Note: Descriptions are shown in the official language in which they were submitted.
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WOOD STRAND MOLDED PARTS HAVING THREE-DIMENSIONALLY
CURVED OR BENT CHANNELS, AND METHOD FOR MAKING SAME
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to wood flake molding.
B. Background of the Art
Wood flake molding, also referred to as wood strand molding, is a technique
invented by wood scientists at Michigan Technological University during the
latter part
of the 1970s for molding three-dimensionally configured objects out of binder
coated
wood flakes having an average length of about 1'/ to about 6 inches,
preferably about 2
to about 3 inches; an average thickness of about 0.005 to about 0.075 inches,
preferably
about 0.015 to about 0.030 inches; and an average width of 3 inches or less,
most
typically 0.25 to 1.0 inches, and never greater than the average length of the
flakes.
These flakes are sometimes referred to in the art as "wood strands." This
technology is
not to be confused with oriented strand board technology (see e.g., U.S.
Patent No.
3,164,511 to Elmendorf) wherein binder coated flakes or strands of wood are
pressed
into planar objects. In wood flake or wood strand molding, the flakes are
molded into
three-dimensional, i.e., non-planar, configurations.
In wood flake molding, flakes of wood having the dimensions outlined above are
coated with MDI or similar binder and deposited onto a metal tray having one
open side,
in a loosely felted mat, to a thickness eight or nine times the desired
thickness of the
final part. The loosely felted mat is then covered with another metal tray,
and the
covered metal tray is used to carry the mat to a mold. (The terms "mold" and
"die", as
well as "mold die" , are sometimes used interchangeably herein, reflecting the
fact that
"dies" are usually associated with stamping, and "molds" are associated with
plastic
molding, and molding of wood strands does not fit into either category.) The
top metal
tray is removed, and the bottom metal tray is then slid out from underneath
the mat, to
leave the loosely felted mat in position on the bottom half of the mold. The
top half of
the mold is then used to press the mat into the bottom half of the mold at a
pressure of
approximately 600 psi, and at an elevated temperature, to "set" (polymerize)
the MDI
binder, and to compress and adhere the compressed wood flakes into a final
three-
dimensional molded part. The excess perimeter of the loosely felted mat, that
is, the
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portion extending beyond the mold cavity perimeter, is pinched off where the
part
defining the perimeter of the upper mold engages the part defining perimeter
of the
lower mold cavity. This is sometimes referred to as the pinch trim edge.
U.S. Patents 4,440,708 and 4,469,216 disclose this technology. The drawings in
Patent U.S. 4,469,216 best illustrate the manner in which the wood flakes are
deposited
to form a loosely felted mat, though the metal trays are not shown. By loosely
felted, it
is meant that the wood flakes are simply lying one on top of the other in
overlapping and
interleaving fashion, without being bound together in any way. The binder
coating is
quite dry to the touch, such that there is no stickiness or adherence which
hold them
together in the loosely felted mat. The drawings of Patent U.S. 4,440,708 best
illustrate
the manner in which a loosely felted mat is compressed by the mold halves into
a three-
dimensionally configured article (see Figs. 2-7, for example).
This is a different molding process as compared to a molding process one
typically thinks of, in which some type of molten, semi-molten or other liquid
material
flows into and around mold parts. Wood flakes are not molten, are not
contained in any
type of molten or liquid carrier, and do not "flow" in any ordinary sense of
the word.
Hence, those of ordinary skill in the art do not equate wood flake or wood
strand
molding with conventional molding techniques.
In the past, relatively flat wood strand molded members have been formed with
channels therein. See for example U.S. Patent 4,408,544 to Bruce A. Haataja,
entitled
MOLDED WOOD PARTICLE PALLET HAVING INCREASED VENDING STRENGTH. The
shaping of the part into the shape of a channel is to be distinguished from
merely
forming a channel shaped groove in one surface of a board, as is shown in
Patent
4,131,705 to Kubinsky.
