Note: Descriptions are shown in the official language in which they were submitted.
113~
FOAM SEAT AND BACK CUSHIONS
Background of the Invention
The present invention relates to seat cushions and backrests and more specific-
ally to cushions and backrests which support the human body without creating
pressure points or areas of contact which support disproportionate weight distribu-
tions. Such disproportionate weight distributions often cause circulatory disfunction
in the extremities after prolonged periods of immobility.
Since the erect position presents many anatomical disfunctions in the lower
portions of the human anatomy, the sitting position creates even more problems of
reduced circulatory efficiency due to restrictions in the normal flow of blood to and
10 from the legs. The normal positions of the arteries, veins, and nerves are squeezed,
pinched, and kinked by the displacement of muscle tissue and fat tissue when a person
sits. These problems are accentuated due to the firmness of the underlying skeleton
supporting the upper torso.
Specifically, the ischial tuberosities and the coccyx exert as much as 80% of
the weight of the torso in an area confined to a triangle about five inches on a side
with its apex to the rear. This force concentration or pressure accounts for "hitting
bottom" or "bottoming out" of a person on a cushion after prolonged sitting or the
premature complete compression of the cushion upon which the person is sitting. As
the body is moved from an erect attitude in or~der to avoid discomfort to these
20 anatomical portions of greatest weight support, the position and subsequent distribu-
tion of weight on the spinal column is changed, thus causing posterior movement of
the vertebrae in the lumbar region. Such movement causes stretching of the deltoid
muscles of the back, irregular pressure on the vertebral discs, emphasis and increased
pressure on the coccyx, and the like. Stretch of the deltoid muscles reduces the
supportive and strength capabilities of these muscles which causes further relaxation
and po6terior curvature of the spine. Such further relaxation and posterior curvature
of the spine causes additional pressure on the anterior side of the discs yielding
nervous stress and subsequent reduction of efficiency. Holding the deltoid muscles in
their stretched position brings on eventual premature fatigue of the individual.
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113B~3~
Since the arteries and nerves are located to the rear of the skeleton and
especially in the extremities (presumably to protect them from injury), it follows that
the sitting attitude of a person causes constriction and squeezing of these arteries
and nerves between the femur and the cushion substructure of the chair or seat being
sat upon, especially on the popliteal region behind the knee-joint. Resulting
circulatory disfunction and nerve pinching are especially experienced by people whose
occupations require long periods of sitting, such as stenographers, technicians in
laboratories, computer operators, motorists, telephone operators, confinees of wheel-
chairs, and the like.
The art is replete in solutions to the above-described problem. Exemplary
patents disclosing chairs specially designed to alleviate the f oregoing problems
include U.S. Patents Nos. 3,193,328, 3,380,410, and 3,261,037. Also, attempts have
been made to specially design beds in order to make them more anatomically
acceptable as in U.S. Patent No. 3,885,258. Relative to the prior art proposals in
providing specially designed chairs, beds, and the like, usually such structures are
extremely complicated in structure and expensive to manufacture. Additionally,
most of the assemblies are based upon an inherent misconception regarding the
underlying theory for solving such problems. This misconception will be amplified on
further below. Suffice it to say that the present invention provides a unique and
20 revolutionary approach toward the comfort in seating people whose occupations
require sitting for extended periods of time.
Broad Statement of the Invention
The invention provides a seat or back cushion comprising cellular elastic
material, this material being divided into segments having different moduli of
elasticity, the modulus of elasticity of each segment being inversely proportional to
the pressure exerted on that segment by the anatomical portion of a person seated
against said cushion to more uniformly distribute the supportive forces exerted by the
cushion over a larger area of said anatomical portions, the segments comprising pre-
formed blocks of the cellular elastic material assembled to form the cushion.
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~3~3~34
The invention also provides a method of forming a seat or back cushion, the
method comprising foaming a plurality of blocks of cellular elastic material, at least
one of the blocks having a modulus of elasticity differenct from that of another one
of the blocks, cutting from each of the blocks at least one segment of the cushion,
and assembling the segments to form the cushion, such that the modulus of elasticity
of each segment is inversely proportional to the pressure exerted on such segment by
the anatomical portion of a person seated against said cushion to more uniformly
distribute the supportive forces exerted by the cushion over a larger area of these
anatomical portions.
