Closed cell foam ground pad and methods for making same

A flexible pad is disclosed for supporting a load above an underlying surface. The pad is thermoformed from closed cell material comprising a plurality of closed cells. A substantial portion of the cells are elongated in a direction generally parallel to ribs and velleys formed in the upper and lower surface and lower surfaces of the pad.

TECHNICAL FIELD 
The present invention pertains to a closed cell foam ground pad which is 
used to support an individual in a prone or sitting position. 
BACKGROUND OF THE INVENTION 
Sleeping pads for outdoor use have comfort requirements similar to those of 
indoor beds and cushions. They also have added requirements for durability 
and portability. The tradeoffs which exist between these three general 
requirements and the materials available for construction have determined 
the evolution and effectiveness of ground pads developed to date. 
Lacking significant thickness or weight constraints, most bed mattresses 
are made of several layers of various foams, textiles and spring 
assemblies. By varying the compliance and resiliency of each layer, indoor 
mattresses can be designed to meet virtually all user requirements. 
For ground pads, direct use of indoor mattress designs are infeasible due 
to the requirement that the ground pad be easily transported by the 
individual. However, a ground pad needs to have enough compliance to feel 
comfortable, but not so much that the individual user "bottoms out" on the 
ground. One method of achieving compliance is by increasing the thickness 
of the pad, but only at the sacrifice of increasing the stored volume and 
weight. 
Ground pads also have special comfort-related requirements which are unique 
to their use environment. Thermal loss due to conduction, convection and 
radiation are important factors, especially due to the fact that most 
ground pads are thinner and rest on colder surfaces than indoor 
mattresses. Because they are often used in wet environments, resistance to 
moisture absorption is also a key consideration in the design of a ground 
pad. 
Relatively early, ground pads were made of natural rubber foams, which were 
both elastic and could be molded to intricate shapes The natural foam 
rubber ground pad offered new features which were only partially exploited 
because of the relatively low compliance of natural rubber. However, 
introduction of latex foam rubber offered a further comfort breakthrough 
for mattresses because of its softness, resiliency and resistance to 
fatigue. 
The natural rubber and latex foam rubbers, as well as urethane foams, 
incorporate an open cell structure. That is the rubber is formed by a 
number of cells which are in communication with each other via openings in 
the cells. Resistance to compression of these foams is mainly due to the 
structural support provided by the cellular walls As the open cell foam is 
compressed, the air within the cells is displaced into the atmosphere. 
An open cell structure has several additional disadvantages. First, it 
promotes the absorption of water from wet supporting surfaces into the 
structure, much like a household sponge (which is commonly made from an 
open cell foam material) increasing the pad's weight and promoting 
moisture transfer to the user's sleeping bag. As a result, many of the 
open cell foam ground pads have an outer water impervious cover to prevent 
their water absorption. Second, the open cell structure is also less 
effective as a thermal insulator due to intercellular openings which 
facilitate heat transfer. Also, open cell foams allow water vapor to pass 
through the foam and to condense on an underlying colder surface such as 
the ground or on the bottom surface of the foam pad, causing the foam to 
get wet and reduce its insulation value. 
A further advance in ground pad design was achieved by the development of 
several soft, low density, closed cell polymeric foams such as a 
vinyl-nitrile copolymer known as Ensolite. Reductions in weight and cost 
of closed cell foam ground pads were achieved through the use of a foamed 
copolymer of ethylene and vinyl acetate, also known as ethylene-vinyl 
acetate (EVA). When used as a ground pad material, EVA foam appears to 
provide the best balance over all other closed cell foams in terms of 
economy, weight, durability and stored volume. 
In addition, not only is the closed cell structure resistant to water 
absorption, but it also reduces heat loss. This is primarily due to the 
individual cellular pockets which are essentially sealed and contain 
therein trapped gases. The presence of the trapped gases, however, tends 
to make the closed cell pad less compliant than the open cell pad, because 
the gases must be compressed when the foam is loaded. 
A number of support mattresses and pads made of foamed material and the 
like, have been disclosed. For example, support devices which are 
configured to be flexible along a specific axis of orientation are 
disclosed in U.S. Pat. No. 4,370,767 (beach mat) by Fraser; U.S. Pat. No. 
4,275,473 (buoyant mattress) by Poirier; and U.S. Pat. No. 4,399,574 by 
Shuman (foam mattress pad). 
Other support apparati which have specific geometries for increasing 
compliance were disclosed in U.S. Pat. No. 4,110,881 by Thompson, where 
the surface of a mattress is slotted so it may not be put in tension; U.S. 
Pat. No. 4,383,342 by Forster, where a plurality of upstanding flexible 
ribs are tilted at selected angles to achieve a traction force; and U.S. 
Pat. No. 3,197,357 by Schulpen, where an open cell or closed cell foam pad 
includes corrugations on at least one surface to increase compressability 
and compliance. Also disclosed are U.S. Pat. No. 2.194,364 by Minor, which 
shows a sponge rubber carpet pad which has ridges and valleys which are 
alleged to entrain air as a cushioning agent; U.S. Pat. No. 2,751,609 by 
Oesterling, which discloses an insulating pad formed by a plurality of 
easily compressible blocks secured to a backing sheet; U.S. Pat. No. 
3,016,317 by Brunner, which discloses a closed cell resilient mat which 
has a number of lengthwise and transverse grooves which are made by a 
thermoforming process; and U.S. Pat. No. 3,814,030 by Morgan, which shows 
a mesh-like support member which is formed in a corrugated manner by 
thermoforming, injection molding, extrusion or the like. 
In addition to the aforementioned disclosures, a number of multilayered 
support apparati have been disclosed, such as U.S. Pat. No. 839,834 by 
Gray (ribbed surfaces oriented at right angles); U.S. Pat. No. 2,953,195 
by Turck (opposing sawtooth configured members separated by an inner 
planar layer); U.S. Pat. No. 4,450,193 by Staebler (mat assembly); U.S. 
Pat. No. 4.476,594 by McLeod (reversible mattress); and U.S. Pat. No. 
4,574,101 (exercise mat containing internal air chambers). 
Also disclosed is a support pad having an exterior cover in U.S. Pat. No. 
4,329,747 by Russell and an inflatable cushion in U.S. Pat. No. 4,076,872 
by Lewicki. 
SUMMARY OF THE INVENTION 
The product of the present invention comprises a flexible pad for 
supporting a load (e.g. a person) above an underlying surface, with the 
pad having an upper surface, a lower surface, a first horizontal axis, a 
second horizontal axis perpendicular to the first axis, and a vertical 
axis. The pad is characterized in that it is made in a thermoformed closed 
cell foam material which comprises a plurality of closed cells. A 
substantial portion of the cells are elongated in a direction having a 
substantial alignment component generally parallel to the first axis and 
also having a substantial alignment component following a contour of at 
least the upper surface. 
At least the upper surface of the pad is formed with a plurality of 
upwardly extending protrusions, separated by upper recesses positioned 
between their respective protrusions. Each of the upper protrusions has an 
upper side surface which slopes upwardly and convergently toward an upper 
peak area, with opposite surface portions of each of said side surfaces 
extending upwardly toward one another at a pad angle of between about ten 
degrees and one hundred and thirty degrees, and with a more preferred 
range of thirty to ninety degrees, in some configurations a pad angle 
between about sixty and one hundred and thirty degrees, with a more 
preferred range between sixty five and one hundred and five degrees and a 
more preferred range between seventy to ninety degrees. 
The pad has a total thickness dimension which is measured from a plane 
occupied by the upper peak areas to a lower plane defined by the lowermost 
portions of the lower surface of the pad. The pad also has a peak-to-peak 
dimension which is equal to a distance between center locations of 
adjacent peak areas of adjacent upper protrusions. The pad has a total 
thickness dimension to peak-to-peak ratio of between about 0.4 and 2, with 
a more preferred range being between about two to three and four to three. 
The pad also has a minimum material thickness dimension which is equal to a 
minimum distance between the upper and lower surfaces The pad has a 
minimum material thickness dimension to a total thickness dimension ratio 
which is between about 0.2 and 0.7, with a more preferred range being 
between about 0.3 and 0.6, and the most preferred range being between 
about 0.35 and 0.5. 
In some embodiments, the protrusions are formed only on the upper surface 
of the pad, while in other embodiments, the protrusions are formed on 
upper and lower surfaces of the pad. Further, in some embodiments, the 
protrusions are formed as elongate ribs, positioned on one or both sides 
of the pad, while in other embodiments, the protrusions each have a 
sloping circumferential side surface enclosing that protrusion. 
In a preferred form of the present invention, the pad is formed with a 
plurality of upper and lower ribs and upper and lower valleys, with the 
upper ribs being offset from the lower ribs in a manner that the upper 
ribs are vertically aligned with the lower valleys and the lower ribs are 
vertically aligned with the upper valleys. 
Each rib is made up of a pair of adjacent wall segments, with the wall 
segments having a minimum material thickness dimension measured between 
that wall segment's upper and lower surface portions. The wall segments 
each have alignment planes centered between the surfaces of that segment, 
and adjacent alignment planes form a pad angle. A preferred range for the 
pad angle is in this embodiment is between about 60 to 130 degrees, with 
65 to 105 degrees being more preferred, and with a pad angle of 70 to 90 
degrees being most preferred. 
The pad of the preferred embodiment has a total thickness dimension and 
also a peak-to-peak distance. The ratio of the material thickness 
dimension to the total thickness dimension is between about 0.2 and 0.7, 
more preferably between about 0.5 and 0.6, and most preferably between 
0.35 and 0.5. 
The pad of the preferred embodiment also has a ratio of the peak-to-peak 
dimension to the rib depth dimension which is between about 0.9 and 4.3, 
with a more preferred range being between about 1.3 and 2.7, and the most 
preferred range being between about 1.4 and 2.5. 