Until the present invention, however, it was thought that one would not be
able
to mold three-dimensionally curved or bent channels into a three-dimensionally
curved
or bent surface of a wood strand molded part because of the difficulty of
expecting the
individual wood strands in the loosely felted mat to slide past one another
such that they
would move relatively uniformly into three-dimensionally curved or bent
channels in a
curved or bent part. It was believed that uneven distribution of the wood
strands would
leave the part weaker and with unsightly gaps.
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SUMMARY OF THE INVENTION
In the present invention, it has been surprisingly discovered that one can
mold a
loosely felted mat of wood strand particles into a three-dimensionally curved
or bent part
having three-dimensionally curved or bent channels. Equally surprising is the
fact that
such molding does not weaken the part, but actually increases the strength of
the part.
Indeed, the formation of the three-dimensionally curved or bent channels in a
three-
dimensionally curved or bent part increases the strength of the part to a
greater degree
than would be expected by simply increasing the section modulus of the part
through the
use of channels. This surprising increase in strength has made it possible to
form such
parts with channels which are not as deep as one would have expected necessary
in order
to increase the strength of the part.
These and other features, advantages and objects of the present invention will
be
further understood and appreciated by those skilled in the art by reference to
the
following specification and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross sectioned, fragmentary perspective view of a loosely felted
mat,
with a portion broken away, in position on a lower mold half which is three-
dimensionally curved to form a three-dimensionally curved part with three-
dimensionally
curved channels, and showing the upper die in position to close on the lower
mold die.
Fig. 2 is a similar view showing the mold halves closing.
Fig. 3 is a perspective view of the part made with a mold of Figs. 1 and 2.
Fig. 4 is a fragmentary cross-sectional view showing the orientation of the
wood
strands in the various portions of the molded part, and highlighting the
tensile strength of
various areas of the part.
Fig. 5 is a perspective view of a part which includes a sharply bent portion.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For purposes of description herein, the terms "upper," "lower," "right,"
"left,"
"rear, " "front, " "vertical, " "horizontal, " and derivatives thereof shall
relate to the
invention as orientated in Fig. 1. However, it is to be understood that the
invention may
assume various alternative orientations, except where expressly specified to
the contrary.
It is also to be understood that the specific devices and processes
illustrated in the
attached drawings, and described in the following specification are simply
exemplary
embodiments of the inventive concepts defined in the appended claims. Hence,
specific
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4
dimensions and other physical characteristics relating to the embodiments
disclosed
herein are not to be considered as limiting, unless the claims expressly state
otherwise.
The reference number 10 (Fig. 1) generally designates a mold of the present
invention. The mold 10 is used in a method of forming a loosely felted mat 11
of wood
flakes 12 into a molded wood flake part 14 (Fig. 3). The mold 10 includes a
top mold
die 16 and a bottom mold die 18. The top mold die 16 includes a surface 20,
and the
bottom mold die 18 includes a surface 26. The surface 20 of the top mold die
16 and the
surface 26 of the bottom mold die 18 define a cavity 30 therebetween.
In the illustrated example, the molded wood flake part 14 is made by
positioning
a loosely felted mat 11 of wood flakes 12 on the bottom mold die 18 (Fig. 1).
The top
mold die 16 and the bottom mold die 18 are then brought together (Fig. 2) and
heat and
pressure are applied to the felted mat 11. The felted mat 11 is thereby
compressed and
cured into the molded wood flake part 14.
The part 14 or 14' formed in accordance with the preferred embodiment includes
three-dimensionally curved or bent channels 15 or 15' (Fig. 3 or Fig. 5), each
comprising spaced sidewalk 15a and a channel base wall 15b (Fig. 4). Channels
15 can
be "V" shaped in cross section as well. When they are "U" shaped, the
sidewalls must
have at least some draft, i.e. make at least a slight angle to the vertical,
in order to
facilitate release from the mold die. Channels 15 are said to be three-
dimensionally
curved or bent in that the wall 16 of molded part 14, into which channels 15
are shaped,
is curved or bent, rather than being flat, such that the part includes
portions on opposite
sides of the curved or bent portion forming an inside angle of about
150° or less,
preferably about 135° or less, and most preferably about 120° to
about 80°. The radius
of curvature on the inside (Ri) of the curved or bent portion can be
essentially zero, for a
sharply bent part, or can be much greater in the case of a more gradually
curved or bent
part. The radius on the outside (Ro) of the part at the curved or sharply bent
portion
will preferably be equal to at least the thickness of the part in order to
facilitate
formation of the part. Preferably, the radius of curvature of the inside of
the channel
(Rci) is at least equal to the depth of draw of the channel. Preferably, the
outside radius
of the channel (Rco) is at least about equal to the thickness of the part plus
the depth of
draw of the part, to facilitate part formation.
