystudy isis toto setset upup aa standstand (inc(includingluding aa containercontainer withwith strainstrain gaugesgauges onon whichwhich heat shrink film is applied) on a belt conveyor before entering the shrink tunnel and starting the heatheat shrinkshrink filmfilm isis applied)applied) onon aa beltbelt conveyorconveyor beforebefore enteringentering thethe shrinkshrink tunneltunnel andand startingstarting thethe conveyor and recording equipment. As the conveyor moves, the tooled container moves inside the conveyorconveyor andand recordingrecording equipment.equipment. AsAs thethe conveyorconveyor moves,moves, thethe tooledtooled containercontainer movesmoves insideinside thethe tunnel, passing through heating sections. As a result of the appropriate sequence of heat impacts, tunnel,tunnel, passingpassing throughthrough heatingheating sections.sections. AsAs aa resultresult ofof thethe appropriateappropriate sequencesequence ofof heatheat impacts,impacts, thethe the film compressed on the container exerts pressure on its walls. Film compression is recorded by filmfilm compressedcompressed onon thethe containercontainer exertsexerts pressurepressure onon itsits walls.walls. FilmFilm compressioncompression isis recordedrecorded byby strainstrain strain gauges as circumferential deformations at various levels. During the testing and calibration gaugesgauges asas circumferentialcircumferential deformationsdeformations atat variousvarious levels.levels. DuringDuring thethe testingtesting andand calibrationcalibration ofof thethe of the test stand, several dozen tests were performed. However, the results of seven of them were testtest stand,stand, severalseveral dozendozen teststests werewere performed.performed. However,However, thethe resultsresults ofof sevenseven ofof themthem werewere presented in the manuscript. presentedpresented inin thethe manuscript.manuscript.

Materials 2018, 11, 2544 7 of 14 Materials 2018, 11, x FOR PEER REVIEW 7 of 14

ItIt should should be be noted noted that that the the specific specific design design and and sh shapeape of of the the steam steam nozzles nozzles in in the the shrink shrink tunnel tunnel is is appropriateappropriate for for the the packaging packaging of of a a particular particular shap shapee and and size. size. With With the the use use of of presented presented measurement measurement methodmethod containers containers of of different different shapes shapes and and sizes sizes ca cann be be analyzed. analyzed. At At the the same same time, time, testing testing the the same same packagingpackaging withwith differentdifferent configurations configurations of theof the nozzle nozzle system system and differentand different nozzle shapesnozzle willshapes optimize will optimizethis layout. this layout. TheThe primary primary test piecepiece waswas a a PET PET bottle bottle of of 500 500 mL mL capacity capacity with with dedicated dedicated heat shrinkheat shrink film. Strainfilm. Straingauges gauges were installed were installed on the wallson the of thewalls bottle. of the The bottle. tests wereThe tests carried were out carried with the out use with of special the use strain of specialgauges strain 1-LY18-3/120A gauges 1-LY18-3/120A manufactured manufactured by HBM (Figure by6 ),HBM intended (Figure for 6), analyzing intended the for deformations analyzing the in deformationsplastic elements. in plastic Strain gaugeselements. were Strain glued gauges using special, were glued elastic using Z 70 adhesive special, manufacturedelastic Z 70 adhesive by HBM. manufacturedExposed wiring by harnesses HBM. Exposed were covered wiring with harnesses two layers were of PU covered 140 polyurethane with two protectivelayers of coatingPU 140 to polyurethaneprotect them fromprotective exposure coating to moisture. to protect Strain them gauges from wereexposure installed to moisture. in a half-bridge Strain configuration,gauges were installedas shown in in a Figurehalf-bridge7. configuration, as shown in Figure 7. ItIt should should be be taken taken into into consideration consideration that that the the influence influence of of connecting connecting wires wires on on the the measurement measurement resultsresults cannot cannot be be excluded. excluded. Determining Determining the the amount amount of of the the influence influence of of these these wiring wiring would would require require measuringmeasuring the strainstrain withwith other other methods, methods, preferably preferab non-contact,ly non-contact, which which in the in conditionsthe conditions of the of steam the steamtunnel tunnel would would be extremely be extremely difficult difficult or can or even can be even impossible. be impossible. However, However, it is also it importantis also important to note tothat note all that tests all were tests carried were outcarried with out the samewith the wiring same configuration. wiring configuration. Thus, the possibleThus, the error possible was always error wasthe samealways and the negligible. same and negligible.

