Pre-pressing corrugated boxes is a critical step in the corrugated box production process. Its core function is to optimize the corrugated structure through physical deformation, providing stable substrate conditions for subsequent forming steps. This process involves applying controlled pressure to the corrugated cardboard using a pre-pressing wheel, causing plastic deformation of the corrugated core. This, in turn, adjusts the internal stress distribution and lays the foundation for the final flatness of the finished product.
From a structural perspective, unpre-pressed corrugated cardboard exhibits significant elastic recovery properties. During the creasing process, the corrugated core experiences localized springback due to the sudden stress, resulting in slight deviations or wrinkles at the creasing position. Pre-pressing uses a gradual application of pressure, gradually releasing the elastic potential energy during the compression process, resulting in stable plastic deformation. This deformation method effectively reduces stress release during subsequent processing and ensures accurate creasing position.
Pre-pressing has multiple mechanisms for controlling flatness. First, uniform pre-pressing distribution eliminates differential shrinkage between the transverse and longitudinal directions of the cardboard. When the moisture content of the face and back paper differs, unprepressed cardboard is prone to unidirectional arching during the forming process. Prepressing adjusts the compression ratio of each layer of paper to achieve consistent shrinkage between the face and back paper, thus preventing wavy deformation after forming. Furthermore, the curvature of the prepressing wheel has a direct impact on the forming effect. Prepressing wheels made of elastic material can buffer pressure through curved grooves, preventing cardboard breakage caused by excessive localized pressure.
Matching process parameters is crucial for effective prepressing. In practice, the prepressing gap must be dynamically adjusted based on cardboard thickness, flute type, and production speed. For five-layer corrugated cardboard, the prepressing gap is typically set at 80%-85% of the cardboard thickness. This setting ensures sufficient fluting while preventing excessive compression that could weaken the cardboard. Furthermore, the timing of the prepressing process and the subsequent creasing process is crucial. If the interval between the two processes is too long, the cardboard's elastic recovery may offset the prepressing effect. Therefore, modern production lines often use integrated prepressing and creasing equipment.
In automated production scenarios, pre-pressing becomes even more crucial. Automated box packaging equipment demands extremely high flatness from corrugated boxes; even a 0.5mm deviation in the forming process can cause suction cups to fail. Pre-pressing, by standardizing the amount of corrugated deformation, significantly improves the structural stability of corrugated boxes during high-speed movement. Experimental data shows that corrugated boxes with optimized pre-pressing can reduce the chance of jamming on automated production lines by over 60%.
In terms of quality defect prevention and control, pre-pressing can effectively prevent two typical problems. First, by controlling the distribution of pre-pressing pressure, it can avoid cracking caused by uneven pressure. If the pre-pressing wheel gap is set too small, the corrugated core paper is over-compressed, which can easily lead to fiber breakage during the subsequent crimping process. Second, appropriate pre-pressing parameters can reduce dimensional deviation during the die-cutting process and ensure accurate slotting. For irregular-shaped corrugated boxes that require multiple folds, pre-pressing can also enhance the structural strength of the creases, preventing paperboard breakage caused by repeated bending.
From a process optimization perspective, the synergistic application of pre-pressing and steam heating can further improve forming quality. Steam heating softens the paperboard fibers, reducing the pressure required during the pre-pressing process while enhancing the corrugated structure's ability to retain its plasticity. This combined treatment is particularly suitable for the production of high-strength corrugated board, ensuring both compressive strength and excellent forming flatness.