Molding wood strand parts is not a question of flowing a plastic material into
a
curved or bent section of a mold. Somehow, the individual strands in the
loosely felted
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mat have to slide past one another such that they will move relatively
uniformly into the
three-dimensionally curved or bent channels in the curved or bent part. One
would have
expected to see part weakening and unsightly gaps form in the three-
dimensionally
curved or bent channels. Such gaps do not occur, however.
It was surprising to find that the parts increased in strength beyond that
which
would be expected merely by forming channels in the part. This appears to be
due to
the orientation of the wood strands or flakes in the channel sidewalls 15a. In
curved or
bent portions 16, the strands or flakes tend to lie in generally parallel
planes, which axe
also parallel to the top and bottom surfaces of part 14. When one applies a
force to open
the bend of part 14, as would be the case if part 14 were used as a chair
shell, for
example, one places the cross section of part 14 in tension perpendicular to
the surface.
Because of the orientation of the wood strands or flakes, the tensile
strengths
perpendicular to the surface of part 14 in wall 16. tends to be determined by
the strength
of the binder used, and is approximately 150 psi.
In contrast, the formation of channels 15 tends to cause the wood flakes or
strands in sidewalls 15a to orient in a direction generally parallel to the
inside and
outside surfaces of walls 15a, but in a direction which is generally parallel
to the tensile
force which is placed on part 16 by a bending moment. The tensile strength of
the
strands placed in tension in the direction of orientation of the strands is
from 3,000-
4,000 psi., as compared to about 150 psi. when the tensile force is applied
perpendicularly to the planes of the strands. As a result, the strength of
part 14 to resist
a bending moment is substantially greater for any given channel depth than
would
otherwise have been expected.
One normally adds channels to a part in order to increase their section
modulus
and thus their resistance to bending moment. However we have found that one
can
design part 16 with a channel depth of only 1/a inch to accomplish significant
strength
improvement.
The number of channels and the depth of draw of the channels are the major
factors which influence back pull strength. The greater the number of
channels, the
greater the area of side wall acting in resistance to the radial tensile
stress. Also, the
greater the channel draw depth, the greater the section modulus. Molding
difficulty
increases with both increased number of channels and increased depth. Both
require
greater mat elongation for uniform material density.
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A configuration of multiple channels and draw depth which required 10 % mat
elongation in a direction laterally of the length of said channels, provided
chair shells
with approximately 25 % back pull strength improvement. Significant strength
improvement could be expected with combination requiring as little as 5 % mat
elongation with strategically located channels. Configurations requiring 30%
mat
elongation can be made with extra mat preparation efforts (i.e. extra material
added to
the mat in areas where channels are to be formed) .
The wood flakes 12 used in creating the molded wood flake part 14 can be
prepared from various species of suitable hardwoods and softwoods used in the
manufacture of particleboard. Representative examples of suitable woods
include aspen,
maple, oak, elm, balsam fir, pine, cedar, spruce, locust, beech, birch and
mixtures
thereof. Aspen is preferred.
Suitable wood flakes 12 can be prepared by various conventional techniques.
Pulpwood grade logs, or so-called round wood, are converted into wood flakes
12 in one
operation with a conventional roundwood flaker. Logging residue or the total
tree is
first cut into fingerlings in the order of 2-6 inches long with a conventional
device, such
as the helical comminuting shear disclosed in U.S. Patent No. 4,053,004, and
the
fingerlings are flaked in a conventional ring-type flaker. Roundwood wood
flakes 12
generally are higher quality and produce stronger parts because the lengths
and thickness
can be more accurately controlled. Also, roundwood wood flakes 12 tend to be
somewhat flatter, which facilitates more efficient blending and the logs can
be debarked
prior to flaking which reduces the amount of less desirable fines produced
during flaking
and handling. Acceptable wood flakes 12 can be prepared by ring flaking
fingerlings
and this technique is more readily adaptable to accept wood in poorer form,
thereby
permitting more complete utilization of certain types of residue and surplus
woods.
