CA1060172A

CA1060172A - Process for producing polymeric film and apparatus therefor

Info

Publication number: CA1060172A
Application number: CA240,492A
Authority: CA
Inventors: Masami Etchu; Hideaki Watanabe
Original assignee: Teijin Ltd
Current assignee: Teijin Ltd
Priority date: 1974-11-27
Filing date: 1975-11-26
Publication date: 1979-08-14
Also published as: DE2553069C3; DE2553069B2; FR2292570B1; DE2553069A1; BR7507892A; FR2292570A1; GB1523629A

Abstract

Abstract of the Disclosure The invention relates to a process and apparatus for preparing a polymeric film which is free from thickness unevenness along its entire width the thickness control being affected by temperature regulation of molten polymer immediately before extrusion from a die slit, the temperature regulation being made along the whole width of the die.

Description

This invention relates to a process for produc~
ing 3 polymeric f`ilm, and an apparatus for producine the film. More specifically, the invention relates to a process and an apparatus for producing a po:Lymeric fi~m, characterized in that the tempera~ure of a molten polymer immediately before extrusion from a die slit is controlled along the whole width of the die in order to obtain a polymeric film having uniform thickness along its entire width. ~ ;
When the clearance between die lips at a die slit is rendered constant along the whole width of the die in an attempt to obtain films of uniform thickness ; from a molten thermoplastic polymer by extrusion, films !
having the intended uniform thickness cannot always be prepared. This i8 because the flowability of the molten polymer fluctuates locally within the die. Various methods have therefore been suggested for producing films of uniform thickness by extrusion.
The first suggested method involves adJusting -the die lip clearance by a mechanical means, for example, using an adJusting bolt so as to render the film thickness uniform along its whole width. This technique, however, is unable to ad~ust minute changes of film thickness in the widthwise direction because of a structural restriction of the material that constitutes the die ` lips. Especially when the changes of the film thickness occur drastically at short intervals, adjustment of the die lip clearance by fastening or loosening a nut on a bolt is unsuitable. Furthermore, this technique cannot , `1 ~

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be applied to the adJustment of the die lip clearance in a continuous automatic operation and in a remote control operation.
With a view to remedying this defect o~ the prior technique, improved methods were sugeested in ,~
Japanese Laid-Open Patent Publication ~o. 16975/73 (corresponding to United States Serial No. 157,216) ~ and Japanese Laid-Open Patent Publication No. 45964/74 ; (corresponding to United States Serial ~o. 280,890) to render the thickness of the extruded film uniform along ~
its entire width. These methods comprise controlling the ~;
temperature of the die slit portion locally by heating ~ ;~
means thereby to locally change the temperature of the molten polymer to be extruded from the die slit, and con-sequently, locally changing the apparent viscosity o~ the molten polymer to adjust the amount of extrusion locally. ~;
These methods do give rise to a great improvement over the first-mentioned conventional technique, but still suffer from various disadvantages. For example, thickness uneveness inherent to these methods sometimes occurs newly, and the methods are not sufficient for the adJustment of minute thickness unevenness that occurs at short intervals.
Furthermore, in an automatic production, a time lag will occur between the detection of the thickness unevenness . -~ . .
~; and the time at which the heating means effectively ~ operates for the adjustment of film thickness upon ; response to the detection of thickness unevenness.
~ These disadvantages are more spsci~ically described below.
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I Firstly, since the adjustment in accordance 5 ~
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with these known methods depends exclusively upon heating means, it is extremely difficult to reduce the thickness of the fi.lm. According to these methods, predete~mined amounts of heat are supplied to all of the heating means, and while extruding a molten polymer into a film form, the amount of heat is reduced at tha1; portion near the die slit which corresponds to a thicker portion of the film, and is increased at that portion near the die slit which corresponds to a thinner portion of the film, thereby to render the thickness of the extruded film uniform. In this case, besides the main heating `
means, auxiliary heating means for thickness unevenness adJustment provided in the vicinity of the die slit partly serve to maintain the die slit at a pre-set standard temperature (base temperature). When a portion having a larger thickness is discovered in the extruded film ~y a thickness tester, the amount of h~at as-cribable to the auxiliary heating means located in the vicinity of the die slit needs to be reduced until that portion has the same thickness as the intended thickness.
In order to reduce the amount of heat by a sufficient degree, it is necessary to pre-set the initial amount of ~ ;
heat (e.g., the amount of electric power) on all of the auxiliary heating means so that the temperature provided will become somewhat larger than the pre-set temperature of the die. However, when the ~mount of heat by the auxiliary heating means is increased, thickness unevenness occurs newly at positions corresponding to the positions of these auxiliary heating means. This is because the - 4 - ~ -~. , . . . ~

