Keep your tack on track! - an article about tack measurement 1 Issued: 2003, vip publicationsKeep your tack on track!an article about tack measurement
Keep your tack on track! - an article about tack measurement 2 Issued: 2003, vip publicationswhat is tack?Over the past decades several denitions of tack have been given by variousexperts in the Industry.In 1967, Prof. Jan Mewis of the Catholic University Louvain, Department ofChemical Engineering in Belgium performed a very interesting rheological studyinto tack. He dened tack as follows:“The resistance which a thin ink lm offers against fast splitting under certainprinting conditions”7 years later, in 1974 the Technical Committee of the Group for Printing Inksof the European Association of Ink and Paint Manufacturers dened tack asfollows:“Tack is the drag force between two rotating rollers caused by the presence ofan ink layer on their surfaces”The ISO 12634:1996(E) denes tack as:“Restoring force between two rotating rollers of a given width caused by thesplitting of an ink or vehicle lm on the roller surfaces”Perhaps the best and most accurate denition is mentioned in the ASTM standardfor tack measurement: D 4361-97:“Tack - a function of the force required to split a thin uid lm of a printing inkor vehicle between two rapidly separating surfaces; it is a rheological parameterindicative of internal cohesion of the uid”These are all defenitions of tack but the term “tackmeter” is perhaps not a gooddescription of instruments which measure tack. Given the scope of operation andthe mathematical uid modelling of thin ink layers on rotational tackmeters, theterm “roller rheometer” would be a better description of this type of instrument.
Keep your tack on track! - an article about tack measurement 3 Issued: 2003, vip publicationsthe importance of tackTack of printing inks controls their high speed transfer properties. It may also bemeaningful as to the ability to predict paper picking and wet trapping in multicolor printing. Tack measurements are a vital factor in the development andproduction of printing inks and the media onto which they are transferred.During the printing process a printing ink is subjected to mechanical forcesof divergent magnitude. In each phase, ink duct, distribution or transfer, anoptimal response of the ink is required. However these responses are difcult tocombine in one material. Therefore a printing ink formulation can be consideredas a compromise between contradictory requirements. With respect to pressbehaviour recent studies begin to describe the effect of all parameters involvedbut by no means can the full complexity of the offset printing process becaptured in convenient models or formulas, at least not untill recently.The measurement of tack helps to a large extent to isolate some of the factorswhich inuence nip ow and transfer and therefore tack remains to be oneof the critical parameters to accurately measure in the formulation and QC oflithographic printing inks.
Keep your tack on track! - an article about tack measurement 4 Issued: 2003, vip publicationsparameters which affect tack measurementWhen Prof. Reed of the American Lithographic Technical Foundation rst pre-sented his mathematical model for measuring tack on a friction less roller systemin the 1930’s, nobody could have imagined what impact this new phenomenawould have on the characterisation of printing inks and their components.Yet, 70 years later and 3 years into the new millennium, the measurement oftack is as important as ever before. A little over three years ago, several usersof tackmeters raised the question whether it would be possible to perform astudy into the human, technical and environmental aspects which inuence theoutcome of tack measurements. The result of the study would perhaps help tobetter understand these inuential aspects and they would form the basis for apossible improvement of the measurement technique.The ultimate goal would be to establish a recommended procedure which, onceobserved and practised consistently, would lead to an enhanced compatibilityof measured results, indifferent from the location on Earth. The reason for thisimproved procedure is simple: everyone seems to appreciate the results oftheir tack meter for intemal QC or R&D work, however communication betweensuppliers and customers is often hampered due to different results obtained ondifferent instruments for the same ink or vamish.The rest of this publication is available in the downloads section of our websitewww.tacknology.US.
