PRINTING INDIA

Processing

Exposure
The special Matt surface of the offset plates provides for quickest possible optimum contact between film and plate- i.e. fast, reliable vacuum. Side lighting is prevented. Offset plates are exposed in standard exposing units. We recommend units equipped with metal halide lamps. Uniform lighting of the exposing frame is essential for good results.

Test exposure
The optimum exposure time depends on the exposing unit and the down printing original. Using a range of exposures, the behavior of the offset plates can be determined quickly and accurately. The UGRA test film has proved efficient for this purpose.

Procedure for test-exposing positive and negative plates:
Paste the test elements (e.g. UGRA test film, grey scale, tone step scale) on a mounting foil about 5 x 11” in size. Use the assembly for a series of exposures, starting with 10 or 20 seconds and increasing the time by factor 1.4 from step to step. Example of a time step series: 10, 14, 20, 28, 56, 112, 160, 224, 320, 448 sec. Now develop the test plates by hand (manual developing).

Assessment
For example, referring to UGRA Test Film 1982: In positive plate printing down, more and more of the very fine line elements will disappear with increased length of exposure, while the gaps in between widen or remain constant. Thus, in positive plate printing down, the K value (i.e. the parameter of printing down) is the one opposite the last line element fully reproduced. In negative plate printing down, with increasing exposure time wider and wider gaps fill in while ever-finer lines are reproduced. The K value in this case is the one opposite the last gap still opens.
In working according to the Fogra concept of standardization, the standard exposure range.

* For short run plates is correct if according to the “Printing down
table for positive plates with a resolution of 5-8 μm”- the K values 12 or 15
are intact (Fig.1).

* For long run plates is correct if-according to the “Printing down table for
negative plates with a resolution down to 7 μm” – the gaps of the LK
values 6 or 8 are open (Fig.2).

To ensure reaching the standard exposure range without film-edge marks, it is recommended to expose offset positive plates with diffusion film. For this purpose, split the exposure time determined from the exposure series and do have this exposure with diffusion film.

Fig. 1

Micro line indication Area coverage of screen fields on plate
Finest reproduced line field

(K value)

7% field (S)

10% field

40% field

80% field (T)

6 μm

=7.0%

=10.0%

=40.0%

=80.0%

8 μm

6.5%

9.5%

39.5%

79.5%

10 μm

6.5%

9.0%

38.5%

79.0%

12 μm

6.0%

8.5%

37.5%

78.5%

15 μm

5.5%

8.0%

36.0%

77.5%

20 μm

4.5%

7.0%

34.5%

76.0%

25 μm

4.0%

6.0%

33.0%

74.5%

Table B2-2: Printing down table for positive plates. Resolution larger than 5-8 μm
= standard range

Fig. 2

Microline indication
Finest reproduced gap field

(K value)	7% field (S)	10% field	40% field	80% field (T)

4 μm	=7.0%	=10.0%	=40.0%	80.0%
6 μm	8.0%	11.0%	42.5%	81.0%
8 μm	8.5%	11.5%	44.0%	82.0%
10 μm	9.0%	12.5%	45.0%	82.5%
12 μm	9.5%	13.0%	46.0%	83.5%
15 μm	10.5%	14.0%	46.5%	84.0%
20 μm	11.5%	15.5%	49.0%	85.5%

Table B2-5: Printing down for negative mono metal plates. Resolution down to 7 μm.
= Standard range

Development
Ready-to-use developers are available for processing offset plates. The optimum processing temperature is 24⁰C +/-2⁰C (75⁰F +-4⁰F).

Manual developing
Using a cotton pad or viscose sponge, apply the positive or negative developer to the exposed plate, evenly disturbing the fluid. After about 30 seconds, squeegee the plate. Repeat the process two additional times. Total development time to be at least 90 seconds. At the end, rinse the plate with plenty of running water, then squeegee it.

Dip-tank developing
Dip time in Offset Positive 0r Negative developer is to be about 60 seconds. When the developer becomes weaker from dissolved layer substance, compensate for the loss by prolonging the dip time. After dip-tank development, treat the plate as after manual development.

Machine developing
In machine developing. The effective life of the offset developers can be maximized by the control options offered by the processors. Given proper maintenance and optimum replenishing, up to 25 m² of plate surface can be developer per liter of developer (some 300 sq. ft per Brit, quart)

Development control
To provide optimum production conditions, exposure and development have to be suited to each other. Manual, dip-tank and machine developments are controlled by means of the grey scale. The rule: on offset Positive plates, density step log 0.60 should be fully developed, density step log 1.80 fully covered; on offset Negative plates, density step log 0.60 should be fully covered, density step 1.05 fully developed. After any interruption over night or longer, it is advisable to check the state of development with a grey scale. In doing so, make sure that the same exposure (time, position, mounting foil) applies to the grey scale, on the basis of the exposure test series.

