U.S. patent application number 09/775741 was filed with the patent office on 2001-08-30 for method of making edge densified lumber product.
Invention is credited to Bassett, Kendall H., MacPherson, Gerald N..
Application Number | 20010017186 09/775741 |
Document ID | / |
Family ID | 23687853 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017186 |
Kind Code |
A1 |
MacPherson, Gerald N. ; et
al. |
August 30, 2001 |
Method of making edge densified lumber product
Abstract
The invention is an edge densified lumber product of improved
strength and stiffness and the method of its manufacture. The
method is based on the parallel lamination of multiple plies of
wood veneer. Narrow longitudinal reinforcing strips are laid along
each edge of the veneer assembly between at least some of the
veneer plies. Additional spaced apart veneer strips, about twice
the width of the edge strips, are laid up at preselected locations
in the mid-portion of the veneer assembly. These strips in the
mid-portion are preferably spaced so that the distance between
their centerlines corresponds to standard lumber widths.
Appropriate adhesives are used to bond the assembly. The assembly
is pressed to a uniform thickness so that the areas along the
narrow veneer strips are densified relative to the adjacent
portions. Longitudinal saw cuts are then made along the centerlines
of the interior veneer strips to separate the assembly into
multiple units of lumber. Bending strength and stiffness is
significantly increased by having the densified areas along each
edge of the resulting lumber units.
Inventors: |
MacPherson, Gerald N.;
(Woodland, WA) ; Bassett, Kendall H.; (Tacoma,
WA) |
Correspondence
Address: |
Keith D. Gehr
Patent Department, CH 2J29
Weyerhaeuser Company
Federal Way
WA
98063-9777
US
|
Family ID: |
23687853 |
Appl. No.: |
09/775741 |
Filed: |
February 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09775741 |
Feb 2, 2001 |
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09425747 |
Oct 22, 1999 |
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6217976 |
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Current U.S.
Class: |
156/298 ;
156/300 |
Current CPC
Class: |
Y10T 156/109 20150115;
E04C 3/29 20130101; Y10T 428/24066 20150115; B27M 3/0053 20130101;
Y10T 156/1093 20150115; E04C 3/18 20130101; Y10T 428/24992
20150115; B27D 1/04 20130101; Y10T 428/24132 20150115; Y10T
428/24777 20150115; E04C 3/14 20130101 |
Class at
Publication: |
156/298 ;
156/300 |
International
Class: |
B32B 031/20 |
Claims
1. A method of making a laminated lumber product which comprises:
providing a plurality of wood veneer sheets of predetermined
dimensions; placing and retaining parallel, spaced apart narrow
reinforcing strips along each longitudinal edge of at least one of
the veneer sheets to form a subassembly; laying up additional
veneer sheets on the subassembly to form a laminated assembly; and
pressing and adhesively bonding the members of the laminated
assembly to an essentially uniform thickness in order to densify
the wood in the locus of the narrow reinforcing strips and form a
laminated lumber product having increased edge density and superior
strength and stiffness in bending.
2. The method of claim 1 which further comprises placing and
retaining additional parallel spaced apart reinforcing strips
longitudinally in the interior portion between the edge strips,
said interior veneer strips being about twice the width of the
interior strips; laying up additional veneer sheets on the
subassembly to form a laminated assembly; pressing and adhesively
bonding the laminated assembly to an essentially uniform thickness
in order to densify the wood in the locus of the narrow veneer
strips; and longitudinally sawing the pressed product generally
along the center lines of the interior veneer strip or strips to
form laminated lumber products having increased edge density and
superior strength and stiffness in bending.
3. The method of claim 1 or 2 in which the reinforcing strips are
wood veneer.
4. The method of claim 1 or 2 in which the spacing of the
reinforcing strips is chosen to correspond to standard lumber
widths.
5. The method of claim 1 or 2 which further includes laying up
veneer sheets on each face of the subassembly to form a multiple
ply product.
6. The method of claim 1 or 2 in which multiple subassemblies are
placed one on top of the other.
7. The method of claim 1 or 2 in which the narrow strips are
prebonded to the veneer sheets prior to forming the laminated
assembly.
8. The method of claim 1 or 2 which includes the use of an adhesive
selected from the group consisting of isocyanates and phenolics to
bond the laminated assembly.
