Saturday, February 16, 2013

Screen Printing The Warhol Way

I want to make silk screens like Andy Warhol in The Factory. I've created graphics that I want to wear and screen printing offers a wide range of possibilities to print fine line drawings, lettering and photographic half-tone positives. So I searched for direction and got it from Mr. Warhol himself:
In August '62 I started doing silkscreens. I wanted something stronger that gave more of an assembly line effect. With silkscreening you pick a photograph, blow it up, transfer it in glue onto silk, and then roll ink across it so the ink goes through the silk but not through the glue. That way you get the same image, slightly different each time. It was all so simple quick and chancy. I was thrilled with it. When Marilyn Monroe happened to die that month, I got the idea to make screens of her beautiful face the first Marilyns.
From that description I surmised that I would need to forge an image into glue applied to a screen. After laying the forged screen on paper or fabric, ink would be pushed through the forged image only, connecting with the paper or fabric below it. There are other options - the fastest, least expensive, and simplest being to use (freezer) paper stencils - but I want the Warhol way so I researched and came up with this procedure.

Preparing a UV-Free Space

A number of steps in the screen printing process involves work that needs to be done in a room where no ultraviolet light is allowed to enter. This UV-free space is where you'll work with chemical emulsion which is sensitive to UV rays. There needs to be some square footage available as you will be emulsifying your screens, exposing your artwork to the emulsified screens, and storing emulsified screens for future use. You will also need space for an exposure unit and screen drying rack. Ideally, this space might be near a washout area and garden hose (with a high pressure setting) to wash out the unexposed emulsifier or reclaim/reuse/clean your screens.

This UV-free space can be created in a small room or shack. First use Rubylith Masking Film to cover up all areas where natural UV rays from the sun or other light source might be able to enter the room. This includes windows or cracks between paneling. Now, swap out all the bulbs in your space to bug lights - yellow incandescent bulbs that can be bought in any hardware store. Lastly, you can hang a black cloth that, similar to a shower curtain, can pull across a closed door if light enters through the door jamb.

Assembling an Exposure Unit

I couldn't explain it any better than Bob Mongiello. This unit must fit in your UV-free space.

These are my preparatory notes from Bob's video. Thanks, Bob!
  • 500 Watt Halogen Work Light ($9) should come with small stand, on/off switch, remove the glass from Halogen light because the glass might have a UV filter in it and you don't want a UV filter
  • Stand - [a] 3/4 inch 24 x 20 plywood for base [b] cut a 2 x 4 to attach, this will hold the light [c] attach 2 x 4 with screws to base [d] attach 20 inch support 2 x 4 from base to stand
  • Use another 20 inch 2 x 4 with a screwed in piece of plywood to hold stand of the Halogen light, attach this to the top of stand, plywood is extra long to allow for adjustments with screen size
  • Attach shelf to hold the screen while you are burning it - shelf is 30 x 24 and must be painted black so no light is reflected back
  • Distance between the light and screen (laying on shelf) should be 12 inches
  • Brace shelf with wood support as you braced the stand
  • Don't touch the Halogen light bulb with your hands
  • Get a 15 x 17 inch piece of commercial glass (with no UV filter) to hold the film positive to the screen while exposing - 1/4 inch thick, edges sanded with sandpaper - no picture frame glass

Getting a Screen Drying Rack

Get or build a drying rack to dry screens after prepping them for emulsion or coating with emulsion. You can also build something on the wall of your UV-free space as this unit must fit in your UV-free space.

Creating the Artwork

The artwork can be created in a vector or bitmap application like Illustrator and Photoshop, respectively. I'll be working with simple one color vector or bitmap images. Artwork with multiple colors needs to be separated - each CMYK color is pulled out from the original image and output in black separately. For printing, the separate screen positives are burned to individual screens. The screens must then be lined up exactly for the complete image to emerge correctly. The base of the press should have pins or an edge that helps to line up each screen; registration marks on the image output is also helpful. (It might be better to use the single station table top press with four screen clamps when printing color separations on one piece of paper or fabric.)

Outputting the Screen Positive

After your artwork is created and/or separated, you will need to convert it to all black and produce what are known as screen positives. A screen positive is any opaque image of (usually) black ink on any transparent or translucent surface (acetate, vellum or transparency). (If your art has more than one color, you will need a film positive for each different color.)

You can use most standard printers to output a screen positive, or outsource the work to a copy shop like Kinko's. (Are Kinko's still around?) You may also use a copy machine but in order to satisfactorily produce a screen positive using a copy machine, the black and white line work must be opaque and a photographic print must have high contrast.

