Common Test Terms and Definitions for Flexible PU Foam

Regardless of whether it is furniture foam, shoe material foam, automotive foam, electronic foam, cleaning foam, bra foam, etc., we often test its physical properties such as hardness, resilience, tensile strength, elongation, tear strength, and so on. Below is a summary of the definitions of these physical properties and the common testing methods for your reference.

Hardness

Hardness is generally divided into surface hardness and indentation hardness.

Surface Hardness: For foam materials, we usually use a Shore C or Shore F hardness tester to directly read the value on the foam sample.

Indentation Hardness: According to national standards, place a flat, skinless foam sample with a typical thickness of 50mm on a foam indentation hardness tester. The foam sample is usually circular or square and generally smaller than the pressure plate. Typically, a circular pressure plate with a diameter of 200mm is used. The force applied to achieve a certain percentage of indentation is the indentation hardness (measured in N). Additionally, compression strength refers to the indentation stress divided by the area of the pressure plate.

Tensile Strength

The maximum tensile stress that a unit area can withstand until the specimen breaks.

For the tensile strength test, we generally use a dumbbell-shaped foam sample that must be undamaged. The maximum force (Fb) endured when the sample breaks, divided by the original cross-sectional area (S) of the sample, gives the tensile strength, measured in N/(MPa).

Elongation

The ratio of the increase in distance between gauge marks on the specimen at the moment of rupture to the initial gauge length.

This is fairly straightforward. If an undamaged foam with an initial length of 1 meter is subjected to a certain force and stretched until it breaks, the final length is divided by the original length (1 meter) and then multiplied by 100% to obtain the elongation percentage.

Tear Strength

The maximum tearing force recorded after pulling apart the cut of a test sample of specified dimensions, divided by the thickness.

A specimen of specific dimensions is notched, clamped in the testing machine, and stretched at a specified speed. The maximum force required to tear the sample beyond 50mm is recorded, then divided by the thickness to obtain the tear strength.

Resilience Rate

Typically refers to ball rebound. A steel ball of specified weight is dropped from a specified height to freely fall onto a sample of specified dimensions. The percentage of the rebound height to the drop height is calculated. For example, if the drop height is 1 meter and the rebound height is 0.6 meters, the ball rebound rate is 60%.