Further, the pad of the preferred embodiment has a normalized area ratio 
which is between about 0.3 and 0.8, With a more preferred range being 
between about 0.5 and 0.75, with the most preferred range being between 
about 0.6 and 0.7. 
The pad has a material elongation ratio which is between about 1.05 and 
2.02, with the preferred range being between about 1.1 and 1.6, and With a 
preferred value being about 1.3. 
Desirably, there are a plurality of support members connecting to and 
extending between at least the upper set of support ribs. These support 
members are oriented with substantial alignment components perpendicular 
to a lengthwise axes of the ribs. Desirably, these support members connect 
to and extend between the lower ribs also. In the preferred form, these 
support members have an outer surface positioned below the peak areas of 
the ribs, in a manner that when pad sections are positioned against one 
another, the ribs of one pad section can become nested with ribs of a 
second pad section, thereby reducing a volume occupied by the pad 
sections. Also, the support members are arranged linearly in the preferred 
form, with axes of alignment of these support members slanted relative to 
a second axis, so that when the pad is rolled in a stowed position, 
support members of different pad sections which are positioned adjacent to 
one another are offset from one another along a first axis. The preferred 
spacing of these support members is that they are no further apart than 
about six inches, and desirably less than four inches, and more desirably 
less than 2.75 inches. 
Desirably, the support members are slanted to the second axis at an angle 
less than about half a right angle, and more desirably at an angle between 
about seven and twenty degrees, and most desirably about eight degrees. 
In the preferred form, the ribs form with the support members enclosed 
pocket recesses which define insulating pocket areas. Desirably, these 
pocket recesses are formed at both the upper and lower surface. 
In another embodiment, the pad is formed with elongate ribs on only one 
side of the pad, while in a further embodiment, such ribs are provided on 
both surfaces of the pad. 
In another embodiment, protrusions having a circumferential side wall are 
provided on one surface of the pad, and in another embodiment such 
protrusions are provided on both sides of the pad. At least a portion of 
the side wall tapers upwardly so that the peak area of the protrusion is 
less than the base area of the protrusion. In one of these embodiments, 
the protrusions on opposite surfaces of the pad are vertically aligned 
with one another, and in another embodiment such protrusions are laterally 
offset from one another. In the latter configuration, in one arrangement 
the surfaces are provided with recesses, with the lower recesses being 
aligned with the upper protrusions, and the upper recesses being aligned 
with the lower protrusions. 
There are preferred configuration and dimensional relationships associated 
with each of the embodiments noted above, and these are described in more 
detail in the following detailed description. 
In the method of the present invention, there is provided a closed cell 
foam polymer workpiece having a known thermoforming temperature and a 
thickness dimension. At least one mold member having a forming surface 
with a plurality of protruding portions is applied to the workpiece which 
is at a temperature at least as high as the thermoforming temperature. 
This forms the workpiece with the desired pattern of raised portions and 
recessed portions Further, the workpiece is formed in a manner that cells 
in the workpiece are elongated in a direction of elongation of the 
workpiece. 
In one preferred form of the process, the mold is at a temperature below 
the thermoforming temperature, thus simultaneously cooling and elongating 
the cells near the surface of the workpiece which is being formed. In the 
preferred form, two such molds are provided. 
In accordance with another feature of the process of the present invention, 
there is provided an edge cutting member and an edge compression member 
positioned adjacent to and inwardly of the cutting member. These engage an 
edge portion of the workpiece to trim the edge portion of the workpiece 
and form a trimmed edge with a relatively narrow compressed edge portion 
which has a relatively high density and a relatively high tear resistance. 
In the preferred form, the workpiece is engaged in a manner to provide for 
the appropriate deformation of the workpiece to form the pad 
configurations as described above, and also to provide the proper 
orientation and elongation of the cells to give desired structural 
characteristics to the pad which is formed. The structure of the mold or 
molds used in the process of the present invention are significant, and 
design parameters of these are given in the following text.

While the present invention is susceptible to various modifications and 
alternative forms, specific embodiments thereof have been shown by way of 
the Drawings and will herein be described in detail. It should be 
understood, however, that it is not intended to limit the invention to the 
particular forms disclosed, but on the contrary, the intention is to cover 
all modifications, equivalents and alternatives falling within the spirit 
and scope of the invention. 
DETAILED DESCRIPTION 
The present invention pertains to a closed cell foam support pad having 
increased comfort and compliance, resistance to tear, and insulation 
properties, as well as a process and mold for making the closed cell foam 
support pad. 
As indicated in the Background, closed cell foams are desirable for their 
good insulating properties, low mass, resistance to moisture absorption, 
and their relative compactness. However, the properties of closed cell 
foams which provide these desirable characteristics, that is the 
individual closed cells, tends to make a closed cell foam structure less 
comfortable. The comfort of a support pad is a direct function of its 
ability to gradually deform when subjected to a compressing force. 
Compliance, or the amount of compression of a material resulting from a 
given load, is a measurable quantity and is useful when comparing the 
comfort of various pads. 
Several open cell and closed cell support pads which have been disclosed in 
the Background utilize various structural patterns to control their 
compliance. It has been found in the present invention, however, that 
compliance of a closed cell foam is a function of (1) the ratio of the 
uncompressed vertical cross-sectional area of the pad to the uncompressed 
height of the pad, (2) expansion of compressed foam at exposed, 
unrestrained surfaces, (3) bending of formed pad members and (4) tension 
in formed pad support members. This will be explained more fully below. 
There is shown in FIG. 1 a model of a closed cell foam structure indicated 
at 10 having a top surface 12, and a bottom surface 14 which is supported 
on an underlying surface 16. This foam structure can be modeled as three 
volumes 10a, 10b and 10c, each of which has a length l and a height h; the 
widths w of each volume being treated as constant for all compressive 
forces and therefore ignored in the following discussion. The uncompressed 
total height h.sub.ut, of foam structure 10 is defined by the vertical 
distance between upper surface 12 and lower surface 14, when the structure 
10 is not subjected to loading. An uncompressed vertical cross-sectional 
area A.sub.u, i.e. that area which lies in an imaginary vertical plane 17, 
is defined as the sum of the vertical cross-sectional areas A.sub.1 
=l.sub.1 h.sub.1, A.sub.2 =l.sub.2 h.sub.2, and A.sub.3 =l.sub.3 h.sub.3. 
In accordance with the present invention, an increased compliance is 
provided by forming a structure in which the ratio of the uncompressed 
area A.sub.u, to the total uncompressed height, h.sub.ut, is minimized, as 
shown by the following analysis. 
During compression of the closed cell foam structure 10 by a downward 
acting force F per unit width w, the rigidity of the cell walls, which is 
very small, is assumed to be zero for purposes of analysis The compliance, 
C, (or softness) of the structure 10 is 
##EQU1## 
where h.sub.ct is the total height of the structure after being 
compressed. It is assumed that the ideal gas law for isothermal 
compression is valid, i.e. P.sub.u V.sub.u =P.sub.c V.sub.c, where P.sub.u 
=the uncompressed pressure of the gas within the cells V.sub.u =the 
uncompressed volume of any structure 10 and is equal to l.sub.i h.sub.u 
per width unit (ignoring the constant w), h.sub.u, which is the same for 
each block in the model, is the uncompressed height of the individual 
block, P.sub.C =the compressed pressure of the gases included in any 
structure 10, and V.sub.C =the compressed volume of any structure 10, and 
is equal to l.sub.i h.sub.ci. 
In general, 
EQU P.sub.c =F/l (Eq. 2); 
therefore by substitution into the Ideal Gas Law, in the compression of 
only one structure, 
EQU P.sub.u l.sub.i h.sub.u =(F/l.sub.i)l.sub.i h.sub.ci (Eq. 3); 
and by algebra 
EQU h.sub.c1 =(P.sub.u /F)h.sub.u l.sub.1 
EQU h.sub.c2 =(P.sub.u /F)h.sub.u l.sub.2 
EQU h.sub.c3 =(P.sub.u /F)h.sub.u l.sub.3 
By summing h.sub.c1, h.sub.c2, h.sub.c3, the total compressed height is: 
##EQU2## 
In the limit as the height of the uncompressed element approaches zero and 
the number of elements i approaches infinity 
##EQU3## 
where A.sub.u is the uncompressed cross-sectional area. 
By substitution of Eq. 5 into Eq. 4: 
EQU h.sub.ct =P.sub.u A.sub.u /F (Eq. 6). 
By further substitution into Equation 1 
EQU C=1-(P.sub.u A.sub.u /Fh.sub.ut) (Eq. 7). 
Thus as the ratio A.sub.u /h.sub.ut increases, i.e. an increasing A.sub.u 
or a decreasing h.sub.ut, compliance decreases. 
Assume structure 10' in FIG. 1 is defined by a unit length L, an 
uncompressed height h.sub.ut and by the constant w. When A.sub.u is less 
than h.sub.ut L; the compliance increases in accordance with equation 7. 
In other words, C increases when A.sub.u is less than h.sub.ut L; or 
stated another way, compliance increases when A.sub.u /(h.sub.ut L) is 
less than 1. 
For ease of discussion, the ratio A.sub.u /(h.sub.ut L) will be henceforth 
termed the normalized vertical cross-sectional area A.sub.n or the 
normalized cross-sectional ratio. 
This analysis shows that the normalized vertical cross-sectional area 
strongly influences the compliance of a pad made from closed cell foam. 
This discovery allows one to select the pad geometry with the best 
compliance from a set of candidate geometries. 
The preceeding analysis ignores the ability of exposed, unrestrained 
surfaces of a structure to expand, or bulge outward when subjected to 
compressive forces. If significant exposed, unrestrained surfaces are near 
or under highly loaded areas, net pad compliance greater than that 
indicated by the above analysis (Equation 7) is possible. Thus, one can 
significantly control the compliance of a pad by controlling the 
unrestrained surface area and its shape. 
