No doubt about it. Screenprinting is a decidedly “multistep” process. It all begins with selecting an image. If that image hasn't already been rendered in digital form, digitize it by scanning or photography.
Bear in mind images often look different on a computer screen than when screenprinted. The reasons are straightforward: monitors use additive color (RGB) whereas ink on paper is inherently a subtractive color process. (CMY/CMYK color is fundamental to printing.) Monitors often display unnaturally brilliant blues and greens. Inevitably these high chroma colors appear darker on paper—the left vs. right images above illustrate the point. Imagine having a crystal ball that accurately predicts what your image will look like as a screenprint. What a big advantage that would be! Well, reality isn't quite like that but using the magic of color science it can come pretty close. It allows the artist to do things like modify image colors, etc., or maybe decide it's better to start off with a different image.
The middle image above shows an attempt to forecast the appearance of the final screenprinted version. It was made by applying a Color Lookup Table (CLUT) to the original image (on the left). In this case the prediction was pretty close to the real outcome, but of course mileage is going to vary. You can try it out yourself, the “prediction” CLUT (aka haldCLUT, though no one seems to know what the “hald” part actually means) is included in the bundle of files available for downloading**.
Image size and resolution are crucial. Each pixel in the image translates to a color “dot” in the final print, so for example, if it's intended to make a print 10x10in at 150 px/in (6 px/mm) then the image size has to be 1500x1500 px. Maybe you'd prefer trying something less demanding, say 4 dots/mm, or even 2 dots/mm. Then the image size would be 1000x1000px @100ppi, or 500x500px @50ppi respectively.
This page describes the steps to take once your art image is ready to go. My methods are geared toward specific software applications—luckily these are open-source, free, cross-platform programs and easy to find on the 'net. Start with these: GIMP, G'mic plugin for GIMP and Scribus. (Alternative apps no doubt exist but likely require significant modifications to the workflow outlined here.)
Artist-screenprinters generate positives needed to make stencils or “screens”. Positives are like big transparencies, pure black/monochrome images inkjet-printed onto to special screenprint film. The best printers for the purpose are made by Epson, specifically wide-format 8 and 10 color “P” series photo printers and large units designed for high-production screen shops. I use a 17in P800 model. Epson printers and inks yield the densest black, a crucial feature for optimum stencils. Other low to medium cost printer brands have proven less satisfactory.
With an image file of the right size and resolution in hand these are the many steps to making a fantastic screenprint. Have fun!
- Open the image in GIMP. Make sure the image is the right size and resolution. Use “Scale Image...” from the Image menu to check. If all is good, just cancel, but make needed adjustments if necessary.
- In the layers dialog duplicate the primary layer. Select the top layer. Open G'mic, select “Apply external CLUT”. Browse for “clut+full-r07.png”** and press “OK”. (This CLUT has the effect of slightly darkening the layer which compensates for lightening due to dithering in the next step.)
- In the GIMP “Color” menu select “Dither”. When the dialog opens, set red, green and blue “number of colors” option to “3”. Make sure dithering method is set to “Floyd-Steinberg”. Press OK. Note that the image lightens up a bit and should match pretty closely the unmodified image in the bottom layer.
- With the dithered layer selected, open “Components” in the color menu. Select “decompose”. When the dialog opens, choose “RGB” which is the default. A new image appears with 3 grayscale layers, red, green and blue. I refer to this channel-separated image unsurprisingly as “RGB”.
- Let's clear up some potential confusion before it gets serious. The separated channels will be labeled Red, Green, and Blue, no surprise there since that's what we asked for. Yet we're proceeding as though we have Cyan, Magenta and Yellow channels. What gives? Pretty simple really, the convention is that white areas in separations stand for 100% full color, black is 0%. However positives for exposing screens are black on clear. So using RBG separations to make screen positives means inverting the original sense of RGB separations. Great! With absolutely no extra effort whatsoever we've created CMY separations for positives! But this is simply the idea of complementary color: red/cyan, green/magenta, blue/yellow are merely opposites on the good ol' color wheel.
