Board Moisture Testing in the Corrugator Lab

A corrugator quality lab earns its keep on moisture before it earns it on crush. Wet liner, dry medium, and a finished sheet that leaves the double backer uneven will show up later as warp, soft bond, or a stack that will not stay flat in converting. Plants that only squeeze a sheet by hand usually discover the problem after the order is strapped. A written moisture method, with a named instrument and a named sample point, turns that argument into a number the next shift can repeat. If the sample sits too long between cutting and weighing, write the wait down: the bench can pick up or lose moisture and the next oven check will not match the handheld reading from the shift. The method sheet names the longest allowed time from sample to scale, and a sample past that time is discarded and taken again. A hand squeeze only finds an obvious wet streak. It does not replace the lab number. Count out-of-window readings on the shift report. When they repeat, check the reel store and the preheaters before anyone changes adhesive grade.

Sample three materials, not one. Incoming liner, incoming medium, and finished board answer different questions. The rolls tell you what the mill and the warehouse delivered. The finished sheet tells you what the preheaters, the adhesive, and the hot plates did with that paper. A lab that records only finished-board moisture cannot say whether the warp started in the reel or on the line. Mark each sample with roll or order number, machine side, and the time it was cut. Bag liner, medium, and finished board separately. Do not stack them in one bag for the lab, or the lab cannot say which layer brought the moisture. The layer on the request must match the bag label. A sample that does not match does not enter the shift’s release record. A sample with no layer named cannot be used to decide where warp started.

Take left, centre, and right on the same sheet, and do not average them into one number before you look. A cross-web spread larger than about 1.5 percentage points on one sheet is a process flag even when the average sits inside the plant window. Operator side and drive side should be named on the card, because “left” changes when someone stands at the other end of the machine. On a stack, pull the top sheet, a sheet from the middle, and a sheet near the bottom. A wet bottom and a dry top is a storage or cooling problem, not a glue-gap problem. Space left, centre, and right across the web. Do not cluster them inside the middle half-metre, or the cross-web spread is made to look smaller than it is. Keep the distance from each point to the edge fixed, so the next shift does not move the spots. Once the positions move, the spread cannot be compared with the previous shift.

Many plants hold incoming liner and medium near 6–9% moisture and treat a reading outside that band as a hold, not as a number to argue with on the bridge. The exact window belongs on the plant’s paper specification. A liner-to-medium gap above about 2 percentage points is a warp flag worth stopping for, even when both reels are inside their own limits. The drier web shrinks differently from the wetter web after the hot plates, and the sheet curls toward the drier side. Record the gap on the same card as the warp note in corrugated-board-warping-prevention. After a reel is held, write the retest time and the moisture that released it. A verbal “run it” from the bridge is not the record. Warp on a reel that went up without a retest is not blamed on the adhesive first. The release moisture and the time it went on the stand must point at the same reel number.

Use an oven dry as the reference method, and treat handheld meters as the shift tool. A practical oven check uses about 50 g of paper or board, dried near 105°C until the weight stops changing, then calculates moisture from the weight lost. Name the dish, the balance, and the temperature on the method sheet so the night shift does not invent a hot-plate shortcut. Do not dry the sample on a corrugator hot plate or in a glue-kitchen oven that also holds adhesive. Those surfaces are not a lab, and the number will not match the next test. Keep the weight before drying and the weight after. A final percentage alone cannot show whether the scale or the arithmetic was wrong. Write the time of both weighings so you can see whether the sample had reached constant weight. Do not pull the sample and report a number while the weight is still falling. A percentage that is not at constant weight does not go on the release record, and it is not used to check the handheld meter.

Pin meters and pinless meters do not read the same zone. A pin meter stabs a small spot and is easy to bias if the pins hit a glue line, a printed patch, or a crushed flute. A pinless meter reads a surface band and will follow coatings, wet streaks, and the liner you happen to face upward. Label the method on every row. A chart that mixes pin readings from Monday with pinless readings from Tuesday will show a moisture swing that the paper never had. Check the handheld meter against the oven at least once a week, and again after a battery change, a drop, or a new paper grade. Set the allowed gap for coated paper and for uncoated liner apart. Do not release one grade on the gap that belongs to the other. Until a new grade has passed that check, release it on the oven number only.

