Humidity is the variable that explains most of the machine problems that appear without anything having been changed. The settings are the same, the board supplier is the same, the operator is the same, and the machine no longer runs as it did three weeks ago.

Relative humidity changes the board. The board determines how it picks, folds, glues and holds its shape. Nothing on the machine has changed, and everything that matters has.

The Short Answer

Corrugated board equilibrates to ambient humidity, and its moisture content moves roughly between 5 per cent at 30 per cent RH and 12 per cent at 80 per cent RH. Over that range, board stiffness, fold behaviour, glue wetting and squareness all change measurably.

The practical bands are these. Below 35 per cent RH board is brittle, cracks at the score, generates static and resists folding. Between 40 and 60 per cent RH board behaves as the machine was set up to expect. Above 65 per cent RH board is soft, folds where stress takes it rather than where the score directs it, takes glue poorly, and loses compression strength.

The correct response is not to chase the machine but to control the room, and then to hold two or three parameters that need to change with the season.

How Board Moisture Content Changes with Humidity

Board is hygroscopic: it absorbs and releases moisture until its internal moisture content reaches equilibrium with the surrounding air. The relationship is not linear, and it is not the same for absorption and desorption.

The practical consequence is that board which has been stored in a dry area and then moved to a humid area takes hours to equilibrate. A pallet moved from a dry warehouse into a humid packaging hall is still dry inside when it reaches the machine. This is why humidity problems often appear after a pallet change, at the start of a shift, or on the first day after a delivery.

What Changes and What to Adjust

Pick-Up and Vacuum

Dry board is more rigid but also more prone to static, which can hold a second blank against the first and cause double feeds. Humid board is flexible and can deflect into the suction cup throat, reducing the sealing area.

Adjustment for dry conditions: verify that the vacuum is not excessive — a higher vacuum pulls harder on a brittle blank and increases the double-feed tendency. Check the static control measures on the machine.

Adjustment for humid conditions: check that cups are not being bridged by a deflecting blank. Where the blank is flexible, a smaller-diameter, firmer cup holds better than a large soft one.

Fold Behaviour

Dry board cracks along the score line when folded; humid board folds but not necessarily where intended, because soft board yields to the direction of least resistance.

Adjustment for dry conditions: reduce fold speed and, where the machine allows, reduce fold rail pressure. A dry score will crack rather than yield, and a slower fold gives the board time to bend at the score.

Adjustment for humid conditions: tighten the fold rail gap slightly so the fold is forced at the score rather than released to the stress path. Check fold rail gap against specification — the usual error in humid weather is a gap that is too loose.

Glue and Adhesion

Glue adhesion degrades at both extremes. Very dry board has a surface that resists wetting; very humid board has a moisture film that forms a barrier between the adhesive and the board fibres.

Adjustment for dry conditions: raise the glue application temperature within the safe window, and reduce the open distance so the adhesive reaches the board before it loses its heat.

Adjustment for humid conditions: verify that the adhesive is specified for high-humidity conditions. Many standard hot-melts lose bond strength above roughly 70 per cent RH. Where the machine cannot change adhesive, increasing compression dwell by 20 to 40 per cent partially compensates.

Squareness

Humid board is soft, and soft board deforms under the forming force rather than resisting it. On a machine set for dry board, humid board produces cases that are out of square in a consistent direction, because the board yields where the force is greatest.

Adjustment for humid conditions: increase compression dwell, and verify that the fold rails are holding the geometry rather than relying on the board’s own stiffness.

Compression Strength and Stacking

Board compression strength falls as moisture content rises — a substantial proportion of the dry strength is lost between 50 and 80 per cent RH. This is not a machine issue, but it is a pallet issue, and it should be checked against the stacking height and the storage time.

Room Control Measures

The measures below are listed in order of cost effectiveness.

Measure the room before adjusting anything. A humidity logger costs little and answers in a week what years of guessing will not. Record RH at the machine, not at the wall. Board handling criteria are set out in more detail in our support section.

Keep board off the floor and away from external walls. The floor is the largest moisture source in most halls, and an external wall in winter is a condensation surface.

Allow equilibration time. Where board is stored in a different environment from the packaging hall, allow at least one shift, and preferably 24 hours, for it to equilibrate before use.

Control the hall between 45 and 55 per cent RH where possible. This is the band in which the machine’s settings are most likely to be correct all year.

Where control is impossible, hold two setting sheets. One for the dry season and one for the humid season. This is the cheapest available adaptation and it is almost never done.

In a cold room or freezer, expect the board to gain moisture. Cold rooms are humid, and board equilibrates at a higher moisture content there than in the dry ambient hall it came from. Settings validated at ambient will not transfer, and machines intended for cold duty need cold-rated belts, cups and adhesive from the outset — see the product range for which configurations carry those options.

Settings that Should Change with the Season

Alarms and Humidity

E-41 Flap Detect Missing clustering at the change of season is the classic signature of a board moisture problem. A flap that folds and then springs back, or that folds at a different place than the score, is behaving as its moisture content dictates rather than as the rail directs.

Maintenance Rhythm

Every shift — check the room RH reading at the machine, check the magazine stack for warp, and verify glue position on a sample case.

Every week — review the humidity log against the previous week, inspect cups for glazing, and check fold rail gaps against specification rather than against last month’s setting.

Every month — measure board moisture at goods-in and compare with room RH, measure squareness on five cases, and verify glue quantity per case.

Every quarter — review the season’s humidity range against machine settings, and adjust the setting sheets if a seasonal adjustment has proven necessary.

Annually — review whether one setting sheet or two is appropriate for the climate, verify that the glue grade is still suited to the humidity range actually experienced, and re-baseline settings against a known reference batch of board.

FAQ

Why does the machine behave differently in summer and winter?
Because the board’s moisture content follows ambient humidity, and board moisture determines fold behaviour, glue adhesion and squareness. The machine is the same; the board is not.

Should I adjust the machine or control the room?
Control the room where possible — it is cheaper and it fixes several problems at once. Adjust the machine only to adapt to conditions that cannot be controlled.

Can I run the same settings all year?
In a climate with a stable controlled environment, yes. In a hall where humidity swings from 35 to 75 per cent, no, and the plant should maintain two setting sheets.

Does humidity affect board strength?
Yes, substantially. Compression strength falls as moisture content rises, which matters for stacking height and for storage duration even when the erecting machine runs perfectly.

What is the most useful instrument to buy?
A humidity and temperature logger placed at the machine. It costs a fraction of a service call and it converts a recurring mystery into a trend line. Standard board and blank values to measure against are in our FAQ, and the fold and compression settings referenced throughout are described on the Single-Piece Carton Erector page.

Getting seasonal variation in case quality? Send your room humidity range, board grade, storage conditions and the settings that change with the season to [email protected] or WhatsApp +86 13681839278. Our engineers will return a seasonal setting sheet and a board specification review within 24 hours.