- Back to the image layers, select the bottom, not-dithered image. Use Components/Decompose on this image layer to produce another RGB set of channel separations, only this one isn't dithered.
- In this new set of separations we're only interested in the blue layer. Open G'mic again, this time find “Posterized dithering”. In the dialog, leave sliders set to default values, except set “Gamma” to 0.45. Make sure to set output to “New layer(s)”. Press OK, and a new layer indeed appears. Despite being composed of only black and white areas/dots it should resemble the original blue layer quite closely. Now delete all layers except the new one. (This ultimately becomes the “yellow” positive/stencil in the process.)
- Returning to the dithered red, green and blue layers, start with green. Use “Control-D” to duplicate the RGB image. In the duplicate, delete the red and blue layers, leaving only green. Now use “Control-D” twice to make 3 instances of the green-only image.
- Make the first green image active. In the “Colors” menu find “Threshold”. The dialog will show two darker vertical bands at around 80 and 170. Leaving the white arrow at 255, move the black arrow above (to the right of) the rightmost vertical band. The green layer now becomes mostly black (typically around 75%). Press OK.
- With the second green-layer image repeat the Threshold process, only this time set the black arrow between the vertical bands, anywhere around the middle of the range. The image black area will be around 50%. Hit OK.
- Use Threshold again on the the third green image. Move the black arrow to the left of leftmost vertical band. Image will be about 25% black. The three processed images represent three levels of magenta that will later be screenprinted.
- Repeat step <7>, this time deleting the green and blue layers, duplicating the red-only image twice for a total of three red-layer images.
- <repeat step 8>
- <repeat step 9>
- Apply Threshold as was done in steps <8> through <10>. Result is three images which will be used for printing cyan layers.
- So now we have 7 black & white (or 1-bit) images, one for yellow, 3 magenta and 3 cyan. Choose a directory (aka “folder”) to store the files for the image that will be screenprinted.
- Save the 2-layer “source-image”. I use this format: <name>-<width>x<height>.xcf. Then I save the dithered channel separation as <name>-<width>x<height>-RGB.xcf. The 1-bit images are saved as follows: the originally blue-only image is saved as <name>-<width>x<height>-Y1.xcf. Green-layered are stored as -M1, -M2 and -M3.xcf, red-layered as -C1, -C2 and -C3.xcf.
- Note that “xcf” is GIMP's native format. One limitation of GIMP is that it doesn't deal with the considerable complications of printers, can't fault GIMP's developers on that account. We'll simply use other software geared to solving the problem. For that purpose we need to export the CMY images in png or jpeg format. For each image (Y1, M1, etc.), activate then hit Control-E (or use File menu item “Export as”), save as .png. Just make sure it's saved to the same directory along with the xcf's.
- I use Scribus to print the 7 Y1 through C3 positives. Generally I screenprint 3 sizes: 5x7in, 7x9.5in, and 15x21in. One sheet of 17x22in film holds 9 5x7 images, four 7x9.5 images, or one 15x21 image. Accordingly, I've made Scribus templates** for these 3 sizes. Open Scribus and pick the appropriate template which opens showing 1, 4 or 9 image frames. Let's assume prep for screenprinting 5x7 images.
- Right-click in the upper left image frame, choose “Get image” from the menu. Browse for the directory where images were saved, select the -Y1.png. The image fills the frame, right click again this time select “Adjust frame to image size”. Repeat this process for the other 6 images. They may be placed in any order, but to minimize confusion place row-wise -M1 to -M3 in middle row, -C1 to -C3 bottom row.
- After template is filled delete the empty frames to prevent spurious error messages when template is sent to printer. Save the filled-in template as <name>-<width>x<height>-<Y1-...>.sla.
