CA2204548C

CA2204548C - Heated die lips system

Info

Publication number: CA2204548C
Application number: CA002204548A
Authority: CA
Inventors: Alfredo Bentivoglio
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-05-06
Filing date: 1997-05-06
Publication date: 2001-06-05
Anticipated expiration: 2017-05-06
Also published as: CA2204548A1

Abstract

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.

Description

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.

2 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.

3 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.

Claims

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 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:

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.