Misregistration on a web offset press is usually a symptom, not a single fault. Colour drifts out of position because something upstream has changed the length of the paper path or the geometry of the plate and blanket package. On roll fed presses four causes account for most of the cases we see in the field: too much fountain solution, excessive web tension, a badly bent plate, and a mismatch in blanket or packing thickness. Each one leaves a different fingerprint on the sheet, so the first job is to read the print before touching a setting.
1. Too Much Fountain Solution Swells the Web
When the ink and water balance has no fixed reference value, operators tend to open the dampening tap to avoid scumming in the shadow areas. The web takes up more water than it can release, the paper fibres swell, and the printed image grows between units. Register then walks from the first printing unit to the last, and the error changes with the amount of ink coverage on the sheet.
Start with the dampening train. Inspect the fountain brush and replace it when both ends are worn, because an uneven brush delivers more water to the middle of the roller. Check the dampening roller pressure and its straightness, and set pressure evenly across the width. A roller that shows a bend or a flat spot must be replaced, since no pH adjustment will compensate for a roller that is out of round.
Once the mechanical parts are sound, tune the fountain solution itself. Keep pH in the usual 4.8 to 5.5 window and watch conductivity as the run goes on. Aim for the smallest water film that still keeps the plate clean at full speed. On a roll fed press this matters more than on a sheet fed machine, because the web cannot pause while the operator corrects a scummed plate.
2. Web Tension Above the Working Range
High tension flattens the dots and pulls longitudinal creases into the web. The web stretches slightly at every printing nip, and the stretch is not identical from unit to unit, so the image arrives at the folding unit out of register. The finished product also loses flatness and gloss, which customers notice before they notice the register error.
Set tension at the value where the web runs flat and steady at production speed, and hold it there. Watch the tension readout while the press accelerates and brakes; a web that is too loose unwinds unevenly, and a web that is too tight can part at the splice. On a shaftless or braked unwind, adjust the damping torque as the roll diameter falls so the tension stays constant from the first metre to the last.
3. Bending Errors in the Plate
On a multi colour web press the gripper edge and the tail edge must sit square, with the image area centred on the plate. If the bend line is drawn inaccurately, or if the bending is done quickly at the end of a shift, the plate sits skewed in the clamp. The skew cannot be corrected with the register controls on the unit, and the operator ends up chasing the error on every job that uses that set of plates.
Take the plate off, flatten it with a rubber mallet, and align it to the centre line again. Find where the error sits, redraw the line and re-bend. A quicker path is to bend a fresh plate against a good printed sheet, matching the position of the image to the sheet rather than to the old plate.
4. Blanket Thickness, Tightness and Packing
A new blanket and a worn blanket do not transfer ink the same way. A new blanket with more squeeze pushes ink outward at the edge of each dot, and the dot gains size. An old blanket that has lost its tightness lets the dot slip, and the image loses sharpness. Packing thickness that differs from unit to unit changes the effective print length, so colours that were in register at make ready drift apart as the run continues.
Keep packing thickness uniform across all units and record it. Check the tightness of older blankets at every wash up rather than only when a job fails. When a blanket is replaced, reduce the packing under the new blanket so the total thickness matches the unit it sits in, then recheck register on a test sheet.
A Short Diagnostic Order
Print a test sheet with register marks and colour bars, then compare the error on each unit. A steady error across the whole sheet points to plate bending. An error that grows along the web points to tension. An error that changes with ink coverage points to water. An error that appears on one unit only points to blanket or packing. Working in that order saves time at the console, and it keeps the operator from adjusting register controls that were never the cause.