Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02342840 2001-03-28
Method and Apparatus for Manufacturing Endless Veneer Laminates
Field of the Invention
The present invention relates to the field of veneer laminates, and more
specifically, to a method and apparatus for manufacturing endless veneer
laminates from
a strand of veneer panels.
Background of the Invention
A method for manufacturing veneer laminates is known from DE I96 27 024
wherein, after leaving the veneer panel placement station, the strand of
veneer panels is
prepressed and preheated in a continuously operating prepress with integrated
RF or
microwave preheating. After leaving the prepress, the strand of veneer panels
is
transferred to a conveyer belt and transported through the remaining section
to the
intake point of the main press. In this area, an undesired rising up of the
basically wavy
veneer panels occurs, thus permitting the glue seams to dry out as well as a
loss of
valuable energy. When veneer laminates (LVL) are manufactured in a continuous
process, during the preheating process temperatures of 65 degrees - 75 degrees
are
currently used in order to increase the production speed, which reduces the
pressing
time in the continuously operating press by 50 percent compared to
conventional
manufacturing processes. Despite this significantly short pressing time that
has led to
an increase in productivity, the continuously operating facilities for the
manufacture of
veneer laminates are approximately 30 - 50 percent more expensive than
conventional
facilities with the same productivity. Moreover, the maximum productivity is
limited by
the technical feasibility of the press length of the continuously operating
press, which is
currently approximately 60 m.
In current facilities, the problem is that the preheating temperature cannot
be
increased any further, since this would result in a procuring or drying of the
glue seams
before the pressure at the intake point of the press can be built up and
before the veneer
panels to be manufactured can be pressed to form a veneer laminate. The
remaining
energy required until the curing temperature of the glue is reached must of
necessity be
introduced into the product by means of the conventional heating of heating
plates and
steel belts. Due to the inferior thermal conductivity of wood, an additional
temperature
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increase in the middle of the veneer panel laminate only occurs after a
certain delay
time, which is necessary for heating the peripheral areas. The subsequent
temperature
increase only occurs very slowly. Since the veneer laminates are made very
thick
(30 mm - 150 mm), the pressing process still takes between 10 minutes and 45
minutes.
Summary of the Invention
The objective of the invention is a method and apparatus by which a higher
quality of veneer laminate can be manufactured in continuous production and by
which
a greater throughput can be achieved through quicker processing.
In accordance with one aspect of the present invention there is provided a
method of manufacturing endless veneer laminates from a strand of veneer
panels made
of veneer panels in multiple layers superimposed on each other and behind each
other
and with intermediate layers of glue by means of gluing and pressing in a
heated,
continuously operating press, said method comprising the following steps: (a)
pressing
the strand of veneer panels together under light pressure before or during
preheating in
such a way that the veneer panels lie flat one on top of the other; (b)
passing the strand
of veneer panels through an RF (radio frequency) or MW (microwave frequency)
heating section until the respective curing temperature of the glue is reached
or slightly
exceeded in the strand; (c) prepressing the heated strand of veneer panels at
a pressure
of 5 to 30 bar surface pressure or 3 - 20 N/mm linear pressure and fed to
within a few
millimetres of and then into the clamping area of the continuously operating
press; and
(d) pressing the strand of veneer panels in the continuously operating press
while
maintaining or increasing the temperature reached in the RF or MW heating
section
until it has reached a desired panel thickness and glue seams have cured.
In accordance with another aspect of the present invention there is provided a
method of manufacturing endless OSB, LSL (laminated strand lumber) or wafer
panels
from a strand of wood particles of long chips or wood waffles, spread in
multiple layers
superimposed on each other and behind each other, by gluing and pressing in a
heated,
continuously operating press, said method comprising the steps of: (a) prior
to
preheating, precompressing the strand under light pressure in such a way that
the wood
particles lie firmly against or upon each other; (b) while maintaining the
precompression
thickness passing the strand through an RF or MW heating section until the
respective
curing temperature of the glue has been reached or slightly exceeded in the
strand; (c)
CA 02342840 2001-03-28
prepressing the heated strand at a pressure of 5 to 30 bar surface pressure or
3 - 20 N/mm linear pressure and fed to within a few millimetres of and then
into the
clamping area of the continuously operating press; and (d) pressing the
partially cured
strand in the continuously operating press while maintaining or increasing the
temperature reached in the RF or MW heating section until it has reached a
desired
panel thickness and is cured.
