Note: Descriptions are shown in the official language in which they were submitted.
2~0~31
METHOD AND APPARATUS FOR NAgING
PRESSED ARTI~LES FRCM WOOD
This invention relates to a method and apparatus for making
formed articles from wood. The method involves conditioning the
wood by plasticization prior to pressing and while the work piece
is substantially uniform in cross section. Conditioning is followed
by pressing the heated wood between forming dies to impress the
desired form into the product. The conditioning and pressing stages
are done in two steps involving apparatus that has separate
conditioning and pressing parts. In another embodiment of the
present invention the apparatus performs the conditioning and
pressing stages within a single working chamber.
Forming of wood by pressing requires the wood matrix to be in
a softened state. In dry, unheated state wood is relatively brittle
and difficult to mold. When exposed to combination of moisture and
temperature wood acquires a varying degree of plasticity. It
undergoes transition from a glassy to a soft state. The temperature
at which wood softens is called the glass transition temperature
and is a function of moisture content.
,
Wood has three major structural components. Cellulose and
hemicellulose form the ~ibre that gives wood its strength. Lignin
is a thermoplastic polymer that binds the wood matrix together. All
three constituents of woody cells are subject to softening when
exposed to temperature and moisture. However, their glass
transitions are different. For lignin it varies from about 160C
when dry to about 100C when saturated. Hemicellulose, on the other
hand, softens at about 180C in dry state and at about 50C in
saturated condition. Cellulose crystallites do not show softening
until about 200C and their glass transition does not depend on th~
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2 ~
!'~ moisture content because of their hydrophobic nature. The amorphous
: .
~5 regions of the elementary fibrils, however, are likely plasticized
by moisture. The glass transition characteristics of the amorphous
cellulose is assumed to be similar to that of hemicellulose.
?
Forming action deflects the wood matrix from its natural
configuration. To preserve the new shape requires the wood
constituents to deflect and to flow into the new shape. It also
requires that the wood matrix is subsequently "locked" in the new
configuration. Plasticized wood flows easily and upon cooling the
~ new shape is preserved.
¦ Large compression of non-plasticized wood can result in
serious damage to its structure. Partially softened wood can also
sustain damage as the higher loads are supported by the
constituents which have not been softened. This may lead to
development of fissures, thus weakening or destroying the wood.
Non-uniformity of heating of compressed wood can be detrimental to
i the product. If cold wood is subjected to full forming pressures
20 before heating, as is the case in the typical manufacturing process
for wood composites, a portion of the compression energy is stored
in the cold mat in the form of potential energy ~elastic and
delayed elastic strain). Some of this energy is released when the
subsequent heating changes the elastic characteristics of some
parts of the wood matrix. A nonuniform heating lowers the stiffness
of the heated areas and this allows some of the stored energy to be
relaxed by compressing the softened fibres. Consequently, the areas
that are receiving preferential heating will be subjected to a
larger amount of compression. This may result in development of
cracks and wood cell collapse. Most heating processes produce large
temperatures gradients during heating.
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For successful shaping of wood, as in wood bending, deep
embossing or forming, the entire work piece and all the
constituents of wood should be in adequately plastic state prior to
pressing. In bending of solid wood the softening is usually
accomplished by steaming the work piece in a pressure vessel for a
period of time before the bending procedure. Plasticization needed
for deep embossing or forming composite products or large sizes of
wood is not as easily achieved on practical basis. A typical steam
treatment of larger cross sections requires prolonged heating
times. Steaming in a pressure vessel would be difficult to apply to
wood compositions containing typical wood binding glues.
Majority of manufacturing methods used for wood composite
products involve simultaneous application of rapid heating and
forming pressure. A rapid and uniform heating of the work piece can
be achieved by high frequency or steam injection heating methods.
However, these heating methods are difficult to apply to non-flat
contoured surfaces. High frequency heating would be highly
nonuniform and the steam injection would be inefficient due to
steam leaks. In present processes the high frequency and steam
injection heating is limited to products with planar surfaces which
are in contact with the heat applicators. The existing art which
applies high frequency for heating is described by Fraser et al.,
US Patent No. 3,888,715 and Lamberts et al., US Patent No.
4,221,950. The disclosed methods are suitable for the manufacture
of flat panel prod~cts. The requirement of a substantially planar
interface between the applicator of high frequency or steam
injection heating and the surface of the heated wood limits the
applicability of these efficient heating methods.
