The prints produced by a web offset press are subject to significant variation due to transients, particularly during start up. This results in waste being produced that could be minimised if these phenomena were better understood. As run lengths become shorter, these become increasingly significant. The objective of this project was to identify the key factors that affect print quality during press start-up transients in order to minimise the waste and optimise the process. Three major experimental trials were carried out on full-scale web offset presses to study basic process variability, press transient response, and global inker thermal transients. Concurrently, a computer model of the inking system was developed to improve understanding of the process mechanisms. Extended transients were measured due to changes in the ink rheological properties caused by shearing in the ink roller train, the effect of which was aggravated due to low operating temperatures. The effect of form roller load was found to be insignificant. The result of ink key adjustment was also influenced by changes in the viscosity of the ink. Ink viscosity changes had the greatest effect on print variation and transient. By pre-shearing the ink, the transient and the print variation were reduced, halftone dot transfer was improved, and dot gain decreased. High press speed increased the temperature in the inker and affected significantly the cross-print density variation. The largest temperature changes of the inker components occurred at cold press start-up, where inker temperature control is critical to reduce transients. The flow of ink and water through the inker removes heat, which changes the ink viscosity and transfer. Inker thermal stability is a function of print duration, press temperature, water temperature, and ink temperature. Excessive temperature settings aid inker thermal stability but reduce print quality. The temperature of the ink was found to be the key factor for both inker stability and print quality. The computer simulation results showed that the key point of the inker is the ink feed mechanism. Subsequently, three different inker configurations were simulated that shared a common ink feed mechanism. The results showed a small influence of inker configuration on transient, but a pronounced effect on steady-state variation. The inker with the largest ink storage capacity was found to produce the least variation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:638397 |
| Date | January 2004 |
| Creators | Paikos, I. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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