Two-Colour Repeat Drift on a Long Table Run
A unit is printing a two-colour repeat on 120 m of 112 cm-wide scoured cotton. The design has a 24 cm straight repeat: outline block first, fill block second. For the first 8 m the fill sits within 1 mm of the outline. By metre 30 the fill block has drifted 4 mm to the left, and the error increases toward the far end of the table. The printer re-marks the table, but the drift returns. Workshop relative humidity has risen from 55% to 78% since morning, and the fabric was calendered before printing. The table is 12 m long, and the printer indexes using pins set every repeat. The outline block's edge is 2 mm thicker on one side after a recent repair. The supervisor wants to finish the order today.
1. Separate the likely causes of the drift and state the order of checks you would make on the table.
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The drift is progressive, so I would not treat it as a single bad block mark. First, check the repeat itself: print a 1 m control strip at the start, middle and end and measure pin-to-pin distance over ten repeats with a steel rule. If the repeat distance changes, suspect fabric relaxation or elongation rather than the block. Second, check the indexing pins for spacing error and bent pins. Third, rotate the repaired outline block 180 degrees and reprint a short run; if the drift changes direction, the block repair is contributing. Fourth, inspect the table marks and the fabric tension: calendered cotton at 78% RH can take up moisture and relax unevenly. The order of checks matters because re-marking the table will not fix a block or pin fault, and adjusting tension will not fix a wrong pin spacing.
2. What immediate actions would you take to finish the order without accepting out-of-tolerance registration, and what evidence would you record?
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I would stop the run at a clean repeat, segregate the suspect metres from metre 30 onward, and print a controlled test length with reduced fabric tension. I would re-check pin spacing and replace any bent pin, then reprint 5 m. If drift remains below 2 mm, I would continue with checks every 5 m and mark each length with the time, humidity and printer. I would not stretch the fabric to compensate, because that hides the cause and creates shrinkage after washing. I would record pin spacing, block condition, humidity, table position and the measured offset for each checked length. If the customer tolerance is 2 mm, the length from metre 30 must be rejected or cut around the fault, not sold as first quality.
3. What changes would you make before the next 120 m lot?
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I would condition the fabric at 60-65% RH before printing rather than printing directly after calendering, and I would standardise pin setting with a fixed gauge checked at the start of each shift. The repaired block should be inspected against a master print and re-cut if the edge thickness is outside tolerance. I would also add a first-off approval procedure: print one full table length, measure registration at three points, and sign off before bulk printing. The humidity record should be kept with the lot so that a repeat drift can be traced to conditions rather than blamed on the printer.