~Comfort and Durability as functions of Firmness and Density~
A frequently heard term when discussing PU foam is “I want that really firm density”.
The truth of the matter is the quality and durability is derived from the density; how much chemical is utilized in making a cubic foot (cu ft) of foam. The higher the density the more durable the foam; up to a point. Long term testing has determined that 1.8 lbs/cu ft density, provides the best balance between fatigue resistance and chemical/foam cost. Improvements in longevity performance are minimal at higher densities when dealing with conventional foam. Firmness of foam (IFD) (Indentation Force Deflection) can be adjusted in the process to achieve similar hardness irrespective of density.
When the properties of foam are reviewed for use in furniture and upholstery it may be that 3-4 different formulations of foam will be used in the construction.
Taking the highest fatigue area, the seating area, the minimum density used should be 1.8lbs/cu ft. The firmness of this component will ultimately depend on the furniture application; task seating will be quite firm around 45 IFD, sofa style would be more plush and much softer at 25-30 IFD.
Trim components such as chair backs are used to create crisp tailoring and are rarely subjected to fatigue loading. In this area foam density of 1-1.2 lbs/cu ft would be cost effective. Inside arms and wraps may be subjected to intermittent load and would typically use a density of 1.2 – 1.5 lbs/cu ft. Firmness here could vary from as soft as 20 through to 40 IFD.
The final consideration is the back support, typically task type chairs will stay with a firmer type material working with the overall design balance between cushion and back. The density can be lowered to 1.2-1.5lbs/cu ft. For plush seating the density will be in this same range but the IFD may be below 20 and may even incorporate some polyester fibers to get the right feel.
In more complex seating the designer may incorporate layers of different hardness; this has the effect of increasing the modulus. It allows for a plush touch but significant support as the load builds.
Post by, Lawrence J Garner