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heating is insufflcient at an intermediate part between the adjoining auxiliary heating means. The new thickness ;
unevenness occurs more with larger amounts of heat to be i~
supplied to the auxiliary heating means, and with nearer distances between the auxiliary heating means and the path `
of the molten polymer.
Alternatively, a method is employed in which the non-uniformity of the temperature is reduced, and the thickness of the film is rendered uniform, by reducing the amount of heat supplied to the auxiliary ; heating means or by disposing the auxiliary heating means :
as far as possible from the path of the molten polymer.
The first-mentioned method results in the narrowing of the range of ad~ustable thickness unevenness. The latter method lends itself to difficult precise ad~ust-ment of local thickness unevenness because of heat diffu-sion, and has the defect that the speed of response of :, . :
;~ adjustment becomes slow ~in other words, the time required from the detection of thickness unevenness until : ' in response to it, the ad~usting means operates effectively ~ ;

is long.) . ~ . . .
In order to reduce the thicker portion of the , film, a method is also performed in which the amount of ;
heat is increased only in that auxiliary heating means which corresponds to the thinner portion of the film without adjusting that auxiliary heating means which corresponds to the thicker portion. However, according ~ ;~
to this method, other auxiliary hea~ing means than the corresponding auxiliary heating means must be heated only `~

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to adJust the thickness of thicker portion, and the method suffers from the defect of the complexity of ; operation, poor precision, and slow response. Further- ~;
more, since the amount of heat is increased over a wide range, thickness unevenness appears newl;y corresponaing to the arrangement of the auxiliary heating means.
Secondly, the conventional methods basically cannot be free from the slow speed of response in the adjustment of the thickness of a portion having a large thickness. In these methods, the lowering of tempera ture for the adjustment of a portion having a larger thickness depends only on the amount of reduction of the heat to be suppliea to the auxillary heating means or the stopping of heating, and no positive cooling is carried out using a cooling means. Since cooling by spontaneous heat releasing is time-consuming, the speed ~
of response is greatly retardea. This hinders the forma- ;
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tion of an automatic control loop in automatic production. ~ ;~
Thirdly, according to the conventional methods, the precision of adjustment is limited. Since the methods rely on spontsneous cooling without using a cooling device, heat diffusion occurs and the local temperature gradient cannot be made greater. This poses a problem in adjust~
ing minute thickness unevenness which occurs at short ~! intervals in the widthwise direction of the film.
It is an object of this invention to provide a process for producing a polymeric film by extrusion which has uniform thickness along its entire width.
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Another object of this invention is to provide ~ ;
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a process and an apparatus for preparing a polymeric film whose thickness uneveness can be very rapidly adjusted in response to its detection by a thickness tester.
; We have now discovered a process for extruding a polymeric film having uniform thickness along its entire length while controlling thickness unevenness, whlch comprises continuously extruding a molten polymer into ~ `
. a film form from a slit of a die while maintaining the temperature of the molten polymer within the die substantial-. ly constant, continuously measuring the thickness of the extruded film along its entire width by means of a thickness -~
tester, and when a portion having a larger thickness than the desired thickness is detected, locally cooling that ~, portion near the die slit which corresponds to the detected ; portion, and when a portion having a smaller thickness than the desired thickness is detected, locally heating ;~
1 that portion near the die slit which corresponds to the ,"J detected portion, thereby to equalize the amount of the . 20 molten polymer extruded from the die slit along the :. entire width of the die. ~
According to this invention, there is also ; `
provided an apparatus for producing a polymeric film free from thickness unevenness along its entire width, comprising a pair of die lips defining a slit of a die, a main heating means for maintaining the temperature of . a molten polymer within the die substantially constant, :~ auxiliary heating means for locally elevating the tempera~., ture of the molten polymer immediately before e~trusion ; ;

