Trouble Shooting On Press

We all hope is doesn’t happen to us, but if it does, it helps to be prepared. Sappi has put out a great guide to help with some common issues that arise. Here are some things to keep in mind if things go amiss while on press.

Precautionary Measures to Take

· Allow time for paper to acclimate to pressroom environment before opening packaging

· Avoid cutting paper for press any sooner than necessary

· Rewrap cut paper as soon as possible

· Open no more paper than necessary during makeready and prior to approval

· Remove the top and bottom sheets, which may have been damaged during packaging, handling or transit

· Save 16 consecutively numbered printed and unprinted sheets with DEFECT CIRCLED.

· Flip 50 sheets prior to production run to assess side-to-side print consistency

· Cover press loads between passes to help maintain sheet integrity and stability

Hickeys occur when contaminating particles adhere to the plate or blanket, causing either a doughnut effect (small solid printed island surrounded by a white halo) or an unprinted void surrounded by printing.

Suggestions: Dip out ink fountain; drop fountain blade and clean; inspect roller condition; add fresh ink from new can or change inks.

If ink is too tacky, or if the coating is defective, bits of coating/fiber are pulled from the paper’s surface. This material adheres to the blanket and leaves a color 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 colors.

Suggestions: Clean blankets, change contaminated ink, reduce impression cylinder squeeze or reduce ink tack. Try a different production run of paper.

Piling/Tail-edge pick occurs when ink builds up on the blanket until it eventually lifts off a portion of the image or pulls the fibers or coating from the sheet. This can be caused by defective paper coating, wrong ink consistency, inadequate film of fountain solution, improper fountain solution mix, excessive impression cylinder squeeze or blanket wash that renders the blanket sticky.
Suggestions: Try a lower tack or lower set rate of ink, and/or increase fountain solution to plate. If above suggestions do not work, replace the blanket or try a different production run of paper.

Loose dust particles on the paper surface adhere to the blanket, take on ink and print as dark specks, or show up as voids in print. Dust deposits can occur during sheeting or trimming operations.

Suggestions: Predust on impression with a dry, blank unit; inspect all four sides of paper for cut quality; wipe edges with a glycerin or tack cloth; trim paper on all four sides or replace with a different production run of paper.

Smashed Blanket
Sometimes a foreign object or a paper defect can actually smash, or render useless, a blanket or a plate. The only remedy is to clear away whatever has caused the damage, spot-check the remaining paper and replace the blanket or plate.

Scratches/Blade Streaks
Scratches 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 other contaminant becomes lodged between the coating blade and the paper. This cuts a groove into the paper’s coating in the grain direction. Very light scratches can be caused by sheeting equipment.

Suggestions: Isolate and replace affected paper.

Delamination occurs when the paper itself pulls apart during printing. The surface of the paper may appear bubbled or lifted. There can be numerous causes of this problem: high-tack inks, partially dried ink on the rollers/blanket or high impression squeeze. Delamination is not to be confused with blistering, which occurs almost exclusively in the dryer unit of a web press.

Suggestions: Reduce ink tack on the rolls, reduce impression squeeze or try a different production run of paper.

Printing dots do not precisely align, causing a blurred image or color variance; register marks are out of sync. This can be the result of gripper slip, loose blankets, high ink tacks, misalignment on the feed table, bowed/scalloped/wavy sheet edges, quality differences between the consecutive sheets or off-square paper.

Suggestions: Adjust and clean grippers, torque blankets to spec, lower ink tack, readjust feed table, reduce impression squeeze, check sheet for wavy/tight edges and relief-cut blanket packing outside image area. Try a different production run of paper.

This occurs when ink transfers to the back side of the next sheet, sometimes causing the sheets to stick together. Offsetting can be due to insufficient spray powder, delivery pile that is stacked too high, excessive ink film, slow drying ink or paper, or a combination of water/ink balance and
humidity conditions.

Suggestions: Increase or change spray powder, decrease height of the delivery stack, run a stronger pigmented ink with less ink film or adjust ink/water balance.

Mottle occurs when the ink lies unevenly on the sheet, especially in an area of uniform color, or cyan/magenta ink trap, like a blue sky.

Suggestions: Pull single prints to identify problem printing unit(s). For cyan/magenta mottle, reverse ink sequence and tack accordingly. Run tack-graded inks with the highest tack down first. Trap heavier coverage down last and put solid colors in latter-down units. Increase press speed, flip sheets or try a different production run of paper.

Blistering, the occurrence of small bubble-like formations on both sides of the web paper, can occur when trapped moisture vaporizes as the printed paper is passing through the dryer on a web press. Causes vary, but are often traced to excessive dryer temperature, slow press speed and/or high-flash/high-solvent inks, paper moisture or form design. Heavyweight coated paper is more susceptible to blistering.

Suggestions: Increase press speed and/or decrease oven temperature, reduce ink/varnish film or use a lower flash ink so oven temperature can be reduced. Isolate problem rolls by machine reel or roll position. Try a different production run of paper.

Wavy paper
Wavy 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.

Suggestions: Keep the paper wrapped until time of printing and cover loads between passes on press. Maintain proper temperature and humidity in pressroom. Try a different production run of paper.

No comments:

Post a Comment