Many people confuse density with hardness  when talking about flexible foam and the two are not the same. Density is an expression of mass (or weight) per unit volume and is normally expressed in Kg/m3 or g/cm3 in Europe and Lb/ft3 in the U.S. The hardness of a foam is a measure of how the material deforms when a compressive force is applied and is expressed as force, e.g. N or kpa.

Several factors within the formulation will effect hardness but when it comes to the day to day use of a two part system, the main influence is the mixing ratio between the polyol blend (A component) and Isocyante (B component); more isocyante will increase hardness and less will result in a decrease. A change in mixing ratio will effect the hardness with no change in weight and therefore density. It is worth noting that a small change in mixing ratio can have a big effect on hardness and other properties essay writing for money, so it is recommended that any changes are make under the guidance of your supplier or by referring to their Technical Data Sheet.

The hardness of flexible foams is generally measured by the Indentation Technique using a method like the one laid down in BS EN ISO 2439:2008 using apparatus such as the one below.

Indentometer from Testometric

Indentometer from Testometric

A change in density (or weight) will also have an effect on hardness but to a lesser extent than mixing ratio. If, for example, an moulding is too heavy, it could be possible to reduce the density and rebalance the hardness by increasing the mixing ratio.

Please remember to be careful when making changes as any alteration to density or mixing ratio will effect other properties and processability.

 

 

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