Lithotronic Torque Reproducibility

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Introduction to a new method for dynamic tack
measurements
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Objective

• Results from the traditional tack determination
  method are essentially effected by
• film formation process
• film splitting process
• the dynamic mechanical properties of the
  elastomer roller
• the surface conditions of the elastomer roller
• A new method has been developed for measuring the
  surface forces in a roller nip. A dynamic
  miniature force sensor is placed on the roller
  surface and scans the forces versus rotation
  angle under typical press conditions. Therefore,
  the effects of the film formation and film
  splitting process are separated and as a result
  the real tack of the ink can be characterized.

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Definition of Tack

• The tack is the force required to split a thin
  fluid film of a printing ink or vehicle between
  two rapidly separating surfaces it is a
  rheological parameter indicative of internal
  cohesion of the fluid.
• Apparent tack is the tack reading obtained at
  specific set of conditions, like temperature,
  shear rate, roller geometry and composition,
  emulsification, time etc.

Reference ASTM D 4361 97 (Reproved
2002) Standard Test Method for Apparent Tack of
Printing Inks and Vehicles by a Three-Roller
Tackmeter
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Significance of Tack

• Quality of ink transfer through the roller
  distribution system
• Statistical variations of color density
• Tendency of paper ripping at the roller based
  offset printing
• Tendency of ink particles or globules ejection
  from the roller distribution system (flying and
  misting)

References 1. ASTM D 4361 97 (Reproved
2002) Standard Test Method for Apparent Tack of
Printing Inks and Vehicles by a Three-Roller
Tackmeter 2. Dipl.-Ing. Helmut Wordel Entwicklung
eines Zügigkeitsmeßplatzes Research Report
Fogra, Munich 1999
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Principal disadvantages of traditional method for
tack determination

• film formation is mainly influenced by inner
  friction but film splitting is mainly influenced
  by inner cohesion
• no separation of forces needed for film formation
  and film splitting as they are combined in the
  measurement
• there is no general correlation between these two
  forces because they are based on different
  physical effects
• influence of dynamic mechanical properties of the
  elastomer roller
• these properties and surface conditions are
  influenced by material aging, solvent
  contamination, mechanical stress on surface,
  process conditions etc.
• this may explain the large errors in tack
  measurement results from instruments of the same
  type

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Advantages of new method in comparison to the
traditional method

• Exact separation between forces needed for film
  formation and film splitting
• force progress versus rotation angle is scanned
  and can be analyzed
• elimination of the influence of the elastomer
  roller
• a lot of additional information is available from
  a single test method
• length of roller gap
• Length of film formation process
• Maximum force and integral force of film
  formation process (indicates inner friction of
  ink)
• Length of film splitting process
• Peak force and integral force of film splitting
  process (TACK, inner cohesion of ink)
• Statistical variations of final splitting angle
  (misting or flying tendency and indication)
• leads to a new level of accuracy, reproducibility
  and reliability in tack measurement

10
New methodology used in the RealTack instrument
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RealTack Capabilities

• The RealTack allows real tack measurements under
  typical conditions on high speed presses
• Independent parameter are
• surface speed
• temperature
• time
• line force
• emulsification capacity (option)
• time
• factory calibrated but can be re-calibrated by
  operator at anytime
• physical unit of tack force is N/m2
• controlled completely by new Windows software
  WinTACK
• easily to clean, all parts that may be
  contaminated with ink are made of stainless steel
  or anodized aluminum
• automatic roller engagement and disengagement
  system for easy handling and roller protection
  after measurement stop
• gearless electronic driving system for continuos
  speed setting
• state-of-the-art technology of mechanics,
  electronics and software

12
Roller and sensor design

• high speed solid state force sensor is integrated
  in roller surface
• forms a common hard plane without any edges
  because it is manufactured in a common process
  with other roller surface
• wireless energy and data transfer increases
  robustness of system
• PT 1000 temperature sensor is integrated very
  close to roller surface
• brass and chrome (optionally) plated sensor
  surface

13
Principle of roller calibration

• Continuous self zeroing system by real time data
  processing
• Calibration parameter are saved in the roller
  data memory

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• closed cycle cooler is not required
• high stability by fast system response
• PT 1000 temperature probe is integrated near
  roller surface
• accuracy 0.1 K

15
Software WinTACK

• controls the complete measurement of the
  instrument
• predefined and user definable programs
• comprehensive data evaluation and presentation
  (2D/3D diagrams)
• data transfer via ASCII tables is possible
• no license is required
• investment certainty by periodical updates

16
Sample measurement

• Peak and Integral Tack versus EC
• integral and peak tack are increasing with higher
  water content
• at 130 water capacity (56 water volume) the
  tack is out of phase

17
Sample measurement

• Film thickness has been increased stepwise from
  12 µm to 50 µm
• peak and integral tack are nearly constant
• the increased inner friction doesnt affect the
  tack result
• ink volume on roller doesnt affect the tack
  measurement

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Summary

• Measurements on roller surface eliminate
  systematic errors caused by differences in roller
  surface conditions
• In addition to this the inner friction of the
  film doesnt affect the tack result anymore
• These advantages are leading to new levels of
  measurement accuracy and reliability
• Real time data acquisition provides rotation
  angle dependent force measurements which can be
  evaluated as force function versus angle
  graphically.
• The physical unit N/m2 represents the real tack
  force that is required for splitting an ink film
  from a defined area
• Extensive tack characterization is possible by
  systematical curve analysis providing a wealth of
  information instead of a single tack value.
• The miniature sensor devices in the roller
  surface enable new insights in the ink film
  morphology and may be used for instance to
  characterize nanostructered formulations.

Bob Cook, Novomatics GmbH