It should also be noted that for the present invention (FIGS. 2 and 3), the 
sharpness of the angle of the corrugations has an effect upon the 
resistance of the ground pad to compression loading Referring to FIG. 14, 
let it be assumed that the corrugations have a relatively narrow angle 
(.gamma. is small), and that a load is applied over several ribs As the 
foam at the peak of the rib is compressed downward, there is resistance to 
this downward movement which is offered by the foam positioned on either 
side of the peak: .gamma. increases to .gamma..sub.1 as the rib members 
bend and t.sub.c increases to t.sub.c1 as the unrestrained rib surfaces 
bulge out. If .gamma. is small, then the ribs would take most of the load 
in compression and very little in bending. If l is increased such that 
little of the load is taken in compression and more is taken in bending 
(of the rib members) then compliance will be increased. This is because 
closed cell elastomeric foam, which is mostly air, is much stronger in 
compression than in bending. 
In review, there are several mechanisms by which compliance may be 
increased: (1) Selective compression of volumes as shown by Equation 7, 
(2) tailoring exposed, unrestrained surface area and shape to allow 
displacement through expansion or bulging, and (3) adjusting the angles at 
which members act on each other so as to result in bending rather than 
compression of the polymer structure. 
The ingenious use of these discoveries, in conjunction with the use of 
tensile support members (to be explained more fully later) can allow the 
systematic engineering of pad compliance given an understanding of the 
material being used. 
It has been found that a preferred compliance in a closed cell foam pad is 
achieved by the corrugated pattern shown in FIGS. 2 and 3, and which is 
formed by the application of the aforementioned normalized vertical 
cross-sectional area. Briefly support pad 20 includes an upper surface 21, 
a lower surface 22, a lengthwise axis 23, a transverse axis 24 and a 
vertical axis 26, as well as an imaginary neutral plane designated by the 
number 27. Neutral plane 27 is located parallel both to the lengthwise 
axis 23 and transverse axis 24, and lies midway between the upper surface 
21 and the lower surface 22 so as to coincide with axes 23 and 24. The 
support pad 20 has a corrugated configuration and includes a number of 
ribs 28 at its upper and lower surfaces and which are separated by valleys 
32 and which extend parallel to the transverse axis. The pad is supported 
by a number of lengthwise extending stringers 33; the structure and 
function of stringers 33 to be described in further detail later. 
More specifically, the support pad 20 includes upper and lower extending 
ribs 28U, 28L (FIGS. 3 and 13), respectively, and upper and lower 
extending valleys 32U, 32L, respectively, the ribs 28U being vertically 
aligned above the valleys 32L and the valleys 32U being vertically aligned 
above the ribs 28L. While the valleys 32 have a V-shaped cross-section, 
each rib 28 has a rounded end surface 34 at the outer apex portion for 
reasons to be explained later. The points of maximum vertical distance 
between neutral plane 27 and each rib 28 define a transversely extending 
ridge line 45. The maximum height of the rib 28 relative to an adjacent 
valley 32 is shown as V.sub.rib (See FIG. 13 ). Each pair of adjacent 
upper ribs 28U, 28U' are separated by an upper valley 32U which is defined 
by surfaces 46U which intersect at a transversely extending valley line 
47U to form an angle .gamma..sub.U. A portion of each rib 28U is also 
defined by the surfaces 46 which terminate at the rounded end surface 34 
of each rib and form an angle .gamma..sub.U '; .gamma..sub.U being equal 
to .gamma..sub.U '. Likewise, each pair of adjacent lower ribs 28L, 28L' 
are separated by a valley 32L which is formed by planar surfaces 46L which 
intersect at a transversely extending valley line 47L to form an angle 
.gamma..sub.L ; .gamma..sub.L being equal to .gamma..sub.U. A portion of 
the rib 28L is also defined by the surfaces 46L which terminate at the 
rounded end surface 34 of the rib 28L and form an angle .gamma..sub.L '; 
.gamma..sub.L ' being equal to .gamma..sub.U '. 
In the present invention, pad angles .gamma., .gamma.' between about sixty 
degrees and one hundred and thirty degrees are preferred; pad angles 
between about sixty five and one hundred and five degrees being more 
preferred; and a pad angle of between about seventy and ninety degrees 
being most preferred. A preferred radius r.sub.v (FIG. 13) at the apex or 
valley line of the valley 32L or 32U is less than 0.3 inches, and more 
preferably less than 0.02 inches. Furthermore, the material thickness 
dimension t.sub.c of the support pad is relatively constant, the thickness 
dimension t.sub.c being defined as the shortest distance between each pair 
of adjacent slanted surfaces 46U and 46L which define a single wall 
segment 48, with each wall segment 48 being a section of the pad extending 
between a vertical plane passing through an upper rib peak line 45U and a 
vertical plane passing through an adjacent lower rib peak line 45L. In the 
present invention, t.sub.c of between 0.15 and 0.75 inches is preferred, 
with t.sub.c of between 0.21 and 0.54 inches more preferred and t.sub.c of 
about three tenths of an inch or between 0.25 and 0.33 inches most 
preferred. 
In a preferred configuration of the present invention, the ribs and valleys 
are parallel to each other and are parallel to the transverse axis of the 
pad. Within the broader aspects of the present invention, the ribs and 
valleys could (1) deviate from a straight line, (2) need not be parallel 
to the transverse axis of the pad, (3) need not be parallel to each other, 
and (4) need not be on both sides of the pad yet still achieve many of the 
benefits of te preferred configuration. 
Also defined in FIG. 3 is a horizontal peak-to-peak distance, H.sub.pp, 
between adjacent upper rib peak lines 45U, or adjacent lower rib peak 
lines 45L; and, a maximum vertical peak-to-peak distance V.sub.pp which is 
the "total thickness dimension", that being vertical distance between a 
plane coincident with the upper ridge lines 45U and a plane coincident 
with the lower ridge lines 45L. In the present invention a vertical 
peak-to-peak distance, between about 0.3 inches and 1.5 inches is 
preferred, with a V.sub.pp between about 0.5 and 1.0 inches more 
preferred, and a V.sub.pp of about 0.7 inches being most preferred. A pre 
horizontal peak-to-peak distance H.sub.pp of less than about three inches 
is preferred, with H.sub.pp less than one and one quarter inches more 
preferred and an H.sub.pp of 0.75 inches or about three quarters of an 
inch is most preferred. 
For a preferred configuration of the present invention having ribs on two 
opposing surfaces, the preferred rib depth V.sub.rib is such that: 0.17 
inches.ltoreq.V.sub.rib .ltoreq.0.84 inches, whereas 0.2 
inches.ltoreq.V.sub.rib .ltoreq.0.56 inches is more preferred and 0.31 
inches.ltoreq.V.sub.rib .ltoreq.0.5 is most preferred. 
Also in accordance with a preferred embodiment of the present invention, 
support pad 20 is made of a polymer material, most preferably an 
ethylene-vinyl acetate/polyethylene copolymer, (EVA) of a density 
preferably between 1 and 25 pounds per cubic foot (pcf), more preferably 
between 1 and 12 pcf and most preferably between 1 and 4 pcf. It is formed 
by a molding process, most preferably by thermoforming. Briefly, the 
thermoforming process of the present invention involves heating a 
thermoplastic polymer slab workpiece having substantially uninterrupted 
upper and lower surfaces to a temperature above that determined to be the 
temperature at which the material begins to become plastic (formable) but 
is not fluid. This is known as the material's thermoforming temperature, 
T.sub.f. The heated workpiece is then placed in a press having upper and 
lower molds. The press is then closed to engage the polymer workpiece 
between the upper and lower molds and with sufficient force to cause the 
heated pad to flow and conform to the mold patterns. The pad is then 
cooled and the thermoformed pad is removed. 
Although in the present invention EVA foam is preferred, other 
thermoplastic foams, such as polyethylene foams, cross-linked polyethylene 
foams, vinyl foams, and the like may be used. Further, foams with uniform 
cell size and uniform cell distribution and uniform density are preferred. 
In the broader range, foams with variations in cell size, density 
distribution, cell distribution, and foam/film and foam/fabric laminates 
may be used. Further, although the bulk of the discussion herein has 
addressed a mold having two portions: an upper and a lower portion, within 
the broader aspects of this patent, the mold may also (1) have portions 
which dependently or independently move in any single axis or combination 
of axes, (2) have only one side and use a diaphragm and pressure and/or 
vacuum to form the pad against the mold, and (3) include a combination of 
compression molding and vacuum thermoforming to form the pad against the 
tool. 
Further, the workpiece from which the pad is made is in the preferred form 
in the shape of a rectangular prism having length and width dimensions 
generally corresponding to the length and width dimensions of the pad 
being formed, and having a thickness dimension which is approximately the 
same as the total thickness dimension V.sub.pp (see FIG. 3) of the pad 
which is formed. However, the thickness of the slab workpiece may in some 
instances be less than the final vertical thickness dimension (V.sub.pp) 
of the pad. The slab workpiece from which the most conveniently provided 
has a cellular configuration where the cells are generally spherical or, 
at most, slightly oblate. When this slab workpiece is formed into the pad 
of the present invention, the cells of the polymer material become 
elongated along a material elongation axis to impart certain improved 
properties to the pad of the present invention. (This will be described 
more fully later herein.) 
