An uneven delivery pile shows up long before anyone counts the sheets. The stack leans to one side, corners arrive turned over, and the jogging blades scrape across the top sheet instead of guiding it. In the bindery the same stack has to be re-sorted by hand, and the count for the run stops matching the count on the job ticket. A sheet-fed press delivery has four simple demands: the sheet lands loosely, settles square, builds a flat pile, and keeps its gripper and tail edges facing the same way on every sheet. When one of those four fails, the cause is usually found in one of the eight areas below.
Too Much Water on the Plate Curls the Sheet
Paper stays flat only while its moisture content sits near the level it was conditioned to. When the plate carries more water than the sheet can absorb evenly, the fibres on the printed side swell and the sheet curls, usually toward the blanket. A curled sheet has no stiffness left at the tail, so the side and back joggers push it around rather than squaring it. Run the press with the least water that keeps the plate clean, and adjust the dampening feed to match how much image area sits on each side of the sheet. A plate with heavy coverage down one edge needs less water on that edge, not more everywhere.
Heavy Solid Areas and Wet Ink
Large solids put down a thick, tacky film that has not set by the time the sheet reaches the pile. The fresh ink layer keeps the sheet from lying flat, and on lighter stock the tail dips downward as the sheet drops. Slow the press slightly on full-coverage work, reduce ink viscosity within the limits the colour still needs, and review the ink sequence so a heavy solid is not the last film down. Fitting a rubber band or a strip of gauze across the jogger blades gives a curled tail something soft to land against, which often squares the pile without touching the press settings.
Jogger Blades Set Wider Than the Sheet
If the side or back stops are set for a larger format than the one on the press, the sheet has room to drift and never gets squared. Set the blades to the sheet size or a millimetre or two more, no further. Check the cam that swings the blades as well: when the swing timing is out of step with the gripper release, the sheet meets a blade on its way in and lands crooked. Time the blade swing to the moment the grippers let go, then the sheet falls into a gap that is already open.
The Sheet Brake Suction Wheels
Sheets that kick upward as they enter the pile point to the sheet brake rather than the joggers. The suction wheels are either clogged with paper dust or running too weak to hold the tail down. Pull the wheel, clear the small holes in its rim, wash off the dust and ink mist, and set the suction high enough that the tail stays flat through the drop. A wheel that has lost its grip lets the tail lift, and the next sheet lands on top of a raised edge.
Gripper Release Timing on the Chain Delivery
Chain delivery grippers that open too early drop the sheet short of the pile; ones that open too late carry it past the back stop and let it snap back. Either way the sheet enters at an angle. Set the release so the sheet is fully free just above the pile, and check the setting on both gripper bars, since a bent gripper finger on one side produces the same crooked landing as a mistimed cam.
Stock Thinner Than the Press Handles Well
Slim stock below about 52 gsm gives the delivery almost nothing to work with. After the sheet passes between the blanket and impression cylinders it picks up water through the blanket, goes limp, and folds or drifts on the way down. Offset work usually runs at quarter-sheet size or larger, so the unsupported span is long. Keep job stock at 52 gsm or above where the job allows it, and when a lighter sheet has to run, reduce water, slow down, and add a light air blast to hold the tail flat.
Flatness, Formation and Wavy Edges
Paper that arrives with wavy edges, a bowed centre or uneven formation will not stack flat no matter how well the delivery is set. Condition the stock before printing: hang it in the pressroom, then stack it in the warehouse or the pressroom under clamp boards weighted with a heavy plate so the sheets settle flat. Stock other than coated art paper benefits from a light knock-up and riffle to relieve the stresses left from the mill reel. On carton board, the same conditioning step matters twice over because a bowed board also feeds badly and prints with uneven pressure.
Dry Air and Static in the Pressroom
In winter, low relative humidity pulls moisture out of the sheet, and dry paper holds static charge. Sheets then cling to each other and to the delivery frame, so they drop in clumps and land out of square. Bring the room back to a normal humidity range, seal the windows rather than opening them for air, and use an ionising bar or static eliminator at the delivery. Sprinkling water on the floor is a rough but effective stopgap when a humidifier is not available. A hygrometer on the wall is worth more than guesswork here, since the same complaint returns every cold snap.
Most delivery faults clear once the cause is isolated, and the eight checks above cover the common ones. Start with water and ink, because a curled or tacky sheet defeats every mechanical adjustment downstream. Then confirm the blades and the brake, and finish with stock conditioning and room humidity. A pile that lands square saves the bindery a full shift of re-sorting, and it keeps the printed count honest.