Mainstream pulp molding methods

1. Molding of Pulp Molded Cushion Packaging Products

Based on the differences in molding, drying, machine structure, and movement methods, there are various types of equipment developed and used for producing pulp molded cushion packaging products. The molding of pulp products involves the deposition of paper fibers on the forming mold net and filtering dehydration, forming wet pulp blanks. This is a key process in the production of pulp products, directly affecting the output and quality of the entire production process. The main molding methods are vacuum molding, hydraulic molding, and compressed air molding.

(1) Pulp molding methods-Vacuum Molding Method

The vacuum molding method is the most widely used method domestically. It utilizes vacuum filtration technology for molding, i.e., vacuum is drawn in the low-consistency pulp (about 1%) forming mold, creating a negative pressure in the mold cavity. Under the vacuum environment, the fibers in the pulp uniformly deposit and attach to the forming net on the surface of the forming mold, while a large amount of water is removed during the vacuum suction. When the required thickness of the product is achieved, the forming mold is removed from the pulp and pressed with the upper mold to dewater, until the product moisture content reaches 65%~75%. Compressed air is then used to release the wet pulp blank from the mold. This method has high production efficiency and uniform product thickness, suitable for producing shallow thin-walled products, such as egg, fruit, and dish liners.

The pulp consistency for vacuum molding is about 1%, the vacuum degree during suction is generally 520Pa, and the air pressure during demolding is generally 0.2MPa. The deposition speed of paper fibers on the mold net largely depends on the fiber type, pulp degree, vacuum degree, pulp consistency, temperature, and the duration of the molding process. For shallow and thin pulp molded products, the process can be completed in 10~15 seconds; for deep and thick pulp molded products, the molding duration is appropriately extended according to the specific product situation. Molding time is one of the important factors affecting production efficiency.

(2) Hydraulic Molding Method

The hydraulic molding method, also known as extrusion molding, uses mechanical extrusion to generate higher molding pressure. The pulp pump sends the pulp from the pulp tank to the metering injector, which then injects the metered pulp into the drainage molder. Then, the upper mold extends into the molder filled with pulp under the action of the hydraulic machine and compresses the pulp together with the lower mold. The water in the pulp is discharged through the mesh holes of the mold, and the fibers form between the upper and lower molds. After molding, the upper mold connects to the vacuum, and the lower mold connects to compressed air, transferring the wet pulp blank to the upper mold under the vacuum suction of the upper mold and the blow-off action of the compressed air from the lower mold. Finally, the wet pulp blank is removed from the upper mold and sent for drying. The pulp consistency during molding is 2.5%, the pressure on the pulp during extrusion molding is 0.6~1.2MPa, the vacuum degree in the upper mold’s vacuum chamber is about 780Pa, the compressed air pressure for blowing out the wet product is 0.1~0.2MPa, and the moisture content of the wet pulp blank after molding is 65%~75%.

The required molding time is determined by the quality requirements of the product and depends on the structure of the molding device, pulp consistency, and fiber pulp degree. This method is suitable for the manufacture of thick-walled pulp molded packaging products. The wet state wall thickness of the pulp molded products produced by the hydraulic method can exceed 60mm, with a density of 0.3g/cm³. When the wet pulp blank is subjected to high-temperature pressing, the density can reach 0.65~1g/cm³. The wall thickness of the pulp molded packaging products produced can reach 5~20mm, and the single piece weight can reach 500g. This method is suitable for producing heavy and dense shallow dish pulp molded products.

(3) Compressed Air Molding Method

The compressed air molding method, also known as pressure molding, uses gas dynamics principles for molding. Unlike the vacuum molding method, the compressed air molding method uses compressed air as the filtration power to accelerate the deposition and dehydration process of paper fibers. The pulp is pumped to the pulp tank by a pulp pump, and then flows under gravity to the mold covered with a metal mesh on the surface. The compressor blows hot compressed air into the container, dehydrating the pulp under pressure. White water is discharged through the metal mesh and mold, while fibers uniformly deposit on the metal mesh surface. The compressed air is evenly distributed to all parts of the mold, enabling uniform dehydration and drying of the product to the required moisture content. The wet pulp blank is then removed and sent to the drying section. The pulp can be prepared using different types and ratios of pulp according to product quality requirements, with a pulp degree generally at 30~40°SR. The pulp consistency in the high-level box is 0.7%~1.5%, and the pulp consistency entering the metering mold container can be diluted to 0.5%~0.8%. The mesh diameter on the mold surface is about 0.5mm, the compressed air pressure is 0.4MPa, and the temperature is generally 377~400℃. This method is mainly used for complex-shaped and highly dense hollow pulp molded products, such as bottles, cups, buckets, and boxes.

2. Molding of Pulp Molded Tableware

Pulp molded tableware mainly adopts the vacuum adsorption molding method, which can be divided into two major categories: injection molding and suction molding based on the method of pulp application on the mold.

(1) Injection Molding Method

The injection molding machine is the earliest developed molding equipment for producing pulp molded fast food boxes. The basic work program is: mold frame closing (downward) → filling water → injecting pulp → vacuum dehydration molding → mold frame opening (upward) → discharging white water → transferring the wet pulp blank with the mesh mold to the next process.

When the mold frame moves to the forming mold with the filter mesh, pulp is injected into the mold frame, and vacuum dehydration is used to obtain the wet pulp blank. The wet pulp blank is then transferred to hot pressing for drying.

Characteristics include:

• Simple equipment, low investment, easy operation, and convenient maintenance.
• Long action time, low efficiency, and product replacement requires changing the mold frame.
• Uniform pulp consistency is difficult to ensure due to the impact of water filling and pulp injection.
• Long suction time easily leads to thin upper and thick lower wet pulp blanks.
• Due to the movable metal mesh on the forming mold, the metal mesh wear is significant during production (generally, one metal mesh can only produce about 300 fast food boxes), making it unsuitable for producing large depth ratio cups, bowls, and other pulp molded tableware products. It is only suitable for medium-batch production of shallow depth pulp molded tableware products.

(2) Suction Molding Method

The suction molding machine represents the development direction for producing pulp molded tableware. The basic work program is: lower mold moves down to pulp tank → vacuum suction pulp → lower mold moves up → lower and upper molds close and press to dehydrate → upper mold transfers the wet pulp blank to the next process.

Pulp is contained in the pulp tank, with the mesh mold fixed on the lower mold. The flow of pulp in the pulp tank and the up and down movement of the lower mold in the pulp tank stir the pulp to ensure uniform consistency. When the lower mold moves down into the pulp tank below the liquid surface, vacuum suction deposits the pulp on the mesh mold, then it moves out of the pulp tank and closes with the upper mold. Pressing and dehydration produce a high-dryness wet pulp blank.

Characteristics include:

• Shorter action time, significantly improved production efficiency.
• Good molding uniformity and flexible product replacement.
• Capable of molding products with greater depth and industrial packaging materials, suitable for large-scale production of various depths of tableware products.