Referring to FIG. 4, there is shown a portion of a hypothetical mold which 
is not used in the present invention, but which is provided to show a 
nonoptimal mold pattern as well as to define several variables associated 
with the mold pattern. In FIG. 4, the mold M includes upper and lower 
portions each having a base B and a number of extending ridges R. Each 
ridge R is formed by opposing sidewalls S which extend from the respective 
bases and terminate at end surfaces P; the lengthwise dimension of the end 
surface P defining a mold plateau width W.sub.P. Each ridge R is separated 
from the adjacent ridge R at the base of the sidewalls S by a horizontal 
distance which is defined as a mold groove width W.sub.g. In accordance 
with the normalized vertical cross-sectional area analysis, it would be 
logical to assume that increased compliance would result from an increase 
in mold plateau width W.sub.p. This is because an increase in mold plateau 
width causes an increase in the valley width of the formed pad, and which 
in turn reduces the normalized vertical cross-sectional area A.sub.n. It 
has been found in the present invention, however, that it is desirable in 
the thermoforming of pad 20 that the value of mold plateau width W.sub.P 
be as small as possible; that is the value of W.sub.p approaches zero. It 
is recognized a plateau width of zero is unachievable, however, a plateau 
width which is as small as may be achieved practically is desirable. 
It has been found that when a mold plateau width W.sub.p of 0.3 inches or 
greater is used, the resulting pad is degraded substantially, both in 
performance and appearance as will be discussed in further detail later. 
By utilizing larger plateau widths, too much material is permanently 
deformed by the ridge plateaus resulting in the degradation of the formed 
pad. 
In carrying out the process of the present invention, there is shown an 
exemplary mold generally indicated at 90 in FIGS. 5 and 11, which includes 
an upper mold portion 92 and a lower mold portion 94. The upper mold 
portion 92 includes a number of downwardly depending transversely 
extending ridges 95U, each of which is formed by opposing angled linear 
sidewalls 96U which join at a transversely extending ridge line 98U to 
form an angle .alpha..sub.u. The base of each sidewall 96U joins with the 
sidewall 96U of the adjacent ridge at a transversely extending groove line 
100U to form an angle .alpha..sub.u '. A vertical distance between ridge 
line 98U and groove line 100U is defined by the variable V.sub.mold. 
When the unformed pad has a more preferred thickness t.sub.u of between 
about 7/16 and about 5/8 inches V.sub.mold is preferably between about 
0.46 and 0.62 inches; more preferably between about 0.52 and 0.56 inches; 
and most preferably about 0.54 inches. A horizontal ridge-to-ridge 
distance on the mold M.sub.HRR between about 0.30 inches and about 0.84 
inches is preferred, and an M.sub.HRR of 0.73 inches is more preferred. 
Preferably the plateau ridges 95 have respective plateau widths which are 
less than 0.3 inches, and more preferable plateau widths W.sub.P which are 
less than 0.02 inches. In order to maximize compliance by decreasing 
normalized vertical cross-section area A.sub.n, the mold groove width 
W.sub.g is also as small as practicable with a preferred mold groove width 
W.sub.g which is less than 0.03 inch, and a more preferred mold groove 
width less than 0.02 inches. The lower mold portion 94 is nearly identical 
to the upper mold portion 92, however, the ridges 95L of the lower portion 
are displaced along the lengthwise axis from the ridges 95U so that the 
ridge lines 98U, 98L vertically align with the groove lines 100L, 100U, 
respectively, during molding of the workpiece. At maximum closure of the 
mold, a minimum vertical distance between the upper ridge line 98U and 
lower ridge line 98L is defined by a variable D.sub.CLOS (FIG. 5). When 
the unformed pad has a more preferred thickness t.sub.u of between about 
7/16 and 5/8 inches, D.sub.CLOS is between about -0.24 inch (a negative 
quantity indicating ridge overlap) and about 0.2 inches a more preferred 
D.sub.CLOS range between about -0.18 inches and about 0.08 inches; a most 
preferred range between about 0.11 and 0.05 inches; and an optimum 
D.sub.CLOS of -0.05 inches. 
It is found that by utilizing the mold of the present invention, that not 
only is there an optimization of compliance, but in addition, the pad has 
increased resistance to tear due to both foam densification and polymer 
orientation within the pad. During the present thermoforming molding 
process, the polymer workpiece is compressed from its initial thickness 
t.sub.u to a compressed thickness t.sub.c. The overall compression of the 
workpiece by the mold causes cells at or near the outer surface of the 
workpiece to be compressed. The resulting increase in density of the 
material near the surface forms a tough skin. This skin has a significant 
resistance to abrasive forces which are typically encountered when the pad 
is supported on a rough surface, such as in a camping environment. It has 
been found that a rib radius r.sub.p, as shown in FIG. 13, achieves a good 
balance between compliance and durability when r.sub.p is preferably such 
that about 3/32 inches.ltoreq.r.sub.p .ltoreq.7/32 inches and more 
preferably 3/32 inches.ltoreq.r.sub.p .ltoreq.5/32 inches. 
In the present invention, utilizing a workpiece having a preferred initial 
thickness t.sub.u between about 3/10 and about 9/10 inches, and a more 
preferred initial thickness t.sub.u of between about 7/16 and about 5/8 
inches, it is preferable to compress the workpiece so that the material 
thickness dimension t.sub.c is less than 9/10 of the thickness, t.sub.u, 
of the initial workpiece and more preferably so that t.sub.c is from about 
five tenths to about seven tenths of the initial thickness t.sub.u of the 
workpiece. Although increased compression results in greater skin density, 
there is a corresponding reduction in support and thermal insulation, 
therefore, when using a workpiece of initial thickness between 7/16 and 
5/8 inches, a compressed thickness (i.e. the material thickness dimension 
t.sub.c) of between about 5/10 t.sub.u and about 7/10 t.sub.u is most 
preferred to provide sufficient thermal insulation and comfort at maximum 
loading. 
In the formation of the ground pad, it is stated above that a compressive 
force is applied to the foam to give it its corrugated pattern. However, 
it should also be recognized that as this occurs there is a stretching of 
the foam to allow it to follow the contour of the mold. In other words, 
since the centerline length of the foam (as measured midway between rib 
surfaces 46U and 46L) is increased by following the convoluted or 
corrugated pattern in a direction perpendicular to the lengthwise 
direction of the ridges and valleys, there is a stretching along a line 
that follows the corrugated pattern. Thus, the individual cells are 
compressed in one direction because of the loading, but are stretched in 
another direction to follow the contour. This stretching causes lengthwise 
orientation of the foam microstructure which further enhances the pad's 
resistance to tearing and tensile stresses Further, in a preferrred 
configuration where both sides of the pad have ribs and where the ribs on 
one surface are substantially parallel though not vertically aligned with 
the ribs on the other side, it has been found (1) that the polymer 
orientation is continuous along the entire elongated centerline dimension 
of the foam and (2) that orientation extends throughout the thickness of 
the formed pad. This results in an increased ability of the formed pad 
members to resist unwanted buckling when under loads which induce 
compression and/or bending in the foam structure. This full-depth 
orientation is a significant finding and improvement over that available 
in thermoformed pads having planes of symmetry which are parallel to their 
neutral axes. 
Further, it has been found that it is desirable to form the initial 
workpiece by use of molds which are at a lower temperature than the 
workpiece being formed (i.e. at a temperature lower than the thermoforming 
temperature of the material). Thus, for a thermoforming temperature of 
above 160 degrees Farenheit, the molds would desirably be at room 
temperature, or in any event less than about 120 degrees Farenheit. 
Further, the molds are desirably made of a material having good heat 
conductive characteristics (i.e. steel or aluminum) so that heat from the 
workpiece is dissipated into the mold during the thermoforming process. 
Further, the mass of the molds should be sufficiently great, relative to 
the total mass of the workpiece being formed, so that the molds provide a 
sufficient heat sink for the heat contained in the polymer workpiece. For 
example, if the polymer workpiece being formed has a total mass of about 
one pound, the mass of the two molds would be at least as great as about 
twenty pounds, and more desirably at least as great as forty pounds. Thus, 
during the thermoforming process, the molds are both forming and cooling 
the foam material into the final pad shape. As an added benefit, it is 
believed that the initial rapid cooling of the surface portions of the 
workpiece contacted by the molds enhances the toughness of the surface 
material of the pad. 
It would be logical to assume that the formed pad would be weakest along 
the valley lines 47 (FIG. 3). This was typically the case in conventional 
corrugated or convoluted pads which were formed by saw cutting a standard 
piece of flat foam. Typically, the reduced thickness and weakening of the 
saw cut portions along the valleys allowed the pad to tear easily along 
the valley lines. In the present invention, however, the valley lines of 
the pad are actually stronger and more resistant to tear than the other 
portions of the pad. During the thermoforming molding process, the 
displacement of the polymer material by the mold ridges 95 produces an 
elongation and an increase in polymer density in a direction which is 
perpendicular to the valley lines 100. It is believed the aforementioned 
polymer orientation and densification result in the increased resistance 
to tear along the valley lines. 
In addition to increasing the tear resistance of the surfaces of a pad, it 
is desirable to maximize the resistance to tear initiation along the pad 
edges. In the present invention foam densification and edge trimming were 
combined into one step which was done concurrently with pad surface 
molding FIGS. 16 and 17 illustrate the details of two edge forming/edge 
trimming approaches which were found to work well. 
FIG. 16 shows a preferred mold configuration having an edge forming member 
120 having a forming surface 126, a compression surface 125 of width 
E.sub.1, and a transition zone 127 which connect 126 and 125. Also shown 
is an edge cutting member 121 haVing an interior forming surface 123, and 
exterior forming surface 122 and a cutting edge 130. The edge forming 
member 120 and the edge cutting member 121 are mounted to the upper mold 
portion 92 so that the cutting edge 130 of the edge cutting member 121 
contacts the lower mold portion 94 at a lower mold cutting surface 124 
when the compression surface 125 of the edge compression member 120 is a 
distance E.sub.2 from the lower mold cutting surface 124. Also shown is 
the vertical mold spacing, S.sub.mv, which determines the thickness of the 
molded pad next to the trimmed edge, and the upper and lower mold vents 
128 and 129 respectively. 
