Understanding Common Flexo Print Defects


Ghosting

 

Over the years, APR has shared many articles that assist in identifying and reaction to print defects such as dirty print, ink spitting, and ghosting. But as technology, equipment, and operating techniques develop, we find it necessary to revisit these areas of concern that continue to challenge many of us on a daily basis. As I look back and think about my experiences with certain pressroom challenges I often remember the first time and place I was faced with a particular challenge. As I trained as a young press operator in 1982, my only exposure to a flexo inking system was a four roll inking system. The press had a metering roll partially submerged in the ink pan, which transferred the ink to an anilox roll; and from what I recall there was never a shortage of ink delivered from plate to substrate. But then again, who knows...I’d never heard of solid ink density and the term “ghosting” had no meaning to me whatsoever. But also introduced to flexo in the 1980’s was a component now commonly known as the (ECDB) enclosed doctor blade system. This unit can be credited with contributing to both better print quality, higher press speeds, and reducing ink consumption. It was also with my introduction to this new component that I had my first experience with ghosting. In Part 1 of this series our primary focus will be on “ghosting”.

What is Ghosting?

The flexographic term ghosting is actually borrowed from the offset printing industry. Mechanical pinholing is the print defect that we have adopted the term ghosting for in flexo. However, it does seem that most of us are in agreement that “ghosting” can be described as a faint image, which is repeated from another part of the design and appears in an area where it is not intended to be in the flexo print process. This type of ghosting is always found in the image area on the printed side of the substrate and most noticeably in large solid areas. Ghosting became more common after the advent of chambered doctor blades.

What Causes Ghosting?

 

In most cases ghosting occurs when the anilox roll cannot recover enough ink to consistently provide the required ink density. Ghosting problems are not necessarily more prevalent in darker colors, but they are certainly more apparent in darker colors like browns, reds, blues, and greens. The idea behind eliminating or reducing ghosting is to improve the ability of the anilox cells to both fill with and transfer ink.

Giving Up The Ghost

 

Both the porosity of an anilox roll’s cell walls and the cell depth greatly affect the transfer of ink to the substrate. As press speeds increase, the ability to consistently and effectively fill the cells on the anilox with ink in order to deliver a uniform ink film thickness to the plate becomes more challenging. The cell depth-to-opening ratio is of critical importance. Deeper cells may mean greater volume. However, if the surface tension of the printing plate is too low to pull the like particles of ink from the cell, then transfer will be very poor and ghosting may occur. Likewise, if the surface tension of the anilox roll is too high, it may easily pick up the ink from the fountain roll or the chamber, but not release ink from the cells resulting in ghosting. Because of the anilox roll surface topology, it is nearly impossible to accurately measure the surface energy or dyne. However, it is possible to test the transfer rates of printing plates with a large dyne range. In addition, proper ink viscosity is also important in reducing ghosting whether you are using solvent, UV or water-based inks.

Possible steps to minimize ghosting problems include:

 

    On colors where ghosting is occurring, change the anilox roll to one with the same cell volume but with a coarser screen. This will bring the same amount of ink to the plate but with reduced ink drying on the anilox as long as the ink transfer rate is comparable.

    With solvent inks and sometimes water-based ink, the viscosity may be too low. A higher viscosity should slow ink drying on the anilox rolls.

    Because the defect is normally seen in large solids with reverse print or windows, you will sometimes have an overprinting white (reverse print on clear substrate). In this case the white can re-wet the ink that the ghosting occurs. So speeding up the white ink could yield favorable results.

    If possible, put 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 air blown from the BC (between color) dryers.

    Reduce blade pressure, which will reduce friction and heat at the sheer point of the ink.

    Some enclosed chamber systems utilize an isometric inner cavity contour that through the use of fluid dynamics fill each cell with ink, and remove the air from the unfilled cells returning it to the feed tank for dispersal into the atmosphere. This process helps prevent ghosting and starvation from occurring, due to air pockets or foam build-up in the ink.

    If possible, try to increase the dwell time between blades or run trouble inks on decks that have enclosed systems with greater openings. This will assist in the rewetting of ink on the anilox roll. A deeper chamber cavity will also reduce agitation inside the chamber, minimizing air pockets that could result in either ink starvation or ghosting. An analogy that always comes to mind is that Lake Erie is both the shallowest and most turbulent of all the Great Lakes, whereas Lake Superior is the deepest and calmest.

    Severe ghosting can sometimes be reduced by adding extender or using a solution of 10% normal propyl acetate/90% ethyl alcohol.

    While never the most popular action to take in a production environment, reducing the press speed can also reduce this effect.

    Another solution, while not very practical, would be using a specialized anilox roll for a problematic 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. This would mean the cfd (complete finished diameter) of the anilox would be the same or a factor of the cfd of the print cylinder/print sleeve (consisting of bare cylinder diameter + the sandwich, i.e. mounting tape, plate, and possibly sleeve).

 

Other Potential Causes

 

Another phenomenon, which can be mistaken as ghosting or even ink setoff, can occur when running a 100% lacquer coat in the last-down deck, especially on paper substrates. Ink setoff occurs as ink transfers to the reverse side of the substrate on top of the printed image. Often the lacquer roller impression is hitting too hard, so it picks the printed image and duplicates this image. Unlike true ghosting, this can occur in either the printed or non-print area of the printed side of the substrate. This is often caused by four things:

 

    The operator has over impression on the rubber roller.

    The rubber roller has low spots and needs to be reground or replaced with a new one.

    The viscosity is way too high causing the over lacquer to be tacky and “pick“ the image, thus duplicating it.

    The print cylinder gear is the wrong size.