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Saturday, May 7, 2016

Introduction to Screen Printing

For some time I've been trying to figure out how to effectively carve stamps for ink printing on fabric without much success. A few weeks ago I remembered silk screen printing and, at a glance, it seemed to be the perfect solution. Because I am not well versed in the process and I would rather not become largely invested in the equipment, I did some figuring and came up with a simpler or smaller solution to parts of the process.

First off, I needed to make the patterns. These need to be printed on transparency film and act in a way like exposing photographs, except the paper will be a photo sensitive resist on the screen. Everywhere I have read, exposing the screens requires a bit of trial and error, so I came up with some designs of varying detail and size to work with.

Here is the first change. Usually the frames for screen printing are rectangular for obvious reasons (the silk and transparencies are both rectangles) but I found that the silks themselves without frames are significantly cheaper. So, instead of getting a frame, I used an ordinary embroidery hoop. Getting the tension was a little tricky but once it was set, I had no problems with it. One thing that can be seen are a few hard creases in the screen, which is an unfortunate result of the shipping. Ultimately, this had no effect in the print.

Next up, the screen needs the photo emulsion resist. This is, once mixed, a sort of light sensitive paint that makes the screen usable. Once exposed to high intensity light for a short period of time (or a lower intensity light for a long period of time), the mixture hardens and its washable characteristics are removed. Areas covered by the pattern are not exposed, and thus able to be washed away, leaving clean screen through which the ink can be passed.

In spite of the very obvious and boldly stated instructions that tell you to add water to the diazo (activator), a great many people complain that the diazo is dried out and does not work. Imagine my surprise when it worked perfectly when I followed the provided instructions.

From a weird blue to an even weirder green, the photo emulsion also changes consistency slightly when the two parts are mixed. Make sure that it is thoroughly mixed, or parts of the screen may not cure properly.

One thing that I could not find anywhere online before physically having the emulsion is the shelf life once mixed. After adding the diazo, it can be stored in a refrigerator for up to four months for later use.

Now then, to prepare the screen, I poured a line of the emulsion over one edge and used a squeegee to spread it thinly and evenly across the screen. Be aware that any uncovered place will allow the ink to pass through it, so covering as much of the screen as possible is recommended. Once I had the one side covered, I flipped it over and smoothed over the back with the squeegee. Removing excess is important, or the emulsion will pool in droplets while drying, which is undesirable for its inconsistency in curing when exposed.

Once the emulsion is spread over the frame, let it dry in a dark, not overly warm place. I shut it in a closet for a few hours, checking on it periodically to ensure there was not any pooling and, if needed, using the squeegee to gently spread out the emulsion again to correct it.

This was the first effort, which eventually did not survive due to under exposure, but the process here is the same. Careful to not tape over the middle (which would cause the emulsion beneath to be unexposed), I taped the negatives on the screen. If you have clear tape, problem solved. When placing the patterns, be cognisant of which side you place them on and in which orientation. The side of the screen that is flush with the frame will be down when printing, so in the orientation I have the stencils currently, the final printing will be mirrored. You can expose from either side depending on how you set up the light configuration, but this is the only side I could use, so I had to have my stencils flipped over because this is the 'underside' of the screen.

In order to keep the stencils flat on the screen, and since I do not have a lightbox, I had to improvise a little. I do not have any clear glassware in my kitchen and, I realized, virtually nothing made of flat glass at all. So, I clamped the screen to the door of my shower, which is why I had to have the negatives on the above side of the screen. Having a piece of flat glass or plastic would do just fine to keep the transparencies flat to the screen, really whatever works. Or, you could probably get away with just taping all the edges down with clear tape as long as the light you are using to expose it with is not overly hot.

Exposing the screen is where the trial and error comes in. I exposed three before getting it good enough to print, and it is still not perfect. As per the instructions of the photo emulsion, using a 250 Watt lightbulb requires 7~8 minutes depending on the size of the screen. This did not work for me, so I tried again at 9 minutes, and it was closer, but still not great. The third time I went for 11.5 minutes, and it was functional but a bit over exposed. The problem with this is the emulsion under the negative becomes harder to wash out and damages the exposed bits in the process.

After exposing, wash the screen. Almost immediately, the unexposed parts begin to appear as the emulsion washes out. Gently using a soft toothbrush, the pressure of the water, or your fingertips to clean the screen, wash until until all the areas to be printed are white again like the original screen.

Now, it's ready for printing. The squeegee presses against the screen, so the side of the screen flush with the frame needs to be down.

With a rectangular frame, it is a bit easier to print because you can use a squeegee that is the same width as the screen, but for a round frame it's not as smooth. Laying a line of the ink down on one side, press firmly on the squeegee and spread the ink over the entire area to be printed, bringing the excess off of the printed pattern. Then, just lift off the screen and there you go!

Once the print is made, it needs to be cured via heat. For cloth, the easiest way is to use a dryer on high heat. Note the ink needs to dry to the touch first or it will get everywhere. After curing, the ink is washable in ordinary settings. This was the first printing and turned out a little rough, but I learned a lot from the process and what to expect.

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