What is Engineered
designers and manufacturers, you know that comfort is an important
requirement for success in the marketplace. Each furniture component
has performance objectives, and when it comes to cushioning, that
means comfort, support and durability. Engineered comfort places
quantitative numbers on comfort by matching the capabilities of
the furniture components with specifications on use. Successful
comfort engineers understand the human body and apply that knowledge
to the design of various furniture components. The intended use
of an upholstered piece sets the design and engineering criteria
that is specific for that application. For example, a dining room
chair is designed for a short term sitting and therefore, has limited
cradling and thin FPF cushioning. An office chair that may be called
to service for hours at a time usually has a harder, thicker FPF
that allows the user to sit comfortably in an erect position. That
cushioning, however, will not be the same as that used in a sofa
or easy chair whose purpose is altogether different. Engineered
comfort takes into consideration all of the features and components
and determines the appropriate response for each design.
Art or Science?
Let there be
no doubt: furniture design is an art. The ability to create a vision
and put it on paper is a skill that only a privileged few enjoy.
But engineered comfort, as part of the furniture-making process,
is very much a science. It is a carefully choreographed process
that examines the details, formulates a strategy and applies them
in a calculated, precise manner. Engineered comfort moves furniture
designers and engineers away from the more traditional, empirical
method of specifying, and toward a more theoretical system based
on scientific data obtained by standardized performance characteristics.
In most cases, the marriage of art and science produces a higher-quality
The key to engineered
comfort is to obtain and utilize performance data early in the design
process. This requires establishing a partnership between FPF and
furniture manufacturers at the design and engineering stages. Selecting
the right FPF for the intended end-use, desired comfort and maximum
durability is no easy task. In the case of cushioning, the right
FPF for the correct fit should be determined at the planning stage
so that specifications can be incorporated into the design/construction.
Doing so can save time and money, and will go a long way toward
manufacturing a superior product.
Line in Furniture Design
about comfort, we have to talk about capillary pressure, or pressure
on the skin surface. The harder the seating surface, the more capillary
pressure, thus blood flow is restricted.Pressure on the capillaries
causes physical discomfort which shortens the length of time consumers
can sit comfortably. In upholstered furniture, the seating comfort
system is a combination of many items including fabric, cushion
construction, FPF, decking materials, springs, frame construction
and system geometry. All of these items provide the synergism that
leads to comfort.
of comfort is subjective; however, understanding the design capabilities
of FPF can play a significant role in the styling and comfort of
the final product. Here are some basic considerations in seating
design for upholstered furniture:
and seat depth. Seat height will vary depending on the function
of the furniture piece. In general, the dimensions of the seat height
and seat depth should add up to approximately 39 inches (fig.1).
If there is a significant variation of the 39" rule, the body of
the person sitting will not be positioned to enjoy the benefits
of ideal construction materials.
Vertical Motion (TVM). Also known as ride, total vertical
motion is a function of how deeply a person sits on and in the piece
of furniture. It is a delicate balance between too much and too
little cushioning and depends on fabric selection, seat cushion
thickness, decking materials, deck cover, deck construction, spring
type and spring installation construction. If a cushion has inadequate
support, or too much ride, the user's buttocks will sit too deep,
forcing a slouch position. The buttocks move toward the front rail,
the knees are bent and the shoulders and back are curved, causing
discomfort because the load is not equally distributed in the chair
Cradling is the property of an upholstered seat system and
back system in which the body weight is distributed uniformly over
the sitting and leaning area of a cushion or back. The uniform distribution
of body weight results in low pressure between the interface of
the seating system and the parts of the body, which are in contact
with the seating system. Low "interfacial pressure" between the
body and the seating system minimizes the restriction of blood flow
in the contact areas (fig.3).
The amount of
cradling varies with the end-use and design of the upholstered piece.
For example, cradling is minimal in typical dining room chairs -
usually because of typically thin cushions and restriction of ride.
Dining room chairs are usually used for short-term sitting. On the
other hand, fully upholstered chairs, love seats, sofas, recliners
and other long-term sitting seats can be designed with good ride,
which usually results in good cradling. Ride is the basic promoter
of cradling, but excessive ride can lead to "bottoming out" of a
sitting system. In long-term sitting, bottoming out can produce
as much discomfort as sitting on a flat, immovable board. The comfort
of an upholstered piece is a direct function of the entire seating
system, e.g., cover fabric, cushion wrap, FPF type, deck fabric,
decking materials, springs, spring mounting system, frame design
and integration of basic design with the overall construction of
the upholstered piece.
