Screen Printing and Screen Preparation

Screen Printing and Screen Preparation

A. DEGREASING THE SCREEN:
Degreasing is not always absolutely required, but it is always highly recommended.
The primary reason for this is to eliminate oils and other contaminates from the mesh
prior to coating. Even new mesh on a screen will have small amounts of dirt, dust and
oils from handling before you received it. Failure to degrease properly will trap dirt,
dust and oils that may cause streaking during coating and numerous other problems,
including “fish-eyes” which are a result of oils (such as from fingers during handling)
which will repel liquid emulsion leaving small open areas. These spots will invariably be
in an area of the screen that is difficult or impossible to touch up or fix later. It’s always
easier to prevent a problem than fix it. To properly degrease silkscreen mesh:
1) Wet both sides of screen mesh thoroughly with water.
2) Apply degreaser. It is possible to use a dishwashing detergent (like Dawn®) but a
degreaser designed for degreasing silkscreens is best because it will not contain
dyes, perfumes and other things you don’t need (those things can leave residue).
3) Using a scrub brush, (the long bristle kind with a handle is recommended, but
almost any kind will do) methodically scrub the ENTIRE surface area of the mesh,
starting with the inside, or squeegee side, first. A good technique is to start at
the upper left-hand corner, using small, counter-clockwise circular motions and
moving left to right in descending rows towards the lower right-hand corner. Then
do the exact opposite, that is, small, clockwise circles working from the lower right
to upper left corner. Next scrub the mesh horizontally left to right working in rows
from top to bottom; then vertically scrub up and down working in columns from the
left side of the screen to the right. Then scrub the whole screen at approximately a
45 degree angle, switch (90 degrees) and scrub in the opposite direction. Next go
around the inside (squeegee side) of the frame, one stroke around all four sides
going counter-clockwise, then the same going clockwise. After that, use the exact
same process on the outside (print side) of the screen. Note that the idea here
is to THOROUGHLY clean the mesh and prevent problems. So, although this is
a good technique, it’s fine to use a technique of your own. But you do want to be
methodical about it and degrease every bit of the mesh.
4) After you’ve lathered both sides of the mesh, flip the screen back over and
thoroughly rinse the squeegee side first and then flip and rinse the print side. Rinse
with low water pressure until there are no more “soap” bubbles.
5) Place the screen in a horizontal position to dry (either side up is ok). The screen
should be suspended so that it is supported by the frame and air is allowed to flow
freely over both sides of the mesh. Make sure the mesh is completely dry before
coating.):
B. COATING THE SCREEN (LIQUID EMULSION
There are basically 2 types of liquid emulsion used in screen printing. Dual cure
emulsion is emulsion that is mixed with a photo sensitizer (Diazo - usually supplied
with it). It generally has longer exposure times than the alternative, pure photopolymer
emulsion, which is ready mixed. There are pros and cons to either type. There are also
different types of scoop coaters. Some scoop coaters have two different coating edges.
One “sharp” edge and one “rounded” edge. The “sharp” edge of one of these type
scoop coaters will deposit a thinner coat of emulsion than the “rounded” side. Some
scoop coaters just have one coating edge that is usually somewhere in the middle with
regard to “sharpness” and coat thickness. Whichever type of emulsion or scoop coater
is used, the techniques for coating are essentially the same.
1) Pour the emulsion into the trough of a scoop coater. Let the emulsion settle so
that it is equal in depth across the length of the scoop coater. The liquid emulsion
should be level.
2) Place the screen so that it is almost vertical, (but slanted roughly 70 -80 degrees.
Usually placing the screen at a slight angle to a wall is fine. The screen should be
braced (at the top) so that it won’t shift while you are coating it. A wooden block
attached to the wall will do nicely, but even nails or screws in the wall will do.
They don’t need to stick out very much at all; just enough to stop the screen from
shifting.
3) Tilt the scoop coater slightly backwards and place it at the bottom of the mesh
on the print side (always coat the print side before the squeegee side). Then tilt
the scoop coater forward so that the emulsion begins to flow toward the mesh.
