EPE foam cushion packaging design in carton and analysis of its G value control (I)

The theoretical buffer packaging design method is the “five-step method” proposed by Lansmont, and it is very effective to adjust the G of the drop impact with the Lansmont equipment test data.

EPE foam is a good choice for protecting products. To ensure that the breakage rate of the product has a minimum requirement for the strength of the product protection package, the cushioning material's resilience can ensure multiple shocks. In other words, the cushioning material we are concerned with is not to ensure that the product is not damaged on the first impact, but rather that the product can provide continuous protective performance on multiple repeated impacts. Such as ETHAFOAM brand products have natural resilience, their own significant resilience can provide good buffer protection performance in repeated shocks; and it has a variety of strength properties of the material, making it suitable for product weight range From a few pounds to a few hundred pounds. This article uses the ETHAFOAM brand EPE foam as an example to illustrate the steps to determine the shape of the buffer structure.

EPE foam buffer packaging design steps and methods The first step to determine the value of the product's fragility, also known as vulnerability, is a strength concept defined by the product to meet circulation needs. The loss of product value caused by poor packaging during the distribution process of packages is called damage. There are many forms of damage, such as the deformation of thin-walled products, the breakdown of fragile products, the malfunction of instruments, and the leakage of containers. Damage is a kind of circulation accident. It is controlled by the inherent attributes of the product (material, density, structure) on the one hand, and it is affected by the external circulation environment on the other hand. The value of this breakage is described as the fragile value. In terms of magnitude, the fragility value is defined as the maximum acceleration value that the product can withstand without physical or functional damage. It is generally expressed as a multiple G of the gravitational acceleration. The impact tester can be used to determine the fragility of the product's mechanical impact, but be aware of the different impact stances of the product and the G values ​​in different directions.

The second step, determining the circulation environment From the circulation environment, it is possible to analyze the situation of the mechanical shock that the product may encounter and thus obtain the possible drop height of the product. The possible drop height of the product is mainly affected by the weight of the package and the handling method (at the same time the weight of the package also influences the handling mode), the value of which can be determined by the relevant manual or experience.

The third step, buffer structure prototype design According to the product's crisp value (impact resistance) and circulation environment (impact strength), packaging engineers should choose reasonable packaging materials and packaging structure, so as to mitigate the impact to achieve the purpose of product protection. When selecting materials for packing, try to use familiar packaging materials (understand the mechanical properties of the materials), buffering the dynamic impact cushioning characteristics (Figure 1) and vibration transmission rate characteristics (Figure 2). Their abscissa PSI is the static load in pounds per square inch, which is the application of the force of the packaged product on the cushioning material. Now suppose that a product is determined to have a drop height of 36 inches based on its weight (20lbs) and its crispness (40g's), and related standards.

Fig.1 Dynamic impact cushioning curveFig.2 Vibration transmission rate characteristic curve Determination of buffering distance (thickness)

Select SealedAir's CelluPlank® 220 Polyethylene Foam as a buffer material. The dynamic impact cushioning curve of this material is shown in Figure 3. This product SealedAir can provide three different thicknesses, namely 2", 3", 4". In the figure, a straight line parallel to the horizontal axis is drawn with the product crispness (brittleness value of 40g's), which is called a product. The crisp line of the buffer, the dynamic impact cushioning characteristic of the cushioning material can reach the protection product under the expected impact when the part below or below the crisp line, and the dynamic impact cushioning curve of the cushioning material is above or above the crisp line. In part, the product will be destroyed under the expected impact. It can be found that the minimum G values ​​of the two thicknesses of 2” and 3” are all above 40g's, so the two thicknesses of 2” and 3” Foam cannot meet the protection requirements of the product. The Foam's dynamic buffering curve of 4" thickness has a portion below the G value of 40g's straight line, so it can meet the buffer protection requirements of products with a G value of 40g's.

Figure 336. Drop-height 2-5 drops average value The CelluPlank® 220 Polyethylene Foam product with a static load value and a buffer area thickness of 4” has a dead load between a1 (0.4psi) and a2 (2.35psi). Products with a G value of 40g's provide the protection needed. Based on a product weight of 20 lbs, the static load at a1 has a buffer area of ​​50 square inches (20 lbs ÷ 0.4 psi) and the static load at a2 has a buffer area of ​​9 square inches (20 lbs ÷ 2.35 psi). This value is reduced by 82% compared to the value obtained when using a1 (0.4 psi), so that a larger static load can be obtained to reduce costs when the static load is chosen between a1 (0.4 psi) and a3 (1.1 psi) Overpacking will occur, which is not promoted.

It is worth noting that if the use of thicker materials to reduce the amount of materials, the results will increase the storage, transportation and handling costs due to the increase in the size of the outer package, often this cost will be greater than the cost of the buffer material saved, Therefore, designers should give priority to selecting the smallest material thickness. By verifying multiple comparisons of several materials, the most economical balance points can be found relatively quickly in material costs and package sizes.

The vibration transmissibility curve of CelluPlank® 220 Polyethylene Foam is shown in Fig. 4. The vibration amplification and attenuation of this material are affected by PSI. After understanding the vibration parameters of the product, the product and the buffer structure will form a new system (package). The ratio of the actual damping in the system to the critical damping coefficient of the system is called the damping ratio (ζ). After the buffer material is determined, the system damping ratio is mainly affected by the buffer structure (buffering distance and buffer area). It is easy to see from Figure 5 that the value of the system designed by the packaging designer should be as close to 1 as possible. The ratio of the amplitude of the forced vibration of the system (package) to the static stress offset represents the effect of the disturbance force on the dynamic action of the vibration system and is called the power amplification factor (β). After the product circulation environment is determined, through package standards (such as ISTA), it is possible to determine the frequency of vibration that the product may undergo during the circulation process. From Figure 6, we can see that when designing the packaging structure, we should try to avoid that the system frequency is equal to or close to the frequency spectrum of the circulation process, and the ratio should be controlled to be greater than that, so that the packaging structure is protected against vibration to protect the product. Due to the fact that the vibration damage does not have a strong impact and destruction, in many cases, the design of the structure does not focus on vibration protection. The complexity of the package system, the limitations of the test equipment, and the ability to control the damping ratio (ζ) and the power amplification factor (β) by changing the structure are not easy to achieve. This is one reason that structural engineers do not look at vibration protection.

Fig. 4 Vibration transmission rate characteristic curve Fig. 5 β-λ curve Fig. 6 The resonance frequency of the Tr-λ curve product and the final resonance frequency of the package can be obtained by the sweep test. Sweep frequency test refers to a test in which one or two vibration parameters (displacement, velocity, or acceleration) are kept constant during the test, and the vibration frequency continuously increases and changes within a certain range. Figure 7 is a record of the 2Hz to 5Hz sweep process under the ISTA test standard. The test equipment used was the Vibration System from Lansmont.

Figure 7 VibrationView

(To be continued)

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