JPS6134974B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the production of seamless inflation tubes of thermoplastic film by melt extrusion.

Thermoplastic resin tubes are manufactured by melt-extruding thermoplastic resin through an annular orifice, applying internal gas pressure to expand the extruded tube to reduce its wall thickness, and cooling and solidifying the resin. can. This tube is then flattened between pinch rolls to form two sheets of film. This flattened tube-shaped film may then be wound onto a cylindrical roll and stored for use as a tube, or the tube may be cut open to form a sheet with the same thickness and twice the width. It may then be rolled up into a single roll, or it may be cut off at both ends to form two sheets of the same thickness and wound up into separate rolls.

One of the major problems lies in the cooling of extruded thermoplastic bubbles. The rate of production of tubes (bubbles) of a given size is limited by the characteristics of the extruded valve. That is, under certain operating conditions, increasing the throughput of the extruder will cause the thermoplastic resin to be formed into a tube at a higher rate. However, since the heat exchange properties of the system remain unchanged, the height of the frost line (the boundary line where the extruded tube changes from the molten state to the solid state) increases accordingly. As a result, the length of the unsupported melt portion of the extrusion valve becomes too long, increasing its instability. Supporting film valves generally allows for increased cooling air impingement and therefore extrusion speeds.

US Pat. No. 3,867,083, by the present inventor, discloses a high speed extrusion process for blown film. This method makes it possible to produce films of excellent uniform thickness at extremely high speeds. This method uses multiple perforated cooling rings with multiple holes to maintain the shape and cool the extruded film. A development of the device described in this US patent was disclosed by the inventor in German Patent Publication No. 2510804,
This uses a removable insert in the air ring chamber.

A major production rate limiting factor common to all blown film extrusion systems is the amount of time required to cool and solidify the extruded film bulb before flattening the tube.

The inventors have now found a way to increase the cooling rate so as to make it possible to increase the production rate. According to the present invention, the extruded tubular film, in addition to the perforated cooling ring, is further blown inwardly and/or upwardly from continuous (i.e., not discontinuous orifices). It is also cooled by an air flow (supplied from a continuous annular orifice around the circumference of the tube). Compared to extruding and cooling blown film using the conventional inflation tube extrusion method,
Tubes can be produced at a manufacturing speed that is approximately 10 to 50% faster.

The equipment used to make the film includes an extruder and an annular extrusion die and means for introducing air or other pressurized fluid into the extruded tube.
The pinch rolls are spaced sufficiently from the annular die to allow sufficient cooling of the tube until it is substantially solid and non-stick. The tube extruded from the die first passes through a cooling housing that provides intense cooling to rapidly cool the molten thermoplastic to a more viscous, self-supporting state. The cooling housing is similar to that described in US Pat. No. 3,867,083 and is generally cylindrical or divergent. Another cooling housing is provided overlying the first cooling housing. The second cooling housing is an annular housing that surrounds the tube and supplies a continuous flow of cooling air to the tube. This airflow is directed outward and upward.

The present invention therefore combines a fast cooling system and an air ring cooling system similar to that described in U.S. Pat. The part near the frost line is cooled by an air ring. This not only substantially increases the cooling rate compared to conventional cooling methods, but also compared to the first fast cooling method described above, it also allows the final film width adjustment to be the same as with conventional cooling methods. , which can be done simply by changing the amount of air in the bubble (in the first fast cooling method mentioned above, changing the film width requires changing at least part of the surrounding cooling housing). Become wider. For the operator, control of the air pressure inside the bubble is no longer necessary, and the operation method becomes similar to a conventional cooling method, so it is easier to control than the original high-speed cooling method.

The invention relates to polyolefins such as polyethylene, polypropylene, polybutene-1, copolymers of two or more of these, polyvinyl chloride or vinylidene, copolymers of vinyl chloride or vinylidene with acrylates, acrylonitrile, olefins or other comonomers, acrylic homopolymers and Applicable to thermoplastic materials such as copolymers, styrene homopolymers and copolymers.

The first cooling housing includes a plurality of pairs of two-by-two rows of cooling holes directed toward the extruded tubes. Each row of holes is arranged along a plane generally perpendicular to the axis of the extrusion die. Each row of holes is associated with a pair of adjacent rows of holes.
The two rows of holes in each pair are oriented in different directions at large angles to form a diverging jet of air (or other cooling fluid). As explained in U.S. Pat. No. 3,867,083,
This cools the molten resin while creating a vacuum that pulls the extruded tube toward the interior surface of the housing. The ducts leading to the individual holes are oriented with their axes at acute angles to the surface of the tube of thermoplastic material. The shape of the duct need not be cylindrical, but may be tapered or nozzle-shaped, for example.

