CA1060164A - Shaping apparatus for the extrusion of tubular thermoplastic film - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Blown tubular film is formed by an extruder using a diverging housing with cooling fluid inlets to cool the film bubble. The inlets are arranged so as to create a partial vacuum which draws the bubble outwards and conforms it to the shape of the housing without bringing the hot film into contact with the housing.

Description

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Thls invention relates to the production of continuou6, seamless blown thermoplastic film tubing and more particularly to the production of thin wall film at higher speeds with improved uniformity of wall thicknes6.
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Blow~, tubular thermoplastic film may be produced by~ continuously extruding thermoplastic resln through an annular die, applying lnternal fluid ~-~
pressure to the e~truded tube and thereby expanding the t~be to reduce itB wall thickness to the appropriate value w~ile cooling and solidifying the resin, The tllbing iB then flatte~ed between pinch rolls. The flattened double thickness film may be wound into a cylindrical roll for storage and subsequent ; ~;
use as a tube or the tubing may be 6plit to form a single thicknefis sheet of double width or two single thicknes~ sheets.

Under certain circumstances the tubing 80 produced lack~ good gauge (thlckness) uniformity, that i~, the thickness of the film i6 not uniform.
If this non-unlform film i6 wound up onto a roll, an uneven, humped roll i8 obtained. The winding ten5ion stretches the film over the humps and 80, if the ¦ resin ha6 cold flow, the film does not lay flat when it is unwound and it has ~ ;~
rlpples or billow~ which nece6~itate special precautions ln its printing, converslon and other uses. Conventionally such problems are ameliorated by rotatlng the die or any cooling ring being used, or both.

l These gsuge vsriations also tend to use a greater weight of resin per i unit area of film because a greater average thickness must be maintalned in.~ . .
order to laintain a given minimum thickness. Therefore, it 18 desirable to r~duce gauge variatlon.

One of the ma~or problems iA to cool the e~truded bubble of thermoplastic j ~sterlal. The production rate for any gi~en tube (bubble) slze i8 limited .
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~5 by the character o~ the bubble belng extruded. Thus, under a given set of operating conditions, increasing the extruder output will cause the thermo plastic material to be formed into the tube at a higher rate but since the heat loss characteristics of the system will not have changed, it will also cause a rise in the height of the frost line (that is, the line where the molten extruded resin becomes solid). This, in turn, causes an increase in the instability ;
of the extruded bubble because its unsupported molten length beco~es too long.
We have now devised a way of improving the uniformity in the wall thickness of extruded tubular films. The extruded film is passed through a device which imposes a chosen configuration on the bubble but which is also designed to prevent the molten resin coming into contact with the device.
According to the present invention, the apparatus for forming the tubular thermoplastic resin film comprises:
B (1) extruder means~ and an annular die which cooperate to extrude a tube of thermoplastic film;

(2) a pair of pinch rollers downstream of the die, adapted to flatten : , :
the tube extruded by the die; `~

(3) means for introducing a fluid under pressure into the extruded tube to expand it and so reduce its wall thickness; and

(4) a housing coaxial with the die and downstream of and ad~acent to, the die and diverging in a downstream direction, said tubular thermoplastic film passing said diverging housing upon exiting from said die, the housing having ;

multiple rows of cooling fluid channels arranged in spaced pairs with each channel terminating in an aperture on the inner surface of the housing, the ; channels of one row of each pair pointing away from the channels of the other row of the pair so as to provide a flow of cooling fluid which diverges away from each pair of rows, ~ fluid outlet passage being provided between the row - pairs, and means for forcing a fluid through said apertures towards the extruded tubing and then away from said tubing, thereby creating a suction and drawing - 30 said tubing toward said housing.-.",.`, :. .

