Process Color Printing On A One-color Press

Are you routinely printing four-color jobs on a one- or two-color press?

Find out how to produce even better quality work with fewer problems.

Printing full four-color process on a single color press can be challenging, to say the least. But it can also be very rewarding to see the final result gradually come alive, one color after the other.

For obvious cost, quality and productivity reasons, most four-color process printing is done on multi-colored presses in one pass. However, there are some advantages to printing full four-color process on a single-color press. For example, there is usually better ink trapping due to dry trapping and less dot gain from blanket and transfer cylinder doubling.

Unfortunately, as always, there are also some disadvantages:

• There is more opportunity for register fit problems due to the paper's lack of dimensional stability when reacting to the relative humidity in the atmosphere.

• There may be more spray powder accumulation and contamination onto the blankets from multiple passes.

• Finally, there is the inability to see the final appearance of the composite color during each intermediate pass.

When I mention trapping, I am referring to the pressroom context of the word, not the graphic designer or pre-press technician's definition. Ink trap is the ability for one ink to transfer and adhere onto another ink, whether that first or bottom ink be wet or dry. Film trap is the amount of overlapping between different but adjacent colors so any misregistration doesn't result in the showing of white paper in the gap.

What's The Sequence?

A common concern for in-plant printers who are printing four-color process on a one- or two-color press is which sequence to use? Another word for sequence is rotation or laydown. When determining which color to print and when, there are 24 different possible combinations for printing four colors. The printer has to pick the best choices from these options.

If printing four-color process on a four-color press, the popular and preferred sequence is cyan first followed by magenta, then yellow and finally black or a CMYK sequence. This sequence reduces the amount of unwanted hue error in the two-color secondary overprint. Hue error is the amount of unwanted contamination a single primary ink has from the addition of a secondary primary ink.

For example, when you overprint a 100 percent solid of yellow and magenta, you expect to get a red hue but you don't. The actual result is an orange hue because magenta often has about 50 percent contamination with yellow. By printing yellow on top of the magenta, the yellow will undertrap during wet trapping. The result will be an overprint hue that should appear more as it should, namely reddish.

Likewise, cyan is contaminated with about 25 percent magenta. That's why an overprint of 100 percent solid cyan and magenta doesn't produce the expected hue of blue but looks purple. Fortunately, yellow is very clean and pure.

When printing four-color on a two-color press, print those colors that are the least important on the first pass. These are obviously yellow and black. Yellow has very little contrast and is difficult to see alone. Viewing yellow ink on white paper through a blue filter, such as a Kodak Wratten #47, makes the yellow ink appear as black, not green. In fact, this is exactly the same filter that is inside PMT (photomultiplier tube) drum or CCD (charged coupled device) flatbed scanners, as well as densitometers. Being so difficult to see, yellow should be controlled with the use of a reflection densitometer.

Black doesn't effect the hue of a color, it only makes the color darker and dirtier. Adding black to a three-color CMY print only adds density to extend the dynamic range and build contrast. Black is not required to achieve gray balance in neutral grays. This is achieved by screening the halftone dots smaller in the magenta and yellow and larger in the cyan.

A typical neutral gray balance consists of a 65 percent cyan and 50 percent magenta and yellow. By the way, whenever black is added to a color the result is called a shade. Likewise, whenever white is added to a color the result is called a tint. The hue remains unchanged. Why do you think they call a screened solid a tint? It's because screening allows the white paper to show through and mix with the color to make it lighter.

Getting back to sequence, to prevent any possible back trap contamination problems, on the first pass print the yellow on the first unit followed by black on the second unit. If there is any back trap contamination because of ink film thickness due to plate coverage or incorrect ink tacks, the yellow will back trap into the black and be unnoticed. You don't want the reverse, which is your yellow ink becoming dirty because it is being back trap contaminated with black.