Emulsifying the Screen

This step requires screen making chemicals and a water source / garden hose. Emulsify the screen in the UV-free space.

Screens used in screen printing consist of a mesh stretched tightly over an aluminum or wood frame. You need a screen onto which a stencil of the artwork can be burned. Originally, the mesh was made of silk and the process was referred to as silkscreening. The fabric of choice for screens now is polyester. (Silk is not good if you want to reclaim the screen. Nylon is non-porous and will stretch with water-based inks.) Polyester mesh comes in various sizes from 6XX to 20XX; the higher the number, the more threads per inch, the tighter the mesh. The tightest meshes are reserved for printing with thin inks while the lower numbered meshes work with specialty inks such as fabric inks with glitter or other additives.

Silk screen frames can be homemade, with the fabric stretched and stapled by hand, or they can be purchased (in bulk) from suppliers. The mesh should be at least 3 1/2 inches larger on all sides than the intended artwork. For example, if the screen is 16 x 20 than the artwork should be no larger than 8x12. The smaller the image compared to available mesh (the mesh INSIDE the frame), the better registration from screen to screen when printing multiple colors. If you're just doing one color/screen, you might push it to 10x14 if your press has decent alignment between the platen and the outer edges of the mesh.

Once you have a screen (whether new or reclaimed), prepare the screen by cleaning it of any grease or debris before coating it with emulsion. Spray the screen with a degreaser and scrub with a brush used solely for this purpose. (Each brush used with a chemical should always be used only with that chemical.)

The emulsion is the light sensitive glue that will be coated onto the mesh and left to dry. (Water based inks require a water safe emulsion; graphic inks for printing to wood, canvas, banners and the like require a graphic emulsion.) Apply one of the following types of emulsion using a scoop coater.
  • Presensitized emulsion is premixed, lasts about a year, exposure time must be exact or the emulsion will not harden correctly
  • Unsensitized (dual care) emulsion requires mixing with sensitizer, lasts about three months, can under or over expose and still good, recommended for beginners
Once dry, the screen positive will be exposed on the emulsified screen to make a stencil. Used screens can be reclaimed - that is, the hard and stenciled emulsion can be removed for use with different artwork - with emulsion remover. Apply the remover and use a second brush to remove the emulsion.

Properly cleaned, a stenciled screen can be reused hundreds of times. A third soft brush can be used to clean the screen without dislodging the stencil. Dehazers and stain removers (like Liquid Renuit) help to remove old ghost images off stenciled screens (or screens being reclaimed). Rub it on, let it sit, wash it off. Be sure not to let the dehazer near the emulsifier as some chemicals tend to interact with the emulsion. Dehazing gel is available to avoid this. Pinhole block out is also available to fix small tears. Screen wash also takes ink off the screen. Put it on a paper towel and use it to spot clean your screen. Depending on the type of ink, mineral spirits or other solvents may be needed to clear the screen.

SCREEN CLEAN-UP - An organized work area will make clean up easy. Here are the recommended procedures: o WATER SOLUBLE INKS Use warm water and a soft brush. These inks will remain water-soluble even after thorough drying. o FABRIC and ACRYLIC INKS Wash IMMEDIATELY after the last print is pulled. Use warm water and a soft brush. o Should ink dry in the screen; spray with Speed Clean by Speedball® or a house hold window cleaner. Rub with a lint-free cloth. When ink is removed, was with a mix of warm water and dishwasher detergent using a soft brush. Rinse with warm water. o DRAWING FLUID Wash with cool water.

Exposing the Screen Positive on the Emulsified Screen

A screen positive transparency is taped to a tightly meshed screen which has been coated and dried with emulsion. When exposed to light emitting UV rays, this emulsion turns hard. The black ink on the transparency prevents light from hardening the emulsion where the black ink covers it. To expose the screen positive, use the exposure unit in the UV-free space.

Place the screen positive on the emulsified screen. Be sure the graphic has the correct placement / orientation and affix the screen positive to the emulsion coated screen with clear Scotch tape. Place a 15 x 17 inch piece of commercial glass on top of the screen positive to hold it in place. (Always make sure the exposure glass is clean.) The hardening time of the exposed emulsion varies based on the type of emulsion and light source intensity so we'll generalize at 3-10 minutes.

