Four-color process printing is where you start with finished full-color artwork and separate out the three subtractive primary colors of yellow, cyan, magenta, plus black. The program creates the separate films that are then printed with special process inks and the print looks like the original artwork or at least it should look like the original artwork. Process color is not the same as basic 4-color printing where you cut overlays for the colors and use halftone dots from sheets of dots to create other colors and patterns. Some people think they are printing process color when they are just printing 4 individuals colors.
Use Process Color?
Because the prints look great if done correctly! Where else can you print with just 4 colors (process colors) and get hundreds and hundreds of colors and also come very close to matching the original artwork? Also, with computer graphics and process inks getting better, process printing will become as commonplace as puff printing, jacket printing, caps or any other commodity item.
Kind Of Artwork Is Needed?
For process printing you must start off with a great piece of artwork because the design is only going to look worse when printed. You just cant take a small photo, enlarge it and hope it will be better on the shirt. The artwork should have good contrast and avoid fine subtleties that can get lost in this process. Artists should also avoid fluorescent colors as they do not reproduce as accurately.
In addition, a high quality computer generated image may be used. Since most art today is transferred from customer to printer via the internet, it is essential that any file destined for process separations be in a high resolution format. To paraphrase the previous paragraph, if you start with garbage, it is hard to produce a great process print
Computer graphic programs default to screen angles that are designed for lithography. They are useless for screen printing. If you are creating your own films, make sure that these angles have been adjusted before the film is produced.
The screen angles, line counts, dot shape and tonal compression are critical for screen printing. Most separators are accustomed to separating for lithography. They are not usually prepared for the different factors that must be taken into consideration for screen printing.
The angles are important because of interference from the mesh. Mesh creates axes that are situated at 90 degree angles. If you use these angles you are almost certain to introduce moir into your design. Moir is the condition where an unplanned wave or line pattern appears in your print. Often, it cannot be seen until ink is actually printed onto the substrate. Using a typical angles (except for yellow, which will not show the moir is one of the factors in avoiding moir.
starting point for your screen angles is as follows: yellow = 5%
cyan = 55 degrees
magenta = 22 degrees
black = 80 degrees
The line count most we use in textile process printing today is 55 lines per inch. This level of definition creates a fairly high level of detail combined with a heavy enough ink deposit to stand up on the fabric.
Screen printing has a major disadvantage as compared to lithography when it comes to process printing. Since process printing requires the use of dot patterns of some type, we are using a printing system that can compound the error possibilities. We print dots through a mesh that may not match the pattern of the dots. Textile printers have the additional disadvantage that graphic screen printers do not face in that they are printing onto a substrate that also has a pattern that can cause interference.
Lithography is capable of printing and holding the halftone dots from about 3 to 97% density. Screen printing cannot achieve such range. The rule of thumb for textiles is that 15 to 85% is achievable. The main reason for the restriction is dot gain/loss. When you print a 80% dot, it usually gains to somewhere above 90%. When you print a 15% dot, you usually lose down to under 10%.
The accepted dot shape for screen printing is elliptical. Of the myriad dot shapes available in modern separation programs, elliptical dots work best with the screen printing process. The main reason is that the dots avoid interference problems with the square holes of the mesh. The result is a minimization of dot gain, especially in the mid-tones.
Union Ink has made available a small computer plug-in file that contains the color values and initial dot gain curves for Adobe Photoshop?. This file is available in the software section of the Union Ink website or on disk from any authorized Union Ink dealer.
Printing 55 line per inch halftones, you can use 305 threads per inch fabric. White printers are most commonly printed with 305 threads per inch and spot prints can use 230 to 305 threads per inch, depending on the ink laydown desired.
The process inks today are excellent at reproducing accurate prints. Union Inks Tru-Tone process inks are recognized as the ultimate inks in achieving accurate true-to-life color reproduction. The colors available in the Tru-Tone pallet include the normal Cyan, Magenta, Yellow, and Black, but also include a Fine White Printer for printing white highlights and a cool and hot Magenta for use in manipulation of artwork that does not quite meet your standards for accurate reproduction.
If spot colors are needed, they should be prepared so that they have a viscosity as close to the same as your process colors so that no excessive squeegee pressure is necessary to print them. Since spots are printed last, it is important to avoid pressing too hard on the print which will increase the dot gain in your halftones.
The quality of process printing is definitely affected by the quality of the substrate. Since you are printing dots, if there is nothing for them to land on, then they do not transfer to the substrate. A tight weave high quality garment is best. In practice, 100% heavyweight jersey will yield the best quality.
Beware of using some of the "super" heavyweight T-shirts. The thread diameter used in some of these shirts to obtain the high weight creates a wale affect that can cause a loss of dots.
The best policy is ? test everything. Test your press, test mesh, test screens, test ink, test squeegees, and test garments. When you test, document what you did and what the results are.