In use a preheated workpiece of thickness t.sub.u is placed on the lower 
mold portion 94. The upper mold portion 92 with edge compression member 
120 and edge cutting member 121 attached are lowered onto the workpiece. 
The cutting edge is first to contact the workpiece and, if it were not for 
the edge compression member 120, the edge cutting member 121 would easily 
shear through the softened foam. However, by proper choice of edge 
compression member width E.sub.1 and edge compression member setback 
E.sub.2, the hot foam can be compressed and densified until the cutting 
edge 130 meets the lower mold cutting surface 124 accomplishing pad 
trimming. This process is shown in stepwise fashion in FIGS. 18a, 18b and 
18c. 
For pads of a preferred configuration having a workpiece thickness t.sub.u 
such that 7/16 inches.ltoreq.t.sub.u .ltoreq.5/8 inches and 
S.sub.mv.ltoreq.t.sub.u it is preferred that E.sub.1 .ltoreq.1/2 inch and 
E.sub.2 .ltoreq.3/4 S.sub.mv, it is more preferred that E.sub.1 
.ltoreq.1/8 inches and E.sub.2 .ltoreq.1/2 S.sub.mv and it is most 
preferred that 1/32 inches.ltoreq.E.sub.1 .ltoreq.3/32 inches and 0.010 
inches.ltoreq.E.sub.2 .ltoreq.3/32 inches. Further, in general it is 
preferred that E.sub.2 e.sub.1 .ltoreq.2. 
Further, it has been found to be advantageous to include upper mold vents 
128 and lower mold vents 129 to aid in the expulsion of trapped air during 
molding. Within the broader interpretation of this invention, it is 
recognized that enhanced air removal and finer molded pad surface detail 
will result from (i) increasing the number of mold vents and/or (ii) 
connecting the vents to a vacuum source. 
In molding a pad as shown in FIG. 18, wrinkles were found to be induced in 
the lower surface of the pad just inside the formed and trimmed edge. 
These wrinkles were eliminated by changing the location of the lower mold 
stepped cutting surface 131 to a location between the upper and lower mold 
surface as shown in FIG. 17. The lower mold stepped cutting surface height 
E.sub.3 is preferably less than 0.95 S.sub.mv, more preferably 0.2 
S.sub.mv .ltoreq.E.sub.3 .ltoreq.0.8 S.sub.mv, most preferably 0.4 
S.sub.mv .ltoreq.E.sub.3 .ltoreq.0.6 S.sub.mv and optimally E.sub.3 
=0.5(S.sub.mv -E.sub.2). The internal step width E.sub.4 is preferably 
such that 0.2 E.sub.1 .ltoreq.E.sub.4 .ltoreq.4E.sub.1, and more 
preferably E.sub.4 =E.sub.1. 
It has also been observed that when using a stepped lower mold cutting 
surface as shown in FIG. 17, lower vents 132 may also be placed in the 
mold step corner to minimize vent detail transfer to the molded surface. 
For purposes of analysis, the pad of the present invention can be 
considered as having a material elongation axis, which is generally 
perpendicular to lengthwise axes of the ribs being formed. In the present 
embodiment, with the ribs being transversely aligned, the material 
elongation axis would be generally aligned with the longitudinal axis 23. 
However, if the alignment of the ribs is changed, then the orientation of 
the material elongation axis would also have a corresponding change of 
alignment. This material elongation axis 50 is illustrated in FIG. 13, and 
it can be seen that it follows a zigzag or corrugated path which is 
centered between the upper and lower surface portions 46U and 46L of the 
pad. The material elongation caused by the mold ridges 95 may be 
determined as the ratio of the initial length of that portion of the 
workpiece that is formed with ridges along a direction transverse to the 
ridges being formed, to the elongation axis of that same portion of the 
workpiece. This can be set forth as an elongation ratio E.sub.R which 
equals L.sub.A /L.sub.B where L.sub.B is the length of the workpiece prior 
to thermoforming, and L.sub.A is the length of the material elongation 
axis after thermoforming. 
In the present invention, an elongation ratio E.sub.R between about 1.05 
and 2.2 is preferred; an elongation ratio between about 1.1 and 1.6 being 
more preferred, and an E.sub.R of about 1.3 being most preferred. It has 
been found that an elongation ratio greater than about 2.2 results in 
degradation of the foam whereas it is believed an elongation ratio of less 
than 1.05 does not provide sufficient comfort or tear strength 
enhancement. The aforementioned increased valley tear strength cannot be 
attributed simply to the presence of additional polymer material along the 
valley lines. It has been found that when the plateau width W.sub.P was 
increased in a test where only one side of a piece of workpiece was 
corrugated, the resulting pad was no more resistant to tear along the 
valley lines than when a smaller mold plateau width was used even though 
additional material was compressed forming the valleys. The implication of 
this is that even very narrow pad valleys increase the tear resistance of 
the pad, thereby allowing relatively smaller rib-to-rib spacing, H.sub.pp. 
Further smaller values of H.sub.pp result in pads with more uniform 
feeling surfaces which are in turn more comfortable. 
In the present invention, it has also been found that the mold angle 
.alpha. is important in achieving an optimum support pad. Specifically, it 
has been found that larger mold angles increase the pad horizontal 
peak-to-peak distance, H.sub.pp, for a constant vertical peak-to-peak 
distance V.sub.pp. At mold angles .alpha. above one hundred and twenty 
degrees which form a pad having valley angles .gamma. greater than one 
hundred and thirty degrees, the larger horizontal peak-to-peak distance 
results in less comfort. That is, the user's body instead of being 
supported on top of the pad ribs 28, sinks between the ribs 28 and into 
the valleys 32, providing an uneven "lumpy" feeling In contrast, at 
smaller mold angles .alpha., there is a degradation in the appearance and 
strength of the pad due to a rupturing or burst-through of the pad skin 
cover. This occurs predominantly at the surface of the pad along the ribs. 
This not only adversely affects the appearance of the pad, but it also 
reduces abrasion resistance by severing the protective skin cover. Small 
mold angles .alpha. also result in smaller pad angles .gamma., which are 
more susceptible to catastrophic buckling rather than elastic compression 
and bending. In addition to resulting in a pad with non-uniform compliance 
characteristics, buckling also results in permanent creases in the pad 
skin, thereby decreasing its durability. So, utilizing the aforementioned 
ranges of workpiece thickness t.sub.u and mold closure distance 
D.sub.CLOS, a mold angle .alpha. such that 45 
degrees.ltoreq..alpha..ltoreq.120 degrees is preferred with 56 
degrees.ltoreq..alpha..ltoreq.90 degrees being more preferred, and 56 
degrees.ltoreq..alpha..ltoreq.80 degrees being most preferred; and a mold 
angle of about sixty eight degrees achieving optimum compliance and 
optimum horizontal peak-to-peak distance, as well as avoiding 
burst-through. 
Earlier in this discussion, the pad angle .gamma. has been described, with 
reference to FIG. 3, in connection with the angles formed by the side 
surface portions 46U and 46L of the line segments 48. With the surface 
portions 46U and 46L being substantially planar and parallel, those pad 
angles are easily identifiable and ascertained. However, for purposes of 
further analysis, reference will be made to a main pad angle, and this is 
the angle formed by alignment planes of two adjacent wall segments 48. An 
alignment plane is defined as a plane centered between, and aligned with, 
the side surface portions 46U and 46L of the wall segment. 
In regard to the present invention, a preferred configuration of the pad 
shown in FIGS. 3 and 13, having a minimum pad thickness t.sub.c, pad angle 
.gamma., rib radius r.sub.p, full thickness height V.sub.pp and horizontal 
peak-to-peak spacing H.sub.pp can be shown to have a normalized vertical 
cross-sectional are of A.sub.n of: 
##EQU4## 
,where A.sub.1, A.sub.2, and A.sub.3 as shown in FIG. 15 are determined 
as: 
##EQU5## 
By substitution: 
##EQU6## 
By example, for a preferred case where .gamma.=80, r.sub.p =0.125 inch, 
H.sub.pp =0.75 inch, V.sub.pp =0.70 inch and t.sub.c =0.30 inch, the pad's 
normalized vertical normalized vertical cross-sectional area analysis, the 
vertical cross-sectional area of the pad is less than the product of the 
pad uncompressed height, V.sub.pp, and a unit length L represented by the 
horizontal peak-to-peak distance H.sub.pp. In the present invention, 
A.sub.n is less than 1 and increased compliance over that of a flat pad is 
obtained. For the present invention, a value of A.sub.n between about 0.3 
and 0.8 is preferred, with A.sub.n between about 0.5 and 0.75 being more 
preferred, and A.sub.n between about 0.6 and 0.7 being most preferred. 
In carrying out the present invention, it has also been found that the 
flexible ribs 28 (FIG. 3) require support along the lengthwise axis of the 
pad to prevent easy flattening of the ribs 28 when they are subjected to a 
downward force. In other words, as a result of loading, the ribs bend 
easily at the peaks and valleys. This tends to increase the lengthwise 
distance, H.sub.pp, between the ribs 28 and decrease the vertical 
peak-to-peak distance V.sub.pp. To prevent this flattening of the ribs 28, 
there is provided in the present invention a number of intersecting 
elongated stringers 33 shown more clearly in FIGS. 2. 6 and 7. The 
stringers 33 have a truncated triangular configuration when a cross 
section is taken perpendicular to their lengthwise axis. The stringers 
include upper stringers 33U (FIG. 6) which are integrally connected to the 
right and left sidewalls 46U of the upper ribs 28U, as well as lower 
stringers 33L (FIG. 7) which are connected to the right and left sidewalls 
46L of the lower ribs 28L; the lower stringers 33L being vertically 
aligned with the upper stringers 33U. The stringers 33 are molded into the 
valleys 32, and each includes a top surface 102, and angled side surfaces 
106 (FIG. 7) which converge upwardly at about ten degrees from a 
lengthwise extending vertical plane. A preferred vertical distance S.sub.v 
(FIG. 13) between the top surface 102U and the bottom surface 102L being 
no greater than with V.sub.pp, with 0.4 V.sub.pp &lt;S.sub.v &lt;V.sub.pp being 
more preferred and 0.6 V.sub.pp &lt;S.sub.v &lt;0.8 V.sub.pp being most 
preferred. The width o string as measured between their side surfaces 106 
(FIG. 7) is preferably no more than 6 inches, more preferably less than 2 
inches and most preferably between 0.1 inch and 0.7 inch. An optimal 
embodiment would include stringers of width of about 5/8 of an inch, as 
measured at the base of the stringer, and about 7/16 of an inch, as 
measured at the top of the stringer, for V.sub.pp of 0.7 inch and 0.27 
inch&lt;t.sub.c &lt;0.33 inch. 
