How to avoid offset reproduction problems

How to avoid offset reproduction problems Knowing what goes wrong in pressrooms makes it possible for you to work hand-in-hand with your printer to achieve better printed results The transparency is perfect. The separator's proof is equally ideal, although the image did have to go through one round of corrections. Now the film is stripped, plates are made, and it's time for the printer to get his hands on this perfect material and, you fear, ruin everything.

If you've ever assumed that printers are ruled by just such diabolical desires, you may find some information in this article that will help you not only make peace with your supplier, but also work hand-in-hand with him to achieve better printed results. Production directors coordinate the work of vendors, and because few of the pre-press tasks and none of the reproduction chores are their direct accomplishments, it often appears that their contribution to quality control begins and ends with selecting the suppliers. Certainly your vendors set some limits on the level of quality in your publication, but on a functional basis there are some steps only you can take to eliminate problems.

Confronting inline conflicts

When you hand over a paginated magazine to a stripper, you have already set up conditions under which even the most fastidious printer can fail you. The imposition of your pages is determined by the printer's equipment and your choice of page placement. Somewhere in even the best-planned imposition, a full-page ad may be positioned directly above some editorial color. If you've neglected to pour out enough libations to the printing god, luck will have it that the ad features a cobalt blue Buick against an even darker background, while the editorial shot is a pastel sunset.

That combination will tax any printer, regardless of his quality standards, because it poses a problem for the press itself. The cyan, magenta, yellow and black ink drawn down for the top half of the plate must serve a dark image, in which the shadow tones should tip toward the cool side. For the bottom half, warm tones are emphasized and a lighter cyan inking level may be necessary to preserve nuances in the reds and yellows. To exaggerate the problem a bit, imagine that the sunset begins to run to the purple side if the car is to appear adequately rich. Or the car might have to veer toward navy blue if magenta is boosted to keep the sunset warm. Either way, the images are in conflict.

They're not at war because both separations weren't well made. And they're not printing with a color shift because the press foreman is off on a coffee break while all that costly paper rumbles through the press. They're incompatible by nature. A single set of ink channels supplies both images, and there simply is no such thing as an all-purpose ink density level that will yield correct results for every image.

What to do? The closest we can come to an ideal layout for a form is a checkerboard, in which only black text runs above or below color images. It's rarely practical, but this paragon is worth understanding. Even if your final imposition falls short of the ideal, the more pages you can position without potential conflict, the better. Notice that text black takes all the heat here, and if type density is to remain consistent, you won't be tinkering much with black to adjust the color images.

A double-page spread poses another sort of inline problem. If the background or image bleeds across the two pages, the ink channels for both pages must be set to deliver a crossover match. Four pages may be affected, as shown in Figure 1. If the facing pages of the car ad are to match, and if you've allowed the color requirements of the page below one side of the ad to dictate the ink level, the fourth page will have its color set by that first page as well. In this example, one page is setting color for three others.

Your instincts might tell you that the best imposition for a spread is to position facing pages in line. This way, the ink channel is shared by the right and left pages. For the most part, this is a sound practice, and the illustration compares the benefits of two versions of the same signature. The first imposition poses just about every inline conflict I can think of, while the second shifts a few page numbers and attempts to make the signature easier to print.

Note that some spreads don't belong in line: The contents page in this imaginary magazine is a spread with a photograph filling the right page and half of the left page. If the spread runs in line, half of page four requires about the same ink as all of page five. The result is a visible shift on page five from dark to light, with the side behind the photo on page four running light. The problem is worsened with darker images, but you will have to accept on faith how glaring a defect it can be because this type of flaw is a ghost, and therefore can't be reproduced photographically.

Why inking poses problems

To visualize the cause of this and other inline problems, imagine a single roller carrying ink. The ink is drawn off the roller by the plate cylinder at high speed, and is effectively split, with some remaining on the roller and some carried onto the plate. The plate draws ink to the printing dots that attract it, while nonprinting areas, receptive to the dampening solution the plate is bathed in, repel it. The plate is pulling ink selectively by area, but the thickness of the ink film it draws is dictated by density control on the press.

Ink density is set in channels, short sections along the ink fountain in which pressure on a doctor blade can be adjusted. The blade pinches against the ink roller, allowing a variable thickness of ink film to be drawn out. Thus the plate's printing dots control where the ink will be deposited, and the density setting of the ink channels dictate how much ink is available in those printing areas.

