PRINTING FRAMES (Pr., Chassis positif, Chassis de tirage, Chassis presse ; Ger., Kopierrahmen) Appliances for keeping the negative and sen sitised paper in close contact during exposure to light in printing. The ordinary pattern, A, is made of teak or other hard wood, and the divided hinged back, the inner side of which is usually covered with cloth or felt, is fastened by means of two metal springs that engage in bent wire staples. To examine the progress of printing, one spring is unfastened and half the back is raised, keeping a firm pressure mean while on the remaining half with the fingers of the other hand in order that the print may not be shifted. Many workers prefer to have the back unequally divided, so that the greater por tion of the print may be examined at once. Vari ous frames are now made with a non-slip back, one of the best being that shown at B, in which projecting metal pegs extend from the hinges and fit in metal grooves at the sides of the frame, effectually preventing slipping. In another pat tern the back is hinged directly to the frame. C shows a heavier type of frame, suitable for large sizes, and holding a sheet of plate-glass, so that a negative of any size can be used ; this has two hinged pressure-bars with springs on their lower side, the bars being fastened down by brass strips which fold over their ends.
For copying plans, etc., large frames are used, with the back divided into several sec tions, each with its own pressure-bar, as at D.
A great variety of frames are specially designed for combination, multiple, and border printing, as well as for postcard, lantern slide, and stereo scopic work.
In process work, considerable inventiveness has been displayed in the design of printing frames. For paper prints, spring pressure is relied upon ; but for printing on metal plates, etc., screw and wedge pressure is resorted to.
The oldest form of frame for printing on plates has a number of wooden screws inserted in several heavy bars across the back (see B). The screws
are now generally iron, threaded into iron bushes let into the wooden crossbars (see P). A fur ther improvement is to make the crossbars of iron and connect them together into a frame or " spider," with crab-like castings attached to the back for taking the thrust of the screws, the number of which has been reduced to two and even to one. The frames are usually made of wood, strengthened by iron damps to take the strain of the crossbars, or heavy steel screw bolts are passed through from back to front. The front is of plate-glass of from in. to if in. thick, while the back is usually of wood heavily clamped, although in America iron backs have been used with the multi-screw frames. For collotype, a wedge pressure is generally preferred, the frame having no back and the wedges press ing directly on the thick glass printing-plate. Frames operating by leverage have come into general use in America ; in a popular pattern the bars are pressed down on to heavy pads of rubber by means of a quick-acting cam lever. Vacuum frames are found effective for blue printing, photo-litho transfers, and for printing direct on thin zinc for photo-lithography. These frames consist essentially of a wooden frame in which is mounted a glass plate to form the front. The negative and sensitive surface are laid upon this and backed by a sheet of india rubber, which is pressed down by another frame around the margin. The air is then exhausted from between the front glass and rubber sheet by means of a vacuum pump, the pressure of the atmosphere on the rubber sheet then pro ducing contact. For thick zinc and copper plates, a pneumatic cushion covering the back of the frame and held down by a heavy back board and crossbars is found more effective ; after the frame has been shut down the cushion is more fully inflated by means of a pump. A hydraulic pressure frame has also been employed.