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Advantages of the present invention include a highly effective solution to the
problem of artery and nerve squeezing and restriction due to a person remaining
seated for extended period of time. Additionally, the seat or back cushion of the
present invention are extremely inexpensive to manufacture and easily adaptable to a
multitude of practical uses. These and other advantages will become readily
apparent from the disclosure of the invention herein contained.
Brief Description of the Drawings
Fig. 1 represents the distribution pattern of body pressures (in p.s.i.) against a
stationary seat;
Fig. 2 depicts a seat cushion embodying the present invention;
Fig. 3 depicts an alternative embodiment of a seat cushion of the present
invention; and
Fig. 4 depicts a back support of the present invention.
The drawings will be described in detail in connection with the detailed
description of the invention which follows.
Detailed Description of the Invention
In order to more fully understand the problems associated with a person in the
sitting position, reference is made to Fig. 1 which shows the distribution pattern of
body pressures (in p.s.i.) against a stationary seat. Note that the greatest body
20 pressure is found in a triangle in the rear center portion of the seat which point
corresponds to the ischial tuberosities and the coccyx of a person seated on such
stationary seat. Note that in areas outside of this triangle of high body pressure, the
pressures decrease significantly. Such uneven pressure of the human torso on a seat
causes the squeezing of nerves and constriction of arteries which lead to extreme
discomf ort to the person seated. The same is also true of the pressures exerted
against the backrest as seen in Fig. 1. The net result of such body pressures is
depicted in the graph at the bottom of Fig. 1 which shows the well-known
anthropological curve of a person in the seated position. Prior solutions to these
problems typically are based upon providing the greatest resistive forces in a seat
--3--
1~3~34
cushion corresponding to the greatest body pressure points and less resistance in the
seat cushion corresponding to those areas of less body pressure. Unfortunately, such
solutions are incorrect and only accentuate the problem attempted to be solved. The
present invention, however, provides segmented cellular elastic material acting
individually and in concert which is systematically arranged to offer true comfort and
support to a person seated thereon. Such arrangement of the material segments
yields a cushion for the human anatomy which takes into consideration the distribu-
tion of weight of a person in the sitting attitude. This allows conformation of the
cushion to the buttocks, and the structure of the skeletal frame, including, the pelvic
10 girdle, the ischium, the ilium, the sacral symphysis, the tuberosities of the ischium, the
coccyx, and the hip joints including femoral tuberosities.
The seat cushion shown in Fig. 2 comprises a first substantially horseshoe-
shaped segment forming both side edges and the rear edge of the seat cushion, a
second, substantially horseshoe-shaped segment disposed within the first segment and
being open at the front edge of the cushion (turned towards the viewer in Fig. 2), a
third, substantially wedge-shaped segment disposed within the second segment and
extending rearwardly from the front edge of the cushion, this third segment tapering
from the front edge towards the rear edge of the cushion and extending more then
half way from the front edge toward the rear edge of the cushion. The seat cushion
20 shown in Fig. 2 also comprises a fourth, substantially elliptical prismatic segment
having its major axis extending substantially across the width of the cushion, this
fourth segment lying within the second segment and surrounding the rear end of the
third segment, the third segment extending into an indentation formed in the forward
wall of the fourth segment. The first, second, third and fourth segments of the
cushion shown in Fig. 2 are all prismatic in shape, the axes of the prisms lying
perpendicular to the flat upper face of the cushion on which the user sits. A fifth
flat segment is provided extending over the full area of the cushion.
In the cushion shown in Fig. 2, the foam of the first and third segments
(designated 11 and 13 respectively) has a relatively high modulus of elasticity, the
30 foam of the second segment (designated 12) has a lower modulus of elasticity and the
foam (designated 14) of the fourth segment has a still lower modulus of elasticity.
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t ''~Y'~
ll~B~34
The foam of the fifth segment (designated 15), should have a relatively high modulus
of elasticity, conveniently the same as that of the first and third segments.
By framing the perimeter of the cushion with the extra firm
cellular elastic material 11 (material having a high modulus of elasticity) as well as
the inside of the legs and the perineum with extra firm material 13, more of the
weight of the torso is supported on the outside of the skeletal frame including the
upper legs where nerves and blood vessels are at a lower concentration. Further
support of the anatomy is accomplished through cellular elastic material 12 which is
horse-shoe in shape and is of lower density or lower modulus elasticity than material
10 11 and 13. Material 12 supports the gluteal region and the long, ham-string muscles of
the upper leg. This area of support reduces the stretch of the ham-string muscles
which otherwise occurs in soft uniform cushions. The variable density or moduli of
elasticity of the various materials reduces fatigue in these lower muscles as well as
the muscles in the back, and further reduces flaccidity of muscle tissue which occurs
in occupation~requiring long periods of sitting without moving from such position.
