Density is weight per unit volume, weight/volume, and is expressed as lbs/ft3 (pcf). Density is usually measured in grams/cm3 and multiplied by 62.4 to convert to lbs/ft3 (pcf). Density can be obtained from any sample size. Density is not related to pore size, i.e., coarse pore and fine pore foams can have equal densities. Density is not a measure of firmness, stiffness, or load bearing capacity. That is defined by Indentation Force Deflection (IFD) or Compression Force Deflection (CFD).
Indentation Force Deflection (IFD)
IFD (previously ILD) is one test method to determine load bearing capacity (firmness or stiffness), and is expressed in pounds force per 50 in.2 at a given percent deflection of the foam. For example: P215-50, FHA grade carpet cushion foam has a target IFD of 50 lbs/50 in2 at a deflection depth of 25% of the original thickness of the sample after a rest period of one minute, 25%R. To obtain the 25%R value, a 50 in.2 circular indenter platen is driven into a 15” x 15” x 4” foam sample, stopping when it reaches a deflection of 1”, 25% of the 4-inch thickness. The testing device records the force in pounds required to hold this foam indented after one minute. The higher the force reading, the higher the load bearing capacity of the foam. This result can be reported in metric and the sample size can be varied. Sag Factor (Modulus) is the ratio of the 65%R IFD to the 25%R IFD values and is expressed in real numbers with one decimal. Sag factor gives an indication of cushioning quality. A high value indicates a resistance to “bottoming out.” Recovery Ratio is the ratio of 25%R IFD released to 25%R IYD initial when measuring IFD values at 25% deflection, 65% deflection, and then released back to 25% deflection. Recovery ratio is expressed as a percentage. Guide Factor is the ratio of 25% IFD to density and is expressed in whole numbers. Guide factor is useful in determining the relative firmness of foams with different densities. It is also used to compare the economy of foams. The higher the guide factor, the more economical the foam because you get a firmer foam with a lower density.
Compression Load Deflection (CLD)
CLD is also a measure of firmness and is expressed in pounds per square inch (psi), at a given percentage deflection. The sample size is 2” x 2” x 1” thick. The entire sample is compressed under the 50 in2 indenter platen in this test. The procedure is the same as IFD. Record 25%R & 65%R values. It is common practice on foam specifications is to specify only the 25%R-value. Both IFD & CFD can be tested at 50%R or any other deflection point based on customer / supplier agreement.
% Compression Set
% compression set is a measure of the permanent deformation of a foam after it has been compressed between two metal plates for a controlled time period and temperature condition. The standard conditions are 22 hours at 70°C (158°F) The foam is compressed to a thickness given as a percentage of its original thickness, usually 50%. Compression set is expressed as the percentage of its original thickness that remained “set”. For example: If a 2” x 2” x 1” sample measured 1.00 inch before compression and 0.95 inch after the test, it is reported to have a compression set value of 5%, i.e., it did not recover 5% of its original thickness.