Electromagnetic beam or table for screen or like printing

A screen-printing or roller-printing machine in which magnetic force generates the printing force against a web, has a support member, e.g. a magnet beam or worktable, which provides the attractive force for a pressure member printing the pattern on the web. The support member has limited bending resistance, i.e. can flex readily and is supported from below by a separate structure thermally insulated therefrom so that bowing of the support member does not occur.

CROSS REFERENCE TO RELATED APPLICATIONS 
The present application is related to the following copending applications 
naming as inventor or inventors, one or more of the present applicants: 
U.S. Ser. No. 539,913--filed Oct. 7, 1983 (now U.S. Pat. No. 4,550,681), 
U.S. Ser. No. 614,412--filed May 25, 1984 (now U.S. Pat. No. 4,557,194), 
U.S. Ser. No. 614,417--filed May 25, 1984 (now U.S. Pat. No. 4,552,778), 
U.S. Ser. No. 623,425--filed June 22, 1984, and 
U.S. Ser. No. 658,127--filed Oct. 5, 1984. 
Reference may also be had to the applications, patents and publications 
mentioned in the files thereof. 
FIELD OF THE INVENTION 
Our present invention relates to an electromagnetic beam or girder-like 
table structure for printing, coating and like web-processing machines 
and, more particularly, to such machines which employ a magnetic force for 
generating the pressing action. 
BACKGROUND OF THE INVENTION As the aforementioned copending applications 
make clear, in recent years the pressure for applying a material to be 
deposited on a web-like substrate, e.g. a fabric or a paper sheet, has 
increasingly derived electromagnetic force designed to draw the pressing 
member against the pressing table or a beam underlying the pressing 
member. 
Typical among such machines is a screen-printing machine in which the 
pattern-carrying screen is a drum which is rotatable with a peripheral 
speed equal to that of the substrate as it is drawn between this screen 
and the support table or beam, the pressing member in this case being a 
doctor blade or roller within the screen printing drum which processes the 
somewhat viscous printing medium, e.g. a fabric-printing ink or dyestuff, 
through the pattern on the screen to print the fabric forming the 
substrate. 
Another application for electromagnetic force can be found in roller 
printing in which, for example, printing ink in a particular pattern is 
applied by a roller magnetically drawn against the substrate or web by a 
magnetic force generated by electromagnets in the underlying beam or 
mounted in or beneath the worktable over which the web is passed. 
In the following discussion, reference to a beam or table which is provided 
with the electromagnetic means, will always be considered to include the 
other when one is specifically mentioned and both the beam and the table 
can be generically considered to be electromagnetic support members 
underlying the substrate and the pressing member which is drawn against 
the substrate by the electromagnetic force. 
Magnetic beams and worktables, i.e. the support members mentioned above, 
are sensitive to bending resulting from heating, such bending being 
generally in the form of an upward bow toward the center of the support 
member. 
As a result of this bending, in the central region the web is pressed with 
a greater amount of force against the pattern drum or printing roller 
while laterally outwardly of this central region, there is less pressure 
between the web and the support member and thus the printing pressure 
decreases laterally outwardly. 
As a consequence, especially in screen printing, but also in the use of a 
printing roll to transfer an ink, because the pressing force is less in 
the lateral outwardmost regions and the printing medium is thereby not 
forced away from these regions, the print is comparatively dark whereas in 
the highly pressure-central region, the print is significantly lighter. 
Obviously the answer to this problem is to prevent bending of the support 
member as much as possible, and indeed the problem has already been 
recognized in the art and special efforts have been taken to ensure 
uniform distribution of heat over the length of the beam or the width of 
the support member, and to brace the support member against bending. 
For normal web widths and for ordinary quality standards, these efforts 
have been successful. 
However, for relatively wide substrate widths, i.e. support beams of 
considerable length and worktables of substantial dimensions parallel to 
the axis of the printing drum or roller, and where high precision is 
required, i.e. the print must be of uniform darkness with considerable 
accuracy over the entire width of the fabric, these techniques have proved 
to be unsatisfactory. 
Indeed, even when nonuniform temperature distributions do no develop in the 
support member, bending of the support member may arise. This is the case 
because the support member and other parts of the machine are especially 
sensitive to different temperatures and because the standstill temperature 
overnight or for weekends may differ from normal operating temperature by 
40.degree. C. and more. These fluctuations in temperature themselves give 
rise to bending, and naturally to the cumulative defects in the printing 
which have been outlined above. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
an improved printing or coating apparatus for the purposes described, 
utilizing a magnetic pressing force, whereby the drawbacks enumerated 
above are obviated. 
Another object of this invention is to provide a printing and coating 
apparatus in which the uniformity of contact is maintained with high 
pressure over the length of the contact line or zone and thus screen or 
roller printing can be effected with greater precision. 
Yet a further object of this invention is to improve upon the earlier 
printing and coating systems of the aforedescribed type so that the 
bending of the support member is minimized not only as to local nonuniform 
temperature distributions which might otherwise tend to develop in 
operation, but also as to the significant temperature variations which 
might be encountered between shutdown and operation, i.e. temperature 
fluctuations to which the entire machine may be subject. 
SUMMARY OF THE INVENTION 
These objects are attained, in accordance with the present invention, which 
utilizes an approach which is directly contrary to the approach used 
heretofore, i.e. the reinforcing of the beam or support member so that its 
bending resistance or stiffness is increased to minimize bending due to 
the thermal action mentioned previously. 
We have found that it is possible to practically completely eliminate both 
the local effects causing beam or support member bending and the effects 
of temperature variation on the entire machine when the support member, 
i.e. the magnetic beam or the worktable, is provided so that it has a 
relatively reduced bending resistance or flexural stiffness and wherein 
the support member is engaged from below by the separate support structure 
of the machine which is thermally insulated at one or more points from the 
support member, i.e. the beam or table. 
In other words, the support member of the invention is designed to bend 
relatively freely, i.e. need not have any significant bending resistance 
or flexural stiffness, provided, of course, that it has the compressive 
strength in the direction of magnetic force application, the beam or table 
being supported by a structure beneath it via thermal insulation support 
points or regions so that the effect of the varying temperatures on the 
machine structure itself are not transmitted from the support structure to 
the beam and conversely there is no heat conduction from the support 
structure to the beam or from the beam to the support structure and 
consequently, the temperature remains uniform in the beam without local 
cool or hot regions resulting from thermal conduction. 
In practice the bowing of the beam is sharply reduced and in most cases 
completely eliminated without any other measures. 
Consequently, beneath the magnetic beam or worktable at least one separate 
structure is provided to support the beam or the worktable and because it 
is thermally insulated from the beam or worktable but otherwise connected 
thereto, bowing of the beam is precluded.

