Lint particle dynamics in the offset printing of newsprint

Linting is considered to be a major print quality problem. It is defined as the removal of fibres and fines from the surface of uncoated paper during offset printing. The accumulation of lint on the blanket will eventually reduce the print quality and affect the pressroom efficiency. Although linting has been studied for many years, no measurement methods for characterizing lint have been standardized. Several major factors that contribute to the complexity in lint measurements are the very small amount of lint that is being removed from the paper surface, the interactions between different variables affecting linting and the lack of understanding on the dynamics of linting during printing. This work is focused on four main parts, which explore the dynamic measurement of linting. In the first part, Weibull statistics have been used to characterize the distribution of adhesion strength of lint particles to paper. Static tests were conducted to measure lint dynamics by repeatedly stopping the press during a run, washing and collecting multiple lint samples from different areas of the blanket. The effect of a change in the size distribution of lint particles with increasing number of copies was examined, in order to estimate relative removal from the blanket as a function of lint particle size. From the analysis, the removal rate of lint particles from the paper decreased with the increasing size of particles. Differences in the bond strength of the lint particle size to the paper substrate were suggested to be responsible for the change. The removal rate of particles from the printing blanket was found to be independent of particle size, indicating that both the adhesion force between the particles and blanket and the removal force due to ink tack are proportional to the particle area. The second part of this work presented the initial development of a dynamic lint measurement technique using a high speed video camera, together with image analysis, to measure the accumulation of lint on the blanket. The area distribution of lint measured from the lint camera was compared to the area distribution measured by washing the blanket. The percentage area covered by lint in dynamic images (when the press was running) was larger than in either the static (when the press was halted) images or the filtered samples. The lint particle size distribution from the static images shows similarities to the distribution of the lint removed from the blanket in filtered samples. The differences in the lint area distribution for the dynamic images were due to the blurring of images and agglomeration of particles. These results showed that the system needs to be improved before it could be used for routine quality control tests. The third part developed the lint camera system further so that it can measure lint dynamically and reproducibly with different colour printing inks. The results of the lint camera system were also validated against other measurements of lint. The correlation between the lint camera and the blanket tape pull method was very good, while the lint measured from the washed and filtered sample gave higher percentage area distribution at small particle sizes than the area distribution measured from the lint camera. The differences in the lint area distribution from the filtered sample at small particle sizes, is likely due to disaggregation of the agglomerated lint particles during the washing and filtering, and thus large particles are being measured as more smaller size particles. When the overall lint deposited and removed from the printing blanket are in equilibrium, there was approximately 5% to 6.5% variation around the measured average number of lint particles, from image to image. This was due to random deposit and removal of lint particles. The final part of the work discussed the effects of the paper and of the printing press variables of printing tone and ink tack on the two rates of adhesion failure in linting, by using the lint camera system developed in part three and also by stopping the press repeatedly to wash the lint samples off the blanket. The first failure (k₁) is for the removal of lint particles from the paper surface to the blanket, while the second failure (k₂) is for the separation of the lint particles from the blanket. The key finding of this study was that these two rates of adhesion failure are generally correlated due to the combination forces of the free ink film, porous film flow in the printing nip and the ink film splitting force. The results showed that improved newsprint produced higher lint than standard newsprint for both small and large particles. The lowest tone of 20% had the highest values of k₁ and k₂. The previous contradictory results from the literature of the maximum lint with tone are due to the different number of copies being printed when the measurements were conducted. Both k₁ and k₂ also increased with higher ink tack for small and larger particles. By conducting these studies, the dynamics of linting in offset printing can be better understood. A quality control system has been developed to measure lint migration from the paper or the printing blanket. The system also provides better correlation between paper measurements and the linting performance of paper products. Knowledge of the dynamics of linting will help paper mills and printers to assess whether the accumulation of lint on the blanket is caused by the paper or by the printing conditions.