Screen Printing 101 Hints & Tips: Air Bubbles
Air bubbles in the mesh can cause pin holes during printing and result in down time due to new stencils having to be be made. In this short guide you will learn how to coat the perfect emulsion and how you can reduce the likelihood of air bubbles during coating.
Several factors influence the coating regime required to produce an ideal stencil. These include mesh count, thread diameter and tension, coating trough edge radius, emulsion viscosity and solids content, coating trough fill, screen cleanliness, coating speed, angle and pressure.
The first coats must be from the print side and full mesh fill should be achieved before proceeding to coat from the squeegee side. Full mesh fill can be seen as a glossy finish on the squeegee side. If, after the print stroke, the squeegee side is matt then the emulsion has not filled the mesh and will risk air being entrapped on the squeegee stroke. These air bubbles may not be visible without magnification, but during printing hidden air bubbles cause weak spots in the screen and may cause pinholes.
The squeegee side coats are applied to achieve the final stencil profile.
Coating too fast can cause air bubbles. This is because the emulsion cannot flow into the mesh fast enough to keep pace with the coating trough. The emulsion begins to roll over itself drawing in air, which will then be deposited on the mesh.
The emulsion’s viscosity and rheology under shear will influence how fast the mesh will fill during coating. Very low viscosity emulsions could drip through coarse mesh.
Coating troughs are typically designed with edge diameters from 1-2.5mm. Sharp edged troughs are not suitable for coating a base coat on coarse mesh because they tend to scrape emulsion away rather than pushing into the mesh.
Trough fill level will influence how quickly the trough can be coated.
The more coarse the mesh, the more emulsion is needed to fill it.
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Name: William Shorter
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