Producing art for silkscreen printing. 

by Michael Robin Cooke

Silkscreen printing is very popular, especially for t - shirts. If you want to create or convert a design to be printed by silkscreen, here's how. 

Basics: 

First, understand that silkscreens are usually made with a photo-emulsion technique, an emulsion fluid covers the screen, a transparency with the art in black is placed on the emulsion – and when exposed to light – the dark area of the design burns the emulsion so that it may be washed away. This way there is a physical screen, with your 1 color design – that can be printed by pushing ink through the screen with a squeegee – on to paper or fabric. 

 If your design has more than one color, you will need a different screen for each color.  

This is why most silkscreen designs are bold flat color designs. We'll talk about how to work with photos later, but understand four color process screen printing is difficult enough many printers will not attempt it (instead there are 'virtual' process techniques). 

We live in age of computers, and it is much easier to create art for screen printing digitally. But if you're working with kids, or for some reason prefer to do it by hand – I'll start us off talking about creating 'separations' (the black images corresponding to each color) by hand. You would take the paper  artwork to a copy shop (like Kinkos) – they can photocopy your art on transparency film for you. My first screen separations were created by hand, it was very educational.

 The drawing you work with should have flat color, like a cartoon. Look at the colors. You want to have to print as few colors as possible, so think about which colors are really necessary – and understand you will replace less necessary colors with the important ones. Your design should usually be 5 or fewer colors. If you're printing on a color shirt – you can use that color too, in your design. 

 Registration marks – often a cross inside a circle – Are used to help more than one print color print in registration with the other colors. Registration marks can be bought as a kind of tape or film from a graphic art store, or you can draw your own. We'll use tracing paper to make the plates, trace or stick the registration marks on all overlays (perfectly over, in registration,  with the previous registration mark) so you can align them and the prints later.

 If you need to recolor the art – trace it and recolor it with your limited colors, use a good quality tracing paper, such as a heavier weight 'vellum' from an art supply store or website. 

On each paper placed over the art or recolored art – trace every instance of the color the plate is for, and fill it with black. You can start with a pencil, when the outline is right, fill it in with a sharpie marker or other opaque pen. 

 Spreading:

 This is when one plate slightly overlays other plates, to close gaps- to lessen the possibility of mis-registration – so the shirt doesn't show through. If you are doing your own printing, this is important – your print registration may not be very tight. If you are using a professional screen printer with multicolor screen printing hardware – their registration may be so tight it may not be necessary to spread at all. But it's never a bad thing to spread your color plates at least a little. A dark color should spread over the lighter color. If you are printing – it's better to print the dark colors after and over the lighter ones. 

 As a rule of thumb, if you are working with a cartoon with a black outline – simply spreading the black lines, making them thicker – will properly spread and troubleshoot registration globally. Just double check and make sure the black outline is closed, has no gaps or breaks in it.

 Choking:  

Choking is the opposite of spreading. If your design is printing on a black or color shirt – you may need to back the entire design, at least the parts that are not already black – with white. If the color inks have some transparency, backing with white may be necessary. But you do not want the white color to show if there is a slight mis-registration, so you would 'choke' the backing – make a gap between the edge of the backing and the edge of the color. 

 Making or getting your screen:  

 You'll want the designs on a transparency – Kinkos or another copy shop should be able to do this for you, if they carry transparency film as a paper option. If you don't want to create your own photo emulsion silkscreen – you can pay any screen printer to do it for you. Look for local resources. In NYC, Pearl Paint will take a white paper photocopy and make a usable screen for you inexpensively. 

 Printing: 

The printing process is simple, put ink in screen and push it through the screen onto whatever the screen is placed on with a squeegee.  

If you're doing your own silkscreen printing, I hope you have a hinge system if you are doing multi-color. A hinge system holds a few screens so you can print as many colors as you have screens. Fidget with the printing screens and how they are positioned until you get everything in as good registration as possible (using your crop marks and a test paper or shirt). 

