Beginner FLEXOGRAPHER CREATING SEPARATIONS FOR FLEXO
Producing quality color seps for flexo requires far more than simply shipping a disk to your service bureau and asking for film output.
 by James R. Kadlec

“Here is a picture and a disk, make me separations.” Wait! How will this be printed? The typical response is, “It does not matter.” Color separators hear this day in and day out. Separations and how t h ey are produced actually play a significant role in the final results of the printed product. Why? Flexo quality has dramatically improved in recent years in part because of the art and science of Flexo separations. In years past, the color separator would make a fine separation for offset printing, yet when it was applied to Flexo, the printed results were poor. The leading culprit here was insufficient knowledge and understanding of the two different printing processes. Offset separations are different, and sometimes considered easier than those produced for Flexo. Reasons include offset’s use of thinner plate thickness and thinner, more concentrated film thickness, which could result in lower dot gain. Flexo’s plate thickness is typically .067" or higher, and the ink is applied evenly across the plate surface using an anilox roll . Because of the characteristics of relief, impact printing with lower density, and more fluid inks, dot gains are typically (but not necessarily) greater than offset.

Dot Gain
All forms of printing have dot gain. This is simply what the actual dot prints versus the actual dot produced on film. Dot gain is a natural aspect of all printing, yet more pronounced in flexography than most other printing methods. For example: In Flexo, a 5% dot may print on press as 11%; a 50% as 70%; and the 85% or greater dot as a solid. The difference between the two is dot gain. Therefore, when separations are being made, dot-gain compensation must be incorporated.
Finding the dot gain of a particular press is called fingerprinting. One of the most popular fingerprint tests is the DuPont CyFOS™ target, although there are many targets used in the industry. The DuPont target uses a set of four plates containing a variety of predetermined screen values and angles to be printed in all the process colors at specified ink densities (cyan 1.3-1.5, magenta 1.3-1.5, yellow 0.95-1.1, black 1.4-1.6). A color densitometer is used to measure ink density on press to establish the ground - zero reference point. A computer program then reads ink density and actual printed dot, then converts this information into a dot-gain curve custom to the specific press and substrate tested (see Figure 2). This dot-gain curve information guides the separator to the proper dot-gain compensation particular to the press . Clean anilox rolls and monitoring of ink viscosity will make the fingerprint test results a viable tool for the separator.

Plates
Flexographic plates have been referred to as high quality rubber stamps because of their greater thickness and rubber - like qualities, as compared to metal offset plates. Understanding plate thickness is a key to producing quality separations for Flexo, because offset plates are typically .006"-.020" thick, with a printing emulsion virtually undetectable. This allows dots from 1%-99% to be clearly held and printed. Flexographic plates range from approximately .022" to .300" in thickness, with a relief of approximately .025"-.045".
Relief creates a problem in Flexographic printing. Often, 1%-4% dots require the use of a 4%-5% dot to avoid break-off of the image, which causes a poor appearance in the high-light areas. When a color separator has fingerprint dot-gain curves, adjustments to the separation can be made during the scanning process, as opposed to attempting to color correct a furnished scan made for offset. Additionally, when scans are made from a high-end drum scanner, the tonal range and detail will remain clean and sharp — much better than those produced with an inexpensive flatbed scanner.
During the separation process, careful consideration must be give n to the gray balance of the overall subject being printed. Gray balance is the combination of cyan, magenta and yellow proportioned to appear as a percentage of black. If the gray balance has, for example, too much magenta, it will appear red. Utilizing a gray - balance bar in the waste matrix will help the press operator identify subtle changes in the inks early enough to take corrective actions while staying within color tolerance, and thus reduce wasted product .

Screen Angles
Flexography has a pattern in the method of ink lay down. Anilox rolls typically have a cell pattern of 30, 45 or 60 degrees, requiring the printing screen to be produced at angles of 7.5, 22.5, 52.5 and 82.5 degrees. This is different than the customary offset angles of 0, 15, 45 and 75 degrees . The 7.5º offsetting of the standard printing screen angles helps prevent conflicts with the anilox rolls that cause moiré, a condition created when two screens are printing on the same angles. Although the anilox roll is not a printed screen, it can cause the same effect. Anilox cells transfer-ring ink are in a screened pattern; depositing ink in a similar pattern as the plate will cause the same moiré pattern as experienced with two different colors having the same angles.
PMS spot colors enhance the overall appearance of Flexographic sticker printing. Using four-color process to generate a PMS color often produces inconsistent results, such as “dirty” colors. For example, PMS 300 appears to be closer to PMS 542 when printed as process. This is not a problem exclusive to flexography; all forms of printing will produce similar results when attempting to reproduce a true color with combinations of process colors. Black is also considered a spot color when screens of black are required to print a subject. Breaking the black-line work away from the screen (process) work allows the operator to have more control over the depth of black used to produce the best picture, while also maintaining strong black-line work .
“It’s on the disk.” Yeah, so? This statement is worthless, because nine times out of 10 the disk is incomplete. Without some kind of guidance from the artist who produced the work, the separator is just guessing. In this modern day of electronic art and the high speed with which projects can now be produced, the biggest problem is simply communication. Informing your color separator of what you are attempting to achieve will assist in gaining outstanding results. For example, when a customer states that he has a six-color Flexo press with two die stations and a sheeting station, the separator can help him decide how to run the 10-color cartoon-character project on that press while reminding him not to forget the UV coating. Better under-standing of the press configuration allows the best suggestions to be made prior to the separator beginning work, reducing the probability of having film and proofs produced that cannot be used.

Proofing Pays Off
Proofing is a common area where some printers try to save money. Skipping the proofing process is foolish, because even though “it is on the disk ,” countless times the artist will produce “ghost” colors or “ghost” knockouts that are not obvious until the job is either on press, or in the customer’s hands. Consider this classic example: A printer decided to accept a $48,000 project that took a week to print, and air-freighted it to the customer without sending a press proof. Needless to say, the small savings realized by not producing the proof led to a huge monetary loss when the unsatisfactory results of the press run were revealed. Proofing from final film is the safest way to communicate between the separator, printer and the end user.
Software is another key element in producing quality separations. The most popular software products used for Flexographic color separations and distortions are Macromedia Freehand , Adobe Illustrator and QuarkXPress . Although Quark does not distort for Flexo, a “plug in” called Flex Scale is typically used to allow distorted out-put. Freehand features ease in typeset-ting, distorting, trapping, designing illustrations and imaging scans. Distortion is a key element in flexography. When a photopolymer plate is wrapped around a print cylinder, the outside (printing surface) stretches beyond its original, flat dimension. Therefore, without distorting the film, a circle will stretch to print like an oval.
Trapping is another key element in producing good separations. Again, communication between the printer and separator is critical, because, for example, an offset press requires a .0035" trap (print overlap), and a narrow - web Flexo requires .007" trap, while wide-web Flexo may require a .012" trap. Without trap, the press operator may not produce a sellable product off the press due to slight movement of adjoining colors.
Color separation means much more than sending a disk to a service bureau and asking for film output. Next time your customer tells you he will supply color separations for Flexo, ask yourself if you are heading for trouble. Scanning, distortion, special PMS separation, stepping, trapping, and even the type of film are considerations required for the Flexo printer’s color separator.

James R. Kadlec is President of Advanced Prepress Graphics, Inc., which specializes in Flexo color separations and plate making. Kadlec has over 20 years’ experience in the printing industry, of which over 15 years have been spent specializing in Flexographic and rotary letterpress printing. For further information on Advanced Prepress Graphics, Inc., call 1-888/GO-FLEXO or fax 630/766-2917.