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Photopolymer Emulsion

(August 2006) posted on Fri Aug 18, 2006

Murakami USA


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With the advent of high-density inks more than ten years ago came a revolution in special-effects garment printing. Nowadays, consumers commonly see multiple effects incorporated into a shirt design, but behind the scenes, the prepress department has to make different types of stencils for these effects. For example, a design may have two or three thick-film stencils for stacking, two water-resistant stencils made of a diazo-type emulsion for discharge and water-based inks, and dual-cure emulsion for high-resolution printing. This assortment of stencils can certainly cause problems for screenroom personnel. Each type of stencil requires different preparations and exposure times, and the confusion that arises from managing so many products and parameters can slow down production.

Murakami’s R&D department formulated Aquasol-HV, a blue colored, PVA-SBQ pure photopolymer direct emulsion to simplify the process of creating stencils for use with a variety of inks and remove variables in stencilmaking that stand in the way of consistent quality. Aquasol-HV is pre-sensitized for immediate use. Murakami says the emulsion is excellent for use with plastisol, water-based, and discharge inks and notes that the product’s specially balanced solids content and high viscosity make it ideal for coating any mesh count.

Photopolymer emulsion can generally be three to four times faster in exposure time than conventional, dual-cure emulsion. However, the speed of a photopolymer needs to exhibit some exposure latitude. Having an emulsion that is extremely fast can present more problems simply because it becomes harder to correctly expose stencils that leave no elbowroom. Murakami points out that even though Aquasol-HV is faster to expose than a dual-cure emulsion, Aquasol-HV still offers exposure latitude that won’t compromise the emulsion’s resolution, definition, and mesh-bridging qualities. 

Mesh color, emulsion type, coating thickness, lamp type, and the age of the bulb all influence exposure time and quality. Keep in mind that using the proper lamp is essential. The spectral sensitivity of pure photopolymer direct emulsions is between 360-420 nm. According to Murakami, using a 5-kW metal-halide lamp at 40 in., a white, 86-thread/in. mesh coated with Aquasol HV can be exposed in 50-80 sec. Using that same setup, a yellow, 350-thread/in. mesh can be exposed in 25-35 sec. 

Aquasol-HV’s high solids content (42%) and high viscosity (8000-10,000 mPas) make it possible to coat a screen wet-on-wet to build up to a 250-micron stencil with what Murakami describes as an extremely flat Rz value. In many cases, the manufacturer explains, a single coat on each side of the screen may be all that’s necessary. Diazo should be added to enhance a stencil’s water resistance (a 15% increase in exposure time is also required). Murakami will provide the correct diazo bottle upon request. Diazo may simply be dissolved and added to the emulsion. Users also may need to fortify the stencil further with hardener. Note that the use of hardener can make reclamation more difficult.

Coated screens may be stored for four to six weeks when kept between 60-75°F (15.6-24°C) and at 40-60% relative humidity. Murakami also suggests drying coated screens that are stored under these conditions for more than two hours to ensure proper exposure.

Murakami recommends immediately washing out exposed screens with cool water and then drying the screens in a horizontal position (with the print side down) to prevent any emulsion from running into the image area. Circulating air and keeping temperatures at 86-104°F (30-40°C) and relative humidity at 30-50% will help ensure complete drying. 

Aquasol-HV is available in 1-, 5-, and 50-gal drums. For more information, contact Murakami USA, 745 Monterey Pass Rd., Monterey Park, CA 91754, 800-562-3534, fax 323-980-0659, Web: www.murakamiscreen.com.
   


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