Successful Emulsion Coating
Coating Screens for Consistent Results
We visit many shops each year to evaluate stencil issues and to shed some light on the various processes involved in making screens. Larger shops often have automatic coaters while smaller shops perform hand coating. First the auto coaters:
Auto Coaters are predictable when set up correctly. By predictable I mean their EOM (percentage of emulsion over mesh) is more consistent. No matter who uses the machine it will consistently apply emulsion the same every time. The key is dialing in coater pressure and speed to achieve the desired EOM.
I have gone into shops where they are coating 3 times on the squeegee side and 2 times on the print side and the EOM is close to 0% because the coater pressure is too high. Coating Pressure, Speed, and the type of emulsion used need to be balanced to achieve the desired EOM. When changing to a new emulsion the EOM should be measured on a test screen and pressure and speed adjusted to meet the target EOM percentage.
More Pressure or More Speed equals lower EOM percentages, or less emulsion on the mesh.
Less Pressure or Less Speed increases EOM.
Auto Coaters can be programmed with different coating recipes. While a 305 may utilize a 1:2 coat with a sharp edged coater, a 60 mesh may need slower coating speeds, and may provide better results with a 2:3 coat using the dull edge. It all depends on the mesh, the coater settings, and the ow qualities of the emulsion. Generally three to four recipes are all that is needed for textiles, and for UV graphics only one or two is needed since mesh counts are usually in the 300-380 range.
Manual Coating
Too often manual screen coating is left for the end of the day. The screen room worker rushes to get 50 screens ready for tomorrow and coats as fast as possible. Worse the night worker and he do not use the same angle or speed. The result? Unpredictable EOM, under or over exposure, and lots and lots of pinholes. Auto coaters coat slowly to keep air bubbles from forming. Hand coaters ignore this and whip air into the emulsion with very fast coating speeds. Slow down coating speeds and use more pressure. Let the emulsion flow through the mesh instead of gliding a fast coat on. EOM can vary by 20% from screen to screen using a fast coating speed, so will the quality of exposure causing breakdown on press.
The diagram below shows the correct placement of the scoop coater, hey the end cap is pointed for a reason! (fyi). If all coating workers use the end cap and coat at the same speed then screen exposure will be predictable. This will preserve crucial details while exposing the emulsion completely for dicult inks like discharge and waterbase that require complete stencil exposure to avoid breakdown on press.