CA1255063A - High stalk blown film extrusion apparatus and method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A high stalk blown film extrusion apparatus and method is described which produces a stabilized high stalk for increasing the production rate of blown, low to high molecular weight polymers, while increasing the film's physical and mechanical properties. The high stalk is stabilized by application of high velocity, low volume flow rate of air over the interior and exterior surfaces of the extruded film.

Description

The present invention relates in general -to a blown film extrusi,on apparatus and method, and more particularly, to such an apparatus and m~thod which produces a stabilized, high stalk between spaced-apart tandem air rings for increasing the production rate of blown, low to high molecular weight polymers, while improving the film's physical and mechanical properties.
The use of spaced-apart tandem air rings has been reported to improve the production rate as well as -the physical and mechanical properties of blown plastic film. These improvements are attributed in part to the creation of a high stalk of a semi-molten state which allows the viscoelas-tic forces of the extruded film to dissipate before expansion. This artificially created condition produces improved drawdown characteristics and allows the molecular re-orientation of the film to provide increased impact strength. However, although an improvement over conventional blown film, the tandem air ring arrangement results in limited film width and production rate capabilities due to instability of the high stalk, i.e., oscillation of the film about its longitudinal axis whereby undesirable wrinkles form in the finally collapsed film. In addition to the formation of wrinkles, severe instability of the high stalk at increased production rates can result in tearing of the blown film which would cause its ultimate collapse during the extrusion processes.
Accordingly, there is an unsolved need for a high stalk blown film extrusion apparatus and method which provides increased stability of the high stal~
between spaced-apart tandem air rings to permit an increase in the production rate, while improving the film's physical and mechanical properties over such film manufactured using either tandem or conventional air ring arrangements.
According to the present inven-tion, there is provided a method for forming a film of plastic material, said method comprising the steps of forming a .. ,, ~?

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-2-progressive~.y advancing unexpanded -tubular film having a substan-tially uni.form firs-t diame-ter about a cylinder arranged along a longi-tudinal axis over a predetermined distance, applying a firs-t gas stream over the exterior surface of said film, applying a second gas s-tream over the interi.or surface of said film within an annular region formed between said cylinder and the interior surface of said film, controlling the velocity and volume flow rate of said first and second gas streams over said unexpanded -tubular film over said predetermined distance for stabilizing said film by preventi.ng the oscillation of said film about said cylinder, and applying a third gas stream over the exterior surface of said film having said first diameter adjacent the exten-t of said predetermined distance for expanding said film from said firs-t diameter to a second diameter thereat.
According to the present invention, there is also provided an apparatus for forming a film of plastic material, said apparatus comprising supplying means for supplying plastic material in a flowable state, a die arranged in advance of said supplying means for forming a progressively advancing unexpanded tubular film having a substantially uniform first diameter along a longitudinal axis over a predetermined distance, a primary air ring arranged adjacent said die and exteriorly of said film, said primary air ring supplying a first gas stream over the exterior surface of said film, means for supplying a second gas stream over the interior surface of said film, a cylinder arranged interiorly of said film and extending from said die along said longitudinal axis over said predetermined distance, said cylinder and the interior sur~ace of said film defining an annular region there-between for receiving said second gas stream, controlling means for con-trolling said first and second gas streams over said unexpanded tubular film to stabilize said film over said predetermined distance by preventing the oscillation of said film about said cylinder, and a 5~ 3

