What is Gas Ghosting in Offset Printing?
Also often referred to as Gloss or Chemical Ghosting, the term refers to the appearance of a “ghost” image from the reverse side of one printed sheet on the face of the adjacent sheet. The ghost may appear as a variation in gloss level, or density and although many research projects have been carried out, it remains a complex problem.
We know that rather than the migration of any process ink or varnish components through the substrate, ghosting occurs as a result of the oxidation by-products of the first pass sheet’s printed ink film, either altering the surface energy (surface tension) of the substrate - which then impacts on the transfer of ink during the second pass (backing up), and/or the same oxidation by-products interfering with the drying of the subsequently printed ink film on an adjacent sheet in the stack.
It’s not possible to isolate one single cause of a ghost; ink formulation, substrate, image format and ambient conditions (humidity and temperature) can all have an impact. It is therefore also difficult to predict when it might occur, although the following considerations will greatly assist in avoiding Gas Ghosting.
- Print heavier image first (may require re-planning as sheet work)
- Keep stacks to minimum height
- Back up stacks in sequence (number stacks)
- Air sheets thoroughly as soon as possible after printing
- Allow first side to dry thoroughly before backing up
- Varnish first pass before printing second pass
- Run printed sheets through press prior to back up to supply fresh air to ink film (also helps to dissipate oxidation by-products)
- Maintain constant optimum temperature & humidity in production areas
- If you suspect ghosting may occur, avoid duct fresh inks and do not use drying additives
- Consider Under Colour Removal (UCR) when heavy ink builds are incorporated in the image
- Clive Harper