Problems with ghosting are common enough in the printing industry, but with a bit of knowledge, they can easily be overcome. There are a number of different ghosts in the printing industry. One of them is mechanical ghosting, the effect that occurs when the ink supply system of a press cannot keep up with the ink demand of the job. This is particularly problematic with line and tone near big solids: a repeat of part of the print design shows in the solid. Then there is gloss ghosting, where a mirror image of type from one side shows as a differential gloss in a solid printed on the reverse side of a sheet. Last, we have gas ghosting, where a coloured stain also appears on the reverse side of a litho print.
A vaguely similar effect to gas ghosting may also occur on coated films where retained solvent from flexo or gravure ink may attack the reverse side of the film in the rewind, leaving a faint but discernible out of register image.
Mechanical ghosting, where the ink supply runs short, is the most common of our three ghosts and can occur across different printing processes, particularly with older machines. Gloss and gas ghosting are specific to vegetable oil based inks, however, and hence turn up only in litho printed work.
A single vegetable oil unit consists of three long chemical chains called fatty acids tied together by a single molecule of glycerol (glycerine). The various oils differ in the chemical structure and the chemical reactivity of these fatty acid chains.
For example, castor oil consists mainly of a fatty acid called ricinoleic, which has no unsaturated or reactive sites, and so castor oil is incapable of drying.
At the other extreme, tung (China wood) oil consists mainly of a fatty acid called eleostearic which has three reactive sites per chain. This is very reactive and inks made from tung oil dry the quickest of all, often too quickly to control.
In between is the vegetable oil most commonly used in litho inks, linseed oil. The main component of this is a fatty acid called linolenic, which has about two reactive sites per chain and so dries well.
Vegetable oils dry by absorbing oxygen from the air. The oxygen ties the fatty acid chains together at the reactive sites to form a cross-linked matrix that binds the ink layer tightly together.
Instead of combining to make a dry film, a small amount of the oils’ fatty chains can split at the unsaturated sites into by-products of small fragments called aldehydes and ketones. These are volatile, and can leave the print as a gas. It should be stressed here that this is a normal part of ink drying, and it is only when these gases are produced to excess or are trapped that subsequent problems can occur.
In a tight stack, the only place such a gas can go is the back of the sheet above the print. When this happens, the gas can be absorbed into the back of the sheet and this can lead to two effects.
Especially in highly filled papers, the gas can stain the colour of the coating to a pale yellow or beige – this is the effect known as gas ghosting. The effect is sometimes mistaken for showthrough, but the colour stain always gives the game away.
In gloss coated papers and boards, the adsorbed gas alters the drying rate of any ink printed over it by speeding it up, so that if a solid is printed over that area, the print over the gas affected area will appear to be more glossy: hence the term gloss ghosting.
This particular version of the gaseous by-products of ink drying shows most annoyingly when type is printed first, and then a solid is printed on the other side. The type then stands out as a glossy mirror image in the dry solid print, and is most evident in black solids.
The conditions that lead to the significant generation of by-product gases are an excess of driers (catalyst) – particularly Cobalt driers – and high temperatures. Running high stacks worsens the effect, as this both retains the heat and compresses the sheets which then prevent the gases escaping out through the sides of the pile.
If gloss ghosting is encountered (and the job cannot be UV coated to rescue it), there are a number of things you can do when reprinting the job. Start by printing the solid first and run short stacks. As soon as it is feasible, also crack the stacks and wind as often as possible, airing the stacks thoroughly before backing up. Make sure that no Cobalt driers have been added to the ink, or litho drying stimulator to the fountain.
With the problem of gas ghosted yellow staining, it is best to consult the paper and ink manufacturers before reprinting, because a different grade of substrate with a less dense coating, and inks with a low vegetable oil content would reduce the likelihood of a repeat. Of course, you’ll need to observe the same cautions about no drier additions and short, well-aired stacks.