The present invention discloses an improved method and
apparatus which allows an efficient use of high frequency or steam
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2~ Q ~
injection heating methods to plasticize the work piece prior to
forming. In the present method the wood is plasticized and formed
in two separate steps. The apparatus of the present invention
achieves a high degree of wood plasticity in one step and presses
the softened wood into a shape by a pair of pressing dies in a
second step. Although this invention is applicable to manufacture
of shaped wood products it can be used to produced flat panel
products which can be densi~ied to a high degree.
;0 This invention pertains to a method of making pressed products
from wood in an apparatus having a working space defined by a
conditioning part and a forming part, the method comprising the
steps of: a) preparing a moisture containing formable work piece;
b) positioning the formable work piece in the conditioning part of
the apparatus comprising an upper and a lower pressure and heat
applicators, within the conditioning part subjecting the work piece
simultaneously to a restraining external pressure and heat and
heating the restrained work piece to the boiling point of the
moisture within the said article, the restraining external pressure
opposing the internally generated steam pressure, the moisture and
heat plasticizing the work piece; c) transferring the work piece to
the forming part of the apparatus comprising àn upper and a lower
forming die assemblies; d) compressing the plasticized work piece
between the forming die assemblies to impress the desired shape
into the plasticized work piece.
This invention further pertains to a two stage method of
making pressed wood composite product, the method having a
conditioning stage and a forming stage, the method comprising the
steps of: a) preparing a composite assembly constructed of a
plurality of wood elements and a wood bondiny glue between the
elements, the glue between the wood elements forming a plurality of
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2100131
~luelines within the composite assembly; b) transferring the
composite assembly into an apparatus having a conditioning part and
a forming part, first positioning the composite assembly in the
conditioning part of the apparatus comprising an upper and a lower
pressure and heat applicators assemblies; c) in the conditioning
part of the apparatus simultaneously subjecting the wood composite
to a restraining pressure and heat and heating the wood composite
assembly to the boiling temperature of the moisture within the said
composite assembly, the restraining external pressure opposing the
internal steam pressure within the assembly wherein the internal
steam pressure in excess of the opposing restraining external
pressure being allowed to vent to the exterior of the wood
composite assembly, the elevated temperature and moisture within
the wood composite assembly plasticizing the wood; d) transferring
the plasticized wood composite assembly from the conditioning part
to the forming part of the apparatus, the forming part comprising
an upper and a lower pressing die assemblies movable in respect to
each other; e) compressing the plasticized wood composite assembly
between the pressing die assemblies to impress the desired shape
~0 into the conditioned wood composite assembly.
`The working space of the apparatus can be pressurized during
the transfer of the conditioned wood article or composite assembly
by air or by steam. Conditioning of the wood article or composite
assembly can be performed by high frequency heating method or by
steam injection heating method. The boiling temperature of the
moisture in the conditioning step can be between about 100C and
about 150C and the restraining pressure in the conditioning step
can be between zero and about 100 psi. Pressing dies can be heated
or can be cooled within the forming part of the apparatus.
This invention further pertains to an apparatus for making
21~0131
pressed articles from wood, the apparatus comprising: a)
conditioning part further comprising upper and lower frame means
supporting mutually opposing upper and lower heat applicators, the
heat applicators being disposed to apply restraining pressure to a
work piece inserted therebetween, the opposing pressing surfaces of
the said applicators being substantially planar and defining a
conditioning chamber, the distance between the opposing planar
pressing surfaces defining the opening of the conditioning chamber;
means for varying the opening of the conditioning chamber; means
for holding the work piece elevated above the lower heat applicator
whereby creating a gap between the work piece and the planar
pressing surface of the lower heat applicator; b) forming part
comprising upper and lower pressing frame means, the upper pressing
frame means supporting an upper pressing die, the lower pressing
frame means supporting a lower pressing die, the upper pressing die
being attached to the upper pressing frame means, the lower
pressing die not being attached to the lower pressing frame means
and being movable and transferable between the forming part and the
conditioning part of the apparatus, said lower pressing die being
slidable into the gap between the elevated work piece and the lower
heat applicator; c) means for depositing the work piece on the
forming surface of the lower pressing die while the said pressing
die is in the conditioning chamber and located below the work
piece; d) moving means for transfer of the lower pressing die
between the conditioning part and the pressing part of the
apparatus; e) means for removing of the pressed work piece from the
forming part.