Given the variety
of factors, one can easily see why it is imperative for FPF manufacturers
to be involved early in the design and engineering of upholstered
Back Pitch Angles.We
all remember how long our grandparents could sit in an unpadded,
wooden rocking chair. The reason that a rocking chair is so comfortable
for a long period of time is because the rocking motion of the chair
produces an instant, or even constant, change of the forces on the
parts of the body in contact with the chair. Even if blood flow
is restricted, it is so for only a short period of time due to the
constantly changing distribution of the body weight from the process
changes the pitch angle of the seat and back of the chair relative
to the floor, which are important factors when designing comfortable
upholstered furniture. If the seat of an upholstered piece has no
pitch, is perfectly parallel to the floor and the back is at an
absolute right angle to the seat, the piece's comfort is totally
dependent on the seat construction. In this instance, it is doubtful
that the piece will be comfortable for long-term sitting because
the seat is bearing most of the weight of the individual sitting
and denying a good load distribution of the body.
This is where
engineering is important in the development of a comfortable seating
system. On virtually every different style/design, the seat pitch
and relative back pitch must be determined so that the back plays
a significant role in bearing some of the weight of the body. One
way to accomplish this is to design the seat frame with some front-to-back
pitch. For example, the front of the seat frame may be higher than
the back of the seat frame. The pitch angle of the seat will then
force the body to lean on the back and, if the back has a moderate
pitch and proper padding under the fabric, it will begin to carry
some of the person's sitting weight. Obviously, there are many different
possibilities for seat and back pitch, and the materials selected
for the seat and back systems are very important to the overall
comfort of the piece (fig. 4).
of Fabric Selection. In most instances, fabric selection
is controlled by the consumer. When designing and constructing an
upholstered piece, it is very important to remember that fabric
selection can positively or negatively change the sitting characteristics
of upholstery. For example, if an upholstered piece is shown in
the retail store in a lightweight cotton print, but the consumer
purchases it in a heavier, stiffer fabric, the sit of the piece
is going to be firmer than the sit on the sample. The opposite scenario
is also true. The differences in sitting characteristics can be
great, so it is imperative to select the proper materials in the
early stages of design development. Proper seat and back pitch,
FPF/padding selection and the engineering of the remainder of the
seating system can minimize the effects of fabrics. One must remember
that the fabric effect can only be minimized, never totally eliminated.
of Firmness. The ratio of firmness is the relationship of
seat firmness to back firmness. The firmness of the seat construction
must be matched to the firmness of the back construction or the
comfort of the system will be negatively affected.
that Affect Design Criteria
Density affects the FPF's ability to provide support, comfort and
durability. It is a measurement of mass per unit of volume and is
expressed in pounds per cubic foot (PCF). It is a primary measurement
used to gauge FPF durability. Generally, as FPF density increases,
durability also increases. Some of the factors related to durability
are loss of firmness (flex fatigue), breakdown in the sitting area
of the cushion (dishing) and fabric bagging caused by loss of FPF
dimensions (compression set and fatigue loss, including loss of
cushion height and IFD).
Force Deflection (IFD). Indentation force deflection is
a measure of FPF firmness that is independent of density. IFD is
determined by measuring the pounds of force required to indent a
4-inch thick FPF sample 25% of its thickness, or one-inch. For upholstery,
25% IFD can range from five pounds to 50 pounds. Firmness affects
the ride of a cushion.
Factor (Compression Modulus). Support factor is the ratio
of 65% IFD divided by 25% IFD. Technically referred to as "compression
modulus," the ratio typically ranges between 1.9 and 3.0. Conventional
FPFs cover the range between 1.9 and 2.1. Filled FPFs (depending
on filler type and loading) are generally 2.2 to 2.6. FPFs based
on high resilience technology cover the range from 2.4 to 3.0 (high
density is required for the upper range). The higher the number,
the greater the FPF's ability to provide support.
comfort, the support factor should be selected to maximize cradling,
allowing a seated person to sit into the cushion rather than on
top of it. When cushions are thick, lower support factor FPFs may
be used to improve cradling and to achieve more even distribution
of body weight. For thinner cushions, where the ride is limited,
higher support factor FPFs can be used to provide softer initial
contact and resist bottoming out.
conventional FPFs with different IFDs and/or densities can provide
a high support factor and soft contact. In fact, when engineered
properly, laminating two FPFs together can attain the desired feel
and performance better than using alternative cushioning materials
such as fiber. You can adjust the support factor by using FPFs of
different density, thickness and IFD.
