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W32- Leaflet | Wet pick – Wet Repellence

Wet pick - Wet Repellence
AMSTERDAM - Multi-purpose printability tester
Version: 2021-11

INTRODUCTION

In offset printing, the fact that water as well as ink is transferred to paper may well lead to complications. Especially in process printing, where the paper is wetted several times, water may affect ink transfer because it changes the surface structure of the paper. For example, it is possible water is weakening the paper surface to such an extent, that paper particles are pulled off from the surface by the tack of the ink. This phenomenon is known as wet pick. Another possible effect of water is that the paper does not accept ink because the water did not completely penetrate into the paper. This is called wet repellence. Both phenomena may occur simultaneously and sometimes it is difficult to differentiate between them, because they both appear as white spots in the print.

During offset printing a moisture film of about 0,2 to 0,3 µm (0,2 to 0,3 g/m²) per colour is applied to the paper. To investigate and check a paper for wet pick and wet repellence a water film of this thickness is needed and sometimes a thicker water film to imitate the multi colour process.
The occurrence of wet pick and wet repellence is also influenced by the time interval between dampening and printing. This is the case in multi colour printing presses. The time lapse between two colours depends on printing speed and the distance between printing units. In practice this time is dependent to the type of printing press and varies between about 0,03 and 1 s.
This leaflet describes a method with a rubber printing disc of 85 Shore A. Leaflet W66 describes the method with a rubber printing disc of 65 Shore A.

PRINCIPLE

An engraved dampening disc of a known volume on the first printing shaft is wetted with an excess of dampening fluid that subsequently is metered with a doctor blade. The paper to be tested is wetted with the remainder of the dampening fluid and then printed upon with a standard pick test ink, that is transferred to the paper with a rubber disc on the second printing shaft. This print is made after a previously set time interval. Here after the printed sample is checked on wet pick or wet repellence. For that a strip of reference paper is mounted on the sector and a print is made with the same rubber printing disc (with the residu of ink).

A part of the sample is damped and another part is not damped. Both areas can be compared with the help of reference paper. It is adviced to use a standard ink and a standard reference paper to be able to compare the results of several samples.

MATERIALS

 

  • [402.354.002] Damping disc 0,25 ml/m² or
  • [402.354.005] Damping disc 0,50 ml/m² or
  • [402.354.010] Damping disc 1,00 ml/m²
  • [435.031.412] Doctor blade holder
  • [180.431.710.001] Doctor blade
  • [404.001.005] Paper packing 55 mm
  • [402.634.720] Printing Disc black rubber 85 Shore A
  • [404.800.001] IGT Pick Test ink no 1 – Low tack or
  • [404.800.002] IGT Pick Test ink no 2 – Medium tack or
  • [404.800.003] IGT Pick Test ink no 3 – High tack
  • [404.009.029] Reference paper, C2846, 55 mm
  • [466.000.710] IGT High Speed Inking Unit (HSIU)
  • [466.003.003] Top roller – 4 segments – conventional inks
  • [408.000.200] IGT ink pipette 0,01 ml resolution

  • 3 strips of substrate to be tested, dimensions 55 x 340 mm
  • 3 strips of reference paper, dimensions 55 x 340 mm
  • Dampening fluid (water or fount)
  • Cotton pads
  • Lint free rags
  • Ethanol or Cleaning Naphtha

TESTING CONDITIONS

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

SAMPLE CONDITIONING AND PREPARATION

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: 1,0 constant
  • though with interval time ≤1 m/s, without ≤3 m/s
  • Printing force: 650N
  • Interval time at choice
  • Ink film thickness 3,2 µm (= volume 0,14 ml)
  • 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): Mode 2

  • Waterbath: 23.0 °C (73.4 °F)
  • Start-up time [10s]
  • Distribution time 1 [20s] | Distribution speed 1 [0.5m/s]
  • Inking time discs [15s]

PREPARATION

  1. Cut paper strips of 55*340 and mark with top and/or bottom side, machine and/or cross direction and a code for the paper.
  2. Select method Wet Pick/Repellence and select preset values or change settings of force, speed and interval to your liking.
  3. If desired, touch the checkbox Scan to Scan and save (not applicable for Basic type Amsterdam)
  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 0,14 ml of ink on a segment of the top roller.
  8. Place the printing disc on a shaft of the HSIU and ink the printing disc.

