CA1046213A - Method of and apparatus for making polymeric film - Google Patents

Abstract

ABSTRACT
A process and apparatus are provided for making polymeric film wherein flowable polymeric material is ex-truded in molten form through a die, having a pair of die lips divided into thermally isolated sections, and which in-clude selectively controlling, by heating or cooling, the temperature of the isolated sections in response to film thickness measurements, whereby to improve thickness uni-formity of the film so formed.

Description

~ 3 Thi8 invention relates to the preparation, by extrusion, of ~ilm of a thermoplastic polymer,sld more partlcularly, to controlling the gauge profile of such ex-truded film.
mere ha~e been numerous attempts to improve thick-neæs propertie~ in ex~ruded ~lms. Numerouæ apparatug and methods, for example, have e~olved to sense thickness devlations and effect ad~ustment of film thickness at æpecific locations usually at or near the point of extrusion o~ the polymeric material in the film forming operation.
In some such methods, the ori~ice o~ the extrusion die is increased or decreased in thicknes~ by ad~ustment screws, ~ither manually or with tors, as described in U.S. Patent 3,122, 782 issued March 3, 1964 to E.J. Moore; U.S. Patent 3,122,784 is~ued March 3, 1964 to C.~. Jolli~fe, and U.S. Patent 3,455,175 issued July 16, 1969 to DeSmedt et al. In other such methods, selected lanes of the extruded film are heated after the film has been removed ~rom the quench drum but before it iB stretched, as de~cribed in U.S. Patent 3,161,711 issued December 15, 1964 to M.C. Tas~ler and U.S. Patent 3,649,726 issued March 14, 1972 to F.L. Knowle~. In yet another method, heaters are poæitioned wi~h-in the main body of the extruæion die ad~acent the entrance pas-~ageway and distribution manifold for the polymer 1n order to heat areaæ of the die to help maintain the thicknesæ of the æheet substantially constant, as ~escribed in U.S. Patent 3,341,388 issued September 12, 1967 to E.D. Bunyea.
All such ~ethods, however, suPfer from one or more defects, lncluding inadequste control of the film gauge pro-ff le, complexity of the apparatuæ, and non-adaptability to con-trol o~ a large number of Yery narrow laneæ of ~ilm. It is de~irable to have a wa~Y to control the thickness of a film in a large number of very narrow lanes. ; . .
According to the present invention, there is provided

2 _ ;'Ç~

iO4ti~3 a process for controlling the gauge profile of a thermoplastic polymerlc fllm which is made by extruslon of the polymer through a dle having a pair of die lip8 having contlnuous surfaces, by adJusting the temperature of the molten poly-mer ln the dle in respon~e to a measurement of the film thick-ness, characterized in that the temperature of the molten poly-mer, by means of a heater element positioned in each of a plurallty of thermally i~olated closely-spaced sections of the dle lips, i9 selectively controlled in indi~ldual lanes as the polymer moves between the die llp9.
~uring the formatlon of the extruded film, an ex-trudate of controlled gauge ls obtalned wlth apparatus, also provided according to the present invention, that includes an extrusion die having a pair of dle lips each havlng a continuous surface ln opposed relationæhip to each other to deflne an extruslon orifice therebetween, wherein each die lip comprises a plurality o~ closely spaced die lip segments, each o~ the seg-ments contalning an independently controlled heater element and a temperature sen~lng element.
In the manufacture of film for a variety of uses, lt is known to improve its thlcknes 8 characteristics by con-trolling flGw of the polymeric material in the extrusion apparatus Nonetheless, in making such films by many known method~, the film, a~ formed, usually has one or more zones or areas of greater or lesser thickness than the rest of the film (i e., gauge variations~ which are caused by polymer flow lmperfections or variations, which, in turn, create nonuniform patterns of the polymeric material ln the extruding device.
In the extrusion of polymeric materials in web ~orm, it is known that gauge or thickness of the film, its unit weight and the roll ~or~ation character~stics of the web, sheet or tube, are all directly controlled by or deter-mined by the uniformity of flow of the polymeric material 2~3 through the extrusion die and other parts of the extrusion apparatus, particularly the die lips. For e~ample, when the web is wound, any continulng~ standing gauge variation would lie one atop the other so that ln cumulative e~fect a hard band is built up, which distorts the roll, thereby impairing its utility and causing waste The manufacture of thin gauge polymeric film, for example, from oriented polyester, polyethylene and polypro-pylene material, u~ually commences with the formation of a relatlvely thick web of material extruded in a thermoplastic condition from a narrow elongated extrusion orifice of a flat plate extrusion die. The molten polymeric material is deposited onto a smooth, cooled, endless moving quenching surface, such as a stainless steel conveyor band or a rotary drum and solidified. The hardened material in the form of -~
a web of film is subsequently removed from the cooled surface and forwarded into a stretcher apparatuæ where it is reheated to just above its glass transition temperature and stretched longitudinally and transversely to the desired thickness. From there, the thln gauge web of film i9 sent to other processing ~tations where it may be given a variety of surface treatments depending on the end use, then suitably slit and wound lnto packages or rolls~ in accordance with customer specifications.
The ~uperior physical properties of oriented poly-ester material are now being exploited in the form of high strength ultra-thin films, These films, ranging from 0 005 mm.
(0 002 in ~ to 0,025 mm, (0,0010 in ) thickne~s~ are finding widespread use in magnetic recording tapes, electrical appli-cations, surface coverings and general thin film packaging.

