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High density foams which have lower mechanical properties give higher total performance. Cell size, resilience and porosity, as a group, are additional physical properties. These properties can affect the initial and hand feel of a cushion, and should be continuously measured and controlled by the foam Manufacturer. These properties, along with the mechanical properties and the compression set properties, which we will 'discuss next are primarily used by the foam manufacturer to monitor more closely the uniformity of his foam production. The basic foam formulation he has designed to give maximum performance will have for these properties a normal or average value. By continuously measuring these properties he can detect additional changes in the uniformity of his foam production. Many people have tried to overemphasize these properties as being the most important properties contributing to performance. The true importance lies in the manufacturer using it as a quality control measurement. Compression set is determined by accurately measuring the height of a foam sample, compressing the sample to a certain percentage of this height and holding it in this position for a given period of time and at an elevated temperature. The compression set properties of almost all urethane foams are more than adequate so failure in service because of this property will not occur. The compression set properties are far superior to almost every other competitive material used in upholstery and cushioning application. In fact, the compression set properties of these com- petitive materials, is generally not even a criteria of rejection. The low density foams, which give the furniture manufacturer the most returns and complaints will generally have the better compres- sion set values. The reasons for this paradox are several. When a slab of foam is manufactured, the exotherm from the re- action releases a great quantity of heat. The internal temperatures in fact can approach 300 degrees F. This heat makes urethane foam self curing. The principal source of heat in the reaction comes from the TDI-water reaction. More water is used in low density foams, more heat is generated, and a more complete cure effected. Higher density foams use less water. However, they generate more than sufficient heat to give a satisfactory cure. Now this does not mean the foam manufacturer should abandon all efforts to maintain low compression set values. Each formulation will have a normal compression set value. Should compression set 10