Ghosting – Possible Causes and Remedies (white paper pulled from All Printing Resources’ Tech Tips section of website)
Anilox Ghosting Problem
Ghosting is usually considered the presence of a faint image of a design in solid printing areas that is not intended to receive that portion of the image. This print defect is always in the machine direction and usually is a repeated pattern. Ghosting was inherited mostly from the advent of the doctor blade chambers. Ghosting problems are usually more prevalent in certain colors such as blues and greens. Ample ink supply to the pan or chamber or even the depth of the chamber has little effect on reducing ghosting. Generally, the root cause of ghosting comes from the anilox not recovering enough ink in the cells to supply adequate coverage to the substrate. The result is an area that lacks density and transfers a negative or ghosted image. The idea behind eliminating or reducing ghosting is to improve the ability of the anilox cells to both fill and transfer ink.
The smoothness/porosity of anilox rolls cell walls and cell depth greatly affects the transfer factor of ink to the substrate. The cell depth to opening ratio is of critical importance. Deeper cells may mean greater volume, but if the surface tension of the printing plate is not great enough (or the surface tension of the anilox is too great) to pull the like particles of ink from the cell, then the transfer factor will be very poor and ghosting may occur. It is important to remember that just because a cell is deeper, that does not necessarily mean that an increase to ink density or a reduction in ghosting will take place. Surface tension only allows a portion of the ink to be drawn from the anilox roll cells and delivered to the substrate. Proper viscosity or fluidity of the ink is also important, in addition to surface tension (dyne) and rheology of the ink regardless of solvent or water-based formulations. While it may be impossible to measure the surface energy of the anilox, it is possible to test transfer rates of printing plates with a large dyne range.
The quality and design of your ink chamber system is critical to reducing ink starvation, especially at high press speeds. Sometimes a longer dwell time between doctor and containment blades will help reduce ghosting. Normally, the wider the space between blades, the more time the anilox has to effectively fill the cells with ink. Some chamber manufacturers promote other systems to ensure the proper refill of anilox cells with ink.
Possible actions to take to eliminate or at least minimize ghosting problems include:
- Change the anilox roll on the color(s) the ghosting is occurring for one with the same volume but with a coarser screen to bring the same ink to the plate but with reduced drying on the anilox.
- With solvent inks (and sometimes water), the viscosity may be too low. A higher viscosity should slow the drying on the anilox rolls.
- If possible, put the ghosting color on a back deck to utilize the full chamber for wetting.
- Make sure the anilox is covered and protected from ambient air or blown air from dryers.
- Reduce blade pressure, which will reduce friction and heat at the sheer point of the ink.
- If possible, try to open the dwell time between blades or run trouble inks on decks that have enclosed systems with greater openings. This will assist rewetting of rolls.
- Severe ghosting can sometimes be reduced by using 10% Normal Propyl Acetate 90% Ethyl Alcohol at a viscosity of 25 seconds and reducing the press speed.
- Another solution, while not very practical, would be a specialized anilox roll for a problem job. If the anilox roll is the same diameter as the print cylinder, or at least a factor of the print repeat, it will synchronize with the design and ghosting will not appear.
- Sometimes what some call “ghosting” can also occur when running a 100% lacquer coat in the last down deck, especially on paper substrates. Often the lacquer roller impression is hitting too hard, so it picks the printed image and duplicates this image. There are three primary causes for this occurrence:
- The operator over impresses the rubber roller due to low spots. In this case the rubber roller has and needs to be reground or replaced with a new one.
- The viscosity is too high causing the over lacquer to be tacky and “pick” the image, thus duplicating the image.
- Having the wrong size print cylinder gear will also produce this effect.