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' ~ ,, , -The housing has rows of channels for cooling fluid; each channel terminating in an aperture on the inner surface of the housing. The channels are arranged in pairs of rows with the channel~ in one row of each pair pointing away from the channels in the other row of the pair. This provides a flow of cooling `

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fluid (normally air) which diverges away from each pair of aperture rows;
Bet~een each pair of rows is a passage through which the cooling fluid may escape. Thus, a circulation of cooling fluid is set up from the ; channels, through the apertures, over the extruded film and out through the `` escape passages.
` The effect of blowing air or another cooling fluid through the apertures at the end of the channels is to create a partial vacuu~ which draws the extruded bubble outwards towards the walls of the housing and so , .
conforms the configuration of the bubble to that of the housing. ~re have ; 10 found that the housing should have a maximum angle of divergence of 48 for best results. - ~ `
The method according to the present invention therefore comprises melt extruding a tube of thermoplastic resin, maintaining pressure inside the tube upon extrusion thereby expanding said tube by the application of the pressure;
cooling said extruded tubing to solidify the resin and flattening the expanded, cooled tube to entrap fluid in the expanding tube, and drawing the expanding extruded tubing outwardly and confor~ing the expanding tubing to a diverging form having a maximum angle of divergence of less than 48, said drawing being effected by utilizing a vacuum created by a plurality of pairs of superimposed `~ 20 diverging air streams surrounding said tube, said air from the air streams being exhausted through annular openings located intermediate said divergent pairs of air streams.
In a preferred embodiment, the housing takes the form of a series of rings along the central axis of the bubble and these rings are spaced apart from each other so as to provide the passage for the escape of the cooling gas.
In order that the invention may be more fully understood, preferred embodiments of the extrusion apparatus and method will now be described by ~ -way of example only, with reference to the accompanying drawings in which~

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~lgure 1 1~ a sectional ele~ation of a ~ubular extrusion apparatus, Figure 2 ls a section view taken on line 2-2 of ;
Figure 1, Figure 3 is an enlarged sectional view of the cooling .~
ring portion of the apparatus of Figure l.

Referring to the accompanying drawings, a thermoplastic resin 10 i~
fed to an extruder lZ in which it is melted and then extruded through a~
~ annular extrusion die 14 to form a tube 16 of molten thermoplastic resin. ;~
j) Conduit 18, in the island 20 of the annular die 22 is provided for feeding ;~
a fluid, normally air, into the thermoplastic resin tube 16. As the tube i proceeds downstream it cools until, at frost line 24, it solidifies lnto ,"1 .: ~:
a dimenslonally stable tubular structure 26. This solid tube 26 i8 ~ collapsed by a guide 28 and then passes through the nip of a pair of rollers ~ 30.

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Multiple cooling ring manifolds 32a, 32b, 32c, 32d, 32e, 32f and 32g whlch are either separate entitle~ in a housing 34 as shown or may be an equivalent aingle monolithic structure are disposed around the axi~ of the bubble. A pump 36 feeds a ~luid, suitably;alr, through a manifold 38 into . ., ,:
j the interior 40 oP the ri~g manifolds 32. This fluid impinges on the ex~
truded tublng through the diverging channels 42a and 42b and apertures ij 43. The fl~w of the fluid from the di~erging nozzles creates a decreased pres~ure bet~een the nozzles (in the area designated 44) and also Porces ~¦ the fluid to pass out of the system through passage~ 46 betwee~ adJacent - ~;
rlng~. The high velocity fluid leaving the proximity of the extruded tube al80 causes a ~light decrease in pressure (in the area designated ~` 48). The~e pressure drops draw the still molten extruded tube 16 toward . ~ !
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the rings 32 but the fluid forms a cushion between the rings and the tubing so as to prevent the tubing from contacting the rings and stick~ng to them while it is still molten.

One of the important features of the apparatus i8 the use of the divergent housing for imposing a particular configuration on the extruded tube from the time it issues from the extrusion die to the time at which it is more in a substantially solid state. This housing is of generally diverging9 e~g. conical, paraboloid or other similar shape. This characterization of the configuration imposition housing does not limit it to an exact geometric reproduction of these mathematical functions.
Rather, these are given as illustrative of the general type of configuration which can be used. All that is required i8 that this housing diverge from an initial, upstream position proximate to and co-axial about the annular ; extrusion die to progressively more down-stream positions less proximate lS to but still co-axial with the annular extru~ion die.