Keep enough pressure with the coater against the mesh to prevent emulsion
from leaking below the coater. Most scoop coaters have an angled piece on both
ends of it to guide you to the best coating angle. If you have keep the emulsion
level even across the length of the coater, the leading edge of emulsion in the
coater should flow towards the mesh and hit it evenly, at all points, approximately
simultaneously. Start the upward movement of the coater immediately when this
happens, maintaining firm even pressure and speed as you move it upward. You
will develop a feel for the required pressure and speed of the coating stroke,
through experience, pretty quickly. One very helpful tip; when you reach the top
of the squeegee stroke, (avoid contact with the top of the frame) give the scoop
coater a slight flip backwards. This will prevent drips.
4) After you’ve applied one coat to the print side, flip the screen upside down and
apply another coat to the print side from the opposite direction.
5) Next flip the screen over (end over end) so that you are now ready to coat the
squeegee side. You should be starting at the bottom of the screen, which was the
top of the screen before you flipped it over. In other words, you want to be coating
from the opposite direction that you were coating when you were coating the print
side. Apply at least one coat to the squeegee side. If another coat is desired flip
the screen upside down so that the next coat is applied in the opposite direction.
6) The coated screen will need to dry in a completely dark environment. A small
amount of warm air flow is helpful. Obviously, it needs to be as dust free as
possible. Place the screen so that it will dry horizontally, squeegee side up. It
would be much easier if it could just rest on the frame with the print side up, but
then the emulsion would tend to settle toward the squeegee side leaving a surface
that is not perfectly smooth for the print stroke. Therefore it needs to be supported
by the edges of the screen frame (print side down) which will allow both sides of
the screen to dry. Time to dry completely will vary according to the particular setup
and conditions. Under ideal conditions, with excellent warm air flow, such as in a
commercial drying cabinet, a screen may be completely dry in 1 hour. In a closed,
cool cabinet with little or no air flow, 12 -24 hours may be required. The emulsion
must DRY COMPLETELY before it is exposed.
C. “BURNING” THE SCREEN:
The basic way an image is “burned” onto a screen is by placing a clear film or
translucent vellum with a (usually) black opaque image between the print side of a
screen coated with a photo-sensitive emulsion and a light source and then exposing it
to light. With the correct exposure time, every part of the emulsion on the screen that is
exposed to the light will “harden” while everything on the screen that is covered by the
opaque image will not “harden” but will, instead, wash out of the mesh when sprayed
with water. Those areas that wash out will leave open areas of mesh that ink may be
pushed through using a squeegee. To burn a screen:
1) Lay the film or vellum (let’s call it film, here) with the side of the film which has the
ink from the printer on it (usually the blackest side) so that that side is in contact
with the print side of the screen. Usually, this is taped to the screen with clear tape.
When viewed from the print side the image will be reversed. The image will be
correct (not reversed) when viewed (through the emulsion) from the squeegee side.
2) Set the image on the exposure unit with the film/print side down. If there is a
vacuum, there should be a cord or string draped so that it will run from inside the
screen frame to the intake for the vacuum. This allows the vacuum to remove the
air from the cavity caused by the screen frame and pushes the vacuum blanket
against the back of the screen holding the screen firmly against the glass of the
exposure unit. If there is no vacuum, some other means of pressing the screen
against the glass should be used. This will prevent light from “leaking” around the
edges of your image, called “undercutting”, which results in fuzzy, rough edges on
the stencil.
3) After the screen is in the exposure unit with the lid closed, (with vacuum on or
other means of pressure in place) begin the exposure. Exposure time should be
calculated well in advance of this step and will vary (often extremely) depending on
the thickness of the emulsion, the density of the black on the film, the intensity of
the light source and the distance of the image/screen from the light source. Once
the exposure is done, open the unit, take out the screen and remove the film.
