A step by step guide
Textile screen printing has traditionally been a two-dimensional art. You could print anything imaginable, but the only directions your designer could travel were East, West, North, and South, so to speak. Puff ink was the only exception. With puff, you could go up, but the amount of puff was difficult to control, even by the best screen printers. Using puff ink also severely limited your ability to render accurate details.
Recently, several ink and emulsion manufacturers, working with a small group of textile screen printers, have developed a system that adds a reliable, controllable "up" capability to screen-printed T-shirt designs. The cluster of procedures that comprise this system lets you print single layers of ink up to 600 microns (roughly 0.02 in.) thick, sometimes even thicker.
To put this in perspective, the thickest single ink layers printed in ordinary production (measuring thickness as the height of the ink film above the surface of the substrate) are usually glitter inks printed on transfer paper. The ink film thickness reported there is usually about 250 microns at the most, and this is on a flat, nonporous substrate with outstanding ink holdout compared to a jersey knit T-shirt.
The ink film thickness is not the most remarkable attribute of 3-D garment printing. What immediately attracts the attention of everyone who sees a good-quality sample printed with this technique is the outstanding resolution and definition of the print. Three-dimensional printing enables you to provide printed ink layers with vertical edges so sharp that they appear as though they've been diecut or even lasercut (Figure 1).
The ink film you get with 3-D printing is unlike the ink film produced on high-opacity transfers, athletic uniforms, and similar applications that are notorious for thick layers of plastisol. With such prints, the edge of the image is thin, often smudged, and at best, resembles the edge of a small puddle of spilt molasses, regardless of how sharp the stencil may be. Puff inks can produce a comparable ink film thickness, but the edge definition of a cured puff print has an irregular, almost organic appearance compared with 3-D printing's clean, sharp, almost machine-cut look.
As an aside, we must commend the men and women who developed 3-D printing for devising the most innovative T-shirt decorating process since process color on textile was systematized back in the 1980s. But we also must say that their language skills leave something to be desired. The common term for this process is "high-density printing." We're not sure why.
The wet ink and printed samples that we've seen do not have a particularly high density. While they are more dense than an ordinary cured puff ink, they are generally no denser than a high-opacity white plastisol. However, the name will probably stick, joining "self-tensioning frames" and "underbase" as industry standard terms.
Printing a 3-D design
Printing a 3-D design requires much more than using a different ink or emulsion. Successful 3-D printing requires that you master several procedures that are different from your normal production routines. They're not necessarily harder than standard production procedures, but they're significantly different and must be linked together into an integrated system to produce this remarkable new effect.
The printing screens you use for 3-D images should consist of 60-thread/in. (24-thread/cm) mesh on retensionable frames tensioned to at least 30 N/cm. Checking with my handy Comparative Screen Fabric Guide (available from the ST Books (www.stmediagroup.com target="_blank"), 513-421-2050, ext. 352), we determined that this mesh is available in five thread diameters, ranging from 120-145 microns. We recommend using the thinnest thread diameter available and, if possible, dyed mesh (mesh this coarse is difficult to find in colors other than white).
The reason for preferring the finer thread diameter is that you want the stencil openings to be filled as much as possible with ink, and as little as possible with mesh. Every micron matters here. We recommend dyed mesh because it will reduce the light scatter inherent in the extremely long exposure times required by the emulsions used in this technique.
Most printers know that stencil thickness is the primary influence on printed ink-film thickness. It is certainly true in 3-D printing, where the thickness of the stencil correlates directly to the thickness of the printed and cured ink film. Tests have shown that if you perform every step correctly, you can achieve a cured ink film that is about 90% as thick as the stencil.
Two-hundred-micron stencils are used routinely in 3-D applications, and 400- and 700-micron stencils are not uncommon (Figure 2). Up to a point, the question is not how thick do you want your stencil to be, but how do you produce a thick stencil? Once you select and master a method, getting additional thickness is usually only a matter of repetition. We'll outline three methods of producing extremely thick stencils and comment on the advantages and disadvantages of each.
Although the emulsion type you select for each of the coating methods listed below may vary somewhat, it must expose quickly to compensate for the extreme thickness of the emulsion layer. Fast-exposing SBQ-sensitized or pure photopolymer emulsions (liquid or film) are preferable to diazo and dual-cure varieties. Suitable liquid emulsions and/or capillary films are available from most stencil-system manufacturers.
