Teaching Aids

PROPERTIES THAT AFFECT FPF PERFORMANCE

1. DENSITY - A measurement of the mass per unit volume. It is measured and expressed in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3).

2. INDENTATION FORCE DEFLECTION (IFD) -- A measure of the load bearing capacity of flexible polyurethane foam. IFD is generally measured as the force (in pounds) required to compress a 50 square inch circular indentor foot into a four inch thick sample, typically 15 inches square or larger, to a stated percentage of the sample's initial height. Common IFD values are generated at 25 and 65 percent of initial height.

3. SUPPORT FACTOR (COMPRESSION MODULUS) -- Support Factor = 65% IFD¸25% IFD determined after one minute of rest or recovery. When the support factor is known it can be used in conjunction with a known 25% IFD value to determine the 65% IFD value. Seating foams with low support factor are more likely to bottom out under load.

4. FLEX FATIGUE (DYNAMIC FATIGUE) -- A softening or loss of firmness. In general, fatigue is measured in the laboratory by repeatedly compressing a foam sample and measuring the change in IFD. Several different protocols, including methods contained in ASTM, ISO, and BIFMA testing standards, may be used. Even ASTM 3574, one of the more widely used sets of testing standards, contains multiple testing methods. In selecting one or more test methods, it is important to consider the intended foam application and to choose a testing protocol that closely simulates the intended application.

5. TEAR STRENGTH -- A measure of the force required to continue a tear in a foam after a split or break has been started and expressed in pounds per inch (lb./in.). This property is important in determining suitability of foam in applications where the material is sewed, stapled, or otherwise anchored to a solid substance.

6. RESILIENCE -- An indicator of the surface elasticity or "springiness" of foam. It is measured by dropping a steel ball onto the foam cushion and measuring how high the ball rebounds.

7. HYSTERESIS -- Measurement of a foam's ability to maintain original support characteristics after flexing. Normally, the firmness (IFD) is tested at 25% indentation, 65% indentation and again at 25% on the way up. Hysteresis is the percent of 25% IFD loss measure as a compression tester returns to the normal (25% IFD) position after measuring 65% compression. Lower hysteresis values, or less IFD loss are desirable. Current research indicates that hysteresis values may provide a good indication of overall flexible foam durability. Low hysteresis in conventional foam is equal to less IFD loss in durability and fatigue tests.

8. AIR FLOW -- Amount of air expressed in cubic feet per minute, that can be drawn through a 2" x 2" x 1" foam sample at .5-inch water pressure differential. Air Flow is measured by a test. High air flow equates to very open foam while low air flow tends to be pneumatic. Air flow can be a critical factor in FPF performance depending on the application. For instance, in normal furniture cushioning applications, a pneumatic foam with low air flow will have poor durability and high compression sets, regardless of density. However, in a packaging foam, a foam with minimal airflow may be desirable as a shock absorber, or in medical applications, slow recovery foams are used to improve patient care. Low air flow foams are also used in gasketing applications in air conditioning and heating.

Note: Details on the laboratory procedures used to measure FPF performance properties can be found in ASTM 3574, and in other testing standards such as those promulgated by ISO and BIFMA. Also, to make a fair comparison of FPF sample properties, the same test methods and testing conditions must have been applied.

 

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