Structural Elements of Pulp Molding Products
1. Design of Cushion Packaging Structural Elements
The structural design of pulp molding products is based on the fundamental elements of cushion packaging, including wave shapes, trapezoidal shapes, and hemispherical shapes, with wave and trapezoidal shapes being the most common. The design process also requires the integration of various reinforcing ribs to enhance the performance of these basic structural elements, thereby improving the overall performance and functionality of the design, making the pulp molded product structure more reasonable and practical to meet the packaging requirements of different products.
2. Performance of Basic Structural Elements
(1) Load-bearing Capacity
Different cushion packaging structures have varying static load and deformation capacities. Ideal cushion packaging structures have load-deformation (P-x) curves that approximate a hyperbolic tangent curve, limiting the maximum force transmitted to the cushioning material and absorbing energy efficiently. Trapezoidal structures have greater load-bearing capacity than wave structures because the sidewalls provide better support in trapezoidal shapes, whereas wave structures tend to bend more easily under pressure.
The addition of reinforcing ribs significantly enhances the load-bearing capacity of both trapezoidal and wave structures. When the front surface of the pulp mold is stressed, the ribs provide cross stability, limiting deformation and increasing load-bearing capacity. The smaller the sidewall inclination, the higher the load-bearing capacity, but too small an inclination can hinder the demolding of wet pulp molds, thus requiring a balance between load-bearing capacity and production convenience.
(2) Absorption of Impact Energy
Cushioning materials absorb external energy during the product flow process, protecting the product. The larger the area under the load-deformation curve, the more energy is absorbed. Wave and trapezoidal structures, as well as structures with added reinforcing ribs, can effectively absorb impact energy. The degree of energy absorption is also related to the physical state of the pulp molding products, such as density, quality, and deformation patterns, which should be controlled during production to improve energy absorption.
(3) Elastic Recovery of Impact Energy
Elastic recovery refers to the ability of cushioning materials to return to their original state and dimensions after unloading. Materials with good elastic recovery can withstand multiple impacts while maintaining contact with the packaged product; materials with poor elastic recovery undergo significant plastic deformation after impact, increasing the likelihood of product damage after multiple impacts. Elastic recovery is described by the rebound rate, determined through static compression tests. Improving elastic recovery requires optimizing both structural design and manufacturing processes.
Structural Design Process of Pulp Molding Products
1. Client Requirements
Clients provide general requirements and needs. Engineers use their experience to communicate and understand specific client requirements.
2. Design Requirement Analysis
The analysis of design requirements includes product positioning requirements, assembly methods of pulp molded products and outer packaging boxes, and cushioning protection performance requirements.
3. Comprehensive Investigation and Research
In-depth and comprehensive investigation and research are essential during the design process. Understanding the product status, including its position in the industry, market positioning, consumer groups, sales channels, logistics, and storage methods, is crucial. Studying existing packaging, brand history, philosophy, culture, and brand identity standards, along with on-site inspections to understand product display, arrangement, opening methods, and protection methods, ensures a thorough understanding of the consumer experience.
4. Conceptual Design Plan
Based on the results of the demand analysis, engineers propose a conceptual design plan. Brainstorming during the concept design phase is recommended to gather diverse ideas. By diverging and converging thoughts, establishing a mental model, and integrating keywords and related elements, the structural design, opening methods, materials, and processes can be rethought.
5. Drawing and Process Review
According to the confirmed conceptual design, product shape characteristics, dimensions, gravity distribution, performance protection requirements, and other determined requirements, combined with the actual process characteristics of each enterprise’s production, the conceptual design is implemented for production. The drawings of the pulp molded products are then created with corresponding technical parameters marked, and a joint review with production personnel is conducted to ensure the design can be mass-produced.
6. Sample Confirmation
Samples can be made in a simplified manner or as single-mold products for direct product fitting or performance verification.
7. Design Modification and Sample Reconfirmation
If the fitting reveals dimensional deviations, redesign and remolding are required, followed by sample production and confirmation.