9. The method of claim 1 or 2 which includes the use of an adhesive
selected from the group consisting of isocyanates and phenolics to
bond the narrow strips to the veneer sheets forming the
subassemblies.
10. The method of claim 1 or 2 in which the grain direction of the
veneer sheets and narrow veneer strips is parallel.
11. The method of claim 1 or 2 in which the reinforcing strips are
retained in place by a hot melt adhesive.
12. The method of claim 1 or 2 in which the reinforcing strips are
retained in place by staples.
13. An edge densified lumber product which comprises a plurality of
adhesively bonded veneer laminae, said product being of rectangular
cross section with edge and central portions, the product having a
greater number of veneer laminae along the length of each edge
portion than are present in the central portion, said product
having been compressed to an essentially uniform thickness so that
the edge portions have a higher density than the central
portion.
14. The edge densified lumber product of claim 13 in which the
densified edge portions constitute less than about 50% of the
volume of the product.
15. The edge densified lumber product of claim 14 in which the
densified edge portions constitute less than about 20% of the
volume of the product.
16. The edge densified lumber product of claim 13 in which the
grain orientation of all the laminae is parallel.
17. The edge densified lumber product of claim 13 in which at least
one interior lamina has a grain angle oriented approximately
90.degree. from the other laminae.
Description
[0001] The present invention is directed to the method of making an
edge densified lumber product formed from a plurality of parallel
laminated veneer sheets. The invention is further directed to the
lumber product formed by the method.
BACKGROUND OF THE INVENTION
[0002] Sawn lumber in standard dimensions is the major construction
material used in framing homes and many commercial structures. The
available old growth forests that once provided most of this lumber
have now largely been cut. Most of the lumber produced today is
from much smaller trees from natural second growth forests and,
increasingly, from tree plantations. Intensively managed plantation
forests stocked with genetically improved trees are now being
harvested on cycles that vary from about 25 to 40 years in the pine
region of the southeastern and south central United States and
about 40 to 60 years in the Douglas-fir region of the Pacific
Northwest. Similar short harvesting cycles are also being used in
many other parts of the world where managed forests are important
to the economy. Plantation thinnings, trees from 15 to 25 years
old, are also a source of small saw logs.
[0003] Whereas old growth trees were typically between two to six
feet in diameter at the base (0.6 m to 1.8 m), plantation trees are
much smaller. Rarely are they more than two feet (0.6 m) at the
base and usually they are considerably less than that. One might
consider as an example a typical 35 year old North Carolina
loblolly pine plantation tree on a good growing site. The site
would have been initially planted to about 900 trees per hectare
(400 per acre) and thinned to half that number by 15 years. A plot
would often have been fertilized one or more times during its
growth cycle, usually at ages 15, 20 and 25 years. At harvest the
35 year old tree would be about 40 cm (16 in) diameter at the base
and 15 cm (6 in) at a height of 20 m (66 ft). Trees from the
Douglas-fir region would normally be allowed to grow somewhat
larger before harvest.
[0004] American construction lumber, so-called "dimension lumber",
is nominally 2 inches (actually 11/2 inches (38 mm)) in thickness
and varies in nominal 2 inch (51 mm) width increments from 31/2
inches to 11/4 inches (89 mm to 286 mm), measured at about 12%
moisture content. Lengths typically begin at 8 feet (2.43 m) and
increase in 2 foot (0.61 m) intervals up to 20 ft (6.10 m).
Unfortunately, when using logs from plantation trees it is now no
longer possible to produce the larger and/or longer sizes and
strength grades in the same quantities as in the past.
[0005] There is another problem with plantation wood lumber that is
not as generally recognized as are the tree size limitations.
Typically, in plantation wood the average wood density is lower
than old growth wood. This, in turn, affects strength and
stiffness. Strength in flexure, otherwise termed modulus of rupture
(MOR), and especially the stiffness measured as modulus of
elasticity in flexure (MOE), may be lower and more variable than
old growth wood. This is a problem for members used in a bending
situation and it can be one for those members used in compression;
e.g. longer wall studs. Typical of bending uses are floor joists,
roof rafters, truss members, and headers over wide windows and
doors, such as garage doors.
[0006] The problems noted above were outlined 20 years ago in a
paper by A. Bendtsen Forest Products Journal 28 (10): 61-72 who
noted the implications for construction lumber but offered no
suggestions how to deal with them.
