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W90 – Leaflet | Linting: Pre-Wet

W90 - Linting: Pre-wet
AMSTERDAM - Multi-purpose printability tester
Version 2022-11

Introduction

Pick is the name commonly given to damage to the paper surface occurring during the printing operation. When the printing form is lifted from the paper, the ink exerts on the paper a force, which increases with increasing viscosity and tack of the ink and with increasing printing speed. When this force exceeds a critical value, which depends on the paper, the surface of the paper is damaged.
The minimum printing speed at which pick occurs is a measure of the pick resistance of the paper.

In the offset printing technique paper is dampened with fountain solution, which means that the surface strength will decrease. For that reason, it can be of interest to carry out the IGT pick test after dampening the paper. This leaflet describes this method.

Sometimes it is not only a matter of surface strength. This test is called “Linting”: a combination of dust, fluff and lint (loosely bound fibers). The minimum printing speed at which linting occurs is a measure of the pick resistance of the paper.

W44 Leaflet Linting with printing disc with rubber of 85 Shore A.
W70 Leaflet Linting with printing disc with rubber of 65 Shore A.
W90 Leaflet Linting: Pre-wet describes the pre-wetted linting test to measure the influence of fountain solution.

Principle

Paper is dampened with dampening fluid at a constant speed and subsequently the paper or board is printed with a pick-test ink of known tack, at constant printing force and at an increasing velocity. For linting the result is observed visually as the fibers (dust, fluff and lint) which has been pulled off from the surface of the paper. Linting velocity and linting resistance are derived from the distance between the starting point of the print and the point on the print where the first damage is observed.

In the case of board when the adhesion between the outer layer and the next layer is too low, delamination between the layers can occur during printing.

Linting: Pre-wet, print

Method of operation

Materials

  • [402.354.002] Dampening disc 0,25 µm  or
  • [402.354.005] Dampening disc 0,50 µm or
  • [402.354.010] Dampening disc 1,00 µm
  • [435.031.412] Doctor blade holder
  • [180.431.710.001] Doctor blades
  • [404.001.005] Paper packing 55 mm
  • [402.687.412] Printing Disc 50 mm black rubber 65 Shore A or
  • [402.634.412] Printing Disc 50 mm black rubber 85 Shore A
  • [404.800.001] Pick test ink low tack or
  • [404.800.002] Pick test ink medium tack or
  • [404.800.003] Pick test ink high tack
  • [404.000.220] Pick Start Viewer and/or
  • [441.000.040.190] Delamination viewer V shape
  • [441.000.040.280] Delamination viewer  U shape
  • [466.000.710] IGT High Speed Inking Unit (HSIU)
  • [466.003.003] Top roller 4 segments for conventional inks
  • [408.000.200] IGT ink pipette 0,01 ml resolution

Testing conditions

Standard test atmosphere according to ISO 187 (23,0 ± 1,0 °C and 50 ± 2% RH)

Sample conditioning

Be aware of the fact that a different way of conditioning of the samples can have influence on the results.

Standard settings

AMSTERDAM – multi-purpose printability tester

  • Printing speed: increasing, end speed at choice
  • Printing force: 625 N
  • Ink film thickness 8 µm
  • Camera lamp high (Not applicable for Amsterdam Basic type)
  • Checkbox Scan activated  (Not applicable for Amsterdam Basic type)

IGT High Speed Inking Unit 4 (HSIU)

  • Water bath: 23,0 °C (73,4 °F)
  • Mode 2:
    • Start-up time [10 s]
    • Distribution time 1 [20 s] | Distribution speed 1 [0,5 m/s]
    • Inking time discs [15 s]

Preparation

  1. Cut the paper to be tested in strips of 55 x 340 mm and mark them with top and/or bottom side, machine and/or cross direction and a code for the type of paper.
  2. Select method Linting: Pre-wet.
  3. If desired, touch the checkbox Scan to scan and save.
  4. Mount the paper packing on the sector.
  5. Fill the ink pipette with the desired IGT pick test ink.
  6. Set the HSIU in Mode 2 (for changing settings and ink application, refer to your HSIU manual).
  7. Apply 0,35 ml of ink on a segment of the top roller.
  8. Place the printing disc on a shaft of the HSIU for inking.

NOTE: Due to the drying properties of the ink it is not advised to add some ink after a test.

Execution

Doctor blade in holder
  1. Mount a strip of paper or board to be tested over the packing on the sector of the Amsterdam.
  2. Place a clean dampening disc on shaft 1 and mount the blade holder onto the two pins of the accessory holder with the blade pointing to the right and downward:
  3. Place a small strip of cotton pad (5 x 50 mm) just left of the doctor blade on the dampening disc
  4. Place the printing disc with ink on the 2nd printing shaft of the tester.
  5. Touch the button [Print] and press both side buttons to rotate the sector, both shafts into the start position and to lower the blade.
  6. Release side buttons and apply dampening fluid onto the cotton pad. When done press both side buttons again.
  7. Keep both side buttons pressed to meter the excess dampening fluid, moisten the paper on the sector and make the print.
  8. Assess the result visually (see section Assessment below.)
  9. Before starting a new test, clean the printing disc and the used segment of the inking unit (or use the next segment)
    and start inking the printing disc again.
  10. Add dampening fluid on the cotton pad if necessary.
  11. It is recommended to perform the test at least five times per sample.

 

Cleaning

  1. Clean the rubber printing disc with lint-free rags and naphtha.
  2. Clean all rollers of the inking unit, including the metal ones with lint-free rags and naphtha.
  3. Make sure the Amsterdam is clean and if necessary clean with some ethanol or cleaning naphtha on a lint free rag.
  4. Clean the damping disc and doctor blade with ethanol.

Assessment

Pick Start Viewer

To view the result of linting, place the test strip under the opening of the pick start viewer. Looking from above into the viewer, assess the test strip and mark the point where linting begins.

To view the result of delamination, place the test strip in the delamination viewer and by carefully moving the test strip along the U-profile or V-profile check where delamination starts and mark this point.

Measure the linting distance between the starting point of the print (= the centre of the initial print nip) and the point where linting or delamination begins at the test strip. Make a record in mm.

If desired derive the linting velocity in m/s from the velocity table or with the formula:

Delamination viewer U and V

For linting: Vp = 0,005 * Ve * dl
For delamination: Vd = 0,005 * Ve * dd

 

Herein is:

  • Vp = velocity at point d (in m/s)
  • Vd = delamination at point d (in m/s)
  • Ve = set end speed (in m/s)
  • dl = distance from beginning of the print to beginning of linting (in mm, ≥50 mm)
  • dd = distance from beginning of the print to beginning of delamination (in mm, ≥50 mm)
Velocity tabel

NOTES

  1. Although the camera can be activated for a scan, this option saves the scanned image for future reference. Assessment can be done visually with a pick start viewer.
  2. If linting or delamination occurs < 50 mm from the starting point of the print the test must be repeated at a lower speed. In case the lowest speed has been applied already a change to a lower grade of the pick test ink is necessary.
  3. If no linting occurs or occurs >180 mm from the starting point of the print, the test must be repeated at a higher speed. In case the highest speed has been applied already a change to a higher grade of the pick test ink is necessary.
  4. Pick test inks have a shelf stock time: <3 years; open time: <1 year.

Randstad 22 - 02

1316 BX Almere

The Netherlands

P.O.box 22022

1302 CA Almere

The Netherlands

+31 20 40 99 300 +31 20 40 99 339 info@igt.nl
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