When multi-color products are printed by
single and two-color offset press, the four-color overprinter deviation is
often a headache for the workers of book
printers in China. The first two color overprint is generally no problem,
but the latter two colors are difficult to control. Usually, we can only reduce
the machine speed, reduce the ink volume, adjust the ink fluidity and spray
powder, but the effect is not great, and the above measures will also affect
the work efficiency and product quality, and bring the labor intensity
multiplied.
In fact, we can divide printing into
"dry" and "wet". The so-called "wet printing"
means that the front and back two overprinting on the substrate are wet press
wet, that is, the overprinting starts just after the first color is printed; while
the "dry printing" means that the next color is printed after the
first color is dry, such as the use of monochromatic machine printing
four-color products.
1. Dry printing
The advantage of this printing method is
that it can ensure the stable positioning of paper feeding in post sequence
printing, and the overprint position is basically fixed. In addition, dry ink
overprint can obtain full color, bright and accurate color. However, due to a
certain downtime, it is easy to cause paper deformation and overprinter, and
the efficiency is not high, so it can not be completed in case of urgent
work.
2. Wet printing
This printing method saves time, and the
paper is overprinted accurately due to its small deformation. But it has many
problems, such as easy to cause back rub dirty, the color of the printed matter
is gray, when the ink volume is large or the printing area is large, it is
difficult to locate the paper in post printing sequence, which affects the
accuracy of overprinting. If there is a four-color offset press, there will be
no such problems. Single or double color offset press can only be solved by
experience. How to reduce the time of wet printing is our goal.
Methods 1,
Check the status of the printing paper to
be printed. The paper with high humidity or air drying deformation cannot be
used. The new paper shall be dried or stored for a certain time to ensure the
temperature and humidity balance with the printing workshop. Cutting paper to
ensure that its edge neat, smooth, to ensure the paper stability;
Methods 2,
The ink does not put or less diluent, toner
oil and other additives containing more fat oil, because it not only affects
the color, but also easily causes the back rub dirty, and the ink is not easy
to dry quickly, which makes the paper positioning difficult in the subsequent
color printing, and directly affects the accuracy of overprinting;
Methods 3,
Before printing, especially before printing
on the coated paper with quick drying ink, it is necessary to ensure that the
blanket is dry and clean, so as to prevent the rubber cloth from adhering to
the paper in the semi dry state and affecting the stability of the paper. When
the machine is stopped in the middle, the blanket shall also be cleaned;
Methods 4,
Take measures to accelerate the drying of
the front printing semi-finished products in book printers in China. Here, the arc-shaped paper protecting frame
outside the paper feeding drum can be modified into an oiling electric sheet
for heating treatment. In addition, the powder spraying device should be used
flexibly. The first two colors do not need to spray powder to maintain the air
drying effect of the air blowing pipe on the printed matter. The air volume is
adjusted according to the thickness of the paper. Generally, the air volume of
the advanced four-color printing paper can be increased appropriately. When
printing white board paper, the air volume can be adjusted to the maximum so
that the printed matter can be fully air dried. After the installation of the
hot air drying device, the ink from the printed matter to the front of the
paper receiving desk has been basically dry, which will not cause the back of
the printed matter to rub dirty, but also ensure the correct paper positioning
during the subsequent color printing. In this way, the wet printing seems to
become a dry printing without time interval, which not only ensures the overprinter
precision, but also solves the problems of the printed matter such as dirty and
poor drying.
The above measures are based on the
requirements of conventional printing of book
printers in China. The printing operator must operate carefully, master the
ink balance, control the ink volume, spot check the overprint at any time, and
stop the machine in time if any problem is found.