[0007] Since loblolly pine (Pinus taeda L.) and its closely related
southern pines are particularly important timber species they will
be used in the following discussion as a non-limiting example of
coniferous trees in general. A frequently used unit related to
density is specific gravity measured as oven dry weight/green
volume. For loblolly pine, near the base of the tree specific
gravity of the first several growth rings surrounding the pith will
typically range around 0.38. By about age 20 the wood being formed
near the bark at the same height will have a specific gravity of
about 0.51-0.56. Density even of the outer mature wood portion of
the tree varies longitudinally along the tree, being generally
lower in the upper portions. Density of woods has been shown to
correlate directly with stiffness, measured as modulus of
elasticity in flexure. This variability has not been seriously
taken into account in the manufacture of lumber products. Current
sawmill procedures make no attempt to specifically deal with these
inherent differences in density. The general assumption appears to
have been that density variability was a factor which was not
subject to any control.
[0008] Solid sawn wide dimension lumber is not without its own
significant drawbacks. In particular, inconsistency in dry
dimensions and strength properties and limited availability of long
lengths are major deficiencies. Decrease in moisture content after
installation causes shrinkage which is not consistent from piece to
piece due to differences in grain orientation. This results in
variability in dry width even though initial width was uniform.
Particularly when the lumber is used as floor joists, inconsistent
width from piece to piece results in poor conformation of sheathing
or subfloor laid over the joists. This is a major contributor to
the cause of annoying squeaks as people walk on the floor.
[0009] Lumber is graded visually by established rules that take
into account many factors; i.e., knot size and placement, density,
grain slope, manufacturing defects, etc. Any piece of lumber within
a given grade is presumed to have some minimum stress rating.
Unfortunately, the actual stress ratings of individual pieces
within any one grade will vary considerably since the rules are
established to ensure that the poorest piece will fall within
grade.
[0010] Many approaches have been taken to engineer structural grade
wood products to take the place of the larger and/or longer lumber
sizes now in short supply. One successful approach is based on
adhesively bonding a number of plies of rotary cut veneer. Unlike
typical plywood products, the grain direction of all the plies is
normally in the same direction. In one way of producing this
product wide panels of appropriate thickness are ripped into pieces
of standard dimension lumber width then finger jointed to the
desired length. Other processes start with relatively narrower
veneer sheets which can be butted end-to-end and continuously
bonded to make units of almost any desired length, width, and
thickness. The butt joints of adjoining plies are preferably
staggered to prevent introducing points of weakness. This so-called
laminated veneer lumber (LVL) has been in commercial production and
use for a number of years, often as the tension members of trusses;
e.g., as seen in Troutner, U.S. Pat. No. 3,813,842. It has the
advantage that defects, particularly knots, do not run entirely
through the piece as they do in sawn wood. This generally allows a
higher stress rating for a LVL member of any given cross sectional
dimensions. Other exemplary products of this type are described by
Peter Koch, Beams from bolt-wood: a feasibility study, Forest
Products Journal, 14: 497-500 (1964) and by E. L. Schaffer et al.,
Feasibility of producing a high yield laminated structural product,
U.S.D.A. Forest Research Paper FPL 175 (1972).
[0011] Many combinations of veneer, solid sawn wood, and
reconstituted wood such as engineered strandboard or flakeboard
have also been explored for use as structural lumber products.
Lambuth, in U.S. Pat. No. 4,355,754, shows a structural member in
the form of an I-beam using a plywood web with solid sawn flange
members. When used as a joist, this is presumably substitutable for
sawn lumber of the same cross sectional dimensions. The web is
friction fit and glued into tapered slots in the flange pieces.
Other very similar constructions use composite wood strips such as
oriented strandboard or flakeboard as the web member.
[0012] Barnes, in U.S. Pat. No. 5,096,765, notes the importance of
stiffness (modulus of elasticity in flexure) (MOE) in lumber
products. The product described uses splinters or strands of sliced
veneer from 0.005-0.1 inch (0.13-2.5 mm) thick, at least 0.25
inches (6.4 mm) wide and at least 8 inches (203 mm) long. These
must be free of any surface or internal damage and have their grain
direction within 10.degree. of the longitudinal axis of the
product. After addition of adhesive the product is pressed to have
"an MOE equivalent to a composite wood product having a MOE of at
least 2.3 mm psi [1.59.times.10.sup.7 kPa] at . . . a density of 35
lbs/cubic foot".
