Source: Chinazhixiang.com — https://www.cnzhixiang.com/news/106/2/6007.html

As autumn and winter approach, and in order to avoid unnecessary losses in the production of corrugated board, cartons and color boxes caused by changes in ambient temperature and humidity, Nine Dragons Paper and Bohui Paper have recently issued their "Guidelines for Preventing Crease Cracking in Corrugated Board and Cartons." Both companies hope to work jointly with packaging converters to take preventive measures and safeguard product quality.

1. Nine Dragons Paper: Letter on Preventing Carton Crease Cracking in the Autumn and Winter Season

Autumn is here, and the air is dry. Drawing on years of industry production experience and taking into account the behavior of base paper under such conditions, and in order to adapt to this seasonal climate change and avoid unnecessary trouble and loss in corrugated board and carton production caused by variations in ambient temperature and humidity, we will work together with your company to take measures to prevent crease cracking.

Base paper: From September through March of the following year, the company will make appropriate adjustments to the base paper production process and raw material furnish, and suitably optimize internal control indicators such as folding endurance, moisture and bursting strength. In addition, from the perspective of paper characteristics, we offer the following friendly reminders regarding board and carton production:

I. Corrugated board production

1. When producing board, please adjust the preheater cylinder temperature, wrap angle, hot plate temperature and suction/exhaust time according to the moisture content of the base paper, so as to avoid excessive moisture loss that would impair the physical properties and appearance of the finished carton.
2. Steam spraying systems may be added before the inner liner enters the single facer, before the top liner enters the hot plates, and before the medium is corrugated, in order to raise the moisture content of the board (it is recommended that board moisture be controlled at 10–12%).
3. Rationally adjust the starch adhesive formula and reduce the amount of borax added.
4. Properly adjust the accuracy of the creasing wheels and the depth of their engagement.

II. Carton production — cracking occurs mainly at the slotting and creasing positions. The following measures are recommended:

1. Determine the slot depth according to actual conditions; generally slots may be cut 3–5 mm deeper. The slot width can also be adjusted to suit the dimensions and shape of the carton. This effectively prevents cracking at the slots.
2. When using an integrated printer-slotter, a pre-creasing approach may be adopted to pre-weaken the flute structure at the crease and control the slotter pressure. Take care to keep the creasing feed rollers and the slotter knife in the same straight line.
3. When die-cutting equipment is used for die-cut slotting, while controlling die-cutting pressure properly, consider adding creasing strips or rubber pads alongside the die-cutting rule to extend the buffering time and the contact time between the cutting edge and the board. Preferably, use a wide-edge creasing/die-cutting rule.

III. Other measures

1. Owing to the influence of weather humidity, it is recommended that subsequent processes be completed within 24 hours after the board comes off the double backer. If this cannot be achieved and the board is prone to cracking, it is recommended to condition the workshop — for example, install humidifiers in the board storage area or sprinkle water around the board to increase the humidity of the air.
2. Given the marked difference between winter and summer climatic conditions, it is recommended that you make reasonable adjustments to the basis weight combination of materials well in advance of the season change: the grammage or grade of the corrugating medium may be appropriately reduced, while the grammage or grade of the linerboard may be increased.
3. If face-layer cracking or crease cracking still occurs after the preventive measures have been applied, it is recommended to spray steam evenly onto the stacked board in the flute direction. This re-moistening method reduces the stiffness of the medium and thus alleviates face-layer cracking.
4. To prevent moisture loss during transportation and storage, it is recommended that the board be wrapped with stretch film after spraying. Downstream customers are also advised to improve their storage conditions and carry out die-cutting and creasing as early as possible.

The problem of crease cracking in corrugated board and cartons must be prevented and reduced through measures taken at every stage, from base paper to board and carton converting. We hope your company will make reasonable production adjustments tailored to product characteristics and seasonal conditions.

