Having problems with exposure of your screen printing emulsion or is your stencil breaking down prematurely during printing ? If you can identify one or more occasions when this has occurred, then this 'How to Screen Print' guide on 'Under exposure and the problems it causes' is what you've been looking for.
Is your screen printing emulsion fully exposed?
Full exposure of screen printing emulsions is more critical than people realise. The following guide identifies how incorrect exposure can effect the durability and the processing of your stencil.
Screen Printing 101 Hints & Tips: Under Exposure and the problems it causes
The correct exposure time is key to getting the stencil to its optimum performance for the press. In this short guide you will learn the effects of under exposure on screen printing emulsions.
Under exposure of a screen printing direct stencil (emulsion) is probably, the single biggest cause of stencil breakdown!
Picture 1 is a cross section of an emulsion with an exposure time of four minutes. If you give this screen only one quarter of the optimum exposure, i.e. one minute, the light will have penetrated only through the first quarter of the emulsion as depicted in blue. During the washout all the unexposed emulsion that has not been hardened will just rinse away. A 'skin' of the image would first appear and then the whole stencil would disappear down the drain.
If exposure time is increased to 2 minutes (Picture 2) the light source will penetrate further through more of the stencil layer and harden some of the emulsion around the fibres. During the washout the stencil would probably stay on the mesh, but because the under exposed emulsion on the squeegee side has washed away, the stencil would have a very low level of durability.
If we increase the exposure time to 3 minutes (Picture 3), more of the emulsion round the mesh fibres will become hardened with a further increase in exposure time. The stencil will have a greater durability, but on the squeegee side of the mesh the emulsion has only been partially hardened.
The maximum wear on any stencil is the squeegee side where the water, solvent and chemicals of the inks combine with the abrasive effect of the squeegee passing backwards and forwards. You must harden the stencil completely to ensure durability. For long print runs it is critical that the entire stencil layer has been fully hardened (Picture 4) which will provide the resistance possible.
If you are still experiencing problems with exposure or stencil breakdown during printing after reading this guide? We recommend that you take a look at these alternative 'How To Screen Print' guides for more information.
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Name: William Shorter
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