In this article we will discuss common problems encountered by press operators trying to feed thin or lightweight stocks. Modern day printing presses utilize air feeders which use a combination of blown air and vacuum suckers to separate a single sheet from a stack, feed it into a headstop, and ride it against a registration guide as it feeds into the printing press.
Stacks of lightweight paper fluff up more readily than regular paper. It is common for the air blast to cause a hump in the stack before transferring to the in-feed wheels. This causes sheets to arrive at headstops a little later than with heavier sheets. Some press models have sheet guides that can be adjusted over fluffed up sheets to minimize the problem. Advancing feeder timing along with minimizing air blast will help.
Excessive airflow into the stack and blowing too deep into the stack can cause doubles to feed. Adjust your air blast height to only fluff up the top ten sheets. This will help the press feed a single sheet at a time. Sometimes a simple adjustment of air can solve a problem that an operator can struggle with down the line for hours.
It only takes a little bit of a drag on a sheet to cause it to lag before reaching the headstops. Using a 3” strip of paper at the headstops instead of a full size sheet is more effective in picking up subtle drags on the sheet. Drag is often caused by the upper sheet guide just before the headstops.
Pickup suckers can cycle too deep into a stack of previously fluffed up paper, squishing the air out of a fluffed stack and causing doubles. Adjusting your feeder foot to pick paper off the top of the stack can make the difference, but remember that this adjustment changes other previous adjustments.
Proper airflow is critical when feeding lightweight paper through a sheetfed press. One item that is commonly overlooked is the air filter. Reduced airflow can make it difficult to feed paper straight down the registration board. Weaker than normal suction will cause the sucker that is farthest from pump to release early which leads to crooked feeding. Another item that can affect airflow is the air solenoid plunger. A dirt solenoid plunger can stick, resulting in symptoms that mimic a weak pump.
Occasionally adjusting the plate height on the pull guide can be beneficial. However, it’s important to be wary of the repercussions of this adjustment because going the wrong way can result in a timing change of the pull guide release, causing the timing to overlap with the swing grippers.
Static electricity is always prevelant with paper. In lightweight stocks static can much more easily induce feeding problems since a lighter and thinner sheet is that much more likely to move. Your first line of defense against static is to allow your stock to acclimate. This means allowing some time for your stock to adjust to the local environment in your building. Due to time constraints this may not always be an option. Slowing the press down will allow the air blast more time to separate sheets resulting in less kick offs and more productivity.