Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VERTICAL GLUING MACHINE FOR CHIPBOARD PRODUCTION PLANTS
This invention relates to those gluing machines used in chipboard
production plants to distribute a sufficient quantity of liquid
glue throughout a mass of wood particles or chips to enable this
mass to be transformed into chipboard panels by subsequent
operations.
Traditional gluing machines comprise a normally cylindrical mixing
chamber of substantially horizontal axis, into one end of which
wood particles of the desired size are fed by gravity in a
suitably metered quantity. A predetermined quantity of liquid
glue of thermosetting type is also fed into the chamber, usually
by spraying. The gluing machine also comprises means for mixing
them together more or less intensely and for transferring the
resultant mass, after a predetermined time, to an exit aperture
through which this latter, called hereinafter the mixture, which
is formed from wood particles wetted with glue, leaves.
2 0 For the subsequent use of the mixture the glue should be
distributed uniformly over the surface of each particle, by
applying to the surface of each particle a glue quantity which is
proportional to its area. It has however been found that in known
gluing machines this does not happen to the optimum degree. In
this respect, in the resultant mixture the glue "wets" the
particles of smaller size with greater uniformity. In any batch
of wood particles, even if of rigorously controlled particle size
distribution, there is a considerable variety of particle sizes,
hence the glue distribution over the surface of the particles does
30 not correspond to the said optimum.
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It is well known to the expert of the art that traditional gluing
machines offer a rather low level of filling of the mixing
chamber, which does not exceed 40~ of the chamber volume, with a
wood particle retention time in the mixing chamber varying from 20
to 30 seconds, depending on the capacity.
To obtain improved glue distribution it has already been proposed
to increase the mixing time of the wood particles with the glue,
and to increase mixing intensity. One manner of increasing mixing
intensity is to increase the speed of the mixing member. In some
known gluing machines the peripheral speed of the mixing member
can in fact reach 14-i7 m/s, but with the result that the wood
particles break, with consequent change in their size
distribution, which is no longer an optimum. High mixing speed
also leads to mixing temperature rise with consequent
polymerization of the glue, which partly loses its effectiveness.
Another solution used to improve glue distribution is to obstruct
the outflow of the mixture from the mixing chamber of the gluing
2 0 machine, which is equivalent to prolonging the wood particle
retention time in the chamber, with the result of prolonging
mixing with the glue. This solution tends to increase mixing
chamber filling, but only in the vicinity of its exit door. The
door setting is indicated in terms of percentage, with 0%
corresponding to the door completely open and 100 to the door
completely closed. However too high a door closure percentage
cannot be used because this excessively increases mixture
temperature, with consequent glue polymerization.
Another method already used to prolong the mixing time is to
instal after the first gluing machine a second gluing machine the
sole purpose of which is to remix the mixture leaving the first.
This solution leads to considerable improvement in the glue
distribution, but at a significant increase in plant and running
costs, in addition to increased overall plant size.
As already stated, all known gluing machines have a degree of
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mixing chamber filling which reaches a maximum of 40~, hence the
mixing chamber must have a volume much greater than that
effectively occupied by the mixture. This results in significant
dimensions which have to be taken into account in designing the
plant.
An object of this invention is to obviate the aforedescribed
drawbacks of known gluing machines.
A further object is to provide a gluing machine which is of simple
construction, reliable operation and easy maintenance and
cleaning.
The said objects are attained by the gluing machine of the
invention, comprising a cylindrical mixing chamber in which a feed
aperture, an exit aperture and a mixing member are provided,
characterised in that the mixing chamber is positioned vertically,
the feed aperture is provided in the lower part of the mixing
chamber and the exit aperture (36) is provided in the upper part
2 0 of the mixing chamber (12), means being provided to feed the wood
particles into the mixing chamber via the feed aperture.
The means for feeding the wood particles into the mixing chamber
preferably comprise a feeder screw.
Conveniently, to facilitate the upward movement through the mixing
chamber of the wood particles fed via the feed aperture, a
vertical screw is provided in the lower part of the mixing
chamber.
The invention will be more apparent from the following description
of one embodiment thereof. In this description reference is made
to the accompanying drawing, on which the single figure shows a
schematic vertical section through the gluing machine of the
invention.
As can be seen from the figure, the gluing machine 10 has a
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vertical cylindrical chamber 12 formed from two concentric
cylindrical steel shells 14 and 16, spaced apart to form an
interspace 18 the function of which is explained hereinafter. The
cylindrical chamber 12 has a feed aperture 20 which communicates
with one end of a motorized feeder screw 22 enclosed in a relative
cylindrical casing 24. As can be seen the screw 22 is
substantially horizontal, although this is not essential. In the
top of the 'Far end of the casing 24 there is provided an aperture
26 through which the wood particles are fed by gravity, these
being thrust into the cylindrical chamber 12 by the action of the
screw 22.
As can be seen in the figure, the lower part of the cylindrical
chamber 12 contains a vertical motorized frusto-conical screw 28
coaxial with the chamber 12, its rotation upwardly urging the wood
particles fed into the chamber 12 by the action of the feed screw
22.
