Footpads had their origins in the 1920s when felt on the feet of furniture kept scuff marks off wooden floors. Later, rubber pads on telephones helped eliminate mars on desktops. Custom-blended urethane materials dominate the footpad market today, with molded solid urethane with an adhesive backing top-ping the list. More recently, diecut footpads made from cellular urethane materials offer designers a new option.
Selection of these cellular or foam footpads relies on factors which include footpad loading, thickness requirements, whether the device will be stationary or slide around, and the level of material "cleanliness" the application demands.
STEP ONE: DETERMINING APPLICATION PARAMETERS
The performance of a footpad depends on loading, compression, material properties such as density, firmness, and pad thickness.
Fortunately, these properties interrelate and are captured on load-compression graphs. Three different grades of Poron cellular urethane help illustrate the selection process. As a hypothetical example, a 20-lb computer monitor needs four circular footpads in 1 /16-in. recesses on the monitor base. Each 1-in.-diameter pad has an area of 0.785 sq in. and experiences a force of 6.4 psi.
First examine load-compression graphs at 25% deflection or strain. Deflection of 25% is considered the standard because urethane at this compression level provides its best protection, maximum longevity, and vibration damping. At the juncture on each graph where 6.4 psi stress meets 25% strain the material stress/strain curve that comes closest to the intersection is Poron 470-40.
Second, a pad thickness must compensate for 25% compression plus the 1/16-in. recess. Obviously the selection hinges on getting a pad thick enough to accommodate the recess when compressed.
The Poron cellular urethanes don't take a permanent compression set under load and retain over 90% of their original thickness when tested under ASTM D3574 guidelines at room temperature. It is important to note however, that compression set test methods for foams don't apply to solid elastomers. Solid materials deform or creep over time and often take on a permanent compression set. They also don't compress as much as foam, often reaching only an upper limit of 15% of their total thickness.
STEP TWO: "SLIP" OR "GRIP"
Material gripping power is described by coefficient of friction. COF is a measure of the relative difficulty with which the surface of one material will slide over another. High COF denotes a better grip.
It is always best to consult with your materials supplier when selecting the COF of a material. However, you can use the following rule of thumb: if the application sees occasional use, such as a kitchen appliance or a battery charger, select a material with a coefficient of friction of 0.4 or less. In cases where the device should remain stationary, select a COF of 1.0 or greater.
STEP THREE: FOOTPAD CLEANLINESS
Many elastomers contain plasticizers for better flexibility at low temperatures. Often, these plasticizers over time migrate to the surface. Stains, contamination, and even surface corrosion may arise depending on plasticizer formulation. Poron polyurethanes contain no plasticizers and re-main flexible to -40°F.
Information for this article was provided by Fred Seidel, Applications Engineer, Rogers Corp., One Technology Dr., Rogers, CT, 06263 (860) 774-9605.