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.