Textile Printing Techniques

Correcting Process Color Images On Press

Correcting Process Color Images On Press
by Mark Coudray

Without proper color balance in film separations, adjusting color and dot gain on press becomes a hit-or-miss gamble.
One of the most perplexing, challenging aspects of printing four-color process is balancing color on press. This difficulty is due to the general lack of color knowledge of press operators and supervisors, most of whom started as entry-level helpers or general production workers. For general line and simple fine detail work, this is not a problem. But when a job requires overprinting colors and controlling transparent shades, problems may arise.

Given printers' lack of experience in complex color and tone control, as well as their limited understanding of four-color process film separations, it is no wonder press personnel become frustrated. The unpredictable, hit-or-miss nature of color correction adds to the distress, sometimes leaving the printer without a clue what to do. It seems that no matter what changes are made, the image never comes into balance.

There are two common responses: Blame the ink or blame the separations. While it is possible that one or both of these responses are true, the challenge almost always lies in understanding the process of balancing color.

The two basic causes of color imbalance are:
  1. Printing inks of the wrong hue and strength
  2. Failing to manage dot gain properly
In the first case, it is imperative to develop a profile of the process-color inks that will be used. No pigments meet the "ideal" theoretical characteristics necessary to reproduce color. All process-color pigments have some contamination and unwanted color absorption that must be compensated for. This unwanted absorption affects overprinted colors and neutrals (red, orange, green, blue, violet, brown, gray), while color strength also affects the neutral balance and shades of overprinted colors.

Dot gain, the second cause of color imbalance, is the growth of the halftone dot, which makes the resulting color appear darker than intended. Dot gain is a fact of life in all printing processes and cannot be avoided. However, it can be controlled and compensated for so printed colors end up looking like they should.

Understanding Color

In Film Separations
Before considering serious ink and press adjustments, it is necessary to understand how separations are made—an important factor because, without balance in the film, it is impossible to adjust color and dot gain on press. All separations are produced with several basic assumptions relating to color balance and tone values.

Gray Balance
The first major factor in process-color separations is gray balance. The separator assumes that relatively equal proportions of yellow, magenta, and cyan will produce a neutral gray. This is the "gray balance" of the separation. It is important to realize the values the separator uses are based on known pigment values, typically reflecting Specification for Web Offset Printing (SWOP) standards for Web offset lithographic printing. They may be based on Chromalin, Matchprint, or Press Match pigment values, or they may represent any one of a number of arbitrary values the color separator selects.

Since screen printing does not have an established standard for process-color inks, there is a high likelihood that the gray balance chosen by the separator will not match the gray balance of the process inks—particularly since no two process-color ink sets in this industry are identical. Experienced industry separators establish gray balance profiles for different inks. These profiles are usually available as PhotoShop CMYK Set-Up values and from most major ink companies. If these values aren't available, give printed swatches of your process colors to the separator to let him adjust gray values to your inks. Even if he does not adjust the values, the separator can tell you in which direction colors will shift.

The gray balance of the color separation is one of the most important factors in successful color balancing. If you can't match a neutral gray with your inks, the separation never will be correct, and there always will be one or more colors you can't match. A classic indication of unbalanced inks are printed separations where most of the image reproduces perfectly but parts are wrong. In this case, you cannot adjust the incorrect areas without throwing the entire image out of balance.

Your inks must match the colors in the proofing materials the color separator provides or the printed piece will show a color shift. There are common shifts in magenta and cyan process inks that often do not match the proofing materials. These differences affect the purity of the overprint colors, with greens, reds, oranges, violets, and blues tending to be grayer and duller on the final print than on the proof.

When a four-layer (CMYK) overlay proof is supplied, these polyester sheets have a color shift toward the gray side for all colors. To determine how close your pigment colors are to the proof, print individual process colors and adjust color strength (ink density) to a slightly lighter value than the overlay material—the value being the relative lightness or darkness of the printed ink film. A color reflection densitometer gives absolute values, but if you do not have one, visual comparison against the proofing material is acceptable and provides a close match if the lighting is good. Look for distinct shifts away from the proof material. If the color matches exactly, you still will have problems on press because the proofing pigments are intentionally darker than your inks to help simulate anticipated dot gain on press.