4) Take the screen to a washout area. Ideally, a pressure washer should be used to
wash out the screen, but a high pressure nozzle will often do. Thoroughly wet the
squeegee side of the screen first with water using gentle pressure. Then flip the
screen over and thoroughly wet the print side using gentle pressure. After awhile,
the areas covered by the opaque image on the film will begin to soften. When
this starts to happen, increase the pressure and concentrate the stream to get an
smaller, intense spray pattern…but don’t let the jet of water dwell too long on any
one area…move the spray in short, jerky back and forth motions across the screen
and pay attention to all areas that you want to open up. The minute you achieve
this goal, stop. There is no need to keep washing after you get all the areas open
that you want to have with nice clean lines and edges. Pay close attention at this
stage, halftones can be especially tricky, but if your exposure is correct there should
not be any problems. An underexposed stencil will have a slimy texture and milky
appearance. Uncured emulsion will run into areas that were meant to be open
and parts of the stencil that were meant to remain may wash out. An overexposed
stencil will be extremely difficult or impossible to wash out. Note that it may be
possible to blow out areas that you intended to keep, even on a properly exposed
screen, by concentrating the water jet in one spot too long or washing out from the
squeegee side.
5) Place the screen in a horizontal position to dry. Either side up is OK. Again, a warm
air flow will speed up this step and allow you to do the final preparations on the
screen and start printing sooner.
D. PREPARING THE IMAGED SCREEN FOR PRINTING:
The final step in preparing the screen for printing is to mask off the edges of the stencil
areas next to the frame where there is no emulsion, and block out pinholes or voids in
the stencil. This will prevent ink from leaking and contacting unintended areas of your
substrate. Using tape intended for this purpose is preferred, that is, tape that won’t
become saturated from ink or ink wash, won’t fail or come off and won’t leave adhesive
residue in the mesh that can be difficult to wash out during reclaiming. Although some
screen printers use regular packing or masking tape, these do not have all of the
advantages listed earlier. Pinholes and voids can be blocked out with any water soluble
liquid block out and a small paint brush. Some screen printers prefer to block out using
their regular emulsion and a brush and then post-exposing it to harden up the repairs.
One advantage of this is that it tends to be more durable on long production runs.
1) First, Using 2” or 3” wide tape, take small pieces of tape (approximately 2”X3” or 4”
each) and fold into sort of an inverted pyramid shape so that each piece can be placed
on the squeegee side of the screen in each corner. The tape should be attached to
the inside wall of the screen frame and connect the frame to the squeegee side of the
mesh, bridging the gap between the frame and the mesh. These pieces of tape should
look like the inside corner of little boxes when properly folded and attached in each
corner of the frame.
2) After all four corners are taped, lay a strip of tape along all four sides of the screen
frame, again bridging the gap between the frame and the mesh and creating an “ink
well.”
3) Next, lay another width of tape (slightly overlapping) along the inside edges of the
first “border” of tape on the squeegee side. You may get by with going around the inside
of the frame only one time, as long as the tape covers open areas of uncoated mesh.
4) Flip the screen over to the print side and lay a border of tape on this side. Match
the width of the entire border on this side to that of the squeegee side…if you made
two rows on the other side make two rows on this side. You will be able to see through
the emulsion to line them up, This doesn’t have to line up perfectly, it’s just that two
layers of tape is that much less likely to allow ink to leak through…and that’s the main
purpose, although it makes cleanup a little easier, also, as ink won’t get under the frame
between the frame and the mesh.
5) The last step is to block out all pinholes and voids in the stencil. Keeping the glass
on your exposure unit very clean will eliminate a lot of this. So will degreasing, but it
invariably occurs anyway. Examine the stencil very closely by placing it on a light table
or holding it up to the light and carefully scanning the entire surface of the mesh with
your eyes. Wherever you see small pinholes or voids, take a small brush, dip it into
the liquid block out and cover the hole by dotting or “painting” it with the brush. NOTE
– BLOCK OUT PINHOLES ON THE PRINT SIDE ONLY. Applying block out on the
squeegee side will impede the print stroke. Allow the block out to dry thoroughly.
6) Finally, you’re ready to start screen printing!