Molded paper pulp products possess the strength, stiffness, and toughness typical of paper-based materials and can be molded into various complex shapes to meet application requirements. Their shock absorption primarily relies on their structural design, making it crucial when used for product packaging.
When molded paper pulp products are used as disposable food tableware, their shapes are relatively simple, and their strength requirements are not stringent. However, when employed for industrial packaging, their designs become more complex and must meet three critical requirements:
Positioning Requirement: The pulp-molded packaging must securely hold the product in place, with a precise shape fit and accurate dimensional alignment between the packaging and the product.
Cushioning Requirement: The pulp-molded packaging must has sufficient toughness and flexibility to prevent surface damage to the product upon contact, while effectively absorbing shocks and vibrations during transportation and handling.
Load-bearing Requirement: The pulp-molded packaging must possess adequate strength and rigidity to support both the product’s weight and external pressures.
Structural Design Details
As an emerging packaging solution, molded paper pulp products have only recently entered the market. Currently, their structural design still relies heavily on empirical methods.
Molded paper pulp products feature two distinct surfaces:
Mold-facing side (smooth surface): Precisely conforms to the forming die’s contours, requiring exact dimensional matching with the packaged product’s exterior shape
Free side (textured surface): Engineered to provide structural support and secure product immobilization through optimized surface texture
Molded paper pulp products are manufactured using vacuum suction molding technology, where both the absolute thickness and thickness variations between different sections are constrained by the molding conditions. Generally, the absolute thickness ranges from 2 to 5 mm. This requires the design of cavities in appropriate areas to ensure overall dynamic cushioning performance, along with a certain number of reinforcing ribs to guarantee structural strength. Although the thickness of different sections in molded paper pulp products can be controlled by adjusting the perforation rate in corresponding areas of the molding die, excessive thickness variations may lead to quality issues during the forming and drying processes.
During the molding process, the wet pulp-molded product has a dryness of approximately 10%. At this stage, the slurry retains a certain degree of fluidity, which can lead to vertical fiber movement, causing localized fiber flocculation that affects molding quality—particularly for larger pulp-molded products. To address this, a draft angle of 2°-5° should be designed along the vertical axis. This angle also facilitates demolding. Additionally, sharp 90° or acute angles should be avoided at surface corners of pulp-molded products, as they tend to create stress concentration. The proper solution is to implement rounded transitions.
Edge structure should also be considered in the structural design of molded paper pulp products. The edge structures include flush edge, flanged edge, double-layer edge, etc. The flush edge is suitable for lightweight disposable tableware with low strength requirements. The flanged edge and double-layer edge are mostly used for industrial packaging, which can improve overall strength, cushioning performance and aesthetic appearance.
Mold Design
The mold is critical in the production process of pulp-molded products. Its design must meet both the structural requirements of the pulp-molded products and the forming process conditions. In addition, it must also comply with mold design and manufacturing requirements.
Vacuum adsorption molding is the most commonly used process in the production of pulp-molded products. In this process, the pulp molded products must undergo multiple entries and exits from the molds before becoming qualified products. The required molds mainly include molding molds, shaping molds, etc.
Molding Mold
The molding mold mainly consists of a male mold, female mold, mold screen, mold base, mold back cavity, and air chamber. The mold screen, attached to the mold surface, is woven from wire mesh (metal or plastic) with a diameter of 0.15mm, typically with a mesh count ranging between 40-65. The mold back cavity refers to the hollow space formed between the mold and mold base. It perfectly mirrors the shape of the mold working surface, with the two connected by evenly distributed small holes, maintaining a specific thickness between the mold base and the working surface. The mold is mounted on the pulp molding machine‘s mold plate via the mold base. On the opposite side of the mold plate, an air chamber is installed, which connects to the back cavity. The air chamber wall features two air intake channels.
1. The dimensions of the molding mold are critical to mold design. Pulp molded products need to undergo drying to remove excess moisture as they transition from wet semi-finished products to finished products. During this process, water evaporation causes shrinkage of the pulp-molded products, with varying shrinkage rates occurring at different locations of the product without any predictable pattern. This results in multi-directional and variable shrinkage changes during the forming process, making it difficult to determine proper allowances in mold design.
2. Currently, most molding molds are manufactured using polyester putty, curing agents, resin, and aluminum alloy. For industrial packaging applications with low precision requirements, the molds can be directly produced by casting methods. However, for high-precision requirements, mechanical cutting and machining methods must be employed. In the molding mold, the male mold (convex mold) is primarily machined from aluminum alloy, while the female mold (concave mold) can be cast directly from the corresponding male mold using resin to ensure manufacturing precision.
Shaping Molds
The wet pulp-molded semi-finished product will deform to varying degrees after drying. When high precision is required for the external surface shape, the pulp-molded product needs to undergo a shaping process. The mold used in this shaping process is called a shaping mold, which is generally made of brass, aluminum alloy, and stainless steel. This type of mold requires heating elements but does not necessarily need a mold screen.
In shaping molds, the dimensions of the male mold are customized according to the external dimensions of the packaged product, while the dimensions of the female mold equal the sum of the male mold dimensions and the thickness of the pulp-molded product.