At IfraExpo 2001, Purup-Eskofot, a member of the Ifra CTP Closed-Loop Measurement Working Group, presented a development for checking the results of computer-to-plate plate exposure and processing.

The objective of the closed-loop project is the automated process control and quality assurance of digital platemaking for newspapers on the closed-loop control system. As a precondition for this, all relevant influencing parameters must be measured and recorded under daily plate production conditions.

Demonstrations were given in Geneva of the Purup-Eskofot DMX 2737 CTP imagesetter with an integrated Nela device that can position a stepped filter wedge at the edge of a CTP plate.
Photos courtesy of Ifra

The system shown by Purup-Eskofot (www.purup-eskofot.com) at the international newspaper trade exhibition in Geneva is now being used and evaluated under regular production conditions at the Darmstädter Echo in Darmstadt, Germany.

The length-of-run capacity of the plate is considered to be an important process parameter that depends to a large degree on the hardness of the plate coating. The Closed-Loop Measurement Working Group concluded the only way to determine the hardness of photopolymer plates after laser exposure of the plate was to use an analogue optical halftone step wedge.

A video camera installed in the punching and bending machine records the control elements in a brief rest period of the plate.

At the Darmstädter Echo, the CTP production that the company started in July 1999 was equipped correspondingly. The newspaper uses two Purup-Eskofot DMX 2737 systems for computer-to-plate, supplied with the exposure data by a bitmap-oriented EskoNet workflow system. The two internal drum-type CTP systems expose Agfa photopolymer plates and are equipped with online plate processing as well as a following Nela Ternes Register Group plate separating, punching and bending machine.

An automatic swivel device for the halftone step wedge is installed in one of the two DMX 2737s. Purup-Eskofot commissioned Nela to produce the device; Purup-Eskofot developed the interface for the electronic control. After the plate is drawn into the DMX drum, the device positions the wedge on the edge of the plate so that the laser can then carry out the exposure through it.

Techkon measuring system

Automatic recording and evaluation of the control element are done with a Techkon measuring system that is supplied with images from a charge-coupled device video camera integrated into the Nela automatic punching and bending machine. For purposes of the evaluation, the Techkon system compares the recorded images with the matching reference image of a plate produced under optimally calibrated exposure and processing conditions.

The control system is now in continuous use in CTP production at the Darmstädter Echo. At present, the burned-in halftone step wedge is recorded at one-hour intervals.

The measured data can provide indications concerning the behavior of plate processing (preheat temperature, brush pressure and developer activity) and of the laser intensity over longer periods. In a next step, it is aimed to incorporate the additional control elements of chess board/line patters and Siemens star that are digitally exposed on to the plate into the long-term measurement and evaluation.