The cooling fluid (eg, air) serves to cool and solidify the extruded tube to the extent that it is no longer sticky and is dimensionally stable. With this feature alone, the tube of extruded thermoplastic material would be free to expand in response to the pressure exerted by the internal fluid as a function of the external cooling rate and the inherent liquid strength of the thermoplastic material. Become. However, in this case, due to the shape and arrangement of the cooling holes described above, the fluid flowing out of the cooling holes creates a local vacuum, which causes the extruded tubes of thermoplastic material to move toward the inner surface of the cooling housing. The tension causes the extruded bubble to conform to the shape of the surrounding housing.

A pump or blower is provided to direct air or other suitable cooling fluid through the cooling holes toward the tube of extruded thermoplastic material. Exhaust passages are provided between each pair of cooling hole rows to allow air and other fluids to escape. That is,
The cooling fluid is fed from the outside toward the extruded tube, hits the extruded tube, and returns to the outside. If the fluid is air, the atmosphere is a suitable reservoir. If other fluids are used, a suitable reservoir and closed system may be provided, or the fluid may simply be vented to the atmosphere through an exhaust line.

The first cooling housing may be of integral unitary construction or may be of stacked ring construction as described in DE 2510804 A1. In any case, a pair of cooling hole rows and an adjacent pair should be separated by about 12 to 100 mm.
The distance between the two rows in each pair may be about 2 to 20 mm. The spacing between the holes in each row is preferably 2 to 6 times the hole diameter, and the ducts leading to the holes are offset by about 50 to 160 degrees, preferably 100 to 150 degrees. The exhaust channels between a pair of rows and the adjacent pair should be approximately 3-12mm to allow for easy exhaust flow of air bouncing back into the extruded tubes.
It is preferable to have a width of . The flow rate or temperature (or both) of the external air may be substantially constant at all cooling hole locations or may be varied as specified by process conditions.

Thermoplastic resins are typically extruded through annular dies with diameters of about 12 to 400 mm and die gaps of about 0.25 to 2.5 mm. The extrusion speed is of course controlled by the extruder used, but the extrusion speed is approximately 0.2 to 1 cm per 1 cm of the final valve circumference.
A flow rate of 2 Kg/hr, preferably 0.7-1.4 Kg/hr can be maintained. The expansion ratio, i.e. the ratio of the final film diameter to the die diameter, is approximately 1.5
.about.5, resulting in a final film thickness of about 10 to 250 microns. The preferred internal pressurized fluid and external cooling fluid is air, although other gases can also be used. The external fluid delivered to the cooling holes is preferably maintained at a temperature of -20 to 90°C and is supplied at a rate of about 23 to 183 m ³ /min per m ² of surface area of the melt tube to be cooled. Part or all of the air blown into the extruded tube may be extracted from the island of the die to form an air circulation system.

It has been found that placing a conventional air ring on top of the top perforated ring can substantially increase manufacturing speed without degrading the quality of the finished film. In some embodiments, the perforated surface of the first cooling housing is tapered outwardly to impart a bulging shape to the semi-molten tube immediately after extrusion. Alternatively, the inner surface of the housing may be plane vertically, ie, substantially parallel to the direction of travel of the extruded tube, so that no expansion of the tube occurs as it advances along successive ring members. No.
Which type of cooling member shape should be selected?
Of course it depends on the desired expansion ratio.

The air ring portion of the cooling system is arranged adjacent to the uppermost cooling ring member. The air ring may be of conventional design, such as a tube extrusion air-cooled ring commercially available for air rings used in dies.

That is, this air ring is not of the type with a large number of holes, but is of the type with an annular orifice that is continuously connected to the inner surface of the ring and goes around the ring. The orifice has an opening of approximately 3 to 12 mm, and the orifice itself is designed to send air inwardly upward, directly inwardly, or directly upwardly toward the extruded tube. Preferably, the direction of flow is regulated. 25-300 for the top air ring
It is preferred to use an air pressure in the range of mm, preferably about 100 mm (water column).

Preferably, there is a gap of about 6 mm between the tube extrusion die and the lower surface of the lowermost perforated ring to allow air to be discharged from the cooling ring member. In addition, the top surface of the die is insulated,
It is also desirable to prevent the top surface from being cooled by the exhaust flow that is exhausted across the top surface of the die.

The details and features of the invention will be explained below with reference to the drawings.