- When ready to print the positive, use “File/print” menu item, or printer icon on the toolbar. (Note that Scribus regards printing images with resolution <144ppi as an error, it's perfectly safe to ignore the warnings, go ahead and print anyway.) Depending on the printer additional dialogs may appear, settings may need to be modified. Refer to printer documentation for details. For the Epson P800, Epson's website recommends specific settings for printing on screenprint film. These settings might well work for other printers too.
- If the positive prints without printer errors, proceed with producing the stencil for the specified CMY color. Of course with small prints only one or two screens will be necessary. Best results require fine mesh, 300-305 is optimum. Screens need to be properly prepared to receive direct emulsion. That means using appropriate micro-abrasive on new mesh or screen prep product. All screens must be degreased and fully dry.
- Coat the screen with suitable direct emulsion. I prefer pure photopolymer, water-resistant products such as Chromaline ChromaTech WR, PC-787, Saati PHU, among others. These are very fast exposing, and preserve fine detail. A proper exposure unit is necessary, LED-UV light sources are particularly effective and consistent. (BTW it's perfectly possible to build one's own LED exposure unit but that discussion will have to happen on its own web page...) The spectrum emitted by LED-UV lamps (generally in the range of 395-405 nm) closely coincides with the peak sensitivity of the photopolymer emulsions.
- Coating technique is crucial to good results. Always work under yellow safelight conditions. Use a clean, undamaged scoop coater. Apply one coat to the print side and then to the squeegee side. Dry print-float down in dust-free environment. It's useful to provide heating to above room temperature (but not above 100°F) and forced air flow to speed drying.
- When screen is thoroughly dry, expose screen with positive. Exposure unit should provide excellent contact between positive and screen. Exposure time has to be worked out by testing with stepped exposure guide or incremental exposure procedure. With photopolymer emulsions and LED-UV, exposure is often 60 seconds or less. Underexposure causes breakdown of stencils, overexposure makes washout difficult and both cause loss of detail in the printout.
- Washout technique is crucial, and more than not little guidance is provided by emulsion manufacturers. The method I use is unconventional but substantiated by published research. After exposure the screen is wet down both sides. Starting with the squeegee side, the fan-spray pressure washer (~1500-2000 psi) is used for one minute side-to-float at a distance of about 2 feet from the screen. Then the print side is similarly washed out for 1 minute. A final spray of the squeegee side of <30 sec completes the washout.
- The screen is again dried using forced air flow at ambient temperature allowing 30-60 minutes for complete drying of the screen.
- While benefit is controversial, some emulsion manufucturers consider it a good practice to “post-expose” the finished stencil before printing begins. This involves exposing the squeegee side to same light source used to expose the stencil for 2 to 4 times the original exposure time.
- Mount the screen in the hinges on the printing table. A vacuum table or platen is extremely useful, I'd consider it a necessity for precision work. (Like other studio equipment a vacuum table can be artist-constructed.) Offset is about ⅛". Of course, before anything is printed the paper to be printed on, and ink to print with have to be prepared.
- Paper is cut to size allowing ample margin for attaching registration tabs. For example, a good size for making 5x7 prints are sheets approximately 8x10in.
- Registration tabs are attached to back side of the paper, but this has to be done in such a way that is consistent. Three ¼" tall registration pins (marked in red in the illustration) are arrayed to align paper sheets. Two .055 in pins (in green) are set in place at 1⅛" from the right/left edges, with the bottom of the pin ⅜" from top edge of the sheet.
- A sheet is “fed” into the alignment pins, weights are carefully placed on the sheet to keep it from shifting. Registration tabs are inserted over the pins at the top, the tabs then fixed in place on the sheet with 2" long pieces of suitable tape, e.g., 1" wide drafting tape. The paper sheet is then removed from the top pins and stacked for printing.
