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The Definitions Of Positive-working And Negative-working Plate
Mar 28, 2018

Positive-Working Plate


One of two basic types of photolithographic printing plates produced by bringing an unexposed plate into contact with a photographic positive of the image to be printed, and exposing it to a high-intensity light. The plate's coated metal surface is exposed in the non-image areas—where light passes through the positive—and unexposed in the image areas—where light does not pass through. The exposure to light renders the non-image areas soluble, and after platemaking the plate is chemically treated to dissolve the non-image areas, rendering that portion of the plate water-receptive, the unexposed image areas remaining hard, durable, and ink-receptive. Some positive-working plates can be thermally cured, which further hardens the image areas, making them suitable for longer than usual press runs. The non-image areas are then treated with a solution of gum arabic (or other gum, either natural or synthetic), which desensitizes the non-image areas, adding a higher degree of water-receptivity. The advantage of positive-working plates is that they tend to reproduce halftone images with less dot gain than plates made from negatives.

 

Negative-Working Plate


One of two types of photolithographic printing plates produced by bringing an unexposed plate into contact with a photographic negative of the image to be printed, and exposing it to a high-intensity light. The plate's coated metal surface is exposed in the image areas (where light passes through the negative) and unexposed in the non-image areas (where light does not pass through). The exposure to light renders the image areas hard and insoluble, and after platemaking the plate is chemically treated to dissolve the unhardened non-image areas, rendering that portion of the plate water-receptive, the exposed image areas remaining hard, durable, and ink-receptive. The non-image areas are then treated with a solution of gum arabic (or other gum, either natural or synthetic), which desensitizes the non-image areas, adding a higher degree of water-receptivity. Although negative-working plates are the most prevalent, a disadvantage of them is that they tend to reproduce halftone images with a higher degree of dot gain than plates made from positives.