SPECIFIC DESCRIPTION 
FIG. 1 shows a magnetic beam 1 whose magnets have been illustrated in 
dot-dash lines at 1a and which draws a magnetically permeable pressing 
roller 3 downwardly to press the printing medium, i.e. a fabric-printing 
dye, through the pattern of a screen-printing drum 4 onto a substrate or 
web 5. 
Screen-printing and roller-printing principles utilizing magnetic force are 
known, e.g. from the aforementioned applications, and are here used to 
apply the printing force over a line perpendicular to the plane of the 
paper at which the roller 3 and the pattern drum 4 are tangential to the 
plane of the upper surface of the beam 1, the web 5 being passed between 
the drum 4 and this beam. 
Obviously instead of a pattern drum 4 and the pressing roller 3, an 
indirect or direct print-transfer roller can be used and can likewise be 
attracted by magnetic force to the beam. 
The beam 1 has a relatively small cross section by comparison to its length 
and has little bending resistance or flexural stiffness, i.e. tends to 
hang down if it is supported at its ends in a catenary pattern. This type 
of sag can be used as a measure of the flexural stiffness or bending 
resistance which is desirable according to the invention. 
Below the magnetic beam 1, we provide a separate structure which, as can be 
seen from FIG. 1, is an H-form body with the cross bar of the H in the 
configuration of a ladder (see FIG. 2), i.e. made up of a number of 
spaced-apart transverse members 6 which are connected by rigid blocks of 
thermal insulation 7 with the underside of the magnetic beam. 
While a number of transverse beams 6 have been illustrated in FIG. 1, it 
will be understood that in many cases a single bar in the middle may 
suffice as illustrated in principle in FIG. 5, assuming the beam is also 
supported by insulating braces at its ends. 
FIGS. 3 and 4 show another embodiment in which the beam 1 is supported from 
below by a further beam 8 extending over the entire length of the beam 1 
and carrying the force thereof through spaced-apart insulating bodies 9. 
It is possible to provide thermal insulating coatings on the parts of the 
support structure 8, 2, 6 which contact the magnetic beam or, 
alternatively, to form the entire support structure as a thermal 
insulation body. 
In the case of round or flat pattern screen-printing machines which utilize 
an endless support beneath the web, it is sufficient to support the 
magnetic beam on a support structure which lies parallel to this magnetic 
beam as shown in FIGS. 3 and 4 so that the framework 2 can be eliminated. 
When no endless underlay for the web is provided, the insulating supports 
can be mounted on the ground. It is also possible to make the supports 
vertically adjustable so as to vary the force with which the magnetic beam 
is supported at each point. 
FIG. 5 shows a simple arrangement according to the invention in which a 
frame 10 carries the magnetic beam 1 and rests upon the ground or floor. 
In the middle of the magnetic beam 1, a single strut 12 is provided whose 
upper end has a head 13 which can brace against the magnetic beam via the 
thermal insulation layer. The strut 12 can be vertically adjustable with 
the vertical position being determined by a spindle with a setscrew. 
In FIG. 6, a worktable 1' with the magnets 1a' is seen to be provided with 
a plurality of vertically adjustable struts 12', the spindle being 
represented at 12a' and the setscrew at 12b'. An insulating layer 7' is 
provided to effect the thermal insulation in the manner previously 
described.