„Printing Press Acceptance Tests in Flexographic Printing with Measurable Criteria“
The performance evaluation of a flexographic printing press based on printed products is problematic and very subjective from observer to observer. Scientific approaches and publications on this subject are scarcely available. A technological guideline for acceptance tests is very helpful during the installation of a flexographic printing press or for maintenance routines. Pre-defined operations and testing methods as well as nominal values allow comparability and build the foundation for a standardized press acceptance testing procedure. It is necessary to clarify that a press acceptance test should only test and compare the performance of the printing press itself. Influences from process parameters like substrate, ink, cliché material etc. should be eliminated, or rather set to a standard.
These requirements were brought to the attention of the Sächsisches Institut für die Druckindustrie (SID) (Saxon Institute for the Printing Industry) by press manufacturers and end users. On this basis, the research project “Recommendations for the Technological Acceptance Test of Flexographic Printing Presses” was begun and successfully completed in mid-2015.
The institute was able to refer to its long-standing experience in the area of printing press acceptance tests in offset printing. Relevant guidelines published by BVDM, IFRA and Fogra have been proven effective after 20 years use in the industry and have since been a reliable resource for the technical assessment of offset printing presses.
Together with project partners from the industry, the project was successfully completed and the developed acceptance test procedure was evaluated. After multiple printing tests using many different printing conditions and parameters, the various influences could be examined and the criteria could be defined. Beside different mounting tapes, printing speeds, ink types, substrates and contact pressures, the printing form properties Are another important characteristic for flexographic printing acceptance tests. The validation of the test procedure as well as the measurement and analysis methods took on an elementary roll in the process. The choice of testing criteria was of great significance. References could be made to the tried and tested characteristics of offset printing, like tonal value spread, tonal value increase or color coordinate differences and from known elements in flexographic printing e.g. contact pressure control or line deformation. Further process related-technological testing criteria for determining the register consistency and alignment and the homogeneity were also tested. A further component for the technological printing evaluation is banding or streaking in the printing units. For testing purposes, a halftone area in cyan is printed on one or more printing units. The difference of the Lab values in the printing direction is metrologically calculated and gives evidence of any vibration present in the printing unit. For this, a SID-developed scanner method for measuring the banding was used.
Based on the tests made during the project, testing criteria were presented in detail that gives the best characterization of the printing quality. Different parameters that will be verified during an acceptance test are, among others, the ink register, the homogeneity in both printing directions, the line deformation, the tonal value spread and banding. The verification of these criteria represents a good balance between effort and benefit.
The register is initially measured using the LUCHS IV register measuring device over 100 consecutive prints. The homogeneity, or the uniformity of the ink transfer, is measured in cross and length direction over the complete print with a densitometer measuring every 2 cm and is given as a percent. To identify dimensional defects or slurring effects, 80 µm wide lines in cross and length direction are placed on all areas of the print and can be used to examine line width changes. Information about the process stability of the printing units and clichés are given by the tonal value spread. Based on the tonal values of the 40 % patch, the printing units can be compared. A precise statement about the banding in the printing unit is given from the determination of the measured values from the banding test form.
The guidelines for the acceptance test process, conditions, target values and test procedures have proved to be promising and are of great interest for use in the industry.
Upon completing the project and in order to present the results in front of industry representatives, SID invited those interested to Leipzig for a workshop on the subject. The participants came from press manufacturers, suppliers and printing companies. Over multiple presentations, the institute introduced the developed recommendations for the acceptance test of flexographic printing presses, explained the meaning of the chosen test criteria and practically demonstrated the measurement technology needed for analyzing the criteria. The closing, intensive discussion of the presentation contents and the further activities resulting from the event between the industry partners and the research institution, underscored the practical importance of this topic.
The transfer of the project results to more practical means will take place through SID in different ways. For one, flexographic press acceptance tests for different kinds of presses will be offered by the employees of the independent institute as a service to the industry. Another is the development of customer specific acceptance test instructions as a basis for internal tests for press manufacturers or also for printing companies as an option for its introduction in practice.