Grafica News - January 2018
The quality of your print begins… before you print!
FOUR COLOUR PROCESS
We can take some general line here under – sometime I will write a huge and complete article dedicated to four colour process and the treatment of files for compensation.
Control tools :
- A good magnifier - 10 times, or more hardly (50 and 100 times),
- The computer, with its calibrated monitor) and the software to use. (Photoshop), knowledge of the used RIPs and their possibilities
- A reflexion densitometer (indispensable)
Indeed, an image in four colour halftone printed or in solvent ink, water based or UV should offer curves of density "compensated" according to the fabric, the type of stencilling, the ink type, the substrate to be printed and the fineness of halftone itself to take into account the result, that is the reproduction of the film through the screen. This transfer modifies the size of dots (gains or losses), as well as the absorption by various types of printed substrate, without speaking about the modification of such or such parameter of stencilling or printing. In four colour, one cannot print a micro-porous plastic with a few or no absorption, in the same manner as a very absorbing cotton in textile printing.
The determination of these " curves of compensation ", the very rich companies can realize them with software as Gretag Profile Maker 5 (expensively) via a measured test " Altona " by on multiple colour patch of colour with a Spectro-photocolorimeter.
The less rich printers can do it using the technique described in my CD. For infinitely less expenses !
So make sure that a good profile of compensation (curve of density for ICC profile) earnings and losses of point was defined for a given work.
Considering the infinity of variables, the goal of the printer from prepress then in stencilling and in printing is to reduce the number of these variables. In halftone printing, I am for the use of finer halftones than those that are usually used by the screen printers - 100 and 150/inch with solvent inks, even 200/inch with UV. Instead of 65 or 75 / inch, classically printed.
A gain, we will see that in more details later, in other articles in Grafica News.
But, remember that the colour Cyan, Magenta mostly and at a lesser degree the Yellow are far from being pure. Each of them is more or less « parasited » by the two others. This is not a new problem, it exists - despite the huge improvements of the primary colours pigments since 1965 - even for the most cold and pure primary colours, the one agreed in the European "cold" chart ECI-ISO standard 1.2647-2, – and this can be even worst in the other printing process and with the use of the warmer American chart SWOP. This problem obliges to compensate these deviations when making the films. This is something that is automatically made by the RIP according to a « global chart ». Software such as Photoshop includes automatically this correction too.
An example, as most of the magenta are parasited with 50% of yellow, this kind of « electronic masking » will eliminate a big percentage of the yellow in the colours that contains both magenta and yellow ! Same principle with the cyan that contains often a little too much magenta : the colours containing both have to be corrected in the same way. In screen-printing, the « purer » colours are the more expensive because the pigments contained are also often much more expensive.
Percentages of parasites in the supposed "pure" colours
Indexed colours are a system mostly used by the textile printer using plastically and printing on carrousels of 10 to 18 colours.
- Good magnifiers, a computer with monitor of course, (MacIntosh advised) and the so
classical software Photoshop,
- Spectrophoto-colorimeter or densitometer (Being very advised)
For printing of multi colour rather than with a classical four colour process, the indexed colour is, for the textile printers having multi colour press, a manner of printing almost " in lines " to reproduce a polychromatic designed artwork or a photographic image with bitmap square dots "juxtaposed" that limits the problems engendered by some new plastisols in « superposition ».
This work of separation in X colours, is a very personal graphic experience and a work difficult to estimate for a third person who would have maybe chosen some other colours... On dark textile it is difficult to estimate the values of the undercoating white and that of the highlights.
Please , verify on the file, then on the film of the undercoating white that it is slightly smaller than the colours which overlap there (2 points maximum). I advise to translate the image in bitmap of 200 pixels / inch for the first white (undercoating white) and 250 pixels / inch for the white of the highlights.
To judge the quality of preparation of this working kind(genre), a very big experience is needed of textile designing and printing !