Irrespective of the particular technique employed for preparing the wood
flakes
12, the size distribution of the wood flakes 12 is quite important,
particularly the length
and thickness. The wood flakes should have an average length of about 1'/a
inch to
about 6 inches and an average thickness of about 0.005 to about 0.075 inches.
The
average length of the wood flakes is preferably about 2 to about 3 inches. In
any given
batch, some of the wood flakes 12 can be shorter than 11/a inch, and some can
be longer
than 6 inches, so long as the overall average length is within the above
range. The same
is true for the thickness.
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The presence of major quantities of wood flakes 12 having a length shorter
than
about 11/ inch tends to cause the felted mat 11 to pull apart during the
molding step.
The presence of some fines in the felted mat 11 produces a smoother surface
and, thus,
may be desirable for some applications so long as the majority of the wood
flakes,
preferably at least 75 % , is longer than 1 1/8 inch and the overall average
length is at
least 1'/ inch.
Substantial quantities of wood flakes 12 having a thickness of less than about
0.005 inches should be avoided, because excessive amounts of binder are
required to
obtain adequate bonding. On the other hand, wood flakes 12 having a thickness
greater
than about 0.075 inch are relatively stiff and tend to overlie each other at
some incline
when formed into the felted mat 11. Consequently, excessively high mold
pressures are
required to compress the wood flakes 12 into the desired intimate contact with
each
other. For wood flakes 12 having a thickness falling within the above range,
thinner
ones produce a smoother surface while thick ones require less binder. These
two factors
are balanced against each other for selecting the best average thickness for
any particular
application. The average thickness of the wood flakes 12 preferably is about
0.015 to
about 0.25 inches, and more preferably about 0.0020 inch.
The width of the wood flakes 12 is less important. The wood flakes 12 should
be
wide enough to ensure that they lie substantially flat when felted during mat
formation.
The average width generally should be about 3 inches or less and no greater
than the
average length. For best results, the majority of the wood flakes 12 should
have a width
of about 1/16 inch to about 3 inches, and preferably 0.25 to 1.0 inches.
The blade setting on a flaker can primarily control the thickness of the wood
flakes 12. The length and width of the wood flakes 12 are also controlled to a
large
degree by the flaking operation. For example, when the wood flakes 12 are
being
prepared by ring flaking fingerlings, the length of the fingerlings generally
sets the
maximum lengths. Other factors, such as the moisture content of the wood and
the
amount of bark on the wood affect the amount of fines produced during flaking.
Dry
wood is more brittle and tends to produce more fines. Bark has a tendency to
more
readily break down into fines during flaking and subsequent handling than
wood.
While the flake size can be controlled to a large degree during the flaking
operation as described above, it usually is necessary to use some sort of
classification in
order to remove undesired particles, both undersized and oversized, and
thereby ensure
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the average length, thickness and width of the wood flakes 12 are within the
desired
ranges. When roundwood flaking is used, both screen and air classification
usually are
required to adequately remove both the undersize and oversize particles,
whereas
fingerling wood flakes 12 usually can be properly sized with only screen
classification.
S Wood flakes 12 from some green wood can contain up to 90 % moisture. The
moisture content of the mat must be substantially less for molding as
discussed below.
Also, wet wood flakes 12 tend to stick together and complicate classification
and
handling prior to blending. Accordingly, the wood flakes 12 are preferably
dried prior
to classification in a conventional type drier, such as a tunnel drier, to the
moisture
content desired for the blending step. The moisture content to which the wood
flakes 12
are dried usually is in the order of about 6 weight % or less, preferably
about 2 to about
S weight % , based on the dry weight of the wood flakes 12. If desired, the
wood flakes
12 can be dried to a moisture content in the order of 10 to 2S weight % prior
to
classification and then dried to the desired moisture content for blending
after
1S classification. This two-step drying may reduce the overall energy
requirements for
drying wood flakes 12 prepared from green woods in a manner producing
substantial
quantities of particles which must be removed during classification and, thus,
need not
be as thoroughly dried.
