In my last two postings I reviewed equipment fragility and the influence of handling and transport on packaging design. The most common approach to protecting equipment within a carrying case, transit case or shipping case is to use foam cushioning. In this posting, I’ll provide an overview of foam materials.
Foam cushioning materials fall under two main categories:
Open cell foams have cells (pores) which are interconnected. This permits gasses (or liquids) to pass from one cell to another, and ultimately escape from the media. A sponge as used in the kitchen or to wash your car is a classic example of open cell foam. An open cell foam’s resistance to compression is influenced by the flow rate of the gasses (or liquids) from cell to cell, and the cell wall’s resistance to deformation (rigidity).
As the name implies, closed cell foams have a closed cell structure. This permits gasses to remain trapped within the cell. Because gasses cannot travel from cell to cell, closed cell foam’s resistance to compression is in part influenced by the cell wall’s deformation and burst (rupture) resistance along with the increase in pressure within the cell (Boyle’s Law). Because the trapped gasses have nowhere to travel, under increased compression (reduced volume) the cellular pressure will increase. Utlimately, the cell walls start to yield and ultimately rupture. So, resistance to compression of a closed cell foam increases until the opposing (resistive) force of the foam equals the compressive force OR the cell walls start to distort and/or rupture. Squeezing bubble wrap is a good example of this.
The density of an object is determined by dividing the object’s mass by its volume. For foam materials, density is expressed in North America as pound per cubic foot (PCF). In Europe and elsewhere, foam density is expressed as kilogram per cubic meter.
From the practical perspective of this discussion, gasses have no mass. I expect scientists and engineers to disagree – and, they are correct. However, the nominal mass of a gas falls beyond of the scope of this discussion. With this in mind, the only way to increase density of a given foam substrate is to have more material – therefore, less gas.
There are two ways to achieve this: 1) more cells (which are smaller) so there is an increase in number of cell walls; 2) Maintain (or reduce) the number of cells but increase the cell wall thickness.
As discussed above, the performance of a foam cushion is affected in part by the rigidity (resistance to distortion) of the cell wall. So, increasing the number of cell walls or increasing the cell wall thickness will increase rigidity. The increase in cell wall rigidity means that – generally – the foam material will be more resistant to compression.
A wide variety of foam substrates are available. The most common are the polyethylene and polyurethane foam variations. Polyethylene foams are typically closed cell. Polyurethane foams are open cell.
The following is a partial listing of the more common foam substrates found in protective packaging.
• Expanded Polyethylene Foams (commonly called “Etha-foam”)
• Cross-Linked (Expanded) Polyethylene Foams
• Expanded Polypropylene Foams
• Polyurethane Ester Foams (commonly called “Ester-foam”)
• Polyurethane Ether Foams (commonly called “Ether-foam”)
• Polystyrene Foams (commonly called “Styro-foam”)
(Ethafoam™ and Styrofoam™ are registered trademarks of the Dow Chemical Company)
Selecting a foam material for you application should be decided with the following factors in mind:
Q. Will the foam perform adequately from a shock attenuation (cushioning) perspective? I will discuss this in a future blog.
Price / Availability
Q. Is the cost within my budget?
Q. Is this product stocked or is there a long lead time?
Q. Does the foam need to be cleaned or decontaminated? If so, then closed cell foam is required.
Q. Does my application need special colored foam? Not all foams are available in multiple colors.
Q. Does my application require specialty foam properties such as being anti-static or fire retardant? Only select foams are available with specialty properties.
In my next blog, I’ll discuss using cushioning curves to assist in selecting the appropriate foam material and thickness for protective packaging applications.