Theory & Practice
By Fred Williams
Editor & Publisher
Published Spring 1979
Novice printers may be warned by professionals "not to monkey" with impression screws. But they were put there for a purpose–to use when absolutely necessary.
No press is built to remain accurate indefinitely on a variety of jobs. Parts may need replacing and adjustments must be made to take cam of wear.
On presses in A-1 condition it is seldom necessary to make adjustments other than to roller sockets and varying the amount of packing. Yet there probably will be times when the unusual form demands special attention.
If possible, the form should be locked with regard to the impressional resistance. A form may be locked in the center of chase and still have the center of impression far from the bed's center. One must take into consideration which is the heaviest part of the form, and lock it so it will come nearer the center of chase than would ordinarily be the case, in order to distribute or equalize the impression efficiently.
For example, a form containing a large halftone at the top and a few type lines below. Naturally, much more impression will be required to print the halftone than the few lines of type. Therefore, the halftone should be very near the center of the chase.
Therefore, sometimes it will be necessary to position a form in such a way as to call for extra care when feeding. A pressman should be able to feed a sheet "dip in" or to a right guide.
To be a good feeder means infinitely more than mechanically sliding sheets of paper in and out.
A "dip-in" sheet is fed with the narrower side toward the bottom guides. This sometimes makes feeding awkward, but as better results are attainable, a printer should not hesitate to work with that aim in view.
In locking a form it is always better to place it a trifle below center than above, as the more rigid impression comes from the bottom screws. "Dipping" a letterhead brings the center of impression of a form nearer to the center of the platen thus equalizing the impression.
Impressional resistance is a real factor in the simplifying of makeready. Not only will the practice of it save time but result in a better quality of print.
Sometimes a press has to take a form which taxes its capacity both as to size and weight. The form may be very heavy on one end or on one corner and the balance of the form comparatively light. As the form is so big that the center of impression cannot be brought to the center of the tympan, the inevitable result is that the heavy end or corner requires more squeeze than the rest of the form. The tyro will resort to the practice of building up the corner by a series of graduated underlays. This method is wrong in principle and may lessen the useful life of the press. It may tip the platen out of true with a heavy impression–making it difficult to makeready the rest of the form. The proper recourse with such a form is to adjust the screws to get good results.
Before beginning makeready or spotting-up, every form should show an equalized impression. To obtain this, it is seldom necessary to adjust the lower impression screws –those down under the platen which are so hard to reach. With all forms possible having the center of impression just a little below the center of the platen, practically all adjusting can be by the upper screws.
Irregardless of the size or type of press used, every printer will benefit if he understands the philosophy of the impression screws.