Screen printing in industrial applications is no different, and it can be taken for granted that precision machinery is essential. Variations in speed, pressure and alignment can all alter the lay-down of the ink, but interestingly it is often the drying and curing process which requires the most attention as this can determine the overall production rate and even the choice of printing equipment.

Common drying and curing methods include radiation curing, such as ultra violet (UV) light and infra-red. Ultra violet energy does not have to have a heating effect, although most emitters produce heat energy as a by-product of producing UV. Efforts are often made to filter out the heat with water cooling, quartz filters and air movement to eliminate substrate distortion. Light emitting diodes (LEDs) produce cold UV but they are restricted in the wavelength that they produce and this  needs to be matched to the chemistry of the printing medium to effect a full cure.

Infra red energy, on the other hand, does emit heat. Short, medium and long wave curing are all possibilities. Short wave is closest to the visible spectrum and is combined with a lot of white light, while medium wave is further away so the emitter glows red. Long wave is the furthest away and the emitter does not give off any light. Short wave curing is dependant on colour of the material and its reflectivity, whereas long wave infra red will penetrate more deeply into a film.

Thermal curing techniques such as forced air and conduction are also an option. Forced air is particularly suited to drying solvent based systems. The heat in the air drives off the solvent molecules and the moving air removes them from the surface of the film. Alternatively, in conduction curing, the substrate carrying the printed film passes over a heated platen to dry the film from bottom to top.

Whichever method is used, control, measurement and recording are a key part of the process. The chosen method will produce vital chemical reactions whose completion is key to the end product.

Solvent evaporation provides a simple example. If the initial drying is too intensive, solvents can be locked into the film below a dried surface. Other examples include polymerisation caused by radiation or thermal curing, which has to be completed in a specific way to achieve the desired outcome. Sometimes film needs to be partially cured so additional layers can be applied, while other times oxygen must be kept away from the curing film as this will alter the finished product. An inert gas such as nitrogen can be used to effect a cure without oxygen contamination.

Dirt can also cause contamination, so incoming air sometimes has to be filtered to submicron levels and drying often takes place in clean room conditions. All these factors mean that with any dying/curing, considerable thought has to be given to the equipment even before you select the printer, as this can affect the overall production.

Another important element of industrial printing is stencil quality. Stencils are created using a mesh stretched on a frame, coated with a photosensitive emulsion which is then dried, exposed and developed. After careful inspection, any non printing areas that are still open mesh are filled.

So how does this differ from a conventional screen printing stencil? The difference is precision in every aspect of production. Frame selection, for example, depends on the mesh specification, the tension, image size and off contact distance. The frame has to be strong enough to maintain its stability as any distortion will push the film thickness outside its tolerance. An unstable frame can distort the image and change the off contact distance, which will in turn alter the printed film.

Mesh has a major effect on film thickness. It meters the printing medium on to the substrate. The theoretical ink volume of the mesh in cubic centimetres per square metre is a guide to the wet film thickness and equates directly to the film thickness in microns.

Mesh specification should also take into account the tension applied to the mesh, depending on the mesh material, mesh elongation, off contact distance, the image size and frame strength. The key issue is to work within the elastic range of the mesh, so it will maintain its tension over a long period. Ideally the mesh should lift away from the wet film as soon as the squeegee has passed a given point to allow an efficient passage of medium from the mesh. Mesh tension should be measured in the middle and four corners of the image area. Deviation of more than one Newton centimetre for most applications would disqualify a stencil.

Stencil emulsion over mesh (EoM) and roughness are also key aspects of stencil selection. The thickness affects the build of ink at the edge of the image and the roughness impacts on edge definition. If liquid emulsion is being used, then additional wet on dry coats should be applied to level the print side of the stencil for good edge definition. Alternatively capillary films provide high frequency roughness that stops the stencil sticking to the substrate but effectively provides a flat stencil for edge definition and controlled EoM.

As this brief examination of drying and curing and stencil issues illustrates, there are a whole host of factors to take into consideration when it comes to deploying stencil printing techniques in an industrial setting. Other aspects 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.

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