Keep your tack on track! - an article about tack measurement 5 Issued: 2003, vip publicationsreproducibilityThere are two major causes for non-reproducible tack measurements: 1. Design parameters of the equipment itself 2. Inaccuracies in the method (see: recommended method for tack testing)A modern tackmeter should feature:Medium to hard rubber rollersSurveys and technical investigations have revealed that tackmeters with rigidmetal measuring rollers have a fairly low accuracy at thinner ink lms. Rubberrollers of intermediate hardness show that the regression coefcients reveal asmaller effect of layer thickness on tack readings. The rollers should stabilisequickly, age slowly and it must be possible to assess when the roller shouldbe replaced.A very accurate waterlevel:When the tackmeter is installed one should pay attention to levelling it in anexact horizontal plane. (And it should be left that way). Deviations from thehorizontal plane will cause the weight of the measuring roller to have an adverseeffect on the measurement of tack. The heavier the roller, the more this adverseeffect will be experienced. And remember: “a horizontal plane is determined by3 points, ...not 4!”A near frictionless measuring system:Older type tackmeters work on the principle that tack is a force which isrequired to keep the measuring roller in its “0” position without displacement,counteracting a dragforce. Some types make use of an oil damper which makesit difcult to determine whether the tack is inuenced by the viscosity of theoil in the damper.Older type tackmeters may also have up to 5 different points in their mechanicalconstruction, which may induce drag, consequently disturbing the measurementof the actual tack related data. If one wishes to measure the splitting force ofa thin ink lm, there should be no parts which can induce drag or inuence themeasurement in any other way.A thin ink lm:This has always been a challenge to manufacturers. Measuring tack with athick ink lm layer is more attractive because small inaccuracies caused by theequipment are lost within a relatively large margin for errors. Besides; a high inklayer thickness leads to the question: ‘do we measure the actual lm splitting ora value which is presumably lower, after subtracting a certain degree of slip?”At lower ink lm thickness the element of slip may be far less but so will bethe margin for errors. Consequently, the construction of the instrument, thechosen rubber compounds, roller weight and diameter, the tack sensors and dataacquisition must all be tuned to perfection and capable of presenting data withina small margin for errors. An ink lm thickness of 4 to 6 micron is recommended.Higher layer thickness requires heavy rollers in order to reduce the slip. Heavyrollers may have an adverse effect (see “water level”).A rigid distribution system:When working with a thin ink lm, the ink will tend to dry and set fasterif not properly distributed at a certain rate. The distribution therefore needsbe proportional to the circumferential speed of the rollers. This will provide asufcient rate of energy to prevent the ink from setting to quickly. The design ofthe distribution mechanism and its characteristic will be crucial in particular whenperforming stability tests. The design of the distribution system on tackmetersrequires a careful balance of roller diameter, distribution rate, drag induced bythe mechanical parts and RPM proportional to the circumferential speed of themain driven roller. Not just any random design will do the job.
Keep your tack on track! - an article about tack measurement 6 Issued: 2003, vip publicationsconversion (abstract)This has always been a tricky issue. Direct conversion is understood here tomean the procedure whereby a single reading on one type of tackmeter isconverted to a single reading on another type of tackmeter. Several studies inthe past have revealed that a layer thickness of 10 micron was the best basisfor conversion.This lead to some controversy since the more recent designs intackmeters were developed to work with thin ink lms (4 - 6 microns) for thepurpose of accurately differentiating performance characteristics of inks.During previous studies, one discovered that at a layer thickness of 10 micronsthe correlation coefcients with a linear regression of one group of tackmeterson another (all different types) were dened in a bandwidth of 0.858 - 0.987.To arrive at the most probable conversion formulae, a study was performed intoconversion of tack-speed curves.The test results suggest that at higher values of ink lm thickness, a directcorrelation of tack readings on another type of tack meter can be found. Whenthe hardness of rubber on the rollers is too different however, the changes oflarge errors are real. Samples with steep tack-speed curves can be convertedwith more condence than others.What about correlation at lower ink lm thickness? This is possible providedthere is not too much difference in roller hardness. Correlation requires a doubleconversion through an algorithmic scale which is based on a tack vs. square rootof speed representation and through linear regression coefcients for each typeof instrument. The scale can present a proportionality factor. This factor will notchange considerably with layer thickness provided the instruments have similarroller hardness and are in good technical condition.