[0013] In the above patent the inventor refers to his earlier U.S.
Pat. No. 4,061,819 which teaches that the strength of wood
composite products is density dependent; i.e., ". . . the higher
[the] density generally the higher the strength of the product for
the same starting materials". The earlier patent describes a very
similar lumber-like product to the above having a modulus of
elasticity approaching or reaching the MOE of clear Douglas-fir at
various densities. Products similar to those described in the
Barnes patents are now commercially available. However, the very
high adhesive usage they require has a significant negative impact
on cost of the products. Also, the strandwood products have
significantly higher density than sawn lumber and are heavier to
handle and more expensive to ship.
[0014] Many other patents teach the manufacture of clear wood
members by various combinations of sawing and edge, end, and/or
face gluing. Exemplary of these are U.S. Pat. No. 1,594,889 to
Loetscher, U.S. Pat. No. 1,638,262 to Neumann, U.S. Pat. No.
2,942,635 to Home, U.S. Pat. No. 5,034,259 to Barker, and 5,050,653
to Brown. Other workers have explored surface densification for
various purposes. Exemplary of these are U.S. Pat. No. 3,591,448 to
Elmendorf and U.S. Pat. No. 4,355,754 to Lund et al.
[0015] Compressed wood products have been known for many years. One
commercially available product is formed of a plurality of thin
parallel grain veneer sheets that have been impregnated with a
thermosetting resin prior to compression. This product is limited
to specialty uses, principally kitchen and table knife handles.
Walsh et al. in U.S. Pat. No. 1,465,383, describe a cross laminated
compressed wood product useful for pulleys and similar items.
Travis, in U.S. Pat. Nos. 4,136,722 and 4,199,632 shows a tool
handle made of parallel laminated veneer sheets. The veneer sheets
at the tool attachment end of the handle are interleaved with
additional narrow veneer strips. The product is then compressed to
uniform thickness so that the tool attachment end is of
significantly higher density than the residual portion of the
handle.
[0016] An earlier development by some of the present applicants,
published as PCT Application WO 98/10157, describes selective
placement of the denser wood from the trees along the edges of
lumber products where it enhances stiffness and bending
strength.
[0017] Most of the products noted above have not found significant
success for one or more reasons. There are exceptions, however.
Laminated veneer lumber and edge and end glued pieces reassembled
to produce clear boards or for use as door cores have been in
commercial use for many years. Composite I-beams similar to those
described in the Lambuth patent are now also widely available. One
such product family manufactured by Trus Joist MacMillan, Boise,
Idaho, is typical of the products which appear to have become an
industry standard.
[0018] The composite I-beams have found considerable acceptance in
the building industry where long spans, consistent dimensions, and
known and dependable strength properties are required. However,
they are not without their drawbacks. Their performance under
common residential dynamic loads is not as good as solid sawn
construction, due primarily to a lack of mass. As a result most
builders use I-joists at a shorter than suggested span or at a
reduced spacing. They cannot entirely be used as a replacement for
sawn lumber. For example, they need reinforcing blocking to fill
out the sides of the web to full width at many loading points.
Their cross section essentially prevents side nailing and they
present a major problem in attaching other members to the sides.
Also, since the flange portions of the I-joist provides most of the
stiffness it cannot be notched as is commonly done with solid sawn
lumber. The nature of the geometry increases shear forces in the
web member to higher values than are found in solid products of
rectangular cross section.
[0019] It is notable in view of the highly heterogeneous nature of
the smaller trees now available that the art has not more seriously
heretofore addressed the problem of producing strong members of
uniform and dependable properties from smaller plantation trees.
The present invention overcomes the noted deficiencies in solid
sawn lumber and composite I-beams. In addition, it results in a
much higher utilization of the tree into useful lumber
products.
SUMMARY OF THE INVENTION
[0020] The present invention is particularly directed to a method
of manufacturing engineered structural wood products. These
products are especially useful in critical applications such as
joists, headers, and beams where predictable and higher stress
ratings in edge loading may be required. The products have the
advantage that they may be handled in the same fashion as solid
sawn lumber. Strength properties are predictable and uniform. The
products do not have the strength variability between and within
individual pieces found in much visually graded solid sawn lumber.
A major advantage of the present method is that it can be adapted
for use in most plywood mills.
