An article by Margaret Barraclough
Most companies in the printing and associated industries frequently have a need for some unbiased technical assistance. Print and paper solutions, with the help of the Print Advisory Service, hopes to solve some of the technical problems that commonly occur in the printing and paper industries.
Problem 1
The job was printed in the four colour process. The layout consisted of large areas of solid blue (made from a solid cyan with a heavy tint of magenta), together with halftones and reversed out type. In some areas of solid blue, there were clearly defined areas that were lighter in shade. These areas always corresponded with the design features of the job. The lighter areas in the dark blue always ran in the direction front to back of the printed sheet, (i.e. parallel to the direction of travel of the paper through the printing machine). The light areas corresponded exactly with the edges of blocks of large areas of colour and strips of virgin paper which were either directly adjacent to, or in direct line with the blue solid print areas causing a darker band.
This problem is referred to in the printing industry as ‘banding’ or sometimes (erroneously) ‘ghosting’.
The problem is not usually caused by any fault in paper or ink, but to the lack of ink replenishment and insufficient levelling of the ink film on the rollers. The design features and layout of the job on the full sheet are putting too great a demand on the printing machine’s ink distribution system. Because of the large, solid areas of ink take-off on this job, the inking system cannot recover sufficiently after each impression to compensate for the extreme unevenness of ink requirement created by this particular design. Although it is impossible to completely cure this problem, there are a few things that can minimize it:
- Make adjustments to the inking rollers to help distribution
- Increase the alcohol level (hence reduce water) in the damping system
- Carry a little more ink
- If possible, change the layout, staggering the blocks of colour across the sheets
Problem 2
Covers for a booklet were printed with solid dark blue, after which they had a spot UV varnish application. Although the job appeared satisfactory after varnishing, upon inspection forty eight hours later, the ink underneath the UV varnish had changed in shade, having a mottled yellowish appearance. This worsened with time and the job was rejected.
By the time the Print Advisory Services examined the job, the inks appeared to be fully dry and the cure of the varnish was satisfactory.
It turned out that the ink supplier had not been told that the blue ink was to be UV varnished and as a result, had not formulated the ink from pigments that would resist chemical resistance with the varnish.
The pigment in the ink had faded when varnished and this fading, being a chemical reaction had worsened with time. Certain pigments are totally unsuitable for UV varnishing and laminating. Once this problem has occurred, nothing can be done to rectify it, but to prevent further jobs suffering the same fate the following rules should be applied.
- Always make sure (with the ink supplier) that any links to be used are suitable for UV varnishing
- Make sure inks are fully dry before varnishing
- UV varnish must be fully cured