Notes on Creating Films for Screen Printing

Notes on Creating Films for Screen Printing
Kevin Haas | www.wsu.edu/~khaas
Notes on Creating Films for Screen Printing
Options for Creating Films
1. Hand drawn images on frosted mylar, or tracing paper.
2. Oiled photocopies, laser prints, or inkjet prints of drawings,
photographs, collages and appropriated material.
3. Inkjet transparencies from files created and/or adjusted in
Photoshop or Illustrator. Photocopier transparencies.
Remember
Laserprinters/Photocopiers = Halftone Dots and Plastic Toner
Inkjet Printers = Random Dither and Water-based Ink
Working with Frosted Mylar or Tracing Paper
The following materials work best for creating opaque images:
China Markers, Litho Crayons, Micron Pens, India Ink, Black
Acrylic Paint, Technical Pens
Be sure to check your drawing on the light table against the
sample drawing in the studio before exposing your screen. You
will often find that you will need to darken much of your
drawing and go over or darken thin lines.
For color separations, cut one sheet for each color to the size
of your paper. The sheets can be placed over a template on
newsprint noting the image size and composition of the print as
a guide for drawing the separations. The Mylar sheets can also
spray booth or outside to vent toxic fumes and lay down
newspaper to protect the spray area.
•
Re-copy a photocopy to make information more coarse and
print better. Larger dots will hold in the screen, smaller ones
will ‘fall through’.
Working with Laserprinters and Photoshop
You can print your grayscale file as is, or adjust the lpi of the
halftone for more accuracy in tone and detail. Think of the
screen as a sieve. If the image dots on your film are finer than
the mesh of the screen, they will not be held in the screen to
print later. This feature only works with pre CS5 versions.
1. You will need a grayscale file that is at least 150ppi at the
size you will print it. This information can be checked by
going to Image > Image Size... in Photoshop.
2. When you are ready to print your image you will need to
adjust the halftone screen settings in the Print with Preview
Dialogue Box. The lpi you choose should be approximately
the screen mesh divided by 5 for reliable results. (195
mesh / 5 = 39lpi). The screen angle should be set by the
chart below.
3. Print your file, oil it if necessary, and expose it.
be punched and registered with pins just as your paper would.
This makes registering your first color easy if you position your
paper under your screen with the corresponding drawing laying
over it also attached to the pins.
Working with Photocopies
•
Tape 2 transparencies together to make an opaque image.
•
Adjust copy lightness/darkness to adjust detail and overall
value or to create a duo-tone with one darker image printed
in a light color and a lighter/detail image printed in black or
a dark color.
•
Rub baby oil into the paper from the back side to make the
paper translucent enough for exposure.
•
If the toner is not dense or opaque enough, spray the
photocopy with Krylon Matte Finish Spray. Use only at a
Typical Halftone Screen Angles for CMYK Offset
Printing
Black/Key Color: 45º
Magenta/Medium Value Color: 75°
Yellow/Lightest or very light colors: 0°
Cyan/Medium Value Color: 15º
Kevin Haas | www.wsu.edu/~khaas
A ʻdiffusion ditherʼ bitmap on the left, and a
halftone on the right. Both images were
created by converting the images to
bitmap mode, Using this feature of
Photoshop will allow you to create
accurate films for screenprinting using an
inkjet printer.
Working with Inkjet Printers
Creating a Diffusion Dither Bitmap
Think of the screen as a sieve. If the image dots on your film are
finer than the mesh of the screen, they will not be held in the
screen to print later. Inkjet printers produce very fine
photographic images by spraying tiny dots of ink onto the
paper. These dots are far too fine and delicate for either the
1. You will need a grayscale file that is at least 150ppi at the
size you will print it. This information can be checked by
photo emulsion to acknowledge, or to be held in the
comparatively coarse mesh of the screen.
Rather than using halftone dots of varying sizes like laserprinters do, inkjet printers use the same size dots scattered in
varying densities to create the appearance of different values in
going to Image > Image Size... in Photoshop.
2. Convert the image into bitmap mode. Go to Image > Mode
> Bitmap. Your output should be the screen mesh divided
by approximately 1.5-2.25, and then round up or down to a
whole number. The method should be set to Diffusion
Dither. An Output Resolution of 130 will work on a 195 mesh
screen.
