Screen Printing 101

What is screen printing?

This information is provided to educate customers on the processes of screen printing and also to be a resource to anyone interested in trying screen printing themselves.

The technical term for screen printing is serigraphy. “Silk screening” is an antiquated term and not accurate. Silk has not been used in screens for 60 years. Polyester has been used since the 1940s and is by far the most common type of screen mesh. Some screens are stretched with nylon mesh, but that is not preferable as it doesn’t hold tension as well.

The process of screen printing starts with artwork, which is then outputted onto a film positive to be “burned” into the emulsion-coated screen. One needs digital artwork to work with and output to films, so it’s best to receive image files electronically from clients and customers. Designs submitted on paper should be scanned and converted to a digital format. Generally, printers use various image editing software, with Corel Draw, Adobe Illustrator and Adobe Photoshop being popular. Designs have to be converted to an ebon dark black before it’s outputted, no matter what color ink will be used. Multiple color designs need to be separated by color and then have each color made flat black, as a film will be made for each color. Printers add crosshairs to the corners of multiple color designs so that he or she can align the screens properly to have proper registration.

Extremely fine lines may not wash out of the screen well in the stencil making process. The higher the mesh count of the screen, then the higher amount of detail that it can hold, given that it is burned properly. Artwork submitted to Vacord should not have lines finer than 2 point.

Film positives are basically opaque sheets, similar to transparency sheets, that the designs are printed onto using a laser printer. The design needs to be as dark as possible, so that it blocks UV light from the exposure unit to the areas behind the design. There are products available that one can spray onto a film that makes the toner/ink on the film a lot darker, not noticeably to the printer, but darker so that it blocks light better. UV light, not visible light, hardens the emulsion wherever it hits. Screens are burned or “shot” for a certain amount of time depending on the screen and the exposure unit, which hardens the exposed emulsion. After burning, the printer will take the screen and wash it thoroughly, usually with a spray hose, which removes the emulsion that was behind the design only. After properly washing out the screen, the printer is left with a stencil in the screen. A good screen is essential to the whole process. It does not matter how well the printer can pull a squeegee, if he or she didn’t make a good screen, then the prints will suffer.

Before you can burn a screen it has to be coated in emulsion and dried. Professionally, emulsion is applied to the screen, at least one layer on both sides, using a scoop coater. Once the screen is coated, it needs to be kept in darkness, as any light could start exposing the screen. All moisture needs to be removed from the emulsion, and this can be done with a fan (potentially a bad idea because dust and dirt can be blown into the emulsion), a heater (slow but effective), or a dehumidifier (the best choice and usually the quickest method). If someone is new to the craft of screen printing and does not have a scoop coater, emulsion can be applied in other methods, which may be adequate but won’t have as good of results. A squeegee with rounder edges can be used, just put emulsion onto the screen using a spoon (after a few times doing this, the printer will know how many spoonfuls are needed, roughly), and then spread it evenly with the squeegee. One can even use a folded piece of cardboard instead of a squeegee if limited on supplies and/or funds.

Screens come in different sizes and mesh counts. Common sizes are 18″x20″, 20″x24″, and 24″x31″, but there are companies willing to make screens to any size needed. What does mesh count mean? The mesh is the fabric stretched over the screen’s frame, and is a weave that has tiny holes in it, which allows the emulsion to harden and adhere to it, and also allows ink to pass through the mesh that is open, IE the stencil. A mesh count is the amount of holes in one linear inch of the screen. Screen meshes range from 86 to 320+. The lower the number, the larger the holes, which matters and determines how much detail the stencil can have, as well as the amount of ink that can flow through the design. 86 mesh is used for glitter and other special inks with a high solid content, as they are less likely to clog because of the large holes. 110 is a common mesh and is good for printing white ink. 156 can hold more detail and is another good general mesh to be using. 230 mesh can be used for process printing (printing full color using CMYK or other methods of color separation), or anytime that fine detail is needed. Anything over 260 would be good for printing onto paper, as it is not necessary to lay down large amounts of ink onto the paper. Beginning printers often use Speedball brand screens, which are available in many art stores and come in the beginner kits. The mesh of these screens is around 120. A lot can be done with a 110 mesh if it is properly burned.