-3-seco~dary air ring arranged spaced frorn said primary air ring and exteriorly of said film having said first diameter, the dis-tance between said prirnary and said secondary air rings defining said predetermined distance over which said film is stabilized, said secondary air ring supplying a third gas stream over the exterior surface of said film for expanding said film from said first diameter to a second diameter therea-t.
In order -that the present invention may be more fully understood, it will now be described with reference to the accompanying drawing, in which the sole drawing schematically illustrates the component parts of the extrusion apparatus of the present inven-tion for the production of a blown film having a high stalk of greatly improved stability.
Referring now to the sole drawing, there is shown a high stalk blown film extrusion apparatus generally designated by reference numeral 100. The extrusion apparatus 100 includes an extruder 102 having a supply hopper 104 containing a polymer 106 to be blown into a thin film by the extrusion apparatus. The polymer lC6 is heated to a molten state wi-thin the extruder 102 and forced under high pressure through an extrusion die 108. The extrusion die 108, being circular in shape, has an annular opening through which a tubular film 110 of polymer in a semi-molten state is pro-gressively advanced. The initial thickness of the tubular film 110 is determined by the size of the annular opening of the extrusion die 108. A primary air ring 112 is arranged adjacent the extrusion die 108 and surrounding the exterior of the tubular film 110. The primary air ring 112 is of the type known as a single lip air ring which prevents the performance of expansion work on the tubular film. The primary air ring 112 is connec-ted to an air blower 114 via a conduit 116. Arranged between the air blower 1l4 and primary air ring 112 is a control valve 118 and a temperature con-tro:L uni-t 120. The control valve 118 is adapted for con-trolling -the velocity and 3l2~;~;i(?63 volume flow ra-te of air from the air blower 114 -to an opening 122 provided in the pr:imary air ring 112. As illustrated, -the opening 122 is cons-tructed and arranged for discharging a continuous s-tream of air a-t uniform velocity and uniform ra-te in a direction parallel -to the external surface of -the -tubular film 110.
A cylindrical mandrel 124 is positioned centrally overlying the ex-trusion die 108 and arranged along the longitudinal axis of the tubular film 110. The mandrel 12~ is constructed to have a smooth uninterrupted exterior surface which defines an annular region 126 wi-th the interior surface of the tubular film 110 in the range of 0.3175 to 3.556 cm, preferably in the range of 0.3175 to 1.27 cm, and in accordance with the preferred embodi-ment, less than 0.635 cm. A passageway 128 is provided internally of the mandrel 124 and communicates between the interior region 130 of the blown film 132 and a conduit 134 arranged underlying the extrusion die 108.
The conduit 134 is connected to an exhaust blower 136 which communicates with the surrounding atmosphere. A
control valve 138 is positioned in advance of the exhaust blower 136 to control the rate of withdrawal of air from the interior region 130 of the blown film 132.
Optionally, a plurality of stabilizing guides 140 are positioned about the exterior of the mandrel 124 and extend into the annular region 126 to provide a restricted passageway 142. The guides 140 are arranged about the mandrel 124 at a location where the tubular film 110 has at-tained sufficient mechanical strength by its partial solidification to prevent damage thereto in the event of contact with the guides. To this end, the guides 140 are provided with a smooth uninterrup-ted e~terior surface to preven-t snagging of the tubular film 110. The guides 140 provide for increased s-tabilization of the tubular film 110 by locking the tubular film thereat in the manner to be described hereinafter.