EXECUTION

FOLLOW THE INSTRUCTIONS ON THE DISPLAY:

  1. Docter blade holder

    Mount a strip of paper or board to be tested 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
  3. pointing to the right and downward:
  4. Place a small strip of cotton pad (5×50 mm) just left of the doctor blade on the dampening disc.
  5. Place the printing disc with ink on the 2nd printing shaft of the tester.
  6. Touch the button [Print] and press both side buttons to rotate the sector, both shafts into the start position and to lower the blade.
  7. Release side buttons and apply dampening fluid onto the cotton pad. When done press both side buttons again.
  8. Keep both side buttons pressed to meter the dampening disc, moisten the paper on the sector and make the first print.
  9. The camera lowers and scans the print, after which the display says to change the sample on the sector. At this time side buttons may be released.

    Bevel

    Mount the second sample on the sector and press both side buttons again to make a second print.

  10. Keep side buttons pressed till a scan has been made and the display says DONE.
  11. Before starting a new test, clean the disc and the used segment of the inking unit (or use the next segment) and start inking the disc again.
  12. Add dampening fluid on the cotton pad if necessary.
  13. It is recommended to perform the test at least three times per sample.

IF THE CAMERA WAS ACTIVATED

The print is scanned and the camera moves upward.
Save or discard the results.
Start from point 1 for a next test. After finishing all tests, press [BACK] and clean and store all parts.

CLEANING

  1. Clean the rubber printing disc with lint-free rags and cleaning 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. Store all parts in the appropriate box.

ASSESSMENT

If a scan was made:

the picture of both prints is saved. Wet repellence in the first print shows as a non-printed area. In the second print the areas C and D have accepted less respectively more ink than areas A and B. In between the two fields of the print, a transition may show as partly dampened and partly printed only.

As scans are saved as black and white images, these are only to be used for illustrative reasons.

If no scan was made:

the assessment may be done visually or with a densitometer.

Visually (d = density)

  • No wet pick / no wet repellence: d A = d B and d C = d D. No paper particles are left behind on the printing disc after the first print.
  • Wet repellence: d A > d B and d C < d D. No paper particles are left behind on the printing disc after the first print.
  • Wet pick: d A > d B and d C > d D. There are paper particles are left behind on the printing disc after the first print.

Densitometrically (d = density)

After the ink has dried measure the density of the areas A, B, C and D. The drying time of the IGT ink is about 30 minutes. Calculate the wet pick and wet repellence with the following formulas:

  • Wet pick + wet repellence: [(d A – d B) / d A] x 100% | (a)
  • Wet pick: [(d C – d D) / d C] x 100% | (b)
  • Wet repellence: (a) – (b) | (c)

Meaning of the calculations above:

  • No wet pick / no wet repellence: (a) = 0
  • Wet Pick: (b) > 0. The higher (b) the more wet pick.
  • Wet repellence: (c) > 0. The higher (c) the more wet repellence.

For example: d A = 1,35; d B = 0,02; d C = 1,10; d D = 1,20. From here follows:  (a) = 98,52; (b) = -9,09; (c) = 107,61

NOTES

  1. Due to the drying time of the ink it is not advised to add some ink after a test.
IGT Testing Systems
Randstad 22 - 02
1316 BX Almere, The Netherlands
P.O.box 22022
1302 CA Almere, The Netherlands
Telephone: +31 20 40 99 300
E-mail: info@igt.nl
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