TIN-PLATE PRINTING--_continued_
Direct Tin Printing--The Machine--Peculiarities of
Impression--Cylinder Covering--Colour Sequence--Printing
Inks--Drying Racks--Air Drying and Stoving.
"Direct tin printing" is not, as the term might suggest, a _direct_
impression of the printing forme on the metal plate. It is in reality a
transfer printing process in which the transferring medium is a cylinder
with an elastic covering. This additional cylinder is one of the chief
characteristics of a direct tin-printing machine, and, apart from a few
accessory movements, it is in this respect alone that it differs from an
ordinary lithographic printing machine. Fig. 25 gives a sufficiently
clear idea of this distinguishing feature. A detailed description of its
mechanical principles is unnecessary, but the following points will
prove helpful.
The impression is made from the printing forme upon the lower cylinder
during the first half of its revolution, and transferred to the metal
plate, which is carried by the upper cylinder as the revolution is
completed. The reciprocal movement of these two cylinders is therefore a
matter which demands the keenest possible attention. Their pitch must be
absolutely accurate, the pressure of one against the other nicely
adjusted, and the brake arrangement on both such as to ensure a
perfectly steady revolution.
The lower cylinder is covered with a three-ply rubber blanket, which
provides a sufficiently elastic impression surface.
[Illustration: FIG. 25.--Rapid tin-plate printing press.]
This rubber covering is a most important feature, and requires both
careful adjustment and intelligent use. Careful adjustment is requisite
to ensure the tension being perfectly uniform over the whole surface,
when the material is drawn taut by means of a tooth-and-ratchet
arrangement. Were it not so, the printing surface presented would
naturally vary in thickness and resilience, in consequence of which the
pressure would be variable and the impression distorted.
A most remarkable peculiarity of this rubber covering is that it has a
very decided influence upon the size of the impression. If, for example,
its pressure upon the printing forme is increased, the impression will
be slightly less than the work on the stone, whilst a lighter pressure
will of course produce the opposite effect. It is advisable, therefore,
when making a first impression, to measure it from back to front and
compare it with the printing forme. This method will ensure a print of
the exact size, and avoid any risk of complications in the subsequent
printings. Such distortion--for a distortion it really is--may
frequently be turned to good account, and under certain conditions it
may prove to be a help rather than a hindrance. Some slight inaccuracy
in the fitting of any part of the design may occasionally be corrected
by inserting patches of thin paper under the cylinder covering, so as to
almost imperceptibly increase the pressure over the required area. This
idea in its application to "making ready" at a tin-printing machine will
suggest many possibilities to the resourceful printer, and if
intelligently treated will rarely prove troublesome. In a lesser degree
the same system of packing may be applied to the upper cylinder, and the
size of the impression to some extent controlled during its transference
to the metal plate. The speed of the machine also affects the
impression, and in a somewhat peculiar manner. Several theories have
been set forth to prove why an increase in speed should produce a
slight enlargement of the impression, and _vice versâ_; but it is
doubtful if any of them are altogether satisfactory. Most probably it is
due to a momentary change in the resilience of the rubber. The character
of the pressure, which is certainly influenced by increasing or
diminishing the speed, would of course effect such a change. The effect,
as just indicated, is, however, only perceptible when a very pronounced
variation in the speed takes place. A rubber blanket is also affected in
a somewhat similar manner by atmospheric changes, though not appreciably
by moisture.
One other point in connection with the cylinder covering is worth
noting. Whenever it is necessary to remove the impression from the
blanket,--and the necessity may arise many times during the course of a
day's run,--let it be done with turpentine or benzole, and as rapidly as
possible. After the superfluous turpentine or other cleanser has been
wiped off, dust over the blanket with French chalk. This will completely
absorb any of the cleanser which may have permeated the rubber, and thus
minimise any injurious effect.