[0021] The method is based on lamination of multiple plies of wood
veneers or strips which will typically range between 1-6 mm in
thickness although thicker laminae are also suitable. In general
the grain direction of all of the plies will be parallel although
it is within the scope of the invention to include one or more
interior laminae with a grain direction about 90.degree. to the
longitudinal dimension. Either sliced or rotary cut veneers are
suitable but rotary cut veneers will generally be preferred. At
some point these will be edge joined by one of the known processes
so that dried veneer sheets having a precise predetermined width
can be supplied. Additional narrow edge reinforcing strips are also
provided. These will most typically be wood veneers of the same
species but may be of a stronger species or of another reinforcing
material; e.g. carbon or synthetic polymer fiber. The terms
"strips, veneer strips, or reinforcing strips" should be considered
sufficiently broad to include these alternative materials. One of
the narrow strips is laid up along each longitudinal edge of a
veneer sheet. In the preferred method of manufacture additional
narrow strips will be laid up in parallel fashion at predetermined
distances between the edge strips. These interior strips should be
about twice the width of those used along the edges. Centerlines of
the interior strips will relate to each other and to the outside
edges in standard lumber dimensions; e.g. about 140, 190, 240 mm
(51/2, 71/4, 91/4 in), etc. or some optimum combination of these
dimensions. The narrow veneer strips will ultimately be adhesively
bonded on both faces to any veneer sheets with which they are in
contact. The width of the narrow strips is not critical and will
depend on the ultimate product characteristics desired. In general
the strips used along the edges will be between about 25-50 mm (1-2
in) with about 35-40 mm (11/2 in) being preferred. As just noted,
the interior strips will be about twice this width.
[0022] A single veneer sheet laid up with the narrow strips as just
described will be for convenience of description be termed a
subassembly. Additional veneer sheets and/or subassemblies are then
laid up above and/or below the initial subassembly to form a veneer
assembly. One or both of any adjoining veneer faces will be
adhesive coated. Preferred adhesives are phenolics, such as those
normally used for plywood construction, or isocyanates, now widely
used for bonding oriented strandboard products. Other commonly used
durable wood adhesives such as resorcinol or melamine based types
are also suitable.
[0023] The veneer assemblies are then placed in a press heated to a
sufficient temperature for an adequate time to ensure permanent
bonding. Temperature will depend on the particular adhesive used.
The pressure used must be sufficient to compress the veneers in the
locus of the narrow strips so that the ultimate product is of
essentially uniform thickness. Typically a maximum pressure of
about 4800-6200 kPa (700-900 psi) is sufficient.
[0024] After pressing, the resulting panels are then sawn
longitudinally along the center lines of the interior strips to
form an edge densified lumber product. The individual boards so
produced may then be end jointed, if desired, to produce lumber in
any required length.
[0025] While this will depend somewhat on the product width, the
edge densified portions of the product will normally comprise less
than about 50% of the product volume, more typically about 20%.
[0026] Where the terms "lumber products" or "lumber-like products"
are used it should be understood that these refer to wood products
that can be used and handled like solid sawn lumber and are similar
in general appearance.
[0027] It is a primary object of the present invention to provide a
process for efficiently making edge densified lumber products.
[0028] It is a further object to provide lumber products having
enhanced stiffness and bending strength from plantation wood
trees.
[0029] It is also an object to provide a process for making edge
densified lumber products using rotary cut or sliced veneers.
[0030] It is yet an object to provide a process for making edge
densified lumber products that is readily amenable to automated
production.
[0031] It is an object to minimize overall product weight by
selectively densifying only the edge regions.
[0032] These and many other objects will become readily apparent
upon reading the following Detailed Description taken in
conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1A, 1B, and 1C represent in vertically exaggerated
cross section alternative ways of preparing veneer
subassemblies.
[0034] FIG. 2 shows in vertically exaggerated cross section a
veneer assembly ready to be pressed.
[0035] FIG. 3 shows in a horizontally compressed exploded
perspective view a veneer assembly ready to be pressed.
[0036] FIG. 4 shows in partial perspective a pressed veneer
assembly (with considerable vertical exaggeration).
[0037] FIG. 5 shows in partial perspective a lumber product
produced by the method.
[0038] FIGS. 6-8 show in cross section alternative layups of the
lumber products.
[0039] FIG. 9 is a partial perspective end view, with considerable
vertical exaggeration, of an experimental panel for producing an
edge densified lumber product and a comparison control sample.
