Volume 39, Issue 2, February  2004

The Big Screen
Achieving Decorative Effects with Screen-printed Glass

by Denise Breard

I
n recent months, the use of decorative glass has become a trend sweeping the architectural community. Numerous techniques for creating such a look are available. Slumped and fused glass, sandblasted and etched glass, decorative interlayers, window films and screen-printing are just a few examples. The screen-printing process involves using a squeegee to transfer ink through a mesh stencil. The printing area is open mesh and the non-printing background areas are masked off with materials that prevent the ink from transferring to areas it should not. 

Direct Screen-Printing with Acid Etch Products
One of the simplest methods used in screen-printing is to prepare a stencil with a film positive. A film positive is an opaque image on a clear film base where the light and dark areas appear as they do in the original image and in the final print. In screen-printing, the ink printing areas correspond to the black solid and tonal values on the positive. Once the stencil is prepared, a squeegee is used to screen-print acid-etching cream directly onto the glass. After the appropriate waiting time, the acid cream should be rinsed, removed or neutralized according to the manufacturer’s instructions. Most etch creams produce light, misty effects that are well-suited for fine lines, details, halftone vignettes and even photographic images. The quality of the film positive and proper screen preparation are essential for fine details.

Screen-Printing with Resists for Texturing Processes
If a stronger, deeper acid etch is required, another method is to prepare a screen with a negative image, the opposite of a normal screen-printing positive. Using a film negative to expose the screen will result in a stencil in which the background will be the printing area, rather than the image, which is left unprinted. Once the negative stencil has been prepared, a squeegee is used to print a liquid resist to mask off the non-etching areas. The resist will harden in the background and non-printing areas so they will not be etched accidentally in the next step. Once the resist is cured or dried, the glass can be etched with stronger acids as needed, or even sandblasted. Some simple resists include conventional, solvent-based screen-printing inks, plastisol inks and even wax. Several printers use screen filler or blockout, which is normally used to spot out pinholes in stencils. Keep in mind that plastisol inks must be heated to 320 degrees Fahrenheit after printing to cure the ink, which will remain slightly rubbery, even after curing.

An interesting option for selective texturing is available after printing and curing the resist onto glass. Plastisol, in particular, can leave a fairly thick ink deposit and the open areas of the glass then can be chipped with relatively fine control by selective placement of animal glue or hide glue. The usual factors of glass type, glue strength, glue thickness, temperature and drying will affect the size and depth of chipping, as they would with any other glue chipping application. With plate glass, the animal glue alone is enough to chip the glass surface, but with harder glass types, a preliminary etch or sandblast will assist the adhesion of glue to glass.

Photographic Stencils Directly onto Glass

Screen-printers use a variety of liquid stencil emulsions and stencil films to produce screens for printing. Many of these stencil products can be applied to glass rather than to screen mesh and then exposed with methods similar to commercial screen production. 

There are several ways in which the stencil emulsion can be applied to the glass, including:
• Coating it on with a printing squeegee or other blade;
• Curtain coating for large volumes;
• Spraying slightly diluted emulsion onto glass with a       typical automotive body paint sprayer; or 
• Printing the emulsion through a fairly coarse screen. 

The key is to ensure a smooth, even coating of emulsion. Some printers prefer to apply or coat several thin layers of emulsion to build up the desired thickness; others apply a fairly thick layer and simply wait longer for it to dry. The simplest method is to apply several layers of adhesive tape along the side edges of the glass, pour a small amount of emulsion near the top and then use a squeegee to draw the emulsion down along the glass in a smooth, even film. The layers of tape control the thickness of the emulsion layer.

Once dry, the emulsion is exposed in the same way a screen would be—with a film positive and an ultraviolet (UV) exposure unit. After exposure, the coated glass is developed similarly to screens, using line pressure for thinner emulsion layers or even a low-pressure washer for thicker emulsion films. Fine details and halftones can be readily reproduced with this method, provided that suitably fine blasting sand is used. After blasting, standard stencil removers will soften or dissolve the emulsion for rinsing it off the glass.

Direct Screen-Printing onto Glass
Let’s discuss an area of glass decoration that’s often overlooked. Many decorative effects can be created with pressure-sensitive vinyl on glass, but some spectacular effects can be achieved by screen-printing images and patterns directly onto glass.

Decoration on to smaller decorative glass panels can be achieved easily with ceramic decals printed with frit-based inks. Normally these decals are applied by waterslide methods. Once dry, the item with the “raw” decal must be fired in a lehr or kiln to burn off the carrier basecoat and fuse the frit (think of ground colored glass) onto the item. Decals also can be used for appliance glass, but direct printing with ceramic frit inks is more common. The frit ink is screened directly onto glass, which must then be fired to fuse the pigments to the glass, just as with decals. 

Not every glass decorator has the equipment necessary for decoration with frit, decals and firing. Also, these methods do not lend themselves to decorating architectural glass that has been installed already, so we shall also take a look at screen-printing with inks other than frit-based. 

Direct screen-printing can be an effective method for architectural glass decoration; several glass enamels and two-part catalyzed epoxy screen inks are available for printing. A favorite with some decorators is to second-surface screen-print a single- or multi-color design with silicone-free, catalyzed epoxy. After curing, the print will accept mirroring readily, and will be protected behind the glass rather than exposed on the first surface of the glass. Decorative bar mirrors are often prepared in this manner, and, as with any screen-printing process, the ability to produce multiple units is attractive.

Other inks will also adhere to glass, but some are not scratch-resistant enough for first-surface use, though they may be fine for second-surface decoration. Recent developments in UV-curable inks have improved their usefulness on glass greatly.

Richly colored transparent and semi-transparent inks can be used to screen-print faux stained glass designs. This can be particularly cost-effective when multiple units are required. The three-dimensional lead effect can be simulated with high-density inks or structured inks printed through thick stencils. 

Screen-printing provides a wide variety of deco effects with techniques that can be mastered easily. Glass decorators should look at modern screen-printing techniques with fresh eyes. 

Operators of glass businesses can start up in screen-printing with minimal capital expenditures. The least expensive method is to convert an existing table into a manual screen-printing press with a pair of hinged clamps that hold the screen in place. Screen-printing suppliers commonly call these jiffy clamps or heavy-duty adjustable hinge clamps. The same suppliers will also have kick-legs that hold the screen up between prints, along with squeegees, stencil emulsions and capillary films. Most suppliers and distributors also provide screen-making services and can provide finished screens ready to print. Mid-range startup packages with multiple color printing systems can cost up to several thousand dollars. At the other end of the investment scale, fully automated screen-printing equipment with associated screen making, exposure and reclaiming systems can run in the several-hundred-thousand-dollar range. 

To learn more, visit the Specialty Graphic Imaging Association’s website, www.sgia.org. 


USG

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