What is silkscreen printing?
The process Disc Makers uses for printing on the surface of CDs and
DVDs is called silkscreen printing. Invented in ancient China, it
is an effective way to print on irregular surfaces, such as t-shirts,
coffee mugs, or compact discs.
In the silkscreen process, film separations are made that represent
each color in the design. The film is laid on a fabric mesh that is
covered with a special emulsion and exposed to light. Where the light
hits the emulsion, it becomes solid; where the film covers the mesh,
the emulsion remains unchanged, and then is washed off. The screen
is then placed on top of the surface to be printed, and ink is pushed
through the mesh using a squeegee. The ink is dried, and the next
color is applied.
Fine details are easily lost in silkscreen printing because of the
mesh that is used. Elements of a design that are too small may end
up sitting on the mesh itself, rather than in a hole in the mesh,
where ink can pass through. Unfortunately, this mesh interference
is an inevitable consequence of the process.
The surface of your CD or DVD
The first hurdle in printing on a CD or DVD
is the surface of the disc itself. Unlike white paper, a disc has
three distinctly different surfaces, and each one has its own issues.
a) The regular surface of the disc is silver
(or reflective aluminum, technically). Depending on the lighting and
angle at which you hold the disc, the surface appears to have a tonal
value of about 15 percent black. That means a halftone printed directly
onto the surface of the disc will lose contrast in the lighter tones.
b) The mirror band. This is a highly reflective ring just outside
the clear center of the disc that contains information about the disc
itself; track information, artist, job number, etc. The issues of
printing onto the surface are multiplied within the mirror band.
c) The stacking ring is a lip in the clear plastic center of the disc,
between the mirror band and the hole. Elements that cross the stacking
ring tend to become distorted. To make things more difficult, the
inks used for silkscreen printing are somewhat translucent. Because
this area of the disc is clear, the translucency of the inks becomes
more obvious, and they will not match the same inks printed on either
of the other two surfaces.
There are two solutions to the problem of printing on the changing
surfaces of a compact disc. The first is to print a flood coat, usually
a layer of white ink that covers the printable surface. This gives
the disc a more consistent printing surface, closer to printing on
paper. The second option is to make sure any halftones in your design
don't cut into the mirror band or stacking ring.
Halftones What is a Halftone?
Put most simply, a halftone is an image (like a photograph) converted
into small dots to fool your eye into seeing shades of gray (or tints
of a color). Depending on the tones present in the image, the dots
may vary in size. The dots start out very small in light areas, and
grow in size as the shade gets darker. Eventually, the dots overlap,
revealing only small amounts of the background. Usually, even the
large dots are so small your eye can’t distinguish them as individual
dots, and they blur together into shades of gray.
The picture on the left is turned into a halftone
(detail shown on the right).
What is a line screen?
Back in the olden days, to create a halftone printers would place
a screen in front of a photograph that was to be imaged onto film.
The number of rows per inch in the mesh would determine the screen
frequency, or linescreen, of the halftone. This is all done electronically
now, but the principle is the same. Line screen, measured in LPI (lines
per inch), determines the detail of a halftone. Lower line screens
result in larger, coarser halftone dots, and an obvious loss of fine
detail. Higher line screens give finer detail because the dots are
smaller and closer together. Disc Makers' standard screen printing
uses a line screen of 100 LPI. This line screen is best suited to
spot-color printing, and CMYK process printing where fine detail is
not a requirement.
Disc Makers also offers deluxe 200 LPI silkscreen printing. This line
screen is the highest available in the industry, and is best suited
to CMYK process printing when reproducing full-color designs with
very fine detail.
Overlapping halftone screens.
We strongly advise against overlapping halftone
screens when silkscreening in a non-CMYK, spot color environment because
of the increased potential for a visible moiré pattern to appear
on your finished discs. A moiré pattern can appear when two
or more halftone screens interfere with each other, a pattern present
in the design, or the silkscreen mesh itself. Rather than overlapping
those spot color screens, we suggest that you set up your images so
that a single halftone prints over a solid color.
This file is a true duotone set up in Photoshop,
set to print with overlapping screens. The color is created by printing
a halftone of the blue ink over a halftone of the red ink. When silkscreened,
this disc would likely have a moiré pattern.
Instead of using overlapping red and blue screens,
this image is created by printing a dark purple halftone over a solid,
light purple background. This is how you should approach designing
a disc.
Tonal range, dot loss, and dot gain
Tonal range refers to the percentage of dots
in an area of a halftone image. Pure white has a zero dot, pure black
a 100% dot. On the silkscreen press, the ideal tonal range for a continuous
tone image is 15% to 85%. Halftone dots with less than a 15% tone
will drop out, or not print. This is dot loss. On the other end of
the spectrum, tonal values greater than 85% will fill in and become
solid. This is called dot gain. Dot gain and loss is an inevitable
limitation of the silkscreen process.
Gradients
A gradient (also known as a blend) is an area
in an image where the tonal range changes gradually, creating a very
smooth transition from one color or tint to another. Unfortunately,
the smooth transition of halftone dots in a gradient area on film
does not reproduce as precisely when the film image is transferred
to the silkscreen mesh. This generally results in inconsistent ink
coverage and sudden tonal shifts that cause banding. Although banding
is most evident in gradient areas that span across a wide tonal range,
the results of any gradient in the CD silkscreening process are unpredictable.
We recommend you avoid using gradients when designing for the CD or
DVD itself, and cannot guarantee the quality of any silkscreened gradient.
Top 5 mistakes on client-supplied design jobs. Learn more.