With reference to the accompanying drawings, in particular figures 1 and 3,
The thermoplastic resin is fed to a rotary screw extruder (not shown) from which it is extruded through an annular die 18 to form a tube 16 of molten plastic material.
is formed. A conduit 22 is provided within the island 20 of the annular die 18 for introducing a fluid (preferably air) into the extruded thermoplastic tube 26 . As the tube moves away from the die, it cools and solidifies at the frost line 24 into a dimensionally stable cylindrical structure 26.
This solid tube 26 is gradually collapsed by a pair of collapsing shields and passed between a pair of take-off nip rolls (not shown) from where it undergoes other processing (not shown).
For example, it is taken up by a bag making machine) or rolled up in the form of a flat film.

Multiple lower rings 32a, 32b and 32c
(which may be separate parts within one housing or an integral unitary structure) are disposed over the die and form a first cooling housing. There is. Pump 36 delivers fluid (preferably air) from manifold 38 to interior 40 of ring 32 . This fluid is then passed through the misdirected duct 4 as clearly shown in FIG.
2a, exits from the hole 43, and collides with the extruded tube 16. This creates a reduced pressure between the rows of holes (areas indicated by 44) and also serves to force the cooling fluid out of the system through the exhaust passage 46 between adjacent rings. This reduced pressure pulls the still molten extruded tube 16 outward toward the ring 32, but the air escaping through the hole acts as a cushion between the ring and the tube while the tube is still molten. This prevents it from coming into contact with the ring and adhering to it.

As shown in FIGS. 1 and 2, the final cooling ring 30 includes porous rings 32a, 32b and 32.
It is superimposed on c. Pressurized air is supplied to the ring 30 from a pump 36a. Air exits the ring through a continuous annular orifice 34 and impinges on tube 16. Air flow is directed inwardly and upwardly from ring 30 by a lip or flange located along the lower border of orifice 34. Alternatively, the air flow may be directed directly inside or above the eyelashes. In the specific example shown in FIGS. 1 and 2, the precooling ring 3
The cylindrical inner surfaces of 2a, 32b and 32c are substantially vertical, ie parallel to the axis of the advancing tube 16. In the embodiment shown in FIG. 3, the inner surfaces of porous rings 32b and 32c are angled outwardly away from the perpendicular direction of extruded tube 16 such that tube 16
Its diameter increases as it passes through c. The tube expands as a result of the local vacuum created by the redirected jets of cooling fluid exiting the holes in the cooling ring.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional front view of a typical tube extrusion device. 2 is an enlarged view of the cooling ring portion of the apparatus of FIG. 1; FIG. FIG. 3 is a sectional front view of another type of tube extrusion device. FIG. 4 is a partial sectional view showing a part of the multi-hole cooling ring shown in FIGS. 1-3. 16, 26...tube, 18...die, 22...fluid blowing pipe, 24...frost line, 30...air ring, 32a, 32b, 32c...perforated cooling ring, 42...duct, 43...hole.

Claims (1)

[Claims] 1. (1) An annular extrusion molding die capable of extruding a tube of thermoplastic resin; (2) A die for flattening the extruded tube and maintaining fluid pressure inside the tube. ,
a pinch roll spaced apart from the die; (3) means for introducing pressurized fluid into the extruded tube to expand the tube and reduce its wall thickness; (4) a connection between the die and the pinch roll; a first cooling housing disposed between and concentric with the axis of the extruded tube, the housing comprising two rows of pairs arranged to jet a jet of cooling fluid directly toward the tube; having rows of holes arranged in a row such that in each pair the jets in each row are oriented in a different direction from the jets in the other row of the pair, thereby drawing the extruded tube outwardly toward the housing. (5) a thermoplastic material forming a localized vacuum, the housing having exhaust passages between pairs of adjacent rows; and (5) means for supplying cooling fluid to the holes in the housing. An apparatus for forming a tubular film of resin, wherein: (6) a second cooling housing is disposed over the first cooling housing and below the frost line; Apparatus as described above, characterized in that the housing has a continuous annular orifice surrounding the extruded tube for directing an annular flow of cooling fluid inwardly and upwardly toward the extruded tube. 2. The apparatus of claim 1, wherein the second cooling housing is adapted to direct an annular flow of cooling fluid inwardly toward the extruded tube. 3. The apparatus of claim 1, wherein the second cooling housing is adapted to direct an annular flow of cooling fluid upwardly towards the extruded tube. 4. The first cooling housing has a cylindrical inner surface facing the extruded tube.
The apparatus according to any one of Items 1 to 3. 5. The device of any one of claims 1 to 3, wherein the first cooling housing has an inverted conical inner surface facing the extruded tube.