Keep your tack on track! - an article about tack measurement 7 Issued: 2003, vip publicationstack measurementTwo geometries have found an application in tack measurement. The rst couldbe considered as a mechanical nger, the second one is based on a set ofrotating rollers. The rst rotary instrument for measuring tack was developed inthe 1930’s. Now, nearly 70 years later the industry should take full advantageof advances in technology in order to improve tack measurement. The industryneeds a greater accuracy in measuring tack. The (offset) printing process knowsmany different variables and parameters, which must be tuned to perfection ina process of unparalleled complexity. The optimising of one parameter may leadto a loss of control of another parameter, elsewhere in the process. Everythinginteracts and is therefore difcult to control, ...but not impossible.Fact is; all components in the printing process are working at the limits of theircapabilities, within tight performance margins. Over the past few years this hascreated a demand for a more precise and reproducible method for tack testingwith repeatable results. The method should distinguish differences in on-pressperformance of inks which cannot be detected with existing methods. This willprovide the printer and his suppliers with an improved level of quality assuranceand process control. And that saves money...Because, if any part of the process fails, the commercial or physical conse-quences can be dramatic. If such failure can be predicted by means of newand sophisticated “ngerprint” techniques in tack measurement, than we havearrived at a new level of technology from which we can implement newprocedures and build reliable databases for ink performance.
Keep your tack on track! - an article about tack measurement 8 Issued: 2003, vip publicationsrecommended procedure for tack measurementAfter three years of joint development involving several users in the industry andconstantly improving the technology we have arrived at an unmatched level ofaccuracy in tack measurement with the following recommended procedure... 1. Switch on the waterbath. 2. Switch on the Tack-o-Scope and select a speed of 50 m/min. 3. Allow a 10 - 15 minute warming-up time for the system. 4. Clean the rollers thoroughly using “Böttcherin Yellow” (supplied by Test- print, INC) 5. Place the measuring roller onto the central (brass) roller. 6. Carefully adjust the waterbath untill you obtain the desired temperature (30°) of the measuring roller. This adjustment takes a little time because the rubber is a natural insulator. Check the temperature of the measuring roller using an infrared temperature probe. (supplied by Testprint, INC) (the minimum accuracy of the infrared temperature probe should be 0.1°C and with a surface response correction factor set for rubber). This adjustment for waterbath and Tack-o-Scope is made once and requires a weekly or monthly evaluation only. 7. Pull the measuring roller forward gently and release it so that it will move to its “zero” position. Read and record the “dry tack” value. This is the value the rubber measuring roller produces without ink. 8. Weigh 0.4 grams of calibration varnish onto a piece of releasepaper or alternatively: ll a pipette with 0.4 cc of calibration varnish. 9. Apply the sample evenly onto the distribution roller. It is important to apply the sample in the same way every time you run a test. 10. Use a spatula to “pre-distribute” the sample evenly around the circumfer- ential surface of the distribution roller. Make sure the spatula is cleared of sample. Important notice! 11. Start the tack measurement by engaging the distribution roller so that it contacts the central roller. Than follow this sequence: The procedure described at item 11 can be ammended to suit your personal specication. a. 30 seconds distribution at 50 m/min b. switch to 150 m/min You may vary distribution times and c. record the tackvalue at 150 m/in and at exactly after 60 seconds. speeds. The Tack-o-Scope offers a free programmable user interface where you can select your own values for time and 12. Disengage the rollers and clean them thoroughly. speed. The computer-controlled version offers 13. Engage the measuring roller again after you have allowed the solvent you the possibility to store a pre-pro- to evaporate from the roller surface. Record the “dry tack” value after grammed procedure and run the Tack- 1 minute. When after thoroughly cleaning the roller surfaces, the dry o-Scope accordingly, in an automated tack measurement mode. tack value appears to be more than three digits higher than its original value (see point 7.), than clean the rollers again. Leave the measuring roller in contact with the central roller after cleaning and in between test procedures. (But you must put the measuring roller in its “rest” position when the machine is not in operation. The rubber roller must not be in contact with the central roller when the machine is switched off!) 14. You are now ready to run the next test, however use the temperature probe to cross check the temperature of the measuring roller. 15. The rst 6 items of this procedure only at initial start-up.
