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Stencil Performance: Understanding Direct Emulsions & Capillary Films
A direct emulsion is one that is applied to the screen in liquid form with a scoop coater. This type of emulsion has specific qualities that differ from capillary film stencils. Capillary film is the type that is applied as a film in roll form with a smooth water squeegee. There are advantages and disadvantages to using either direct emulsions or capillary films.
Direct emulsions are difficult to apply consistently without an automated system. And because they are more than 50% water, direct emulsions can dry unevenly as the moisture evaporates. As the water evaporates out of the emulsion, it contracts and shrinks slightly. As it does, it conforms easily to the surface of the mesh and imitates its surface characteristics. These two factors combined can make it difficult to obtain a smooth emulsion surface with direct emulsions.
Believe it or not, the smoothness of the emulsion surface on the substrate side will affect your print results. The smoother, more consistent the surface, the tighter and more complete the stencil will come in contact with the substrate providing a good "seal". This ultimately restricts ink flow and will increase sharpness and resolution.
Direct emulsions are more durable. This is one of the reasons they are chosen most often in textile print shops. A direct emulsion is better suited for long duration print runs for excellent stencil life. Because direct emulsions encapsulate the mesh from both sides, it has better adhesion qualities and better durability. Direct emulsions also have the benefit of being able to be layered or coated to different thicknesses or emulsion over mesh values, (EOM).Capillary films have a predetermined thickness, (EOM), and surface qualities because they are manufactured to factory specifications. It is almost like a fruit roll up in that it is a film of light sensitive emulsion that is adhered to the screen. But these roll ups are made in varying widths and are perfectly smooth. Obviously this makes getting a smooth surface very easy. As stated above, this can affect print results.
Capillary film also has the disadvantage of being applied from only the substrate side. This means it does not have the same adhesion qualities as direct emulsions. Therefore stencil wear and durability can be an issue with capillary films on high volume print runs.
All direct emulsions are evaluated and/or rated by the following standards which will affect stencil performance:
Resolution - the ability to reproduce fine detail.
Definition - the ability to accurately reproduce the edges of the artwork as it appears in the film positive.
Bridging Characteristics - the ability to span or "bridge" the open spaces between mesh threads.
Viscosity - refers to a liquid's ability to flow. This is important during coating. Low viscosity emulsions flow easily, which can help fill in the tiny mesh openings in higher mesh counts.
Solids Content - refers to the solid particulate count in the emulsion. Emulsions that have higher solids content generally do better at bridging mesh openings.
Rz Value - the surface difference between the hills and valleys of the woven mesh caused by contraction and shrinking of the emulsion during drying is expressed in a numerical measurement.
Bridging characteristics describes the emulsion's ability to span or "bridge" the open spaces between mesh openings smoothly. Stencils that tend to take a longer zigzagging route around the mesh opening will create a saw-toothed edge. This means poor stencil resolution and definition. An emulsion with higher solids content will perform better in this respect. Higher solids content emulsions will also experience less contracting and shrinking during drying.
A dual cure direct emulsion of high quality will do the best job with fine detail and halftones. It will also have a wider exposure lattitude meaning you can vary the time depending on your film and light source. That will allow you to achieve the best results with minimal drawbacks in the washout booth. Capillary films are the absolute best for high specification detailed printing; these films will produce the best halftone stencils.
As it is plain to see, choosing your stencil source wisely can help you achieve the results you desire. Deciding whether a direct emulsion or a capillary film is best for your printing application will facilitate and increase the quality of the resulting print. And finally understanding direct emulsion characteristics in relation to the final stencil will also help you get the best results easily.