The mesh count determines the amount of ink laid down, so it is best to use a higher mesh when doing something easy like black ink on white shirts. A higher mesh also helps prevent bleeding problems with thinner inks. Lower mesh counts should be used with inks like white, gold, silver etc, as one wants to avoid clogging and lay down more of these inks since they are often printed on dark apparel.

There are two basic categories of ink, plastisol and water-based. Plastisol is a plastic based ink that most large scale printers use. It does not air dry, it has to be cured with a conveyor belt oven, which means that the ink does not dry in the screens (this is probably the biggest complaint about water-based). A printer using plastisol could stop in the middle of a print run, go to lunch, and come back and the ink would still be the same, not having dried. To dry or cure, it has to reach around 300 degrees Fahrenheit, which bonds the ink to the shirt and solidifies it. Water-based is, as it sounds, water based ink. It is a better choice for beginning printers as it can be cured without a conveyor belt oven. If a new printer is really pressed for equipment, he or she can just use a very hot iron on the print for a couple of minutes to cure. A flash cure unit can also be used for curing. Some water-based inks, such as the Union Aerotex series, come with a catalyst which can make the ink cure just by sitting out. Whenever trying a new curing method,wash-fast tests should be done to make sure that the design does not fade in the washing machine or dryer. Water-based ink can go straight down the drain, while plastisol requires mineral spirits or an ink degradeant for it to be drain safe. The debate of which is better, water-based or plastisol, could go on for days and days between a group of printers. It is really up to the printer to figure out which works better for his or her needs. Water-based can have a softer hand (hand refers to how soft the ink feels on the shirt), but there are additives for plastisol which can also produce a soft hand. Some people will argue that water-based produces more vibrant colors. The debate may never stop.

Vacord uses only water-based ink currently, but we’re interested in plastisol in the future. There is a new line of ink made by Matsui in Japan, and it is amazing stuff. It’s basically a hybrid between water-based and plastisol, having the good qualities of both. We fell in love with the white ink as soon as we used it. The white stretches, doesn’t clog easily, doesn’t crack. White ink used to be the bane of our existence, but this white is a pleasure to work with. We could not recommend it more highly.

Squeegees are, of course, critical for spreading the ink over the stencil and pushing it through the open mesh onto the shirt. They come in many, many different sizes. Lawson Screen Supply even sells a six foot squeegee, assumingly for cutting down to needed sizes. The stiffness of the blade is referred to as “durometer” and a stiffer squeegee lays down less ink than a softer squeegee. Squeegee blades should be kept sharp. One should either replace squeegees as they dull or purchase a squeegee sharpener. A rounded blade will lay down more ink than a sharp one.

A beginner can start printing with nothing more than a screen with a designed burned into it, ink, squeegee, and some blank shirts. Vacord’s printmaster started on an ironing board with a small speedball kit. This method is fine for dark-on-light prints but could not work for light-on-dark as there is no way to register (line up) the screen with the first design in order to do the second layer. Art stores with screen printing goods will sell hinge clamps, which one can use to build a basic screen printing table. One clamps in the screen and that way the design will come down in the exact same space, allowing for a second layer, such as white ink on a black shirt. Someone interested in screen printing as a hobby could get a very basic set up going for just a couple hundred dollars.