Consider that single roller again. As it begins its rotation, it's loaded up with ink, but as soon as it encounters a hungry dot on the plate, ink is peeled away. Before the roller can complete its rotation and receive a fresh supply of ink, thousands of dots compete for ink. And the competition isn't particularly fair: It's truly a matter of first come, first served. The result is some ink starvation in areas receiving ink late.

Presses are designed with a set of rollers in the inking assembly. Ink is transferred from the fountain to the plate over intermediate rollers that resupply ink swiftly because they're small in diameter. Some rollers move from side to side, distributing the ink in an even film. Throughout the process, ink is milled down into a manageable thickness, because during each transfer, it must be split onto a new roller. This is why, by the way, a change in the ink density setting takes many impressions to affect the printing image.

Back to our inline double-page spread. Even with a number of rollers feeding ink to the plate, the ink density setting remains constant, sending out the same amount of ink with each rotation. The principle of offset printing presumes that we can force ink to behave in strict accordance with the presence or absence of printing dots, and though that remains true, the density of ink we get in a printing area is not quite as selective. It's a binary control, in the end: The dots can turn the ink off if you want none of it, and turn it on if you need it, but within the ink channel you can do nothing to change the amount from spot to spot. To the degree the number of printing dots controls the density of an image, we're fine. To the degree ink density is responsible for the nature of the reproduction, we're playing a dangerous game.

Ghosting

The inline conflict is one manifestation of press ghosting, a broad topic that concerns printers, ink manufacturers and, of course, customers. Mechanical ghosts are undesired images that sneak onto the printed page, and although their appearance may be a bit of a surprise, they can generally be traced to elements of the printing image that either print too strong or too weak. In the first case, excess ink intensifies or repeats a portion of an image where it doesn't belong; in the second, a dark area is selectively robbed of ink and prints with a lighter hole corresponding to the shape that stole the ink.

Both these types of ghosts are mechanical in origin, and arise when page design and imposition place extremes of light and dark in the same ink channel. A prototypical dark print ghost occurs when a reverse area directly precedes a dark area in the journey through the press. No ink is pulled from the roller for the reverse shape, but as the plate continues to revolve and seek ink for the darker spot, excess ink is available in the specific shape of the reverse. In effect, the density control of the press is undermined by the plate's ink requirements. The result is a ghost of the reverse, printing within the dark area as an even darker image.

You can visualize the light print ghost as the opposite phenomenon. If a small, dark image hits the ink roller first, it depletes the ink supply selectively. If it's followed by a large solid, the solid encounters a momentary ink shortage in the shape of the preceding image, and prints with a weak hole corresponding to the image. Figures 2 and 3 shows simulations of both types of ghosts.

Knowing why these mechanical ghosts occur is not going to help you very much when the job is plated and running. Yes, you'll be able to mutter knowledgeably about where the dreadful results came from, but it will be a bit too late to correct them. The remedies for such problems are in job planning and design, and although it sounds as if press manufacturers have conspired to limit graphic designers to web-length pennants of identical ink consumption, there are some useful ways to diminish ghosting.

Production managers have two controls over mechanical ghosting: imposition planning and printer selection. Although some ghosting conditions are so extreme that they tax any printing equipment, some presses can handle ghosts better than others. The principal tool is a large ink capacity with numerous rollers in the ink assembly. The dark print ghost, for example, is defeated when ink replenishment occurs swiftly enough to compensate for the potential starvation. On press, the printer can ward off the problem by maintaining maximum ink strength; this means that attention to the problem on press contributes to correction.

But it's the imposition that will really be the hero or the goat here. When planning an issue's impositions, production managers should have tear sheets or color proofs of ads in front of them, and should review the editorial use of color closely. Within a page, designers may throw down gauntlets that no production manager can pick up, but he can accept the challenge of laying out the form to minimize ghosting opportunities. Figure 1 shows an imposition that tackles both ghosting and inline problems.

The best antighost tactic is unavailable to those disinclined to waste paper. Ghosting bars are solids printed in the trim area of a sheet that balance out ink usage across an ink channel. They're commonly used in sheetfed printing, and are therefore available to those printing covers sheetfed, but intelligent use of the web press's cutoff argues against them in long-run work. Figure 4 shows a common ghosting situation, and the ghosting bars that would minimize the problem.

