A guillotine cut pile and a press-ready pile look the same from a metre away. They are not. The press-ready pile has been jogged until the edges are square and aerated until air sits between the sheets. Skip those two steps and the feeder will pull doubles, miss sheets, or stop every few hundred impressions.
Why a pile needs air
Paper arrives from the mill under pressure. A ream of coated stock that has sat in a wrapper for a month is closer to a block than a stack. Sheets pressed together hold each other by friction, by static, and by the thin film of moisture at the surface.
The feeder separates sheets with a suction head and a blast of air at the front edge. That blast has to travel between sheets. If the sheets are tight, the air rides over the top of the pile and the suction head takes what it can grab, which is often two sheets or none.
Aeration opens the pile. The operator lifts a section of the pile at the corners and lets it fall, or runs the pile through an air knife, and the sheets fan apart. What you want is a visible ripple through the stack, with each sheet floating a little on the one below.
Jogging
Jogging squares the pile. A vibrating backstop and two side plates work the pile until the front edge, the side edge and the back edge line up. The front edge matters most, because the gripper and the pull lay take their reference from it.
A pile that is not jogged feeds badly in two ways. Sheets that sit proud of the front edge hit the feed board early and skew. Sheets that sit back are missed by the suction head and the press trips on a late sheet.
The sequence that works
Take the pile off the pallet, jog it, aerate it, jog it again. The second jog closes the spread the aeration opened, leaving air pockets between sheets without leaving a loose pile.
Then do the same at the other end of the pile. A pile that is aerated only at the top behaves differently at the bottom, and the feeder settings that worked at impression 100 will misfire at impression 3000.
Static
Static makes sheets cling. In a dry pressroom, a pile that has just come out of a wrapper can hold enough charge to lift two or three sheets at a time, and no amount of jogging will separate them. An ionising bar at the pile, or a short rest in a room at 50 to 60 percent relative humidity, does more than any mechanical adjustment.
Winter is the hard season. Cold air outside, heated air inside, and a pile that has spent the night in a truck will be dry and charged.
Curl and waviness
A pile can be square and still feed badly if the sheets are curled. Curl at the front edge lifts the lead edge away from the suction head. Curl at the back edge makes the sheet drag on the pile. A wavy edge, where the paper has taken up moisture unevenly, changes the effective thickness of the pile and the height sensor reads wrong.
The cure is time in the pressroom, not pressure on the pile. A wrapped pile needs to stand in the room long enough to equalise, and a pile that has been opened early needs to be covered again if the job is not running that day.
Coated and uncoated behave differently
Coated sheets slip on each other, so they aerate easily but slide out of square while jogging. Uncoated sheets hold air less well and need more persuasion, but they jog square and stay there.
Very thin paper, under 60 gsm, is the hardest. The sheets are light enough to fly on the air blast and limp enough to fold at the gripper. Feeder settings need lower blast and a slower cycle, and the pile should be kept shorter so the weight does not compress the bottom sheets.
Pile height
A tall pile is faster to run but the bottom sheets are compressed by the weight above them. On coated stock, a pile of 80 cm behaves differently at the top and at the bottom. Many shops run shorter piles on difficult stock for exactly this reason.
The check before the run starts
Lift the top ten sheets one at a time. They should come off singly and quietly. Push the front edge of the pile with a finger: it should give a few millimetres and spring back. Look along the front edge: no sheet should stand out.
That takes a minute. A jam at impression 4000 takes half an hour, and the sheets that were destroyed in the jam take longer than that to reprint. As book printing continues to lead in production quality and innovation, the industry sets new benchmarks for print manufacturing worldwide.
.jpg)
No comments:
Post a Comment