We have found that the shape imposed during the expansion and cooling of the bubble 6hould have a maximum divergence angle of less than 48~
preferably less than 38. Further, for any given frost line to extrusion die distance, it is preferred to operate with an imposed bubble shape and size ~uch that the neck of the bubb:Le (that ls, the part of the bubble from the die to the point where it starts to dlverge) i~ longer than in con-ventional bubbles (i.e. bubbles whose configura~ion is not controlled~ to such an extent that the divergent point is higher by 5 ~o 15~ of the total die to frost line distance. The gauge variation in film thus formed is `~
signiflcantly reduced as compared to the gauge variation observed under otherwise eubstantially identical processing conditions without imposing such a speci~ic shape prior to solidifying the extruded film.

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In the preferred form of housing, each row of apertures lies along a plane generally normal to the axi.s of the extrusion die. The indivldual channe}s 42 and their apertures 43 in each pa~r of rows are directed apart at 8 angle, preferably the widest angle possible, in the direction to the axls of the apparatus i.e. in the direction of fil~ ~ovement. Thu6, the axes o the apertures are positioned at a small acute angle with - respect to the surface of the thermoplastic tube. Although it is probably :
the most efficient configuration, the referred to channels need not be cylindrical in shape but can be a converging no~zle or other configuration.

It i8 only lmportant that these channels cause fluid flowing therethrough to diverge, upon emergence from each cooperating pair of apertures.

Pas~ages are also provlded between the pairs of aperture rows or the air or other fluid to e6cape. Thus, a circulatory ~ystem iB provided forcing fluid from the outside toward the e~truded tube, passing the fluid along the extruded tube and then passing the fluid bacl~ to the outside.

If~a fluid other than air iB used, a suitable reservoir and closed system can , ~ , be provided or the outward passage can simply vent the fluid to the atmosphere.

Thi~ fluid c0018 the extruded tubing and solidifies it to a sufficient extent to render it non-tacky and dimensionally stable. If this were the only function performed by the cooling fluid, the tube would expand according to the pressure applied by the internal fluid and as a function of the cooling rate applied and the inheren~ liquid strength of the thermo-plastic resin in use. The apparatus does more than this. The particular configuration of the channels and their apertures causes the fluid flowing out of them to act as an eductor and 90 forms a partial vacuum in the space between the apertures. This draws the extruded tubing toward the configurat~on imposition housing and causes the extruded tubing to conform closely to the shape of the housing. If the temperature and flow rate of the ~luid are adJusted with due regard to the particular thermc~plastic .