Gray balance

At some point during most press checks, a color fidelity problem arises that threatens to get out of hand. While a face tone is being corrected somewhere, every other image in the ink channel wanders off course--and before you know it, even the type is printing inconsistently. There's a tendency to think that the press check can be used for a trial-and-error probe of ideal inking conditions, but on-press corrections should be confined to remedies you can clearly expect to resolve problems. If you need any incentive to restrict yourself, the great ribbon of paper you're using for your science experiments should be an adequate constraint.

Naturally, you can't always anticipate the solution for a color problem. The pressman at your elbow should have some suggestions, and it's best to point out to him what's wrong and what you think the correct color should be. Note that if you tell the pressman what you think the solution is, all you'll get is the adjustment you describe. But if you tell him what the result should be, you'll get his method for achieving it and the result itself.

This respectful conversation occurs only after you've spotted the problem in the first place. Your most objective tool in detecting color fidelity difficulties is not the magnificent color proof in the viewing room, but the gray balance patches that are running on your form. It is this handy strip of ink overprints that can serve as the best guide to the source of the problem, too. Before you indulge in frantic adjustments to fix a single tone, check the gray balance strip. These tones will let you know if you're working on an overall ink balance problem, or if you're trying to skew the color away from a neutral gray. In short, gray balance information helps you adjust color with a sense of where you'll be moving other images, and it reduces the amount of pure trial and error in your search.

The pursuit of gray balance starts long before printing. Your separation proofs should include a proofing bar with tint overprints indicating gray balance. Neutral gray is the absence of color cast, and is crucial to good reproduction. As it happens, equal dots of cyan, yellow and magenta generally do not yield a neutral gray when printed. Under most printing conditions, the cyan dots must be a bit larger than yellow and magenta to achieve gray balance. Your separator needs to know about your paper and ink and your printer's standards to prepare film that will print without color cast.

On press, the proofing bar should include patches of overprinted process colors in several dot percentages. Under 5000 K lighting, you can make an informed visual judgment about the overall inking on the sheet. It's usually quite clear if balance has shifted to either cool or warm gray, but remember that the culprit can be either too much of the wrong ink or too little of the right one. Rely on the pressman to decide the best method of correction.

If you look through the untrimmed signatures from an issue whose color was absolutely perfect, I expect you'll find that the gray balance patches betoken all sorts of manipulation. Part of adjusting color is allowing shifts from neutral gray. The gray balance patches are handy tools, but they can't set color for you. They can tell you a great deal about where you are and where you might be going, and with practice you can use them to project the results of an ink adjustment.

Diagnosing and solving problems Whether you're looking at sheets fresh from the chopper folder on a press check or thumbing through the samples your latest courting printer has piled on your desk, you're trying to determine if the job is well printed or not. Some flaws are downright conspicuous, and others could slink by undetected. But for all of you who'd like an idea about what caused a problem, this section will present some reproduction flaws that arise in the printing process itself, and are caused by paper, plate, ink or press.

These problems are specifically out of your control, and your entire contribution to avoiding them will be noticing them before the printer has bound all the copies and tossed them in the mail truck. I suggest you identify problems in this sequence: symptom, probable cause, short-term correction, long-term solution. The short-term corrections are those efforts you and your printer can take while the job is in production to minimize the damage, up to re-stripping film. Long-term solutions are those discoveries you make that remind you of Scarlett O'Hara vowing, "As God is my witness, I'll never go hungry again." Remedies here involve changing vendors and production specs--options that aren't available until the next issue rolls around.

1. Curl: If you've hopped on the film and UV coating bandwagon for your covers, you may flirt with the curl problem. Paper curls when there is a marked difference in coating or moisture content from one side to another. When paper is coated or laminated, the coating density and drying conditions must be carefully controlled. Film lamination can easily curl a cover running grain long if the paper fiber is so long and stiff that the grain direction is pronounced. The curl distortion can also arise when the paper is forced to absorb too much moisture from the press. Sadly, you usually won't know if you've got a curl problem until the job is dried and bound, putting this problem's correction in the long-term solution category.

Your tactics include changing the density level of the coating or the thickness of the laminate, improving printing and drying conditions, or changing paper stocks. You can also consider running covers grain short, but his requires sheetfed production. (And at that, you'll probably inherit a new problem when you bind against the grain.)

2. Dot gain: Dot gain is different from the other problems described here because it's unavoidable and not the result of poor materials or shoddy workmanship. Limiting dot gain is the goal. Mechanical gain is the expansion of the printing dot under real-life printing conditions; optical gain is the apparent expansion of the dot when light is reflected from the paper through the ink. In both cases, color fidelity is affected because midtone dots gain more than quarter tones. (If gain operated proportionally, it would parallel changes in ink density.)