Keep your tack on track! - an article about tack measurement 9 Issued: 2003, vip publicationstack and water balanceThe correct water “uptake” of an offset ink is one of the vital properties neededto achieve satisfactory lithographic print quality and performance. Too high ortoo low water sensitivity of the ink can be equally undesirable, and to assistin determining the rate of water uptake and release, a new procedure wasdeveloped on the basis of a tackmeter.This new method comprises of a system for applying water or fountain solutionin a controlled manner. This consists of a roller placed in a special water ductwhich contains the fountain solution. The roller can be placed in contact with orremoved from a chromium spiralled brass centre roller by means of an actuator.The effect of the water is then reected in the variation in the recordedtack reading. Careful study of the comparative traces of different inks, resins,vehicles, etc can reveal signicant differences in the rate of uptake or releaseof water during the test. When the water feed is disengaged, one can observethe tack of emulsied ink.The brass centre roller is equipped with a chromium deposit embedded in thesurface. The surface is smooth but the differences in surface structure enablesthe operator to simulate the effect of fount formulation on the offset platewith the different metals having ink attracting and repelling characteristics. Thisassists in studying the effect of fount formulation in relation to plate sensitivity.The free running and inking up of the “plate” can be simulated and observed.These additional features are very meaningful additions to tack measurementenabling the operator to draw a lot more information from a single test.The test method itself is available in the downloads section of our websitewww.tacknology.US.
Keep your tack on track! - an article about tack measurement 10 Issued: 2003, vip publicationsthe ideal tackmeterA modern tackmeter should be compact, robust, durable and with operatorsconstantly battling for space, it should require as little benchspace in the labas possible.It must be easy to clean the tackmeter and its rollers should be rubber covered,ideally with a hardness which lies between 70 to 80 shore. The rubber compoundmust be formulated for fast conditioning, slow ageing and resistance to cleaningdetergents such as: white spirit or even vegetable oil based degreasers.The tackmeter should be fully automated in order to expel minor operatorinuences and to enhance repeatability. It should work preferably with small inkquantities which create a layer thickness of 4 - 6 micron for minimum “slip”and maximum product dependent tack performance differentiation. High speedoptions should include a selection which simulates circumferential speeds in theink train of modern newsprint presses. It should be possible to carry out ameaningful misting test at these speeds with a means to prevent particles get-ting airborne. Tackmeters of the type “Tack-O-Scope” (model W) can carry outmeasurements which record the tack as function of plate sensitivity, practicallysimulating the clearing and inking up of the printing plate which is a very usefultest providing extra information about the inuence on tack of different fountformulations.It must be possible to capture the data by means of dedicated computersoftware and make easy comparisons. If it is possible to present tack dataagainst xed reference graphs on screen in real time....ideal. Export of datato excel is a must. If the computer can also control the instrument instead ofone way data acquisition, than that is another advantage but a manual speedselection on the instrument itself should also be possible. The tackmeter mustbe provided with a dedicated cooling system. It is not recommended to putmore than 1 instrument on an external water bath, neither paralel nor in series.In addition, it should be noted that the capabilities of external water bathsare usually a bit overkill for the application and an additional, relatively largeinvestment. It must be possible to measure the surface temperature of therubber covered measuring roller for consistency in measurements and increasedreproducibility. Calibration should be a simple and straight forward procedurepreferably with readily available tack graded stable calibration pastes which donot mist.The ideal tackmeter offers the capability to measure tack according to commonlyaccepted US and European standards practically avoiding the purchase of twodifferent instruments in order to comply. This can only be made possible throughan algorithmic conversion and a proportionality factor in the instrument’s embed-ded software. Switching between standards and correlating results is then madepossible.For a peek at new and ideal tackmeters visit www.testprint.com/tack.
