Joint Industry Foam Standards and Guidelines
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.
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