~' A:ir is supplied over -the interior surface of the tubular film ~10 within the annular region 126 by a single annular nozzle 144 arranged within the annular region overlying the ex-trusion die 108. The nozzle 144 is arranged such that the discharged air flows in an upward direction parallel to the interior surface of the tubular film 110. An air blower 146 supplies air to -the nozzle 144 through a conduit 148. A control valve 150 and temperature control unit 152 are provided within the conduit 148 between the air blower 146 and nozzle 144.
The control valve 150 and temperature control unit 152 function in the same manner as the control valve 118 and temperature control unit 120 of the primary air ring 112.
That is, the velocity and volume flow rate of air from the air blower 146 is controlled by the control valve 150, while the temperature of the air is controlled by the temperature control unit 152. As thus far described, the temperature, the velocity and volume flow rate of a stream of air flowing in a parallel direction over the interior and exterior surfaces of the tubular film 110 may be controlled.
A secondary air ring 154 is arranged spaced-apart in tandem with the primary air ring 112. The secondary air ring 154 is arranged a predetermined distance above the primary air ring 112 to define the extent of the tubular film 110 over which the tubular film is stabilized in accordance with the present invention. To this end, the secondary air ring 154 can be adjusted upwardly and downwardly by its support upon height adjustment member 156 as shown. The secondary air ring 154 is located adjacent the predetermined distance over which the tubular film 110 is stabilized to provide a location for film expansion. The secondary air ring 154 is of the dual lip -type adapted to perform substantial expansion work upon the tubular film 110 to provide the blown film 132. The secondary air ring 154 is provided with a pair of spaced-apar-t openings 15~, 160 for the discharge of air at a high velocity and high volume rate as to opening 158 and a-t a high velocity, low volume rate as to opening 160 to create a negative pressure adjacent the exterior surface of the -tubular film 110 -to perform the required expansion work. Air is supplied to the secondary air ring 154 by an air blower 162 connected to a conduit 16~. A control valve 166 and temperature control unit 16~ are arranged within the conduit 164 between the air blower 162 and secondary air ring 154.
The control valve 166 controls the velocity and volume flow rate of air being discharged along the exterior surface of the tubular film 110 by the secondary air ring 154, while the temperature control unit 168 controls the temperature of the discharged air. It should -thus far be understood that the primary air ring 112 is adapted for stabilizing the tubular film 110, while the secondary air ring 154 is adap-ted for expansion of the tubular film to provide the blown film 132. Although the primary air ring 112 performs a modest amount of controlled cooling of the tubular film 110, the primary cooling function is performed by the secondary air ring 154.
The extrusion apparatus 100 provides for the greatly increased stabilization of the tubular film 110 over a predetermined distance by the use of high velocity, low volume flow rate of air discharged over both the interior and exterior surfaces of the tubular film between the tandemly arranged primary air ring 112 and secondary air ring 154. The na-tural venturi vector forces keep the external air next to the exterior surface of the tubular film 110, while the mandrel 124 maintains -the low volume flow rate of air within the annular region 126 at a suf~icien-tly high velocity -to keep the tubular film from oscillating about its longitudinal axis. The guides 140 prevent buffeting and actually make gentle contact with -the solidified inside surface of the tubular film 110. The secondary air ring 15~, being -charac-terized as a high intensity cooling device, provides in-tensive cooling and expansion of -the tubular film 110 at a location where desired, -that is, providing the -tubular film with a predetermined stalk height which is stabilized in accordance with the present invention.
The velocity of air inside the high s-talk, outside the high stalk and at the high intensity cooling and expansion area, i.e., the secondary air ring 154, are separately controlled to balance and stabilize the -tubular film 110 and blown film 132.
In accordance with the method of the present invention, the ex-trusion apparatus 100 is adapted to manufacture blown film from a variety of low to high molecular weight polymers. For example, such polymers include high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polypropylene, low density polyethylene (I,DPE), polystyrene, PVC, polycarbonate, polysulfone, polyester, nylon, and the like. In producing films of such material, solid polymer is provided in the supply hopper 104 to be extruded in a molten state through the extrusion die 108 by the extruder 102. The thus formed tubular film 110 is stabilized over a predetermined height by the application of high velocity, low volume flow rate of air over the interior and exterior surfaces of the tubular film. In this regard, the primary air ring 112 applies such a stream of air over the exterior surface of the tubular film 110 at a control temperature by temperature control unit 112 and at a controlled rate by control valve 118.
Similarly, a stream of air is applied over the in-terior surface of the tubular filrn 110 by the annular nozzle 144 at a controlled tempera-ture by temperature control unit 152 and at a con-trolled rate by control valve 150. The high velocity of the air s-tream over the interior and exterior surfaces of the tubular film stabilizes the film by preventing i-ts oscilla-tion abou-t the mandrel 124 and abou-t its longitudinal axis. The .
.

