Screen E-Book (.PDF) - MacDermid Autotype

You can have the best ink, the best mesh,the best press and the best film/emulsion,but if you don’t get the exposure right, youcan get poor image quality on your stenciland/or poor print-life on the press. Thischapter starts with a review of exposurescience and ends with a set of tips andtricks for getting the right exposure.ExposureDefinition : ‘On exposure to UV, light thesensitizer within the photostencil materialsreacts to crosslink the molecules in the stencilto produce a layer insoluble in water’.The practical mechanics of exposure arefairly simple. The photostencil material isheld in intimate contact with a film positiveand exposed to an ultraviolet lightsource for a pre-determined time.Photo-stencil sensitivityThe emulsions used to produce a photostencil(liquid or film) use light energy toactivate the sensitizer which initiates thecrosslinking reaction that in turn cures thestencil.The wavelength of light that the sensitizerabsorbs will depend upon it’s chemical nature.Typically emulsion sensitizers absorblight between 300 and 450nm, commonabsorption curves are:Diazo – alone in single-cure emulsions.Acrylate sensitiser – present with diazo indual-cure emulsions.Photopolymer – used in ‘One Pot’ emulsions.61