Molded pulp tray packaging, as a relatively new packaging material, has only been used for cushioning for a short time. Currently, its structural design is still in the empirical design stage. The design of the molded pulp packaging tray should be rationally determined based on the external dimensions of the packaged product and the dimensions of the outer packaging box.
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Polyethylene Foam and Molded Pulp Tray Packaging Design
What is the difference between polyethylene foam and molded-pulp trays? Traditional polyethylene foam mainly relies on the elastic modulus of the material for shock absorption and cushioning (generally speaking, when an external force is applied to an elastomer, the elastomer will change shape—deformation. The elastic modulus is the stress divided by the strain in that direction under uniaxial stress. From a macroscopic perspective, the elastic modulus is a measure of an object’s resistance to elastic deformation and is an important performance parameter for engineering materials). Molded pulp packaging trays, however, primarily relies on their cavity structure for shock absorption. Therefore, the structural design of the pulp molded packaging determines its cushioning performance.
3 Requirements for Molded Pulp Packaging Tray Design
The structural design of molded pulp tray includes both the pulp mold design and the forming and transmission mold design. These 2 aspects need to be designed simultaneously and are interdependent. When designing molded pulp disposable fast-food utensils, their shapes are relatively simple, and the strength requirements are not high. However, the industrial inner packaging shapes are usually much more complex, and the design must meet the following 3 main requirements:
Positioning Requirements: The molded pulp tray must ensure that the packaged item has a fixed position within the packaging box. The shape of the pulp tray and the contents must match perfectly, with close contact and accurate dimensions. It must provide good support and fixation within the product’s outer packaging (such as a cardboard box).
Cushioning Requirements: The molded pulp tray must have a certain degree of toughness and flexibility to absorb impacts and vibrations during transportation. The smooth surface of the pulp molded product (the surface that contacts the molding die) must match the outer surface of the packaged product in shape and size. The surface should also be slightly soft to avoid scratching the surface of the contents.
Load-Bearing Requirements: The pulp tray product must have sufficient strength and stiffness, so that it can support the contents and the pressure from external stacking.
3 Key Designs for Molded Pulp Tray Structures
Fixed Support Surface: The fixed support surface is determined based on the product requirements, such as external dimensions and the internal dimensions. For example, when designing a pulp-molded packaging tray for an electrical product, the length, width, and corresponding structural form of the pulp-molded tray can be determined based on 2 factors: the external dimensions of the packaged product and the internal dimensions of the outer packaging, thus determining the fixed support surface.
At the same time, it’s crucial to decide whether the pulp tray packaging prioritizes strength or cushioning. For pulp molded packaging for heavy, high-strength products prioritizes strength, but cushioning performance must also be maintained. If pulp molded packaging is designed for valuable products, prioritizes cushioning, while strength comes second. In one word, the paper tray design must appropriately combine these two aspects, selecting suitable reference points based on the product’s external structure, center of gravity, and strength requirements, and then using these reference points to generate the fixed support surface.
Ribs structure: The pulp package must consider shock absorption on each side. Therefore, rib structures are used to achieve cushioning and vibration absorption, ensuring that the cross-sectional lines on each side are zigzag lines, and improving the pulp tray axial strength. The strength of a molded pulp tray is inversely proportional to its cushioning performance. The more ribs added, the greater the strength, but the lower the cushioning performance.
Cavities and Reinforcing Ribs: The cushioning effect of a molded pulp tray is realized in the following method: the elastic deformation of its paper wall upon impact to mitigate and offset external forces. The pulp tray material itself has relatively low elasticity; it primarily depends on the reinforcing ribs and the cushioning cavities they form. The design of the cavities and reinforcing ribs plays a crucial role in the elastic deformation. The basic basis for the design of cavities and reinforcing ribs is the shape and feature of the product itself. Reinforcing ribs are added to increase the inherent strength of the paper tray.
When designing the structure of a molded pulp tray for packaging, cavities are incorporated to ensure its dynamic cushioning performance. Several simple-shaped cavities with good dynamic cushioning performance are evenly distributed within the paper tray structure, serving to cushion, stabilize, and increase strength. For problems such as structural instability and strength reduction caused by large-area cavities, several or symmetrical reinforcing ribs, or continuous or staggered cavities, can be used to solve this problem.
Increasing the perimeter of a single cavity enhances the structural load-bearing capacity; conversely, increasing the load-bearing height reduces it. Experience shows that the ratio of perimeter to height has a positive impact on load-bearing capacity, becoming more pronounced when this ratio exceeds 5, leading to a rapid increase in load-bearing capacity. The design of cavities and reinforcing ribs should fully consider ease of demolding, simple processing, easy stacking, uniform distribution, and even weight distribution of the product.