In the triangular area which normally exerts the greatest torso pressure on the
cushion, the weight is distributed over a larger area by the cellular elastic
material 14 which, as already mentioned, is in substantiaUy the form
of an elliptical cylinder. Material 14 has a low density or low
20 modulus of elasticity and allows the unyielding ischial tuberosities and the coccyx to
contact this soft material to relieve pressure on these anatomical portions until the
balance of the seat cushion area has met and been compressed to the planned degree
of compression; and then material 14 offers support to these anatomical portions of
pressure. Suppt for the anatomy is attained uniformly without severe pressure
points. The body weight rests on a cushion of variable density or moduli of elasticity
uniform in its distribution of weight thereon.
The seat cushion depicted in Fig. 3 is generally similar to that depicted in Fig. 2
except that the elliptical fourth segment of cellular elastic material 14 is omitted,
the second segment being expanded to take the place of the missing fourth segment.
30 Thus, the flat segment of material 15 becomes the fourth segment of the cushion.
The distribution of weight on the cushion depicted in Fig. 3 is not as uniform as is
~13~34
obtained by the seat cushion depicted in Fig. 2; however, the supportive forces
exerted by this cushion are distributed over a larger area of the anatomical portions
of a person seated thereon than occurs in a cushion of uniform construction. Material
15 in Figs. 2 and 3 preferably is the same material as material 11 and ensures a
uniform thickness to the cushions and ensures stability for all of the various
materials, i.e. reduces strain on the bonded faces of the materials.
Placement of the variable density cellular materials in forming the seat cushion
of the present invention advantageously can be used to dictate an erect posture and
support for the lower back now can be gauged to offer predictable placement of a
10 back support. Preferably a backrest is an integral part of the seating system when
the cushion of the present invention is used in a chair or seat assembly.
The back cushion of the instant invention shown in Fig. 4 comprises a first,
substantially cuboidal segment forming the major portion of the cushion; in Fig. 4
although the first segment is substantially cuboidal, the upper corners of the segment
are rounded. An indentation is formed in the upper edge of the first segment and
extends across the center line of the cushion, while a pair of apertures, each of which
has the form of a rectangle with its upper outer corner rounded off, are formed in the
upper part of the first segment, this pair of apertures being symmetrically disposed
on either side of the vertical center-line of the first segment. A second segment of
20 the cushion is disposed within the indentation of the first segment, while third and
fourth segments of the cushion are disposed in the pair of apertures in the first
segment. A cuboidal fifth segment of the cushion is disposed along the lower edge of
the first segment and extends forwardly beyond the forward face of the first
segment. In this particular cushion, the forwardly projecting part of the fifth
segment has the form of a triangular prism having one of its side faces lying
substantially in the plane of the front face of the first segment and a sixth segment is
provided covering the two exposed side faces of this triangular prismatic portion.
The triangular prismatic portion of the fifth segment is, in this particular back
cushion, a right triangular prismatic portion having its hypotenuse in the plain of the
30 front face of the first segment.
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~13~3~34
The various foam materials used in the back cushion shown in Fig. 4 are
identified by the same reference numerals as in Figs. 2 and 3. Thus, the first and
sixth segments of the back cushion are formed from the intermediate-density foam
material 12, while the second, third, and fourth segments are formed from the low-
density material 14. The second segment accomodates the head of the user, while the
third and fourth segments accomodate the scapulae. The fifth segment is formed of
the extra-firm material 13 to accomodate the lower back and lumbar region, the
intermediate-density covering of the sixth segment providing a comfortable feel
when the cushion is in contact with a user's back. The triangular prismatic portion of
10 the fifth segment desirably extends about three inches forwardly of the front face of
the first segment (measured perpendicular to that plane) and the sixth segment is
desirably about one and one-half inches thick. Optionally, the back rest in the lower
back area may be adjustable in the direction of the seat area to further accomodate
the person's lower back. Such adjustment can be provided by a seat back which is
hinged horizontally in the center or by providing a movable insert.
