Vacuum molding method is the most popular method in China. This method is formed by vacuum suction technology, that is, vacuum is pumped in low-concentration (about 1%) molded paper pulp forming mold to form negative pressure in the mold cavity. The fibers in the molded paper pulp are evenly deposited and attached to the molding net on the surface of the mold in a vacuum environment, and a large amount of moisture is vacuumed. When the required thickness of the part is reached, the forming mold is removed from the molded paper pulp and closed with the upper mold for extrusion and dehydration until the water content of the part is 65% – 75%, and then the wet paper mold of the part is demoulded with compressed air. This method enjoys high efficiency, and the thickness of the products are uniform, which is suitable for producing less deep and thin-walled parts, such as cushions for packaging eggs, fruits, plates, etc. Its working principle is shown in Figure 4-1.
1——Lifting cylinder 2——Mobile platform 3——Upper mold 4——Lower mold
5——Air tube 6——Overflow tank 7——Molded paper pulp storage tank
8——Bottom plate 9——Upper plate 10——Moving guidepost
The concentration of vacuum molded paper pulp is about 1%. The vacuum is generally 520Pa, and the air pressure is generally 0.2MPa. The deposition speed of paper fiber on the mold largely depends on the type of fiber, pulping degree, vacuum degree and molded paper pulp concentration, temperature and the duration of product molding, etc. For shallow and thin pulp molded products, generally, it can be completed from 10 to 15s. However, for deep and thick pulp molding products, the duration of molding should be extended according to the specific product. Forming time is one of the important factors affecting production efficiency.