Negatives for photopolymer plates

 

Negatives for many of the solid material plates have to be prepared on a matt surface film. This is to ensure maximum contact between the negative emulsion and plate surface during exposure, otherwise pockets of air will be trapped and retard exposure, which in turn will result in these areas breaking down during the washout operation. Some manufacturers have built the matt effect into the plate material surface so that all types of negative film can be used.

Techniques required for producing photopolymer plates differ from those required for producing similar results on metal. It has long been established that this type of printing surface has a greater ink transference than metal which allows for less impression and therefore less ink squash. The effect of this greater ink transfer is to reduce the contrast between the main shadow area and the lower middle tones, particularly with monochrome work. For example, if the same screen negative is used to produce two plates, one on Nyloprint and one on zinc, when printed side by side on the press using the correct weight of ink for the zinc plate, the Nyloprint plate will print with some filling in of the lower middle tone areas. On the other hand, if the Nyloprint plate is printed with the correct weight of ink, the zinc plate will be lacking in contrast. Some newspapers that use Nyloprint solely for illustrations mount them on the rotary stereo a few thousandths of an inch below the level of the printing surface.

 

 

Moulding photopolymer plates

Some photopolymer plates are suitable for newspaper and all types of commercial printing; some are concentrating mainly on the newspaper market, book printing and business forms; others have been produced specifically for the flexographic industry. The volume markets are considered to be newspaper and flexography.

 

 

Photopolymer flexo plates

 

The most recent development in the photopolymer field has been the introduction of new plates for flexographic printing. Although they are initially more expensive than rubber plates it is significant that some of the companies now manufacturing plate materials for flexography have been traditional suppliers of the materials for the production of rubber duplicates.

Laminates control the relief depth. For example, the first layer is laminated to the base plate, exposed to a line positive, washed out in a spray machine and then, after drying, the operation is repeated until the necessary relief layer thickness is achieved. The average mould requires six to seven minutes to produce.

After the relief has been formed on the final laminate the mould has to be cured in a special oven for six to eight hours before being used.

The vertical relief duplicate rubber plates produced from Foto-Flex moulds allow much longer press runs with no loss of quality because as the relief wears

down, there is no thickening of the image. The company used one set of plates for a 10 million press run on cartons and although the rubber relief had worn down by 0.152 mm there was no thickening of the image as there would normally be with rubbers produced from original photoengravings.

 

 

Laser plates

 

Several manufacturers of photopolymer plate materials have for some months been experimenting in the field of lasers, which will allow direct copy-to-plate exposures, thereby eliminating the camera. The first commercial operation in this field is in Elmira, New York State, where a system known as Laser-Graph is producing all the full-page plates for the local daily newspaper.

Two main items of equipment are being used to produce a Laser-Graph plate direct from the compositor’s paste-up, a Scan-Scriber unit and an Ablation unit. The planned page, protected by a sheet of clear foil, is placed on a vacuum base at the scanning end of the Scan-Scriber and at the scribing end is placed a blank Laser-Plate, which is made up of the following: a 0.254-mm thick aluminium base to which is laminated a 0.508-mm thick layer of plastic material which forms the relief in the finished plate; an extremely fine layer of copper foil (non photosensitive) is laminated to the plastic relief layer.

As the paste-up page passes into the unit a helium-neon laser scans the copy and simultaneously an argon laser removes the copper foil covering the future non printing areas of the blank plate. The Scan-Scriber operation takes two minutes for one full newspaper page, and at this stage the plate has a visible copper image but no relief. The plate is then transferred to the Ablation unit, where a carbon dioxide laser vaporizes the plastic non-image areas of the plate in four minutes to form the relief. It is then punched and crimped at both ends, and without requiring to be pre-curved is ready for the press room. Total plate production time is six minutes.