The combination of dominant color shift (hue error) in the ink and the combined color shift (grayness) determines which direction your printed image will go compared to the proof. For instance, if you have a magenta shift in the cyan and a yellow shift in the magenta, the printed neutral gray shifts toward orange. Added grayness may accompany the shift, which might be acceptable when printing a warm-tone subject such as a model or perfume advertisement. However, it is unacceptable when working on a piece that has a great deal of green in it because greens will shift toward the brown side, resulting in olive green instead of clean green.

When your contamination is complementary to the color you are printing, the image tends to lose color saturation (purity). (See Figure 1 for more info on complementary relationships.) If the color shift is adjacent to the color you are printing, the result is a dominant cast in the direction of the contaminated color. For instance, a marine blue ocean shifts toward jade if the cyan ink contains too much yellow.

Tone Range
The second major factor in process-color separations is the tone range, which is the difference between light and dark. When you order a set of separations with a range of 15% to 85% (a typical range), the separator provides film with dots no smaller than 15% and no more solid than 85%. For the printed piece to accurately reproduce these values, the ink's color strength must be correct.

If the ink is too weak, the values in the shadowy (dark) portion of the image will not be dark enough. If the ink is too strong, the lighter tones will be too dark. Adjust the ink strength or density to closely match the values on the proof, remembering that these adjusted values should be slightly lighter than the proof's solid process-color values.

Print a test with the same color sequence that will be used during the run, and overlay all three process colors on each other. If the gray balance is correct, a color close to neutral dark gray should be achieved. It most likely will be slightly cast to the brown side, due to the high-level yellow contamination in all magenta inks—which is why black is added to the printing sequence.

If the resulting three-color gray value (also called three-color black) is lighter than the desired target, increase the color strength. There are two ways to adjust it.
  1. The first method is adding pigment toners, available from your supplier. These highly concentrated colors are mixed into the inks to increase the color load.
  2. The second method is to increase the ink film thickness by using a thicker stencil or coarser mesh. If possible, avoid this solution because it can cause dot gain problems.
It is much better to print a thin ink film with high color concentration than to increase ink film thickness. However, if you do not have access to the necessary toners, this method can be a solution of last resort.

To summarize, so far the key points and steps in balancing color are:
  1. All separations are produced with a normal gray balance that is established for the proofing material used by the separator. (Gray balance is achieved by printing the three primary process colors at a specific color strength.)
  2. Compare your ink colors against the proof material colors to make sure no color shifts occur
  3. If possible, have the separator adjust the gray balance in accordance to your inks
  4. To evaluate a printed image accurately, the printed ink film must be of equal value to the color strength on the proofing material
Understanding and following these points provides a foundation for the next step.

Analyzing the Printed Image

Determining color and tone balance is one of the most difficult aspects of printing four-color process. To be truly proficient, you must avoid adjusting an image's color and instead direct your attention to the control bars printed on the edges of your process separations. These bars are your roadmap to proper color reproduction. Remember, process color ink values do not change with each piece of art; you will always correct to the same values. The values we are concerned with are how light or dark the ink prints, and the colors that result when we print the process colors together. They are determined by the software program you use to make your separations and are compared to the printed proof of the separated image before it is printed. Determining whether the separation, ink, or printer is at fault requires an understanding of these values.

Process color is created by overprinting transparent inks to form secondary and tertiary colors. The efficiency with which the colors overlay and the purity of the resulting overprint is referred to as trapping. (This is not to be confused with spreading an image to aid registration, also called trapping). Because inks are not perfectly transparent, the resulting overprint colors are less than ideal. It is your job to determine how close to the idea colors the overprinting will be.

