It is the simplicity of the screen printing process that makes it as attractive as a manufacturing process. If you have a need to lay down controlled thicknesses of material over a closely specified area screen printing could be your ideal solution.
The initial criteria are to create a mixture of the material in the form of a liquid or paste that you wish to deposit onto a substrate. The ranges of materials that suit this are vast. From chocolate to ceramic in solutions of sugared water to sulphuric acid. Of course conventional inks consist of a pigment (solid,) solvent (liquid,) resin (dissolved in the solvent) and additives These will dry with the application of heat and moving air. Other systems use Ultra-Violet energy or occasionally Electron Beam or even Microwave energy. Whichever system is used the aim is to create a dried and cured film that is fit for purpose.
Having decided what material you want to deposit and that it can be formulated so that it will pass through a mesh stencil or a stencil mask (More on these later,) you next need to consider how you are going to dry and or cure the material. This will depend on its end use and the required condition of the substrate after drying and or curing. Substrates can be rigid, flexible solid or woven. They even have been known to be hard deep frozen when printed flexible when at room temperature and hard ceramic when fired. The key is that at the point of print the substrate has to be stable. A woven textile is extremely flexible but it is held so that it is stable when it is printed.
Already we have a huge array of options in terms of material to deposit, substrate and curing techniques. So let us address the issue of mesh stencil or stencil mask.
A mesh stencil is the most common form of stencil. The image is created using a photosensitive emulsion that is coated onto the mesh and then dried. Conventionally a photopositive is attached to the coated mesh and then the coated mesh is exposed to Ultra Violet light. The exposed area hardens and the unexposed area is washed out with water creating a very accurate image area that allows the material printing medium to pass through the myriad mesh openings onto the substrate. There are alternative methods of creating the image using Computer to Screen systems.
A Stencil mask has no mesh but consists of a metal plate that has the image area cut out normally with a laser or chemically etched. Stencils masks are widely used in the deposition of solder pastes in Surface Mount Technology (SMT).
The mesh stencil is the most adaptable of the methods and the stencil mask is used mainly where paste or higher viscosity materials have to be applied.
With a mesh stencil it is possible to apply a 0.300 mm film thickness and sometimes higher, with a stencil mask it can be significantly more than this but the printing medium must be suitable. The printing mechanism of mesh stencils allows higher speed printing up to 4500 sheets per hour as against stencil masks only hundreds per hour. Speeds are dependant a whole range of factors, particularly drying and curing. It is unwise to assume a production speed before the project has been considered in depth. If you intend to use screen printing as a mass production deposition process you must take into account the need to formulate the material so that it is suitable for printing. If you don’t know how to do so there are companies who can help you.Some of the Materials Currently Screen PrintedCeramicsChocolateMagnetic IronElectrically ConductiveDielectricsCementEnzymesAluminiumPlatinumOxidesInkSilverAcidsGraphiteGoldAlkalisAntibioticsDyesSiliconeAdhesivesSolder PasteResistetcetc
If your material is not in this list don’t worry, if it can be suspended in a solution or paste it is likely that it is printable.
In industrial applications the drying and curing element of the process is crucial. Sometimes it is necessary to dry materials in inert atmospheres such as nitrogen. In less complex applications it is still necessary to apply the drying mechanism in a very controlled fashion as this can affect the final characteristics of both the printing medium and the substrate. Modern dryers provide data on the conditions throughout the dryer. In some cases cooling or air-conditioning is used to bring the substrate and ink back to room temperature. One of the main constraints in drying and curing is time. The length of the dryer and the speed of transport through the dryer directly affect the print speed. This assumes that drying takes place inline with the print. Occasionally it is necessary to dry offline in static ovens but this should be avoided where possible. There are many different solutions to inline drying and curing. When printing onto sheets dryers can take the form of horizontal conveyor ovens or wicket dryers. In roll to roll applications vertical multi-pass dryers are used. There are many different solutions intended to provide the necessary drying/curing conditions for a prescribed time.
For any ink (printing medium) to achieve an acceptable bond to the substrate it must be capable of wetting the substrate. That means that the ink must have a lower surface tension than the surface energy of the substrate (ideally 10 Dynes difference,) if it is not so the ink will not flow onto the surface and may not even transfer. Pre-treatment of the substrate to increase the surface energy may be necessary or wetting agents need be added to the ink. Pre-treatment methods include flaming, corona discharge, plasma or even liquid priming.
Getting back to the basics of screen printing as stated before the process appears very straight forward but like every process the secret is reducing the variables and maintaining precise process control. The biggest variable is generally the printing medium. Having a consistent mix at all times is obvious but people who use the process have an unfortunate habit of altering the mix on the printing press by adding thinners or other additives that in turn screw up the whole process. Any changes must be controlled by weight and recorded and their effects noted down. My advice is to mix your printing medium and use it in volumes that mean no adjustments ever take place on the press. This will often mean you have to control the ambient conditions.
- Posted by aplmach
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