Process Overview and Industry Background
Molded pulp products are an environmentally friendly packaging solution. Their cushioning performance can rival some foam plastics. Compared to traditional plastic foam, these products have a smaller volume and excellent stackability. This significantly improves transportation efficiency. Therefore, their use is growing rapidly in packaging for industrial products like electronic components, home appliances, and power tools. The market is showing increasing recognition for them.
Wet-press molding is a key process in molded pulp production. The main stages include pulp preparation, forming, hot-press forming, and trimming. In the pulp preparation stage, recycled fiber materials like corrugated paper, bagasse pulp, or reed pulp are typically used. These are broken down, screened, and cleaned. Then, functional chemicals (such as water repellents or strengthening agents) are added to create pulp that meets requirements. The forming stage often uses vacuum suction. Fibers are deposited evenly on a mesh-covered mold to create a wet blank.
Unlike the dry-press process, the unique aspect of wet-press is that the wet blank, containing high moisture, goes directly into a hot press. Inside the press, it undergoes simultaneous pressing, shaping, and drying under high temperature and pressure. This process compresses the fibers while they are still relatively mobile. Consequently, the products usually have a denser and smoother surface quality. This makes them especially suitable for high-end product packaging, such as for mobile phones and computers.
However, although the wet-press process produces high-quality items, it faces challenges in actual production. Because the industry started relatively late, it lacks mature, unified theoretical guidance and process standards. This leads to significant issues with production stability and consistently high product defect rates. Based on practical experience, the following sections analyze common problems in the wet-press process and propose countermeasures.
Hot-Press Mesh Cracking
In the wet-press process, dense exhaust holes are drilled into the hot-press lower mold. These holes allow the large amount of steam generated during pressing to escape. A special metal mesh sheet is attached to the mold surface. This prevents fibers from blocking the holes. This mesh must first be pressed in the hot press to match the product's shape. However, during this pre-forming, the mesh frequently cracks. This happens especially on product sidewalls or complex curves with minimal draft angles. The mesh cracks under high tensile stress. If a cracked mesh is used for production, it will leave clear marks on the product surface, causing visual defects.
Countermeasures:
Before formal pressing, try to pre-shape the mesh manually. Make it conform as much as possible to the mold's contour. This reduces initial stress during pressing.
If local cracks appear after pressing, repair them using spot welding equipment. The goal is to restore the mesh's integrity.
Evaluate the hardness of the current mesh material. Consider switching to a more flexible and softer mesh type. This improves its ability to adapt to deformation.
Surface Marks on the Product
Sometimes, regular patterns appear on the surface of wet-pressed products. This primarily originates from the earlier forming stage. The forming mold (lower mold) surface has small holes for vacuum suction. When fibers are being deposited, the suction force is stronger at the hole locations than in areas without holes. This causes slightly more fiber buildup at the hole sites, creating tiny raised points. This microscopic thickness variation becomes visible as surface patterns after hot pressing.
Countermeasures:
Consider reducing the diameter of the vacuum holes on the forming mold. This decreases the concentration of suction force at each point.
Appropriately increase the unit pressure of the hot press. Use higher pressure to smooth out minor surface irregularities.
Add one or more additional auxiliary mesh layers on the forming mold. This helps distribute the vacuum suction force more evenly. It leads to more uniform fiber deposition and reduces the pattern at the source.
Localized Blackening of the Product
Localized blackening, particularly in complex structures or corners, is another common issue in wet-press production. There are two main reasons. First, during the forming process, fibers can accumulate excessively in specific narrow areas. This results in a wet blank thickness there that is much greater than elsewhere. During hot pressing, this area experiences abnormally intense pressure. The fiber structure gets damaged and carbonizes, turning black. Second, improper machine parameter settings, like excessive pressure or overly long pressing time, can also cause the entire product or parts of it to overheat and blacken.
Countermeasures:
For the first issue (localized thickness due to structure), thin the mold base at the corresponding position. This can be done by manual grinding or machining. It reduces the actual pressing force in that area.
For the second issue (machine parameters), fine-tune the hot press pressure and time. The key is finding a balance. The pressure must be high enough to smooth the surface and ensure thorough drying, but not so high it causes scorching. The time must allow complete moisture evaporation, but not be so long it causes overheating. Operators need to experiment repeatedly to find the optimal process window for a specific product.
Edge Cracking of the Product
Cracks sometimes appear on the sidewalls of products during hot pressing. This is often related to operational factors and mold design. For example, if the wet blank is not placed perfectly aligned with the lower mold, misalignment occurs. This can lead to uneven force during pressing and cause tearing. Furthermore, the wet blank shrinks after forming. Its initial thickness is much greater than its final, dry thickness. If the hot press mold cavity gap is designed for the final thickness, the mold's sidewalls will contact the still-thick blank early in the closing process. It then forcibly squeezes the blank downwards. This shear force easily causes the sidewall material to crack.
Countermeasures:
Standardize operations. Ensure each wet blank is placed accurately and flatly in the designated position on the mold.
Address mold design. Appropriately increase the product's draft angle. This provides more space for wet blank shrinkage and fiber flow.
For areas prone to cracking, manually pre-press these spots after placing the blank. This makes them fit the mold more closely and reduces the initial gap.
The wet-press molded pulp process holds great potential in the high-end packaging market due to its superior product quality. However, the production process involves multiple stages and is influenced by many complex factors. Problems like mesh cracking, surface marks, product blackening, and edge cracking are just a few examples. There is no single, universal solution.
Solving them requires production personnel to deeply understand the process principles. They must continuously explore, summarize, and optimize based on practical experience. By persistently improving mold design, meticulously controlling process parameters, and standardizing operating procedures, production stability and product yield can be gradually enhanced. This will drive the molded pulp industry towards more mature and standardized development.