photoblocker
A substance that stops light.
photoblocker: UV-blocking additive for screen mesh
A photoblocker is a light-absorbing chemical compound mixed into screen emulsion or applied as a separate layer to block ultraviolet light from reaching unexposed areas of the mesh during photographic exposure. In screen printing, the emulsion hardens only where UV light penetrates; photoblockers prevent premature hardening in areas that should remain soluble for ink flow, sharpening image definition and edge clarity.
Photoblockers work by absorbing UV wavelengths in the 365 to 405 nanometer range, the typical output range of mercury vapor and LED exposure units. Carbon black is the most common photoblocker in direct emulsion systems, either pre-mixed into the emulsion or applied as a separate light-blocking layer. Some shops use opaque tape or photomasks as physical blockers instead, but chemical photoblockers offer finer control for detailed halftone work and small text.
The substance matters most in high-resolution applications. Screen printing of fine detail (below 4 point type, or halftone screens finer than 85 lines per inch) demands tight photoblocking to prevent undercutting, where light scatters under stencil edges and hardens emulsion it should not. Photoblockers reduce lateral light diffusion and improve edge definition, reducing ink bleeding and improving registration on multi-color jobs.
Photoblockers can introduce brittleness or poor ink release if overused. Excessive carbon black in emulsion may cause the hardened stencil to crack during squeegee pressure or block ink flow entirely. Storage stability matters too; some photoblocker additives settle or degrade over months. Mixing photoblockers unevenly into emulsion creates uneven hardness across the screen, causing patchy ink transfer.
The chemistry and effectiveness of a photoblocker depend on screen mesh count, emulsion thickness, UV lamp wattage, and exposure time. A 156-mesh count screen with 1 mil emulsion needs different blocking strategy than a 48-mesh screen with 6 mil emulsion. Screen printers typically adjust photoblocker type and concentration empirically rather than by formula.