Pulp molding materials are formed by using a certain concentration of pulp and adding appropriate amounts of chemical additives. The fibers are evenly distributed on the surface of a mold with a filtering screen through vacuum or pressure, thus wet press molded pulp materials with predetermined shapes. These materials are then dehydrated, demolded, dried, and finished to become cushion packaging materials.
Pulp molded products are typical green packaging materials that can provide cushioning performance comparable to foam plastics to some extent. They have a smaller volume compared to foamed plastics, can be stacked, and are convenient for transportation. They can be widely used in industrial product packaging, such as electronic components, household appliances, and power tools industries. Therefore, pulp molded products have gained increasing popularity among consumers due to their own advantages. However, due to the relatively late start of the pulp molding industry in the country, both product design and production lack mature theories and standards, resulting in a considerable rejection rate of the produced items. Through a six-month internship at a certain company and participating in actual production and sampling multiple times, I have identified some common issues in the wet press molded pulp process of pulp molding and proposed some solutions based on theoretical research, hoping to provide assistance to the practical production of the paper-plastic industry.
Introduction to Wet Press Molded Pulp Process
Currently, there are two main processes for pulp molding production: wet pressing and the commonly used traditional process (i.e., dry pressing). The pulp molding wet pressing process mainly includes the following steps:
Pulp preparation includes pulping and formulation. The main raw materials for pulp molding currently include corrugated paper, recycled newspapers, bagasse pulp, reed pulp, etc. Pulping refers to pouring the pulp molding raw materials into a hydrapulper, using the hydrapulper to disintegrate the pulp board into fiber pulp, and then purifying the disintegrated fiber pulp through screening and sand removal processes. The fiber pulp is then refined to improve the fiber bonding strength. Formulation involves adding fillers, pigments, as well as functional additives such as waterproof and oil-proof agents to the purified pulp to prepare a pulp mixture suitable for product quality requirements. Sometimes, the pulp consistency also needs to be controlled. To ensure continuous production and product uniformity, it is necessary to control the liquid level in the pulp chest and formulation chest, and periodically add fillers, water, and other ingredients.
There are currently two main methods for molding pulp molded products: vacuum forming and extrusion molding. Vacuum forming is commonly used. The basic principle of vacuum forming is to extract the air from the cavity of the lower mold with a filter screen immersed in the pulp. Under negative pressure, plant fibers in the pulp are adsorbed on the outer surface of the lower mold, while water is extracted to form the shaped product. The basic principle of extrusion molding is that the cavity of the lower mold with a filter screen is filled with a suitable amount of pulp, and under the pressure of a plunger or compressed air, plant fibers in the pulp are pressed onto the inner surface of the lower mold, and water is squeezed out to form the shaped product.
Due to the drying or air-drying process, wet pulp blanks undergo varying degrees of deformation, and the product surface may have wrinkles. Therefore, shaping is necessary after drying. Shaping involves placing the product on a shaping machine equipped with molds and subjecting it to high temperature (usually between 100°C and 250°C) and high pressure (typically between 10 and 20 MN). This process results in products with more regular shapes and smoother surfaces. Since the wet pressing process involves shaping the product without drying, direct hot pressing is carried out to ensure sufficient drying of the product. Generally, the hot pressing time is at least one minute or more (the specific hot pressing time depends on the thickness of the product).
The products obtained through the above steps often have uneven edges or debris. Therefore, for products with higher quality requirements or specific structural requirements, trimming is required. Trimming is performed using a trimming machine equipped with a product cutting die.
The dry pressing process has a similar process flow to the wet press molded pulp process, with the main difference being the presence of a drying step after forming. In the dry pressing process, the formed products are placed in a drying machine, and hot air is used to dry the products. The drying machine is equipped with a conveying device. In the first half of the drying machine, the temperature is generally set within the range of 200°C to 300°C, and in the second half, the temperature is reduced to between 100°C and 200°C. The products pass through different temperature zones in the drying machine and are finally dried to a moisture content ranging from 10% to 13%.
