Printing Problems
Printing problems are usually caused by the paper, the press, the printing plate, or the ink. If the problem is serious, the press may have to be stopped until a correction is made, and someone-the printer, the client, or the paper merchant-will have to pay for the delay.
The following are some of the more common printing problems that may arise:
Chalking. Also called Powdering, A condition that occurs when the printing ink is not properly bound to the paper and can be easily rubbed off as a powder. This condition is never noticed until the ink has had time to dry, and by that time it is too late to correct the factors that caused it. However, the job can be saved by using the same printing plate to print a transparent size over the entire job.
Crawling. This printer's term is so ambiguous that the same word can have opposite meanings within one plant. To some printers, crawling means the contraction of ink film on a surface that the ink has not wet completely; to others, it means the expansion of ink, such as might occur when a "soupy" ink overprints a wet ink film.
Crystallization. (Not to be confused with the same term, meaning the formation of crystals, used in chemistry.) A dried ink film repels a second ink that must be printed on top of it. For example, when a four-color job is printed on a two-color press, the first and second colors are printed on the first pass through the press; the third and fourth colors are printed in a subsequent pass, allowing enough time between passes to avoid smudging. It is during this time that crystallization can occur-the inks may dry too hard or some of the ink's components may rise to the surface and cause poor trapping. Crystallization can be avoided by using an ink formulated to dry properly on the specific paper used and by paying attention to the time intervals between subsequent printings.
Doubling. As the name suggests, doubling is two impressions of each dot, which causes the printed image to appear heavier, or fuller, than it should. Doubling occurs in wet multicolor printing when the first color printed is picked up on the blanket of the second cylinder, which in turn, prints it back onto the next sheet. If it is in exact register this second imprint will not be seen, but if it is slightly off-register it will print as a light "ghost" dot next to the original dot, thus causing variations in tone and color values.
Drying, Poor. When ink requires an unreasonable amount of time to dry after printing. Poor drying can be caused by the kind and amount of dryers in the ink, dryer dissipation, too much moisture in the paper, too much humidity in the printing plant, too much water being run, or too acid a fountain solution.
Emulsification. In off set printing, the dispersion of the fountain solution (acid, gum arabic, and water) in the ink. Emulsification affects the color strength of the ink and its ability to dry with a gloss, resulting in a washed-out appearance.
Flocculation. Also called orange peeling. Clumps of pigment particles are surrounded by clear vehicle, which causes the solid areas to resemble an orange peel. The condition can be caused by improper handling of the press, but it is generally regarded as an ink defect.
Ghosting. The printed image appears faint where not intended. This is caused by an abrupt change in ink take-off on the rollers. This may occur when a border is involved, for example: because the vertical sides strip off more ink than the horizontal, a solid printed behind will be starved for ink and print lighter in that area. Ghosting is one of the few printing problems that the designer should be able to foresee and control. One solution is to have the solid areas well distributed thereby giving the ink a chance to build up again on the rollers. Also, avoid running light tints of color: choose a lighter color and run more of it.
Halation. Although not a press problem, halation may be mistaken for one. It appears in halftones (black and white or color) as a light, halo-like area around a very dark area. Halation is caused by improper automatic photographic developing, and new plates must be made to correct it.
Hickies, Ink and Paper. This is probably the one printing problem with which most designers are familiar. Hickies may be caused by foreign matter in the ink or by loose paperfibers. Ink hickies are identifiable as small doughnuts (ink spots with white rings around them), while paper hickies appear as clean white specks.
Ink hickies occur when dirt or solids in the ink adhere to the blanket: ink is transferred from the particle to the paper, with the edge of the particle leaving a small uninked ring. Ink hickies continue to appear in the same place from sheet to sheet, and the only way to eliminate them is to do a complete wash-up and replace the ink in the fountain.
Paper hickies, on the other hand, are caused by clumps of paper fiber or dust adhering to the blanket (see Picking). As they absorb water they tend to repel the ink, becoming whiter as they repeat. The only way to eliminate paper hickies is to find out where they come from and remove them at the source. Otherwise, frequent press wash-ups will be necessary.
Moiré. A moiré (pronounced moh-ray) is an undesirable pattern created by the optical meshing of screen dots when screens are superimposed on one another. This can happen whenever two or more screens are used. A common cause of moiré is when a printed halftone is rescreened for subsequent reproduction. For example, if you wished to reproduce the halftones on this page, and you shot the already screened halftone through a second screen, the result would be a combination of the two screens. This could cause a moiré. To avoid a moiré, the second screen must be set at a different angle from the first so that the angles of the two screens are about 30 degrees apart.
There is another way to reproduce a previously printed halftone that will avoid creating a moiré. Because the halftone already has dots, it can be photographed as line art. This is a good solution if the dots are sharp and if the halftone does not have to be enlarged or reduced too much; overly enlarged dots become coarse, and if reduced too much the dots may be so close together that the halftone will fill in.
Mottle. An uneven or speckled appearance caused by the ink's inability to form a smooth, uniform film. Some forms of mottle are noticeable while the ink is wet; others become noticeable as the ink dries. Mottle can be caused by a number of factors: ink not properly adjusted to stock, stock not receptive to ink; uncoated stock with wild formation; coated stock with nonuniform ink absorbency; running too much ink on hard-surface, nonabsorbent stock; and too much dampening water. In some cases, mottle can be reduced or overcome by modifying the ink or keeping the dampening water to a minimum.