The segments of cellular elastic material used to form the seat cushion and
back support of the present invention have different moduli of elasticity such as
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1138134
measured by Young's modulus of elasticity. For cellular elastic material, though,
density more often will dictate the relative stiffness or firmness of the material and
will be a useful gauge in determining the modulus of elasticity of the segments to be
utilized in forming the seat cushion of the present invention. The modulus of
elasticity of each segment is inversely proportional to the pressure exerted on such
segment by the anatomical portion of a person seated on the cushion as is shown in
the description of the seat cushions depicted in the drawings. Such arrangement of
the segments and their inverse relationship to the pressure exerted thereon provides
more uniform distribution of the supportive forces exerted by the cushion over a
10 larger area of the anatomical portions of a person seated on the cushion. This
translates into a change in the force vector of the cushion which balances the force
of a person seated on the cushion. The force vector of the cushion is tilted more to
the front portion of the seat cushion by virtue of additional support being provided by
material 11 and 13 as shown in Figs. 2 and 3. An almost buoyant effect is created
thereby.
The cellular elastic material forming the seat cushion typically is about 11/2 to
about 3 1/2 inches in thickness. Thicknesses of less than about 1 V2 inches are
insufficient for preventing bottoming out of virtually the entire areas of the cushion
being sat upon using present day commercially available cellular elastic materials.
20 Thicknesses greater than about 3 1/2 inches are unnecessary as compression of the
materials usually does not exceed this value and, thus, additional thickness merely
provides an ostensibly non-compressible bottom support which preferably is used for
the seat cushion of the present invention. It will be appreciated that depending upon
the density or modulus of elasticity of the particular cellular elastic materials of
choice in forming the seat cushion that the foregoing thicknesses of the cushion and
back rest may vary significantly and such variance is within the spirit of the present
invention. Preferably, the thickness of the seat cushion ranges from about 2 inches
to about 3 V2 inches. In determining the precise materials and their corresponding
modulus of elasticity in forming a seat cushion according to the precepts of the
30 present invention, certainly, the particular foam used in forming cellular elastic
material 14 of Fig. 2 and/or material 12 of Fig. 3 should be of sufficient firmness or
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~138~34
have a sufficiently high modulus of elasticity that such material will not be fully
compressed for a bottoming out effect for the designed weight load of the seat
cushion. Since weights of people vary greatly, the seat cushion of the present
invention can be designed for typical weights of those people for which the seat
cushion is designed. That is, the seat cushion can be designed for the weights of
people in certain occupations, such as seat cushions for truck drivers wherein
normally heavier men will be using the seat cushion and for chairs intended for
secretaries wherein lighter weight women normally will be using them. Clearly, the
foregoing is an exemplary description only. Alternatively, the seat cushion can be
10 designed for general distribution and use wherein the typical average weights of the
population will be used in determining the particular cellular elastic materials for
manufacturing the seat cushion and back rest of the present invention.
The cellular elastic materials useful in the present invention typically are
foamed elastic materials. The term elastic is used herein to describe a substance
that assumes its original shape after a force, causing distortion, is removed. The
preferred material for construction of the seat cushion and back support of the
present invention include polymeric cellular materials such as foamed plastics
including, for example, foamed polyethers, foamed polyurethanes, and the like.
Additional suitable materials include elastomeric cellular materials such as, for
20 example, various natural and synthetic elastomers including natural rubber, poly-
chlorobutadiene, polybutadiene, polyisoprene, styrene-butadiene rubber, butyl rubber,
nitrile rubber, ethylene-propylene copolymer, fluorene elastomers, polyacrylates,
silicone rubbers, polysulfide rubbers, halogen substituted rubbers, and the like. These
materials are commercially available presently and many are already in use for
forming conventional seat cushions and back supports as is well known in the industry.
It should also be appreciated that different materials may be used for forming the
various segments of the seat cushion and such varied materials even may have the
same density, but due to their inherent physical property differences one may still
obtain a differential in the moduli of elasticity of such segments for forming the seat
cushion and back rest of the present invention.
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~13B~34
In manufacturing the seat and back cushions of the present invention, the
variable density materials most simply can be bonded together by adhesives or
lamination, though other manufacturing techiques may be used as is necessary,
desirable, or convenient. Economy and expediency often dictate the manufacturing
technique of choice. Occassionally, the segments of different density (com-
pressibility) will be required to be used in different thicknesses. Under these
circumstances, additional firmer or higher density material may be placed under the
less thick areas to provide an overall same thickness to all segments of the cellular
elastic material which forms the seat or back cushion of the present invention. Also,
it may be desirable to adhere or laminate a layer of average density material to the
entire lower or back area of the cushions.