In accordance with yet another aspect of the present invention there is
provided
an apparatus for carrying out the method according to any one of claims 1 to 5
for
manufacturing endless veneer laminates from an endless string of veneer panels
or for
manufacturing OSB, LSL or wafer board from an endless string of spread wood
particles, comprising a feeding device for the strand of veneer panels or the
strand, an
RF or MW heating device, a prepress and a continuously operating press, the RF
or MW
heating device having an entry and an exit shielding device and that a
prepress is located
downstream from it whose pressing range, including a transfer nose and a
transfer plate,
extends almost to the clamping area of the continuously operating press, and
that the
material to be pressed can be subjected to a pressing pressure of between 5
and 30 bar
surface pressure or between 3 and 20 N/mrn linear pressure when passing
through the
prepress.
More specifically, according to an embodiment of the present invention, a
method for the continuous production of LVL panels or veneer laminates lies in
the
implementation of the following steps:
First, the strand of veneer panels is pressed together before or during
preheating
in such a way that the veneer panels lie flat one on top of the other. In a
second step, the
strand of veneer panels passes through an RF or MW heating section until the
respective
curing temperature of the applied glue is reached or slightly exceeded in the
strand.
Following this, the heated strand of veneer panels is prepressed at a pressure
of 5 to 30
bar surface pressure or 3 - 20 N/mm linear pressure and fed to within a few
millimetres
of and then into the clamping area of the continuously operating press.
Finally, the
strand of veneer panels is pressed in the continuously operating press while
maintaining
or increasing the temperature reached in the RF or MW heating section until it
has
reached the final panel thickness and the glue seams have cured completely.
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More specifically, according to an embodiment of the present invention, an
apparatus for carrying out the above method (for example) includes a RF or MW
heating device having an entry and an exit shielding device and that a
prepress is located
downstream from it whose pressing range, including the transfer nose and (if
applicable)
the transfer plate, extends almost to the clamping area of the continuously
operating
press, and that the material to be pressed can be subjected to a pressing
pressure of
between 5 and 30 bar surface pressure or between 3 and 20 N/mm linear pressure
when
passing through the prepress.
Compared to state-of-the-art technology, the advantage is that the process and
facility according to the invention can significantly increase the
productivity of the
continuously operating production facility as well as provide a more gentle
compression
and, therefore, improved product quality. To this end, the strand of veneer
panels is
initially covered between two conveyer belts and the waviness of the veneer
panels is
pressed flat under light pressure, so that the individual veneer panels lie
snug against
each other. In this state, the strand of veneer panels is preheated to the
respective curing
temperature of the glue or slightly higher by means of RF (high frequency) or
MW
(microwaves). Only after preheating is the strand of veneer panels compressed
in a
continuously operating prepress under a pressure of 5 to 30 bar surface
pressure or
3 to 20 N/mm linear pressure. The rollers of the prepress are arranged so
close together
that no significant spring-back of the strand of veneer panels occurs between
the rollers.
If compression of the veneer panels occurs after preheating, the wood fibres
are already
in a plastic state due to the high temperature and can be compressed with
minimal force
and without breaking. It was determined that this procedure results in the end
product
having measurably better flectional strength with the same veneer laminate
quality.
However, it is crucial that the preheating temperature can be increased to the
required
curing temperature of the glue, thus ensuring that the reaction of the glue
begins
immediately after leaving the preheating section. The strand of veneer panels
is
compressed and pressed directly at the start of the curing reaction. This
prevents
premature curing or drying of the glue seams. The facility is designed in such
a way
that the transfer components of the prepress project far into the intake mouth
of the main
press and transfer the strand of veneer panels (which at this point is already
partially
cured) almost without transition to the steel belts of the main press.
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Since the required curing temperature has already been reached in the strand
of
veneer panels, no significant temperature increase is necessary in the main
press, but
rather a moderation or minor increase in the temperature of the steel belts in
order to
prevent the veneer panels from cooling and to cure the glue.
The strand in the continuously operating press is pressed and cured at the
usual
temperature and pressure used in the manufacture of veneer laminates or OSB,
LSL and
wafer board.
When applying the model according to an example of the invention for
manufacturing OSB, LSL (laminated strand lumber) or wafer board in a first
step, the
strand is compressed under light pressure prior to preheating in such a way
that the
wood particles lie firmly against or upon each other. In a second step, while
maintaining the precompression thickness, the strand passes through an RF or
MW
heating section until the respective curing temperature of the applied glue
has been
reached or slightly exceeded in the strand. Following this, the heated strand
is
prepressed at a pressure of 5 to 30 bar surface pressure or 3 - 20 N/mm linear
pressure
and fed to within a few millimetres of and then into the clamping area of the
continuously operating press, and the already partially cured strand is
pressed in the
continuously operating press while maintaining or increasing the temperature
reached in
the RF or MW heating section until it has reached the final panel thickness
and is
completely cured.
