A slit line that moves towards the printed image is often treated as a knife-setting problem. That is only one possibility. The printed image may be wandering against the web edge, a splice may have entered out of line, or the edge guide may be correcting an unstable reference. Moving the knives before separating these conditions can produce one good roll and leave the real fault in place.
Measure two references, not one finished roll
Keep sequential samples from before, during and after the movement. On each sample, measure from the web edge to a fixed printed feature and from that printed feature to the slit edge. Mark the operator side, roll direction, lane number and approximate roll position. This shows whether the print moved inside the incoming web or the slit moved against a stable print.
Use the approved measuring method and the same reference points on every sample. A ruler placed against a soft roll edge is not enough for a narrow tolerance. Record the instrument, units and acceptance limits from the job specification. If the customer controls label position from the liner edge, measure that relationship directly rather than relying on artwork-centre estimates.
Map all lanes across the web. If every slit shifts by the same amount, investigate the incoming web path and common knife carrier before adjusting individual knives. If one lane alone changes while the others remain stable, inspect that knife position and local web condition. The pattern decides where the next check belongs.
Prove whether the printed master roll is already wandering
Unwind enough of the affected master roll under controlled conditions and trend print-to-edge distance through the fault. Note any sudden step, slow drift or repeated movement. A sudden step near a splice needs a different response from a gradual change over several hundred metres.
Check whether the print, die-cut or register mark was held to the same web edge on the upstream process. A web edge is not automatically a print reference. If the press guided to one edge while the slitter guides to the other, variation in web width or edge quality can change the final slit-to-print relationship even when both machines appear stable.
Do not use the finishing machine to hide a printed-web error without documenting the decision. If the print position varies beyond the job tolerance, preserve samples and the roll map for production review. A web guide can align an edge or line chosen by the system, but it cannot make a wandering image constant against both edges at once.
Treat the splice as a measured part of the web
The Flexographic Technical Association notes that a slight shift in the incoming web can put slit edges out of alignment with the printed image. Its slitter-rewinder guide also explains that a splice table helps the operator keep the expiring roll and new material aligned and prepare a smooth, low-profile splice.
Inspect both ends of every splice that passes through the affected section. Record edge offset, overlap, tape width, thickness and whether the joint pulls square under tension. A splice that enters at an angle can create a correction at the web guide and a short section of off-position slit even when the knives have not moved.
Reproduce the plant's approved splice on trial material if the fault repeatedly follows joins. Run it at setup speed first, then at the normal production condition while watching the edge-guide response and web position at the knives. Do not reduce the tension or disable guiding only to make one difficult splice pass unless that changed condition is approved and recorded.
Check what the edge guide is actually following
Identify whether the machine is guiding to the left edge, right edge, a printed line or another reference. Clean and inspect the sensor by the equipment instructions, then record its position, reference value and correction trend. Check that the selected reference matches the job specification and the way the master roll was produced.
PRATI's digital finishing configurator lists an ultrasonic edge guide before the buffer for web realignment. That location matters when diagnosing a fault. Record where the sensor sits relative to the unwind, buffer, slitting unit and inspection system, because web movement introduced after the sensor may not appear in the guide reading.
Watch the guide through acceleration, steady running, a splice and a planned stop. Frequent full corrections, hunting or a sudden position change are evidence to investigate. Do not increase guide response only because the slit line looks unstable. First confirm sensor reference, web tension, roll centring, roller condition and the position of the printed image against the selected guide reference.
Inspect the knives only after the web reference is stable
With the web held to a proven reference, stop and isolate the machine according to its safety procedure. Check the knife-position scale or digital setting, carrier lock, cassette seating and any spacer or shaft arrangement. Compare the actual lane widths with the approved slit map and record every position before making a correction.
Inspect the blade and mating component for damage, contamination and side loading. A poor edge can make the measured slit look inconsistent, but it does not explain a common sideways movement across all lanes. Keep edge-quality diagnosis separate from lateral-position diagnosis so that a blade change is not credited with fixing a web-alignment problem.
After adjustment, make one controlled change at a time. Produce sequential samples at setup speed and normal speed, and measure the same print-to-edge and print-to-slit references used at the start. If both values remain stable through the trial, the correction has evidence behind it. A single sample taken immediately after threading does not prove the full roll.
Release the job with a roll map and downstream checks
Confirm each finished roll against the specified slit width, image position, rewind direction, core size, count and roll build. Labels and Labeling explains that slitting divides the web into narrower rolls for the following process. The final check therefore has to use the requirement of the applicator or next converting stage, not only the slitter display.
Keep a short roll map showing where the fault began, where any splice passed, what adjustment was made and which finished rolls contain the affected section. This prevents a mixed group of rolls from being released on the assumption that the final roll represents the entire master roll.
Royal Coat represents PRATI inspection, slitting, rewinding and finishing technology across India, Bangladesh, Sri Lanka, Nigeria and Kenya. For a slit-register review, send the Royal Coat team the printed master roll width, slit map, lane tolerances, guide reference, sensor location, unwind and rewind direction, tension settings, speed, splice photographs and sequential samples showing where the movement begins.
Questions from production and purchasing teams
What to ask about slitting register and web alignment
Should the slitter knives be moved as soon as a slit line approaches the print?
No. First measure print-to-web-edge and print-to-slit distance on sequential samples. The result shows whether the incoming print, web alignment or knife position has moved.
Can a splice cause a short section of slit misalignment?
Yes. An angled or laterally offset splice can make the edge guide correct sharply and move the web at the knives even when the knife positions remain fixed.
Which edge should the finishing machine follow?
Use the reference defined for the job and consistent with the upstream process. Guiding to the opposite edge can expose web-width variation as slit-to-print movement.
Why record the edge-guide sensor location?
Movement introduced after the sensor may not appear in its reading. The sensor position relative to the buffer, rollers and slitting unit is part of the diagnosis.
What proves that the slit-position correction is stable?
Sequential measured samples at setup and production speed, including a splice and stop-start where relevant, should hold both the print-to-edge and print-to-slit requirements.