To set up a professional-quality shop, one has to make a much bigger investment. A nice 4-color, 4-station press (allowing one to load four shirts at a time and to do a four color design) costs between $2000 to $5000. Everything on such a press rotates, allowing for quick production. Load four shirts, print one, spin it under a flash cure unit for six to twelve seconds as you print the next shirt, and so on. Flash cure units use infrared heat to dry the ink to the touch quickly so that it may receive another layer of the ink or a new ink color in a multiple color design. It is not the best way to cure prints, but a flash cure unit can be used to do the final cure of a shirt, laying the shirts under the unit one by one, letting it raise to 320 degrees Fahrenheit for a couple of seconds for water-based ink. The main problem with using such a unit for a final cure is that the unit can have hot spots and uneven heating, so that some identical shirts may take longer amounts of time to reach the desired temperature than others. Anyone seriously printing should invest in an InfraRed thermometer gun to monitor the temperatures of printed ink. The main annoyance with using a flash cure unit for a final cure is the fact that the printer has to stand at the unit placing every shirt under it for 20 to 60 seconds, one by one. This means that if you can print 50 shirts in an hour, then you will have between 20 minutes to an hour of standing there at the unit curing shirts, without being able to do much else, other than tasks that can be worked on for 30 seconds at a time.

The best and most professional way to cure shirts is with a conveyor belt oven, which is a gas or electric IR oven unit with a conveyor belt that goes through it, allowing the printer to finish a shirt, placing it on the belt, and it will pass through the unit, being properly cured in a hands-off method, then leaving the heat chamber and falling into a box or tub on the other side. Conveyor dryers can range from a six-foot belt to huge dryers forty feet long.

There are other ways to cure shirts that are not professional nor time-effecient, but they will do the job. If someone getting into screen printing has access to a thermal press, such as one used for doing plastisol transfer prints, that can be used to cure. Another way is to use a very hot clothing iron, on the highest setting, having laid a sheet of wax paper on the dry print, going slowly over and over the design. This works for water-based printing and can result in tolerable cures with some inks.

There are brands of water-based inks that claim to “air cure” and that just letting them sit out for a few days will make them wash fast. Other ink brands come with a catalyst that, when mixed into the ink before printing, results in a wash fast print without any heat related curing process. Be aware that adding in these catalysts can drastically reduce the shelf life of the inks.

There are countless set-ups that can be used for burning a screen. Beginner kits may suggest using a 150 watt bulb from a certain distance to expose the screen. This is not a bad way to start out, and may result in good burns with fast exposing emulsion. Any source of UV light will work. Many budget-minded printers build their own light boxes using florescent light strips, employing white blacklight tubes or bulbs designed to give out large amounts of UV light, such as “day glo” or bulbs designed for growing plants. High wattage metal halide bulbs can work too, as well as the Sun itself. Take your coated screen outside with the film positive taped on and hold it in the sunshine for a minute and it will expose. It does work but would be hard to maintain consistency.

After exposing a screen and washing it out, resulting in a proper stencil in the mesh, pat it with paper towels to remove excess moisture and set it aside to dry. If the emulsion looks or feels slimy after washing out, the exposure time was too short. If the emulsion comes off everywhere, not just behind the design on the film positive, but on areas that are supposed to harden and stay, then the burn time was too short. If the design won’t wash out, then the exposure time was possibly too long.

Once the printer has a good screen, it is time to make some shirts. Load the shirt(s) on the platen(s), a platen being the platform that shirts or other items lay on to be printed. Clamp the screen and pull it down to make sure it will come down to the shirt properly. There should be an off contact so that the screen doesn’t lay flat on the shirt, but instead has around 1/8th of an inch of space. If the printer does not have a proper press and is just using a screen, he or she can tape coins to the corners of the screen for the same effect. Put some ink at the top or bottom of the screen. Lift up the screen and spread the ink over the openings in the mesh. This is called a flood stroke and will fill up the stencil. Lay the screen back down and pull the squeegee over the stencil with adequate but not extreme force, holding the squeegee at a 70 degree angle. Do another pass if desired. Lift up the screen and see that the shirt has had a good layer of ink laid down evenly. Spin it under the flash cure unit, or use a heat gun to dry the print to the touch. Remove the shirt, load another, and repeat.

Once the print run is finished, the screen can be reclaimed, meaning that the emulsion and the stencil are removed. This can be done with a power sprayer or with chemical reclaimer. Often, the ink will have dyed the mesh leaving aghost image. These do not necessarily get in the way of printing when the screen is re-used and may be removed with a dehazer.

This ends the screen printing 101. If you have comments or questions please contact us.

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