Measurement of local variation of ink tack dynamics

Backtrap mottle is a common printing problem and is linked to a nonuniform ink tack development during multicolor offset printing. A novel method and device are proposed here to quantify the point-to-point variation in the ra te of ink tack development at the millimetre length scale. This device meas ures the force required to pull a small inked probe off the paper surface a nd quantities the rate of tack buildup and its local variation during the p rocess of ink setting An empirical equation is found to describe the time-t ack force curve. Coated papers with different degrees of backtrap mottle nl e differentiated by the point-to-point variation of the initial time-tack s lope. A I. I mm probe is found to give results that correlate well with the subjective rating of backtrap mottle. Other standard laboratory tests are found to riot correlate with backtrap mottle problems for the samples studi ed This method and device are a new way to screen papers immediately after production quality problems.