SYSTEME A LEVRES DE FILIERES CHAUFFEES

HEATED DIE LIPS SYSTEM

Abrégé français

Cette invention portant sur un système à lèvres de filières chauffées est conçue spécialement pour être utilisée dans des filières d'extrusion de film. L'utilisation de ce type de système permet de faire fonctionner des extrudeuses de film à des vitesses de production plus élevées avec des écartements de filières plus petits, tout en éliminant les propriétés de rupture de la masse fondue. Ce système à lèvres de filières chauffées est installé dans la zone de filière finale de la filière d'extrusion, sur chaque côté de l'écartement de filière. Il applique une chaleur directe à la masse fondue de polymère lorsqu'il quitte les lèvres de filières, et réduit la viscosité de la masse fondue. La valeur de « contrainte de cisaillement critique » est ainsi réduite. Cela se traduit par le report de la propriété de rupture de masse fondue du film et les vitesses de production augmentent sans rompre la masse fondue. De plus, ce système a été conçu de façon à l'isoler du bloc principal de la filière, limitant ainsi la dissipation de la chaleur et offrant ainsi un système fermé pour le chauffage homogène des lèvres de filières.

Abrégé anglais

This invention of Heated Die Lips System is specifically designed for use in Film Extrusion Dies. Use of such a system allows Film Extruders to be run at higher production rates with narrower die gaps, while eliminating the melt fracture properties. This Heated Die Lips System, is installed in the final die land area of the extrusion die, on each side of the die gap. It applies direct heat to the polymer melt as it exits the die lips, and reduces the melt viscosity. Thus the 'Critical shear stress' value of the melt is reduced. This results in postponement of the melt fracture property of the film and higher production rates are achieved without melt fracture. Furthermore, this system has been designed so as to insulate it from the main block of the die, minimizing heat dissipation and providing an enclosed system for uniform heating of the die lips.

Revendications

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Claim 1. Apparatus for use in producing plastic film by extrusion, wherein:
the apparatus includes an inner die and an outer die;
the outer die has an inside cylindrical surface, and the inner die has a
complementary opposing outside cylindrical surface;
the inside and outside surfaces together define an annular nozzle;
in respect of at least one of the dies:
the die comprises a massive bulk of material of generally cylindrical-annular
form;
the die is provided with a deep, annular heater-groove, which is located in
close
proximity to the cylindrical surface thereof;
the heater-groove includes a lip-side-wall and a bulk-side-wall, the lip-side-
wall being
closer to the said cylindrical surface;
a lip of the die comprises the portion of the die that lies between the said
cylindrical
surface of the die and the said lip-side-wall of the heater-groove;
the lip is annular, and is of narrow radial width;
the apparatus includes a lip-heater-assembly, which includes a heater element,
which is located in the heater-groove of the die;
the apparatus includes a spring-means;
the spring means includes means for forcefully pressing the heater-element in
a
radial direction, into heat-transmitting contact with the lip-side-surface of
the
heater-groove;
the spring-means includes means for pressing the heater-element against the
lip-
side-wall of the heater-groove at many individual and discrete points;
the spring-means includes means for pressing the heater-element uniformly and
evenly, with respect to the circumference of the heater-groove, in that the
said
many points are pitched in close-spaced adjacency, regularly and evenly
around the circumference;
and the spring-means includes means for pressing the heater element against
the lip-
side-wall with a force that is substantially the same at each of the said many
points.
Claim 2. As in claim 1, wherein the many points are approximately fifty or
more in

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number.
Claim 3. As in claim 1, wherein the many points are pitched approximately 2 cm
apart around the circumference.
Claim 4. As in claim 1, wherein the spring-means comprises many individual
spring-
clips, which are arranged in a contiguous series within the heater-groove,
between the heater-element and the bulk-side-wall of the heater-groove.
Claim 5. As in claim 4, wherein wherein the individual spring-clips comprise
respective pieces of metal, each folded to a Vee-shape, defined by arms and
an apex.
Claim 6. As in claim 5, wherein, in respect of each spring-clip, the apex of
the Vee
lies in contact with the heater-element, and the arms of the Vee lie in
contact
with the bulk-side-wall of the heater-groove.
Claim 7. As in claim 1, wherein the apparatus includes a heat-insulation
means, for
insulating the bulk-side-wall of the heater-groove from the heater element.
Claim 8. As in claim 7, wherein:
the spring-means comprises many individual spring-clips, which are arranged in
a
contiguous series within the heater-groove, between the heater-element and
the bulk-side-wall of the heater-groove;
and air spaces between the spring-clips comprise the said heat-insulation
means.
Claim 9. As in claim 1, wherein the heater-element comprises a length of
heater
cable, arranged in a coil of two or more turns, and the turns lie against the
lip-
side-wall of the heater-groove in a vertical stack.
Claim 10. As in claim 4, wherein:

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the heater-element comprises a length of heater cable, arranged in a coil of
two or
more turns, and the turns lie against the lip-side-wall of the heater-groove
in a
vertical stack;
and the spring-clip and the heater cable are loose in the axial sense within
the
heater-groove.
Claim 11. As in claim 10, wherein the cross-sectional configuration of the
spring-clip
at any one depth within the heater-groove is the same as the cross-sectional
configuration of the spring-clip at any other depth within the heater-groove.
Claim 12. As in claim 1, wherein the lip is formed integrally with the
material of the
bulk of the die, and is joined thereto by a narrow root.