2. Bohui Paper: Reminders on Preventing Kraft Linerboard Cracking in the Autumn and Winter Season

We are now in the autumn and winter season. As temperatures fall, air humidity drops accordingly, and many customers have not yet switched over their board material combinations and converting processes — this is precisely when seasonal surface cracking and crease cracking are most likely to occur. We therefore offer the following reminders: jointly manage storage, transportation, the processing environment and the process itself, so as to avoid unnecessary losses.

Lower air humidity causes the paper in contact with it to lose moisture. On the one hand, the folding endurance of the top and inner liners declines while board stiffness increases, raising the likelihood of carton crease cracking. On the other hand, the flat crush strength of the medium increases and the flexibility of the liners decreases, so the liners are easily damaged during creasing, resulting in cracking of the board or finished carton. To mitigate this risk, the following suggestions are offered for reference:

I. Adjust the corrugated board converting process to ensure a proper moisture content. Specific measures:

1. Appropriately reduce the contact area between the base paper and the preheater to prevent excessive evaporation of moisture on the preheater cylinder.
2. Moderately increase the glueapplication volume. This effectively raises the board's moisture content and also improves bonding to a certain extent.
3. Adjust the suction/exhaust time appropriately according to the materials used and the actual moisture content. During forming of the medium, board moisture is recommended to be controlled at 10–12%; after the corrugated board is formed, moisture is recommended to be controlled at 10–15%. If the moisture of board coming off the corrugator is below 9%, the risk of cracking is high and re-moistening is recommended. Re-moistening may be done by spraying a water mist or by running the board through the printing press with water (replacing the ink with clean water). After re-moistening, wrap the board with PE film to ensure uniform moisture recovery.

II. Board that is hard and low in moisture is especially prone to cracking during creasing — please pay particular attention during production. To reduce the risk, the following improvements are recommended:

1. Properly adjust the accuracy of the creasing wheels and the depth of the creasing impression.
2. Where conditions permit, apply a pre-creasing pass before the main creasing operation.
3. If the above operations are still insufficient, apply water to the creasing area of the board before creasing.

III. For the crease cracking commonly seen at the slotting position in carton production, the following is recommended:

1. Determine the slot depth based on actual conditions; generally slots may be cut 3–5 mm deeper, and the slot width can be adjusted to suit the carton's dimensions and shape. This effectively prevents cracking at the slots.
2. When using an integrated printer-slotter, control the slotter pressure and keep the creasing feed rollers in line with the slotter knife.
3. When using die-cutting equipment for die-cut slotting, while controlling die-cutting pressure properly, consider adding rubber pads on both sides of the creasing rule to extend the creasing buffer time; the compressed fluted paper then folds and forms more easily. Preferably, use a wide-head creasing rule.

IV. Regarding the interval between board coming off the line and carton forming, and regarding transport protection:

1. After board comes off the double backer, it is best to complete subsequent processes within 8 hours, and to keep the storage environment relatively enclosed, avoiding direct sunlight and drafts.
2. If this cannot be achieved and the temperature and humidity of the storage environment cannot be controlled, it is recommended to humidify the workshop or humidify the board directly — for example, sprinkling water around the board or using a mobile spray humidifier to increase air humidity; or wiping the crease area of the carton with a damp towel, or using a spray gun to moisten the folded corners of the carton.
3. It is recommended that board and cartons be protected with enclosed box trucks or PE film during transportation and storage, to reduce moisture loss.
4. Storage of base paper rolls should likewise be properly protected.

V. Regarding adjustment and trial runs of board material combinations, the following is recommended:

1. Avoid a large disparity in grammage between the top liner and the bottom liner.
2. Adjust the base paper materials appropriately according to board strength, seasonal characteristics, the ambient temperature and humidity of the local area, and end-use requirements (testing standards, load-bearing requirements, etc.).
3. Whenever board materials are changed, carry out proper testing and evaluation.
4. Base paper production switches between winter and summer standards according to temperature and humidity changes — please follow the first-in, first-out principle when using it.