Prevention of Board Curl

Paperboard curl is one of the major problems faced in many of the offset printing presses.Curl leads to further other problems during corrugation and other post-press operations.The current practice is to press the printed stocks are for a period of about two days using rope and wood, in order to make it flat. This paper aims at providing practical solutions to eliminate the tendency of the paperboard to get curled.

Initially, various factors that lead to the problem were studied followed by the comparative study on coated and the uncoated boards in terms of 'moisture release' and 'water absorbing' abilities of them respectively. Paperboards were conditioned in environmental test chamber at constant temperature (press room temp) and different RH so as to study the behaviour of board. Since drying time and temperature plays a predominant role in dimensional stability of paperboard, dwell time of the paperboard is also determined at different printing speeds. The sudden change in atmospheric condition while drying causes the paperboard to lose its moisture content rapidly, thus causing it to become dimensionally unstable leading to curl.

Lack of unique device to measure curl has driven us to fabricate a warp measurement device to quantify the curl produced in the paperboard. This device consists of a measuring unit (Ultrasonic sensor assembly) and microprocessor controlling unit. The data obtaining using the device enables us to draw a line graph that would give a picture of the pattern of the curl produced across the board.

Based on the experiments conducted and literatures reviewed we arrived at two possible solutions. Paperboards have to be made acclimatized to press conditions before being printed and extended type dryer has to be used to increase the dwell time with corresponding decrease in temperature.