Above the vertical frusto-conical screw 28 there is provided a
mixing member 30 consisting essentially of a coaxial motorized
shaft 32 from which mixing arms 34 radially extend, these being
indicated very schematically in the figure, but of conventional
type. The mixing arms 34 can have different shapes depending on
the height at which they are positioned. As in the specific case
shown in the figure, the mixing member 30 can be rigidly connected
to the vertical screw 28 (so that both are driven by one and the
same motor), this latter being keyed onto the shaft 32, which is
rotatably supported at its ends by supports 42, 44. It is however
apparent that the shaft of the mixing member and the vertical
3 0 screw can be driven independently, even at different speeds, by
relative motors. It should also be noted that the vertical screw
can be of a shape different from frusto-conical (for example
cylindrical).
As can be seen from the figure, an exit aperture 36 is provided
close to the top of the cylindrical chamber 12. In the
illustrated example, radial "blades" 46 are provided on the shaft
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32 to facilitate discharge of the mixture from the mixing chamber
12 through the exit aperture 36 by overflowing.
The side walls of the chamber 12 and the mixing member 30 are
cooled in conventional manner to prevent the mixture, as a result
of heating caused by remixing, from reaching a temperature which
would cause the glue to polymerize, one result of which would be
the rapid fouling of those parts of the gluing machine 10 which
come into contact with the mixture.
A gate 38 which closes a relative aperture is provided in the
bottom of the casing 24 of the feeder screw 22, in this specific
case the gate 38 being operated by a conventional pneumatic device
indicated overall by 40 in the figure.
The glue is injected into the interior of the mixing chamber 12 at
various points distributed over various heights (as indicated
schematically in the figure, in which three nozzles 48 located at
different heights are visible, fed with liquid glue) in order to
2 0 wet the wood particles with the glue in successive stages. As is
well known to the expert of the art, injecting the glue in
successive stages enables the so-called particle "surfacing"
effect to be obtained, by which glue absorption by the particles
is reduced, with consequent reduction in the amount of glue
required.
The operation of the aforedescribed gluing machine is described
briefly below, but should be clear to an expert of the art from
the aforegoing.
As stated, the mass of wood particles is fed by gravity through
the aperture 26 provided in the casing 24 of the screw 22. By
rotating this latter the wood particles are gradually urged
towards the mixing chamber 12, then through the feed aperture 20
and into the bottom of the chamber 12, where they encounter the
vertical screw 28 which when rotated urges them upwards. During
their upward movement the wood particles are struck in successive
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stages by jets of glue leaving the nozzles 48, and are
simultaneously remixed by the mixing member 30. The overall
effect due to the action of the mixing member 30 and the vertical
screw 28 is a combined horizontal and vertical remixing enabling
optimum results to be obtained, ie a very uniform glue coating on
the wood particle surface.
The~mixture of wood particles and glue finally leaves from the
exit aperture 36 by overflow aided by the action of the blades 46.
The gluing machine is cooled by circulating a cooling fluid, such
as water, through the interspace 18 between the cylindrical shells
14 and 16. Cavities (not shown) are also provided in the mixing
member 30 for circulation of a cooling fluid.
It is important to note that with such a gluing machine, 60-70%
filling of the mixing chamber 12 can be achieved, this being
considerably higher than known gluing machines (40% at most).
Such a degree of filling enables good mixing to be obtained with
iow rotational speed of the mixing member. Specifically, the
peripheral speed of the mixing member 30 can be maintained at
around 7-10 m/s, enabling the wood particle size distribution to
be maintained while at the same time achieving a decidedly higher
particle/glue mixture retention time within the mixing chamber 12,
of the order of 60-70 seconds, compared with known gluing
machines. Such a high retention time would be counter-productive
in a traditional gluing machine in which the mixing speed is much
higher (14-17 m/s). The particle temperature would in fact rise
with consequent pre-polymerization of the glue.
From the aforegoing it will be apparent that the gluing machine of
the invention has a rather simple structure, and is therefore easy
to construct. As no fouling occurs on those parts which come into
contact with the mixture of wood particles and glue, maintenance
and cleaning are much simpler than in known gluing machines. In
addition, as the installed power requirement is less, such a
gluing machine consumes significantly lesser energy than known
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gluing machines, besides being of much smaller overall plan
dimensions.
A further advantage of the gluing machine of the invention is that
as it extends vertically and has its mixture exit aperture at the
top (ie at a much greater height than in known gluing machines),
some of the mechanical conveyors (not shown) required to transfer
the mixture from the exit 36 of the gluing machine to a panel
forming machine (not shown) can be eliminated, this latter being
located in the relative plant at a much higher level than with
known gluing machines. Shortening the conveyor transfer path
reduces transfer time and hence the tendency of the glue contained
in the mixture to polymerize.
When the gluing machine 10 is to be emptied for maintenance
purposes or for an extended shut-down, this can be achieved very
easily by reversing the direction of rotation of the vertical
screw 28 and horizontal screw 22 and opening the gate 38, on which
the material contained in the mixing chamber 12 flows out from the
2 0 gate aperture.
