Over the years I have learned that one of the major secrets to good printing is good color separations. As you become a better printer, it doesn’t take long to learn that in most cases if the separations are done correctly, the print looks great. Yes, yes, you need to use properly tensioned screens, good press setup, proper ink viscosity, etc. – but it all starts with color separations.
This article lists 25 tips for better separations. In order to fit this in the allotted space, many tips are brief suggestions that will may require referring back to your software manual to figure out. Others are just good nuts-and-bolts suggestions that can make or break a set of seps.
Vector Based Program Tips
Vector based programs like Corel Draw, Adobe Illustrator and Macromedia Freehand are generally used for spot color images or images with a hard edge and more of a cartoon look. The funny thing about printing shirts is that sometimes spot color images where colors touch colors are often HARDER to print than photorealistic images made up of halftone dots. Here are some vector based program tips:
- Spend time on trapping.
When printing spot colors, any out of registration will show up as gaps between colors. If your “films” are output on a laser printer, they many not line up. Trapping is the overlapping of darker colors (like black) ontop of undercolors. A trap can be as small as 2 points for shops that can hold tight registration, or as big as 6 points for simple images on low-end equipment. If you spend time adding traps, images will appear to be in register when they are not. A trap is generally done by adding an outline/stroke to undercolors to make them slightly fatter (figure 1).
- Allow for Dot Gain.
If using halftone dots or tints of color in an image, when output, these areas are made up of small dots called halftones. When you print a halftone dot, it will grow in size 30% to 50%. You need to allow for this by using a lower percentage tint in shading areas so when it is printed it will look correct.
- Use custom registration targets.
The “stock” registration targets in all programs are generally too small and you don’t have any control over their placement. Make your own custom targets and assign them a color of “Registration Color” that means they will print on each separation.
- Scan line art at high resolution. When scanning lineart, scan at resolutions of 800 to 1200 dpi in order to have your tracing program like Streamline or Corel Trace, give you a much more accurate bitmap to vector conversion of the image.
- Choke underbases for better registration.
When printing on dark shirt colors, you need a base plate of white. This is generally the entire image converted to one color. For better on-press registration, you should make the underbase from 2 to 4 points “skinnier” – the opposite of a trap (figure 2).
- Soft underbases print easier.
Some images that are solid spot colors need a solid white underbase. Other images that have more shading and gradations, print easier and feel better if the underbase is actually halftone dots. Do this by making the underbase plate a 60% tint of black and then output it as halftone dots.
- Spend time with tints and gradations.
Don’t stand for flat images with solid spot colors when you can use gradations and shading to give the image more depth (figure 3).
Adobe Photoshop Tips
- Scan at proper resolution.
It is widely written that the scan resolution for photorealistic images needs to be 1.5 to 2 times the final halftone line-count. This rule works for offset printed images that use frequencies of 150 to 175 lpi. Since garment screen printing uses halftone line-counts of 55 and 65 lpi, you need to scan at 175 to 250 dpi – at the final image size. If your image has hard edge graphics and if you really want crisp edges (without having to take the image into a vector based program), then use 300 dpi.
- Working with low quality JPG files.
In the age of e-mail, we all get 72dpi file that have been compressed using the JPG format. This format loses much of the detail in an image. Yes, you can upsample an image to the proper resolution, but the little “average” square boxes that JPG uses, just get worse. You can improve these files slightly by using a Gaussian Blur on select areas of an image or blurring the most problematic channel of Red, Green or Blue. Yes, the solution is to not use these files but that is another story.
- Increase color saturation.
Many files arrive from clients very dull looking. You almost wonder what the heck they were thinking. Some files are made dull or “artsty” on purpose. When separation these images, it is harder for Photoshop to “find” the proper colors. Take time and boost the Hue/Saturation (not too much) of flat files.
- Sharpen files.
Files also arrive very soft. It is very rare when I don’t apply Unsharp Masking to a Photoshop file. Start off with settings of 150 Amount, 1 pixel Radius and Threshold of 6. For very poor originals, be bold and see what happens when you apply 400% Amount.
- Increase contrast.
File also can lack good contrast. If an image is flat, it will just print flatter. Take time to use the Tone Curve to improve a files contrast. An “S” curve does a nice job of make a flat file, jump off the screen (figure 4).
- Improve sharpness of hard edge elements.
If your image has lots of type and hard edges, you can improve the edge definition of these elements by adding then to the photorealistic elements in a vector based program. Using DCS2.EPS file format, you can bring a “channel” separation from Photoshop directly into a vector program.
- Don’t neglect depth (shadows).
If you are building an entire image in Photoshop, improve the “depth” of the file by using Layer Style shadows and glow effects on various elements (figure 5).
- Allow for dot gain.
Tips #2 applies to Photoshop also. Images will always print darker than they display on the monitor. When in doubt, think “light” when adjusting images in Photoshop. If a portion of the image has subtle shadow detail, it will be lost when printing, unless you take time to lighten this section.
- Make a great underbase.
Images on dark shirts generally “work” if they have a good underbase white plate. The underbase is EVERYTHING on dark shirts. A typical underbase is a grayscale image of the file that has been inverted. A GREAT underbase is higher contrast and has been boosted where colors like red and blue print on the underbase (figure 6).
- Preview with proper ink opacities.
If building color separations using Spot Color Channels, it is helpful to assign the proper ink color to each channel. It is also more helpful to assign the proper ink Opacity to these channels. An all-purpose ink designed for white shirts has an opacity of around 5%. If you assign this number, when previewing these colors on a white underbase, they will look correct when the colors falls off the underbase onto a black shirt background.
- Build color proof profile.
It doesn’t do much good to show the customer an inkjet proof of the separations if they don’t look like that when printed. Take time to learn how much “off” your print is to what you see on screen and then make an adjustment Tone Curve that you can apply to a file that will make it look more like the print.
- Learn to trust the Info Palette.
Your eyes can lie to you. When separating images it is not uncommon to “think” the image has solid black in it, yet that same area is halftones on the film. Use the info palette to make sure areas of solid black read Zero levels of R, G, B (figure 7).
- Use ink company’s ink values.
If doing process color seps in Photoshop, make sure to get the Process Ink Colors Ink Values from your supplier. These can be “loaded” into Photoshop and will be used during the conversion of the RGB file to CMYK and the final print will be more accurate.
Assorted Color Separation Tips
- Use center targets to help production.
Use only two registration targets on the films. Place on in the center/top and one in the center/bottom. Make sure the targets are places in a location that is dead center to where the print should fall on the shirt. This helps eliminate guesswork on print placement for odd shaped images.
- Take time to “look” at the separations.
This one is too easy. How often have you output films and never checked to see if they lined up. It is much easier to re-output a film than to re-burn screens.
- Improve output – vellum, acetate, inkjet.
If using vellum or laser acetate to burn screens, try pre-shrinking the blank pages by running them through the laser one time BEFORE imaging them – to improve registration and minimize shrinkage.
- Specify details, mesh, sequence, etc.
If you “built” the separations then YOU know how you think it should be printed. Don’t be shy about dictating mesh counts, color sequences, ink colors and more, AND place this information on each film.
- Use automated color separation programs.
Even if you are an experienced color separator, don’t let your ego get in the way of improving your productivity. With a handful of automated color separation programs, on the market, it doesn’t take long to see that they can do the basic “grunt” work of building separations and then you can apply your own knowledge and intuition to tweak them.