There are numerous descriptive terms by which foam products can be tested, described, rated, & specified, including (but not limited to):
is a measure of the ease with which air will pass through a foam sample. (Test Method ASTM D3574).
is a test procedure used to measure the surface resiliency of flexible polyurethane
foam. The test involves dropping a steel
ball of known mass from a predetermined height onto a foam sample. The rebound height attained by the steel
ball, expressed as a percentage of the original drop height, is the ball rebound resiliency value. (Test Method ASTM D3574).
is the number of cells per linear inch or centimeter, expressed as pores per inch or pores per centimeter.
Cell Size is the average diameter of the cells in the final flexible polyurethane
foam product, often measured in micron units.
Constant Deflection Compression Set
is a test used to determine the amount of foam recovery from a static or fixed compression. Test method is described in ASTM D3574.
is the mass of a substance divided by its volume. In the United States, density is accepted as the weight of a substance divided by its volume. Foam density is often expressed as pounds per cubic foot or kilograms per cubic meter. (Test Method ASTM D3574)
is an instrument used to measure hardness of elastic materials. Durometer is also used to reference a scale of hardness; ie, a low durometer implies a soft material.
is the percentage that a specially shaped sample will stretch from its original length before breaking. (Test Method ASTM D3574).
- GF = 25% IFD divided by density determined after one minute rest.
is a Synonym for the 50% IFD value. Some furniture designs are for a maximum 50% indentation while some are for only a 20% indentation, ie., chairs versus bar stools.
is the ability of a flexible polyurethane foam to return to its original support characteristics after it is compressed. Hysteresis = (25%)
- 25% IFD after compressing 65% of initial height)/25% IFD initial * 100.
Indentation Force Deflection (IFD)
is 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 no smaller than 24 inches square, to a stated percentage of the sample's initial height. Common IFD values are generated at 25 and 65 percent of initial height. Reference Test Method ASTM D3574.
- IM = (40%IFD-20%IFD)/20%IFD. IM is the force required to produce an additional 1% indentation between the limits of 20% IFD and 40% IFD determined without the one minute rest. The slope of this line represents the resistance of the cell struts to post buckling. The slope of the linear portion of the stress-strain curve is defined as the indentation modulus.
Indentation Residual Deflection Force
(IRDF) is a test method used with seating foam to determine how thick the padding is under the average person. The amount of deflection is determined by measuring the thickness of the pad under fixed force of 4.5 Newtons, 110 N, and 220 N on a 323 square centimeter circular indentor foot.
Initial Hardness Factor
- IHF = 25%IFD/5%IFD determined without the one minute rest. This ratio defines the surface feel of a flexible foam. Soft surface foam will have a high IHF value, while stiff or boardy surface foams will have a low IHF value.
is the density of a foam sample at its center. Generally, a foam will form a density gradient, with the highest density being at the outer, or skin surface, and the lowest density being at the core
of the foam sample.
Modulus Irregularity Factor
- MIF = 2 * 20%IFD - 40%IFD. The MIF is the extrapolated intercept of the stress axis, or y axis, of the linear portion of the stress-strain curve. If the MIF is zero, the indention modulus is essentially constant, and the stress-strain curve is linear and passes through the origin. If the indentation modulus varies at low levels of strain before reaching a constant value at above approximately 10 per strain, the MIF will either be a positive or negative value. The degree of deviation of the MIF from zero describes the shape of the lower end of the stress-strain curve, and thus the performance of cushioning for some seating applications.
is accelerated fatigue aging of flexible polyurethane foam by cyclically compressing samples to a specified percentage of their original height and releasing for a specified number of repetitions.
is the loss in load bearing properties of a flexible polyurethane foam sample under constant compression of 75% for 17 hours at room temperature. See Test Method ASTM D-3574
- Support Factor = 65%IFD/25%IFD determined after one minute of rest or recovery. When based on 25% IFD values, the support factor indicates the 65% IFD values that will be attained by the foam. Seating foams with low support factor are more likely to bottom out under load.
- The pounds per square inch of force required to stretch a material to the breaking point. Reference ASTM D-3574.
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