Replenishing
(Applies only to offset Positive machine developer.)
As the effectiveness of the Positive machine developer diminishes in the course of time and due to saturation with dissolved layer substance, metered addition of replenisher will restore or maintain the original state. For further information on replenishing, please refer to the operating instructions for our plate processors. No replenisher is available for offset Negative developer.

Gumming
Offset gum comes ready to use and is universally applicable in plate making and with the plate on the press. Spread as an evenly thin layer on the plate, the gum will improve the ink acceptance, dampen ability and all other run properties of offset plates. For use in two-compartment plate processors, a modifier is available. For further information, please refer to the operating instructions for the offset plate processor.

Correcting
Removal of layer elements (subtractive correction) is easily done with the offset Positive or Negative correcting fluid. Plates to be corrected, gummed or not, have to be completely dry. Let the correcting fluid act upon the plate for at least 30 seconds. After correction, see to complete removal of dissolved layer. Adding layer elements (additive correction) is possible with lacquer pens. The diazo pen has proved effective with Positive and Negative plates. For long run life, it is advisable to prepare the spot (e.g. with film cleaner or correcting fluid) and dry it thoroughly. The added part of layer must be completely dry before printing is resumed. A hot-air blower or pistol is useful to speed up the drying process.

Baking
Baking an offset Positive plate gives its printing layer added hardness. Long runs imposing high wear stress and jobs printed with special inks (e.g. neutralization inks, UV inks, inks with metal pigments) then pose no problem.
Baking conditions: 230^oC (450⁰F), 8 min holds time. Use only faultlessly developed and corrected plates. Applying the offset baking fluid without structure will make additional plate treatment before printing unnecessary.

Printing

          Offset plates with their fine porous surface are best suited for printing
      coated papers. The plates accept the ink immediately at the start of the
      run and achieve the proper running conditions right away, without extra
      treatment in the press.

         Offset plates are compatible with any conventionally used dampening
      additive. The plate properties become particularly effective with the
      modern alcohol dampening systems in sheet offset.

           Offset plates are easily treated should a failure occur during the
      production run. Cleaning or activation with offset plate cleaner or offset
      gum will take care if any difficulty.

Faults and Remedies

Faults detected	Possible causes	Remedies
Poor ink acceptance	* Unsuitable gumming method.	Do not gum in developer tray.
	* Plate stored with gumming.	Use preserver.
	* Gum film too thick.	Use offset gum, apply as thin,
		even film.
	* Over Development.	Determine correct
		development by means of
		grey scale.

Filling-in-of shadows	* Wrong pH value of the	Establish pH value 4.8–5.5
	dampening solution.
	* Ink film to thick.	Re-establish ink-water
		balance.
	* Incorrect metering of the	Observe manufacture’s
	dampening.	instructions solution additive.
	* Excessive emulsification of	Excessive feed of dampening
	the ink.	solution, suitable ink.

Corrected elements reproduce in print.	* Plate was damp when	Parts to be corrected must
	corrected.	be dry; let the correcting
		fluid act on the plate for
		30 sec.
	* Correcting fluid applied	Correcting fluid to applied
	only once.	twice, removed by brush
		shortly after.
	* Correcting fluid was not	Let the correcting fluid act
	allowed to act long enough	on the plate for at least 30
	on the plate.	sec.
	* Dissolved later substance	Rub out corrected parts with
	dried on again.	sponge.
	* Plate not gummed after	Gum plate with offset gum
	correction.	after correction.

Premature plates wear.	* Faulty cylinder rolling.	Set to press specifications.
	* Excessive printing pressure.	Check and set correctly.
	* Over development or over	Check with test scale,
	exposure.	determine development with
		grey scale.
	* Cleaner to aggressive.	Use cleaner recommended
		for offset.

Smears after baking.	* Soiled plate baked.	Make sure plate is clean
		before baking.
	* Uneven application of	Apply baking fluid as a thin
	baking fluid.	and even film

Scumming.	* Baking fluid applied too	After baking, treat plate with
	thickly.	sponge and water, then Gum.
	* Developer residue not	Rinse plate thoroughly, use
	neutralized.	offset gum.
	* Neutral dampening solution.	Establish pH value 4.8 – 5.5
		increase buffer effect.
	* Plate not gummed.	Gum with offset gum.

Preserving the plates

If offset plates are to be stored for re-use, the water-carrying areas must be clean, and no ink residue is to remain on the printing layer. Method of preservation with the plate on the press: After the run, apply preserver with a sponge. Immediately remove ink with roller cleaner and rag. In a second operation, remove residue of dried-on ink. Take the plate out of the press and store it in a dry place.

Environmental and working protection

Avoid skin contact with the processing chemicals. For manual and dip-tank developing, for preparing and pouring the solutions, it is advisable to wear protective gloves. If a chemical has come in contact with the skin, rinse the affected part with plenty of water. Change any garment spilled with chemical.