After exposure, take the screen to a wash tub where the unhardened portions of the emulsion (what was under the ink during exposure) can be blown out with a high pressure water hose. Remember you still can't expose the unhardened portions of the screen to UV rays. Apply a forceful spray of water (body temperature) to both sides of the screen. DO NOT USE HOT WATER. Concentrate this spray on the top side of the screen. After a few minutes, these areas will open. Continue spraying until all unwanted emulsion is gone. Once you have completely washed the screen, let it dry thoroughly in a level flat position. Hold the dry frame to the light and check for pin holes. (These can be covered.) There is now a stencil on the screen that can be used to print.

Printing with Your Screen

For this process, I figured I would need a rather large table with clamps to hold the paper or fabric taut, or one of the following table top presses with one screen clamp for one screen.

Each one of these manual presses has a base which provides a firm surface on which to push when applying ink through your screen. (Spray tack can be used to keep a tee shirt securely fixed to the base while printing.) The base might have pins or an edge that helps to line up when printing with multiple screens. A single station table top press with four screen clamps for color separations is also available.

You can use Plastisol inks or water-based inks. If you are using Plastisol (recommended for longer lasting graphics), you will need some sort of dryer either a flash unit with heat control box or a conveyor dryer to either flash cure between colors or cure one color prints. Some have had success with a heat gun to flash dry the print and a heat press to cure the ink. If you use water-based you can probably get by with a hand held heat gun with a one station four color press although it is definitely not the most effective way to dry but an option.

The type of ink used in screen printing depends on the type of printed object. The opacity of the ink is also an important consideration. Simple, one color designs may require opaque ink to really stand out. But it's just as important for other inks to be translucent. Just four colors--cyan, yellow, magenta and black--when properly layered, blend to create every color of the rainbow.

Squeegees push the ink through your stencil on the mesh and onto the shirt or paper. The squeegee should be as wide as the interior of the frame. The squeegee pushes the ink, it doesn't spread the ink. This means that the squeegee should be held at an almost 90° angle to the screen.

Important notes for printing with fabric inks:
  1. Usually two (2) people should work on the printing process – one holding the screen frame tightly against the fabric, and the other doing the printing.
  2. On articles like T-shirts, a piece of foamcore, cardboard or paper must be put inside each garment to act as a barrier.
  3. To improve the lubricity (slipperiness) of the ink, you may add the Transparent Base. To slow drying or to prevent screen clogging, add the Retarder Base (1-2 tbs. to 8 oz. of ink).
  4. Wash-up of screens and tools must be done immediately after use. If they are allowed to dry on your screen or tools, they are difficult or impossible to remove.
  5. After the fabric ink dries on the fabric, set a household iron at the highest dry heat (no steam) that will not scorch the fabric and with a cloth or paper between the iron and printed material, iron on each side for 3 – 5 minutes. This will make the ink withstand repeated washings.
  6. Use only fabrics that can be subjected to temperatures of at least 275 - 375° F. Do not use on non-porous fabrics such as nylon.
  7. Pre-test all fabrics. Fabrics with sizing must be washed prior to printing to assure proper adhesion of the fabric ink to the fabric.
  8. If inadequate wash fastness occurs, the print needs to be heat set longer.
  9. Put screen wash on a paper towel and use it to take ink off your screen.

Curing Your Print

The information in this section is interpolated
from this excellent article on curing.

One thing you can NOT do is print a tee shirt, let it dry and voila, you have a screened tee shirt! The Plastisol ink must reach a temperature of 320° to be considered fully cured. A print that might only be dry to the touch (or gelled) and is not fully cured will not stand up to a washing. The ink will come off during the washing cycle because the ink was only gelled and not thoroughly cured. In other words, the surface of the ink may seem dry but the ink between the surface and the shirt itself (the entirety of the ink deposit) may not be. The process of curing your ink is basically deep drying it using a heat source that is at least 320° for about 20-30 seconds.

People use all kinds of things to cure. One option would be to use the halogen light constructed for the screen exposing step. Another might be to affix a room heater so it is horizontal to the screen and turn it on. How long you need to heat the screen is something to learn based on the room heater being used. The latter seem somewhat precarious and is not recommended by me. Other options (blow dryer, heat gun, oven) are discussed in the article previously referenced and this video.

