Teaching Aids

FIRMNESS AND SUPPORT IN FLEXIBLE POLYURETHANE FOAM (FPF)
LESSON PLAN FOR 50 MINUTE CLASS

OBJECTIVES:
1. The student will be able to define terms related to firmness and support in flexible polyurethane foam (FPF).
2. The student will be able to calculate the support factor of a foam sample if given the 25% IFD and 65% IFD.
3. The student will be familiar with how firmness and the support factor affect the choice of foam for various applications.
4. The student will be able to give examples of FPF versatility and describe how it can be modified to perform in different applications.

TEXT:
1. InTOUCH; Volume 3, Number 1 May 1993
2. InTOUCH; Volume 4, Number 3, July 1994
3. Flexible Polyurethane Foam Glossary

PROCEDURE:
I. (7-10 minutes) Give the students a short quiz on FPF terms assigned during the previous class period. (A sample quiz follows this lesson plan.) After collecting the quizzes, go over the terms with the students as a means of reviewing the material from the last class and introducing today's topic of firmness in FPF.

III. LECTURE (20-25 minutes)

A. INDENTATION FORCE DEFLECTION
The Indentation Force Deflection (IFD) number represents the pounds of force required to indent a foam sample by a specified percentage of its original thickness. This is measured with specially designed equipment. (Put "Indentor" transparency [T5] on overhead projector) In most cases, a 50 square inch round indentor plate is attached to a vertical piston-like cylinder having a precisely calibrated stroke.
Firmness is measured on foam samples having a square surface area of at least 15" by 15". Surface firmness is measured at 25% sample indentation (25% IFD). As an example, using a 4" thick sample, the 25% IFD reading would be made while the foam is indented to 3" height under the piston foot plate (25% deflection of the sample height).

B. SUPPORT FACTOR
Support (also known as compression modulus) is a key FPF property. (Put transparency [T6] "IFD and Support Factor" on overhead.) In many ways, it is the most important function foam can provide. In upholstery, good support from foam means that cushions don't "bottom out" or compress to a point where they no longer hold up the weight of a person. It also means that the cushion is capable of adequately distributing the weight of the person. As we have already learned, the most common foam specifications are density and 25% IFD (firmness). However, it is also possible to evaluate the deep down support of the foam. This is done by measuring the support factor, also known as "compression modulus".
Support factor is defined as the ratio of a foam's ability to support force at different indentation (or compression) levels. Support factor is measured by dividing the 65% IFD by the 25% IFD. These measurements for FPF typically range from ratios of 1.8 to 3.0. Two factors commonly affect the support factor. First of all, the higher the density of the foam, typically, the higher its support. Second, the foam chemical formulations and the manufacturing process often affect FPF support.
Foam producers can alter support by changing foam chemistry slightly during the manufacturing process. Specialty chemicals or fillers may be added or the manufacturing process may be adjusted.

C. HOW SUPPORT AFFECTS FOAM FUNCTION
Why be concerned about support? There are a number of reasons why different support levels can improve the comfort, support, or durability of finished goods using foam. In upholstery, support can affect a number of key design factors. Proper support enhances cradling, or the ability of the cushioning to distribute body weight and reduce pressure to the skin. Proper cradling distributes body weight so there are virtually no areas of body contact where the weight/load is concentrated to restrict blood flow and cause discomfort.
If cushions are thick, then softer, lower compression modulus foams may be used to improve cradling and to achieve more even distribution of body weight. If cushions are thin, higher compression modulus foams may be used to distribute the weight enough to prevent the cushion "bottoming out" against decking.
Seating systems -- foam, steel, springs, webbing, and fiber -- all soften with use. However, the specification of a higher support level can help cushions continue to feel "new" even after some surface softening of the foam and other materials takes place. This has the net effect of improving foam durability, an important feature of high quality foam.
It's also possible to laminate hard and soft foams together to improve the support of the composite foam structure. However, the difference between the firmness of the two foams cannot be too great, or the softer foam may seem to "bottom out" on the firmer foam.

IV. (15-17 minutes) Have the students return to their groups from the last class period. Give them the handout with various 25% IFD and 65% IFD measurements. Have the groups work together to calculate the support factor for each foam sample and then, using the list of flexible polyurethane foam applications, recommend which application(s) this particular type of foam is best suited. Encourage them to be creative and come up with applications not on the handout. Have them consider specialty applications such as healthcare, sporting equipment and so on.

V. (3-5 minutes) Summarize the results of the groups as a whole on the blackboard. Discuss any discrepancies in the groups' findings.

MATERIALS NEEDED:
1. Additional copies of the Flexible Polyurethane Foam Applications handout
2. Worksheet for figuring Support Factor
3. Overhead transparencies
4. Overhead projector

ASSIGNMENT:
Look around your home for various ways that polyurethane foam has been used. Be sure to also check around school and other public places too. Compare the different densities and firmnesses of the types of foam used in the different applications.

 

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