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FLEXO Magazine : October 2013
• Dual Durometer (Capped Plate) • Strippable • Compressible Con- struction PLAte eXPosure The basic process of making a printing plate is similar for liquid and sheet photopolymer. In both cases, the back is exposed to UV light to establish the floor. The face is then exposed through the imaging mask, which establishes the printing surface. Finally, the plate is light finished to cure all remaining photopolymer. The final plate will have six dimensional attributes that affect the print characteristics of a photopolymer plate: • Caliper • Floor • Image Area • Shoulder • Plate Backing • Relief uv LigHt sources UVA, 400-315-nm., is the primary source for the main exposure of a photopolymer plate. UV initiators have a broad absorption band between 380- and 340-nm., with a peak of approximately 360-nm. After the processing and drying of the exposed photopoly- mers, the polymers exhibit a tacky surface. Special germicidal lamps are used to eliminate this tack from the surface. Fluo- rescent lamps without a phosphorescent coating to convert UV to visible light emit UV light with two peaks at 253.7-nm. and 185-nm. due to the emission of the mercury within the bulb. Roughly 85 percent to 90 percent of the UV light produced by these lamps is at 253.7-nm., while only 5 percent to 10 percent is at 185-nm. Germicidal lamps use quartz, i.e. glass, doped with an additive to block the 185-nm. wavelengths. Such low pressure mercury lamps are used extensively for disinfection, and in standard form have an optimum operating temperature of about 30 degrees Celsius or 86 degrees Fahrenheit. Use of a mercury amalgam allows operating tem- perature to rise to 100 degrees Celsius or 212 degrees Fahrenheit and UVC emission to about double or triple per unit of light-arc length. These low pressure lamps have a typical efficiency of approximately 30 percent to 35 percent, meaning that for every 100 watts of electricity consumed by the lamp, it will produce ap- proximately 30 to 35 watts of total UV output. bAcK eXPosure Back exposure is necessary to fully adhere the polymer of the plate to the polyester backing and establish the relief. No image mask is used during back exposure and the photopoly- mer is cured by exposure to high intensity UVA light through the polyester backing sheet. During the back exposure, the polymer cures to form a solid in a progressive migratory manner. The longer the exposure, or more UVA energy absorbed, the thicker the floor becomes. Variables that can affect proper exposure include differences in UV sensitivity for the photopolymer and UV energy output, especially as the UV lamps age. Back exposure step tests must be conducted regularly to establish the rate of cure for particular combinations of photo- polymer materials and exposure equipment. This simple test exposes a sample of the plate material through the polyester backing sheet on the exposure equipment. Exposure times are stepped according to the plate material and the equip- ment manufacturer’s recommendations. After processing, the resulting thickness steps produced in the plate material are recorded and charted to form a back exposure guide for the particular material and lamp output. The main exposure, i.e. imaging exposure, transfers the image from the image mask to the printing face of the photo- polymer. This is done by selectively curing the photopolymer with UVA light through the clear areas of the mask. Curing is progressive. Rate of cure is dependent on many factors: • Sensitivity of Photopolymer • Image Detail • Mask Transparency • UV Energy Output of Exposure Unit Flexographic Image Reproduction Specifications and Toler- ances (FIRST) recommends using the plate exposure target, available in five resolutions, depending on the linescreen used. Selecting and incorporating the appropriate resolution control target is important. After exposing and processing the plate, the micro-line must be straight to the touch. Failure to hold this line straight is an indication of underexposure. Main exposure stepped test graph- ics containing a variety of copy detail and tonal values are available from FIRST. Once the desired back exposure is established, images are exposed for various times necessary for plate production. With any of the direct-to-plate imag- ing technologies, exposure and polym- erization take place in the presence of oxygen, which inhibits polymerization at the plate’s surface. As a result, the im- ages that form in the plate are smaller than the image that was cleared on the carbon black mask. The shoulder is not as sharp when compared to a film-masked made plate imaged from the same electronic file. Often referred to as dot sharpening, this exposure process has played a signifi- cant role in the development of plate imaging technologies. Exposing plates without oxygen pres- ent has been done for many years in the flexographic industry using film-masked platemaking techniques. In film-masked platemaking, the negative acts as a barrier between the photopolymer and the UV light. As a result, the images that form in the plates are often slightly larger than the image on the film nega- tive. One advantage to this process is the shoulders of the printing plate are sharp and well-defined. However, a disadvantage of the film- masked platemaking process is the need to create a vacuum for holding the film negative in place. The film and vinyl cover sheet create a light diffusion effect resulting in wider shoulders. n Editor’s Note: Flexog- raphy: Principles & Practices 6.0 (FP&P) features more than 600 pages, 29 chapters, hundreds of illustrations, a detailed glossary and extensive troubleshooting guide. For more information on purchasing a copy of FP&P, visit www.flexography.org/fpp. Back Exposure Step Test The Components of a Photopolymer Plate www.flexography.org OCTOBER 2013 FLeXo 67 66 FLeXo OCTOBER 2013 www.flexography.org Your finishing operators know this too. The new VLI delivers greater precision and speed of inspecting, slitting and rewinding. And that means quicker turnaround and perfectly finished rolls, every time. Speed and precision. www.rotoflex.com IT’S NOT ONLY THE SPEED OF THE STEEL,BUT THE PRECISION OF THE CUT AT THE CUTTING EDGE Do you make the cut? Find out at rotoflex.com/thecut RF_FlexoMag_4.5625x7.375.indd 1 4/16/12 4:29:48 PM ULTRAVIOLET WAVELENGTH CHART ULTRAVIOLET WAVELENGTH CHART ULTRAVIOLET WAVELENGTH CHART ULTRAVIOLET WAVELENGTH CHART Name Abbreviation Wavelength Range (In Nanometers) Energy Per Photon (In Electronvolts) Ultraviolet UV 400–100nm 3.10 – 12.4 eV Ultraviolet A UVA 400–315nm 3.10 – 3.94 eV Ultraviolet B UVB 315–280nm 3.94 – 4.43 eV Ultraviolet C UVC 280–100nm 4.43 – 12.4 eV Near Ultraviolet NUV 400–300nm 3.10 – 4.13 eV Middle Ultraviolet MUV 300–200nm 4.13 – 6.20 eV Far Ultraviolet FUV 200–122nm 6.20 – 10.16 eV Hydrogen Lyman-alpha H Lyman-a 122–121nm 10.16 – 10.25 eV Extreme Ultraviolet EUV 121–10nm 10.25 – 124 eV Vacuum Ultraviolet VUV 200–10nm 6.20 – 124 eV Table 1 UV Wavelength Range Founda ti on of FlexographicTechnicalAssociation TechnicalEducationServicesTeam 3920VeteransMemorialHwySte9 BohemiaNY11716-1074 Phone:(631)737-6020 Fax:(631)737-6813 www.flexography.org FLEXOGRAPHYPrinciplesandPractices6.0