~l.~S~3 applica-tion of a low volume flow rate of air res~llts in only modest cooling of the unexpanded tubular film thereby allowing for the control of the stalk height to a predetermined dista~ce.
~ s a result o~ this created stalk heigh-t, the amoun-t of machine direction drawdown of the tubular film 110 takes place at a greatly reduced rate over -that of conventional tubular film extrusion processes. This permitted relaxation of the polymer stresses within the high stalk provides a uniformly stressed film for expan-sion having better uniformity of thickness and physical and mechanical properties. In addition, the high s-talk height allows for randomization and interweaving of the long polymer molecules, rather than keeping them aligned parallel to the extrusion direction. This randomization and interweaving gives the blown film 132 greatiy improved tensile and tear strength properties. In addition, by controlling the temperature of the air streams being applied over the exterior and interior surfaces of the tubular film 110, the film temperature over the high stalk may be maintained at an optimum temperature for ultimate blowing by the secondary air ring 154, and while being stabilized. This stabilization of the high ~talk is further enhanced by the guides 14 which create the narrow passageways 1~2 to increase the velocity of air flowing in the annular region 126. This increased velocity of air has the tendency of locking the tubular film 100 about the guides 140, thereby increasing the stabilization of the tubular film over the predetermined distance of the thus created high stalk.
The size of the bubble of the blown film 132 is controlled primarily by the exhaust blower 136 and control valve 138. Generally, under steady state opera-tion, the mass in of air via air blower 146 is equal to the mass of air being extracted from the interior region 130 by the exhaust blower 136 through the passage-way 128 extending through the mandrel 124. In order to increase or decrease the size of the blown film 132, the .:

:~5S~ 3 internal pressure within the in-terior region 130 is momen-tarily increased or decreased, so as -to affect the size of -the blown film 132, which size is sensed by means of sonar sensors 170. Once the blown film 132 has achieved its predetermined size, the mass in and mass ou-t of air within the interior region 130 is against balanced for steady state operation. The -tubular film 110 is expanded and intensely cooled adjacent the frost line 172 in a conventional manner using the secondary air ring 156.
In accordance with the method of -the present invention, the following Table I discloses the contem-plated range for the various operating parameters in producing blown tubular film from -the di-fferent disclosed polymers using the apparatus of the present invention.
The operating parameters are based on the use of a 10.16 cm diameter extrusion die. In this regard, the mass flow, inside air flow, outside air flow and secondary air ring flow are directly proportional to the die diameter.
Thus, for a 20.32 cm diameter extrusion die, the mass flow would be 91-545 kg/hr, the inside air flow would be 1132-33984 liters per minute, the outside air flow would be 1132-45312 liters per minute and the secondary air ring flow would be 3398-113280 liters per minute.

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~ZSSt~63 AS thus far described, the ex-trusion apparatus 100 and method of manufacturing blown tubular film in accordance wi-th the present inven-tion provides for increased bubble stability, improved gauge uniformity, reduced gauge standard deviation, improved optical properties, improved impact streng-th, improved tear strength, improved tensile strength, improved down gauging capability, and increased output.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that the embodiments are merely illustra-tive of -the principles and application of the present invention. For example, air from the interior region 130 of the blown film 132, which is suppl.ied by air blower 146, could be recirculated through conduits 134, 148 thereby eliminating the exhaust blower 136 and control valve 138. It is therefore to be understood that numerous modifications may be made in the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims (19)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method for forming a film of plastic material, said method comprising the steps of forming a progressively advancing unexpanded tubular film having a substantially uniform first diameter about a cylinder arranged along a longitudinal axis over a predetermined distance, applying a first gas stream over the exterior surface of said film, applying a second gas stream over the interior surface of said film within an annular region formed between said cylinder and the interior surface of said film, controlling the velocity and volume flow rate of said first and second gas stream over said unexpanded tubular film over said predetermined distance for stabilizing said film by preventing the oscillation of said film about said cylinder, and applying a third gas stream over the exterior surface of said film having said first diameter adjacent the extent of said predetermined distance for expanding said film from said first diameter to a second diameter thereat.

2. Method according to Claim 1, wherein said stabilizing includes applying said first gas stream at a sufficiently high velocity, low volume flow rate, over the exterior surface of said film and applying said second gas stream at a sufficiently high velocity, low volume flow rate, over the interior surface of said film to prevent the oscillation of said film about said cylinder.

