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Professionals in the corrugated box industry frequently encounter a recurring issue during rainy seasons: customer complaints regarding soft and weak cardboard boxes. This challenge is particularly difficult to address, as the fibers used in base paper are inherently hydrophilic, meaning they readily absorb moisture from the surrounding environment. During periods of high humidity or heavy rainfall, corrugated boxes exposed to ambient air tend to absorb moisture, resulting in a noticeable loss of structural rigidity. Facing customer complaints, returns, penalties, and compensation claims, resolving the issue of cardboard softening has become a top priority for carton manufacturers.
Below is a comprehensive approach to addressing the problem of cardboard softening.
I. Moisture resistance of paper and paperboard
Base paper and corrugated paper, particularly flute paper, exhibit strong water absorption properties. When exposed to humid conditions for extended periods, the moisture content of the paper increases, leading to a significant reduction in transverse ring crush strength. At a relative humidity of 85%, this strength can decrease by up to 60%.
Solution
It is recommended to use base paper with enhanced water and moisture resistance, especially flute paper. To assess the water resistance of base paper, its surface water absorption can be measured using a Cobb instrument. Generally, a surface water absorption value below 50 grams per square meter (after 60 seconds) indicates satisfactory moisture resistance. In the absence of testing equipment, a simple water droplet test can be performed: sprinkle a few drops of water on the surface of the paper and observe whether penetration or discoloration occurs after 60 seconds. If no penetration is observed, the paper demonstrates adequate water resistance.
To improve the water resistance of base paper, paper mills typically increase the proportion of waterproofing additives in the surface sizing formulation during the papermaking process. Due to the high cost of these additives, water-resistant base paper is typically priced 50 to 100 yuan per ton higher than standard paper, with the cost increasing as the grammage decreases. Therefore, carton manufacturers should proactively communicate with customers and adjust pricing accordingly.
II. Corrugated cardboard production process
The primary solution to the problem of softening cardboard involves selecting appropriate materials, followed by optimizing the production process. The main cause of softening is excessive moisture absorption. Therefore, during the manufacturing process, it is crucial to minimize moisture retention in the cardboard.
Solution
The raw paper storage area should be isolated from external humidity as much as possible. Install dehumidifiers in the warehouse to maintain a relative humidity level below 65%. When base paper is stored in a humid environment, it absorbs moisture. To counteract this, increase the preheating wrap angle of the base paper on the drying cylinder. If this proves insufficient, reduce machine speed and increase steam pressure.
During adhesive preparation, increasing the powder content while reducing glue application can lower the overall moisture content of the cardboard. Additionally, increasing the proportion of borax can enhance the tactile quality of the corrugated board.
Reduce the amount of cardboard stacked on the overpass, slow down the conveyor speed, and ensure thorough dehumidification after stacking.
III. Storage environment
After production, residual steam in the cardboard should be dissipated to prevent condensation caused by temperature differences. Cardboard and cartons should be stored in a well-ventilated and breathable environment. Avoid placing them directly on the floor; instead, use pallets or wooden supports. Maintain adequate spacing between stacks to facilitate air circulation and heat dissipation.
Solution
It is recommended to use wooden supports, which can absorb moisture from the ground and are appropriately sized for cardboard storage. Maintaining a well-ventilated storage environment and using stretch film for external protection can effectively reduce and isolate moisture-induced softening.
Many manufacturers now employ dehumidifiers. If the warehouse's relative humidity can be maintained below 65%, the rigidity of the cardboard will be significantly improved.
IV. Transportation environment
During product delivery, some shipments are transported via open-top trucks, while others use enclosed box trucks. Generally, box trucks provide better protection against moisture, reducing the risk of cardboard softening.
Solution
It is recommended to use enclosed box trucks for transportation, which is the standard practice among major suppliers.
V. Carton processing technology
Corrugated cardboard should proceed to the next processing stage immediately after production. Prolonged exposure increases the likelihood of moisture absorption and softening.
Additionally, during multi-color or large-area solid color printing, the pressure exerted by the printing plate can cause softening of the carton.
Solution
Minimize the dwell time of cardboard during processing to prevent moisture absorption. The production workshop should be partially isolated from the external environment to prevent humid air from entering directly.
Before printing, ensure that the corrugated cardboard is sufficiently rigid and avoid applying excessive pressure during the printing process. In the die-cutting stage, avoid using rotary die-cutting methods that significantly compromise the flat crush strength of the cardboard.
In conclusion, the softening of corrugated cardboard is primarily attributed to moisture absorption. Only by implementing comprehensive moisture control measures throughout the supply chain-from raw paper storage and corrugated board production to carton processing, warehousing, and transportation-can the structural integrity of the cardboard be preserved.