Perhaps a little too often, random printing problems are blamed on gain. The most important reason to be conversant with the effects of dot gain is to be able to distinguish controllable problems from gain so that corrections can take place. If you're seeking a color match, don't give up too soon because you believe you've hit the brick wall of dot gain. You or the printer can check for gain by taking densitometer readings from the midtone and quarter tone color patches on the form's color bar. The GATF/SWOP proofing bar includes a visual dot gain target next to a slur gauge. The latter helps you see if gain is directional, while the target itself roughly indicates the effective gain on the 50 percent dot. Gain, no doubt, is affecting your sheet. In the end, you're trying to make a color match under gain conditions; let the conditions rob you of some nuances, but don't admit defeat too early.

3. Doubling: Doubling is a problem in wet-on-wet printing that is most evident only when registration shifts slightly from one impression to the next. If the first unit's color is carried from the paper to the next unit's blanket, this bonus dot will print on the following sheet. The image will be a little heavier and, if the next impression shifts a bit, will include a visible double dot.

Detection is fairly simple if you include a star target on your trim area, for the rayed circle takes on a fairly psychedelic appearance when doubling occurs. The printer corrects doubling by adjusting a range of press controls, and can generally usher the problem out the door while running or with brief downtime.

4. Drying problems: So many things can go wrong with ink drying that one is tempted to return to hand copying. Ink manufacturers work valiantly to eliminate these problems, but ink drying is affected by several factors: humidity, moisture in the paper, pH balance and level of the dampening solution, drying temperature, and, of course, the driers in the ink itself. Ink can blister underneath an inline aqueous coating; the ink on stacked, folded signatures can emit gaseous by-products in the drying process that affect the gloss on the adjacent sheet; the critical paper/ink combination can go awry and defeat ink setting--the list here is truly too long to complete.

I'll spare you Ink 101, but I'll mention that even if you graduated, your ability to pin down the precise cause of an ink drying problem would still be rather slight. Defer to your printer, and note that he may be scratching his head, too, since so many factors bear on the problem. The symptoms that mean the most to you are diminished ink gloss and slow drying, allowing scuffing.

Remedying the problem

Remedies include changes in the paper/ink combination, changes in ink and fountain solution, and, of course, publishing only in the winter months. Note that even though dozens of things can go wrong, the quality of inks and printers is now so high that few of them consistently do go wrong.

5. Emulsification: Slowly but surely, the dampening solution can make its way into the ink. If it does, ink color and gloss are affected, resulting in a washed-out image with diminished gloss. Emulsification looks not unlike inadequate print contrast, but you can spot it by noting the reduced color strength in the proofing bar: If ink density is down, the color should still be pure; if ink is emulsifying, the color will be directly affected.

The short-term correction here is chucking the contaminated ink and starting over. The long-term solution is changing conditions so that fountain solution isn't carried back to cause the problem. A certain level of contamination is inherent because the plate contacts both sets of rollers, and as far as rollers are concerned, they're as happy taking as giving. But for dampening solution to worm its way back through the entire ink assembly, a major problem must occur, and no well-maintained press should allow it.

6. Filling in: The universe is very big, but your magazine may have as many dots in an issue as there are stars in the cosmos. Controlling all these dots from film to platemaking to printing is only a little easier than counting them. When ink starts appearing between dots, it's called filling in, or plugging up. The overall effect is reminiscent of gain, in which dots enlarge, but here we have dots out of control. The most likely cause is an inking level too high to be managed. You'll spot the problem with your linen tester.

The first line of defense is lowering ink density. In many cases, the shadow dot will fill in simply because so much ink surrounds that pinpoint that it's powerless to resist. You may be tempted to beg a printer to let this happen to allow intense enough inking and contrast, but he'll wisely warn you that if you lose dot control, you lose everything. If you can't print with adequate intensity without filling in, you've got a problem with your specs or pre-press suppliers.

7. Gusseting: Before the signature tumbles through the folder section, it receives a light edge perforation to allow air to escape when the pages are pressed together during folding. If the folder pressure is too strong, the number of pages in the signature too great, or the thickness of the paper in any way unaccommodating, a wrinkle appears, usually at the tightest corner in the fold.

