Use of an on-press acoustic sensor to monitor coldset offset printing of newspaper

A newly developed acoustic emission on-line monitoring technique (Voltaire J. et al. 2004) provides insight into the dynamic interactions occurring between paper, inkfount emulsion, and rubber blanket in offset printing. The technique uses a microphone placed in the vicinity of the exit of the paper-blanket nip. Through digital signal processing of the measured sound pressure it is possible to distinguish between machinery sound and that caused by the tacky ink splitting. In this study, printing of coldset inks on newsprint was carried out on a two-unit sheet-fed offset press. In line with earlier results, an increase in print density was found to correspond to increased sound pressure at the nip exit for higher frequencies (above 15 kHz). Depending on the status of the press at startup, i.e. ink and fount condition and temperature, an increase or decrease towards a more stable value of the sound pressure was detected during the early running of the press, and in both cases this correlated well with the evolution in print density to its target value. This and related results can prove useful for monitoring, and adjusting by feedback, the initial press equilibration before the print-ready stage. Moreover, the acoustic emission can directly reveal destabilised conditions at longer running times that by other means would not be detected until later.