Mechanical ghosts are usually visible as soon as the press sheet lands in the delivery section of the press. There are three types of mechanical ghosts: starvation, blanket, and plate.
Starvation ghosts
A "light print ghost" appears in large solid or dark halftones either as a light or dark image of another part of the press form. It is caused by the layout of the press sheet elements combined with the limitations of the press inking system.
This can happen when graphic elements contact the form rollers on the press just ahead of large areas of heavy solid ink coverage. The graphic elements remove more ink from the form roller than the press can make up before these rollers come in contact with the solid. As a result the solid receives less ink in those areas and the graphic elements appear as a light image in the solid.
Strategies to avoid this problem include: rotating or "cocking" the press form relative to the press sheet so that the graphic elements are no longer in the same inking zones, changing the imposition or design to eliminate the problem, running the job on a larger press with greater inking capacity, making sure that water levels are run at a minimum, running on a larger press sheet and adding "take-off" bars - graphic elements - in the non-live image area of the sheet to even out ink usage.
A "dark print ghost" occurs when knocked out graphic elements immediately precede an area of heavy ink coverage.
This is the opposite of a light print ghost. Here, the knock out can cause excess ink to remain on rollers in the reverses causing a dark image of the graphic to appear in the large solid.
The same strategies used to avoid a light print ghost are also used to avoid a dark print ghost.
A "plate ghost" or a "blanket ghost" appears when graphic elements intrude into the printing as latent images.
This problem usually occurs on multicolor presses where the press form on one of the color units sensitizes the plate of the following color unit. Proper desensitizing of the affected plate and ink adjustments should correct the problem, however, sometimes the plate and/or blanket may need to be replaced.
Comment below submitted by "Otherthoughts"
For what it's worth? The how, why and when that I've used ink take-off bars in the past.
Solid ink Borders were the most prone to a starvation type of mechanical (ghosting?) in my experience, especially PMS spot color borders.
In my sheetfed experience, often there was a bit of space available for ink take-off bars, our standard sheet size was 19" x 25". Despite there being some space available, ink take-off bars were never used to make a job easier running and nicer looking as a matter of course by anyone other than myself. I employed them as a stripper because I knew that the pressman would appreciate the help (being a former pressman myself), and because I knew it would produce a better printed result.
The type of job that really needs them, doesn't come along all that often, here's an example.
Lets say the form below shows a reflex blue to be run as a fifth color. And lets say that the reflex blue borders will trim out to be 1/2" wide on all four edges.
Without ink take-off bars, ink usage ranges from 17.125" to 2.125", a ratio of 8 to 1 With ink take-off bars, ink usage ranges from 17.125" to 3.562", a ratio of 4.8 to 1
Layout Details
Limited the bleed at both the gripper and tail to 1/16" each. Butted the pages together at the circumferential center line mark and finally shrunk the Color-bar down to 1/8". This leaves 1 7/16" to implement the take-off bars on a 19" x 25" sheet.
Gripper margin = 00.312
8.5 x 11 x 2 = 17.000
Bleeds x 2 = 00.125
Color bar = 00.125
Total = 17.562
Notes As you well know, ink take-off bars are useful with low total ink coverage scenarios as well, but such scenarios have nothing to do with ghosting/starvation. Regardless, prepress failed to employ ink take-off bars in this scenario just as well. If there was no room for ink take-off bars on a job, we did without them, which was essentially "No Change" from what we did in all cases. :)
Gordo's response
The iconic National Geographic magazine cover border is a great example of this problem - but uses a different solution. They don't have enough space on the press form to add ink take-off bars, so instead, in anticipation of the increase in darkness at the top and bottom of the picture window, they use a screen tint of the solid spot color in those sections of the border.
The screened part of the spot yellow bar gets darker on press due to the mechanical ghosting effect and ends up the same color as the solid yellow on either side.
Chemical ghosts are related to the chemical activity of inks as they go through their normal drying process. Their appearance is usually unpredictable and, unfortunately, become evident only after the job has been printed and in the press delivery pile for a period of time.
Chemical ghosts may appear as reproductions of one side, or part of a press form, in the solid area of another part. Their appearance can be erratic - showing up in one area of the printed sheet but not in a similar or duplicate part. They usually appear as a dull ghost on a glossy background or as a glossy ghost on a duller background.
What makes an ink glossy or dull.
When an ink appears glossy, it is the result of enough ink vehicle forming a film on the surface of the paper to provide a smooth covering layer for the pigment particles in the ink.
When an ink appears dull, it is the result of the ink vehicle draining from the surface into the paper so that the irregularities of the individual pigment particles and paper surface are not covered with a thick enough film of ink vehicle to create a gloss appearance.
How inks dry
When ink is printed on a press it goes through two distinct phases:
1 - Setting - the drainage and leveling of the ink vehicle into the paper coating or fibers. This causes the ink to "gel" and become immobilized.
2 - Oxidation - the polymerization of the ink vehicle into a solid mass creating a hard film.
As the film of ink oxidizes it releases gaseous by-products. As a result, the ink printed on the second side of the press sheet can be exposed to the gaseous by-products from the oxidation of the first side if it is printed at a critical point in the first side's ink drying cycle. Those gasses affect the drying rate of the second side ink selectively as the sheets are stacked in the press's delivery. This results in an ink film that has been immobilized at two very different rates, and therefore has areas of high or low gloss which reflect the image on the other side.
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