To coat the wood flakes 12 prior to being placed as a felted mat 11 within the
cavity 30 within the mold 10, a known amount of the dried, classified wood
flakes 12 is
introduced into a conventional blender, such as a paddle-type batch blender,
wherein
predetermined amounts of a resinous particle binder, and optionally a wax and
other
additives, is applied to the wood flakes 12 as they are tumbled or agitated in
the blender.
Suitable binders include those used in the manufacture of particle board and
similar
2S pressed fibrous products and, thus, are referred to herein as "resinous
particle board
binders." Representative examples of suitable binders include thermosetting
resins such
as phenolformaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, urea-
formaldehyde, urea-furfuryl and condensed furfuryl alcohol resins, and organic
polyisocyantes, either alone or combined with urea- or melamine-formaldehyde
resins.
Particularly suitable polyisocyanates are those containing at Ieast two active
isocyanate groups per molecule, including diphenylmethane diisocyanates, m-
and p-
phenylene diisocyanates, chlorophenylene diisocyanates, toluene di- and
triisocyanates,
triphenylmethene triisocyanates, diphenylether-2,4,4'-triisoccyanate and
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polyphenylpolyisocyanates, particularly diphenylmethane-4,4'-diisocyanate. So-
called
MDI is particularly preferred.
The amount of binder added to the wood flakes 12 during the blending step
depends primarily upon the specific binder used, size, moisture content and
type of the
wood flakes 12, and the desired characteristics of the part being formed.
Generally, the
amount of binder added to the wood flakes 12 is about 2 to about 15 weight %,
preferably about 4 to about 10 weight % , as solids based on the dry weight of
the wood
flakes 12. When a polyisocyanate is used alone or in combination with a urea-
formaldehyde resin, the amounts can be more toward the lower ends of these
ranges.
The binder can be admixed with the wood flakes 12 in either dry or liquid
form.
To maximize coverage of the wood flakes 12, the binder preferably is applied
by
spraying droplets of the binder in liquid form onto the wood flakes 12 as they
are being
tumbled or agitated in the blender. When polyisocyantes are used, a
conventional mold
release agent preferably is applied to the die or to the surface of the felted
mat prior to
pressing. To improve water resistance of the part, a conventional liquid wax
emulsion
preferably is also sprayed on the wood flakes 12 during the blinding step. The
amount
of wax added generally is about 0.5 to about 2 weight % , as solids based on
the dry
weight of the wood flakes 12. Other additives, such as at least one of the
following: a
coloring agent, fire retardant, insecticide, fungicide, mixtures thereof and
the like may
also be added to the wood flakes 12 during the blending step. The binder, wax
and
other additives, can be added separately in any sequence or in combined form.
The moistened mixture of binder, wax and wood flakes 12 or "furnish" from the
blending step is formed into a loosely-felted, layered mat 11, which is placed
within the
cavity 30 prior to the molding and curing of the felted mat 11 into molded
wood flake
part 14. The moisture content of the wood flakes 12 should be controlled
within certain
limits so as to obtain adequate coating by the binder during the blending step
and to
enhance binder curing and deformation of the wood flakes 12 during molding.
The presence of moisture in the wood flakes 12 facilitates their bending to
make
intimate contact with each other and enhances uniform heat transfer throughout
the mat
during the molding step, thereby ensuring uniform curing. However, excessive
amounts
of water tend to degrade some binders, particularly urea-formaldehyde resins,
and
generate steam which can cause blisters. On the other hand, if the wood flakes
12 are
too dry, they tend to absorb excessive amounts of the binder, leaving an
insufficient
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amount on the surface to obtain good bonding and the surfaces tend to cause
hardening
which inhibits the desired chemical reaction between the binder and cellulose
in the
wood. This latter condition is particularly true for polyisocyanate binders.
