Acoustic investigation of cavitation noise from offset ink film splitting

The acoustic signal from the nip during film splitting between inked rollers and on ink transfer to paper was investigated on laboratory scale, using two sheet-fed offset inks (based on mineral oil and vegetable oil) and two coated fine papers (glossy and matte). Film splitting emits a broad noise-like acoustic spectrum in the range 5-50 kHz, from which the two simplest measures of average power and average frequency were derived. Using these two measures, four characteristic regimes of film splitting on the rollers could be distinguished as a function of ink load. Moreover, for intermediate amounts, average power was found to be accurately predicted by a simplistic model of sound produced by cavity expansion due to the under-pressure in the nip exit, thus providing a physical interpretation of the nip noise emission. For printing, the average power was higher for the glossy paper than the matte, presumably due to a more efficient sealing of the nip, or onset of ink setting. The vegetable oil-based ink gave higher power than its mineral oil counterpart. These results were found to be consistent with longer-time measurements of tack evolution from the Ink Surface Interaction Tester. This correlation and the verified theory thus provide support to practical applications of acoustic emission as an on-press monitoring tool.