Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~32257
DESCRIPTION
~xtrusion d_e for the production of hollow-chamber panels
. The invention relates to an extrusion die for the production
of hollow-chamber panels from thermoplastic, for example, from
poly(methyl methacrylate), polycarbonate or pol~vinyl chloride.
Known extrusion dies for this purpose contain according to
DE-A 15 04 800 two slits 1, formed by lips 4, 5, running in a
straight line and parallel over the width of the extrusion die,
for the extrusion of two parallel plane outside walls of the
hollow-chamber panel and a core piece 2, which is placed between
slits 1 and in which a multiplicity of cross-slits 3 are cut,
which come out into slits 1 and serve for extrusion of webs which
connect the outside walls of the extruded hollow chamber to one
another. Preferahly cross-slits 3 are placed at right angles to
slits 1.
The hollow-chamber panels produced with such extrusion dies,
which are also identified as web panels or web double panels,
have achieved a significant place in construction. Often
rectangularly limited sections of such hollow-chamber panels are
laid horizontally or slightly inclined so that their lateral
edges, running in the extrusion direction, rest on a suitable
substructure and the intervening surface of the section is freely
supported. This assumes a sufficient crosswise rigidity, which,
besides its own weight, optionally must take into account
additional live loads such as wind pressure or snow load. To
guarantee its crosswise rigidity at a width of 1 to 1.2 m, a
2 0 3 2 ~ ~ 7
wei~ht per unit area of at least 4.7 kg/m2 is prescribed in the
building permit for hollow-chamber panels made of PM~A. This
regulation starts from the assumption that a like crosswise
rigidity at a lower weight per unit area could not be achieved.
It sets a bottom limit to the requirement of plastics for the
production of hollow-chamber panels.
Object and achievement
The object of the invention is to provide an improved
extrusion die with which hollow-chamber panels can be produced of
like crosswise rigidity at lower weight per unit area or higher
crosswise rigidity at the same weight per unit area.
With the known generic type extrusion dies the outside walls
of the hollow-chamber panels are produced by rigid lips. The
feed of the thermoplastic molding compound within the extrusion
die to the lips is generally controlled by a choke bar, which for
design reasons has considerable rigidity, but is adjustable to a
limited extent to guarantee a uniform delivery of molding
compound over the entire width of the extrusion die. Differences
in the flow resistance of the molding compound between the center
and ends of the extrusion die can be largely equalized by the
choke bar so that the thicknesses of the outside walls are
basically the same over the entire width of the extruded hollow-
chamber panels.
It has now been found that the crosswise rigidity of the
extruded hollow-chamber panel can be improved if for their
production an extrusion die of the generic type is used, in which
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one of the two lips 4, 5, which form slit 1, is designed flexibly
and the flexible lips 4, 5 can be adjusted by a multiplicity of
control elements 7 which are independent of one another. During
extrusion flexible lips 4, 5 can be adjusted with great accuracy
to the required thickness of outside walls.
The crosswise rigidity of the hollow-chamber panel depends
decisively on that of the outslde walls. It is again affected by
the thickness variations of the outside walls. I~ was determined
that the crosswise rigidity of a plastic panel of uneven
thickness is not greater than tha~ of a completely even panel
which is as thick at the thinnest spots of the uneven panel.
The material, which is contained in the thickness variations of
the uneven panel, is lost for the crosswise rigidity.
By the adjustable flexible lips according to the invention
present in the extrusion die the thickness of the outside walls
can be adjusted to the required thickness substantially more
accurately then with usual extrusion dies, which have rigid lips.
The crosswise rigidity of the hollow-chamber panel thus extruded
corresponds substantially to the required thickness and is not
reduced by thickness minima, which unavoidably occur with the
usual extrusion dies. The thickness variations of usually
produced extrudates is due to melt throughput variations over the
width of the extrusion die, which can have a series of causes.
These include defects in the design or embodiment of the
manifold, temperature differences in the molding compound melt or
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in the die body or deviations from the originall~ provided
operating point of the die.
