Joint Industry Foam Standards and Guidelines

SECTION 4.0

Published: 7/94

INDENTATION FORCE DEFLECTION (IFD) STANDARDS AND GUIDELINES

4.1 To the furniture manufacturer and final user of a piece of furniture, one of the most important quality questions is related to the firmness of the seat cushions. The firmness of a polyurethane foam cushion is measured by a physical property called the indentation force deflection (IFD).

 

4.1.1 The history of describing firmness is very interesting. Prior to the advent of polyurethane foams, rubber latex foams were in wide use for furniture cushions. The term used to describe the firmness or softness of foam rubber was RMA, which stood for Rubber Manufacturers Association. RMA was measured only slightly different from the way IFD is measured today.

4.1.2 When polyurethane foams arrived on the scene, they weren't associated with the rubber industry, so the acronym "ILD" was developed. "ILD" stood for "indentation load deflection." During the drive for conversion to the metric system in the late seventies, the American Society of Testing and Materials (ASTM) decided that in all of their publications and test methods, the metric system would be used. Because the ASTM insisted on the use of the word "force" rather than "load," the term "IFD" came into common use--replacing "ILD." IFD stands for "Indentation Force Deflection and the actual test method is basically identical to the older ILD test.

4.2 It should be noted that the foam IFD is only one of the contributors to the comfort of a furniture seat cushion. There are many other contributors, and some of these have already been discussed.

 

4.2.1 In this publication, The Joint Industry Committee has purposely avoided using the word "comfort" directly associated with IFD or IFD properties. Suffice to say, IFD is a part of the comfort equation, but IFD is not always related directly to comfort. For example, one cannot say that a 25% IFD of 26 lbs/50 in2 always produces comfort, while a 25% IFD of 40 lbs/50 in2 does not produce a comfortable seat. Comfort is not directly related to the magnitude of the IFD number alone.

4.2.2 IFD is defined as the amount of force, in pounds, required to indent a fifty square inch, round indentor foot into a predefined foam specimen a certain percentage of the specimen's total thickness. IFD should always be specified as a number of pounds at a specific deflection percentage on a specific height foam sample, e.g., 25 pounds/50 insq. at a 25% deflection on a four inch thick piece. Different IFD values will be obtained if a different percentage deflection is used or if the height of the test specimen is different. It is also necessary to report the entire sample size. Sample size, in addition to thickness, can drastically influence IFD readings. Flexible polyurethane foams can be made in a very wide range of IFD's. To get a good feeling of the potential uses of each of the various IFD ranges, the following chart should be of some assistance:

 

IFD @25% DEFLECTION USE (pounds/50 insq. on 20 "x 20"x 4")

6--12--------------Bed pillows, thick back pillows

12-18--------------back pillows, upholstery padding, wraps

18-24--------------thin back pillows, tufting matrix, very thick seat cushions,wraps

24-30--------------average seat cushions, upholstery padding, tight seats, certain mattress types, quilting

30-36--------------firmer seat cushions, mattresses

36-45--------------thin seat cushioning and firm mattresses

45 and up----------shock absorbing foams, packaging foams, carpet pads, and other uses requiring ultra-firm foams.

The above table should only be used as a beginning guideline. The actual IFD required is a function of many things, such as the design type, spring type used, and other parameters within the actual furniture construction.

4.3 IFD varies significantly with foam thickness. On the exact same foam, the IFD increases as the thickness increases, as the following chart illustrates:

All samples are 20" x 20" x stated thickness.

 

Sample Thickness (inches) IFD at 25% deflection (lbs/50 in squared)

4---------------------------28.0

5---------------------------31.0

6---------------------------34.5

7---------------------------38.5

8---------------------------43.0

IFD values in the above table were obtained from testing actual foam samples. These values should not be used as anything but a guide. The actual magnitude of the IFD versus thickness change must be determined for each foam type. A simple "rule of thumb" on the degree of change is rarely accurate.