A flash cure unit is intended for flash curing (getting only the surface layer of the ink up to 320°) before applying a second print stroke to achieve opacity. With the right ink, you can flash cure a print in 7-10 seconds. To completely cure a print with a flash cure unit, you will need to expose it for 35-45 seconds. If you are going to use a flash cure unit to do the final cure of your items, you should invest (about $80) in a non-contact temp gun with laser pointer. If you're printing with Plastisol, most Plastisol cures at about 320°F. (Clothes come out of the dryer at about 500°F. Overcuring will result in a print that will crack and fade prematurely.) Be sure the Plastisol reaches the cure temp all the way through to be fully cured. Also, you should use something other than your printing platen to place the item upon that you wish to fully cure because the platens are not designed to handle that much heat constantly. When flashing Plastisol (as in a print-flash-print operation), you only want to "gel" the surface of the Plastisol which will usually occur at about 270°F. Much lower than the temps you'll see of about 350-400°F needed to insure the Plastisol has completely reached the necessary fully-cured stage.

Some things to note:
  • Some cure the print on the outside first and then turn it inside out to heat the inside of the shirt and ensure the Plastisol is fully cured.
  • Infrared, quartz and hallogen flash units are available. Quartz is choice since the bulb technology delivers a much more uniform heat across the print.
  • When a flash cure unit is operating, it reaches a temperature of around 600°.
  • Always let your flash cure unit warm up prior to beginning production. Even if it has a temperature read-out, it is still a good idea to let the unit warm up. A 15-20 minute warm-up is usually sufficient. However, if in doubt, it's not a bad idea to check the temperature with a temperature heat gun. (See below.)
  • Some flash cure units do not come with an on/off light.
  • Never touch the heating element on the unit to see if it's on.
  • Never leave the unit hovering over anything when it is on.
  • Keep a record of dryer settings, temperature settings, notes about inks, and other important details that are specific to your printing equipment and capabilities.
  • A flash will cure water-based ink fine. Just pull the print away from the platten as you want the water to evaporate away. Set the flash a bit further away than you do with Plastisol and flash. You should get steam as the heat dries the water out. Water-based inks take a much longer time to dry/cure than Plastisol. Generally about 2 minutes.

As previously indicated, the ink deposit on the shirt must reach a temperature of 320° to be completely cured. After a warm up, adjust the flash cure unit so that it hovers 2 to 2¾ inches above where the shirt is laid out. There are several methods that can be used to help with the determination of whether or not a print has been fully cured.
  • The fabric type and color affect the curing time. Dark colored shirts will cure faster than white and light colored shirts. White, cotton shirts will take longer to cure than black (or other dark) 50/50 blend shirts.
  • Temperature tapes are little strips of specially made paper printed with temperature readings that will indicate the temperature as a garment is being cured. These paper thermometers were the standard way to test temperature for years and are still used. Although they are only accurate within 15 degrees (if you are lucky), they are the least expensive and close enough item we have. If you can get one to turn black at around 330 degrees, then you know you are probably running AT LEAST 300 degrees based on the heater height and temperature. Temperature strips are inexpensive and available at most screen printing supply vendors; you can cut them in half so the cost per unit is even less.
  • A non contact pyrometer is a useful device for checking the temperature of the print when being flashed. Simply point the gun at the ink when the garment leaves the dryer and it will read the surface temperature of the ink. While the pyrometer does not give the final word that a shirt is completely cured, it is useful for measuring temperature and providing a reasonable assurance that the shirts are curing properly.
  • Stretching the print on the shirt to see if it cracks has a limited value for checking for curing of the print. The idea behind this test is if the print cracks during stretching it is most likely under cured. It is suggested using this test for spot checking only.
  • The only guaranteed way to determine if your prints are curing properly is to do a wash test. It is a good habit to periodically wash a test shirt to see if your dryer is working as expected. What can you tell from the wash test? If the print was only partially cured, some or most of the print would have washed off. If the print was severely undercured, the print will most likely be completely gone.

Choosing A Flash Cure Dryer is an excellent article with technical tips. A clothing iron would do the trick as well but, you will have to press on the design for 4 - 5 minutes, until there is a lot of steam. Cover the print with a tea cloth before ironing.

Checking Up

In addition to the tools mentioned, you might benefit from the following.
  • Ink solvents to clean up ink and clean screens or tools used to handle inks
  • Chemical resistant spray bottles for screen making chemicals
  • Ink scoops or spatulas to get ink form the bucket to the screen and back again.
  • Ruler and t square for marking pallets with a Sharpie and aligning film on screens.
  • Masking tape or screen tape to block off the sides of the screen or unwanted portions of stencil on the screen
  • Spot cleaning gun for removing unwanted spots of cured plastisol ink from printed shirts.
  • Inkjet or laser film for making your film positives from your artwork
  • Spot cleaning fluid for the spot cleaning gun
  • Scorch out to remove yellowing from white shirts that turn yellowish from excessive heat
Let me know how it goes.

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