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resin being extruded and with regard to the dimensions of the extruded tube as well as the extrusion die, the ~rost line of the extruded tubing can usually be positioned within the con- ~
figuration imposition housing, generally toward its downstream ~ ~-end.
It should be clear that many of the processing variables are interdependent and are not independently variable. The principle of the process is to impose a particular rate of expan~
sion and a particular shape upon the extruded tube of thermoplas~
tic material while cooling it to a dimensionally stable and non-tacky condition.
The thermoplastic material is usually extruded through an ;
annular die having a diameter of about 10 to ~0 mm. and a die gap o about 0.25 to 2.5 mm. Ext~usion rates are, of course, dependent upon the extruder used. Blow up ratlos, that lS, the ratio of final film diameter to die diameter/ of about 1^.5 to 5 are suitable as are final film thicknesses of 0.01 to 0.25 3 mm. The preferred internal pressurizing fluid and external ;~`
cooling fluids are air, however, other gases can be used. The internal fluid should be maintained at about or slightly above atmospheric pressure. The external cooling fluid is suitably maintained at a temperature of about 20 to 100C and is suit-ably fed at a rate of about 20 to 150 M.3 (0,760 mm.Hg) per ! s~uare meter of surface area of the molten tubing being cooled.
We have found that using certain conventional conditions `
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directly in the present imposed-shape process may result in `~
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`j inoperability. It has been found that when certain conventional ;
conditions are used, the molten extrudate will become slack as it leaves the die, allowing it to fold over on itself or flow outward between the vertically spaced cooling ring elements, so -~ that it sticks, or in some way becomes caught on the .1 ,~ ~
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surface~ of the cool~ng rin~ elements immediately adiacent the die. The pos~lbll1ty of thi6 happening can be sub~tantially reduced or eliminated by increasing the die orifice gap to form a gap which is wlder than would normally be employed in conventlonal tubular extrusion operation~. In a conventional cooling process, the usual gap may be ln the range of 0.06 to 0.1 mm, for example, whereas the orifice gaps for the present process of the pre6ent lnvention are preferably 1 to 1.25 mm. For example, a - 66 cm. lay-flat tubular width tl32 cm. circumference) film, 0.07 ~m.
thick, is normally made with a 1 mm. gap on a conventional prncess, but require~ a 1.25 mm gap with the present process. By using a larger dle orlfice gap, the molten film i~ thicker as it leaves the die, and mu~t be drawn down with greater force to achieve desired final film thickness at the frost line. This greater force overcomes the tendency of the molten bubble to fold over on itself or flow outward between the cooling rlng j elements. Although the exact mechanlsm is not understood it ls theorlzed that the enlargement of the orifice gap and the proportlonate increase in the draw down force eliminates the problem of tube hang-ups, slnce as a result of increasing the draw down force a downward drag exerted by the cooling ring elements on the molten tube is overcome. For purposes of the pre~ent invention it has been found that a ratio of dle oriflce gap to final fllm thicknes~ ~hould be above 10:1 and preferab~y above 15~

Fluid flow conduits may be provided within the island portion of the extrusion die to extract some or all of the air iniected lnto the extruded tube ~o as to form a circulating sy~tem.

The configuration imposition housing may be composed of a single mono-llthic ctructure of such height a8 to extend from the plane of the extrusion die to a point downstresm of the frost line, or it may be 6eries of ~tacked riQgs of ~imilar cumulative height. In elther c~se~ the apertures ~hould ,j ~.
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be spaced about 1 to 20 mm. apart with the two rows in each pair spaced sbout 10 to 100 mm. apart. The apertures themselves and thelr correspondlng channels should preferably be in radial plane~ and should be apaced apart about 2 to 6 aperture dlameters apart in each row and ~he channel pairs should diverge about 50 to 160 included angle, preferably 100 to 150.
The air return passages be~ween aperture row pairi should be about 3 to 15 mm.
wide ~o as to permi~ ready flow of the air back from the extruded tubing.
It should be noted that the veloclty and/or temperature of the e~ternal cooling air can be substantially constant at all aperture position3 or it may be varied e.g. in an ascending or descendi~g gradient, a~ processing cond$tions dictate.

The thermoplastic resin may be for example, a polyolefin such as polyethylene, polypropylene~ polybutene-l, copolymers of two or more of these with or without other olefin3, polyvinyl or vinylidene chloride, vinyl or vinylldene chloride copolymers with acrylates, acrylonitrile or olefins~acrylic homo and/or copolymer~, styrene homo and/or copolymers~
and in general, such other polymeric materials as may be melt extruded ~nto tubular f.lms.
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Claims (11)

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

1. Apparatus for forming extruded tubular thermoplastic resin film, which comprises:
(1) extruder means and an annular die which cooperate to extrude a tube of thermoplastic film;
(2) a pair of pinch rollers downstream of the die, adapted to flatten the tube extruded by the die;
(3) means for introducing a fluid under pressure into the extruded tube to expand it and so reduce its wall thickness; and (4) a housing coaxial with the die downstream of, and adjacent to, the die and diverging in a downstream direction, said tubular thermoplastic film passing through said diverging housing upon exiting from said die, the housing having multiple rows of cooling fluid channels arranged in spaced pairs with each channel terminating in an aperture on the inner surface of the housing, the channels of one row of each pair pointing away from the channels of the other row of the pair so as to provide a flow of cooling fluid which diverges away from each pair of rows, a fluid outlet passage being provided between the row pairs, and means for forcing a fluid through said apertures towards the extruded tubing and then away from said tubing, thereby creating a suction and drawing said tubing toward said housing.

