Page 20 - Demo

ON-PRESS TROUBLESHOOTINGRecommended Pressroom EnvironmentIt is important to monitor relative humidity and temperature. The ambient temperature and humidity of the pressroom may be too high or too low, which may affect ink tack and drying and cause static, tight or wavy edges, paper picking and delamination.Ideal pressroom climate control is around 52% (%u00b1 5%) RH at 21%u00b0C.%u2022 Paper will acclimate on pallets and in reams. Do not open until going to press.%u2022 Paper acclimation time is relative to environmental extremes. The industry accepted best practice is 24-48 hours, or even longer depending on temperature differential and volume of paper.%u2022 Properly conditioned paper runs with a broader operating window on press. Cold paper in a warm pressroom is prone to condensation, which can lead to wavy edges. Cold paper also has greater susceptibility to picking, delamination and slow ink dry.Picking / ContaminationIf ink is too tacky or if the coating is defective, bits of coating / fiber are pulled from the papers surface. This material adheres to the blanket and leaves a colour void, or surface crater, in the printed sheet where the pick-out first occurred. Subsequent sheets show partial filling, or may continue to show absence of one or more colours.Suggestions: Clean blankets, change contaminated ink, reduce impression cylinder squeeze or reduce ink tack. Try a different production run of paper.Scratches / Blade StreaksScratches or blade streaks are generally of short duration and are usually isolated to a small portion of the paper order.They sometimes occur during the coating operation in the paper mill when a particle of grit or contaminant becomeslodged between the coating blade and the paper. This cuts a groove into the papers coating in the grain direction. Very light scratches can be caused by sheeting equipment.DelaminationDelamination is not to be confused with blistering, which occurs almost exclusively in the dryer unit of a web press.Suggestion: Reduce ink tack on the rolls. Reduce impression squeeze or try a different production run of paper.MottleMottle occurs when the ink lies unevenly on the sheet especially in an area of uniform colour, or cyan/magenta ink trap, like a blue sky.Suggestion: Pull single prints to identify problem printing unit(s). For cyan/magenta mottle, reverse the ink sequence and tack accordingly. Run tack-graded inks with the highest tack down first. Tap heavier coverage down last and put solid colours in latter-down units. Increase press speed, flip sheets or try a different production run of paper.Wavy PaperWavy paper can cause problems with wrinkles, misregistration, dot slur or the aesthetics of the final printed sheet. This condition can be aggravated if the paper is exposed to extreme temperature or humidity changes.Suggestion: Keep the paper wrapped until time of printing and cover loads between passes on press. Maintain proper temperature and humidity in the pressroom. Try a different production run of paper.Climate and PaperEspecially in periods of critical climate circumstances printers are advised to follow these guidelines:%u2022 Paper is not an efficient heat conductor. Therefore, allow for sufficient time to let the paper adapt itself to the temperature in the workshop.%u2022 Do not open the paper wrapping until printing is about to begin. The wrapping protects the paper from fluctuations in temperature and humidity.%u2022 Infrared drying, which drastically reduces the relative humidity of the paper, should be used sparingly.%u2022 During drying, the paper should not be exposed to extremely low temperatures, as this would significantly extend drying times.%u2022 Avoid damaging the paper wrapping and carefully re-wrap remaining pallets.Folding and CreasingHow can the pressroom avoid coating cracking?%u2022 Do not print dark areas or images over the fold.%u2022 Always make the final fold parallel to the grain.%u2022 Set the pressing force of the folding rollers exactly to the thickness of the folded product.%u2022 The relative air humidity in the finishing and storage rooms should be about 50%.%u2022 Avoid using UV varnish in the folding area if possible, since this can make the paper more brittle.%u2022 Thorough creasing should take place before printing across the fold in basis-weights of 170g/m2 and over.20