~ 'ith the specified requirement on the crosswise rigidity the
required thickness of the outside walls can be di~ensioned as low
as the thickness minimum of a usual hollow-cham~er panel with
uneven outside walls of correspondingly higher required
thickness. With the use of usual extrusion dies with rigid lips
thickness variations up to ~ 20%, at times up to ~0%, on the
outside walls occur; they cannot be avoided either by an
elaborate temperature control or by a fine control of the choke
bar adjustment. By the extrusion die according to the lnvention
the thickness variations can be reduced to + 5~. As a result,
the required *hickness can be reduced by up to 15~. In view of
the weight of the webs, a reduction of weight per unit area of
the hollow-chamber panel up to 10% with constant crosswise
rigidity results. A particular advantage of the extrusion die
according to the invention resides in the fact that a choke bar
can be completely dispensed with, since the molding compound flow
can be controlled soley by the flexible lips.
Embodiment of the invention
A suitable configuration of the extrusion die according to
the invention is seen in figure 1, which shcws a cross section
through the die.
Extrusion die 10 contains an intake channel 11 for the
thermGplastic molding compound~ which can be connected to an
extruder. Intake channel 11 inside the die is divided into two
2 ~ ~ 2 2 ~ 7
feed channels 12, from which the molding compound reaches the
manifolds 13 placed crosswise over the entire die width. ~rom
there the molten molding compound goes into slits 1 on both sides
of core piece 2. One part of the molding compound flows out of
slits 1 into cross~ise slits 3, which are cut into core piece 2,
and, on leaving the crosswise slits, forms the webs of the
hollow-chamber panel. On lips ~/5 the molding compound leaves in
the form of the outside walls of the hollow-chamber panel. The
thermoplastic hollow extrudate is cooled below the softening
temperature in a vacuum forming channel (not shown), which is
placed tight behind extrusion die 10 and thus obtains its final
configuration.
According to the invention one each of the two lips 4, 5,
which form slit 1, is designed flexible. For this purpose, lip 4
is placed on the end of a tongue 14 extending over the die width,
a tongue whose length and thickness are dimensioned so that lip 4
can be pushed into slit 1 by control elements 7, as a result of
which its inside width is reduced. Such an arrangement is
identified in technical language as "flexlip" and is used
otherwise only in the extrusion of thin sheets.
The elastic bending clearance of the flexlip under the
action of control elements 7 is on the order of O.O1 to 0.5 mm.
In the extrusion of thin sheets the molding compound flow of the
extruded sheet web can be reduced locally, for example, by up to
50% by a flexlip. But the outside walls of an extruded hollow-
chamber panel are substantially -thicker, e.g., 0.5 to 3 mm.
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Consequently, the effect of the flexlip on the thickness of the
extruded web is relatively much smaller; it is on the order of
30~ at most.
Suitably the ex*rusion die according to the invention is
designed symme*rical to core piece 2 so that both slits 1 can be
adjusted in the same way. For the adjustment of the slit width
it is sufficient if one each of lips 4/5, which form slit 1, is
designed flexible. Preferably it is outside lips 4.
The exposed external sides of extrusion die 10 as well as
the exposed sides of ton~ues 14 can be provided with thermal
1nsulations-or heating jackets to guarantee a uniform temperature
control.
Screw bolts can be used as control elements 7, which can be
adjusted by head 15. The closer control elements 7 are placed
next to one another, the finer the slot thickness can be
adjusted. The center-to-center distances of the control elements
can be, e.g., 8 to 40 mm. Preferably short screw bolts 16 are
used for rough adjustment, while for fine adjustment electrical
expansion pins or piezotranslators can be used, which can be
placed coaxially to screw bolts 16. They can be connected by a
control unit to thickness measuring sensors, which measure the
thickness of the outside walls in the relevant area of the
extruded hollow-chamber panel and indicate a corresponding
measured quantity to the control unit. For the thickness
measurement, for example, an absorption measurement in a spectral
range, in which the extruded molding compound absorbs a radiation
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as a function of the layer thickness, is suitable. In this way,
a setpoint for the provided thickness of the outside walls of the
hollow-chamber panel, which is input in the control element, is
automatically compared with the measured actual value and is
converted in a suitable control quantity for the adjustment of
the electrical expansion pins or piezotranslators.