When manufacturing veneer laminates from veneer panels of high quality, i.e.
with the characteristic of greater waviness, and when manufacturing OS or LSL
or
wafer board, it is advantageous to precompress the strand of veneer panels for
veneer
laminate with a pressure of 0.0004 to 2.0 bar and the strand for OSB/LSL
panels with a
pressure of S to 30 bar surface pressure or 3 to 20 N/mm linear pressure and
to feed
them into the RF or MW heating device with this precompression thickness.
Brief Description of the Drawings
Further features and advantages of the present invention will be described in
the
detailed description, taken in combination with the appended drawings, in
which:
Figure 1 shows a side view of the facility for carrying out the process
according
to the invention; and
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Figure 2 shows the facility according to Figure 1 with a prepress that
projects
back into the RF or MW heating device.
Detailed Description of Embodiments of the Present Invention
In Figures 1 and 2, the apparatus for carrying out the process is shown as a
complete side view. Its main components are a feeding device 2 for an endless
strand 1
of veneer panels or long chips, waffles, etc., an RF or MW heating device 8
with
shielding devices ''HA", a prepress 12 and a continuously operating press 3.
In order to
simplify the drawing and the following description, the endless strand of
veneer panels 1
according to the application of the invention according to the manufacture of
veneer
laminates and the endless strand 1 according to the application of the
invention
according to the manufacture of OSB, LSL or wafer board are each identified by
the
reference symbol 1. Further, the term ''HA" represents an RF or MW heating
device
with a shielding device; "HV" represents HA plus a prepress and HS represents
an RF
or M W heating section.
To further clarify the invention, the example of an embodiment shows a
continuously operating press 3, which is designed as a dual-belt press with
circulating
steel belts 15 and 16 and heated pressure/heating plates 17. The heated and
prepressed
strand 1 is fed into a press slot of the continuously operating press 3 via
idler drums 18
and 19, the press slot being adjustable in relation to a stationary press
table 20 by means
of a press rammer 21 that can be raised and lowered. A prepress 12 is located
directly in
front of ingoing steel belts 15 and 16 of the continuously operating press 3.
The
illustration of RF or MW heating device 8 shows the allocation of an upper
circulating
plastic belt 5 and a lower circulating plastic belt 4 to the strand 1 passing
through, the
upper plastic belt 5 enveloping the upper surface of the material to be
pressed and the
lower plastic belt 4 enveloping the lower surface of the material to be
pressed, whereby
the upper plastic belt 5 is guided over a non-conductive Teflon plate 7
serving as a
slideway lining. Both the plastic belts 4 and 5 are each guided over idler
rollers 24 and,
in the intake area of the continuously operating press 3, around transfer
noses 13 and, if
applicable, a transfer plate 25 in the press slot of the prepress 12 and, in
the heating
tunnel, circulate around the EZF or MW heating device 8 with shielding device
''I-IA" and
prepress ''HV" (high frequency or microwave heating device). The strand 1
arriving in
feeding direction 9 at the apparatus from a placement device or spreading
station is
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taken over by the feeding device 2, the lower plastic belt 4 taking over the
strand 1 and,
while the upper plastic belt 5 holds it down, feeding it into the RF or MW
heating
device 8 with shielding device "HA" and the prepress ''HV". If necessary, the
strand 1 .
is slightly precompressed by means of precompressing rollers 6 and then fed to
the RF
or MW heating device 8. In order to prevent any damage from RF or MW rays, an
entry
and an exit shielding device 10 and 11 is arranged upstream from the RF or MW
heating
section "HS".
The prepress 12 includes multiple rollers 14 that form the press slot and pick
up
the preheated strand 1 by means of an arrangement similar to mangles, presses
it and
then feeds it to the continuously operating press 3. 'the rollers 14 are
mounted in an
upper and a lower press spar 23, whereby the upper press spar 23 can be raised
and
lowered in the press uprights 22. Depending on the product requirements, the
press
spars 23 can project back into the exit shielding device 11 and into the
heating section
''HS", i.e. it may be necessary to extend the pressing range of the prepress
12 back to the
RF or MW heating section "HS", where a temperature between 70 degrees Celsius
and
85 degrees Celsius, the maximum temperature in the strand 1, has already been
reached.
