Troubleshooting color

Achieving consistency in color on press is sometimes easier said than done. Color in itself can be a slippery customer: it may agree with set numerical values, but when it is visually reviewed and approved for a particular job it is not unknown for the customer to voice color accuracy concerns. The reason is: the way we each interpret color differs.

That’s not all. Problems that arise when color is being evaluated can include geometric metamerism, a phenomenon exhibited by colors that appear to be a match at one angle of illumination and viewing but when the angle of illumination is changed they no longer match. Geometric metamerism can normally be traced to issues surrounding gloss and substrate surface texture.

Certain colors when used on specific substrates can produce unexpected results. For example, in situations such as flexo printing on transparent films for bread wrapping, the scattering characteristics of an opaque white ink layer greatly affects the appearance and color of the finished product.

Printing and package converting can sometimes seem to be a constant struggle to combat process variables. Color inconsistency is a major cause of production bottlenecks, delays, waste of energy, materials, labor and time, and missed deadlines.

Color variations are of course far from being the only variable that the flexographic package printer has to contend with. There are many others that can occur, and the root cause of the problem may at first be difficult to pinpoint.

Sometimes it’s more about eliminating what the problem isn’t. In other words, methodically working against a checklist, eliminating the obvious until what remains is the solution.

Think carefully before panicking: has something changed? Defects such as dirty prints – where clumps of ink larger than the half tone image are transferred to the moving web – may be caused by increased press speeds and using inks that are not optimized for that purpose. Or the necessary changes have not been made to the dryer to compensate for higher speeds, for example.

Challenges

UV curable inks can present several challenges such as spitting and dive-in. The latter can generally be resolved by adjusting the speed of the press in relation to the curing lamp wattage. Spitting can be controlled by lowering the viscosity of the ink, though that is not always possible, and by adjustment/replacement of a suitably optimized doctor blade.

Color may sometimes appear more intense than it should be. In this instance it’s often down to one of two causes: too much ink is being carried by the metering roll/doctor blade; alternatively the problem is related to the color of the ink being too concentrated during mixing.

Adding an extender in controlled amounts will enable an ink kitchen operative to resolve this problem. If the problem has been identified as one of too much ink, the volume can be reduced by adjustment of the metering system or by adding thinners.

While color may appear too intense when a proof is made, the reverse can occur as well: color can look washed out or paler than expected. If this occurs it can be due to the ink being too thin: insufficient ink is being picked up which may be due to a clogged up anilox. To resolve this problem the flexo printer can increase the viscosity of the ink with fresh and un-thinned ink, clean the anilox or increase press speed – or a combination thereof.

Good housekeeping, which includes cleanliness and routine maintenance checks, is important. It is also worthwhile having troubleshooting and other devices that help to identify and resolve potential problems in place. This is especially important when the printer is working against the clock and the customer is awaiting delivery of an order.