Unlike the Transfer Process in Direct Printing, the colour sequence is
practically the same as for paper printing. There are, of course,
essential points of difference, and these may fitly mark the next stage
of progress in the discussion of this subject. In the first place, a
bright metal plate does not present an altogether suitable printing
surface, and for several reasons. The sheen of such a plate will show
through many of the printed colours with a dull metallic lustre. The
surface, again, is hard and excessively smooth, or, as it is sometimes
expressively described, without _tooth_. As can easily be seen, this is
far from an ideal printing surface.
White also plays a prominent and effective part in many designs, and is
frequently indispensable. Its presence, when necessary in Direct Tin
Printing, must therefore be arranged for in some form or other. One
white printing is seldom sufficient to produce a perfectly clean and
solid ground. Two printings, or even three, may be necessary. Purity of
tone is a most important point, and therefore a pigment should be chosen
which will not only remain unchanged by its contact with the metal, but
which will be unaffected by the heat applied during stoving. The
appearance of this white may be improved by the addition of blue, as in
Transfer Printing (p. 70), and here also the smallest possible quantity
will be sufficient.
Gold Lacquer Printing is peculiar to Tin-plate Decoration, and its
advent indicated a vast progress in artistic display and effect.
Printing lacquer is a transparent pigment of about the same consistency
and character as printing ink. It is usually prepared in three shades of
colour--red, citron or orange, and pale gold. A combination of these in
suitable proportions will produce almost any strength or shade of gold
which may be required. Except as regards their unusually brilliant
transparency, these lacquers much resemble yellow lakes, and in paper
printing might even be used as economical substitutes for the more
expensive pigments. This, however, is only a suggestion, and has little
if any practical bearing upon their application to Tin-plate Printing,
where they completely transform the bright sheen of the highly polished
metal plate into a brilliant and most effective gold. Gold lacquer is an
exceptional pigment in many respects. It works exceedingly well, and
gives a sharp, flat impression where most other pigments would fail. It
also has the power of conveying many of its own good qualities to any
printing ink with which it may be incorporated.
In Transfer Printing the question of rapid drying is very important, and
in Direct Tin Printing it is even more so. Arrangements of a somewhat
extensive character must be made for the reception of metal plates
immediately after printing, so that the air may freely circulate about
them, and thus assist in the drying. The room in which the printing
sheets are thus stored must be free from dust, for it will readily
adhere to the tacky printing ink, and afterwards prove a source of
endless trouble.
[Illustration: FIG. 26.]
Drying racks of various descriptions are used for the storage of printed
metal plates. One of the best is constructed on the lines shown in Fig.
26. The shelves A A are adjustable so as to carry plates of different
sizes. The printed sheets are set up on end in the grooves B B, and by
arranging them back to back in pairs a large number can be accommodated
without hindering the drying. Other types of drying racks are shown in
the sectional drawings (Figs. 27A and 27B).
[Illustration: FIG. 27A.]
The question of air-drying _versus_ stoving has ever been a contentious
one, and admits of considerable diversity of opinion. Air-drying _is_
preferable where convenient. It is much more economical than stoving, of
course; but, on the other hand, it might be a better plan to stove a
printing than to wait, perhaps for some hours, until it dried naturally.
With some printing inks and under certain climatic conditions both may
be necessary, so it is almost impossible to lay down any hard-and-fast
rules as to the adoption of either plan.
[Illustration: FIG. 27B.]
Some knowledge of their respective disadvantage should be acquired,
however, in order to avoid, or at least minimise, them.
[Illustration: FIG. 28.--Convenient method of stacking decorated metal
plates to distribute their weights and prevent injury.]
Take, for instance, a course of eight printings, each one absolutely
necessary to the design, and each one to be dried by stoving. It is only
reasonable to suppose that the first and second printings will be
seriously affected by the subsequent stovings. They will undoubtedly
harden to such an extent as to render the super-position of other
colours a difficult matter, and their purity of tone will most probably
be affected. According to the same line of reasoning, some sheets would
be more affected by the heat than others, owing to their position in the
racks, the heat of stoves being greatest near the top.
Air-drying can only be accomplished by adding to the printing ink a
proportion of some suitable drier. (_See_ Transfer Printing, p. 71.)
Naturally, it requires a much longer time, but it is equally effective,
much less troublesome, and generally more satisfactory.