Liquid emulsion The first possible method is to coat the screen repeatedly with liquid emulsion. You may need as many as 15-20 coats on the substrate side of the screen to produce the necessary stencil thickness. Use this procedure:
1. Apply the first coat of emulsion to the substrate side of the screen, and dry it thoroughly. To enhance screen drying times, your drying room should have a dehumidifier to keep the relative humidity at 40-50%.
2. After you apply and dry the first layer of direct emulsion, place a frame of tape around the image area on the substrate side of the mesh in order to build up the stencil thickness faster.
3. Apply and dry another coat of emulsion, then build up the frame of tape with another layer.
4. Repeat steps 2 and 3 until you reach the desired stencil thickness.
The advantages of this method are that you can do it with familiar techniques and use materials you already have on hand. The disadvantages are the length of time it will take to produce one screen (several days, at least) and the lack of control you have over the thickness of the emulsion layer. This final difficulty is due to irregularities in coating methods and the fact that around the 15th coat, you're likely to lose count of the number of coatings you've applied.
Capillary film buildup A faster method of creating an extremely thick stencil is to laminate layers of capillary film to a screen. The appropriate material for this application is the thickest, pure photopolymer capillary film available. Capillary direct films up to 150 microns thick are already on the market, and 200- to 250-micron films could follow soon.
Adhere the first layer of capillary film to the mesh by using the direct/indirect method. This procedure is now rarely used in most screen departments, so unless you have been working in this industry for a couple of decades or work for an emulsion manufacturer, you may not be familiar with it. The procedure goes like this:
1. On a flat table, create a "platform" smaller than the inside dimensions of the screen, but larger than the outside dimensions of the image area. This platform should be at least 1/4-in. high, and the surface should be absolutely smooth and flat. We recommend using plate glass with rounded edges and corners for this. The platform will ensure close overall contact between the mesh and the film.
2. On top of the platform, lay a sheet of capillary film, emulsion side up.
3. Place the screen, substrate side down, on top of the capillary film.
4. Pouring a thin bead of liquid emulsion on the inside of the screen along one edge of the sheet of capillary film.
5. To adhere the capillary film, spread the liquid emulsion over the entire area of the mesh in contact with the film.
6. Dry the emulsion layer.
Note that you must use a liquid emulsion compatible with the capillary film. By this, we mean a liquid emulsion that uses the same sensitizer as the capillary film and has the same basic chemistry. Since capillary film emulsions are generally only liquid emulsions coated on a plastic backing sheet and dried, most emulsion manufacturers will be able to supply you with compatible liquid emulsions and capillary films.
After you let the capillary film/direct emulsion layer dry and strip the polyester backing sheet from the capillary film, you're ready to continue building up the stencil thickness by adhering additional layers of capillary film to the layer already there. Here's the procedure you should follow:
1. Make a coating solution of 1 part liquid emulsion to 15 parts water.
2. Mask out the open areas of the mesh by applying a layer of tape to the substrate side of the mesh.
3. Coat the capillary film already on the mesh with the coating solution by flowing the solution over the capillary film while the screen is leaning upright against the back of a washout sink.
4. Apply the next layer of capillary film, emulsion to emulsion, over the film you just wet with the liquid emulsion.
5. Allow this layer to dry, then strip off the polyester backing film.
6. Continue to build up the emulsion layer by repeating steps 3-5 until you reach the desired emulsion thickness.
The advantages of this method are that it results in a stencil of any thickness you desire, with a flat, even surface. It also gives you excellent control over the thickness of the emulsion layer. The drawback is that it's a slow procedure, although not as slow as the liquid emulsion method.
Thick film method The fastest method of producing a thick emulsion layer is to start with the thickest possible film. Currently, one manufacturer (Murakami Screen USA, Inc.) offers thick photopolymer films in a range of thicknesses from 100-700 microns. These are not capillary films and require a special application process:
1. Select a piece of film of the desired thickness and cut it so that it is 1 and 1/2- 2 and 1/2 in. larger than the image area of the screen.
2. Peel off the protective plastic sheet from one side of the film and lay it, emulsion side up, on a platform similar to the one described in the "capillary film buildup method."
3. Place the screen, substrate side down, over the film on the platform.
4. Reduce some of the liquid emulsion (provided with the photopolymer film by the manufacturer), using 2 parts emulsion for each part water.