On the second pass, print the two colors that are the most important, namely cyan and magenta. Cyan and magenta are responsible for making secondary hues, such as blue skies and water. The cyan on the second pass is also partly responsible for making green grass and tree leaves when mixed with the yellow. The magenta on the second pass is also partly responsible for making flesh tones, as well as food items likes bread and meats when mixed with yellow.

When printing four-color process on a single-color press, my recommendations are similar. The only exception might be to print black first.

Don't Use Old Information

Some people make decisions about ink sequences based on old and obsolete raw materials and supplies. Though raw materials like inks, paper, plates, blankets, rollers and fountain solutions have steadily improved, the practices and procedures accompanying them have remained frozen in time.

For example, in the "old" days (usually stated as "in the good old days") process yellow ink was not as transparent as it is today. Therefore, yellow had to be printed first down. This obviously is no longer true today. Likewise, people often choose a sequence that would start with the least amount of ink coverage and progress to the most coverage so the inks would trap better.

The fact of the matter is, ink trapping is a complicated issue affected by several different but related factors. Trapping is affected by the absorbency or hold-out of the paper (coated vs. uncoated), the speed of the press (fast web vs. slower sheetfed), dwell time or distance between impressions (unitized in-line or common impression cylinder CIC design like a T-head), as well as the ink's tack, pigment-to-vehicle ratio (called loading) and ink film thickness (IFT). Tack is the force required to split an ink film in half. An inkometer is a device that measures the ink's tack. All of these factors must be considered when discussing trapping.

Generally, one and two color presses have less dot gain than multicolor presses. I say this because one source of dot gain, and there are many, is dot shapes that have some how become distorted. This special type of dot gain is called doubling and slurring.

Slurring occurs when the round dot or other dot shape is no longer symmetrical but becomes elongated, like an oval or comet tail. Doubling occurs when an already printed dot prints slightly out of register on the next unit's blanket. The next revolution then prints a faint ghost dot of lower density next to the darker dot. Doubling is most readily seen in the fine highlight dots when viewed under magnification.

Of course, many color bars have visual targets, such as GATF's Star targets or RIT's concentric circles. Both of these visual targets are very sensitive to slurring and dot gain. If you are encountering a lot of dot gain on a multi-color, one way to troubleshoot is to pull a single impression and see if the dot gain has decreased. If it has, the cause of dot gain is coming from subsequent blanket contact.

Most modern sheetfed presses print relatively sharp and clean with only about 18 percent dot gain. Dot gain is usually progressively higher for earlier printed colors because of some minor slurring and doubling on the subsequent blankets. Duplicators that don't have heavily constructed cylinders with bearers will likely have more dot gain.

Earlier, we said that one of the disadvantages of printing four-color process on a single-color press is the inability to see the final appearance of the composite color during each pass through the press. If one of the colors is improperly printed, either too dark or light in ink density or with too much dot gain, there is little that can be done.

On a multi-color press, the sheets in the delivery are complete and it is very easy to visually evaluate the color and take corrective action. Can you think of any way of simulating what the final color might look like between press passes? If you purchase or make any type of a multiple-sheet overlay proof, you can lay each independent color layer over the inked press sheets to predict what the color should look like when done. It works amazingly well.

Because the final color result is not known until the last pass is completed, press operators must depend more on instrumental evaluation with a densitometer for each of their intermediate colors. Some multi-color press operators have become somewhat lazy about using a densitometer because they can see the final composite image. Single-color printers, out of necessity, must use a reflection densitometer to monitor and control the printing process. This procedural discipline works to their advantage and can provide them with less color variation throughout production than if they evaluate the color by eye.

Common solid ink density specifications for sheetfed printing on coated paper are to run the cyan at 1.30, magenta at 1.40, yellow at 1.00 and black at 1.80. These are general numbers. Each shop may need to tailor them to its unique conditions. Also, a common tolerance limit is ±0.05 density units above or below that target value.

by STEVE SUFFOLETTO

Steve Suffoletto is a consultant for the printing and graphic arts industry. He instructs at several public seminar programs at Rochester Institute of Technology in Rochester, N.Y. Steve can be reached at (716) 648-8789.