[0040] FIG. 10 is a longitudinal edge view of the right hand edge
of the panel of FIG. 9, again with considerable vertical
exaggeration.
[0041] FIG. 11 shows an alternative construction in which only two
reinforcing strips are used.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] An understanding of the method and the configuration of some
of the possible products is readily seen from reference to FIGS.
1A, 1B and 1C. Veneer sheets 2 are edge glued, stitched, or
otherwise joined if necessary to provide full width sheets; e.g.,
up to 4 fit in width, which are then trimmed to precise widths.
Narrow veneer strips 4 are placed adjacent to each longitudinal
edge of the veneer sheets and interior narrow veneer strips 6 are
placed at desired intervals between and parallel to the edge strips
4. A layer of an appropriate adhesive 8 will serve to ultimately
bond the narrow strips to the wide veneer sheet. This may be coated
on the lower face of the narrow strips, as shown, on both faces of
the narrow strips, or on the full upper surface of veneer sheet 2.
The narrow strips may be tacked in place with a hot melt glue,
stitched, or held with staples 7 as shown in FIG. 1C. Interior
strips 6 will be about twice the width of edge strips 4. The narrow
strips 4, 6 may be the same thickness as the larger veneer sheets 2
or may be of some different thickness. The combination of the
plurality of narrow veneer strips with a single full sized sheet of
veneer forms a subassembly. The narrow strips 4, 6 may simply be
placed on the wide veneer sheet 2 and bonded later, they may be
prebonded, or they may be prepressed as seen in FIG. 1B where both
the narrow strips and underlying veneer sheet is compressed and
densified in the locus of the narrow strips. The exact treatment
will depend somewhat on the chosen process equipment and adhesive.
Certain isocyanate adhesives; e.g., PMDI (polydiphenylmethane
diisocyanate) are particularly advantageous since they have a very
long open assembly time that permits coating only the back surface
of the narrow strips at the subassembly stage. The subassemblies
may then be fully coated with adhesive at a later time.
[0043] Reference to FIGS. 2 and 3 show one possible product layup
using a single subassembly. Scale in FIG. 3 is significantly
exaggerated. A three ply product is represented for simplicity.
Such a product is quite practical using thick veneers. However,
preferred products will have additional laminae as will be
subsequently shown. A single subassembly of veneer sheet 2 and
strips 4, 6 is shown as an interior ply. In this case, the upper
face of veneer sheet 2 has been uniformly coated with adhesive 12.
An upper veneer sheet 14, coated on its lower face with adhesive
16, and a lower veneer sheet 18, coated on its upper face with
adhesive 20, completes the assembly. After the assembly is laid up
it is placed in an appropriate hot press to bond the components and
densify the area in the locus of the narrow strips so that the
resulting product is of essentially uniform thickness.
[0044] FIG. 4 represents a vertically exaggerated four ply assembly
22 in which three subassemblies and an additional veneer sheet have
been pressed and densified in the areas along the narrow strips. It
is ready to be cut into lumber products. Veneer sheets 24, 26, 28,
and 30 have been interlaid with narrow edge strips 32 and interior
strips 34 and pressed to a uniform thickness across the width of
the product. Lines where longitudinal cuts to form individual
boards will be made are shown along the center-lines of interior
narrow strips 34.
[0045] FIG. 5 represents a multi-ply product 40 using fifteen
veneer sheets 42, six of which are subassemblies with narrow veneer
strips 44, 46 along the edges. This is a particularly useful
configuration for a product made with a standard lumber thickness
of 38 mm (11/2 in) thickness using 2.5 mm ({fraction (1/10)} in)
veneer. Thickness is uniform across the width but the density along
the edges has been increased by about 40% over the uncompressed
central portion.
[0046] FIGS. 6-8 show some of the variations that can be made in
construction of the products made by the present method. FIG. 6 is
a product 50, similar to that of FIG. 5, in which the narrow strips
52 are of the same width throughout the thickness. This need not
necessarily be the case. An illustration in FIG. 7 shows a
gradually increasing width of the narrow strips 62 toward the
center of the product 60. FIG. 8 shows a construction 70 that is
particularly advantageous from the standpoint of product
dimensional stability. Outer veneer sheets 70, 72 on both faces
have their grain direction oriented longitudinally, as has been the
case with all of the examples described to the present. However,
there are two central sheets 74 with their grain oriented
90.degree. or transverse to the longitudinal axis. This may be done
to increase dimensional stability in a moist environment since wood
is known to expand much less parallel to the grain than it does
perpendicular to the grain. These transverse sheets need not be
adjacent and the number, if they are used at all, is not limited to
two but may be one or more.
