9 Principles For Consistent Screen Coating

Screen coating techniques vary widely among print shops, with some employing complex methods involving up to seven or eight coats per screen. However, similar results can often be achieved with a straightforward one-on-one coating approach. By applying basic physical principles, you can better understand the screen coating process and optimize it for efficiency.

Principle 1: Emulsion Accumulates on the Opposite Side in Wet-On-Wet Coating

Wet-on-wet coating simplifies the process for many applications. Start with at least one coat on the substrate side, though two or three coats may be needed to fully fill the mesh and displace air. A glossy emulsion layer on the squeegee-side indicates success. Coarser meshes require fewer coats, while finer meshes (e.g., those with 15% open area) need more coats than coarser ones (e.g., 25% open area) due to differences in emulsion flow. After coating the substrate side, apply at least one wet-on-wet coat on the squeegee-side to build emulsion thickness on the substrate side, creating sufficient relief.

Principle 2: Finer Meshes Need More Coats

The same logic applies to squeegee-side coatings. Finer meshes, like 355, require more coats than coarser ones, like 196/200, to achieve the same emulsion thickness because smaller mesh openings slow emulsion flow.

Principle 3: Coating Trough Design Impacts Results

The coating trough is critical but often overlooked. A deep V-shaped trough with a round edge ensures optimal emulsion buildup. Smaller edges result in insufficient buildup, while larger ones deposit too much emulsion. For example:

Sharp-edge troughs with small fill volumes are ineffective for building thickness but are ideal for wet-on-dry face coats.

Principle 4: Keep the Coating Trough Well-Filled

Some shops rotate screens after each coat or notice uneven emulsion buildup, often due to improper trough design. A trough with low emulsion volume depletes quickly, leading to uneven coating (thinner at the top, thicker at the bottom). A high-volume trough maintains consistent emulsion flow, ensuring even thickness. Refill the trough as needed during coating.

Principle 5: Use Thin Coatings for Fine Details

For fine details or halftones, thin emulsion layers are essential. Wet-on-wet coating may not yield a smooth, thin enough layer, so consider using an emulsion with higher solids content or different viscosity. Alternatively, apply a thin base coat (e.g., 2 substrate + 1 squeegee), dry it, then add a wet-on-dry face coat on the substrate side. This adds only 1-2 microns, filling voids and smoothing the surface. Repeat up to three times for optimal smoothness.

Principle 6: Sharp-Edge Troughs Are Best for Face Coating

Face coating requires a sharp-edge trough, which scrapes over the dried base coat and deposits emulsion only in surface voids. A dual-edge V-shaped trough (one round edge for base coats, one sharp for face coats) with a deep profile ensures consistent results. Such troughs, like Lawson’s aluminum emulsion coater, allow versatile and reliable coating.

Principle 7: Consistent Angles Are Key

Emulsion flow varies by type, but the angle between the trough, screen, and screen tilt must remain consistent across coatings. Tilting the screen or trough too much increases emulsion flow, resulting in thicker buildup. A emulstion coating stand that holds the screen and a trough with guide-equipped end caps help maintain uniformity.

Principle 8: Wet-On-Wet Suits Most Applications

Following these principles, wet-on-wet coating achieves reliable results with a tolerance of ±2 microns, sometimes even tighter. Face coats allow precise micron-by-micron thickness adjustments.

Principle 9: Dry Screens Substrate-Side Down

For wet-on-wet coatings, dry screens horizontally with the substrate side down to keep emulsion on that side and ensure a smooth finish. Drying substrate-side up causes emulsion to flow to the squeegee-side due to gravity. Face coats can be dried in any position.

Conclusion

Choosing the right emulsion can eliminate complex coating procedures, often requiring just a 2 + 1 wet-on-wet coat for specific mesh counts. Streamlining mesh types and selecting appropriate emulsions reduces costs and speeds up screen preparation. Modern emulsion technology makes outdated practices, like rotating screens or using lacquer thinner, unnecessary. Screen coating can be simple, efficient, and consistent with the right tools and techniques.