The IFD increases with cushion thickness as you read from left to right. For example, if the IFD of a 2 inch thick cushion is 6.7 lbs/50 in sq., one could expect the IFD of that IDENTICAL foam, if it were 8 inches thick, to be approximately 16 lbs/50 in sq.. However, the rate of IFD increase with increasing cushion thickness also varies with increasing IFD. Note in Figure 1 how the slope of the IFD-thickness line increases as the IFD increases, which shows that the foam thickness effect is essentially "compounded" at higher IFD levels. Please keep in mind that the values represented on this graph are approximate and are displayed here only for visualization of the concept.

 

4.3.1 The question arises, why does the 25% deflection IFD increase with sample thickness? The reason is that, as the thickness increases, the physical amount of deflection increases. For example, to obtain a 25% IFD on a 4" thick sample, the 50 insq. deflector foot is indented into the foam one inch; and to obtain a 25% IFD on an 8" thick sample, the 50 in sq. deflector foot is indented into the foam 2". Obviously, even on exactly the same foam, the 8" sample IFD reading will be higher because the foam is being indented (deflected) much more. The following chart demonstrates the IFD/thickness concept more clearly:

 

Foam Sample Thickness (inches) Amount Of Deflection (inches) Required For 25% IFD

4-----------------------1.00

5-----------------------1.25

6-----------------------1.50

7-----------------------1.75

8-----------------------2.00

4.3.2 The key point to remember is that when measuring IFD's, the actual thicknesses and deflection values must be accurately measured. One can never assume that a 4" sample is exactly 4" and run the deflection accordingly.

4.3.3 In the ASTM test method, the original sample height is measured by using a one-pound load on the sample using the 50 in sq. indentor foot and the height measuring equipment on the IFD machine. This procedure is called "a one pound pre-load" and is done to attempt to cancel any small variations in height just under the indentor foot.

4.3.4 There are also significant problems in trying to cut flexible foams to exact dimensions, yet the IFD test requires very exacting dimensional measurements. This fact must be kept in mind when setting up any type of testing program for IFD. There are many potential errors inherent in the IFD test itself. It is of supreme importance that one accurately controls all of the dimensional, force, and time measurements specified in ASTM D 3574.

NOTE: For the sake of simplicity, from this point forward, it will be assumed that unless specifically stated otherwise, all IFD and IFD associated measurements were made on a minimum 20" x 20" x stated thickness samples, and that the indentor foot is always round and 50 insq. in area. Thus, henceforth in this standards and guideline document, the IFD values will only be reported in lbs at the stated deflection; and the "per 50 in2"will be omitted.

4.4 Temperature and Humidity Effects on IFD

 

4.4.1 Little has been published in the public or industry domains regarding the quantitative effects of varying temperatures or humidities on the IFD properties of flexible polyurethane foams. There is, however, a great deal of inferential and observational information on IFD variation caused by varying temperature and humidity within the polyurethane industry itself.

4.4.2 There are two distinctly different--and often confused--effects of temperature and humidity on the IFD of flexible polyurethane foams; a. effect when pouring the foam b. effect when measuring actual IFD.

4.4.3 The effects of temperature and humidity during the actual pouring of the flexible foams are called "summertime IFD regression." In the summertime, the IFD's of flexible foams regress or decrease on the average from their wintertime values. For example, let's say that a particular foam grade had been averaging a 25% IFD of 32 lbs during the months from October to June. From June through September, if the same foaming formulation were used, the average 25% IFD attained would be something significantly less than 32 lbs. Although no significant research has shown the actual quantitative, numerical relationship between summertime temperatures, humidities and IFD, it is a well known fact that foamers must (and do) adjust their formulations to compensate for the differences. Lacking quantitative relationships, most of the formulation adjustments are empirical and experience related.

4.4.3.1 The key point about summertime IFD regression is that it is not reversible once the foam is made. In other words, once the foam is poured in a condition that produces IFD regression, the IFD amount lost is lost forever and is not recoverable.

