Tuesday, 30 October 2012


screen printing definition:  Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate. A fill blade or squeegee is moved across the screen stencil, forcing or pumping ink into the mesh openings for transfer by capillary action during the squeegee stroke.

Screen printing is also a stencil method of print making in which a design is imposed on a screen of polyester or other fine mesh, with blank areas coated with an impermeable substance. Ink is forced into the mesh openings by the fill blade or squeegee and onto the printing surface during the squeegee stroke. It is also known as silkscreenserigraphy, and serigraph printing. A number of screens can be used to produce a multicoloured image.        

Screen printing (also known as silk screening) is one of the oldest methods of printmaking, with examples dating back to the Song Dynasty in China.  The process involves creating a stencil of an image on a screen of porous mesh, traditionally made of silk.  A roller or squeegee is used to pull paint-like ink over the stencil, forcing it through the mesh onto the paper being printed.  Unlike the inks used in some other forms of printing, screen printing ink sits right on the surface of paper, resulting in incredibly rich, vibrant colour.

The screen printing process has multiple steps, starting with the process of creating the screen.
The screens are coated with a light sensitive emulsion, and exposed using a positive image.  Your positive can be created in a variety of ways, from digitally printed film, hand-cut rubylith, or hand drawn with ink on acetate.  The positive is positioned directly on the surface of the light table, and the screen placed over the positive, print side down.  The emulsion hardens when exposed to light, and remains soft and water-soluble where the positive blocks the light.  After exposure, we take the screen to the wash-out sink, and rinse away the soft emulsion.  Once the screen has dried completely, we lock the screen into hinges that are mounted onto our print surface.
We align the paper for printing, and mark the location with registration tabs.  Ink is applied directly to one end of the screen in a long bead, ready to be pulled over the screen with the squeegee.
A nice, firm pass with the squeegee forces ink through the mesh, visibly showing on the print side of the screen.  The screen is lowered on the hinges, and the squeegee is used to press the inked mesh flat against the paper, transferring a thin, even layer of ink to the page.
The amount of ink transferred to the paper is controlled by the thickness of the emulsion, so crisp images need a fine, even coat of emulsion to maintain their detail.
Mixing the second colour for this particular job was a challenge; we wanted to create the illusion of a 3rd color in the print, so the second ink needed to be transparent and overlay the first color to create a pleasing effect.  We settled on a yellowish green that would create a darker green where it overlapped the blue.
Registration was tight!  The blue and yellow-green had to line up perfectly along the sides of the image.

Like most hand-printing methods, screen printing has a very distinctive look.  Even though the surface is flat, the velvety finish and extreme vibrancy of the ink cannot be replicated with any other technique.  Screen printing can also be used on a variety of surfaces, so anything that has a flat surface can be printed; paper, chip board, fabrics, wood, leather and metal are all viable candidates!

Like any other printing process, screen printing definitely has specific limitations, which makes it better suited for some projects (and not so well suited for others).  Fine details or delicate text can be lost or broken up in the printing process, and large blocks of text can be difficult to print consistently.  Light ink on dark paper works beautifully, but textured papers are out.  Thin papers also present difficulty, as the ink could cause them to buckle or warp.

Ever wonder how t-shirts are made that feature photographic quality design, in full color? This articles takes an in-depth look at the subject of cmyk printing and color separations, and their uses in screen printing. This casual approach to a difficult subject, should provide you everything you need to know to start producing full color artwork for fun and profit.
CMYK:  CMYK is a subtractive colour model used in colour printing.  This whole CMYK thing is based on the mixing of specific pigments in particular percentages to create a wide range of colors. Often times referred to as 4 color process. These specific pigments are as follows: 
  • C=cyan
  • M=magenta
  • Y=yellow
  • K=key (black)
Now, the reason this is considered a “subtractive color model” is because the higher percentage of cyan, magenta, and yellow I smear onto a white sheet of paper, the less amount of light that will reflect through, ultimately creating a black (in this case, smudge) on our paper.

Let’s start at the end and work back to the beginning. Mesh Count and thread diameter. Mesh count is the number of threads, per inch, that makes up your screen. Professional screen printers generally utilizes a screen with a mesh count of about 255 threads per inch and above, but typically mesh counts can range anywhere from 110 to 305. To make matters worst, there is also different thread diameters for screens. The higher your thread diameter in combination with your mesh count, the finer detail your artwork can contain, but also, the less ink that gets pushed through the screen. Inversely, the lower the thread diameter, the more ink that can be pushed through the screen, but the ability to maintain high details is lost.

So how do you go about choosing a mesh count? Well, for the sake of this article, and the process of 4 color printing on a screen, we want to go with the highest possible mesh count with the largest diameter of threads suitable for the material we are printing to. The higher our mesh count, the tighter our dots can be when we create our halftone.
Now, this is where the magic happens. In order to output a proper halftone, using the highest possible quality for our screen, we need to do some math.
First, we need to look at our mesh count, we can call that M. In order to figure out the optimal dot size of our halftone for our screen, we need to divide M by 3.5. Why 3.5 you ask? Mostly, because I like that number and have good result with it. But, because there are differing ideas of what number to use, most ranging anywhere from 3 – 5, feel free to experiment and let me know what works best for you.
The quotient of this formula will become our LPI, or lines per inch.
M / 3.5 = LPI
Our LPI will dictate how many lines per inch we will have in our halftone. The higher our LPI, the more dots we can fit in per inch in our halftone, allowing us the ability to print a higher quality image.
The LPI of your halftone can have a huge impact on the quality of your print. Below is a quick break down of typical LPI standards for reference.

  • -Screen Printing 45-65 lpi
  • -Laser Printer (300dpi) 65 lpi
  • -Laser Printer (600dpi) 85-105 lpi
  • -Offset Press (newsprint paper) 85 lpi
  • -Offset Press (coated paper) 85-185 lip
Let’s say I have a screen with a mesh count of 220 threads per inch. If I divide that 220 mesh count by 3.5, I get a quotient of roughly 63. Looking at our reference table, that will produce a lower quality halftone than used in newspapers, but fairly average for screen printing. While it might be ok for this example, I would probably look at buying a higher mesh count screen to improve my halftone frequency.  The final key to producing accurate 4 color halftone separations for output is the screen angle. Not the silk screen, the halftone screen! The series of dots that create the halftone are referred to as a screen. It is this screen, that once overlapped with the other color screens creates the final image. Each separation screen is printed at its own angle to prevent what is commonly referred to as the moiré effect. Moiré produce a sort of distorted, dizzying effect, and can ruin a good print job. To prevent moiré patterns in your prints, a general rule of thumb is to offset each screen angle by 15 to 30 degrees.


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