The heat applicators in the conditioning part of the apparatus
according to the present invention can be high frequency heat
applicators or can be steam injection heat applicators. Holding of
the elevated work piece in the conditioning part to facilitate its
2~01~1
transfer by the movable lower pressing die can be done by
mechanical means by engaging the edges of the wood article or
composite assembly, or can be done by vacuum applied at the
interface between the planar surface of the upper heat applicator
and the wood article. The apparatus can be internally pressurized
by air or steam pressure at least during the transfer of the wood
article between the conditioning and forming parts.
This invention further pertains to an apparatus for making
pressed articles from wood comprising: working chamber defined by
upper and lower heat applicators spaced apart to provide
therebetween a working opening, the said heat applicators having
substantially planar heating surfaces disposed to apply pressure
and heat to a work piece within the working opening of the chamber,
the improvement comprising means for elevating the work piece above
the lower heat applicator and retaining the said work piece in
elevated position above the lower heat applicator thereby
generating a gap between the work piece and the planar heating
surface of the lower heat applicator, further comprising means for
inserting a lower pressing die into the said gap between the work
piece and the lower heat applicator and lowering the work piece
onto the inserted lower pressing die, means for inserting an upper
pressing die into a space between the work piece and the upper heat
applicator, means for applying pressure to the work piece
therebetween the said pressing dies, means for sliding the upper
and lower dies in and out of the conditioning and pressing chamber.
The substance and nature of this invention in certain
embodiments is illustrated in the follow~ng drawings. The drawings
should not be interpreted as restricting the spirit or scope of the
invention in any way:
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21001~1
Figure 1 is a schematic cross sectional view of an apparatus
according to the first embodiment of the present invention in the
conditioning phase of operation
Figure 2 is a schematic cross sectional view of an apparatus
according to the first embodiment of the present invention in the
transfer phase of the operation.
Figure 3 is a schematic cross sectional view of an apparatus
according to the first embodiment of the present invention in the
forming phase of the operation.
Figure 4 is a schematic cross sectional view of an apparatus
according to the second embodiment of the present invention.
The method according to the present invention involves
plasticization of a wood work piece followed by forming of the
plasticized wood by pressure between a pair of forming dies. High
degree of plasticization is achieved by heating the wood work piece
while restrained by pressure of between zero and about 100 psi to
temperature of between about 100C and about 150C. The heating
methods suitable for practising the present invention should
provide a uniform and fast heating of the entire work piece. It
should allow an efficient heat transfer to the work piece or heat
~eneration within the work piece while the work piece is under
restraining pressure that is generally lower than the pressure
needed to form the final product.
Efficient and rapid heating methods, which also provide
uniform heat distribution and can be used to heat wood work piece
under lower restraining pressure, are the high frequency and steam
injection heating methods. However, both of these methods work well
only with substantially planar interface between the respective
heat applicators and the wood work piece. This is easily achievable
in manufacturing of flat panel products. In processes involving
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210~31
forming, such as in deep embossing, a simultaneous heating and
forming using high frequency, steam injection or sometimes a
conventional hot press heating methods is difficult or impossible
to apply because the pressing dies are by definition not planar. A
non-planar interface between the heat applicators and the wood work
piece prevents adequate heat transfer, induces various fringing
fields and does not allow steam pressure development due to an
unsealed interface. The result is a less efficient and more
nonuniform heating.
'
The above described difficulty in applying an efficient,
uniform and rapid heating in manufacturing o~ pressed wood products
is overcome by the present method. The improved method allows
separation of heating and forming. It also achieves high degree of
plasticization prior to forming which results in improvement of the
product's quality and improved forming process.
The method comprises two separate, but coupled steps. In the
first step the wood work piece is conditioned in a heating
~O apparatus. The heating can be by high frequency heating device or
it can be by a steam injection heating device. Such devices are
widely available in the industry. The heat applicators of these
devices can be made in pairs and placed so they mutually oppose
each other as in upper and lower positions and are movable in
respect to each other to close or open the gap between them. The
wood work piece is placed between the heat applicators and
subjected to a small restraining pressure between the heat
applicators of about zero to about 100 psi. In most situations the
external pressure applied during conditioning will be between about
~0 5 psi and about 20 psi.
