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Keep your tack on track! - an article about tack measurement 1
Issued: 2003, vip publications

Keep your tack on track!
an article about tack measurement


what is tack?
Over the past decades several denitions of tack have been given by various
experts in the Industry.

In 1967, Prof. Jan Mewis of the Catholic University Louvain, Department of
Chemical Engineering in Belgium performed a very interesting rheological study
into tack. He dened tack as follows:
“The resistance which a thin ink lm offers against fast splitting under certain
printing conditions”

7 years later, in 1974 the Technical Committee of the Group for Printing Inks
of the European Association of Ink and Paint Manufacturers dened tack as
follows:
“Tack is the drag force between two rotating rollers caused by the presence of
an ink layer on their surfaces”

The ISO 12634:1996(E) denes tack as:
“Restoring force between two rotating rollers of a given width caused by the
splitting of an ink or vehicle lm on the roller surfaces”

Perhaps the best and most accurate denition is mentioned in the ASTM standard
for tack measurement: D 4361-97:
“Tack - a function of the force required to split a thin uid lm of a printing ink
or vehicle between two rapidly separating surfaces; it is a rheological parameter
indicative of internal cohesion of the uid”

These are all defenitions of tack but the term “tackmeter” is perhaps not a good
description of instruments which measure tack. Given the scope of operation and
the mathematical uid modelling of thin ink layers on rotational tackmeters, the
term “roller rheometer” would be a better description of this type of instrument.


the importance of tack
Tack of printing inks controls their high speed transfer properties. It may also be
meaningful as to the ability to predict paper picking and wet trapping in multi
color printing. Tack measurements are a vital factor in the development and
production of printing inks and the media onto which they are transferred.

During the printing process a printing ink is subjected to mechanical forces
of divergent magnitude. In each phase, ink duct, distribution or transfer, an
optimal response of the ink is required. However these responses are difcult to
combine in one material. Therefore a printing ink formulation can be considered
as a compromise between contradictory requirements. With respect to press
behaviour recent studies begin to describe the effect of all parameters involved
but by no means can the full complexity of the offset printing process be
captured in convenient models or formulas, at least not untill recently.

The measurement of tack helps to a large extent to isolate some of the factors
which inuence nip ow and transfer and therefore tack remains to be one
of the critical parameters to accurately measure in the formulation and QC of
lithographic printing inks.


parameters which affect tack measurement
When Prof. Reed of the American Lithographic Technical Foundation rst pre-
sented his mathematical model for measuring tack on a friction less roller system
in the 1930’s, nobody could have imagined what impact this new phenomena
would have on the characterisation of printing inks and their components.

Yet, 70 years later and 3 years into the new millennium, the measurement of
tack is as important as ever before. A little over three years ago, several users
of tackmeters raised the question whether it would be possible to perform a
study into the human, technical and environmental aspects which inuence the
outcome of tack measurements. The result of the study would perhaps help to
better understand these inuential aspects and they would form the basis for a
possible improvement of the measurement technique.

The ultimate goal would be to establish a recommended procedure which, once
observed and practised consistently, would lead to an enhanced compatibility
of measured results, indifferent from the location on Earth. The reason for this
improved procedure is simple: everyone seems to appreciate the results of
their tack meter for intemal QC or R&D work, however communication between
suppliers and customers is often hampered due to different results obtained on
different instruments for the same ink or vamish.

The rest of this publication is available in the downloads section of our website
www.tacknology.US.


reproducibility
There 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 rollers
Surveys and technical investigations have revealed that tackmeters with rigid
metal measuring rollers have a fairly low accuracy at thinner ink lms. Rubber
rollers of intermediate hardness show that the regression coefcients reveal a
smaller effect of layer thickness on tack readings. The rollers should stabilise
quickly, age slowly and it must be possible to assess when the roller should
be replaced.

A very accurate waterlevel:
When the tackmeter is installed one should pay attention to levelling it in an
exact horizontal plane. (And it should be left that way). Deviations from the
horizontal plane will cause the weight of the measuring roller to have an adverse
effect on the measurement of tack. The heavier the roller, the more this adverse
effect will be experienced. And remember: “a horizontal plane is determined by
3 points, ...not 4!”

A near frictionless measuring system:
Older type tackmeters work on the principle that tack is a force which is
required to keep the measuring roller in its “0” position without displacement,
counteracting a dragforce. Some types make use of an oil damper which makes
it difcult to determine whether the tack is inuenced by the viscosity of the
oil in the damper.

Older type tackmeters may also have up to 5 different points in their mechanical
construction, which may induce drag, consequently disturbing the measurement
of the actual tack related data. If one wishes to measure the splitting force of
a thin ink lm, there should be no parts which can induce drag or inuence the
measurement in any other way.

