Vacuum Printing Down Frame

 The vacuum printing frame holds the negative and plate in intimate contact during exposure. The frame consists of two wooden or metal frames one of which holds a corrugated or channeled rubber blanket with a rubber-bed gasket around its edges. The other frame contains a sheet of flawless plate glass. In smaller vacuum frame the two frames are usually hinged together on one side. The rubber blanket is connected to a vacuum pump by a flexible rubber tube.

Operating the vacuum frame:- When the printing frame sis open, the blanket is horizontal and the glass is raised up out of the way. The sensitized plate is laid on the blanket with the coated side up. The positive or negative or a stripped up flat is laid on the plate in exact position, emulsion side down. The glass frame is lowered and two frames are locked together. The vacuum pump is then turned on. The pump sucks the air from between the blanket and glass, thus forcing the sensitized plate and the negative or positive together. When this has been done, the printing frame is rotes so that the glass faces a controlled light source which when turned on, gives the necessary exposure. On a flip top frame the glass is inverted in angle of 180° since the exposing light is underneath.

Whirlers

            Plate whirler are still needed for conventional deep-etch and bi-metal plates.

            The plate whirler is designed to coat lithographic plates. The whirler is actually an oversized turntable. The plate is fastened or mounted on the turntable, the sensitized coating is poured on and centrifugally distributed over the plate while it is turning. The whirler should be equipped with a positive variable speed drive and accurate controls for setting and measuring whirler speed. The coating is dried by heat and/or forced air as the plate revolves.

            There are two types of whirlers in general use:-

            Horizontal and Vertical.  The horizontal whirler distributes the coating solely by centrifugal force. The Vertical whirler which has the turntable placed at an angle of 15° to the perpendicular, distributes the coating by a combination of centrifugal force and the gravity. There is little difference in the quality of the coatings produced by these two types of whirler neither produces a perfectly even coating over the entire plate. Coatings are generally slightly b   thicker in the centres and on the edges of the plates. The vertical whirler has the advantage of taking up less space. The horizontal whirler is better suited for coating fragile materials such as plastics and glass.