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through the die slit, and cooling means for locally lowering the ~ :
temperature of the molten polymer immediately before extrusion through the die slit, said main heating means being disposed sufficiently remote from the die slit to maintain the temperature oP a molten polymer within ~ :
-the die substantially constant, and said auxiliary heating means and said ~ ~
cooling means being disposed in a plurality of ho:Les provided along -the ~ :
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entire width of the die in at least one row at substantially equal intervals and being operable independently from each o-ther.
According to another aspect of this invention there is provided a pxocess for preparing a polymeric fiLm free from thickness unevenness along its entire width, which comprises continuously ex-truding a molten polymer into a film form from a slit of a die while maintaining the molten polymer within the die at a subs-tantially constant temperature, continuously measuring the thickness of the extruded film along its en-tire width b~
S means of a thickness tester, and when a portion having a larger thickness than the desired thickness is detected, locally cooling that portion near the die slit which corresponds to the detected portion, and when a portion ~ ~ :having a smaller thickness than the desired thickness is detected, locally ;~
.~ heating that portion near the die slit which corresponds to the detected portion, thereby to equalize the amount of the molten polymer extruded from the die slit along the entire width of the die. .
~he process of this invention can be applied to processes for ~
preparing orainary polymeric films using a die of the type adapted to ~`
extrude a molten polymer from a die slit through a pair of die lips. :
~ In principle, the process of this invention is applicable to : any thermoplastic polymer, examples of which are polyolefins such as ``. polyethylene or polypropylene, polyes-ter such as polyethylene terephthalate, polytetraethylene terephthalate or polyethylene-2,6-naphthal~te, polyamides such as nylon 6, nylon 66 or nylon 10, polycarbonate, and polyacetal. . `
In the process of -this invention, a molten polymer maintained ~

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:, '.':'~ ~ ., , . ' ': ' ' ~ ~ .' ', '' ' at a predetermined temperature within the die flows down between a pair of die lips and is extruded from the die slit. By locally adjusting the temperature of the vicinity of the die slit so that the amount of the molten polymer extruded becomes constant along the entire width of the die, the temperature of the molten polymer immediately before extrusion is locally changed. The temperature ~; , of the molten polymer is either elevated by heating or lowered ;~
by cooling. Specifically, a required number of auxiliary heat-ing means and cooling means are provided along the entire width of the die in the vicinity of the die slit~ and an auxiliary ~;
heating means or a cooling means at a position corresponding to that portion of the extruded film which has thickness unevenness iB operated to locally change the temperature of ; the molten polymer immediately before extrusion. This causes the chanee of the viscosity flow of the molten polymer, ~nd '~
renders the amount of the molten polymer extruded constant along ~ ~:
the entire width of the die slit. Consequently, films having equal thickne~s along their entire width can be obtained.
A heating medium or electric heating can be : ~"
used as the main heating means. The main heating means ;(, is adapted to heat the entire die uniformly at a position remote from the die opening. Consequently, the amount ` ;
of heat to be supplied to the molten polymer becomes ~ substantially equal along the entire width of the die, - and the molten polymer is maintained at a substantially constant temperature. ;
The auxiliaryheating means and cooling means for adJusting the thickness of film are designed such that the amount of heat for heating or cooling can be ;
minimized and they are arranged as near as possible to the inner surface of the die over which the molten polymer `~

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flows down. In consequence, the local heating or cooling of the molten polymer can be performed easily and rapidly. ;
Electric heating is suitable as a heat source for the auxiliary heating, and a low temperature fluid such as air or water is suitable as a cooling medium. It is essential in the present invention that the time required ~ ;
for heating or cooling that part near the die slit which corresponds to the thickness unevenness of the film should :, , be as short as possible. To achieve this end, a nubber ``
of holes are provided along the entire width of the die in the vicinity of the die slit, and these holes receive the auxiliary heating means and cooling means therein. ;
The holes can be provided in at least one row alorg the width of the die in various formations, for example, . ., ,: .
in a straight_line or zig-zag fashion. Preferably, the end of each hole approaches the molten polymer (to a position several millimeters from the die lip surface) ~;~
so as to permit an easy local adjustment of the temperature of the polymer It is essential that the auxiliary heating means and cooling means should be operable independently from each other. When a thinner portion is detected in the extruded, that portion near the die slit which corres-ponds to the detected portion is heated to increase the flow rate of the molten polymer passing that portion, and thus to enable thickness unevenness to be removed from the film subsequently extruded. Furthermore, when a ;~
thicker portion is detected in the extruded fil~s~ that ~-portion near the die slit which corresponds to the detected ``
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portion is cooled to reduce the flow rate of the molten polymer passing that portion and thus to enable thickness unevenness to be removed from the film ~ extruded subse~uently.
: As a result of providing the cooling means, the larger thickness of the extruded film can be positively reduced. Accordingly, in the present invention, at an initial stage of production where no adjusting operation has yet been started, the temperature of the molten polymer can be main-tained by the main heating means alone. Since the main heating means is provided at a position sufficiently remote from an extrusion exit for the ; 10 molten polymer, the temperatures of the die and the die opening can be main~
tained substantially constant by heat diffusion without the formation of local non-uniformity in temperature.
The extruded film before the starting of the ad~usting operation may possibly have various thickness unevennessos because of the non-uniformi- -; ty o~ the molten polymer in the die or slight stresses exerted at the die 1 ~;
slit. In the present invention, the use of the main heating means can main- ! ' ~ tain the molten polymer within the die at a substantially constant standara - temperature (starting temperature). The film is extruded at a temperature ~ -~ near the standard temperature. When a thickner portion is detected in the -"`'J` 20 extruded film, the thickness of the film can be made uniform by positively ~`
cooling that portion near the die slit which corresponds to the detected portion. On the other hand, when a thinner portion is detected in the extruded film, the thickness o~ the film can be made uni~orm by heating that portion near the die slit which corresponds to the detected portion.
In the present invention, the speed of response in thickness adjust-ment is rapid both in the case of heating and cooling. As a result of uti- -~
lizing positi~e heating and cooling in this invention, the effect of temper-ature ad~ustment extends over a wider range, and the ad~ustment can be so `~ precise as to be able to cope with minute changes in the thickness of the i 30 film in its widthwise direction.