The overprint colors to be evaluated are red, green, and blue (the secondary colors), and brown and gray (the tertiary colors). Because ink fails to deliver ideal pigment values, red usually prints as orange; blue prints as violet or purple; and green is relatively pure. By evaluating the printing order, you can choose the sequence that yields the best possible range of overprinted colors.

There are two trapping methods for printing process color, each of which is dramatically different. The first method is dry trapping, commonly used by graphic printers. In this method, each process color is printed and dried before the next color is applied. The second method, wet trapping, is used by textile printers. It is wet-on-wet printing, which results in overprint colors that are dependent on the transparency of the pigments and the amount of "touch-off" when printing. Touch off refers to the amount of ink that is lifted off the printed shirt as subsequent printing screens contact the wet ink surface.

To evaluate an image on press, use the gray scale and color bar. The grayscale and color bar can be manually produced, or will be automatically applied to the edges of your separation when you set-up the separation parameters for your image output. See the manual of your respective software for directions on how to do this. The gray scale shows the progressive tonal steps from white to black and is based on the neutral gray generated by the three primary process colors. The color bar shows the solid ink values for the three primary process colors (cyan, magenta, and yellow) and the resulting overprints of red, green, blue, brown, and three-color black. (Three-color black is often considered gray because it approaches the value of solid black.)

The first step of toward evaluating a color bar is to print solid ink values of the three primary colors and match them to the corresponding solid color values on the proof. This allows you to evaluate the trapping efficiencies of the overprinted colors.

Colors that are not transparent have a definite shift toward the last color printed. In the color sequence Y+M+C, reds are more orange, greens are darker, and blue is cast to the purple shade. If the sequence is changed to Y+C+M, red is dominant and green is weaker, while blue is cast to the violet shade. Changing the sequence to M+C+Y makes greens lighter and cast to the yellow shade. Blue will be more neutral, and red will be much more yellow-orange.

One challenge in determining acceptability of process color inks is that yellow is the least transparent of the process colors. In fact, it is translucent, casting everything toward the yellow shade and resulting in a predominant yellow cast. (It is possible to obtain transparent yellow, but the pigment is considerably more expensive.)

When printing wet-on-wet, yellow must be printed at the beginning of the sequence. The reason is simple: Yellow is the dominant color in the separations covers the largest amount of area in the separated image. Because yellow is the lightest color, any contamination is readily apparent. If it is printed last, or even later in the sequence, the colors printed first will contaminate yellow's purity. If printed after magenta, it becomes orange. If printed after cyan, it becomes green. If printed last, it shades to brown.

The ink's color strength also is important factor in correcting color. Color strength determines contrast in the image, and the point at which balance adjustments are made. (Contrast is the purity of the whites and the darkness of the solids.) If ink is too weak, the solid color looks washed out. For example, dark cyan blue looks medium cyan, and black shadow areas are affected, making it impossible to attain good black values.

Decreasing color strength to lighten an image, or "basing back," is one of the most common mistakes printers make. Basing back is used to compensate for color shift on press. This shift is due to dot gain, which is the growth of the film dot on the printed substrate, making an image look dark and muddy. As the size increases, the contrast of the printed image decreases. The dot in the lighter area gets bigger, and the point at which the tone goes solid decreases. This phenomenon is called tonal compression, and it is the most common reason separations look muddy. (See figure one.)

Figure 1 - These three examples show the primary colors: Yellow, Magenta, Cyan with their complementary colors. It should be noted to create a complementary color you combine to two remaining primary colors. For example Primary Magenta, Complementary Green (Cyan + Yellow). If the Gray Balance is correct, you will achieve a true Neutral Gray at some point in the gradient. True Neutral Gray is where the values in the RGB Info Window are equal R=G=B) or Saturtion = 0% in the HSB Info Window.

