Key Points for Pulping Control
The pulping stage is a critical step that affects fiber quality and the final moisture content of wet-pressed pulp trays. A common issue occurs when operators, aiming to improve defibering efficiency, excessively increase the pulping concentration. This can lead to over-cutting of the fibers. While defibering may be achieved, the strength of the fibers is significantly reduced. This, in turn, hurts water drainage during the forming process. Shorter fibers can also result in uneven deposition on the mold surface.
This creates areas of locally high moisture content, which affects the stability of the subsequent hot-pressing stage. Therefore, pulping should follow the principle of "thorough separation while preserving fiber length." Avoid solely pursuing high concentration or high-speed pulping. Experience shows that controlling pulping time and speed, and maintaining an appropriate concentration, can create good conditions for dewatering in later stages without damaging the fibers.
Balancing Suction Time and Efficiency
Suction is a key step for dewatering during the forming of wet-pressed pulp. However, two common mistakes are often seen in practice: too short or too long suction time. If the time is too short, water inside the mold cavity is not fully removed. This leads to high moisture content in the tray. This not only affects strength but also makes the product prone to mold growth.
On the other hand, excessively extending the suction time can remove more water initially. But after a certain point, the reduction in moisture content becomes very small. Instead, it increases the load on the vacuum system, raises energy consumption, and hurts overall production capacity. Therefore, suction time needs to be adjusted dynamically based on mold structure, pulp characteristics, and vacuum capacity. The goal is to know when to stop, balancing dewatering efficiency with energy savings.
Proper Setting of the Vacuum System
Vacuum level is the core parameter determining the pressure difference inside and outside the mold. It directly affects the dewatering result. When the vacuum is too low, the pressure difference is insufficient. The suction force is weak, leaving a large amount of water trapped in the wet blank. This causes the moisture content of the tray to exceed standards.
Conversely, if the vacuum is too high, dewatering is stronger. However, it can over-compact the fiber structure, leading to moisture content that is too low. This affects the plasticization effect during hot-pressing.
The final product may be too light in weight and have poor strength. Therefore, the vacuum system needs fine-tuning according to the product's grammage and structural complexity. It is generally recommended to use segmented control within a medium-to-high range. This ensures a moderate pressure difference, effectively removing water without damaging the fiber matrix.
Optimizing the Backflush Process
Backflushing is mainly used to clean the mold screen holes, preventing clogging and ensuring smooth water absorption. However, if the backflush time is set too long, it can cause water to accumulate in the mold cavity. This can even lead to re-wetting and deformation of the formed wet blank. In severe cases, water seeping back into the tray edges can cause warping, affecting demolding and subsequent transfer. On the other hand, if the backflush time is too short, the pulp slurry may not distribute evenly on the mold surface.
Fibers can clump together, causing areas of the tray to be too thin or uneven in thickness. This lowers the first-pass yield of qualified products. Therefore, a "short and frequent" strategy is recommended for backflushing. The settings should be flexible, based on the actual degree of mold clogging, balancing cleaning effectiveness with forming stability.
System Coordination and Process Synergy
Controlling the moisture content of wet-pressed pulp trays is not the responsibility of a single process step. It is the result of the synergy between multiple stages: pulping, forming, dewatering, and the mold system itself. For example, when increasing the vacuum level, it's also necessary to check the seal between the suction hood and the mold.
When adjusting the suction time, attention must also be paid to whether the drainage performance of the pulp has changed due to variations in fiber quality. Furthermore, seasonal changes in temperature and humidity can affect dewatering efficiency, requiring corresponding fine-tuning of process parameters. Only by treating all these factors as a dynamic system can moisture content be controlled at a fundamental level, improving product consistency.
High moisture content in wet-pressed pulp trays results from the interaction of multiple factors. Solving this problem cannot rely on a single method. It requires starting from key points like pulping, suction, vacuum, and backflushing. A systematic approach with repeated adjustments and verification is needed. Only through continuous data collection and parameter optimization in practice can moisture content be effectively controlled. This ensures stable performance of the trays during subsequent hot-pressing and demolding, ultimately improving product yield and quality.