Open-cell flexible foam: Flexible foam with closed cell volume less than 25%.
Closed-cell flexible foam: Flexible foam with closed cell volume greater than 25%.
Principle: A steel ball of a certain mass and diameter is dropped from a fixed height onto the surface of the sample. The height of the ball's rebound is measured, and the percentage ratio of the rebound height to the drop height is calculated.
Method A, test apparatus and main parameters:
Sponge ball rebound resilience tester, including a transparent tube with an inner diameter of 30mm to 60mm, a steel ball with a diameter of 16mm ± 0.5mm and a mass of 16.8g ± 1.5g, released by a magnet or other device.
No rotation during the drop, always centered.
Drop height is 500mm ± 0.5mm. The top of the ball should be 516mm from the sample surface.
Therefore, the origin for zero rebound is the diameter distance of the steel ball above the sample surface.
If the tube is not vertical, it may cause measurement errors. If the steel ball touches the inner wall of the tube during the drop or rebound, the measurement is invalid. Use a level or similar device to calibrate the hard reference surface to ensure it is horizontal, and place the transparent tube and stand vertically.
Manual reading device:
On the back of the tube, percentage scale lines are marked in an orderly manner, with a major division every 5% (25mm) and a minor division every 1% (5mm), in a 120° arc. This complete circumferential marking is an essential and important part of the instrument, as it eliminates parallax errors.
Automatic reading device:
An instrument that can electronically display the rebound height of the steel ball, which has been proven to yield the same results as manual reading. The rebound height can be calculated from the speed of the ball's rebound or the time interval between the ball's first and second contact with the foam surface. The installed electronic device should display the height with an accuracy of ±1% (5mm). The tube of such a device does not need scale markings.
Method B, test apparatus and main parameters: Same equipment as Method A for foam ball rebound resilience tester, with both manual and automatic reading devices. Digital display ball rebound tester, with an instrument accuracy of relative error less than 1.5%. Main parameters: steel ball diameter same as Method A, steel ball mass 16.3g (0.5g lighter than the steel ball in 5.1). Important different parameter: the drop height of the steel ball is 460mm ± 0.5mm. This makes the rebound values measured by Method A and Method B not directly convertible.
Test samples (polyurethane material):
Samples should have parallel and flat upper and lower surfaces.
Sample area 100mm ± 100mm, height should meet 50mm. If the sample thickness is less than 50mm, it should be stacked to 50mm, but adhesive should not be used. For molded products, the top skin should be removed.
Note: For soft materials, if the results have large errors, thicker samples can be used without being limited to 50mm thickness. For ultra-low density materials, test results may be problematic due to the sample itself. For multi-layer sheet samples, interlayer sliding is likely to occur; it is best to use samples with a larger area to overcome this.
Number of samples: Three samples per group. The three samples can be taken from the same block or from different blocks of the same batch.
Conditioning: After the material is made, it must be stored for at least 72 hours before testing. If it can be proven that the difference between the results obtained 16h or 48h after production and those obtained 72h after production does not exceed ±10%, testing is allowed 16h or 48h after production.
Before testing, samples should be conditioned for more than 16h in any one of the following environments:
23°C ± 2°C, (50 ± 5)% relative humidity;
27°C ± 2°C, (65 ± 5)% relative humidity;
When testing 16h after production, the conditioning time may include part or all of the post-production storage time.
For quality control testing, samples can be stored for a shorter time after production (down to a minimum of 12h) and, according to any of the above environmental regulations, tested after a shorter conditioning time (down to a minimum of 6h).
Test procedure and result expression:
Pre-compression conditioning: Open-cell flexible foam should undergo pre-compression conditioning before testing. The method involves compressing the sample to 75% to 80% of its original thickness at a speed of 0.4mm/s to 6mm/s, pre-compressing twice to prestress condition the sample, then allowing the sample a recovery period of 10min ± 5min.
Note: Prestress conditioning is not applicable to closed-cell flexible foam mentioned in 3.2.