In the present invention, the stringers are spaced apart from one another 
to not only prevent the separation and flattening out of the ridges, but 
also to support the user's body to prevent the pockets from collapsing. To 
accomplish this, preferably the greatest transverse distance S.sub.D (FIG. 
7) between the sidewalls 106 of adjacent stringers is not greater than 
about six inches, more preferably no greater than about 4 inches and most 
preferably no greater than about two and three-quarters inches. Each 
stringer 33 has a relatively small height and width dimension, and they 
are spaced apart at relatively wide transverse locations. By using the 
stringers 33, optimum compliance is achieved by (i) minimizing the height 
and width dimensions of each stringer, and (ii) maximizing the transverse 
spacing between adjacent stringers so as to limit the increase in 
normalized vertical cross-sectional area A.sub.n caused by the presence of 
the stringers; while providing sufficient tension along the lengthwise 
axis to prevent the aforementioned deformation and flattening out of the 
pad ridges under projected loading conditions. The vertical dimension of 
the stringers is somewhat less than the vertical dimension of the ribs 28, 
i.e. stringer top surface 102 is preferably spaced below ridge peak 45, in 
order to minimize the normalized vertical cross-sectional area A.sub.n, 
while providing sufficient support for the ribs 28U, 28L. The stringers 33 
are formed by the aligned notches 111 in the ridges of the mold 92, and/or 
94 as shown in FIG. 11. 
In the preferred configuration of the present invention, the stringers are 
located on both sides of the pad which have ribs. Within the broader 
aspects of the present invention, the stringers could be on only one side 
of a pad which has ribs and still achieve some of the advantages of the 
preferred configuration over that of a purely ribbed pad. 
The combination of the stringers 33 and the ribs 28 form pockets 110 (FIG. 
6). The pockets 110 are formed by the sidewalls 106 of adjacent stringers 
33, and the valley walls 46. When the pad supports a downward loading, the 
more compliant ribs deform somewhat, however there is very little 
deformation of the less compliant stringers so that the pocket 110 retains 
its basic shape. The stringers 33U forming the pockets 110 on the upper 
surface of the pad are engaged by the user's body or filled by sleeping 
apparel, while the stringers 33L forming the lower pockets engage the 
underlying support surface. The pockets act as (i) barriers to prevent 
thermal transmission between the user's body and the typically cold 
underlying surface, and (ii) to prevent thermal convection along the pad 
valleys. 
Additional insulation is also achieved during expansion or bulging of 
exposed, unrestrained surfaces under and near the loaded area as the foam 
moves so as to partially fill the valleys resulting in greater effective 
foam thickness which reduces conductive heat losses. (See FIG. 14) 
It has been found that when a pad of a preferred configuration 20 is used 
on a very soft support surface such as sand, snow or the like, the ribs 28 
and stringers 33 can form indentations in the softer support surface when 
the pad is under load. The interference between the pad surface and the 
deformed underlying support surface results in an increase in the static 
coefficient of friction between the formed pad and its supporting surface 
relative to that achievable between the flat workpiece from which the pad 
was made and the supporting surface An example of the usefulness of this 
discovery is that a user of a pad similar to 20 could use the pad on 
inclined surfaces of a greater angle than those allowable with pads of 
flat or modestly contoured surfaces. 
In furtherance of the present invention, the ground pad 20 is adapted to be 
stored when not in use by rolling it about its transverse axis and 
securing it by a strap or the like about its outer circumference. 
Compactness is achieved by at least partial mating of the ridges 28 of one 
surface within the valleys 32 of the opposing surface (FIG. 2). 
Compactness is further achieved by the location of the stringers on the 
support pad so that when the pad is rolled as shown in FIG. 8, the 
stringers at one surface rarely engage the stringers at the opposing 
surface. This is accomplished by locating the stringers so that the 
longitudinal axis of each stringer is at an angle .beta. from a line 
perpendicular to the rib. In the preferred configuration the intersecting 
stringers 33 form a number of end-to-end diamond patterns (FIG. 2). As the 
pad overlaps when it is being rolled, the lower stringers 33L engage the 
upper surface 21 of the pad. However, due to the constantly changing 
transverse separation of the stringers 33U of each diamond, the lower 
stringers 33L generally engage the pad upper surface at locations which 
are transversely adjacent to the upper opposing stringers 33U. In this 
manner, the stringers 33L, 33U rarely overlap during rolling, thus 
allowing a more compact roll. Specifically, .beta. is preferably no 
greater than about one half of a right angle (about forty five degrees); a 
stringer angle of about forty five degrees providing approximately seventy 
percent of the lengthwise support of a stringer located parallel to the 
lengthwise axis. At angles less then five degrees, there is insufficient 
transverse separation between the stringers to fully prevent the lower and 
upper stringers from overlapping when the pad is rolled about an axis 
parallel to the rib peak line. More preferably, the stringer angle is 
between about seven degrees and about twenty degrees, and most preferably 
the stringer angle is about eight degrees. 
Because the valleys 32U are vertically aligned with the ridges 28L, and 
S.sub.v is less than or equal to V.sub.pp, a degree of nesting is obtained 
when several pads 20 are stacked vertically in a flat configuration. 
To describe the operation of the present invention in supporting a load, 
references made to FIG. 14. When a person lies on the pad of the present 
invention, certain portions of the person's body will exert a downward 
compressive force on the pad. During the initial loading where the 
compressive force is rather small, there is first a moderate flattening of 
the rounded peak areas 34 With further compressive force being applied, 
there is relatively little compression of the foam material in a vertical 
direction. Rather, adjacent wall segments 48 begin to deflect angularly in 
a downward direction to increase the main pad angle .gamma. toward 180 
degrees. Each wall segment that is subject to the downward compressive 
force becomes compressed along a direction parallel to the middle 
alignment plane of that wall segement 48 so that compression occurs in a 
direction parallel to the lengthwise orientation of the cells. (This 
lengthwise orientation of the cells follows the material elongation axis 
50, as shown in FIG. 13.) At the same time, there is a moderate amount of 
outward bulging of the side surface portions 46U and 46L, so that the 
material thickness dimension T.sub.c increases to some extent. As the 
compressive load per unit area increases further, the wall segments 48 
totally flatten out so that the lower valley lines 47L come closely 
adjacent to the underlying ground surface. When this occurs, the 
resistance of the pad to further compression increases substantially. 
However, the resistance provided as the pad compresses from its initial 
uncompressed position to the position where the main pad angle approaches 
a value close to 180 degrees is such that a desired cushioning effect is 
obtained, and this particular area or zone through which the pad 
compresses toward a totally horizontally aligned configuration can be 
termed a "comfort zone". 
To analyze further the resistance provided by the pad of the present 
invention, let it be assumed that the pad angle .delta., with the pad in 
its unstressed position, is 90 degrees. Let us further assume an idealized 
situation where as a downward compressive force is applied to an upper 
peak area 45U, the adjacent lower peak areas 45L do not shift laterally. 
Under these conditions, for a downward incremental unit of travel of that 
portion of the pad at the vertical plane extending from the upper peak 45U 
to the valley line 47L immediately below, each of the adjacent wall 
segments 48 compress along their respective alignment planes by a value 
equal to about 0.7 of the incremental unit of downward travel. As the main 
pad angle increases to, for example, 120 degrees, then a further downward 
incremental unit of travel at the area of the upper peak 45U to the lower 
valley line 47L causes a further compression of the two pad segments 48 
which is equal to 0.5 of the incremental unit of travel. As the main pad 
angle becomes yet larger, the amount of compression of the wall segments 
48 decreases further. 
However, there is another contributing factor, and this is that with 
greater downward deflection, the pad offers increased resistance in 
bending. It has been found that the resistance provided by the downward 
deflection of the pad of the present invention by the interaction of these 
forces is such that a very desirable programmed resistance to such 
downward deflection is achieved, with this following a desired comfort 
curve. There are quite likely other phenomena involved in the downward 
deflection resistance provided by this pad, and quite likely the above 
analysis is a somewhat simplified explanation. For example, there are 
likely other factors relating to the manner in which these forces are 
reacted at a cellular level, and there is the further consideration that 
the elongated cell configuration of the pad of the present invention 
enhances the interaction of the force reaction at the cellular level. In 
any event, regardless of the correctness of the above analyses and 
regardless of whether the above analyses may or may not be complete, it 
has been found that the pad of the present invention provides a relatively 
deep comfort zone, relative to the total depth of the pad, and that the 
resistance to the downward deflection provided by the pad occurs in a 
pattern which provides a relatively high comfort level. 
From the above analysis, it can be recognized that within certain limits, 
the configuration of the pad can be optimized to maximize the depth of 
this comfort zone relative to the total depth dimension of the pad. To 
carry on with this analysis, there is a rib depth to total thickness 
ratio, with the total thickness or depth being the dimension V.sub.pp, and 
with the rib depth V.sub.rib being the vertical d between the plane 
defined by the upper peak ridge lines 45U to the plane defined by the 
upper valley lines 47U or the vertical distance between the plane defined 
by the lower peak rib lines 45L and the plane defined by the lower valley 
lines 47L. Desirably, this ratio would be greater than 0.2, and more 
desirably between about 0.45 to 0.65. Preferred values would be between 
0.55 and 0.57. 