2. Apparatus according to claim 1 in which the channels of one row of each pair diverge from the channels of the other row of the pair at an included angle of from 50° to 160°.

3. Apparatus according to claim 2 in which the channels of one row of each pair diverge from the channels of the other row of the pair at an included angle of 100° to 150°.

4. Apparatus according to any of claims 1 to 3 in which each pair of rows of channels is disposed along the inner periphery of a ring manifold, a plurality of the ring manifolds defining the housing with spaces between the ring manifolds to provide the fluid outlet passages.

5. Apparatus according to claims 1 to 3 in which the apertures in a row are spaced apart by a distance of 2 to 6 aperture diameters.

6. A method of producing extruded tubular thermoplastic resin film, which comprises melt extruding a tube of thermo-plastic resin, maintaining pressure inside the extruded tube to expand the tube by the application of the pressure, cooling the extruded tube to solidify the resin and flattening the expanded, cooled tube to entrap fluid in the expanding tube, and drawing the expanding extruded tube outwardly and conforming the ex-panding tube to a diverging form having a maximum angle of divergence of less than 48°, said drawing being effected by utilizing a vacuum created by a plurality of pairs of super-imposed diverging air streams surrounding said tube, said air from said air streams being exhausted through annular openings located intermediate said divergent pairs of air streams.

7. A method according to claim 6 in which the angle of divergence is less than 38°.

8. A method as defined in claim 7 wherein the pressure inside said tube comprises an internal pressurizing fluid and said cooling comprises a cooling fluid, both of said fluids comprising air.

9. A method according to claim 8 wherein said external air is at a temperature of about 20° to 100°C. and is fed at a flow rate of about 20 to 150 M2 (0° 760 mm Hg) per square meter of surface area of the molten tube being cooled, and said internal air is at slightly superatmospheric pressure.

10. In an apparatus for forming tubular thermoplastic film comprising extruder means; annular extrusion die means, adapted in combination to extrude a single tube of thermoplas-tic film; pinch rollers spaced downstream from said annular extrusion die adopted to flatten said tube; and means for introducing a fluid under pressure into said tube whereby to biaxially expand said tube and reduce the wall thickness thereof;
the improvement whereby guage non-uniformity is reduced in said film, which comprises a configuration imposition assembly comprising a diverging housing member disposed concentric to said extruded tube downstream of, and adjacent to, said annular die, said tubular thermoplastic film passing through said diverging housing member upon exiting from said die, said assembly being further characterized by having multiple rows of channels therethrough, the channels being arranged in spaced pairs, each of said channels terminating in an aperture on the inner surface of said housing, the channels of one row of each pair pointing away from the channels of the other row of the pair so as to provide a flow of cooling fluid which diverges away from each pair of rows, a fluid outlet passage being provided between the row pairs, and means for forcing a fluid through said apertures towards said extruded tubing and then away from such tubing, thereby creating a suction and thus drawing said extruded tubing toward said assembly.

11. In the process of producing thermoplastic film by melt extruding a seamless tube of thermoplastic material from an annular die orifice, increasing the pressure inside said tube upon extrusion;
biaxially expanding said tubing as a result of increasing said pressure;
cooling said extruded tubing to a solid state and flattening said expanded, cooled tube utilizing a pair of rotating draw rollers so as to entrap fluid between said flattening and said annular die orifice; the improvement of which comprises drawing said expanding extruded tubing outwardly to a greater extent utilizing a vacuum created by a plurality of pairs of superimposed diverging air streams surrounding said tubing, said air from said air streams being exhausted through annular openings located intermediate said divergent pairs of air streams, and conforming said expanding tubing to a substantially diverging form having a maximum angle of divergence of less than 48°.