Cushioning

Different plastic foams are used in many different products meeting different performance requirements (Chapter 1). In the packaging industry they provide different performances that includes cushioning. They are used in different weights to meet different product requirements. Regarding cost and performance it is sometimes believed that the lower density closed-cell foams that are usually priced lower provide superior cushioning performance. This assumption is usually incorrect as shown in Fig. 4.34. Even though it contains less plastic, the fabricating rates, the amount and cost of the blowing agent, and the amount and cost of the base plastic all influence the final cost. As a

Cushioning effect of polyethylene foam density is influenced by loading

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8 40

Static Loading (psi)

result, very low-density foams can actually be more cosdy to make than others. Cost of a foam is usually not proportional to its density.

The same type and thickness closed-cell polyethylene foams with differing densities have been subjected to dynamic static load cushioning conditions during impact. Curves relate mechanical shock experienced during an impact where the lower the curve goes, the greater the cushioning efficiency. Curves for foam densities above 6 pcf (lb/ft3) the maximum cushioning efficiency of each material is not significandy different, however a dramatic change occurs with a change in the applied load.

If a 40 g package were to be designed according to Fig. 4.34 using a 6.0 pcf foam, the foam would measure 3 in. thick resulting in 40 g shock. If a similar package were then produced using a 2.2 pcf foam, its shock performance would not go as low as 40 g but would instead produce about 60 g's, or 50% more shock or a 50% loss in shock efficiency.

To meet 40 g's, the 2.2 pcf package would need to be redesigned. Greatly increasing the thickness of the pads constructed from the lower density foam can be used to provide adequate protection. However the result would of increasing the package size, impair handling and shipping efficiency, and possibly result in higher costs. The 6.0 pcf foam could, however, be reliably used.

To be at 40g level and keep the 2.2 pcf foam thickness the same, reduce loading from 1.35 to 0.87 psi can be used. Although this approach

! igut 'L ~ ~ Comparison of different foam densities

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Static Loading (psi)

keeps the package size the same, nearly twice as much foam must be used to meet the lower loading. Lowering the density produces a considerably higher deceleration and reduces cushioning performance. Fig. 4.35 compares the effect of density. The lower density foam cost less than half on a cost-per-unit volume basis resulting in a cost savings. Below a density of about 2.2 pcf the cushioning efficiency can begin to change with the density.

Significant in this figure is the narrower range of usable static loadings at the bottoms of the curves that resulted when the density is reduced. Important consideration in comparing foams of different densities is their compressive creep resistance, and their ability to resist undergoing a permanent thickness loss during their time under load. As the density decreases, so does the creep resistance.

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