Generally, the moisture content of the furnish after completion of blending,
5 including the original moisture content of the wood flakes 12 and the
moisture added
during blending with the binder, wax and other additives, should be about 5 to
about 25
weight % ,. preferably about 8 to about 12 weight % . Generally, higher
moisture
contents within these ranges can be used for polyisocyanate binders because
they do not
produce condensation products upon reacting with cellulose in the wood.
10 The furnish is formed into the generally flat, loosely-felted, mat 11,
preferably as
multiple layers. A conventional dispensing system, similar to those disclosed
in U.S.
Pat. Nos. 3,391,223 and 3,824,058, and 4,469,216 can be used to form the
felted mat
11. Generally, such a dispensing system includes trays, each having one open
side,
carried on an endless belt or conveyor and one or more (e.g., three) hoppers
spaced
above and along the belt in the direction of travel for receiving the furnish.
When a multi-layered felted mat 11 is formed, a plurality of hoppers usually
are
used with each having a dispensing or forming head extending across the width
of the
carriage for successively depositing a separate layer of the furnish as the
tray is moved
beneath the forming heads. Following this, the tray is taken to the mold to
place the
felted mat within the cavity of bottom mold, by sliding the tray out from
under mat.
In order to produce molded wood flake parts 14 having the desired edge density
characteristics without excessive blistering and springback, the felted mat
should
preferably have a substantially uniform thickness and the wood flakes 12
should lie
substantially flat in a horizontal plane parallel to the surface of the
carriage and be
randomly oriented relative to each other in that plane. The uniformity of the
mat
thickness can be controlled by depositing two or more layers of the furnish on
the
carriage and metering the flow of furnish from the forming heads.
Spacing the forming heads above the carriage so the wood flakes 12 must drop
about 1 to about 3 feet from the heads en route to the carriage can enhance
the desired
random orientation of the wood flakes 12. As the flat wood flakes 12 fall from
that
height, they tend to spiral downwardly and land generally flat in a random
pattern.
Wider wood flakes 12 within the range discussed above enhance this action. A
scalper
or similar device spaced above the carriage can be used to ensure uniform
thickness or
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11
depth of the mat, however, such means usually tend to align the top layer of
wood flakes
12, i.e., eliminate the desired random orientation. Accordingly, the thickness
of the mat
that would optimally have the nominal part thickness 100 preferably controlled
by
closely metering the flow of furnish from the forming heads. The. mat
thickness that
would optimally have the nominal part thickness 100 used will vary depending
upon
such factors as the size and shape of the wood flakes 12, the particular
technique used
for forming the mat 11, the desired thickness and density of the molded wood
flake part
14 produced, the configuration of the molded wood flake part 14, and the
molding
pressure to be used.
Following the production of the felted mat 11 and placement of the felted mat
11
within the cavity 30 of the mold 10, the felted mat 11 mat is compressed and
cured
under heat and pressure when the top mold die 16 engages the bottom mold die
18.
The felted mat 11 is then compressed and cured between the top mold die 16 and
the bottom mold 18 to become the molded wood flake part 14. After the molded
wood
flake part 14 is produced by the method of the present invention, any flashing
is
removed by conventional means.
The resulting part 14, 14' comprises a curved or bent surface 16, 16' having
three-dimensionally curved or bent channels 15, 15' molded therein. Such a
part is
useful as a chair shell, for example, having a seat portion which curves
gradually into a
back portion. The three-dimensionally curved or bent channels afford
surprising
strength to such a part, due to the orientation of the wood strands or wood
flakes in a
direction generally parallel to the channel sidewalls 15a, and therefore
generally parallel
to the tensile force placed on part 14 when it is subjected to a bending
moment, as for
example when someone sits in a chair shell and leans back against the back of
the shell.
The above description is that of the preferred embodiment only. Modifications
of the invention will occur to those skilled in the art and to those who make
or use the
invention. Therefore, it is understood that the embodiment described above is
merely
for illustrative purposes and not intended to limit the scope of the
invention, which is
defined by the following claims as interpreted according to the principles of
patent law,
including the Doctrine of Equivalents.