Keep your tack on track! - an article about tack measurement 11 Issued: 2003, vip publicationsmaintenanceA tackmeter really is a specic kind of rheometer. Usually companies haveagreed calibration and maintenance contracts for their viscometers and rheom-eters. There is a good reason for that: these instruments are required tomeasure data within very small tolerances with very little play for errors. This isonly possible when dedicated sensing techniques are applied in combination withsophisticated embedded machine software.Intensive daily use however and improper procedures, inadequate cleaningand lack of knowledge of the equipment can all be causes for deviations inmeasured results consequently leading the company’s conformity with productspecications off track. Preventative maintenance and calibration services helpto identify areas of increased technical attention adequately and in a timelymanner. Possible deviations are prevented rather than corrected.This same philosophy also applies to a “roller rheometer” (= tackmeter). Butwhether it is the instrument’s resemblance with a 3-roll mill or not, a tackmeteris often too easily compared with a 3-roll mill. Regular maintenance is oftenleft to the production maintenance department instead of well trained dedicatedinstrument engineers from the manufacturer. Generally speaking; tackmetersare unique in the sense that they combine a robust and durable design withvery sensitive measuring and data acquisition technology. This combinationdemands the skills of a special type of engineer with a multi-disciplinarytechnical background which combines ne mechanical and electronic knowledge.Recent surveys have revealed that tackmeters maintained by the manufacturers’representative “live twice as long” as tackmeters without scheduled preventativemaintenance.
Keep your tack on track! - an article about tack measurement 12 Issued: 2003, vip publicationscalibrationThere are different philosophies and approaches in the calibration of tackmeters.Perhaps we should rst answer the question: “why is calibration necessary?”. Theanswer lies in a number of cumulative reasons which cause the tack sensors notto return to their calibrated “zero” setting. There can be a pure mechanical orelectronic “drift” as reason for this phenomenon, or a combination of both. Theeasiest way of dealing with this is to “zero” a tackmeter manually before themeasurement commences. This however has the disadvantage that we electroni-cally compensate for a deviation which a mechanical cause can be accountedfor. Even poorly cleaned rollers may be a cause although they seem perfectlyclean. “Zero-ing” does not eliminate these causes, it merely moves them “outof sight”.There are also tackmeters where one cannot zero the non tack reading, whenthe tacksensors are in a static position. Although this may be annoying to theoperator at least it is telling us that something is wrong and should be corrected.What is crucial in calibration is linearity of the measuring scale. This scale shouldbe presented as a straight line through a minimum of two but preferably threexed points. It is possible to measure linearity by using three weights of knownaccuracy which simulate a tack force and which will cause the tack sensor todisplay 3 known values. A line through these values should be straight. Thisdoes not say anything about the zero point of this line. It may represent alinear scale but starting at 15 or 150 tack. It is important to use a special rollerwhich can be hooked to the tack sensor. Running this roller should representa reading of “zero tack” on the display. If not, the scale should be moved upor down to zero.There is one important thing we have overlooked right now. That is the conditionof the measuring roller. If the rubber is hard or glazed, rough or cracked, it maylead to considerable changes in tack readings compared from one instrumentto another. With other words, the happy medium lies in a calibration procedurewhich takes the condition of the measuring roller into account. This requirescalibration with the measuring roller in place. This in turn requires a calibrationmedium (paste) of which the tack is graded and extremely stable. Moreover, thecalibration paste itself must be chemically stable and it should notMore InformationFor more information about this article or vip publications, please visit ourwebsite at www.tacknology.US.VIP publications is a new portal for the printing industry provided by Testprint. We offer news, articles,links and detailed information about new technology. The emphasis is placed on distribution of newinformation about testing technologies, performance monitoring of printing materials, six sigma, testingstandards and results of research projects.Once a month VIP publications will publish interesting articlesabout relevant technology issues in our Industry.