- Ink preparation requires a reliable scale with resolution of <=0.01 gram. First is mixing the base. Several mixes are possible, but combining speedball acrylic base with Golden airbrush medium, Golden acrylic retarder, Liquitex thickener works well (for porous media like paper, not as good for plastics, etc.). Like many printing inks in bulk it's quite slow drying, but thin layers on absorbant media dry reasonably fast.
- Colors are transparent, made by mixing pigment (supplied as an aqueous dispersion) with the above-described base. Three pigments are used: Hansa yellow medium, quinacridone magenta, phthalocyanine blue (green shade). The pigment dispersions are made by only a few suppliers, the brand I've used (Createx Pure Pigments) is no longer manufactured, but alternative suppliers exist. Obviously dispersions can vary in concentration and other characteristics, so testing by the printmaker is necessary.
- Yellow is laid down first because it's the least transparent color. Only one layer of yellow is needed, hence the basis for creating the positive as it was. (Yellow is very light color so our vision doesn't easily detect differences in yellow strength, so we can get away with a single “consolidated” layer.) Therefore the one layer of yellow is printed with “full strength” color. Pigment concentrations of layered color have to be determined by testing since effect varies by pigment dispersion characteristics,paper used, mesh size, squeegee pressure, and artist preferences.
- Typically, on well-sized, less absorbant stock, pigment dispertion concentrations of 1, 3, 9% by weight are a good starting point. Using those numbers, yellow would be printed with 9% yellow pigment in base. Calculate the amount of ink needed. On average 0.025 gram/sq.in meets requirements . Thus, 30 5x7in prints have 1050 sq.in. At 0.025 g/sqin 26.25g will be used. However the printable or open area is typically about 75% for Y1, M1, C1 stencils. So actually about 20g of mixed ink will likely be consumed. In practice, an additional amount has to be allowed for the reserve pool on the screen, squeegee, etc. For 5x7 prints 10g is probably sufficient. Mixing 30 grams is reasonable.
- Real-life measurements seldom come out to nice round numbers. A convenient way to prepare ink is weighing a disposable cup and writing the weight on the side. For example a 5 ounce cup might weigh 5.46g. Using the scale's tare feature, the scale is zeroed and ink base put in, say it turns out to be 27.56g. OK, next question is how much pigment needs to be added to make 9.0% by weight? Easy! Just use the formula:
- Plugging in the example values, it's 27.56 × 9 ÷ (100 - 9) = 27.56 × 9 ÷ 91 = 2.73. IOW, 2.73g pigment dispersion in 27.56g base makes 9.0% concentration by weight. Not too complicated! Now mix the measured pigment with the base and the ink is ready. Next step is blocking out open areas of the screen that shouldn't be printed. Various methods can be used to fill the gaps but with multiple stencils on a screen, blockout material needs to be removeable. Masking tape plus any kind of sturdy plastic film is simple, cheap and works well in most instances.
- High-precision printing requires getting registration right, though it can be challenging to achieve. There are many ways to go, but what I use now is printing on a clear substrate placed over the pilot print. Very thin but dimensionally stable material (like 2 mil mylar) is taped in place over the print. A vacuum platen/table is a huge help. The vacuum is on full force when the color is printed on the clear film, then the vacuum is reduced allowing the print to be moved underneath the clear film until registration is satisfactory. Then the vacuum is restored to full throttle, the clear overlay removed and registration pins inserted under and into the registration tabs of the pilot. The pins are then attached with tape to the platen. At that point the vacuum can be safely turned off.
- A potential problem with this registration method is ink thickening in the screen while aligning overlay and print. One choice is filling the screen with extra ink to retard drying. Alternatively, clean the screen before continuing the registration procedure taking care to avoid disturbing ink on the overlay. If using the first method, a heavy layer of ink, extra passes in the proofing stage may be needed to clear the screen.
- Before going forward with the actual prints, it's worthwhile to print a few proofs. Newsprint is typically used, sheets are printed to make sure there are no leaks or pinholes to mar subsequent prints. As noted above, after registration several passes may be needed to clear the stencil of thickened semi-dry ink in the small openings of the stencil.
