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Screen June 09 P10-11:Screen Feb09 P12-14 15/05/2009 08:15 Page 11
SCREEN: INDUSTRIAL
wave curing is dependant on colour of the material and its reflectivity, whereas Frame selection, for example, depends on the mesh specification, the tension,
long wave infra red will penetrate more deeply into a film. image size and off contact distance. The frame has to be strong enough to
Thermal curing techniques such as forced air and conduction are also an maintain its stability as any distortion will push the film thickness outside its
option. Forced air is particularly suited to drying solvent based systems. The heat tolerance. An unstable frame can distort the image and change the off contact
in the air drives off the solvent molecules and the moving air removes them from distance, which will in turn alter the printed film.
the surface of the film. Alternatively, in conduction curing, the substrate carrying Mesh has a major effect on film thickness. It meters the printing medium on
the printed film passes over a heated platen to dry the film from bottom to top. to the substrate. The theoretical ink volume of the mesh in cubic centimetres per
Whichever method is used, control, measurement and recording are a key part square metre is a guide to the wet film thickness and equates directly to the film
of the process. The chosen method will produce vital chemical reactions whose thickness in microns.
completion is key to the end product. Mesh specification should also take into account the tension applied to the
Solvent evaporation provides a simple example. If the initial drying is too mesh, depending on the mesh material, mesh elongation, off contact distance,
intensive, solvents can be locked into the film below a dried surface. Other the image size and frame strength. The key issue is to work within the elastic
examples include polymerisation caused by radiation or thermal curing, which range of the mesh, so it will maintain its tension over a long period. Ideally the
has to be completed in a specific way to achieve the desired outcome. mesh should lift away from the wet film as soon as the squeegee has passed a
Sometimes film needs to be partially cured so additional layers can be applied, given point to allow an efficient passage of medium from the mesh. Mesh
while other times oxygen must be kept away from the curing film as this will tension should be measured in the middle and four corners of the image area.
alter the finished product. An inert gas such as nitrogen can be used to effect a Deviation of more than one Newton centimetre for most applications would
cure without oxygen contamination. disqualify a stencil.
Dirt can also cause contamination, so incoming air sometimes has to be Stencil emulsion over mesh (EoM) and roughness are also key aspects of
filtered to submicron levels and drying often takes place in clean room stencil selection. The thickness affects the build of ink at the edge of the image
conditions. All these factors mean that with any dying/curing, considerable and the roughness impacts on edge definition. If liquid emulsion is being used,
thought has to be given to the equipment even before you select the printer, as then additional wet on dry coats should be applied to level the print side of the
this can affect the overall production. stencil for good edge definition. Alternatively capillary films provide high
Another important element of industrial printing is stencil quality. Stencils are frequency roughness that stops the stencil sticking to the substrate but
created using a mesh stretched on a frame, coated with a photosensitive effectively provides a flat stencil for edge definition and controlled EoM.
emulsion which is then dried, exposed and developed. After careful inspection, As this brief examination of drying and curing and stencil issues illustrates,
any non printing areas that are still open mesh are filled. there are a whole host of factors to take into consideration when it comes to
So how does this differ from a conventional screen printing deploying stencil printing techniques in an industrial setting. Other aspects
stencil? The difference is precision in every aspect of production. include the printing medium, substrate handling and its suitability for
printing as well as squeegee and flood coater settings. Striving for
precision in each and every one is the key to success.
Developed for the photovoltaic/solar panel sector,
BTU’s Tritan firing furnace features a triple-belt/dual
speed conveyor, dual lane capability, wavelength
optimisation technology and full chamber access
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