A thin ink lm:
This has always been a challenge to manufacturers. Measuring tack with a
thick ink lm layer is more attractive because small inaccuracies caused by the
equipment are lost within a relatively large margin for errors. Besides; a high ink
layer thickness leads to the question: ‘do we measure the actual lm splitting or
a 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 be
the margin for errors. Consequently, the construction of the instrument, the
chosen rubber compounds, roller weight and diameter, the tack sensors and data
acquisition must all be tuned to perfection and capable of presenting data within
a 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. Heavy
rollers 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 faster
if not properly distributed at a certain rate. The distribution therefore needs
be proportional to the circumferential speed of the rollers. This will provide a
sufcient rate of energy to prevent the ink from setting to quickly. The design of
the distribution mechanism and its characteristic will be crucial in particular when
performing stability tests. The design of the distribution system on tackmeters
requires a careful balance of roller diameter, distribution rate, drag induced by
the mechanical parts and RPM proportional to the circumferential speed of the
main driven roller. Not just any random design will do the job.


conversion (abstract)
This has always been a tricky issue. Direct conversion is understood here to
mean the procedure whereby a single reading on one type of tackmeter is
converted to a single reading on another type of tackmeter. Several studies in
the past have revealed that a layer thickness of 10 micron was the best basis
for conversion.This lead to some controversy since the more recent designs in
tackmeters were developed to work with thin ink lms (4 - 6 microns) for the
purpose of accurately differentiating performance characteristics of inks.

During previous studies, one discovered that at a layer thickness of 10 microns
the correlation coefcients with a linear regression of one group of tackmeters
on another (all different types) were dened in a bandwidth of 0.858 - 0.987.
To arrive at the most probable conversion formulae, a study was performed into
conversion of tack-speed curves.

The test results suggest that at higher values of ink lm thickness, a direct
correlation of tack readings on another type of tack meter can be found. When
the hardness of rubber on the rollers is too different however, the changes of
large errors are real. Samples with steep tack-speed curves can be converted
with more condence than others.

What about correlation at lower ink lm thickness? This is possible provided
there is not too much difference in roller hardness. Correlation requires a double
conversion through an algorithmic scale which is based on a tack vs. square root
of speed representation and through linear regression coefcients for each type
of instrument. The scale can present a proportionality factor. This factor will not
change considerably with layer thickness provided the instruments have similar
roller hardness and are in good technical condition.


tack measurement
Two geometries have found an application in tack measurement. The rst could
be considered as a mechanical nger, the second one is based on a set of
rotating rollers. The rst rotary instrument for measuring tack was developed in
the 1930’s. Now, nearly 70 years later the industry should take full advantage
of advances in technology in order to improve tack measurement. The industry
needs a greater accuracy in measuring tack. The (offset) printing process knows
many different variables and parameters, which must be tuned to perfection in
a process of unparalleled complexity. The optimising of one parameter may lead
to a loss of control of another parameter, elsewhere in the process. Everything
interacts and is therefore difcult to control, ...but not impossible.

Fact is; all components in the printing process are working at the limits of their
capabilities, within tight performance margins. Over the past few years this has
created a demand for a more precise and reproducible method for tack testing
with repeatable results. The method should distinguish differences in on-press
performance of inks which cannot be detected with existing methods. This will
provide the printer and his suppliers with an improved level of quality assurance
and 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 new
and sophisticated “ngerprint” techniques in tack measurement, than we have
arrived at a new level of technology from which we can implement new
procedures and build reliable databases for ink performance.


recommended procedure for tack measurement
After three years of joint development involving several users in the industry and
constantly improving the technology we have arrived at an unmatched level of
accuracy 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
specication.
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.


tack and water balance
The correct water “uptake” of an offset ink is one of the vital properties needed
to achieve satisfactory lithographic print quality and performance. Too high or
too low water sensitivity of the ink can be equally undesirable, and to assist
in determining the rate of water uptake and release, a new procedure was
developed on the basis of a tackmeter.

This new method comprises of a system for applying water or fountain solution
in a controlled manner. This consists of a roller placed in a special water duct
which contains the fountain solution. The roller can be placed in contact with or
removed from a chromium spiralled brass centre roller by means of an actuator.

The effect of the water is then reected in the variation in the recorded
tack reading. Careful study of the comparative traces of different inks, resins,
vehicles, etc can reveal signicant differences in the rate of uptake or release
of water during the test. When the water feed is disengaged, one can observe
the tack of emulsied ink.

The brass centre roller is equipped with a chromium deposit embedded in the
surface. The surface is smooth but the differences in surface structure enables
the operator to simulate the effect of fount formulation on the offset plate
with the different metals having ink attracting and repelling characteristics. This
assists 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 measurement
enabling the operator to draw a lot more information from a single test.

The test method itself is available in the downloads section of our website
www.tacknology.US.


the ideal tackmeter
A modern tackmeter should be compact, robust, durable and with operators
constantly battling for space, it should require as little benchspace in the lab
as 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 compound
must be formulated for fast conditioning, slow ageing and resistance to cleaning
detergents such as: white spirit or even vegetable oil based degreasers.