Off Color. When the job appears to be a different color from that specified. This can be the result of many things, the most obvious or which is a poor color match on the part of the ink manufacturer. This in turn could be caused by an inadequate color specimen, an incorrect paper sample, or insufficient information regarding the type of press, job, or form. Other factors that can affect color are the thickness of the ink flow- the thicker the flow, the darker the color will appear; whether or not the press rollers, fountain, and printing plates are properly cleaned; in offset, if too much water gets in the ink; and the color of the paper. Also, colors, especially tints and pastel shades, may darken or fade after printing.
Paper Curling. In off set, this can be caused by too much fountain water, which causes the sheets to curl up. (This is especially common in one-side printing such as labels.) In other cases, when the printing ink is too tacky, the back edge of the printed sheet has a tendency to curl down if heavy ink coverage extends too close to the back edge of the sheet (tail-end hook). Extremes of humidity in the printing plant can also cause paper to curl.
Picking. When the pull of the ink (the tack) is too strong for the paper, the surface of the paper is ruptured, or "picked." Picking may appear as small white specks in the solid printed areas of a paper's surface. It is caused by inks that are too tacky or papers that are too weak. Picking can often be corrected by adding a solvent or compounds to reduce the ink's tack, by slowing the press, or by changing to a paper with a higher pick strength. A lot of trouble can be avoided if paper is tested for pick strength before going to press.
Piling, Ink. Ink must transfer uniformly at each point in the printing system. If it does not, an accumulation of ink gradually builds up on the press rollers, making continued printing impossible. Among the causes of piling are inks that are not properly ground, and too much moisture on press which causes the paper coating to become tacky.
Piling, Paper. A problem in offset printing, where coated papers come in contact with moisture on the blanket. The coating is loosened and particles adhere to the blanket, causing halftones to become sandy and highlight dots to be lost. To avoid piling, the fountain solution may have to be modified or the paper changed.
Registration, Poor. In printing, the accurate positioning of one printing plate over another is extremely important. When plates are printed off-register, the printed image is fuzzy. The two most common of the many reasons for poor registration are poor platemaking and paper that is not stable (has a tendency to stretch). Also, the more colors used, the more chances there are for poor registration. Perhaps the most difficult job to control would be a job requiring tight registration, printed by a high-speed web press on cheap paper.
Scuffing. If an ink contains too much non-drying oil or varnish it rubs, or scuffs, easily even when dry. This is a particularly disastrous problem in packaging, where printed cartons or labels tend to rub against one another. Scuffing is also common in printing done on highly absorbent coatings such as Kromecote. Scuffing can be minimized by using a scuff proof ink, or by varnishing after printing, which is usually done on press.
Scumming. In offset printing, the adherence of ink to the non-image area. Scumming occurs only in lithography because the image and non-image areas of the printing plate are on the same plane. When the non-image area loses its ability to repel ink, the ink will be accepted and then transferred to the nonimage area of the print. This may be uniform over the sheet, but it is more likely to occur in the form of blotches at the ends and sides of the sheet.
Although scumming used to be a common problem, advances in lithographic plates have now made it a rarity.
Set-off. (Formerly called offset, a term now reserved for offset lithography printing.) Set-off occurs when the ink on a printed sheet fails to set in time and transfers to the underside of the next sheet. This can be brought about by any combination of many factors that inhibit ink from setting.
Show-through. When the low opacity of a paper permits the printing to be seen from the other side of the sheet. Showthrough can be avoided by printing on a more opaque sheet.
Slurring. The filling in of halftones and of reverse lettering, a common problem in offset printing, especially when printing on coated stock. Slurring is caused by too much ink or by slippage between the paper and either the printing plate or the blanket. Slur, or "drag," is most apt to happen when printing solids or halftones on a smoothcoated paper because the ink acts as a lubricant, aggravating the slippage,
Snowflaking. The failure of a printed ink to form a continuous film. Snowflaking is visible in the form of small holes, or voids, in the printed area. It is caused by water droplets in the ink which prevent the uniform transfer of a solid to the paper.
Spreading. A thickening, or enlarging, of the printed image. This may be due to a number of things: a poorly made plate, running too much ink, excessive plate blanket squeeze, etc.
Sticking. Also called blocking. Set-off carried to a point where the sheets actually stick together.
Strike-through. When the vehicle of the ink penetrates through the sheet so that it is visible from the opposite side. Strikethrough is more apt to occur on an absorbent stock than on a nonabsorbent stock. A possible solution is to change the stock or print with an ink that has a higher hold-out formula.
Tinting. Found only in offset printing; occurs when pigment particles migrate to the fountain solution, producing a uniform discoloration of the background. Tinting differs from scumming in that it usually shows up over the entire plate rather than in streaks. When washed from the plate, the tinting instantly reappears.
Trapping, Poor. Trapping is the ability of an ink film to property accept a subsequent ink film. To ensure good trapping in wet printing, the colors first printed must be tackier than subsequent colors; in this way, the first ink down helps pull the subsequent ink off the plate or blanket. If the ink manufacturer knows how a multicolor job is to be run, he can adjust the ink tack to minimize the likelihood of poor trapping.
Another form of trapping that is of concern to the designer is where two color areas meet. To avoid a white line between these two areas, a slight trapping, or overlapping, of the two colors is necessary.