In-Depth Primer ~ Foam
The ideal foam for mattresses or futons and seating will provide a soft to the touch surface feel and deep down firmness that does not bottom out.
Density is measured as the weight of a cubic foot of foam. Density is the single most important property for consideration by the foam buyer.
Indentation Load Deflection
ILD or IFD is the foam industry's unit of measure to express a degree of firmness. Based upon a calculation that determines how much a fifty square inch indenter must weigh to compress a 4-inch thick piece of foam to three inches (known as the 25% ILD reading), and how much a fifty square inch indenter must weigh to compress a 4-inch thick piece of foam to 1.4 inches (known as the 65% ILD reading), the ideal ILD reading would be 25 to 40 pounds at 25%, and 45 to 100 pounds at 65%.
The comfort factor, or "support ratio" is the ratio of the 25% to 65% ILD readings. A good comfort factor is between 1.8 to 3.5.
Resilience for foam is the measure of its springiness. It is measured by determining the percent rebound of a steel ball dropped from a height of three feet. Mattress foams should range from 40% to 70% resilience.
Other factors like Compression Set, the permanent deformation of foam due to usage, Static Fatigue, the relative degree of softening over the products life, and Tensile Strength the greatest stress in pounds per square inch that foam can bear without rupture are also considerations when purchasing foam for mattresses.
This information was compiled by Bedding Magazine (now Bed Times) using materials supplied by several foam industry sources and the Society of the Plastics Industry (SPI). They give special thanks to North Carolina Foam Industries - Chuck Wittenaur, Dr. Bradley and NCFI's Bulletin 201.