- A goal of printing technique is that ink is deposited consistently from print-to-print. Squeegee angle and pressure, and use/style of flood strokes all affect results. Generally flood stokes are a good idea, but with transparent colors it can lead to progressively greater ink transfer. (But not flooding at all can cause noticeable loss of positive detail, also undesirable.) Using a “split-stroke” approach can reduce these problems. The idea is after a flood stroke only part of the ink is used for the next print stroke, the remainder is used to flood the stencil. It's a more complicated stroke pattern but could improve results.
- After all prints are printed (this color), time to clean up! Remove ink remaining on the screen and squeegee to a separate container, don't immediate place in the unused ink supply. Inspect the used ink for contaminants, particularly dye leached from the emulsion. The squeegee portion is most likely to show this, not surprising given the friction of the squeegee against the emulsion surface. Some portion of the ink may have to be discarded, the good part will keep for weeks in a sealed container.
- Wash the screen with plain water or dilute detergent/degreaser. Protect the surface of the platen with polyethylene sheeting. Remove tape, etc. used for blocking out screen areas and wash to remove ink/pigment. Residual pigment may remain in the screen, particularly where the most highly concentrated colors were used—the residue will be removed in a later step.
- If screen has multiple stencils let the screen dry thoroughly, then prepare the next stencil (step <39>). In terms of color progression, as noted earlier, start with “full-strength” yellow, then M1 (1%), M2 (3%), M3 (9%), C1 (1%), C2 (3%), C3 (9%). (Exact ink concentration varies, running tests is advised.) The basic steps are the same for each color, that is, ink preparation, screen blockout, registration, print run, screen cleanup.
- When all colors have been printed for all stencils on the screen (and that may be only one stencil), the screen may be “recovered”, that is, cured emulsion removed and the screen prepped for recoating. Make sure all ink is removed from the screen, in any case wet down the screen. Apply stencil remover liquid (which can ususally be conveniently diluted and sprayed), let stand for a minute or two then wash out with the pressure washer, this time at close range. It's important that the stencil remover not dry in the screen because it will become difficult or impossible to remove.
Important! Pressure washers are loud enough to damage hearing, and splashback of emulsion and haze removers is hazardous to eyes. Please make sure to wear hearing and eye protection during screen cleanup.
- Apply ink/haze remover. There are many brands, just follow directions on the container. After several minutes rinse off with water at low pressure, then remove the residual with the pressure washer. With most products a separate degreaser step isn't necessary. Dry the screen in a dust-free airflow.
- Between colors prints need at least a few hours to dry. Overnight is better. When all 7 colors have been printed, carefully remove the registration tabs, peeling tape off slowly and evenly. If necessary, apply gentle heat to help with tape removal. (With good quality tape there are seldom problems over short intervals.) The tabs can be reused dozens of times.
- An issue with water-based inks is buckling of paper, though this varies quite a bit depending on individual paper characteristics. Paper can be flattened. Very slight dampening and moderate pressure will work. Dry the print surrounded by absorbant paper or fine weave fabric. Under no circumstances allow printed side to be in contact with glossy surfaces. Doing so may have a “ferrotyping” effect which is difficult or impossible to undo.
- Final points. On inspection, prints may show minor defects such as pinpoint voids and the like. IMO it's completely acceptable to touch up small areas if the artist wants to take the time. Use high-quality materials for touchups (for example, pro-grade water color) to match colors. And after all steps have been accomplished, assuring the work is properly stored and displayed is definitely in the artist's best interests.
So that's screenprinting in a nutshell. Not too bad, just 50 short steps, though some steps are compounds of several substeps, and some steps are repeated several times. Then again, making great art has always been an expression of devoted energy and effort. To quote a famous philosopher, “all things excellent are as difficult as they are rare”.
**Download (CLUTs, Scribus templates, 28.8Mb): Screenprint steps files (zip)