 Digital Design: 

 Okay, you're probably working on a computer. Why am I explaining how to create separations as if we lived in the 1970's? Because I don't know what application you're using, and the theory is universal. 

Computer printers can print film, and there's is 'RIP' software that will create separations for you – that you may use especially if you find yourself working in a professional context. The most popular software programs include Adobe Illustrator, Adobe Photoshop and Coreldraw.

 Vector drawing tools, like Illustrator and Coreldraw are very good for screen printing. But if you know Photoshop and understood this article so far, you understand Photoshop can be used too. Even if you are using Coreldraw or Illustrator, there are some things Photoshop does that those programs cannot. 

 If you are working with a RIP software, it is important that every color used is specified as a SPOT color in the color picker. A spot color represents a single color ink as opposed to a process color. Iin Illustrator's color picker, you can define any given color as a spot color. It may not automatically make all objects the original color spot color though, be prepared to recolor all those objects with your new spot color (the 'select same fill color' option is handy here). The rip system, unless it is ripping CYMK process color separations – may not print any color not specified as a spot color. 

 Your registration marks should be specified as “registration” color – to print on all the color plates. 

Spreading and choking digitally: 

 In a vector application, you would stroke an object with a fine line the same spot color as the fill to spread it.  

You may need to create separate layers for each color, and print/rip them individually.  

 Ignore all this if you're working with a printer with registration tight enough spreading is not necessary. 

 To make a white plate, select every non black color and fill with the spot color specified for “white”. To choke, stroke the object(s) with a non printing color. 

 When I did a lot of separations in Illustrator, I'd have each color element by itself on a separate layer spread or choked, the layers ordered in print order – so that when all layers are visible I'd have a digital preview of the print with the spreads in place. 

 There are no strokes in Photoshop but you can do a lot with the selection tools, expanding and contracting selections of colors will do the trick. When expanding a selection, fill with the spot color (or black, if your program doesn't support spot color) and this will make your spread. Contracting the selection and then selecting the inverse and deleting will choke your plate very well, but will delete all other elements – so make sure the late is on a layer by itself and other elements are safe on a separate layer. 

 Digital Halftones and Gradients for silkscreen prints: 

 Photos and half tones. You CAN print a photo, even if you're printing just one color! But you must make a half tone version of the image first.

 The thing to keep in mind about half tones, is that the image is being broken down into dots, and a dot that is too small, may not stay affixed to a tee shirt – may not even print properly if the silk screen itself is too dense. 

 Half tones are single color bitmaps in Photoshop, they can be treated as vector objects in Illustrator and recolored with a spot color. What there is to remember about a bit map is that it really is one channel single color – no anti-aliasing, every curve, every slanted line will  feature blocky 'steps'. So if you want a half tone 'dot' to actually look like it could be round, the steps – or pixels – need to be very small indeed.

 In Photoshop, to make a halftone bitmap, first convert your photo to gray scale format. Then convert your image to bitmap format – when doing so you will have the option to choose 'halftone'.  

 You will have many halftone options to choose from, all are useful. An ellipse at 45 degrees is a good default. The frequency should be something the silkscreen can handle, 35-40 dots per inch works with many screen meshes. A lower dot per inch will make the individual dots more apparent, this can be exploited for stylistic flourish. Dots per inch of the halftone are not pixels per inch, digitally the pixels are what the 'dot' is composed of. 

 The 'output' resolution, this refers to pixels per inch. Pixels are the digital squares making your dot that you want to make small so the printed dots can look round. Start with 800 pixels per inch and be prepared to go higher. That seems very high, but because the image is in bitmap mode it will not make a very big file size. 

 With halftones and silkscreen there is no one right way. If the dot is small, it can be imperceptible enough to suggest smooth grayscale art. If the dot is bigger it is more perceptible, but also more permanent, durable - and a perceptible screen dot is now a popular design feature. 