3 As the material is made in the ultra-thin range, it has been lO~jZ~3 found that gallge nonuniformities become increaslngly critical in the formation of a high quality product Specifically, small variations such as narrow gauge bands and 3treaks here-tofore tolerable are now exceedingly troublesome and defy correction on conventional extrusion apparatus by mere ad~ust-ment of the d~e lips, for example. The origin of these streaks and gauge band~ can be traced to numerous sources that in some instances may include the polymer preparation and transfer systems as well as the extru~ion die apparatus itself. Stlll other sources of these nonuniformities are thought to be the re~ult of sporadic surges in the pumping ~ystem, uneven heating of th~ molten material and, even, inherent dif~erenceæ in the composition of the polymeric material.
This invention i8 directed to a method of and appa-ratus for making film by extrusion and, more particularly, to a method of and apparatus for controlling the temperature of molten polymeric material in a novel manner in the extrud-ing device by the use of novel isolated die lip parts which assures controlled flow of the polymer and also sub~tantial film thlckness uniformity.
By following the method of this invention and by using the novel polymer flow controlling device thereof, polymeric films of excellent properties and gauge uniformities may be produced capable of meeting untold numbers of use requirements, Fig. 1 shows an extrusion die of this invention wlth vernier heated die llps tied into an automatic closed loop gauge control system (with parts omltted for clarity~ for eontinuous high productlvity operation~ The system includes 3o a traversing web thlckness 3canner, an analog-to-digital 10~6~13 signal converter, a temperature feedback means on the heated die lips, a power supply lnterface and a digital computer.
In the ~ystem, the computer receives web thickness varistions and temperature readings from the individual heated parts of the dle lip~, differentiates the variatlons relative to a desired thickness profile and initiates corrective signalæ
to the power interface which regulates electrical power to the heater elements Fig. 2 i~ an isometric view of a preferred embodi- -ment of the extrusion dle of this lnvention (with parts omitted for clarlty~ showing thermal die lip part~ separated by dead air spaces and minimum thickness sections along the die lips which serve thermally to i~olate the thermal lip parts from each other and the main body of the die.
Fig. 3 shows a typical thermal boundary layer formation through the molten stream of polymeric material as it moves, unidirectionally7 through the eontrolling temper-ature zone wherein the heaters are positioned ad~acent the die lip surfaces.
Flg. 4 is a partlal front view ~howing a close-up o~ the frontal area of the top and bottom body members.
Fig. 5 shows an extrusion die of this inventlon with full web width pinnlng means, along wlth knowr. parts of a film making apparatus lncluding machine and cross-machine fllm stretching meanæ and web windup me~ æ
Referring to the drawings, there is shown an appara-t~ of this invention for feeding flowable molten polymeric material and extruding lt in the form of a web of polymeric material to provlde film, such as polyethylene terephthalate fllm, having enhanced physical properties ~04~f~3 Referring to Figs, 1-3 ln particular, it will be seen that polymeric materlal M is supplied to the apparatu~
of thi~ invention from a supply source, not shown, and moved or fed into a die adapt;er through which the material flows to a novel extruslon die 10 of this invention, from which it i~
extruded in the form of a web W of polymerlc material.
The extruQion die 10 includes die lips 11 which define an extrusion orlfice 12, with each llp 11 including a plurality of closely spaced heated llp sections or part~ 13.
Each lip part 13 includes a heater element 14 and a temperature sensor 15 which is connected to and independently controlled by a regulated power supply 16 so that each lip part 13 is substantially thermally isolated from the other. Prlor to a productlon run, the area of influence of each thermal lip part 13 on the web W of polymeric material is identified and calibrated, Power level to selected heated lip parts 13 i8 then manually adjusted by an operator with the aid of a suitable web gauge measuring device 17. One such device commonly used and commercially available i8 a beta-ray ~canner which is mounted on a traversing mechanism located at æome distance downstream of the extrusion die, preferably at a point after the web W of polymeric material has been solidified into a web of fllm F, The scanner læ moved back and forth across the width of the web of film F and records thickness variations which can be displayed conveniently as a thickness pro~le on a cathode ray tube. By reference to the proflle trace, the operator makes appropriate adjustmentæ to the power level of the heaters influencing the area of the gauge variation.