The tackmeter should be fully automated in order to expel minor operator
inuences and to enhance repeatability. It should work preferably with small ink
quantities which create a layer thickness of 4 - 6 micron for minimum “slip”
and maximum product dependent tack performance differentiation. High speed
options should include a selection which simulates circumferential speeds in the
ink train of modern newsprint presses. It should be possible to carry out a
meaningful 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 out
measurements which record the tack as function of plate sensitivity, practically
simulating the clearing and inking up of the printing plate which is a very useful
test providing extra information about the inuence on tack of different fount
formulations.

It must be possible to capture the data by means of dedicated computer
software and make easy comparisons. If it is possible to present tack data
against xed reference graphs on screen in real time....ideal. Export of data
to excel is a must. If the computer can also control the instrument instead of
one way data acquisition, than that is another advantage but a manual speed
selection on the instrument itself should also be possible. The tackmeter must
be provided with a dedicated cooling system. It is not recommended to put
more 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 baths
are usually a bit overkill for the application and an additional, relatively large
investment. It must be possible to measure the surface temperature of the
rubber covered measuring roller for consistency in measurements and increased
reproducibility. Calibration should be a simple and straight forward procedure
preferably with readily available tack graded stable calibration pastes which do
not mist.

The ideal tackmeter offers the capability to measure tack according to commonly
accepted US and European standards practically avoiding the purchase of two
different instruments in order to comply. This can only be made possible through
an algorithmic conversion and a proportionality factor in the instrument’s embed-
ded software. Switching between standards and correlating results is then made
possible.

For a peek at new and ideal tackmeters visit www.testprint.com/tack.


maintenance
A tackmeter really is a specic kind of rheometer. Usually companies have
agreed calibration and maintenance contracts for their viscometers and rheom-
eters. There is a good reason for that: these instruments are required to
measure data within very small tolerances with very little play for errors. This is
only possible when dedicated sensing techniques are applied in combination with
sophisticated embedded machine software.

Intensive daily use however and improper procedures, inadequate cleaning
and lack of knowledge of the equipment can all be causes for deviations in
measured results consequently leading the company’s conformity with product
specications off track. Preventative maintenance and calibration services help
to identify areas of increased technical attention adequately and in a timely
manner. Possible deviations are prevented rather than corrected.

This same philosophy also applies to a “roller rheometer” (= tackmeter). But
whether it is the instrument’s resemblance with a 3-roll mill or not, a tackmeter
is often too easily compared with a 3-roll mill. Regular maintenance is often
left to the production maintenance department instead of well trained dedicated
instrument engineers from the manufacturer. Generally speaking; tackmeters
are unique in the sense that they combine a robust and durable design with
very sensitive measuring and data acquisition technology. This combination
demands the skills of a special type of engineer with a multi-disciplinary
technical 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 preventative
maintenance.


calibration
There are different philosophies and approaches in the calibration of tackmeters.
Perhaps we should rst answer the question: “why is calibration necessary?”. The
answer lies in a number of cumulative reasons which cause the tack sensors not
to return to their calibrated “zero” setting. There can be a pure mechanical or
electronic “drift” as reason for this phenomenon, or a combination of both. The
easiest way of dealing with this is to “zero” a tackmeter manually before the
measurement commences. This however has the disadvantage that we electroni-
cally compensate for a deviation which a mechanical cause can be accounted
for. Even poorly cleaned rollers may be a cause although they seem perfectly
clean. “Zero-ing” does not eliminate these causes, it merely moves them “out
of sight”.

There are also tackmeters where one cannot zero the non tack reading, when
the tacksensors are in a static position. Although this may be annoying to the
operator 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 should
be presented as a straight line through a minimum of two but preferably three
xed points. It is possible to measure linearity by using three weights of known
accuracy which simulate a tack force and which will cause the tack sensor to
display 3 known values. A line through these values should be straight. This
does not say anything about the zero point of this line. It may represent a
linear scale but starting at 15 or 150 tack. It is important to use a special roller
which can be hooked to the tack sensor. Running this roller should represent
a reading of “zero tack” on the display. If not, the scale should be moved up
or down to zero.

There is one important thing we have overlooked right now. That is the condition
of the measuring roller. If the rubber is hard or glazed, rough or cracked, it may
lead to considerable changes in tack readings compared from one instrument
to another. With other words, the happy medium lies in a calibration procedure
which takes the condition of the measuring roller into account. This requires
calibration with the measuring roller in place. This in turn requires a calibration
medium (paste) of which the tack is graded and extremely stable. Moreover, the
calibration paste itself must be chemically stable and it should not

More Information
For more information about this article or vip publications, please visit our
website 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 new
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standards and results of research projects.Once a month VIP publications will publish interesting articles
about relevant technology issues in our Industry.

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