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Technical background information - murakamiscreen.com Brochures... · ColorGATE...

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Technical background information
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Technical background information

Screen printing mesh – carrier of the stencil

Images show different meshes with and without printing stencils

• The mesh consists of a piece of porous, finely woven fabric made of nylon, polyester or steel.

• One of the most important parameters is the mesh count. Typical are mesh counts from 10 to 200 threads per cm.

• Mostly used for 4 color screen printing are meshes with 120, 150 threads/cm. • The mesh quality is defined by the thread diameter. For example a 150-31 mesh

has a thread diameter of 31µm (microns).• The mesh opening and the screen open area in percent are calculated from the

mesh count and thread diameter and are responsible for the resolution a given mesh is capable to reproduce.

The stencil types and creation• The stencil is the carrier of the printing information• The print stencil is the final developed form in the

frame.• Hand cut or using a CAD plotter• UV projection for large formats (disadvantage: no

good sharpness)• Round stencils for rotation machines• Ink Jet systems printing directly on the mesh

(Computer-to-Screen)• Photographic films (image setter – disadvantage:

limited size, chemistry usage and high costs)• Thermal mechanical stencils (expensive system

with high fail rates) • => Using Ink Jet for film creation

Creation of the printing formThe printing form consists of the mesh, the frame and the stencil that holds the printing information.

Important remarks for 4 color halftone printing• The printing quality is dependant from the quality of the stencil and the

parameters of the printing mesh.

• The mesh count should at least be 4 times larger then the screen ruling if a halftone range from 5% to 95% is required.

• If fine details shall be printed high pressurized water development shall not be used.

• A moiré kind of artifact is mostly the result of not optimized mesh count, screen ruling and coating thickness.

Important remarks for 4 color half tone printing

When selecting the screen angle the thread geometry must be taken into account to avoid moiré.

The mesh, stencil and the colors used must fulfill some requirements to gain a desired quality printing result.

Typical for screen printing applications are 24 to maximum 48 lines per cm and mesh counts up to 200 threads/cm.

A moiré is an unwanted structure that results from overlaying periodical structures.

Screening anglesOffset angles:C: 15°

M: 75°

Y: 0°

K: 45°

Offsetrosette

Screening angles in screen printing

• C: 22,5°

M: 82,5°

Y: 7,5°

K: 52,5°• The screening angles are the same as in offset but with a twist

of 7,5 degrees from the thread direction of the mesh.

Inks

• Inks for half tone printing • Transparent inks• Good sharp dot formation without running• Density must be controllable• Euroscale (was DIN 16539) is now ISO 2846-1 norm• The color raster screen printing is defined in ISO 12647-5

• Solvent and UV inks• Using solvent inks a higher stencil profile can be used as with UV inks.• The solvent evaporates fast and with this a small coating thickness is printed

reducing moiré.• UV inks are more robust, drying faster and are easier to handle.• UV inks tend to build up higher coating thickness in printing which may lead

to unwanted ink trapping.

• Printing sequence: C-Y-M-K or C-M-Y-K

Standardization in screen printing• Ideally all parameters influencing the

print quality shall be reproducible.• Using the following tools help you

reach this goal:• Printing and measuring color bars• Dot gain control• Tone reproduction curve• ICC profiles• Proof print incl. dot simulation

Sharpness

• Sharp details on the film help for the correct exposure on the stencil and the final print.

• Screen printing requires a positive, reading right film.

• In this context, reading right means that the artwork must be reproduced the right way round on the emulsion side of the film.

• This is important, because it allows the emulsion side of the film to rest directly on the stencil emulsion during exposure.

• This is the opposite to films intended for indirect reproduction with offset printing (reading wrong).

Densitometers

• Measuring instruments for detecting reflecting and transmitting light.

• Reflecting densitometers are used for controlling evenness and consistency in the final print.

• Transmission densitometers are used for measuring the film.

• Reading parameters are optical density, screen percentage, dot gain and relative printing contrast

• Measuring apertures typical 3 mm GretagMacbeth D200

DensityPhysical value describing the coating thickness of ink on the substrate. Optical density is the absorbance of an optical element for a given wavelength. In photographic terms density describes how dark a print is.

UV density max required is 3,0

Agfa SelectJet Screen Film• Dmin

(visual 0,06, UV density 0,12)• Dmax

(visual up to 3, UV density up to 4,7)* (visual up to 3.5, UV density up to 3,7)**

* with piezo print heads and pigmented inks** with piezo print heads and dye inks

Linearization

• The linearization controls the amount of ink printed on the film.

• In FILMGATE5 the dot coverage percentage is needed.

• The actual dot coverage of the dot area is dependant from the screen technology and the screen angle.

• The „Murray-Davis“ equation is used to calculate the screen percentage.

• The instrument is calibrated on the film to determine the zero and 100% screen percentage.

• For linearization a simple optical transmission densitometer is needed (no UV densitometer).

Screen rulingl/pi, l/cm:

The screen ruling is indicated in lines or dots per cm or inch. It depends from the physical printer resolution and the desired half tone steps.