3. Method according to Claim 2, wherein said stabilizing includes applying said first gas stream over the exterior surface of said film in a direction parallel thereto and applying said second gas stream over the interior surface of said film in a direction parallel thereto and within said annular region.

4. Method according to Claim 1, further including exhausting at a controlled rate at least a portion of said second gas stream from the interior of said film through a passageway provided within said cylinder and recirculating said at least said portion of said second gas stream into said annular region and over the interior surface of said film.

5. Method according to Claim 1, wherein said first gas stream has a flow rate in the range of 56 to 2230 liters/minute/cm of die diameter and a velocity in the range of 305 to 7315 meters/minute.

6. Method according to Claim 1, wherein said second gas stream has a flow rate in the range of from 56 to 1672 liters/minute/cm of die diameter and a velocity in the range of 305 to 7315 meters/minute.

7. Method according to Claim 1, further including controlling the temperature of said first and second gas streams, said first and second gas streams have a temperature in the range of -29 to 149°C.

8. Method according to Claim 1, further including controlling the width of said annular region, in a radial direction, in the range of 0.3175 to 3.556 cm.

9. Method according to Claim 1, wherein said predetermined distance is in the range of 1 to 20 times the diameter of said film.

10. Method according to Claim 1, further including controlling the temperature of said third gas stream in the range of -29 to 65.5°C, its velocity in the range of 610 to 7315 meters/minute and its flow rate in the range of 56 to 5591 liters/minute/cm of die diameter.

11. Apparatus for forming a film of plastic material, said apparatus comprising supplying means for supplying plastic material in a flowable state, a die arranged in advance of said supplying means for forming a progressively advancing unexpanded tubular film having a substantially uniform first diameter along a longitudinal axis over a predetermined distance, a primary air ring arranged adjacent said die and exteriorly of said film, said primary air ring supplying a first gas stream over the exterior surface of said film, means for supplying a second gas stream over the interior surface of said film, a cylinder arranged interiorly of said film and extending from said die along said longitudinal axis over said predetermined distance, said cylinder and the interior surface of said film defining an annular region therebetween for receiving said second gas stream, controlling means for controlling said first and second gas streams over said unexpanded tubular film to stabilize said film over said predetermined distance by preventing the oscillation of said film about said cylinder, and a secondary air ring arranged spaced from said primary air ring and exteriorly of said film having said first diameter, the distance between said primary and said secondary air rings defining said predetermined distance over which said film is stabilized, said secondary air ring supplying a third gas stream over the exterior surface of said film for expanding said film from said first diameter to a second diameter thereat.

12. Apparatus according to Claim 11, wherein said controlling means controls the velocity and volume flow rate of said first and second gas streams to prevent the oscillation of said film about said cylinder by controlling said first gas stream at a sufficiently high velocity, low volume flow rate, over the exterior surface of said film over said predetermined distance and by controlling said second gas stream at a sufficiently high velocity, low volume flow rate, over the interior surface of said film within said annular region and over said predetermined distance.

13. Apparatus according to Claim 11, further including guide means arranged about said cylinder within said predetermined distance and extending into said annular region for guiding said film thereover.

14. Apparatus according to Claim 11, wherein the exterior surface of said cylinder over said predetermined distance is substantially uninterrupted.

15. Apparatus according to Claim 11, further including means for exhausting at a controlled rate at least a portion of said second gas stream from the interior of said film through a passageway provided within said cylinder communicating between the interior of said film and the surrounding atmosphere.

16. Apparatus according to Claim 11, wherein said controlling means controls said first gas stream over the exterior surface of said film in a direction parallel thereto and said second gas stream through said annular region in a direction parallel to said interior surface of said film.

17. Apparatus according to Claim 11, further including means for controlling the temperature of said first, second and third gas streams.

18. Apparatus according to Claim 11, further including means for changing the distance between said primary and secondary air rings.

19. Apparatus according to Claim 11, wherein the width of said annular region, in a radial direction, is in the range of 0.3175 to 3.556 cm.