 When working in Photoshop, the history control panel will allow you to go back to a grayscale state and try converting it to a halftone screen again. If this is your first time, expect some trial and error. 

 Making 3 or more colors with 2 ink colors: You can mix colors optically. If you have a red color and a yellow color – printing a 30% screen of red over a backing of yellow will result in an orange color optically. If the shirt has a color you can use this too and mix it into new colors with halftone overlays of color ink.

 Printing Gradients. If you have a rip system, a gradient between two spot colors will be separated automatically in the rip software. Just make sure all colors in a gradient are spot colors (unless you intend a color to be non printing). The Rip system should be set up to deliver a coarse enough halftone to print easily.

 If you're doing this the hard way, say in Photoshop with no rip system, you'll have to construct halftone screens for your gradient. Do each color separately, copy and paste into new document, recreate the gradient as black into white – and generate a half tone accordingly.  

 Why Process color is so difficult for silkscreen. 

 You may have to see it for yourself. Newspapers are printed cheaply, find one with a full color cover and look at the color photos closely with a magnifying glass. 

Real printed process color means printing four small dots of color in a very tight pattern, or 'rosette', all over the image. Look at the magnification of the printed color photo – you should be able to see a 'rosette' of the four ink colors printed repeatedly. The ink dots need to be very small, the 'full color' effect depends on the dots being imperceptible to the naked eye. The very small dots need to be in perfect registration with each other or the image will look very wrong. There is almost always 'gain', meaning certain colors print stronger than they should – usually the black, often the cyan and magenta as well. The artwork needs to be adjusted for process color gain, an advanced production artist skill.

 Silkscreen printers are challenged by the very tight registration necessary for process color, and a very small dot of ink may not stick well to a shirt. The challenge of color correcting images for process color printing is a headache unique to that silkscreen process.  

 “Virtual Process” and Advanced Techniques: 

 Virtual Process Color, is essentially attempting to emulate process color without having to print in process color. There is no one technique first of all. When a designer tries to make a color photo printable close to 'photo real' using any technique other than authentic process, it is virtual process. 

 One way of creating a virtual process file is to use Photoshop's 'multi-channel' mode. Start with a CMYK version of your photo, copy each channel into a new document – and convert to a bitmap with a 'diffusion dither'. Copy these bitmaps into your multi channel file, a new channel for each color channel. Specify the preview color of each channel to be the color of the ink you want to use. When you have the channels all added and turned on, you'll see a preview of what the channels together look like. It won't look as good as actual process color – but the colors will blend optically, and it's something that can be printed without precise registration. 

 There are other ways too, Some people with artistic talent will 'draw' in a multichannel Photoshop file with a diffusion dithered brush setting, in channels for specific colors. As long as each channel is previewing the intended ink color, you can roughly preview what the end result will look like. 

 Other 'virtual process' technique exist, and many designers create their own, Photoshop presents almost limitless possibilities for this sort of thing. Do be mindful of mixing patterned halftone dots of different colors, this can yield a 'moire' – which is a pattern that occurs when two halftone screens overlay. 

 Adobe Illustrator's 'Live Trace' tool (IllustratorCS2, CS3 only) will convert a photo into vector art. If you customize the settings, you can limit the number of colors and export the colors to the color swatches dialog box - where you can specify them as spot colors and make a print ready file. The effect looks 'posterized' rather than photo-real. This is an easy, effective and quick way to convert a color photo into a good looking color silkscreen print.  

 Illustrator has a 'gradient mesh' tool.  It's not easy to work with, but it is possible to almost paint photo-realistically with the tool. If you are an artist and can create with this tool, it may be possible for you to recreate a photo with the tool, especially if you have the photos placed in your Illustrator file for reference. I believe if you use spot colors exclusively with this gradient mesh too – a rip system will separate the image into screen printable films. This would prove labor intensive even for an expert however.

 Conclusion: