Designing Polyurethane Products
Given an elastomeric shape (where the top and bottom surfaces are flat and parallel) which is compressed between parallel platens, the loaded surfaces of the elastomer want to spread or slip laterally, thus increasing the effective load-bearing area. If on the other hand, the loaded surfaces in contact with the platens are restricted from lateral movement, the compression-deflection behavior of the piece will be different than if the surface is free to spread laterally.
A surface that is oil or grease lubricated will exhibit almost no resistance to lateral slippage top and bottom. If the surfaces are bonded to metal plates, no lateral slippage will occur. Bonded vs. unbonded surfaces result in different compressive stress-strain curves for the same elastomeric part.
How to Use Stress-Strain Curves to Estimate Load-Bearing Capacity
To effectively use compression-deflection data in design, the designer must know the test conditions under which the data were obtained. For uniformity, we have shown all the compression-deflection data obtained with the loaded surfaces bonded to metal. To fully use the data, it’s important to understand the significance of shape factor.
The curves on the following charts were obtained using specimens whose loaded surfaces were bonded top and bottom to steel plates. In unbonded situations where lateral slippage is expected, such as in an oil or water environment, a lab test will be needed to correct the graphs. You should test a specimen with the appropriate shape factor in a simulated condition. The compression-deflection characteristic of a fabricated part may vary as much as +/- 10% from the curves shown here for a compound of that hardness.
Effect of bonded vs. unbonded load surfaces on same compound and same shape factor (click to enlarge)