A printer with 2880 dpi physical resolution could print 180 lpi (assuming 256 half tone steps).

dpi Square root (G)

lpi =

√∑

G = half tone steps per raster cell

dpi = physical printer resolution

Dot shapes

The most used dot shapes are:

• Ellipse• Dot• Rhombus• Square• Line

Clustering of dots

The unwanted clustering of dots using the elliptical dot shape is reduced to only 2 half tone steps which makes it suitable for screen printing.

The raster cell

• Each raster dot is a combination of pixels.

• The size of the raster cell depends on the physical printer resolution, the screen ruling and the desired half tones steps.

Resolution Correlation between screen ruling, physical printer resolution and half tone steps

Screening algorithm• There are two different types of half-toning

methods a modern RIP should handle: AM and FM

• In FILMGATE the amplitude modulation screening will be used with the PostScript and Super Rosette screening.

• In addition to further reduce moiré a Hybrid Screen is offered combining the AM screening for CMK and printing Y with the FM screening method.

Frequency and Amplitude modulationFrequency modulated screening (FM): The dots are as small as possible but the size is constant. The number of dots per area is variable.

Amplitude modulated screening (AM): Resolution in a constant number of dots per area, the dot sizes are variable.

In screen printing FM screening is not yet of such a big importance because of the reduced resolution and the larger dot size that can only be printed.

Resolution of meshes and screen rulings• The smallest dot must have a chance to stick on the mesh. • This is the reason why the mesh and the screen ruling

must fit to each other.• In screen printing, screen rulings between 8 L/cm till max.

48 L/cm can be printed, depending on the mesh used.• The smallest dot must have a minimal diameter of two

threads and one mesh opening. • With Hybrid Screen you have the possibility to define the

minimum dot size of the FM screen. The dot size variants are in the range of 80-130 µm.

Important remarks for 4-color half tone printing

A moiré is an unwanted structure that results fromoverlaying periodicalstructures and is one basicphenomenon which alwaysappears but is not alwaysvisible.

Moiré – Possibilities to minimize

When selecting the screen angle the thread geometry must be taken into account to avoid moiré.

The mesh, stencil and the inks must fulfill some requirements to gain a desired quality printing result.

Typical for screen printing applications: 24 to 48 lines per cm and mesh counts up to 200 threads/cm.

Moiré – Possibilities to minimize

• In an ideal world the angles of the separations should have a distance of 30°°

• 90°

divided by 30°

allows you 3 colors only – what happens with the 4th color?

• The pattern that is created by 2 or 3 separations look like a rosette and is ideal for our human perception

• Changes of this “ideal world“ originates annoying Moiré• A divergence of 1°

is already enough to destroy a harmonious rosette pattern

Moiré – Possibilities to minimize

• To address this issue(4 separations – 3 Angles), the “weakest“ color (typically yellow), will be set 15°

between two other separations

• A visible Moiré in the separations does not mean to be visible in the final screen print

• Traditionally the color with the highest contrast (mostly black) will be set on the ideal visual angle of 45°

• However, an experienced person will lay the dominant color on the visual angle and place black on 75°

or 105°– provided that it is a “normal“ black and not a dominant color

Moiré – Possibilities to minimize

• The dominant colors lie of course also ideally in an angle of 30°

to each other. E.g., yellow and magenta for skin tones, with an exchange of magenta and black

• Also the method of black generation (GCR, UCR) can help to minimize the effect of Moiré in the seperations

• UCR (Under Color Removel) in Moiré with black• GCR (Gray Component Replacement ) in Moiré with

color separations

Screen printing angles: C: 22,5°°

M: 82,5°°

Y: 7,5°°

K: 52,5°

Moiré – Possibilities to minimize

• HybridScreening combines the screening method of AM and FM (stochastic screening)

• HybridScreening contains the same advantages as a “pure“ FM screening, also to prevent the effect of Moiré…like in the following examples.

• 96 lpi, Offset angles, Standard

• diagonally Moiré

• 96 lpi, Offset angles,Cyan and

Yellowexchanged

• diagonally Moiré, a littlemore narrowly

• 96 lpi, screen printing

angles, Cyan and

Yellowexchanged

• smooth Moiré,very narrowly

• 106 lpi, screen printingangles, Cyan and Yellowexchanged

• No Moiré

• HYBRIDSCREE NING

Usage of SuperRosette (SR) screenswith Moire : Exchange angles

In skin tones between black and magentaIn green tones between black and cyan

Selection of the correct combination betweenscreen resolution and screen

screen resolution x 4 = mesh count of screen

Moiré – Possibilities to minimize

Limitations of digital screenings• With digital systems it is not easy to implement the desired

screening angle and frequency in the pixel matrix of the printer. • The reason is that in digital only certain steps can be used. • As a result there are quantization effects because the screen

cell can not be easily positioned to the desired angle.

Rational Tangens Screening for OffsetThe angles are 18,4 or 71,6 degrees instead of the standard angles 15 and 75 degrees. In addition the frequency per color changes.

The principle of the Super Cell• If you have larger screening cells the screen angle can be

reached more effectively. • That is the reason why the super cell technology gathers

multiple cells together to create a new larger unit. • With this “trick” the ideal angles and

similar frequencies can nearly be reached.• The disadvantage: the calculation time

increases a lot.• Therefore the screen cells are already

predefined in FILMGATE5.• FILMGATE5 uses the super cell principle

with the Super Rosette screening.

www.colorgate.com [email protected]


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