Page 22 - SCREEN PRINTING FOR BEGINNERS

The solids content in an emulsion The step coating procedure (Fig. 2)
does not always reflect in the is used to coat a screen to find
viscosity An example would be your best coating technique for
that pure photopolymer emulsions that mesh count. The step coating
are available in very high solids procedure starts with a stroke on
content with low viscosity. the face of the mesh. Then coat

Emulsion viscosity can also change the squeegee side once over the
with temperature. Lower entire screen. Coat again the
temperatures cause the emulsions squeegee side on only two-thirds
to thicken. Solids content less than of the screen, then coat again one-
30% with low viscosity are often third of the squeegee side (all wet
difficult to coat without a mess. In on wet).

addition, lower solids percentage Dry your screen as normal. When
will require multiple coatings to dry, face coat with the sharp edge
achieve reasonable mesh of the squeegee two-thirds of the
coverage. screen from a crossing direction
Emulsion Over Mesh or EOM is a (perpendicular) then dry face up.
measurement of the emulsion When the screen is dry apply the
thickness on the face or substrate last coat of emulsion on the face

side of the mesh. EOM is a side covering only one-third of the
percentage of the mesh thickness. same direction as the last face
(Fig. 1) Too low of an EOM ratio will coat, then dry. With the face
prevent a good gasket seal, coatings, there will be nine
prevent good detail resolution, and examples of coating thicknesses.
increase chances of sawtooth Pick an exposure time that fits the

edges. Manufacturers recommend median coating technique and
an EOM ratio of 10 to 20 percent. expose the screen with a test
With an emulsion stencil, more is positive that covers all of the
not better. Too much emulsion on coating changes.
the face of the screen can cause
difficult ink transfer and details
can break down in a run. Emulsion

drip from the mesh while drying is
a definite indication that the
coated emulsion is too thick.

Fig. 20 Emulsion Over Fig. 21. Step Coating Procedure
Mesh

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