5. Pour a bead of the reduced emulsion across one edge of the film.
6. Squeegee the emulsion evenly across the entire surface of the capillary film. It is important to get a thorough, even coverage.
7. Card the excess emulsion off the open mesh area and allow the screen to dry.
8. After the screen is thoroughly dried, coat the squeegee side of the mesh again using a scoop coater filled with the undiluted liquid emulsion.
9. Dry the screen again, peel off the second protective plastic sheet from the film, and you're ready to expose it.
The obvious advantage to this method is that it's the fastest way to produce a 3-D stencil. But suitable stencil materials are presently available only in limited quantities.
Needless to say, screen exposure times will be extreme no matter which coating method you use, and it's very easy to underexpose these stencils. Considering the cost of a set of test screens, an exposure calibrator is not just a wise investment, but a necessary one.
Although emulsion type, thickness, coating technique, and exposure device will influence exposure time, the following curing specifications should give you some idea of what you're up against: Assuming you use an exposure unit with a 500-kw metal-halide lamp that is positioned 40 in. from a screen with a 700-micron-thick stencil coating, exposure time will be approximately 7.5-8 min.
After exposing, wet down both sides of the screen and allow the emulsion to soak for a few minutes. Then rinse again. The washout process cannot be rushed. Keep soaking and rinsing until the entire image is washed out.
If you are using a sharp, dense film positive, the degree of detail that can be developed will surprise you. These screens take much longer than normal to wash out and you must be extremely patient.
Three-dimensional designs are easier to print on automatic presses because of the greater control of screen off contact, which is crucial in this process. Manual presses can be used, but it's more difficult to produce a consistent print and nearly impossible to create an ink film thicker than 500 microns. The instructions that follow are for automatic press operations, but they can be adapted to manual press printing.
1. Place the screen in the press with minimum off-contact distance.
2. Set the flood stroke speed at slightly slower than normal, use firm flood-bar pressure. If the floodbar angle is adjustable, use a smaller angle between floodbar and screen (tipped more in the direction of the flood stroke) to fill the stencil with ink.
3. Use a medium-hard to hard (75-80 durometer) squeegee, and set the squeegee pressure to provide just enough pressure to transfer the ink from the screen to the garment. Set the squeegee speed at slightly slower than normal and the squeegee angle so that, like the floodbar, the squeegee is tipped more in the direction of the printing stroke.
4. Although the ink used in 3-D printing is generally not unusually thick and flows well, it's a good idea to use slower than usual press speeds. For the best result, concentrate on achieving the best print possible with one clean, smooth print stroke. Double stroking the print may result in a print that is smooshed or smeared.
If you want to increase the thickness of the printed ink film, the best way is by increasing the thickness of the emulsion layer on the screen. You can't achieve greater ink film thickness by printing, flashing the ink, than reprinting the same screen again. The initial ink layer, now cured, will fill the openings in the stencil (Figure 3), preventing the screen from printing a significant layer of ink on the second print stroke.
However, one method you can use to build up a thicker ink layer is to make two thick stencil screens, with the second screen having a slightly choked version of the image on the first screen. By printing the two screens in succession (choked screen second) and flashing between screens, the ink from the second screen will be deposited on top of the ink printed by the first screen (Figure 4). The result is an ink layer that is roughly twice as thick as you would get with one screen.
The 3-D element in the design should be printed last or flash cured immediately after printing. However, the thickness of this image is such that if the 3-D ink is printed, flash cured, and subsequent colors are printed, any other element printed close to the 3-D ink could become distorted.
We have tested some designs with underprinting under the 3-D ink (Figure 5). You may question why this is necessary because, certainly, underprinting is not needed to improve opacity. Some tests seem to indicate that underprinting improves adhesion to the fabric. On the other hand, other test results seem to show that underprinting may cause a loss of sharpness in the print. This is one area of the 3-D printing process that needs further exploration.
Inks appropriate for 3-D designs are available from most major plastisol ink manufacturers (refer to the 1998/99 Screen Printing Buyers' Guide). Some manufacturers provide additives for use with inks already in general distribution, while other manufacturers have developed new lines of finished inks. A word of caution: Remember that these are not puff inks. In fact, any noticeable degree of puff will destroy the sharp, three-dimensional effect.
All of the samples of three-dimensional inks that we have tested cured at temperatures in the range of standard plastisols (300-340
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