4.4.3.2 It is thought, however not quantitatively proved, that the summertime IFD regression is more a function of "absolute humidity" rather than "relative humidity."

4.4.3.3 See Appendix A3.0 for further explanation of humidity and temperature.

4.5 Another very significant source of variation of IFD is with testing equipment, i.e., from one piece of testing equipment to another. IFD is very difficult to reproduce even when using the same machine. When the additional element of different testing machines is added, the complexity increases, and the ability to reproduce test results decreases.

 

4.5.1 To investigate this variable, the following interlaboratory round robin was run.

4.5.2 Samples taken from the same bun location and cut to exactly the same thickness and lateral dimensions were sent to five testing laboratories who had the same testing machine models.

4.5.3 The labs involved were accurately temperature and humidity controlled. The samples were 15x15x4 inches and were all cut on the same piece of cutting equipment. Samples which did not measure 4.0" plus or minus 0.05" with the one pound preload were discarded. All samples were measured by the same person on the same piece of measuring equipment.

4.5.4 The testing labs were requested to condition all samples for a minimum of 36 (not 24) hours, and the preflex and indent speeds to be used were specifically defined. It was also asked that each lab should calibrate both preflex and indent speeds as well as to recalibrate the load cell (load measuring device). So, in this test, every known and controllable variable has been defined and calibrated. The results were as follows:

Lab Number Sample 125% IFDlbs Sample 225% IFDlbs Sample 325% IFDlbs
1 28.0 27.0 28.5
2 27.5 26.0 26.5
3 27.0 27.0 28.0
4 28.5 27.5 27.5
5 28.0 26.0 27.0

 

Even with all of the most careful controls, using the best available testing equipment, there remains some variability in the IFD results.

4.5.5 There are enough problems in the reproducibility of the IFD test to give rise to a question of the inherent accuracy of the test. Analysis of the data in 4.6.4, as well as other similar data, indicates that the reproducibility of the IFD measurements even under the best of circumstances is approximately plus or minus one pound. However, under normal testing and production circumstances, the variability is substantially greater.

4.5.6 Another source of IFD variation is from the foam manufacturing process itself, i.e. within a foam run and from run to run. The chemicals used to make foams vary from lot to lot; and the production pumps, temperature controls, and mixing equipment also have tolerances of variability. A variety of processing variables can create variations in all of the foam's physical properties. Changes made by the foam chemists and engineers during a run of foam may create additional variations in the final physical properties of the foam.

4.5.7 Another source of variation in IFD is the variation from top to bottom and side to side within the manufactured buns. There is also potential IFD variation from front to back within a bun. The following chart exemplifies the variations in IFD in a typical bun cross section. The horizontal lines represent 4" slices, and the vertical lines represent segmentation side to side into three 26" sections. A leveling cut of 2.5" was cut from the top of the bun; one inch thick side trim was removed from each side of the bun, and a bottom skin of 1.0" thickness was removed. The average density of the foam within the bun was 1.82 PCF. The 25% IFD's are noted in each 4" thick section:

25.0 25.5 24.5
25.5 26.0 25.0
26.5 27.0 27.0
27.0 28.0 28.5
28.5 28.0 27.0
29.5 29.0 29.0
29.5 29.5 28.5

Note: The nearest 0.5 pound is reported as a matter of tradition only and does not reflect the precision of the test.

Note: The magnitude and position of the variation in the above example is only indicative of the bun that was actually tested. Other buns of the same foam type are likely to show more or less variation. Examining the bun IFD data in the above chart indicates that there is a 4.5 pound maximum variation in IFD from top to bottom, and there is a 1.5 pound variation from side to side within this particular bun.

