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Vectors & Halftones
This is a very basic introduction to what vectorizing and half toning entail. If you are struggling with artwork and need some help , I do offer an Artwork Service with very reasonable rates.
Vectorizing Artwork
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When I started screen printing I cut all of my stencils by hand. This is incredibly rewarding when you spend hours cutting a design and finally see it printed. I used to use photostat machines to enlarge images and then stuck designs and text together before cutting them out. Many years later I started screen printing designs for customers and was starting to struggle to keep up with the workload, cutting designs was taking a very long time especially when there was lettering.
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A friend was making signage and suggested cutting my Rubylith in his vinyl cutter. At first I didn't because it would take a long time to vectorize the design for the vinyl cutter to cut. I did start cutting designs with a lot of text on the vinyl cutter and slowly learnt how to do other things like trace images. Learning to use a vector program also opens up a lot of design possibilites as it can be inspiring to see what tools are available.
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A vector is a line/path that the vinyl cutter's blade follows when it cuts. You take your design and draw all of the lines that you want cut. This is called vectorizing. Once a design is vectorized you can change the colour easily and change the size without any loss to detail. Text can simply be typed and then converted to paths. Cutting designs into Rubylith with a vinyl cutter is very fast and easy.
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A stencil of the following image could be cut quite easily and quickly by hand with a blade. If you were making a positive you would need smoother edges and you would need to make the design Rich Black - if you have a vector file then doing this process will be a lot easier.
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I use Coreldraw to make vectors - there are other programs like Adobe Illustrator that can be used. I would put the image into Coreldraw and trace around the edges. The path/curves are blue and the white blocks are nodes.
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If you are printing a positive on transparency paper you would need to make your vector the exact size that you want and then change the mode to CMYK and change the colour to be rich black (C-75/M-67/Y-68/K-90) and then export a PDF file for printing.
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If you are going to cut a stencil in Rubylith or vinyl with a vinyl cutter, you would size your design and then export the kind of vector file your cutting program uses.
Positives Artwork
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When using a positive to expose a screen you need to make sure your artwork is in the right format and is of a decent quality. Images with pixelated edges and grey areas will not give the best results. If your positive is not in Rich Black (C=75/M=67/Y=68/K=75) then it will not be as dark as you need it to be. During exposure your positive stops UV light from reaching the emulsion, if your positive is not very dark or has grey areas, UV light will start to harden the parts it can get to. If you use Rubylith as a positive, the red blocks UV light. A good dark positive will allow you to expose your screen for long enough to get a good result and avoid under exposure of your emulsion.
Ways to get better positives would be to change the artwork to be CMYK Rich Black, vectorize the artwork, or change the contrast and touch up areas and even Halftone images.
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Halftones
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Grey areas on a positive will let light through and this grey detail will be lost. Halftones are used when people want to print more photographic type images which have shading. These images can be halftoned in a program like Adobe Photoshop. When halftoning an image, you would first change it to greyscale mode and then convert it into a bitmap.
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The amount of detail would be determined by the mesh count that is being used. There are a few different methods for working out the best Frequency and Angle settings based on the mesh count. One quick way is to take your mesh count in US measurements (Imperial) and divide by 4. For example if you were printing with a 43T mesh (43 threads per centimetre), the US count is 110 (lines per inch) and you divide 110 / 4 = 27.5 so I would round off and use 30 for my Frequency and 30 for my Angle. The process is a lot more involved than this but for beginning this is a good start. The more common choices for shape are Round, Ellipse and Diamond.
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For more about mesh counts and conversions have a look at my Mesh Count Blog post.
Example of a half toned image exposed on a screen (left) , half toned postive (right)
Vinyl Cutter with Rubylith loaded for cutting
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