(b)

(a) (c)

FigureFigure 6. ConfigurationsConfigurations ofof the the attached attached strain strain gauges gauges on teston test container: container: (a) a ( bottlea) a bottle with strainwith gaugesstrain gaugesglued onglued three on different three different heights; heights; (b) a single (b) a strain single gauge; strain (gauge;c) compensating (c) compensating strain gauges. strain gauges.

Materials 2018, 11, 2544 8 of 14 Materials 2018, 11, x FOR PEER REVIEW 8 of 14

Materials 2018, 11, x FOR PEER REVIEW 8 of 14

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 (Figure4). A single bottle is fitted with three strain gauges mounted at three different heights (Figure 4). Further inA thissingle paper bottle these is fitted gauges with arethree numbered, strain gauges starting mounted from at thethree bottom different of theheights bottle: (Figure the bottom4). Further in this paper these gauges are numbered, starting from the bottom of the bottle: the bottom gauge,Further the middle in this paper gauge these and gauges the top are gauge. numbered, It is worth starting mentioning from the bottom that, of as the in massbottle: production,the bottom the gauge, the middle gauge and the top gauge. It is worth mentioning that, as in mass production, the bottlegauge, is filled the withmiddle liquid gauge (water) and the which top gauge. prevents It is wo permanentrth mentioning deformation that, as in of mass the production, material in the contact bottle is filled with liquid (water) which prevents permanent deformation of the material in contact withbottle hot steam. is filled The with study liquid was (water) carried which out prevents in a shrink permanent tunnel deformation (Figure8a) equippedof the material with in threecontact steam with withhot steam.hot steam. The The study study was was carried carried out out inin aa shrink tunnel tunnel (Figure (Figure 8a) 8a) equipped equipped with with three three steam steam sections with each section composed of four systems of steam nozzles in an adjustable position for a sectionssections with with each each section section composed composed of of four four systemsystemss ofof steam steam nozzles nozzles in inan anadjustable adjustable position position for a for a total of twelve steam sections. Each of valves allowed independent control over the amount of steam total totalof twelve of twelve steam steam sections. sections. Each Each of of valves valves aallowedllowed independent independent control control over over the theamount amount of steam of steam generated and the position of each section from the control panel (Figure8b). generatedgenerated and and the theposition position of ofeach each sect sectionion from from the controlcontrol panel panel (Figure (Figure 8b). 8b).

(a) (b)

FigureFigure 8. Photographs 8. Photographs of: of: (a )(a the) the shrink shrink tunnel tunnel inin which the the test test was was carried carried out; out; (b) the (b) control the control panel panel (a) (b) of theof steam the steam flow flow rate rate in ain sample a sample section section of of the the tunnel.tunnel. Figure 8. Photographs of: (a) the shrink tunnel in which the test was carried out; (b) the control panel Thisof theThis solution steam solution flow allows rate allows in adjusting a sampleadjusting the section steamthe steamof the flow tunnel.flow to packages to packages of varyingof varying heights heights and and shapes. shapes. During the tests,During the the steam tests, nozzlethe steam systems nozzle weresystems arranged were arranged at equal at distancesequal distances along along the lengththe length of theof the tunnel. However,Thistunnel. thesolution However, vertical allows the position vertical adjusting of position the nozzlesthe of steamthe wasnozzles flow adjusted was to adjustedpackages to the to shape theof shapevarying of the of tested the heights tested object. object.and It shapes. shouldIt should be acknowledged that tunnels with different numbers of steam sections are used in industry. beDuring acknowledged the tests, the that steam tunnels nozzle with systems different were numbers arranged of steam at equal sections distances are used along in industry.the length of the The “0” value set on the valve of each section corresponds to the total steam cut off, while “10” tunnel. However, the vertical position of the nozzles was adjusted to the shape of the tested object. It Theindicates “0” valuethe maximum set on the stream valve flow. of eachThe tests section were corresponds carried out for to various the total vapor steam flow cut rate off, settings while “10” indicatesshouldin eachbe the acknowledged section, maximum however stream that in tunneline flow. withls The with the tests tunnel different were operator’s carried numbers long-term out of for steam various experience, sections vapor which are flow used means rate in settings industry.that in eachThethe section, values “0” howevervalue set on set the inon control line the withvalve panel the of were tunneleach closesection operator’s to thecorresponds values long-term that to had the experience, been total used steamwhich so cutfar meanswithoff, while good that “10” the valuesindicates set the on themaximum control panelstream were flow. close The to tests the were values carried that had out been for various used so vapor far with flow good rate shrinkage settings 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

Materials 2018, 11, 2544 9 of 14 quality. The values of settings used for the tests are shown in Table1. Cut sleeve film with a diameter of 68Materials mm was 2018 used, 11, x FOR for testing.PEER REVIEW 9 of 14

shrinkageTable quality. 1. Valve The values settings of forsettings individual used for steam the sectionstests are inshown subsequent in Table measurements. 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 settings1 for 2individual 3 4steam 5 sections 6 in 7 subsequent 8 9 measurements. 10 11 12 1Valve 2.5 Settings for Individual 2.5 Steam Sections 2.5 Measure Series 21 52 3 4 5 6 7 58 9 10 11 12 5 31 72.5 2.5 72.5 7 42 05 5 2.55 2.5 53 07 7 2.57 0 64 20 2.5 42.5 7 75 70 2.5 40 2 6 2 4 7 7 7 4 2 3. Results The3. Results tests consisted in continuous recording of deformation changes in three strain gauges affixed to the containerThe tests walls consisted while in the continuous container recording was moving of deformation inside the changes shrink tunnel.in three Atstrain the gauges same time, it shouldaffixed be to noted the container that the walls maximum while the ambient container temperature was moving ofinside the the gauges shrink during tunnel. the At teststhe same (during the steamtime, blow)it should recorded be noted by that thermocouples the maximum varied ambient by temperature 2–3 ◦C for subsequent of the gauges repetitions during the and tests was in the range(during of the 70–90 steam◦C. blow) Therefore, recorded no by additional thermocouples results varied of this by 2–3 temperature °C for subsequent analysis repetitions are presented and in this report.was in Itthe is worthrange of emphasizing 70–90 °C. Therefore, that after no leaving additional the steam results tunnel, of this thetemperature container analysis was cooled are to a 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 containerthe shrink-wrapped at the tunnel outletcontainer and at after the tunnel cooling outl iset shown and after in cooling Figure9 is. shown in Figure 9.

(a) (b)

FigureFigure 9. The 9. The stand stand with with the the bottle bottle and and shrunk shrunk film:film: ( (aa)) at at the the steam steam tunnel tunnel outlet; outlet; (b) after (b) after cooling. cooling.

The firstThe first few few tests tests were were intended intended to to check check thethe entireentire measuring measuring system system for forcorrect correct function function and and mademade it possible it possible to obtain to obtain an an overview overview of of the the functionfunction of of the the individual individual shrink shrink sections sections of the of tunnel. the tunnel. It shouldIt should be emphasized be emphasized that that due due to to the the violent violent impactimpact of of steam steam at at elevated elevated temperatures, temperatures, water 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.sharp increase This in entailed deformations. an immediate At the same strain time, gauge due response,to the increased which temperature, is clearly indicated the material by aof sharp increasethe container in deformations. (PET) shrank, At thewhich same was time, also recorded due to theby strain increased gauges. temperature, In order to isolate the material the effects of the containerof increased (PET) shrank,pressure whichinside the was container also recorded and the container by strain material gauges. heat In ordershrinkage to isolate from the the effects effects of

Materials 2018, 11, 2544 10 of 14 increased pressure inside the container and the container material heat shrinkage from the effects of

film compression,Materials 2018, 11, x a FOR series PEER of REVIEW measurements was carried out for the container with no film10 andof 14 with the shrinking film on it. Each time the container was cooled down to a temperature of approximately 30–31of◦ C,film which compression, prevented a series the continuous of measurements heating wa ofs carried water inout the for bottle the container in subsequent with no measurements film and and thuswith thethe repeatabilityshrinking film of on the it. measurements. Each time the Eachcontainer series was consisted cooled ofdown six measurements:to a temperature three of for a containerapproximately with no 30–31 film and°C, which three forprevented a container the continuous with shrink heating film on of it.water The resultsin the bottle of these in tests 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. compressingThe charts film. The other charts, i.e., Figure 10b,d,f,h,j,l,n pertain to measurements with the film on.

(a) (b)

(c) (d)

(e) (f)

Figure 10. Cont.

Materials 2018, 11, 2544 11 of 14 Materials 2018, 11, x FOR PEER REVIEW 11 of 14

(g) (h)

(i) (j)

(k) (l)

Figure 10. Cont.

Materials 2018, 11, 2544 12 of 14 Materials 2018, 11, x FOR PEER REVIEW 12 of 14

(m) (n)

Figure 10. AveragedAveraged results obtained for different valv valvee settings controlling the individual steam sections: ( a,b) series 1;1; ((cc,,dd)) seriesseries 2;2; ((ee,,ff)) seriesseries 3; 3; ( g(g,h,h)) series series 4; 4; ( i(,ij,)j) series series 5; 5; (k (,kl),l series) series 6; 6; (m (m,n,)n series) series 7. 7. 4. Discussion 4. DiscussionThe results of all series of tests show that the steam impact on the container passing through the firstThe activeresults section of all series caused of tests a sharp show increase that the in steam deformation impact on at allthe three container levels passing analyzed. through In most the firstcases, active the maximum section caused deformation a sharp levelincrease at three in deformation different heights at all isthree similar. levels However, analyzed. the In level most of cases, these thedeformations maximum isdeformation different for level different at three series. different A sharp he increaseights is similar. in deformation However, is duethe tolevel an of increase these deformationsin the pressure is insidedifferent the for container different as series. a result A sharp of water increase being in heated deformation inside theis due container. to an increase After the in theincrease, pressure a sharp inside decrease the container in deformation as a result occurs. of Thiswater should being be heated explained inside by the heatcontainer. shrinkage After of the the increase,material ofa sharp which decrease the container in deformation (PET) was occurs. made.However, This should this be shrinkage explained takes by the place heat a momentshrinkage after of thethe material pressure of increase which duethe container to the heating (PET) of was water made. inside However, the container this shrinkage and is clearly takes dependent place a moment on the aftervalve the settings pressure of the increase individual due to sections. the heating Any furtherof water decrease inside the in deformationcontainer and is is much clearly more dependent gradual ondue the to valve the persistence settings of of the the individual elevated temperaturesections. Any at further a relatively decrease constant in deformation level. Yet, the is much oscillations more gradualof the deformation due to the persistence waveforms of over the time elevated can betemp seenerature clearly at asa relatively the container constant is passing level. throughYet, the oscillationssubsequent of steam the deformation sections. This waveforms involves aover slight time decrease can be in seen temperature clearly as inthe the container areas between is passing the throughsteam nozzle subsequent outlets ofsteam the individualsections. This sections. involves This isa particularlyslight decrease evident in temperature in Figure 10 a,c,e,gin the whereareas betweeneach of the the peaks steam corresponds nozzle outlets to theof the vapor individual impact insections. each section. This is Eventually,particularly inevident each case in Figure strain 10a,c,e,ggauges always where confirmedeach of the the peaks presence corresponds of compression to the vapor in the impact container in each walls. section. The Eventually, degree of this in eachcompression case strain depended gauges onalways such factorsconfirmed as the the steam presence flow of rate. compression in the container walls. The degreeFor of identical this compression settings ofdepended all steam on sections such factors (Figure as 10 thea–f), steam a steam flow blowrate. is clearly marked at the heightFor of identical each section. settings Such of clearall steam regularity sections cannot (Figur bee 10a–f), observed a steam in the blow other is figures. clearly Increasingmarked at the heightsettings of from each 2.5 section. to 5 and Such 7 caused clear regularity a decrease cannot in the maximumbe observed value in the at other the first figures. rapid Increasing load variation. the settingsIt can be from explained 2.5 to by5 and the 7 fact caused that a a decrease higher temperature in the maximum caused value a faster at the container first rapid material load variation. response, Itwhich can be compensated explained by for the the fact faster that increasea higher intemperature pressure due caused to water a faster being container heated. material At the same response, time, whichfor the compensated same first three for series, the faster the value increase of the in compressivepressure due deformation to water being at the heated. end of At the the measurement same time, forincreased the same as the first settings three increased, series, the both value for theof containerthe compressive with and deformation without the shrinkat the filmend applied.of the measurementThis is mainly increased due to higher as the shrinkage settings increased, of the container both for material the container at higher with temperatures. and without the However, shrink filmthe effect applied. of film This compression is mainly is clearlydue to noticeable higher shrinkage here. Compression of the container values for amaterial container at withhigher the temperatures.film applied are However, always higher the effect than of for film film-free comp containers.ression is clearly The biggest noticeable differences here. areCompression always for valuesthe upper for gauge,a container while with the smallest the film ones applied are for are the always bottom higher gauge. than An increasefor film-free in the containers. settings caused The biggesthigher divergencedifferences betweenare always the for gauges the upper located gauge, at three while different the smallest heights. ones are for the bottom gauge. An increaseThe charts in the shown settings in Figurecaused 10 higherg–j correspond divergence to between the situation the gauges of shortening located at the three steam different tunnel heights.by four sections (Figure 10g,h) and by eight sections (Figure 10i,j), while maintaining a low steam flowThe rate. charts For only shown four in middle Figure sections10g–j correspond in operation, to the there situation wasa of sharp shortening increase the in steam deformation tunnel by at 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

Materials 2018, 11, 2544 13 of 14 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. Materials 2018, 11, 2544 14 of 14

References

1. Izdebska, J. Applications of printed materials. In Printing on Polymers; Izdebska, J., Thomas, S., Eds.; Elsevier: Philadelphia, PA, USA, 2016; pp. 371–388. 2. Raheem, D. Application of plastics and paper as materials? An overview. Emir. J. Food Agric. 2013, 25, 177. [CrossRef] 3. Rudnik, E. Compostable properties and packaging applications. In Plastic Films in Food Packaging; Ebnesajjad, S., Ed.; Elsevier: Cambridge, MA, USA, 2013; pp. 217–248. 4. Averous, L.; Moro, L.; Dole, P.; Fringant, C. Properties of thermoplastic blends: STARCH–polycaprolactone. Polymer 2000, 41, 4157–4167. [CrossRef] 5. Matzinos, P.; Tserki, V.; Kontoyiannis, A.; Panayiotou, C. Processing and characterization of starch/polycaprolactone products. Polym. Degrad. Stab. 2002, 77, 17–24. [CrossRef] 6. Mo, X.; Sun, X.S.; Wang, Y. Effects of temperature and pressure on properties of soy protein polymers. J. Appl. Polym. Sci. 1999, 73, 2595–2602. [CrossRef] 7. Srinivasa, P.C.; Tharanathan, R.N. Chitin/Chitosan—safe, ecofriendly packaging materials with multiple potential uses. Food Rev. Int. 2007, 23, 53–72. [CrossRef] 8. Tserki, V.; Matzinos, P.; Pavlidou, E.; Vachliotis, D.; Panayiotou, C. Biodegradable aliphatic . Part I. Properties and of poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91, 367–376. [CrossRef] 9. Tserki, V.; Matzinos, P.; Pavlidou, E.; Panayiotou, C. Biodegradable aliphatic polyesters. Part II. Synthesis and characterization of chain extended poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91, 377–384. [CrossRef] 10. Wang, X.L.; Yang, K.K.; Wang, Y.Z. Properties of starch blends with biodegradable polymers. J. Macromol. Sci. Part C Polym. Rev. 2003, 43, 385–409. [CrossRef] 11. Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein sheets. Polymer 2001, 42, 2569–2578. [CrossRef] 12. Zhang, X.; Do, M.D.; Hoobin, P.; Burgar, I. The phase composition and molecular motions of plasticized wheat gluten-based biodegradable polymer materials studied by solid-state NMR spectroscopy. Polymer 2006, 47, 5888–5896. [CrossRef] 13. Szusta, J.; Karaka¸s,Ö.; Tomczyk, A. Experimental investigation of thin films with various overprints used for packaging labels. Theor. Appl. Fract. Mech. 2018, 97, 467–477. [CrossRef] 14. Patterson, R.; Kandelbauer, A.; Muller, U.; Lammer, H. Crosslinked thermoplastics. In Handbook of Thermoset Plastics; Goodman, S.H., Dodiuk-Kenig, H., Eds.; Elsevier: Philadelphia, PA, USA, 2014; pp. 697–737. 15. Makuuchi, K.; Cheng, S. Radiation Processing of Polymer Materials and Its Industrial Applications; John Wiley & Sons: Hoboken, NJ, USA, 2012. 16. Breil, J. Biaxially oriented films for packaging applications. In Plastic Films in Food Packaging; Ebnesajjad, S., Ed.; Elsevier: Philadelphia, PA, USA, 2013; pp. 53–70. 17. Kondratov, A.P.; Varepo, I.G.; Nagornova, I.V.; Ermakova, I.N. Transparent Layered Materials Based on Variable Color Polyolefins. Procedia Eng. 2015, 113, 423–428. [CrossRef] 18. Schäfer, C.G.; Viel, B.; Hellmann, G.P.; Rehahn, M.; Gallei, M. Thermo-cross-linked Elastomeric Opal Films. ACS Appl. Mater. Interfaces 2013, 5, 10623–10632. [CrossRef][PubMed] 19. Timoshenko, S.; Goodier, J.N. Theory of Elasticity; McGraw-Hill: New York, NY, USA, 1951. 20. Watson, B.R. Bonded electrical resistance strain gages. In Springer Handbook of Experimental Solid Mechanics; Sharpe, W.N., Ed.; Springer: New York, NY, USA, 2008; pp. 283–334.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).