Split process control from customer specification. For process control, cut the finished-board sample within about 15 minutes of the sheet leaving the double backer, before the stack has time to equalize in the aisle. That number tells the crew whether this order is drifting. For a customer moisture specification, condition the retained sample in the lab climate you have named — many plants use a waiting period of about 4–24 hours — and test it again. Do not ship the 15-minute number as the customer result. The two readings answer two questions, and substituting one for the other creates claims. Process samples and customer retains go in separate columns. Do not later rewrite a retained result as if it had been taken as the sheet left the machine. An order with both columns blank does not ship as moisture-passed.

Adhesive cannot correct a wet reel. If the medium is far above the plant window, adding glue, raising viscosity, or opening the glue gap usually makes a wetter bond line and a slower dry. If the liner is far below the window, the sheet can look bonded on the line and then warp as it cools. Hold the paper, slow the order, or change the preheater wrap before you rewrite the starch recipe. Application notes on /products/corrugated-board-glue are a discussion input after the moisture card is current, not a substitute for the card. The same rule applies to concentrate powders and water-only corrugator powders: their mix ratios do not absorb a 3-point moisture error. When moisture is already outside the window, write the reel number before the shift debates a formula change, so the note is not filled in afterwards. A moisture card with no reel number cannot explain a later warp complaint.

Preheaters and hot plates move moisture; they do not erase a bad reel. Record wrap and steam or electrical heat on the same sheet as the moisture readings when a cross-web wedge appears. A dry edge and a wet centre often track a wrap that does not cover the web, or a hot plate that is hotter on one side. Fix the heat pattern before you chase adhesive penetration. Incoming roll storage, warehouse time, and how long a reel sits on the mill-roll stand are covered in base-paper-moisture-management-corrugator. The lab test is the check that those steps worked. After a heat or wrap change, retest the cross-web moisture on the next good sheet and confirm the dry-edge, wet-centre wedge is gone. If the wedge is still there, do not cover it by adding adhesive.

Recycled and high-filler sheets swing more than a stable virgin liner. Sample them more often: at the start of the reel, after a splice, and again if the bridge crew reports a soft bond or a sudden warp. A single reading at the outside of the reel misses the wet centre that arrives an hour later. If two reels of the same nominal grade differ by more than about 2 percentage points, do not run them as if they were one item. Split the order or reset preheat between reels, and write the reason on the moisture card. Keep a row before the splice and a row after it, so you can tell a wet core inside the reel from a tension or stop at the splice. If only the row after the splice goes bad, check the splice before you scrap the whole reel.

Write the result so another shift can audit it. Each row needs the order number, reel number, grade, test method, instrument identity, sample position, moisture percent to one decimal, tester, and time. A pass/fail stamp without the number is not a lab record. Keep the oven worksheet with the handheld log for that week. When a customer asks why a stack warped, the card should show the liner, the medium, the cross-web spread, and the finished sheet, not a recollection that “the paper felt normal.” A row missing the order number or the instrument identity is unfinished. A pass stamp added later does not release that board. The stamp does not replace the moisture number.

Train the test, not only the button. New lab staff should run one oven check and one handheld check on the same sample under supervision, and the two results should sit close enough that the plant has defined the allowed gap — often within about 0.5–1.0 percentage point once the meter is set for that grade. If the gap is wider, the meter is not released for that grade. Show people a wet streak, a glue-line false high, and a coated liner false low, so they stop testing the convenient spot in the middle of a clean patch. When the gap is outside the allowed band, handheld data for that grade is paused until the oven check passes again. While it is paused, the shift report for that grade records the oven result.

Xuegong New Materials Group can help a plant connect moisture windows with adhesive choice once the lab card is real. Final test methods, customer specifications, and instrument calibration stay with the plant. Use base-paper-moisture-management-corrugator for reel storage and bridge practice, and corrugated-board-warping-prevention when the sheet will not lie flat. Close both cards with a moisture number, so a glue change is never the first answer to a wet or unbalanced reel. A glue suggestion with no moisture number is not a reason to release finished board on that shift. Write the trial result back onto the same moisture card so the next shift can follow it. If the card is missing, do not write the order up as moisture already under control. The oncoming shift reads the card before it keeps the previous shift’s glue.