Common Issues and Solutions in Wet Press Molded Pulp Process
Wet pressing process products contain a large amount of moisture before hot pressing, and the fiber connections within the pulp are still sparse at this stage. During hot pressing, the pulp fibers are more prone to flow, resulting in a smoother and more even product surface. In terms of quality, the wet pressing process is significantly superior to the dry pressing process. Therefore, many customers, especially those producing high-end products (such as mobile phones and computers), explicitly require the use of the wet pressing process for pulp molding as the packaging material. However, in practical work, there are many issues associated with the wet pressing process for pulp molding, leading to a significant amount of waste. Therefore, analyzing the common problems in the wet pressing process for pulp molding is crucial for improving production efficiency and reducing production costs.
Cracking of Hot Pressing Mesh
In the wet pressing process, as the products contain a significant amount of moisture before hot pressing, this moisture evaporates during the hot pressing process. Therefore, it is necessary to create uniform vent holes (commonly using a 2.5mm drill bit) on the hot pressing mold (usually the lower mold) to allow the evaporated moisture to escape. To prevent pulp from clogging the vent holes, a mesh film (usually around 0.5mm thick) is attached to the lower mold. To ensure that the mesh adheres completely to the mold, the mesh is pressed into the shape of the product on the hot press machine. However, it is common to encounter issues with mesh cracking during the pressing process. If a cracked mesh is attached to the mold and the product is hot pressed, especially on the backside, a visible trace will be left along the crack, affecting the appearance of the product. Since significant tension is applied to the sides during mesh pressing, mesh cracks mainly occur on the sides, particularly on surfaces with a small demolding inclination where the mesh undergoes a significant change, leading to more frequent mesh ruptures.
Solution: 1) Prior to mesh pressing, manually shape the mesh as closely as possible to the product form on the mold. 2) If it is not possible to press a complete mesh film, the cracked portion of the mesh can be repaired using a spot welding machine. 3) If the previously selected mesh is too rigid, consider using a softer mesh.
Product fracture mainly refers to the phenomenon of a crack appearing on the side of the product during the hot pressing process. The main causes of fracture are as follows: a) Improper placement of the product on the mold, resulting in a gap between the product and the mold, leading to fracture. b) The product undergoes shrinkage after forming, for example, the thickness of the product is initially about 2-3 times larger than after drying, and the hot pressing mold is designed with a gap based on the actual product thickness. During the hot pressing process, the mold first contacts the side of the product, causing the side pulp to move downward, resulting in fracture.
Solution: 1) Ensure proper placement of the product during the process. 2) Increase the demolding inclination of the product. After placing the product on the mold, manually apply pressure in areas prone to fracture to reduce the gap difference and ensure tighter connections of the pulp in those areas.
Surface markings refer to the presence of lines on the product’s surface after hot pressing. These lines are primarily caused by uneven pulp suction on the forming mold. The forming mold (lower mold) used in pulp molding has a row of evenly spaced small holes. By applying a vacuum to these holes, the pulp adheres to the forming mold. Since the vacuum force at the hole positions on the forming mold is greater than that at the non-hole positions, the product formed in the hole positions protrudes due to more pulp accumulation, resulting in uneven thickness and the appearance of surface markings.
Solution: 1) Use smaller drill bits for creating the holes. 2) Increase the pressure of the hot press machine, which can effectively improve the appearance of surface markings. 3) Add additional layers of mesh film on the forming mold if feasible (2 or 3 layers), which helps to disperse the vacuum force at the hole positions.
Product darkening is one of the most common problems in pulp molding. It is primarily caused by two reasons: firstly, in areas where the product structure is relatively small, excessive pulp accumulation occurs during forming, resulting in a significantly larger thickness compared to other areas. During hot pressing, these areas are compressed too tightly and turn dark. Secondly, it can be caused by excessive pressure and prolonged hot pressing time of the hot press machine itself.
Solution: For the first scenario, simply use a file or machine to remove a layer from the corresponding mold part where darkening occurs. For the second scenario, both the pressure and time of the hot press machine need to be adjusted. It is important to find a balance because if the pressure is too low, the product surface will not be sufficiently compressed, and if the time is too short, the product will not be properly dried. Careful adjustment of the machine is required to achieve a balance point.
Pulp molding has experienced rapid development in recent years due to its environmental advantages and good cushioning performance. However, due to the lack of theoretical research and standardized design and production processes in the industry, many issues arise during actual production. This article provides an overview of the wet pressing process in pulp molding and proposes solutions to common problems encountered. It is hoped that this information will be helpful for the practical production in the pulp molding industry.