The cushions must be provided with an ostensibly non-compressible or fixed
bottom or back for use in chairs or similar articles of manufacture. Springs or the
like may be used under the cushions if desired. The cover for the cushions must be
somewhat elastic as a non-elastic cover would distort the density (or modulus of
elasticity) differential in the segments thereby distorting the beneficial variable
support achieved by the present invention. Cloth or similar material of sufficient
elasticity are the preferred materials of choice to cover the cushions of the present
invention.
By way of example, the cushions shown in Figs. 2-4 may have the materials 11
and 13 of a polyurethane foam of grade P-7900, the materials 12 and 15 (when present)
of a similar foam of grade P-3500 and the segment 14 (when present) of a similar
foam of grade P-1300 all foam grades being measured by ASTM-D-1564. The main
properties of these foam grades are set out in the following table:
Property P-1300 P-3500 P-7900
-
Density(lbs./cu. ft.) 1.10-1.20 1.20-1.30 4.0-4.5
ILD on 4" at 25%
deflection (lbs.) 17.0-23.0 28.0-34.0 60.0-75.0
Resiliency (%) 41.0 min. 40.0 min. 40.0 min.
Tensile (PSI) 10.5 min. 12.0 min. 10.5 min.
Elongation (%) 250 min. 190 min. 105 min.
Tear (lbs./in.) 1.75 min. 1.70 min. 1.0 min.
Compressibility Set at 1s6F
90%, 22 Hrs. (% Set) 10.0 max. 10.0 max. 10.0 max.
90%, 6 Hrs. (% Set) 7.0 max. 6.0 max. 6.0 max.
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~13B134
For further illustration of a preferred seat cushion of the present invention, the
following dimensions of a cushion like that shown in Fig. 2 is given. The cushion
measures abcut 18.75 inches in width and 17 inches deep. Material 12 has a width at
its widest point of about 14 inches and is about 15 inches deep. The two small leg-
sections of material 12 at the front of the cushion are about 2.5 inches wide each.
The front width of material 13 is about 5.5 inches and its depth is about 12 inches
(composed of a lower larger region of about 7.5 inches and a narrower upper region of
about 4.5 inches). Material 14 measures about 9 inches in length and 5 inches wide. It
will be appreciated, of course, that the foregoing measurements are examplary only
10 and are not a limitation of the present invention.
SUPPLEMENTAL DISCLOSURE
Some of the prior art proposals for supporting the human body without creating
pressure points involve varying the density of the elastic material from which a
cushion or mattress is formed in direct proportion to the pressure upon any particular
portion of the cushion or mattress; a typical example of such prior art proposals is
U.S. patent 3,751,111. We have now discovered that such proposals are incorrect and
exacerbate rather than ameliorate the problem. Providing less elastic material at
points of high pressure only increases the pressure at these points, thus placing even
greater pressure upon the arteries and nerves and causing even greater circulatory
20 disfunction and nerve pinching. It should be noted that this extra pressure
concentration caused by putting the hardest parts of the cushion or mattress in
contact with the parts of the body exerting maximum pressure is not described in the
literature and was only discovered during our researches leading up to the instant
invention.
A further problem with some of the prior art proposals for variable-density
cushions, including the aforementioned U.S. patent 3,751,111, is that they propose to
construct the composite cushion by placing preformed blocks of one type of elastic
foam material (usually the higher density material, since it is normally the higher
density material which is in the form of discontinuous inserts within a matrix of
30 lower density material) in a mold and then foam the lower density material around
the preformed blocks of higher density material, thus securing the blocks in the
_g_
~ - .
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113~34
correct relative positions. Although this has not previously been described in the
literature, we have found that when one type of foam is foamed in situ around a
preformed block of a second type of foam, a chemical reaction takes place between
the foam which is being formed and the pre-existing foam, and this chemical reaction
leads to the formation of a skin of material along the surfaces where the two foams
are in contact, this skin being considerably harder and less resilient then either of the
foams from which it is derived. Where, as is inevitably the case at some points, this
skin of hard, non-elastic material extends perpendicular to the flat surface of the
cushion on which the user sits, the user receives the very uncomfortable sensation of
10 sitting on a sharp edge of hard material, which is not only uncomfortable but which
further exacerbates the problems with circulation and nerve pinching already
mentioned.