3 The adjustment of the power level can be either - 104~ ~13 to increase or decrease the temperature of the selected thermal lip part 13 in accordance with the type of defect belng corrected~ By ad~ustment of the power level upward, the temperature of the molten stream opposite the selected thermal llp part 13 is raised. This causes a localized decrease in the melt vi~cosity of the molten material M and a corresponding local increase in the mass flow rate, Because the residence tlme of the material ln contact with the high temperature zone is very brief, no significant degradation of the polymer occurs.
Depending on the compo~itlon and behavior of the polymeric material and the desired gauge correction, the heating geometry c~n be tailored either to increase or to decrease the thickness o~ the thermal boundary layer or the entire thickness of the material can be raised to an ele-vated temperature.
Preferably, the extrusion die 10 is operated with all the thermal lip parts 13 initially adjusted to a uniform temperature level slightly higher than the inlet temperature of the molten stream. The temperature of one or more thermal lip parts 13 opposite a web defect can then, as in the case of a thickened gauge band, be reduced causing a localized increase in the relative ViSCoQity of the material and a corresponding localized decrease in the mass flow rate. Gauge correction can be obtained by either heating or coollng the thermal lip parts 13 with good results, The gauge control concept is particularly well suited for use in an automatic closed loop system controlled by a digltal computer prevlously programmed with a desired web thickness profile. As shown in Fig. 1, such a system lO~Z13 include~ the aforementioned extrusion die 10, a PDP-8/E
computer 18 made by the Digital Equipment Corporation which is connected through analog-to-digital converters l9a and l9b to the outputs of temperature sensors 15 and the beta-ray scanner 17. Computer 18 receives the input data, compares the information with the de~ired thickness profile previcusly programmed into its memory and calculates the amount of electrical power that needs to be supplied to the vernier heater elements 14 to achieve the deslred gauge profile.
The power ls distrlbuted by the co~puter 1~ through a power supply lnterface 16 which i8 connected to the heater elements 14 and to a standard current source. Power supply interface 16 ls connected to the heaters 14 and to the computer 18 through solid state switching circultry which enables the computer 18 to regulate the power level individually and collectively to the heaters 14.
In a preferred method, the computer 18 ls pro-grammed to distribute through the power ~upply interface ;6 an initial uniform power level to all the vernler heaters 14.
This establishes a heated zone 21 having surface temperatures adjacent the molten polymeric material M about 10C above the entr~nce temperature of the materlal. As best shown in Fig. 3, movement through the heated zone 21 produces a boundary layer 22 along the top and bottom surfaces of the viscous material M and a higher local mass flow rate. Where a gauge band or streak develops, the control system corrects the defect as follows: The computer 18 receives both the thickness variation signal from the scanner 17 and the tempera-ture from the thermal lip part 13 opposite the defect. The 3 computer 18 then compares the variation~ ln thickness with -` lO~Z13 the desired programmed proflle and calculates the amount of correction in the power level necessary to change the mass flow rate of the material M in the defective region in accordance with previously determined values A power level slgnal ls then transmitted by the computer 18 through the p~wer supply interface 16 to the selected heater element 14.
As previously mentioned in the preferred method, the corrected power u~ually amounts to a reduced input which has the effect of cooling the thermal section 13. This cooled section 13 produce8 a localized cooling of boundary layer 22, a corres-ponding lncrease in the vlscosity of the material and a reduced local mass flow rate. In actual practice, there are several methods of smoothlng out a gauge band The aforementioned technique of reducing power to the heaters 14 ln the locale of the gauge band and, thus, decreasing the local mass flow rate ha~ an equally effective conterpart. The thickened gauge band may also be smoothed out by increasing power to all the thermal lip parts or sections 13, except those opposite the gauge band, thereby redistributin~ the excess flow of the gauge band substantially across the extrusion orifice 12.
The method of smoothing out gauge bands will depend on the amount of gauge correction, the existing set of extrusion conditions, including extrusion rate, draw rate and accuracy of gauge. Thus, it will be seen that surface defects (rese~bling peaks and valleys~ are read~ly corrected by either heating or coollng the thermal sections 14, selecti~ely In the preferred embodiment the extru9ion die lO
comprises top and bottom body members 23 and 24 which are essentially rectangular-sh~ped blocks made of a dimensionally 3o stable metal alloy suitably doweled and bolted together . .. ~
. , .