Gusseting may come and go, so an isolated signature with the problem may indicate nothing serious. But if all your press check signatures are coming back to you with wrinkles, seek a solution. The printer can increase the frequency or depth of perforations, or adjust the folder. A 64-page signature of exceptionally thick or exceptionally thin stock poses trouble he may not be able to remedy. (Thin paper gussets because it responds so readily to pressure, thick paper because it resists so much that greater pressure is necessary.)

8. Halation: Halation is a stripping or platemaking problem that may not be detected until a job is running. It appears as a light corona around a dark image area, and occurs because of a problem in the contacting or plate development process. To avoid halation, strippers try to build flats of even thickness, because any variation in height allows hot spots in contacting. This is one major reason why halftones are stripped on a separate carrier sheet, and why checkerboarding is important when two tints are to butt. Halation can also be caused by improper plate development.

The remedies send you at least back to platemaking, if not to restripping. You might have slight halation in type stripped below a halftone on the same carrier, and because the correction involves breaking down the flat, you might accept the defect. A plate development flaw, however, calls for immediate correction.

9. Ink hickeys: The uninitiated usually perceive the ink hickey as the definition of poor printing, and though it is reprehensible, it's one flaw that is simply bound to happen. Good printers fight off hickeys with reasonable success, and poor printers let them pop up more than they should. But if you render your judgment about a printer's technical skills based on a single hickey in a single copy of a magazine, you haven't sensed how inevitable they are.

An ink hickey is caused by a bit of foreign matter on the blanket. If ink adheres to this thicker spot, the paper receives a little blob of ink and loses contact with the blanket right around it, resulting in a white ring around the blob. Some hickeys obediently mill themselves out; others can be removed only with a wash-up. Identifying hickeys on a press check is a little like telling Danny DeVito that he's short--the pressman already knows. It doesn't hurt to spot them, but I suggest you refrain from pointing them out in a tone of righteous indignation.

10. Paper hickeys: Most hickeys start out as bits of paper or dust that cling to the blanket. If the crud takes on dampening solution, it repels ink and prints whiter and whiter as it repeats; these are called paper hickeys. If they accept ink, they graduate into ink hickeys. The solution is a press wash-up.

11. Moire: Moire appears when two screen angles overlap incorrectly, and can pop up on anything from a rescreened halftone to a badly stripped four-color job. Screens can form several interesting patterns, from rosette to outright plaid; it's the rosette that is desirable and least noticeable. In a tint consisting of only two or three process colors, your stripper can position screens dot-on-dot. When this works, even the rosette is gone, but if angles shift a millimeter, you're treated to a distracting light show. Thus misregistration on press and through stripping can create moire.

Remedies may require nothing more than improved press register, but moire problems may lurk back in separating. If the dot pattern was incorrectly angled, you can't rescue the image with anything but new film. When rescreening halftones, the cameraman must set the new screen at least 30 degrees apart from the original. If he hasn't, new film, again, is your only cure.

12. Picking: Ink and paper are slammed together, and the particular consistency of the ink dictates whether it will succeed perfectly in its mission to peel off the roller and stick to the paper in a split second. If the ink's tack is too strong for the paper at hand, it will rupture the paper surface, leaving behind small white specks within an image. And, yes, this is a perfect breeding ground for hickeys, too.

The immediate correction may be the addition of solvents to the ink to lower the tack. Long-term solutions call for the printer to measure the paper's pick strength and adjust ink tack accordingly.

13. Trapping problems: In wet-on-wet printing, ink is laid down in sequence and each color must adhere to the ink beneath it. If inks don't trap to each other properly, less ink transfers to the previously printed ink than to unprinted paper. The result is a mottled image, which, under magnification, will reveal broken dots. Proper trapping requires the printer's attention to tack sequence, ink film thickness, temperature, the paper's absorption properties, and press speed. To see if poor trapping is causing a reproduction problem, the pressman consults the color bar on the form, which should include overprints of the process colors in pairs. By taking a densitometer reading of the solid magenta and the solid cyan and then reading their overprint, he can calculate the percent of effective trap for those two inks.

If trapping is poor, the printer may be able to improve it by slowing down the press to allow tack to build. But in general, a trapping problem is solved with an adjustment in ink type or tack, or a change in paper stock.

There are easily a dozen more reproduction flaws we could catalog, but this inventory of calamities must be stopped lest we become worrywarts. A good printer will spot problems and know how to correct them, and it should never be the production manager's job to scurry around the pressroom like a master detective.

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