Description

CA 02204548 2000-09-07
1
Title of the invention: HEATED DIE LIPS SYSTEM.
DESCRIPTION OF THE INVENTION
Manufacture of plastic film requires use of Film Extruders. Plastic resins are
introduced through the hopper to the extruder. Resin melts as it travels
through the
barrel (enclosing plasticating screw), to the gate area, through the die and
finally
exits from the die lips area. This molten polymer is than air blown to form a
bubble,
which is cooled and collapsed to provide the final product, a plastic film.
During the process described above, by the time resin enters the extruder and
exits
the die lips, the molten polymer has gone through many changes. It has
accumulated
thermal and shear history which affects quality of the film produced.
One critical area, influencing film quality and limiting production rates has
been Melt
Fracture or surface imperfection of the film, considered an undesirable
characteristic
in film industry. Much work has been undertaken by plastic industry over the
years to
understand and eliminate melt fracture phenomenon. Some of the techniques used
to eliminate or at least delay the onset of melt fracture have been, use of
processing
aids with resin (for lubricating affect), use of various material of
construction for die
lip area (such as metallic alloys and ceramic coatings), or elevating die
temperatures
to reduce viscosity of the polymer melt.
This invention of Heated Die Lips System is a technological advancement in
solving
the melt fracture problem. The continues loop heating elements of the device
provide
smooth and even heating of the circumferential die lips in melt exit area. It
is
accepted fact that the die lip heaters should heat just the surface and not
the bulk of
the extruding polymer melt. Hence, the effect of lip heaters should be focused
on the
lip area of the die nozzle, for maximum benefit of such an invention. With
varying
dimensions of extrusion dies, keeping heat restricted to desired die lips and
melt exit
area can be a challenge for the designer, larger the dimensions, worse the
problem.

CA 02204548 2000-09-07
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This invention has successfully addressed the problem for large dimension
dies.
Heated die lips are somewhat isolated or insulated from the main bulk of the
dies.
This helps die lip heaters to reduce the temperature difference between the
melt that
is in contact with the lips and the temperature of the bulk of the polymer
flowing
through the die exit nozzle. Such an insulation also helps reduce heat
dissipation to
the surrounding die block assembly of the die. Thus keeping most of the heat
focused in die lips area and maximizing the potential benefit available from
die lip
heating.
In this invention, the inner and outer lips of the die {or die nozzle area)
are provided
with the rings (or loops) of heat conducting materials. The ring is let into a
groove,
cut in the material of the die, close to the die lip. The ring is accompanied
by a
heating means, which may comprise a coil of heating wire. These coils are
placed in
such a way that they are towards the die lips for maximum efficiency. Clips
can be
used to assure a secure fastening of rings and coils inside the grooves. Thus
the
rings are lying in grooves on each side of the die gap in the final die land
area, close
to the die lips. Rings and grooves are continues and uninterrupted around the
circumference in such a way that their cross-section does not vary around the
circumference. The groove is made deep enough to completely accommodate the
ring, so that the ring is not exposed to the surface. Since back of the
heating unit is
thermally insulated, any heat loss to the main block of the die is minimal and
most of
the heat is directed to die lips. The temperature is monitored with
thermocouples
embedded within the heating elements. An SCR (silicon control rectifier)
system
maintains temperature settings. One advantage in view, for example is that the
same
die with lips designed for LLDPE extrusion can also be used for LDPE without
changing the die lips.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-section of the extrusion die (showing only left half of
the
schematic).
Figure 2 is face view of the die lip area as seen from the top.

CA 02204548 2000-09-07
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DESCRIPTION OF THE DRAWINGS
The die set apparatus, as shown in Figure 1 is used to describe location of
the
Heated die lips system. Molten polymer is fed under pressure through a conduit
(1 ),
through a spiral channel (2), and into a collecting channel (3). The melt then
passes
through the annular nozzle (4) to exit the die lips.
Air is blown into the center of the annulus through the passage (5), and the
air
inflates the extruded tube to make a bubble of polymer melt. This plastic
bubble is
cooled and collapsed to form the plastic film.
The nozzle (4) has an inner die lip (6) which is formed on an inner die (14),
and an
outer die lip (7) which is formed on an outer die (15).
In each of the inner and outer dies (14, 15), heater grooves are formed which
accommodate respective Aluminum heater rings (8, 9) and their coils. Adjacent
to
heater rings (8, 9), thermal insulation (10, 11 ) is installed to isolate
heater rings from
main block of the die, thus minimizing heat dissipation to block and providing
heat
consistency to die lips.
Design and construction of the grooves has to be such that heating Aluminum
rings
be a tight fit. This is an important consideration because the rate of
expansion of the
Aluminum of the rings is different from that of the steel of the dies. Thus
Aluminum
ring must be tightly constrained for an intimate heat-transferring contact.
Figure 2 shows a face view of the die lip area from the top of the die.
Annular nozzle
opening (4) or die gap is shown between inner and outer die lips (6, 7)
respectively.
Annular grooves cut into inner and outer dies are visible, though covered by
fastening clips (12, 13) respectively on inner and outer dies. These clips are
used to
fasten heating rings and coils lying below the clips, within respective
grooves.

Dessin représentatif