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Understanding FOAM properties
| UDM UPHOLSTRY DESIGN &
MANUFACTURING | DECEMBER 1993
udm -- The magazine for upholstery industry managers
www.udmonline.com
This article discusses foam as used in the upholstery industry but there are key points that apply to all foam applications.
Foam Highlights
Forms Expanded Polymers come in open-cell and closed-cell forms.
Density of Weight
-
measured in pounds per cubic foot, abbreviated PCF
- directly related to foam cost
- As the density of unfilled flexible polyurethane foam
decreases so does its durability.
- As a rule of thumb, the higher the density, the higher the cost,
and the better the performance.
Durability The durability of filled foams
varies according to the density of the foam before the filler is
added
Hardness or Softness
- measure of load bearing and is expressed in pounds load per 50 square inches at
a given percentage deflection of the foam.
- measured in:
Indentation Load Deflection, abbreviated (ILD)
Sometimes referred to as RMA (Rubber Manufacturers Association)
- the second
most important foam property.
Modulus is another commonly used physical measurement for foam
Compression set is the tendency of foams to remain
compressed after the force of compression is removed
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Article
Being knowledgeable about the different physical
properties of foam can help you make a more educated
buying decision.
Polyurethane foams are manufactured in a complex process
in which virutally hundreds of different chemical reactions
take place simultaneously. Several non-chemical, physical
reactions also occur.
Although the manufacture of polyurethane foam requires
complex chemistry, it isn't necessary for you to understand all
the details of that chemistry to produce furniture. One point
that's readily understandable is that advanced foam
technology isn't always unpredictable.
As foam manufacturing has evolved, it has grown in its ability
to control the major physical properties of furniture foams.
These physical characteristics potentially hold far more
interest for furniture manufacturers than chemical reactions.
Lession #1
The first physical property you should be concerned with is
density. Traditionally, polyurethane foam density has been measured in pounds per cubic foot, abbreviated PCF. One
good way to get a frame of reference for density is to think in
terms of cubes of foam measuring exactly 12 inches on each
side. This 12-inch cube is exactly one cubic foot in volume.
If this 1.0 cubic foot weighs 1.0 pound, its density is said to
be 1 pound per cubic foot, or 1.0 PCF. If this cube weighs
2.0 pounds, it's said to have a density of 2 pounds per cubic
foot or 2.0 PCF.
Density is directly related to foam cost. When you buy foam,
you're buying chemicals. The higher the density you specify,
the more chemicals you'll get, and the more it will cost.
Density also is related to the durability of field performance
of foams. As the density of unfilled flexible polyurethane foam
decreases so does its durability.
As a rule of thumb, the higher the density, the higher the cost,
and the better the performance.
In the past, although lower density foams were less
expensive, they produced complaints in the field because
they were extremely light in weight. To solve the weight
problem, foam technicians added inert fillers to foam
formulations. These inert fillers raised the density and the
weight of the cushions, but performance of the filled foams
left much to be desired.
Subsequent research has found the durability of filled foams
varies according to the density of the foam before the filler is
added. In general, an unfilled 2.0 PCF foam is more durable
than an unfilled 1.2 PCF foam.
Filling a 1.2 PCF foam to a density of 2.0 PCF generally will
not make the 2.0 PCF-filled foam any more durable than the
1.2 PCF foam. To some degree, fillers detract from the
physical properties of foam. Because of what fillers do to
foam formulations, the 2.0 PCF filled foam in the example is
likely to be less durable than its 1.2 PCF counterpart.
The hardness or softness of the foam is probably the second
most important foam property. This is measured by a
property defined as the IFD of the foam. IFD stands for
indentation force deflection. By definition it's the amount of
load, in pounds, necessary to press a 50-square-inch
indenter foot into the foam sample to a percentage of the
foam thickness.
The most used IFD value is the 25 percent IFD. On a
4-inch-thick piece of foam, the 25 percent IFD would be the
amount of force required to indent the 50-square-inch
deflector foot exactly 1 inch into the piece. Any IFD value
must be accomplished by a percentage deflection value, a
sample thickness value, and a sample length and width
because all of these values affect the outcome.
Another commonly used physical measurement is called
modulus or support factor. In addition to the 25 percent IFD
measurement, a second IFD measurement is taken with the
foam compressed to 65 percent of its original height. The
ratio of the two IFD measurements - the 65 percent IFD
divided by the 25 percent value - is the foam's modulus.
Measuring Support
A polyurethane foam with a 25 percent IFD of 28 lbs./50
square inches and a 65 percent IFD of 56 lbs./50 square inches would have a modulus of 2.0.
This measurement gives a reading of the relationship
between the foam's ability to support weight and prevent
bottoming out. The bigger the difference between the 65
percent IFD and the 25 percent IFD, the higher the modulus,
and the better the foam is at providing support.
There is a school of thought contending that foams with a
modulus less than 1.85 are undesirable. This can't be
scientifically supported, however it's known that as modulus
values approach 1.0, foams become "mushier." At this level
the materials of foam cushion is resting on, the springs for
instance, can be felt.
Try this test
The next most important foam property is fatigue softening.
Fatigue softening is one of the measurements of durability
and is extremely important. To understand how quickly
foams fatigue-soften, take a cushion and hand flex a corner
as hard and as fast as you can for about 1-1/2 minutes.
Then wait for one minute and feel the foam on another,
unflexed corner. Compare the feel of the unflexed corner to
the flexed corner. The flexed corner will feel softer. Density is
one of the major controllers of fatigue softening.
Compression set is the tendency of foams to remain
compressed after the force of compression is removed. This
is another important property of furniture foams. As the
compression set value increases, regardless of the method
of measuring that value, the tendency for corresponding
foams to show problems with compression set increases.
The magnitude of a "good" compression set value and how
it is measured is open to discussion. But manufacturers
should always be at least suspicious of compression set
values above 15 percent.
A basic understanding of these foam characteristics is vital
for effective upholstry manufacture. To get the maximum
benefit from the application of any these values,
manufacturers should work closely with their foam suppliers
to develop limits and ranges that both parties find
acceptable.
Posted with the approval of
Michael Chazin, Associate Publisher
mchazin@chartcomm.com
All Contents Copyright © 2007-2008 jobst incorporated
P.O. Box 8 Prior Lake, MN 55372
Phone: 952.447.3904 Fax: 952.447.3709.6121