Laminating Stencils
Roger L. Jennings
All shops can expand their screen printing opportunities with simple stencil making procedures that are sure to surprise even the veteran screen printer with the graphic results.
For those shops using liquid emulsion, each mesh count produces a different coating thickness, and therefore dimension to the ink deposit. A 305 monofilament is typically 35 microns thick. 110 mesh is 80 microns, and 60 is 125 microns. Emulsion fills the mesh creating a depth to the ink well.
The exposure time for liquid emulsion and capillary film varies with thickness, chemistry (such as, diazo, dual cure and photopolymer), and manufacturer. With liquid emulsion the coating thickness is also affected by the angle of the scoop coater, pressure and speed of application, quality of coating instrument, and screen tension. On soft screens the liquid emulsion can be thicker and darker in the center compared to the outside edges where there is better support for the mesh. Those are a lot of variables affecting exposure time and ink deposit.
Capillary film is a coating of liquid emulsion on a plastic sheet at a precise thickness as marked on the packaging by the manufacturer. Thicknesses from 10 to 1000 microns are available. All the uncertainties of liquid emulsion have been replaced by an exposure time and ink dimension that is exactly predictable. Welcome to quality control!
For those shops that had to expose a second screen when the first screen was unacceptable, you can now eliminate that extra work. An investment of about $35 in an exposure calculator allows measuring the exact exposure time required for each thickness, chemistry and manufacturer of capillary film. Under exposed screens with scum in the image, blisters and ragged edges, and over exposed screens that are hard to wash out are replaced with "goof-proof" screens. Precise light exposure is particularly important when using gray images on inexpensive vellum rather than dense black images.
The impact on the printed image can be truly astounding. Here are some samples. An image created with 305 or higher mesh and 20 or thinner capillary film can be printed so that the image can be seen, but not felt. If colors are overlapped to make registration easier, using a slightly thicker stencil for the color on top adds opacity to reduce the risk of the first color bleeding through the second color. Image edges created with hard edge capillary film have better resolution than ink deposits influenced or blocked by mesh in the image rather than above the image.
Some of the most exciting graphics, however, are on textured garments and with textured images. Ink printed through a screen coated with liquid emulsion follows the texture like pique resulting in ragged edges. The same images exposed in a film coated under the mesh produces hard, straight edges when using ink with three dimensional properties. A 100 micron capillary film will bury the texture of pique golf shirts and 600 denier tote bags so that the texture and color of the substrate does not show through the ink film.
Textured images have more than a width and length. They stand off the garment. The amount of that third dimension is determined by the stencil thickness. So realistic bumps can be laid on a basketball surface that is printed on corduroy (see the February 2003 Printwear cover). A mascot like a tiger can be printed with deep socket eyes by printing with thicker stencils around the eyes. Corporate logos can be photographic copies of corporate advertising, signs, packaging and stationary.
Stencils of various thicknesses can be created with almost any exposure unit. One piece of capillary film is exposed, washed out, and then covered with a second piece of capillary film to build the stencil thickness, and therefore ink deposit. The key to success is locating the positive exactly in the same spot for each exposure. For those who don’t have pin registration systems, a screen registration guide is the equivalent.
A screen registration guide is simply a flat board larger than the screen and with a 2" x 2" board affixed to one edge. A screen is butted against the 2x2, and the frame is used to locate a second 2x2 at the edge of the frame and perpendicular to the first 2x2. Every time a screen is pushed against these two 2x2’s, the screen will be exactly in the same spot. Now draw the vertical and horizontal center lines on a screen and on a piece of graph paper. Lay the screen against the 2x2’s and locate the graph paper under the screen with the center lines of the graph paper matching the center lines of the screen. Tape the graph paper in place, cover with a sheet of clear plastic, such as from capillary film, and tape the plastic to the board. Done!
Now, make two copies of the positive and tape one where the image is to be exposed in the screen. Tape the second positive over the first in precise registration and secure in place with one piece of adhesive tape. Add 1" strips of tape that is adhesive on both sides in each of the four corners of the top positive. Butt a coated screen against both 2x2’s, and then down on the double stick tape. Pick the screen up, and the top positive is sticking to the bottom, or print side, of the screen. After exposure, the second positive can again be located exactly in the same spot using the first positive for registration. The process can be repeated as many times as desired to create a stencil of any thickness. This is simple, and free!
Here are some tips. When building thick stencils, the first piece of film should be 50 or 100 microns thick. Thinner film like 50 microns exposes and washes out easier. Selecting a film with double the thickness more than doubles the exposure time. After this first piece of stencil has been washed out, and while it is still wet ("the wetter, the better"), lay a second piece of capillary film over the image like covering a hamburger with a slice of cheese. The water will bond the two slices like superglue. After the second layer of capillary film has dried, the plastic sheet has been removed, and the image exposed again, both sides of the image should have water run over the image, but not sprayed with pressure. Water will absorb into the stencil from both sides to speed the washout process.
If a thick stencil is the objective, that first, thinner stencil, holds subsequent thicker pieces off of the mesh like a gasket to facilitate washout. Also, thinner films dry quicker, because the moisture evaporates out of only the emulsion side of the capillary film. A 400 micron film, for example, can take a long time to dry. Washing out thinner films is easier and more like the experience of washing out liquid emulsion. Drying can be expedited with a heater fan.
Washout out can also be accelerated by laying a screen in a horizontal position, laying a paper towel over the image, and spraying with water. The wet paper holds the moisture on the image so water will be absorbed into the stencil. The paper towel should be removed within 1-2 minutes, or the water could get between the stencil and mesh causing blisters or images to lift up. If that happens, dry the mesh from the ink side of the screen with a towel and then store the screen to dry completely. The film will re-adhere to the mesh. Then wet the screen and continue the wash out process until the image is clean.
Never squeegee capillary film into mesh, because that pushes soft emulsion into the screen and the ink deposit will be less than as marked on the stencil packaging. However, if removing the plastic liner to capillary film is difficult, either moisture is still in the stencil or the stencil should have been squeegeed into the mesh around the edge of the film, and beyond the image, to create a mechanical bond between the capillary film and mesh without adversely affecting the thickness of the ink deposit.
Laminated films add a new dimension to your work, fun and profit potential. Your customers will be impressed.
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