There are several important things to note about the above bun IFD variation data:

 

a. Variances shown by this data are slightly better than average, in that, top-to-bottom IFD variances of 6.0 pounds (and even more in some cases) have been measured; and side to side differences of 2-4 pounds have also been measured.

b. It should be remembered that the above bun IFD data also contains the typical errors normally involved in measuring IFD.

c. The key point to remember is that the IFD will vary significantly. One should work with the vendors involved to develop reasonable IFD specifications.

4.5.8 Another significant source of IFD variation is variation with the size of the sample tested. For example, on the exact same piece of foam, the IFD on a 24" x 24" x 4" will be higher than on the same piece of foam cut 15" x 15" x 4". In this case, the furniture manufacturer is in a quandary. Because the ASTM standard test method for IFD permits the use of a 15" x 15" x 4" size sample, many foamers have established their background data bases on 15" x 15" samples.

Background and databases notwithstanding, it is much more accurate for the furniture manufacturer to calibrate seating comfort and the day-to-day replication of seating comfort using foam sample sizes that are closer to actual seat cushion sizes. Thus, it is recommended that the furniture manufacturer should specify IFD's based on a minimum sample size of 20" x 20"x purchased thickness. The following chart demonstrates typical variation of IFD with sample size:

Sample Number Sample Size25% (inches) IFD (lbs)

1------------ 15 X 15-------- 24.0
2------------ 16 X 16-------- 24.0
3------------ 17 X 17-------- 24.3
4------------ 18 X 18---------24.8
5------------ 19 X 19---------25.5
6------------ 20 X 20---------26.0
7------------ 21 X 21---------26.0
8------------ 22 X 22---------26.3
9------------ 23 X 23---------26.0
10----------- 24 X 24---------26.3
11----------- 24 X 24---------26.5
12------------25 X 25---------26.3
13------------26 X 26---------26.5
14------------27 X 27---------26.5
15------------28 X 28---------26.3
16------------29 X 29---------26.3
17------------30 X30---------26.3

This chart demonstrates that most of the IFD variation occurred on sample sizes under 20" x 20". There was some variation from 21"x 21" to 30" x 30", but it was small compared to the variance under 20" x 20". The above data is the average of 10 replications.

4.5.9 One may question the cause(s) of variation of IFD with sample size. The predominant reason for IFD variation with sample size is "edge effect." If one watches the edge of the round indentor foot while measuring IFD's, it is seen that all of that foam under the foot moves downward relatively uniformly, but the foam immediately adjacent to the compressed foam is stretched to varying degrees. The sample width-length, the elongation of the foam being tested, and the stiffness of the foam will impact the edge pull. The edge effect approaches a constant factor as the sample size is increased beyond 20" x 20".

 

4.6 The 65% IFD And Support Factor

To this point, only the 25% deflection IFD has been discussed because the 25% deflection value is usually the value used for specifying the foam grade. Because IFD is stated at a percentage of the thickness of the foam being tested and used, any percentage IFD could theoretically be used. For example, in Europe, instead of using the 25% IFD for day to day definition of foams, the 40% deflection IFD is used.

In the United States, the 65% IFD is commonly also measured but is not typically used in specifying the foam. The 65% IFD value is used to calculate another foam property --support factor. The 65% IFD is the amount of force necessary to deflect the sample 65% of its original thickness after obtaining the 25% deflection value as directed in the IFD procedure.

 

4.6.1 The support factor of any foam is defined as the unitless value obtained when the 65% IFD is divided by the 25% IFD of the same piece of foam. For example, if a particular piece of foam has a 25% IFD of 30 lbs and a 65% IFD of 60 lbs, it has a support factor of 2.0. From a practical standpoint, support factor values run from 1.7 to 3.0.

4.6.2 Support factor provides an indication of support characteristics not correlated with any other foam property. It is very rare for foams under 1.4 PCF density to demonstrate support factor values over 1.8 to 1.9.

4.6.3 Support factor can be related to comfort of furniture. Higher support factor foams of the same 25% IFD will provide more load bearing at higher deflection values. It has also been claimed that with higher support factor values, softer foams may be used in cushions.