Indexed colour in 9 colours for printing on garment
Some special effect from the SENSATION book of FESPA
At the level of prepress, one is little in terra incognita and here, we need to choose the kind of special effect wished. Expending, 3D, thermo or photo chromic, glittering, nacre , gold or silver coloured, metallic, revealed in the water, photo thermic black, etc.
It is here a purely graphic work to prepare one or several files necessary for these special effects which file which depends naturally on the wished effect. To my opinion, it will be necessary that these files are prepared by the creator or designer of the basic file made for the whole image.
There are three possibilities:
1. Your customer, or its studio, or its graphic service, supply you with the needed films.
2. Your customer supplies a file (TIFF, EPS, PDF - " Certified PDF " would be good). It will be then your job to image the file if you are equipped with an image setter or to subcontract this part of the work at a photoengraver who can make the needed films.
3. Still unusual, you are equipped in CTS (Computer To Screen) and the file will serve for making directly one or several needed screens. It is necessary, here as well that the file is treated and the compensations made according to the needs of your workflow.
- analogue silver
- digital silver
- thermic films
- films ink jet
ANALOGUE FILM - ARGENTIC
The former, if I may say, classical argentic system was made through the use of a argentic classic by photographing the original with a big camera, to obtain a negative, then a positive (contact or enlargement) remains a system still used by some screen-printers. With generally manual « trays » development and fixing.
Here the appreciation of the quality is based on the criteria seen higher : definition (quality of edges), resolution, contrasts, etc. it can be visual with the magnifier or the microscope.
FILM IN DIGITAL IMAGING – ARGENTIC
Now furthermore used, often in subcontracting at a photo-engraver, with image setter Agfa, Scitex, Barco or other, but remaining still " argentic " with automatic developing machine coupled with, after RIPing and imaging. The same quality criteria as above.
THERMIC FILMS AND INK JETED FILMS
Thermic films are almost not used anymore.
For ink-jeted films, even with the best Epson press and especially treated polyester sheets or rolls - although now often used by some printers because this type of equipment is less expensive and is so called "more ecological" – take good care of resolution and definition. For example the best you can expect in terms of halftone film is 115 LPI.
And you will have to use a very precise amount of light in stencilling to take the best or those films.
Ink jet of films with Epson
As matter of fact,
- Resolution and definition remain often too weak compared to the wanted and expected
- In a lesser title, the dimensional stability, particularly for thermic films, is not guaranteed, especially for big sizes what can put register problems between colours,
- films made with the ink jet (on special polyester treated substrates from Autotype or Agfa) made by using generally Epson high definition press.
A problem is the opacity – often bad – in the black parts of films made that way, unless printing them twice successively what harms, in the fine lines and halftones dots, the sharpness and the quality of image (some « cloud effect » in the halftone dots. Without speaking about risks of pixelization or poor definition.
CTS (from Computer To Screen)
Another "maybe premature" here, case in the domain of the screen-printing - most of the companies of average size in the world have not yet reached this point !
If however this system of stencilling " from the computer to the screen " exists and are successfully used in some large European companies. it is already necessary to understand this « near future » that are those systems either in UV – Kiwo ScreenSetter, SignTronic, CST - or in blue/violet laser expected soon. But they are still quite expensive !
I am not in favour of the inactinic wax or ink systems directly ink jetted on the screen, indeed far from the wanted quality criteria here, particularly in relatively small dimensions of image. So a lot of caution and, once more, it is necessary to see and to check carefully in companies using it, the way these techniques are used, if it is the case.
The direct projection shown on the left, despite still used in some countries for bilboardprinting is for me quite obsolete.
The control on the quality of screens will be made in the same conditions as we are going tosee below: always the same problems of resolution and definition and naturally of choiceof screens according to the graphic needs.
Michel CazaFrance Past President. FESPA,
Recipient of Howard Parmele Award from SGIA, USA.
Michel is a great supporter of DMI since its inception and his technical articles are published in Grafica News India.