Related to this rib depth to total thickness dimension ratio is the minimum 
material thickness (t.sub.c) to total thickness dimension V.sub.pp ratio. 
If this ratio is made too small, then the wa segments 48 will tend to 
buckle under compression, thus destroying the desired cushioning effect 
where the resistance increases along a more predictable curve. On the 
other hand, if this minimum material thickness to total thickness 
dimension ratio is made too large, then the pad allows smaller amount of 
downward deflection under compression, thus reducing the total depth of 
the comfort zone. The preferred minimum material thickness to total 
thickness ratio is desirably between about 0.2 to 0.7, and more desirably 
between about 0.3 to 0.6. Preferred values are between about 0.35 and 0.5. 
It should also be recognized that by orienting the cells so that the 
lengthwise axis of the cells generally follows the material elongation 
axis 50, the cells become oriented so that the wall segments 48 are better 
able to resist bending (thus being more resistant to buckling), and also, 
it is believed, contributing to the overall effect of providing a proper 
comfort curve. 
Having generally described the support pad 20 as well as the process for 
molding the support pad and the mold utilized in forming the support pad, 
the following examples are provided in order to describe the pad and the 
process for forming the pad in greater detail. 
EXAMPLE 1. 
A workpiece made of ethylene-vinyl acetate/polyethylene copolymer (EVA) 
foam known as Trocellen XD 200, manufactured by Dynamit-Noble, and having 
the approximate dimensions somewhat greater than forty eight inches by 
twenty inches with a thickness dimension of one half inch, was provided. 
This workpiece had a rectangular configuration with planar upper and lower 
surfaces A conventional commercial convection oven was heated to the 
desired temperature and the workpiece was placed in the oven and heated at 
350.degree. F. for four minutes. Preferred and most preferred ranges of 
temperatures and heating time are shown in the graph of FIG. 9. After 
being heated, the workpiece was removed from the oven by hand, and placed 
on the lower mold 94 of a conventional four post press with at least a 10 
psi compression capability over the area of the workpiece. The mold 
minimum ridge to ridge distance, D.sub.CLOS was 0.02 inches; this interval 
being set by stop blocks between the moving upper platen and static lower 
platen of the press. 
The prototype mold upper portions and mold lower portions were made from 
maple wood. The dimensions of the upper and lower mold portions were 
approximately as follows: 
EQU .alpha.=.alpha.'=68.degree. 
EQU V.sub.mold =0.50 inches 
EQU W.sub.P =0.02 inch 
EQU W.sub.G =0.001 inch 
The heated workpiece was loaded from the oven into the press as 
expeditiously as possible, and the press immediately closed. Preferred 
oven to press times were from ten to fifteen seconds, with thirty to forty 
seconds being the maximum. The press remained closed for about sixty 
seconds, and then opened and the formed pad removed. 
The formed pad had a slightly different configuration than the mold itself. 
More particularly, the angle .gamma. of the rib sidewalls was about eighty 
degrees .+-.3 degrees, with the ribs being somewhat rounded and having a 
radius of about 11/64 inch. The valleys of the pad formed an angle .gamma. 
of about eighty .+-.3 degrees, with the sidewalls of the valleys forming a 
sharp angle at the valley lines. The rounded configuration of the ribs was 
due to the inherent resistance of the polymer material to flow completely 
into, and remain in, the grooves of the mold during compression. The 
minimum pad thickness t.sub.c was about 0.32 inch, resulting in a vertical 
dimension through the rib walls of 0.45 inch. The formed pad had a smooth, 
continuously formed skin along the ribs and valleys with no bubbles 
observed in the valleys and no burst through along the ribs. 
EXAMPLE 2 
Having formed the pad in the manner described in Example 1, the resistance 
to tear of the pad valleys was measured. This test was performed by first 
determining the tear strength of the unmolded workpiece of Example 1, by 
initiating a tear through about 50% of the width of the workpiece and then 
anchoring one tear section to a wall and attaching a force gauge to the 
other tear section. By manually pulling on the force gauge the tear was 
continued at a rate of about twenty inches per minute until the two 
sections were torn in two. The average force was measured during the tear. 
Four samples were tested in this manner which resulted in an average tear 
resistance of 4.88 (standard deviation=0.52) pounds or an average tear 
resistance of 9.38 (standard deviation=1.00) pounds per inch of pad 
thickness. 
For comparison, six support pads manufactured in accordance with the 
process of Example 1 were tested in the same manner. A tear was initiated 
along the length of 50% of a valley line before attaching the load gauge. 
An average tear resistance of 5.3 (standard deviation=1.05) pounds or a 
tear resistance of 14.5 (standard deviation=2.84) pounds per inch of pad 
thickness was measured. None of the tears remained in the valley lines. 
These comparison tests not only showed the improved overall strength of the 
support pads produced by the process of the present invention, but the 
portion of the pad most resistant to tear was along the valley lines. 
As discussed previously, a small mold ridge plateau width W.sub.P is 
important in avoiding unwanted degradation of the pad. This is illustrated 
by the following examples in which a mold having a large ridge plateau 
width was utilized. 
EXAMPLE 3 
A 12 inch.times.12 inch.times.0.7 inch workpiece of two ply laminated EVA 
foam Trocellen XD 200 was molded by an upper mold having the following 
dimensions. 
Design #153 
W.sub.P =0.2 inches 
W.sub.G =0.001 inch 
.alpha.=.alpha.'=54.degree. 
V.sub.mold =0.6 inches 
The lower mold had a flat surface such that only one side of the pad was 
molded. The molding process was performed in accordance with the steps of 
Example 1, except that the mold was used to form a minimum pad thickness 
of 0.1 inches at the valleys. The formed pad upon removal from the mold 
had bubbles which formed beneath the skin along the pad valleys. These 
bubbles were believed to be caused by gases which had been displaced from 
the ruptured cells of the foam by the molding process. 
Also discussed previously was the increased insulation provided by the 
pockets at the upper and lower surfaces of the pad. This increased thermal 
insulation was verified in the following example. 
EXAMPLE 4 
A six inch by six inch by six inch block of ice was removed from the 
freezer of a refrigerator, and placed in an insulated chest. The foam pad 
under test, a six inch by six inch by one half inch piece of a closed cell 
foam material having flat upper and lower surfaces was placed on top of 
the ice block and a one inch thick sleeping bag section of polyester 
batting contained between two nylon sheets was compressed on the surface 
of the pad by 0.5 psi to simulate body load. A thermocouple was placed 
between the sleeping bag section and a piece of urethane foam insulation 
having a thickness dimension of twelve inches. The temperature indicated 
on the thermometer was recorded as a function of elapsed time and 
displayed on a graph in FIG. 10. 
To determine the thermal insulating properties of the support pad of the 
present invention, a support pad formed from a six inch by six inch by one 
half inch piece of workpiece by the process of Example 1 was tested in the 
aforementioned manner and the temperature as a function of elapsed time 
was recorded on the graph of FIG. 10. It is clear from the graph that the 
support pad of the present invention has superior thermal insulating 
properties to a one half inch thick closed cell pad having substantially 
flat upper and lower surfaces. 
EXAMPLE 5 
To verify a significant increase in compliance of the present invention 
over a typical unformed closed cell foam pad, deflection versus load 
measurements were taken. The pad under test was deflected a known amount 
by pushing a ten square inch circular disc into it; the greater the 
deflection caused by a given load, the more compliant the pad. The force 
was then measured with a force gauge; deflection in inches being plotted 
as a function of force in pounds in FIG. 12. A pad formed by the 
procedures set forth in Example 1 was measured in this manner and the data 
plotted in FIG. 12 as a curve designated by the letter A. Curve B in FIG. 
12 shows the deflection versus load measurements for an unmolded one half 
inch thick piece of Trocellen XD 200. Finally, curve C shows the 
deflection versus load measurements for a conventional flat closed cell 
ethylene/vinyl acetate copolymer foam ground pad known as BEVALITE. The 
deflection measurements of the formed pad, as shown by curve A, as 
compared to the unformed pads, as shown by curves B or C, illustrate the 
greater compliance of the formed pad of the present invention. 
A second embodiment of the present invention is illustrated in FIG. 19. 
There is a pad 200 made of a closed cell polymer foam material, as in the 
first embodiment. The pad 200 has a planar lower surface 202, and an upper 
surface 204 formed with a plurality of elongate ribs 206, with each 
adjacent pair of ribs 206 defining related valleys 208. Each rib 206 is 
formed by two substantially planar sidewall portions 210 which extend 
upwardly toward one another to a rounded peak rib area 212. The sidewall 
portions 210 from adjacent ribs 206 that form the related valley 208 meet 
at a valley line area 213. 
The pad 200 of the second embodiment is thermoformed in substantially that 
same manner as in the first embodiment, except that one of the molds has a 
planar surface so that the ribs 206 are formed only on one side. In the 
thermoforming operation, the cells of the material making up the pad 200 
become elongated in a direction having an alignment component transverse 
to lengthwise axes of the ribs 206. Thus the valley line areas 213 are 
formed in such a manner that, as in the first embodiment, there is a 
relatively high resistance to tear at the valley line areas 213. 
With regard to the preferred dimensions of the pad 200, the total vertical 
depth or thickness dimension V.sub.t is desirably between about 0.3 to 1.5 
inches, more desirably between about 0.5 to 1.0 inches, and preferably 
about 0.7 inch. The peak-to-peak spacing distance H.sub.pp (measured 
between peak center lines 214 of adjacent peaks) is less than about 3 
inches, preferably less than about 11/4 inches, and in the preferred 
embodiment about 3/4 of an inch. 