As explained previously, wood is plasticized by both moisture
2100131
and heat. To adequately plasticize the entire wood matrix, all
components of wood cell should be fully plasticized. Depending on
the moisture content of the wood, lignin, hemicellulose and
cellulose soften at different temperatures. To achieve softening of
all components at the typical production moisture contents of
between 5% and 25%, the conditioning temperature should be selected
between about 100C and about 150C. The preferred conditioning
temperature range is between about 120~C and about 140C. At these
temperatures the moisture within the wood work piece will be at the
boiling point for at least a portion of the heating time. The steam
within the wood structure will be at an elevated pressure. The
external restraining pressure can be used to control the boiling
point and the evaporation rate of the moisture within the wood work
piece. The control is based on the well known relationship between
saturated steam pressure and temperature. This controlling feature
is especially useful when the wood work piece is a composite
assembly comprising wood elèments with thermosetting glue dispersed
between the elements. By selecting the proper external restraining
pressure to act on the work piece will establish a specific boiling
temperature. Thus, the external restraining pressure is controlling
the boiling point and the plasticizing temperature of the heated
wood. A substantially planar interface between the heat applicators
and the wood work piece is important in the conditioning process
also to maintain an adequate control of the plasticization
temperature and the moisture evaporation rate. Control of the
conditioning process may be designed to prevent cure of the glue
~ystem that may be used as a component of the work piece. A number
of glue systems, such as the common phenol formaldehyde glues,
allow heating to the preferred conditioning temperature without
curing, providing the moisture evaporation is controlled. Limited
strength of the glueline is, however, desirable during the transfer
of the work piece. Therefore, at the end of the conditioning phase,
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210013~
~he restraining pressure can be temporarily increased to
consolidate the work piece.
~ n the second step the conditioned and plasticized wood work
piece can be transferred to a forming apparatus comprising a pair
of forming dies, or the pressing dies can be inserted into the
conditioning device. The upper and lower forming dies have the
desired shape and apply the required pressure to form the wood. The
dies can be heated in order to transfer additional heat into the
pressed wood or to prevent heat loss during the pressing period if
it is so re~uired. With certain products it may be beneficial to
cool the product immediately after the form is achieved. In this
case the quenching can be achieved by cooling the pressing dies or
the entire pressing apparatus. It should be obvious that this
method allows any combination of heating and cooling of the pressed
product within the forming apparatus and any such combination is
considered within the scope of this invention.
'~
At certain conditioning temperatures the internal steam
2~ pressures may be higher than the internal strength of the heated
wood work piece. A possible steam pressure damage could result
after the release of the external restraining pressure at the
completion of the conditioning step. This problem can be overcome
in practising the present invention by providing the apparatus with
a sealing means which can pressure seal the conditioning and
forming apparatus such, that the conditioning and forming steps are
performed within the pressure-tight working space of the said
apparatus. The working space of the apparatus can be pressuri~ed by
air or other gas before releasing the external restraining
pressure. The pressure within the working space can be such as to
balance the internal steam pressure within the restrained wood work
piece. It may be possible to select the air or gas pressure within
2100131
the sealed working space at a lower value than the internal steam
pressure such that the resulting differential pressure is less than
the critical damage causing pressure. Any suitable sealing method
can be used to provide the required sealing of the working space.
Figure 1 shows schematically a cross section of a two chamber
heating and pressing apparatus according to the present invention
comprising a conditioning part 10 and a forming part 11. The heat
applicators indicated as 14 and 15 in the conditioning part 10 can
be heating electrodes for high frequency heating, sometimes
designated as a radio frequency heater. Other high frequency
heating method which can be used to practise the present invention
is the industrial microwave heating method. In addition to high
frequency heating the apparatus can incorporate steam injection
heating method or other conventional heating methods.
The conditioning part 10 comprises an upper frame means 12
which can be nonmovably supported and a movable lower frame means
13. The upper heat applicator 14 and the lower heat applicator 15
are shown as high frequency heating electrodes incorporated in the
frame means 12 and 13. The electrodes are preferably electrically
isolated from~ the main body of the conditioning part 10 by
isolating means 16 and 17. However, in practising the present
invention the isolating means 14 and 15 can be omitted. Electrical
connections are provided b~ connectors 18 and 19 which are, in
turn, connected to a source of high frequency energy. The wood work
piece 21 is shown located within the conditioning part 10 between
the electrodes 14 and 15. The frame means 12 incorporates vacuum
ports 20 through which vacuum can be drawn within the common
interface between the heat applicator 1~, isolating means 16 and
the wood work piece 21. A suitable ~eans for feeding the wood work
piece 21 into the conditioning part is not shown but any known
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i 21~0131
method can be incorporated.
The forming part 11 is shown in juxtaposition with theconditioning part 10. It comprises an upper pressing frame means 22
which can be nonmovably supported and a movable lower pressing
frame means 23. Pressing die 24 is shown nonmovably supported by
the upper pressing frame means 22. The lower pressing die 25 is
movably supported by the lower pressing frame means 23. The support
of the pressing die 25 is such that it can slide on the surface of
the pressing frame means 23 between the conditioning part 10 and
the forming part 11, when the lower frames of the conditioning part
and the forming part are suitably aligned. Channels 26 can be used
to heat or cool the upper and lower pressing frame means 22 and 23.