For best results
when combining FPFs, the IFD must be close enough so that when a
person sits on the cushion, the transition from the softer FPFs
to the firmer FPFs is not apparent. As a general rule, the highest
support factor values are achieved when 50% of the cushion thickness
is of the firmer FPF and higher density, and 50% is of the softer
FPF and lower density. This combination allows for more design features
and greater durability. For reversible cushions, the laminate must
be the same on the top and bottom.
It is possible
to achieve even higher support factors if conventional, softer FPFs
are combined with high resilience (HR) technology FPFs. This can
be used in thin cushions to improve initial plushness of the cushion
and prevent bottoming out.
Fatigue. Flex fatigue is the loss of FPF firmness after
flexing the FPF a predetermined number of cycles. It is an important
measure of durability because it indicates a cushion's long-term
ability to provide the proper cradling and TVM. FPFs that have good
flex fatigue values tend to retain their original firmness and support
levels, which means that the cushion can retain more of its original
characteristics.It is important to remember when designing upholstered
furniture that even well-made, good-quality flexible polyurethane
seat-cushion grade FPFs will lose some IFD with time in actual use.
According to current Joint Industry FPF Standards and Guidelines,
an acceptable level of flex fatigue is 25 to 30 percent loss. While
this number may seem excessive, it is much less than that found
in alternative materials where flex fatigue loss can be as high
as 70 percent.
Resilience is an indicator of the surface elasticity of
FPF. It is measured by dropping a standard steel ball onto the FPF
cushion from a given height and measuring what percentage the ball
rebounds. FPFs with high resilience feel alive; FPFs with low resilience
feel dead and mushy. Both have a role in upholstered furniture,
depending on a product's end-use. For instance, low resilience FPFs
would be used in back cushions or pillows, while FPFs with high
resilience are typically found in seat cushions. High resilience
FPF will have a ball-rebound of at least 55%.
in the Next Millennium
As we enter
the 21st century, the furniture industry, like so many others, has
the benefit of increased and enhanced technology. Innovations continue
to improve today's standards and set new heights for tomorrow's
modernization and design.
The FPF industry
has a proud history of adapting to change and moving forward with
technological innovations. One example is in the early 1990s, when
consumer and design trends led furniture designers away from firm
cushioning toward alternative cushions that provided a softer feel.
The FPF industry set its sights on developing a softer grade of
FPF that provided more supple and comfortable properties without
sacrificing support and durability. The net result was super-soft
FPFs, which have an IFD within the five-to 10-pound range, a softness
comparable to the feel of fiber. FPF/FPF cushions, time and time
again, have outperformed fiber/FPF cushions in retention of original
properties and appearance.
more consistent FPFs have appeared with the emergence of FPFs that
yield less variation in IFD from top to bottom and side to side.
High-resilience FPFs offer designers the ability to get the looks
they want and the consistency to maintain that look over time.
appetite for innovation and a keen desire to be responsive to their
customers guarantees that FPF manufacturers will continue their
quest for quality and versatility in FPF. But as design/styling
trends change and technology in components evolves, it becomes increasingly
imperative that strong partnerships with furniture manufacturers
be forged earlier in the process. With better education and understanding,
oversight errors will be reduced and end products will be inspired.
Together, these partners will lead the way into a new millennium
of comfort, support, value and durability.
Flexible Polyurethane Foam (AFPF)
Arlington, VA 22209
attn: Neeva-Gayle Candelori
The Alliance for the Polyurethanes Industry (API)
Arlington, VA 22209
attn: Fran W. Lichtenberg
Polyurethane Foam Association
334 Lakeside Plaza Loudon, TN 37774
Contact: Robert Luedeka
* In Touch(r) - a teaching bulletin with information about FPF
* PFA Glossary
* FPF Performance slide show
To receive a
copy of the American Furniture Manufacturers Association's "Flexible
Polyurethane Foam Voluntary Test Standards & Performance Guidelines,"
send $30.00 (per copy) to:
P.O. Box HP-7
High Point, NC 27261