Computer-to-plate has become established as a production process at newspaper plants primarily in Europe. To address the consequent need to find possible measuring and control solutions for improving the quality of CTP production, an Ifra Working Group on CTP Closed Loop Measurement, comprised of users and manufacturers involved in a CTP production line, was formed in 1999.

Ifra Special Report 2.32, “Automated process control in CTP production: A feasibility study” authored by Dieter Kleeberg of Kleeberg & Stein in Germany, documents the research of this Ifra Working Group from November 1999 to March 2001.

The working group concentrated on photopolymer plates that are imaged in the visible light range. The decision to do so was based on the fact that photopolymer plates have a large market share among European newspaper CTP installations and quality control is more extensive there than, for example, in the silver-based plates market.

In the following extract from Ifra Special Report 2.32, the working group discusses problems of CTP quality assurance.

Problems of CTP quality assurance

The switch from computer-to-film to CTP is taking place today at much faster rate than some years ago. Instead of one-year test and parallel production phases, the technology changeover can be carried out in about three months in many cases. Besides the increased reliability of the CTP systems, the greater digital-orientation of the workflow is undoubtedly the main reason for this development. However, the implementation of CTP is confronting users — not only at newspaper operations — with new quality assurance problems that they did not have to face with CTF. Due to the enormous time pressure they work under and their high throughput, newspaper printing operations cannot afford to lose any time due to errors in plate production.

Example of a control element that should be exposed in the edge-bending area of every plate.
Graphic courtesy of Ifra
click to enlarge image

Any interruption of the print run due to the need to replace faulty plates and the subsequent search for the cause of the fault increases the time pressure. It happens seldomly, but it has happened in the past that faulty plates were not detected, or indeed could not be detected by the means available and thus reached the pressroom.

There are several reasons for stability fluctuations in the CTP process that are not immediately clear:

• insufficient calibration of the imager/raster image processor system,

• change in the system status due to changed environmental influences during production, such as temperature, humidity, etc.

• use of a new plate batch or fluctuations in the production quality of the plates used,

• too low laser intensity due to a defect, aging, or dust and dirt contaminating the laser or optical paths (lenses, mirrors), etc.

• temperature fluctuations (re-heating, developer addition) in the plate processor,

• activity (level of exhaustion) of the developer in the plate processor,

• washing quality of the plate surface,

• combination of several of the above.

The following controls should be carried out at regular intervals:

• Laser intensity control with the aid of an analog control wedge (UGRA/FOGRA 1982 test strips).

• Expose halftone test patches across the full plate format and check the consistency of the reproduced tones.

• At greater intervals (e.g. weekly) and at the time of equipment acceptance, a digital wedge should be exposed and measured at nine different positions on a plate.

Furthermore, a digital control wedge should be positioned in the edge-bending area of every produced plate. This includes special control patches that provide information about the CTP process stability (e.g. FOGRA/UGRA Digital Plate Wedge or Agfa DigiControl).

The third step must be completely intact (Agfa N91).

From the point of view both of quality assurance and process automation, the aspect of “exposure calibration” is every bit as important for CTP as it was for CTF. The CTP calibration result should not differ from the result that was achieved formerly on the CTF-copied plate (assuming that the ISO 12647-3 standard was already observed at the film production stage).

Good calibration continues to be one of the decisive preconditions for safe, automatic plate production. Besides the basic setting of the imager, RIP “linearization” is the most important procedure in imager/RIP system calibration. Dot gain in newspaper printing is usually compensated in the reproduction (according to ISO 12647-3, dot gain in print should be 30 percent in relation to a 50 percent patch). Linearization means a one-to-one transfer of the tonal values of the exposed file to the plate. Thus, 20 percent in the file will be measured as a 20 percent dot on the plate, 30 percent as 30 percent, 50 percent as 50 percent, etc.

UGRA/FOGRA wedge 1982 for controlling plate hardening.
Graphic courtesy of Ifra
click to enlarge image

The RIP setting should counteract dot gain on the plate so that the percentage values measured on the plate are linear. ISO 12647-3 permits dot gain on a conventionally imaged negative plate of 3 percent in the mid-tone range. For reasons of compatibility with CTF and in order to avoid problems when processing CopyDot files, this value should also be aimed at for CTP.

The characteristic curves are recorded with the assistance of a plate-measuring device. It is recommended to try to calibrate all existing imagers with one and the same curve.

If regular monitoring reveals a changed dot gain on the plate, this can be due to one or several of the aforementioned reasons. Independent of this, at the time of changing to a new plate batch or a different plate brand, it is important to match the linearization. However, changed dot gains or unstable plates can occur within the same plate brand. The properties of a new batch can have changed to a large degree, thus disturbing production continuity. Especially in the case of photopolymer plates, a storage time of at least one week from the date of manufacture, should be allowed in order that the polymer layer has sufficient time to mature.

A changed linearization curve gain cannot be attributed to a plate that has not been allowed sufficient time to mature before imaging, and is very difficult to see after imaging and developing. It becomes evident only when it is too late: in a too-short useful life in the press. Plates that have been stored for too long can also cause problems.

Experience has shown that frequent causes of unstable plates are too-low laser intensity, too-low preheating temperatures and the state of the developer. To date, however, there is no reliable method to automatically localize and name such faults, and possibly predict whether the CTP plate will have reached the end of its useful life already after 10,000 printed copies. The good optical impression given by a plate, or the tonal value transfer characteristic curves recorded by a plate measuring device on the plate cannot guarantee that the plate will actually reach the length of run stated by the manufacturer. Despite this, reliable measuring methods for all other parameters are a prerequisite for a continuously controllable and even closed loop CTP plate production.

It was this deficit – in conjunction with the described problems – that led to the forming of the Ifra Working Group “CTP Closed Loop Measurement.”

The members of the Working Group include: Thomas Altrath, Verlagsgruppe Rhein-Main, Mainz; *Thomas Beyer, Fuji Photo Film GmbH, Düsseldorf Germany; Helmut Britsch, Nela Ternes Register Group; Brüder Neumeister GmbH, Lahr; Wolf Buchholz, Buchholz & Partner, Schönberg; Frank Dieckhoff, Darmstädter Echo, Darmstadt; Andreas Dreger, Techkon GmbH, Königstein; Stephan Esenwein, Agfa-Gevaert AG, Wiesbaden; Elke Horak, Druckerei Konstanz GmbH, Konstanz; Reinhard Kappen, Darmstädter Echo, Darmstadt; *Ulrich Krzyminski, Techkon GmbH, Königstein; Gerhard Raab, Esko-Graphics, Willich; Michael Rettig, Darmstädter Echo, GmbH, Darmstadt; Michael Schultz, AdvancedTEC Software Schönberg; Wieland Schwarz, Fuji Photo Film GmbH, Düsseldorf; Joachim Spiess, Glunz & Jensen A/S, Ringsted; Manfred Werfel, Ifra, Darmstadt; Andy Williams, Ifra, Darmstadt; Uwe Junglas, Ifra, Darmstadt (Project Leader).

(*In a different position at a different company.)