.
3 operation of the preferred embodiment of the apparatus
j according to the present invention is depicted sequentially in
~ Figures 1, 2 and 3. Figure 1 shows schematically the conditioning
! phase of the manufacturing cycle. The wood work piece is restrained
¦ by a low external pressure between the upper heat applicator 14 and
20 the lower heat applicator 15. rhe isolating means 16 and 17 may
form a part of the pressing surface if the wood work piece is
larger than the pressing area of the heat applicators 14 and 15.
Heat is supplied to the wood work piece 21 until the desired
plasticizing conditions within the work piece are achieved.
Additional pressure may be applied to the conditioned work piece 21
to further consolidate it, and at about the same time vacuum is
applied within the interface between the heat applicator 14 and the
work piece 21 by means of ports 20. If the work piece is a wood
composite assembly, the glueline should be sufficiently formed at
~0 this stage to retain the work piece as one piece without an
external restraint. Next, the lower frame means is lowered to open
the gap between the upper and lower frame means. The vacuum above
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2~ 001~1
the work piece should be sufficient to hold it in contact with the
heat applicator 14 as the applicator 1~ is being lowered along with
the frame means 13. Mechanical means can also be used to retain the
work piece 21 in position so it is not lowered when it is no longer
supported by the lower heat applicator 15.
Figure 2 depicts the phase in the operating sequence when the
work piece 21 is held by vacuum in the elevated position and the
resulting gap between the work piece 21 and the lower heat
applicator 15 is sufficient to allow insertion of the lower
pressing die 25. When the lower pressing die is inserted between
the work piece and the lower heat applicator 15, the vacuum can be
released and the work piece allowed to rest on the lower pressing
die. The working openings of both conditioning and forming parts
can be sealed from the exterior of the apparatus by any suitable
sealing means to form a common cavity which can be pressurized. The
pressurization can be done by air and the air pressure would
counteract the internal steam pressure within the wood work piece.
In the following step the movable lower pressing die 25 is moved
into the pressing position within the forming part, as indicated in
Figure 1. The wood work piece is transported with the lower
pressing die while it remains located on the s`aid pressing die.
Figure 3 shows a cross section of the apparatus according to
the present invention with the wood work piece 21 located in the
pressing location within the forming part 11. The conditioned and
plasticized work piece is compressed between the pair of upper and
lower pressing dies 24 and 25 and is formed into the desired shape.
The pressed wood work piece can be further heated by heat
transferred from the heated pressing frame means 22 and 23. The
additional heat can be applied to further stabilize the wood
product. An alternative treatment of the pressed wood work piece 21
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210013~ ~
is cooling by dissipating the heat from the work piece in~o cold
pressing frame means 22 and 23. Heating or cooling of the pressing
frame means can be achieved by supplying heating or cooling medium
through channels 26.
Figure 4 illustrates another embodiment of the present
invention which comprises a press 28 and a pair of upper and lower
pressing dies 33 and 34 both movable into and out of the press 28.
The press 28 comprises upper frame means 29 and lower frame means
30, the upper frame means supporting an upper heat applicator 31
and the lower frame means supporting a lower heat applicator 32.
The work piece 21 is conditioned as described in relation to Figure
1. The conditioned work piece is retained in elevated position in
the working opening of the apparatus by means of vacuum or other
means to allow the lower pressing die 33 to be inserted into the
gap between the work piece 21 and the lower heat applicator 32.
With the lower pressing die in location within the press, the work
piece is lowered onto the lower pressing die 34 by releasing the
vacuum or any other means which may be used to hold the work piece
in the elevated position. The lower frame means 30 is subsequently
lowered to provide a gap between the upper heat applicator 31 and
the work piece 21 into which the upper moving pressing die 33 can
be inserted. When the upper and lower moving pressing dies 33 and
34 are in place the lower frame means applies forming pressure to
the work piece between the upper and lower pressing dies. The press
28 can be suitably sealed and pressurized to balance the internal
steam pressure within the work piece.
As will be apparent to those skilled in the art in the light
of the foregoing disclosure, many alterations and modifications are
possible in the practice of this invention without departing from
the spirit or scope thereof. Accordingly, the scope of the
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2100131
invention is to be construed in accordance with the substance
defined by the following claims.
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