1. Hot-Pressing and pulp molding Shaping Mechanism

To achieve the required dryness, higher flatness, and strength of pulp-molded products, hot-pressing and shaping methods are commonly used to process the dried pulp-molded products. Hot-pressing and shaping is the final step in the forming process of pulp-molded products and determines the final quality of the product.

Thick Products: For thicker pulp-molded packaging products, drying and hot-pressing are separate processes.

Thin Products: For thinner pulp-molded packaging products and tableware with high shape requirements, drying and hot-pressing are often completed in the same section.

High-precision upper and lower molds are tightly fitted under a pressure of 10 MPa. Heat transfer through contact rapidly vaporizes the moisture inside the pulp mold. The water vapor is removed by a vacuum system. The temperature of the hot-pressing mold is controlled between 180–200°C, determined by the waterproof and oil-proof agents added to the pulp-molded product. Within the first 6 seconds after the mold is closed, drying occurs at a constant rate with a large amount of water vapor being discharged. At this stage, the vacuum system is used to remove the water vapor. Subsequently, drying enters a deceleration phase with minimal water vapor discharge, and vacuum suction is stopped to avoid heat loss. The total hot-pressing time is controlled between 18–20 seconds to ensure good shaping and strong bonding between the waterproof and oil-proof agents and the fibers. After hot-pressing, the thickness of the pulp-molded product is reduced to 70% of its original size, and the density increases.

The shaping process often includes an additional edge-cutting step. The shaping and edge-cutting processes involve hot mold pressing and calendering to remove the mesh marks left by the mesh mold, resulting in smooth inner and outer surfaces. Text and patterns can also be pressed onto the product according to customer requirements. Additionally, burrs on the edges are trimmed, and creases are pressed to facilitate the folding and opening of the lid, ensuring a neat and aesthetically pleasing appearance.

2. Hot-Pressing and Shaping Methods

For pulp-molded products, due to the filtration resistance of the fiber layer, the dryness of the wet pulp mold after vacuum filtration typically only reaches 20–30%. The wet pulp mold must be dried to meet the required dryness, with the final dryness of pulp-molded products generally exceeding 90%. The drying process consumes significant energy, accounting for 15–30% of the production cost. In-mold drying is the primary method used in the production of pulp-molded tableware, allowing for both drying and shaping. The main hot-pressing and shaping methods include:

(1) One-Step Hot-Pressing and Shaping
The formed tableware blank is transferred to the hot-pressing mold, where drying and shaping are completed in one step. This method combines shaping and hot-pressing into a single process, simplifying the shaping procedure and resulting in a high yield rate.

(2) One-Step Hot-Pressing and Shaping After Drying
The wet pulp mold is dried in a drying tunnel before undergoing hot-pressing and shaping. If the wet pulp mold is directly dried to the required dryness, the product surface becomes rough, and the shape becomes unstable, necessitating an additional hot-pressing step for shaping. The dryness before shaping should not exceed 70%. This method involves complex hot-pressing and shaping processes, requiring sophisticated equipment, high investment costs, and significant maintenance. The products tend to have poor stiffness, feel soft, and have a relatively rough surface.

(3) Multi-Step Hot-Pressing and Shaping
The formed wet pulp mold is sequentially placed into multiple shaping stations to complete the hot-pressing and shaping process step by step. This method divides the hot-pressing and shaping process into stages, resulting in high productivity. However, it requires high precision in the molds and poses challenges in transferring the wet pulp mold between stations.

Before hot-pressing and drying, the wet pulp mold often undergoes cold pressing to reduce energy consumption and drying time. During cold pressing, the vacuum filtration mold serves as the lower cold pressing mold (male mold), and the upper cold pressing mold (female mold) is tightly pressed against the lower mold at 10 MPa to squeeze out water, which is then removed by the vacuum system. After cold pressing, the dryness of the wet pulp mold can reach about 60%. The upper cold pressing mold also acts as a transfer mold, moving the wet pulp mold to the lower hot pressing mold. If the wet pulp mold undergoes a vacuum cold pressing step before hot pressing, the moisture content entering the hot pressing mold is reduced, significantly lowering the thermal energy consumption of the hot pressing process. Additionally, cold pressing compacts the wet pulp mold, bringing the pulp fibers into closer contact, enhancing the strength, density uniformity, and surface smoothness of the wet pulp mold. During the hot pressing and drying process, a large amount of water vapor is generated, so the molds must maintain constant heat and temperature and quickly discharge the steam.

3. Heating Methods for Hot-Pressing and Pulp molding shaping mechanism

Similar to the heating methods used for in-mold drying of pulp-molded products, hot-pressing and shaping can employ electric heating, steam heating, thermal oil heating, and heat superconduction technology. The principles and characteristics of these methods are not detailed here.