EXAMPLE
[0047] Nominal 0.1 in (2.5 mm) western hemlock veneer was used to
form a 16 foot (4.88 m) panel 251/2 inches (648 mm) wide with a
finished thickness of 1.5 inches (38 mm). Individual veneer sheets
were clipped to a 25.5 in (648 mm) width and 101 inch (2565 mm
length). The veneer was dried to approximately 5% moisture content.
Weighted input veneer MOE was 1.73.times.10.sup.6 psi
(1.19.times.10.sup.7 kPa) and the weighted density was 25.88
lb/ft.sup.3 (41.46 kg/m.sup.3). The assembly was made of seventeen
layers of full width veneer. Four densification strips 2 inches (51
mm) wide were placed along one edge and a similar number of strips
31/2 inches (89 mm) wide were placed in the interior with center
lines of the interior strips about 10 inches (250 mm) from the edge
of the assembly. The densification strips were placed between
veneer sheets 1 and 2, 5 and 6, 12 and 13, and 16 and 17. The
adhesive used was 6% PMDI based on total assembly weight. Geometry
of the panel is more readily understood by reference to FIGS. 9 and
10. The assembled panel was then placed in a hot press under a
maximum pressure of 5520 kPa (800 psi) at a temperature of
190.degree. C. (375.degree. F.) for 30 minutes followed by a 10
minute three-stage decompression cycle.
[0048] FIG. 9 is a partial perspective end view of the test panel
with considerable vertical exaggeration. Seventeen veneer sheets 82
were laid up one on the other. The four narrow edge strips 84 and
four interior strips 86 were placed to form subassemblies as
described above and shown in the figure. After pressing, narrow
trim strips were taken off each edge along cut lines 88, 90.
Further cuts were made along cut lines 92 and 94 to produce boards
96, 98 which were 91/4 inches (235 mm) wide, 11/2 inches (38 mm)
thick, and 16 feet (4.88 m) long. Piece 99 cut out of the center of
the panel was considered waste for purposes of the present example.
Board 98 is an edge densified product while board 96 is a control
sample without edge densification.
[0049] FIG. 10 shows a longitudinal edge of board 98, again with
considerable vertical exaggeration. Since the veneer sheets were
only slightly longer than half the length of the ultimate products
it was necessary to make end joints along lines 100. These were
staggered along the length as shown in the figure. End joints were
formed by overlapping adjacent veneer sheets by about 25 mm.
[0050] The control and edge densified boards were tested for
stiffness and strength with load applied both to the edge (as a
joist) and face (as a plank). Results are shown in the following
table.
1 Density, Load applied as plank Load applied as joist Sample
kg/m.sup.3 MOE, kPa MOR, kPa MOE, kPa MOR, kPa Control 51.26 1.28
.times. 10.sup.7 -- 1.32 .times. 10.sup.7 5.44 .times. 10.sup.4
Edge Densified 54.63 1.43 .times. 10.sup.7 -- 1.50 .times. 10.sup.7
6.36 .times. 10.sup.4
[0051] The gain in strength and stiffness of the edge densified
product is significant and cannot be accounted for by the slightly
increased overall density. By comparison, solid sawn hemlock lumber
of dimensions equivalent to the test samples, loaded as a joist,
has an MOE that generally falls within the range between about
1.1.times.10.sup.7 and 1.5.times.10.sup.7 kPa The edge densified
product clearly falls at the high end of this range while the
control sample falls at about the average value.
[0052] FIG. 11 shows an alternative method of construction in which
a product of lumber width is being initially formed. This uses one
or more base sheets 110 and employs one veneer strip 112 along each
edge and but omits the interior strips. A top sheet 114 completes
the assembly. Additional sheets 110 and strips 112 may be laid up
one on toop of the other until a desired final product thickness is
obtained. The veneer sheets would initially be clipped to the
approximate width of the desired lumber product, allowing only a
slight excess for trimming after pressing.
[0053] It will be apparent to those skilled in the art that many
variations can be made, both in the product and its method of
manufacture, that have not been described here. These are regarded
as being fully within the scope of the invention if encompassed by
the following claims.
* * * * *