Attempting to adjust for this problem by basing back only further reduces the contrast of the image. The resulting print is flat, muddy, and lacks contrast. Strong ink color is the key to achieving good tonal range. If you have to resort to lightening ink, you have too much dot gain. To minimize dot gain, try reducing ink thickness by using :
  • less squeegee and flood bar pressure
  • higher mesh tension
  • finer thread diameter
  • a thinner stencil
  • a harder squeegee
  • lower off-contact distance
  • or all of these options
If you still are unsuccessful in controlling dot gain, the only correction is to adjust the separations.

One of the easiest ways to tell if color strength is too weak is to look at the printed image's gray scale. Small differences in color are much easier to see in lighter areas than in darker areas. It is easy to see a 1% difference in highlights, while it is difficult to tell a 5% difference in shadow areas.

A gray scale with ink that is too weak has highlights cast in one direction and shadow gray in the complementary color. For instance, a weak magenta has highlights that look too magenta and shadows that are green. The reason for this is that dot gain increases faster visually in the highlights while you attempt to obtain neutral gray in the shadows. This gain in the highlight shifts the neutral gray to magenta. In the shadow areas, it is not possible to increase dot size to compensate for the weakness of ink colors, so the gray scale stays green.

When the realationship between Yellow, Magenta, and Cyan are correct, and the Dot Gain value is corrrect, a neutral gray tone scale will result when all three colors are printed together. This represents the correct Gray Balance of the separation. This is critical to achieving a great looking accurate print.

These three example show what the Neurtral Gray Scale looks like when each of the primary colors is too weak.

These three example show what the Neurtral Gray Scale looks like when each of the primary colors is too strong.

Try to achieve the correct neutral gray in the shadow area first. Establishing this gray balance is crucial. If all the colors are too weak, you never will get the darkest gray as shown on your color proof. If you get the darkest gray but the lighter grays are too dark, you have a dot gain problem that needs to be dealt with in the separations.

When strengthening or diluting colors, never assume the ink has the same color strength as the last batch you received from your vendor. While it would be great if this were not the case, there is indeed variation. Start with a printed sample of the ink you will be using on sample material through the same mesh at the same tension. This is sometimes referred to as a draw down. For instance, use 355 PW at 25 N/cm with 1/32" off contact and a 75 durometer squeegee. Use this setup to check an incoming batch of ink. Pull three or four samples of the same proofing material and compare the printed color against the prior batch. If it is different, adjust to match the prior batch.

The comparison can be as simple as a visual check, or use a densitometer or colorimeter for greater accuracy. A densitometer measures color strength, and the colorimeter plots the actual color for a match according to a predetermined color space.

Also, make ink adjustments with concentrated color or halftone extender base, available from suppliers and balanced for the ink you are using.

Once gray balance and tone balance are achieved, the press run can begin. All the work done up to this point is useless if you can't control consistency of color and tone during the run. To help maintain consistency, tape approved printed samples at the unloading station of the press and at the end of the dryer. Make sure these control samples are signed, dated, and free of fingerprints and smudges. This way, each printed piece can be compared to these approved samples, and small shifts in color and tone can be easily noticed and corrected.

As you become more accustomed to viewing and analyzing color, the mysteries of controlling process color will dissolve slowly. Successful process color reproduction doesn't involve secret tricks, but requires paying serious attention to the many required details.

Mark Coudray

About the Author

Mark Coudray
Mark is founder and President of Coudray Serigraphics, a textile screen printing company and Coudray Graphic Technologies, a digital imaging and prepress supplier to the industry. He was inducted into the Academy of Screen Printing Technology in 1989 and has served as Chairman of the ASPT twice. In addition, Mark is the recipient of 18 SGIA Golden Image Awards, 2 Swormstedt Awards, Magnus Award, and the Parmele Award (2001). He is past SGIA Chairman (2000). He has served a total of 24 years as an SGIA Director. Mark is a noted industry author with Screen Printing, Impressions, Print Wear, Images, and other trade publications with over 275 articles, columns, and technical papers as well as an industry presenter at tradeshows, conventions, and technical symposiums. He is best known for his extensive work in controlling color halftone printing, quality, and production management of the screen printing process.

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