along a common interface, not shown.
Along the interface, the die 10 iæ provided with a polymer passageway 29 that terminateæ downstream a~ the narrow elongated extrusion orifice 12. The passageway 29 consi~ts, in sequence, of an inlet and a distribution header, not shown, a flattened preland 31 and, finally, a constricted land 32, The distrlbution header, of a type kno~n to the art, performs the function of distributing the molten polymer acros~ the width of the die. Preland 31 per~orms the functlon of insurlng substantially unidirectional flow (i.e., machine direction flow) of the material M prior to extru~ion; whlle the land 32 establl~heR the basic gauge of the web W of material extruded through the extrusion orifice 12. By establishing such unidirectional flow in the preland 31, the subsequent selected heatlng of the required lane of the molten material is ef~ective slnce it8 location can be determined by in-line reference to the deviating lane in the film, The passageway 29 is connected to a continuous source of molten polymer (not shown~ from which suitable polymeric compo~itions such as polypropylene, polyethylene, polyamides and, preferably, polyesterc are readily processed through the die The extrusion orifice 12 essentially i8 defined by inner surfaces 35' and 36' of the die lip9 11 (for clarity the upper and lower die lips are also de~ignated as 35 and 36~
which, in turn, define the preland 31 and the land 32. Ad~ust-ment bolts, as known to the art, may be used for preliminary gauge ad~ustments, if required.
Each die lip 35 and 36, as previously described, is lO~tjZ13 divided into a plurality of closely spaced lip thermal sections or parts 13 each containing 6.35 mm (0.25 in.) diameter car-tridge-type re~istance heaters 14. Each thermal lip part 13 is separated or isolated from its neighboring lip part by a narrow open slot or alr space 41. Adjacent each heater 14 is the -~
thermocouple-type temperature sensor 15. In the p~eferred em-bodiment, a 20-inch long orifice 12 contalns fifty-two sections 13.
As best shown in Fig, 2, each lip thermal part 13, for all intents and purposes, is thermally isolated from other llp part~ by lip parts isolating meang (i.e., the slots 41) which extend into the lip9 35 and 36 until only a thin rib or llp sectlon connecting means or parts 42 connects the ad~acent lip parts 13 thereby defining the continuous lip surfaces 35' and 36~.
At the base of each isolated lip part 13, the air spaces or slots 41 terminate into enlarged circular apertures 43, whlch are remote from the frontal area of the die, and each of which is separated from the other by a thin rib 44.
The connecting partæ 42 maintain the lip parts 13 as an integral part of the respective lips and assure that a smooth continuous surface 35' or 36' i~ exposed to the molten material M. Conversely, the ribs 44 serve as heat dams to reduce the heat flux into the main body of the die.
This is a critical feature of the apparatus and method of this invention for without the interposed dead air spaces 41 and the reduced ribs or connecting parts 42 and 44 the outputs of the heater elements 14 would be rapidly dissipated into the main portions of the body members 23 and 24 of the die and into ad~acent lip part~ producing diminished l~ne -lO~Z13 re~olution and prohibitively long response time. With the aforementioned configuration, the temperature of a typical thermal lip part 13 can be varied by as much as 25C be~ore a 1C rise i~ attained in the adjacent lip parts, The specific conditions employed in the extruslon apparatus of this invention depend on the nature of the material being extruded, as are known to the art. For example~ in ex-truding polyethylene terephthalate material, the cond~tions re-quired for doing this are set forth ln U S. patent 2 828 421 issued March 25, 195~ to A. Stecker et al.
While the extruslon apparatus described above can be satisfactorily employed in casting a web of film at slower rates, higher production rates may require selected pinnlng means as adjuncts for effective guenching and gauge profile control, as described in U.S. patent 3 ~83 279 is~ued May 13, 1975 to D.E.
Heyer. In the invention of such patent, the web material W
is fully pinned across its width, as by an electrost~tic pinning wire extending across the full width of the web, as disclosed in U.S. patent 3 223 757 issued December 14, 1965 to J.E.
owens et al. and U.S. patent 3 o6~ 52~ issued December 1~3, 1962 to J.E. o~ens, or by a pneumatic pressure pinning method as dis-closed in U.S. patent 3 779 682 issued December 18, 1973 to Huskey et al, In methods employing these type~ of quenching as~ists, generally designated 50, the web W is sufficiently well anchored throughout its width to a typcial moving quenching surface S (i,e., to quenching ~urface S of quench-ing drum 40, as ~hown in Fig. 5) to prevent any transverse movement o~ the polymer mass. This assures that the location of the proper lane or lanes in the molten material may be 04ti;~13 accurately determined by in-line reference to the devlating film lane or lanes, Thus, the response to the corrective action of a ther~al gradient is essentially that which would be expected from vi9c08ity changes in the flowing polymer.
The following examples show quantitatively the degree of gauge control obtainable by the vernier heated llp concept, Each example describes a series of tests made with a standard flat sheet extrusion die similar to a Series 102 unit manufactured by F,W, Egan Company, The die was connected to a continuous source of molten-polyethylene terephthalate polymer and operated in a normal manner in conJunction with a chilled rotary castlng drum, The vernier heating concept of this invention was only incorporated at the mid-center region of the upper dle lip a~ thirteen thermal lip parts spaced 9,5 mm (3,8 in,) apart, Each lip part or sectlon was thermally isolated from the next in the manner previously descrlbed and was adapted with a cartridge type heater and thermocouple sensor, The heaters were connected to independently controlled power units ~imllar to the type manufactured by the Electronics Control Systems, Inc, At start-up~ prior to measurement of the film, the extrusion orifIce opening o~ the die was mechanically ad~usted to a 1,5 mm, (60-mil) gRp height, Polymer throughput was ad~usted at 181,4 kg, (400 pounds~ per hour under a shear rate of 230 reclprocal seconds across the land portion of the die, The drum on which the molten web was solidified was ad~usted to a speed o~ 28,5 m, (31 yards) per minute so as to lnduce an approximate 10:1 draw ratio and thereby form a 0,18 mm (0,007 in,) thick cast film, These settings were then held 3o con~tant for each test run which are described below, Example 1 1046Z13 Initial casting of the film was commenced with the heater power off so that the die and die lips were stabilized at 2~5C, with power to the vernier heaters off. A profile trace of the cast film thickness was determined. The trace was obtained by an electromicrograph type thickness gauge similar to the type manufactured by the Pratt and Whitney Company.
Heaters H-l through H-13 were then energized raising the temperature of each heated lip part to about 20C, above the initial temperature o~ the molten polymer, After the system stabilized at these conditions, a gauge profile trace was made of the cast film, The power level to heaters H-5 through H-9 was then ad~usted to increase the temperature of the corresponding lip part 10C. Following a stabilization period of 10 to 15 minutes, a profile trace of the cast film was made. The power level to heaters H-5 and H-9 was again read~usted raising the lip temperature another 10C, increment and another profile trace of the cast film was made, The above procedure was repeated at 10C intervals until a final lip temperature of 400C. was attalned, After obtaining a final profile trace, the power level to heaters H-l through H-13 was then cut back, lowering the temperature of the lips to 315C, Analysis of the proflle traces showed that the 0,18 mm (0O007 in,~ thick cast film increased at the mid- ;
section (directly opposlte the heated lip portion~ in thickness 0.0236 mm, (0,00093 in.~, a 13.3~ increase over a 115C. change in temperature, This represents an average change in gauge of 0.11% per C, temperature rise that can be induced by heated lips, The test runs also revealed . . . . . .
.