3. Print your file and expose your film to your screen.
an image. Because these dots are so fine, the image must be
altered before printing a film from an inkjet printer, to assure
that the dots are large enough, and dense enough.
There are two methods of doing this, both which require that
the image be converted to a Bitmap. The first is the ‘squaredot’ or diffusion dither method. With this method, the values in
the image are created by the frequency or density of the dots.
The size of the dots remain a consistent size, which is
determined by the final resolution of the file. The second
method converts the image into a halftone pattern, which
creates the appearance of different values through a grid of
varying sized dots. The halftone is the most common method
for translating a continuous tone photograph into a printable
image.
Just like laserprints, inkjet prints on paper can be saturated
with baby oil to make them transparent enough to expose to
your screen. A lightweight matte paper made for inkjet printing
works best, such as Epson Singleweight Matte.
For quality films however, you must use an inkjet transparency
film, such as Pictorico Premium OHP Transparency Film, or
PosJet Ink Jet Film.
It is also important that you print your films at the highest
quality your printer can print at. For Epson printers, print quality
should be set to 1440dpi.
Creating a Halftone Bitmap
1. You will need a grayscale file that is at least 150ppi at the
size you will print it. This information can be checked by
going to Image > Image Size... in Photoshop.
2. Convert the image to Bitmap. Go to Image > Mode >
Bitmap. Set your Method to ‘Halftone’ and your Output
Resolution to 720. The Output Resolution should be at least
your lpi x 16, and evenly divisible by your printer’s maximum
output resolution. For example, a 35lpi halftone needs an
Output Resolution of at least 560, but 720 is the next higher
resolution that is the inkjet printer’s resolution of 1440
evenly divided by 2.
3. Print your file and expose your film to your screen.
Kevin Haas | www.wsu.edu/~khaas
A grayscale image on the left, and a posterization
on the right.
Posterizations in Photoshop
Rules of Thumb
You can make posterizations in Photoshop at the click of a
button, but to get them set up properly for screen printing takes
a few more clicks. Posterizations work best with grayscale files,
and can be used to print the file in several grays or colors to
extend the tonal range of the image. Once your grayscale
ppi = pixels per inch (the resolution of your file)
lpi = lines per inch (lines of halftone dots)
dpi = dots per inch (pertains to printers)
image is ready, complete the following steps. When you set the
number of levels, remember that one of them is the white of the
paper, provided that there is white in the image. This means
that typically you will have one more level than the number of
colors you want to print.
It is important to set your lpi and ppi accurately for screen
printing because it will affect the amount of information that
ends up in your print, and how trouble-free your printing will be.
If the lpi is set too high, very small dots will be lost since there
will be no screen mesh to hold them, and you will probably also
get a moire. You want to be certain that even the finest dots in
your halftone or bitmap are large enough to be held by the
1. Adjust the levels of your file to assure good contrast in your
file. Go to Image > Adjustments > Auto Levels.
screen mesh. You will inevitably loose some information,
usually in the lightest and darkest 5% of your image.
2. To posterize your image, go to Image > Adjustments >
Posterize...
General Formulas
3. Save your file as ‘myimage-POSTER.psd’ (or anything that
optimal lpi for screenprinting = mesh ÷ 5. (approximately)
A 225 mesh screen should print a 45 lpi image well regardless
you can remember later), to keep your original file
unchanged. Now that the image has been broken down into
several values, you will need to ‘extract’ each one to create
a film to expose your screen with.
4. Go to MB>Image>Adjust>Threshold. The space between
the lines will indicate a particular color with the darkest on
the left and lighter colors on the right. Move the slider under
your first color to be printed on the right and click OK. Print
the image.
5. Go to MB>Edit>Undo Threshold or use the history palette
(MB>Window>History Palette) to return to the index color
version of your file. Repeat the Image>Threshold function to
isolate the next color for printing. Keep repeating these
steps until all of your films are printed.
of higher printer dpi or file ppi.
optimal dpi for bitmaps = screen mesh ÷ 1.5 (approximately)
An Output Resolution of 130 will work well on a 195 mesh
screen.
optimal resolution for halftone bitmaps = lpi x 16.
A 45 lpi Halftone Bitmap should have an Output Resolution of
720 ppi.
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