It might be thought that the problems associated with the hard skin formed
when one foam is foamed in situ around an insert of another foam could be avoided by
foaming the first foam in contact with a solid former of a material to which the foam
will no adhere, and then removing the solid former and replacing it with the desired
isert of the second foam. However, our researches have shown that this also is an
unsatisfactory procedure in many cases because when commercial polyurethane and
some other foams are foamed in contact with a solid former a tough skin is formed
20 adjacent the former and this tough skin again produces localized, almost non-
deformable areas in the cushion which cause discomfort to the user and exacerbate
the aforementioned circulatory disorders and nerve pinching.
Note that, contrary to the teachings of the prior art, in the instant cushion the
modulus of elasticity of each segment is made inversely proportional to the pressure
exerted on the segment by a person in contact with the cushion. In other words,
those segments of the instant cushion in contact with the points of maximum pressure
exerted by a person in contact therewith are the softest segments of the cushion, not
the hardest as in the prior art. This makes the instant cushion a highly effective
solution to the problem of artery and nerve squeezing and restriction due to a person
30 remaining seated for an extended period of time. Moreover, the seat or back cushion
--10--
~ ~13~3134
of the invention is extremely inexpensive to manufacture since the
manufacturing process is simple and can be pergormed by unskilled labor and the
cushion is easily adaptsble to a multitude of practical uses.
As mentioned above, in most cases it is advantageous to secure the various
segments of the cushions of the invention to one another with adhesive. However, in
some cases, especially when the cushion is immediately enclosed within a cushion
cover, it may be possible to dispense with such adhesive since most commercially-
available foam materials will adhere to one another sufficiently to enable them to be
assembled together to form the cushion, which can then be inserted directly into the
10 cushion cover, which will thereafter hold the various segments in their correct
relative positions.
With reference to Figs. 2 and 3, it should be noted that the presence of the thin
layer of material 15 is optional and in some cases this layer can be omitted without
substantially effecting the properties of the cushion. In use, the flat segment lies
adjacent the surface on which a user sits or lies (for ease of illustration, Figs. 2 and 3
show the cushions in an inverted position); the flat segment helps to provide a
uniform "feel" to the cushion so that the user is not aware of the differing foam
densities therein.
The seat cushion shown in Fig. 2 may be easily and economically manufactured
20 by forming three different slabs of material of correct elastic moduli for the
segments 11/13, 12 and 14 respectively. These slabs are formed having a uniform
thickness equal to that of the first, second, third and fourth segments in the final
cushion (if the cellular elastic material used to form the cushion is produced by
foaming and the foam is of a type which produces a relatively hard "skin" where it is
in contact with the mold in which it is formed, this hard skin should be removed
from the slabs before the various segments of the cushion are cut therefrom). The
first, second, third and fourth segments of the cushion are then cut from the
appropriate slabs using commercially available equipment (for example of the jig-
saw type), the cuts being made perpendicular to the flat parallel faces on either side
30 of the slabs, so that the segments are of prismatic form. The segment are then
assembled together in the correct spatial relationship to form the cushion, adhesive
being placed on at least one of the faces where faces of different segments come
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,''~,
113~3~
into contact with one another. The flat fifth segment, which can be prepared in any
conventional way known for forming flat pieces of elastic cellular material, is then
secured to the tops of the other four segments by means of adhesive.
In the cushion shown in Fig. 4, the sixth segment of material is optional and
may be omitted if desired.
Although cushions as shown in Figs. 2-4 using foam materials of grades P-1300,
P-3500 and P-790U (as described above) are adequate for some purposes, after
extended use in certain applications foam fatigue has been observed when cushions
made from the above materials are exposed to lengthy wear. Accordingly, we prefer
10 to make the instant cushions from a high-resistant polyurethane foam. Preferably, in
the cushions shown in Figs. 2-4, the materials 11 and 13 are of foam grade 50HR the
materials 12 and 15 (when present) are of foam grade 30HR and the material 14 (when
present) is of foam grade 20HR, all foam grades being measured by the afore-
mentioned standard ASTM-D-1564. The main properties of these high-resistent
polyurethane foams are given in the following table:
Property 20HR 30HR 50HR
Density(lbs./cu. ft., min.) 2.00 2.00 3.00
ILD on 4" at 25%
deflection (lbs.) 17.0-23.0 27.0-33.0 45.0-55.0
20 Resiliency (% min.) 55.0 55.0 55.0
Tensile Strength (PSI, min.) 10.0 10.0 10.0
Elongation (%, min.) 150 150 150
Tear strength
(lbs/in., min.) 1.20 1.20 1.20
Compressibility Set
Method D (%, max.) 10.0 10.0 10.0
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