Indentation Force Deflection (IFD) tests measure firmness of flexible polyurethane foam cushions. High IFD test results imply increased hardness. Low IFD results indicate soft foam products. Note that the term IFD, or indentation force deflection, is interchangeable with ILD, or indentation load deflection.
The IFD test is one of multiple test methods that are all defined in the foam industry’s most well recognized standard test method – ASTM D3574. Within ASTM D3574, the indentation force deflection procedure is Method B1 which measures the force (in pounds) required to indent an eight inch diameter steel plate (called an indentor foot) into a foam sample to a stated percentage of the test sample's initial height which is commonly four inches. The test procedure, for example, compresses a four inch thick sample of foam until it reaches three inches thick, and the compressive load needed to displace it is measured. Common IFD values are generated at 25 and 65 percent of initial height. As an example, 35 lb IFD foams are associated with medium firm foam mattresses, and 45 lb IFD foams are associated with very firm mattresses.
Comfort Support Factor or Compression Modulus
One of the measured outcomes of an IFD Test is termed ‘Comfort Support Factor’, which is the ratio of indentation force deflection measurements 65% IFD/25% IFD. Support factor is an important indicator of the point at which foam cushions bottom out. Low support factors are more likely to hit bottom.
When the support factor is known, it can be used in conjunction with a known 25% IFD value to determine the 65% IFD value. Good support (high ratios) from foam products can mean that the cushion doesn’t compress to the point where they no longer hold up the weight of a person. High values also mean the cushion is capable of distributing the weight of the person for maximum comfort. Comfort support measurements are more valid measurements of foam’s cushioning ability than other specifications because one sits deeper than 25% into a seat cushion.
Varying IFD values will be obtained if a different percentage deflection is employed or if the thickness (height) of the test specimen is different. The actual thickness must be measured accurately. Another critical measurement is test sample size (15 in x 15 in or 20 in x 20 in being common), which should be maintained for comparable values. Note that it is common practice to test samples sized close to end product sizes, rather than to hold to the standard. This works as long as comparison of test results occurs on like sized samples. When comparing IFD results between runs, lots or cushions, the most important consideration is that both sample sets are made of the same exact size.
Manufacturing process IFD values also vary, both within foam runs and run to run. Note that a variety of processing variables can impact final physical foam properties. Results may also vary depending on where the indentor foot is placed on the foam product (top to bottom, side to side or front to back).
Performing lots of indentation force deflection tests are common -- driven partly by the fact that variations in foam hardness or firmness can come from the raw material, from process variations, or different testing details like sample thickness, sample width or test machine settings. Another contributing factor is the long value chain from material supplier to seat assembly maker. The complete IFD testing process involves a constant check to confirm incoming lot qualification is acceptable.
Test Machines engineered for IFD testing
The main challenge of IFD testing is to produce consistent IFD test results between production runs of foam. There are multiple variables involved and considerable focus is needed to resolve differences.
The indentation force deflection test is used in both research and production line testing applications in spite of the fact that the ASTM D3574 Method B1 procedure can take a long time (12 minutes in the case of a four inch foam slab) because of the hold time specified in ASTM D3574.
In many production testing applications, shortened test derivations can quicken the quality assessment of the material and the process. Fast moving production processes often develop unique internal tests to shorten test times.
Load Frames engineered for indentation force deflection tests
TestResources load frames are modular and can be configured to meet sample sizes ranging from 15x15, 20x20 and on up to load frames sized for mattresses. Of course, each frame is supplied with a baseplate with air holes to match. Our cantilever arm load frame features a collapsible test table for testing irregular shaped cushions and complex product forms. Load frames can be configured with actuators sized to match specific speed and load requirements. Standard actuator – frame solutions are available with force capacities from 250 lb (1.1 kN) to 1000 lb (4.5 kN).
Special IFD Test Fixtures
ASTM D3574 Method B1 requires an eight inch diameter, 50 in2 indentor foot, which is most common. In addition, TestResources engineers special fixtures as needed for the whole range of different tests and test samples, including bolster tests that employ hemispherical indentor feet.
Controller and IFD Software
The IFD software and controller automate the full testing procedure and are interfaced to a precision load cell and indentor foot position sensor. This ensures the IFD machine load and position channels are controlled using the operator’s commands which are entered into an easy to use IFD software program. As the test initiates, the original sample height is measured using a one pound preload, cancelling small height variations under the indentor foot. In the case of IFD testers, TestResources supplies a single dedicated IFD software program that enables the user to run tests according to ASTM D3574 B1 and also variants to that test.
Applicable Testing Standards
- ISO 2439 Indentation Hardness of Flexible Cellular Plastic Foam
- ASTM D3574 Testing Equipment for Flexible Cellular Urethane Foams
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