_~ 104~iZ~3 -the rate of heating was nonlinear and rapid with a rate up to 60% goal temperature taking place at 0.344C. per second and from 60~ to 90~ goal temperature at a rate of 0.177C.
per second. Conversely, the tests showed the rate of cooling was considerably higher than the heating rate. When the power to the heaters wa~ reduced, the temperature of the lips cooled at a rate of 0.407C. per second at 60% goal temperature and 0.252C. per second at 90~ goal temperature. The faster coollng rate ls believed to be attrlbuted to the high heat absorblng capacit~ of the molten polymer stream.
Example 2 This example illustrates the narrow or flne lane resolutlon capability of the heated llp concept of this invention. The equlpment and ca~tlng condltlons were substan-tially identical with those of Example 1.
An ~nitial gauge profile trace of the ca~t film was made wlth the heaters off after the die and die lip temperature was stabilized at 285C. for control purposes.
Power to heaters H-l through H-13 was then turned on and the -temperature of the llp elevated to about 330C. A proflle trace of the cast film was then made after the system stabilized. Power to heater H-7 was then raised ln incre-mental amounts 50 that the temperature of the indivldual thermal ~ection contlnually increased. Simultaneously, the temperature of thermal sections corresponding to heaters H-6 and H-8 was monltored. The temperature of the heater H-7 thermal section was raised untll a 1C. change ln temperature of heaters H-6 and H-8 was detected. Thls test shows that a temperature dlfferential of 25C. could be lmposed between ad~acent thermal llp sections or parts without a disturbing influence greater than 1C. Thus, at a typical heating rate inducing a gauge change of 0.11% to 0.22~ per C., it is po~sible to induce a 2% to 5% varlation in gauge correction ~rom lane to lane.