The ratio of the total thickness dimension to the peak-to-peak dimension is 
desirably between about 0.6 to 1.4, more desirably between about 0.7 to 
1.3 and in the preferred form about 0.8 to 1.2. 
It should be noted that in this text when a ratio is expressed as a single 
numerical value, the ratio is understood to be the ratio of that numerical 
value to one. For example, when it is stated that the ratio of the total 
thickness dimension to the peak-to-peak dimension is desirably between 
about 0.6 to 1.4, this is understood to mean that the ratio is between 
about 0.6 to 1 and 1.4 to 1. This same procedure is followed elswhere in 
this text. 
There is also a rib height dimension V.sub.r which is the vertical 
dimension between a plane occupied by the peak portions 212 to a plane 
occupied by the valley line areas 213. This rib height dimension is 
desirably between about 0.2 to 1.1 inch, more desirably between about 0.3 
to 0.6 inch, and in the preferred form between about 0.35 and 0.5 inch. 
The ratio of the rib height V.sub.r to the total thickness dimension 
V.sub.t is desirably between about 0.3 to 0.8, more desirably between 
about 0.4 to 0.75, and in the preferred form about 0.5 to 0.7. 
As in the first embodiment, the sidewall surface portions 210 are slanted, 
with the sidewall portions 210 of each rib meeting at a pad angle m. 
Desirably the pad angle m is between about 60 to 130 degrees, more 
preferably between about 65 to 105 degrees, and in the preferred form 
between about 70 and 90 degrees. 
While the second embodiment of FIG. 19 does not offer all of the advantages 
of the first embodiment, it does provide a good deal of comfort to the 
user, and also some of the functional benefits of present invention. 
A third embodiment of the present invention is illustrated in FIG. 20. 
Components of this third embodiment which are similar to components of the 
second embodiment will be given like numerical designations, with a prime 
(') designation distinguishing those of the third embodiment. 
The pad 200' of the third embodiment of FIG. 20 is similar to the first 
embodiment except that in addition to having the top surface 204' formed 
with upper ribs 206U', the bottom surface 202' is also formed with lower 
ribs 206L'. Each upper rib 206U' is vertically aligned with a related 
lower rib 204L', and the upper valley area lines 212U' are vertically 
aligned with related lower valley area lines 213L'. 
The pad 200' of the third embodiment is thermoformed in substantially the 
same way as the pad of the first embodiment except that in this third 
embodiment, the ribs of the mold are vertically aligned with one another. 
The total thickness dimension V.sub.pp is preferably between about 0.3 
inches to 1.5 inches, more preferably between about 0.5 inches to 1.0 
inches, and most preferably about 0.7 inches. The peak-to-peak spacing 
distance H.sub.pp is preferably less than about 3 inches, more preferably 
less than about 1.25 inches, and the most preferred dimension is about 3/4 
inches. The rib height V.sub.r is preferably less than about 3/4 inches, 
more preferably less than about 5/8 inches, and most preferably less than 
about 3/8 inches. 
The sidewall portions 210' of each upper rib 206U' converge upWardly, and a 
preferred pad angle m' is between about 60 to 130 degrees, more preferably 
between about 65 to 105 degrees, with a most preferred range being between 
about 70 and 90 degrees. 
The total thickness dimension to peak-to-peak ratio (V.sub.pp /H.sub.pp) is 
desirably between about 0.4 and 2, more desirably between about two thirds 
and four thirds with a most preferred ratio being between about 0.8 and 
1.1. There is also a rib height (V.sub.r) to total thickness dimension 
(V.sub.pp) ratio, and this is preferably between about 0.1 an 0.45, more 
preferably between about 0.15 and 0.45, with a preferred ratio being 
between about 0.2 and 0.4. 
As with the second embodiment, while this third embodiment does not 
incorporate all the advantages of the preferred first embodiment, it has 
been found that the pad 200' of this third embodiment does provide 
relatively good comfort, while having certain functional advantages of the 
first embodiment. 
A fourth embodiment of the present invention is illustrated in FIGS. 21 and 
22. There is a pad 220 which is formed with a closed cell foam polymer 
material. However, instead of forming the upper and lower surfaces 222 and 
224 with elongate ribs, in this fourth embodiment, the upper and lower 
surfaces are formed with upper and lower protrusions 226 and 228, 
respectively. 
Each of the upper protrusions 226 has the overall configuration of a cone, 
with a conically shaped side surface 230 and a rounded peak portion 232. 
The surface portion that is opposite each peak portion 232 is formed with 
a related recess 234 so that the material thickness t.sub.c of the pad 220 
is, as much as possible, substantially uniform. 
The angle "n" formed by the cone side surface 230 (i.e. this angle "n" 
being formed by the lines which are formed by the intersection of a plane 
coincident with the vertical center axis of the cone shape and 
intersecting the side wall 230) is preferably between about 60 to 130 
degrees, and more preferably between 65 to 105 degrees. The ratio of the 
total depth dimension (V.sub.pp) to the minimum peak spacing distance 
(H.sub.pp) is desirably between about 0.4 and 2, and more preferably 
between about two thirds and four thirds. Further, it has been found that 
the ratio of the minimum pad thickness to total depth dimension is 
preferably between about 0.2 and 0.7, more preferably between about 0.3 
and 0.6 with ratios between about 0.35 and 0.5 being most preferred. 
The method of forming the pad 220 of this fourth embodiment is generally 
the same as described with reference to the first embodiment, in that this 
is accomplished by thermoforming between two molds contoured to properly 
form the protrusions and recesses. The cellular structure of the closed 
cell foam material is stretched so that the cells become elongated in a 
direction generally paralleling the contours of the surfaces of the pad 
220. While this fourth embodiment does not provide all of the advantages 
of the first embodiment, this pad of the fourth embodiment (shown in FIGS. 
21 and 22) does provide a relatively high degree of comfort and does 
incorporate some of the functional benefits of the present invention. 
While the slanted side surfaces 230 are shown to be cone shaped, obviously 
the surface configuration could be varied within reasonable limits from an 
ideal conical configuration. Further, in all embodiments and variations 
described herein, it is understood that, due to the nature of the forming 
process, the peaks of the ribs will not be sharp, but rather, will have 
radii. It is natural and conceivable that many of the preferred 
embodiments could have ribs with more nearly full radii in cross-section. 
A fifth embodiment of the present invention is illustrated in FIGS. 23 and 
24. There is a pad 300 made of a closed cell polymer foam material, as in 
the prior embodiments. The pad 300 has a lower planar surface 302, and an 
upper surface 304 formed with a plurality of protrusions 306. In the 
preferred form, these protrusions are each formed with an upwardly 
tapering conically shaped side surface 308 and a rounded top surface 310. 
While this is the preferred shape, obviously, the surface contour can be 
varied to some extent. 
The pad angle "p" formed by the side surfaces 308 (i.e. this angle "P" 
being formed by the lines which are formed by the intersection of a plane 
coincident with the vertical center axis of the cone shape and 
intersecting the side wall 308) is preferably between about 10 to 120 
degrees, and more preferably between 30 to 90 degrees. Also, the pad 300 
has a total vertical thickness dimension V.sub.t, and also a peak-to-peak 
distance H.sub.pp, which is the distance between vertical center lines of 
adjacent protrusions. The ratio of the total thickness dimension to the 
peak-to-peak dimension is desirably between about 0.4 to 2, with a more 
preferred ratio range being between 2 to 3 and 4 to 3. 
The pad 300 has a material thickness dimension t.sub.c which, as 
illustrated in FIG. 23, is the minimum distance between the upper surface 
304 and the lower surface 302. The ratio of the material thickness 
dimension to the total thickness dimension is desirably between about 0.2 
to 0.7, more preferably between about 0.3 to 0.6, and most preferably 
between 0.35 and 0.5. 
While this fifth embodiment of FIGS. 23 and 24 does not offer all of the 
advantages of the first embodiment, it does provide a good deal of comfort 
to the user, and also some of the functional benefits of the present 
invention. 
A sixth embodiment is illustrated in FIGS. 25 and 26, where there is shown 
a pad 400 having a lower surface 402 and an upper surface 404. Both of 
these surfaces 402 and 404 are formed with a plurality of protrusions, 
with each upper protrusion 406U being vertically aligned with a matching 
lower protrusion 406L. These protrusions 406U and 406L are shaped 
substantially the same as the protrusions 306 Of the fifth embodiment, 
except that the dimensions of these protrusions 406 are made smaller for a 
given total pad thickness 
The pad angle p for the protrusions 406 fall in the same ranges as the pad 
angles for the fifth embodiment. Also, the ranges for the ratio of the 
total thickness dimension to the peak-to-peak dimension, as well as the 
ranges for the material thickness to the total thickness dimension ratio 
are substantially the same as in the fifth embodiment. 
The pads 300 and 400 of the fifth and sixth embodiments are thermoformed 
from a closed cell foam in generally the same manner as described 
previously. 
Common to the six embodiments previously mentioned is a comfort-related 
requirement which balances overall compliance with local pad morphology. 
As the horizontal peak-to-peak spacing is increased, all else being equal, 
the user will become more aware of the individual protrusions. Within the 
present invention, the ratio of the horizontal spacing of pad protrusions 
to the height of those protrusions (relative to adjacent valleys) is a 
useful design parameter. Specifically, it is preferred that this ratio be 
between about 0.9 and 4.3, more preferably between about 1.3 and 2.7, and 
most preferably between 1.4 and 2.5. 
It is to be understood that various modifications could be made to the 
products and methods of the present invention without departing from the 
basic teachings thereof. 
Also it is to be understood that the terms "upper" and "lower" are not 
intended to be limiting so as to mean that the upper portion of a pad is 
always positioned upwardly. On the contrary, what is designated as the 
"upper" area or portion could in actual use be placed at a lower location 
so as to be against an underlying support surface.