8. Performance Verification
Performance tests such as drop tests and vibration tests are conducted based on requirements. If the tests fail, the design is adjusted, and samples are remade and verified until the tests are passed.
9. Final Production and Batch Feedback
All design documents are archived and released for production. During subsequent batch supply, the actual usage effects are continuously monitored, and feedback is used for ongoing optimization and improvement.
Key Points of Structural Design of Pulp Molding Products
1. Fixed Support Surface
This is the most basic point in structural design. The appearance surface and the fixed support surface (usually the inside opposite to the appearance surface) are defined based on product requirements. The fixed support surface is determined by the shape and dimensions of the packaged product and the internal dimensions of the outer packaging. For example, for packaging an electrical product, the fixed support surface is determined by the product’s shape and the outer packaging’s internal dimensions. The design must consider whether the packaging focuses on strength or cushioning, and combine both aspects appropriately.
2. Cavities and Reinforcing Ribs
The cushioning effect of pulp molded packaging mainly relies on the elastic deformation of its walls upon impact. The elasticity of the pulp material itself is not high; rather, the design of reinforcing ribs and the resulting cushioning cavities play a crucial role. Cavities ensure dynamic cushioning performance, while reinforcing ribs increase strength and enhance dynamic cushioning. Simple, well-distributed cavities improve cushioning stability and strength. Reinforcing ribs solve stability and strength issues caused by large cavities. The design of cavities and ribs should consider production ease, simple processing, and even distribution to evenly disperse the product’s weight. Generally, cavity dimensions should exceed 8mm to facilitate demolding.
3. Related Parameters
Dimensions and Wall Thickness
The primary requirement of structural design is to ensure a tight fit with the packaged product to prevent movement during transit while considering cushioning and shock absorption. Dimensions should be compact, achieved through the integration of fixed support surfaces, cavities, and reinforcing ribs. Wall thickness, influenced by usage conditions and fiber type, impacts strength. Increased wall thickness raises raw material consumption, reduces production efficiency during vacuum forming, increases drying energy consumption, and can lead to quality defects. Therefore, wall thickness should be minimized while ensuring strength. Vacuum forming yields wall thicknesses of 0.5-6mm, and pressing forming yields 3-20mm.
Demolding Angle
During molding, wet pulp adheres to the mold mesh, with fibers embedded in the mesh. A reasonable demolding angle facilitates the transfer of wet pulp. Larger angles ease demolding, while smaller angles can cause surface marks or cracks but reduce size accuracy and affect load-bearing and elastic recovery. Generally, demolding angles range from 3° to 6°, adjusted based on specific product requirements. Modern technology allows for zero-angle demolding. The demolding angle also affects the nested stacking during transport, aiding easy separation and insertion of products.
Transition Radius
The inner and outer surfaces of pulp molded products, and the connections between ribs and the main body, should use rounded transitions to avoid sharp edges. Rounded transitions aid mold manufacturing, mesh attachment, pulp flow during forming, and prevent stress concentration that could damage packaging. Transition radii depend on the product, pulp performance, and packaging requirements, typically ranging from 2-5mm, with some precision molds allowing for radii as small as 0.2mm.
Practical Tips for Structural Design of Pulp Molding Products
1. Utilizing Reinforcing Ribs
Cavities are fundamental for protection, and reinforcing ribs enhance cavity performance. Ribs connect and divide original structures, forming a “convex-concave” structure that changes the stress model, enhancing resistance to collapse or damage.
2. Utilizing Edges and Flanges
Structural design can incorporate reinforcing rib shapes on edges or flanges to alter the stress model of edge structures or create fixed support surfaces for other products.
3. Using Die-cutting Holes
Pulp molding structural design can create die-cutting holes for use with other products or fixed support surfaces, enhancing stability and performance.
4. Using Corners
Structural design can utilize corners to alter the stress model of edge structures or provide fixed support surfaces for other products, improving overall performance and protection.