Claims (10)

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

1. A process for controlling the gauge profile of a thermoplastic polymeric film which is made by extrusion of the polymer through a die having a pair of die lips having contin-uous surfaces, by adjusting the temperature of the molten poly-mer in the die, in response to a measurement of the film thick-ness characterized in that the temperature of the molten poly-mer, by means of a heater element positioned in each of a plurality of thermally isolated closely spaced sections of the die lips is selectively controlled in individual lanes as the polymer moves between the die lips.

2. The process of Claim 1 wherein the flow of molten polymer within the die is established substantially exclusively in the machine direction prior to selectively controlling its temperature.

3. The process of Claim 1, wherein the temper-atures of the individual lanes are selected so as to make a film of substantially uniform thickness across its width.

4. The process of any one of Claims 1 to 3, wherein at least one selected lane of polymer is heated.

5. The process of any one of Claims 1 to 3, wherein at least one selected lane of polymer is cooled.

6 The process of any one of Claims 1 to 3 wherein the measurement is carried out in said individual lanes before transverse stretching or after transverse stretching.

7, Apparatus for carrying out the process of Claim 1 including an extrusion die having a pair of die lips each having a continuous surface in opposed relationship to the other to define an extrusion orifice therebetween and film thickness measuring means for measuring the thickness of the film in individual lanes across the width thereof, wherein each die lip is divided into a plurality of thermally isolated closely spaced sections, each of which contains an independently controlled heater element and a temperature sensing element.

8. The apparatus of Claim 7, wherein adjacent seg-ments of each die lip are separated by a slot having inner and outer portions, the inner portion positioned adjacent to and spaced from the surface of the die lip defining the extrusion orifice.

9. The apparatus of Claim 8, wherein the portion of each slot remote from the frontal area of the die terminates in a circular cylindrical portion having a diameter greater than the thickness of the remainder of the slot.

10. The apparatus of any of Claims 7 to 9, wherein the film thickness measuring means is adapted to transmit a signal that is a function of the thickness, and wherein there is additionally provided control means adapted to receive signals from the temperature sensing elements in the thermally isolated closely spaced sections of the die lips and from the film thickness measuring means and adapted to regulate a power supply to each heater element with reference to a preferred film gauge profile,