When making a part out of polyurethane, the first two properties to consider are usually rigidity and density. Rigidity, which correlates somewhat with durometer (hardness), is mostly determined by chemical composition, and can be anywhere from that of the hard plastic shell of your computer mouse (about 85 Shore D) to the supple softness of a bass worm lure (about 40 Shore A). Density, as in pounds per cubic foot (pcf), is varied by foaming--adding a blowing agent to the mix that gasses out during the chemical reaction, producing a cellular foam structure--much the same way bread rises from the CO2 that the yeast produces--and can be anywhere from solid (which, for most urethanes, is around 70 pcf) to feather-light like a foam ice bucket (around 2 pcf).

The chart below visually illustrates the combined variability of rigidity and density. If you want to see how this relates to specific applications, click on any of the examples in the list.


The PRUNE Zone

While most combinations can be achieved, not all can. When urethane is foaming, the chemical reaction produces heat. So the gas in the foam cells is hot. When hot gas cools, it contracts. If it contracts in a closed chamber, the chamber pressure goes down, and air pressure outside the chamber now pushes inward with more force. Rigid urethane foam tends to form closed cells, which is OK because the cell membrane is stiff enough to resist the force. Flexible foams have to be formulated to produce open cells that will allow pressures to equalize. If the cells don't open, your molded foam part slowly turns into a prune as it cools on the shelf. Unfortunately, the more rigidity you formulate for, the harder it is to create open cells. Consequently, there are some combinations of density and rigidity that are not practical to create. Click on the prune icon to see where this region falls on the chart.


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