`,'~'! Since in the present invention, the tempera~ure of the dle is ;i maintained uniform by th~ main heating means, and this temperature is made .~
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a standard in adjusting thickness unevenness of the film, the auxiliary heating means and cooling means can be provided sufficiently near the die lip surface over whiCh the molten polymer passes. In addition, no problem of thickness unevenness incident to it arises.
The invention will be described in greater detail by reference to the accompanying drawings in which:
Figure 1 is a schematic cross-sectional view of an apparatus for producing a polymeric film in accordance with this invention, Figure 2 is a cross sectional view of a temperature adJusting lO element as one example of auxiliary heatlng means and cooling means;
Figure 3 is a graphic representation illustrating the relationship between the degrees of heating and cooling and the changes in the thickness of the film;
Figure 4 is a graphic representa-tion showing the rates of heating and cooling;
Figure 5 is a schematic view illustrating the changes of film thickness in the process of this invention and the conventional method; and ;-7 Figure 6 is a cross sectional sketch showing another embodiment of the film producing apparatus used in this invention.
As shown in Figure l, a main heating means 33 for heating a die 31 as a whole is provided at a posi-tion sufficiently remote from a die slit 32.
A molten polymer P is maintained at a substantially constant temperature, and a~ter flowing down between a pair of d~e lips 30, is extruded into a film from the die slit 32. The tem~erature of the molten polymer P is ad~usted by auxiliary heating means (not shown) and cooling means (not shown) which are inserted in holes 34, 35 and 36 provided near the die slit in one or more rows (three rows in the drawing) along the entire width of the die.
One exa7~ple o-f the auxiliary heating me&ns and cooling means to be ~ `~
30 inserted in the holes 34, 35 and 36 is shown in Figure 2. Figure 2 shows a temperature ad~usting element (to be referred to as a cartridge heater) 40 including an electric heater 43 therein and being provided with a hole 41 .

i~V~7~ -at its center and grooves 42 at its per1phery. When the cartridge heater 40 is inserted in the holes 34, 35 and 36, the hole 41 and the grooves 42 `;
form a passageway for a cooling medium.
An experiment was conducted using a die in which along the entire width of the die, holes 34 and 36 are arranged at intervals of 15 nm and holes 35 are arranged at intervals of 40 mm. The results obtained are de-!` " ~ .
scribed below.
In this experiment, ordinary polyethylene terephthalate was meltedat a temperature of 280C., and extruded at an extrusion rate of 15 ~g/hour into a non-stretched sheet having a width of 410 mm and a thickness of 160 , microns at a take-up speed of 3 meters/minute. A cartridge heater of the type shown in Figure 2 was inserted in each of the holes. As auxiliary heating means, the electric heater 43 included in each of the cartridge heaters was used, and as cooling means, air at room temperature (25C) was blown by a noz~le into the holes 41 and the erooves 42. The degrees of heating and cooling in these holes are plotted in Figure 3 against the resulting changes in film thickness. In Figure 3, the axis of ordinates represents the amount of change in film thickness in percentage based on the average thickness of the film as a whole. The axis of abscissas represents :; 20 the degree of heating or cooling, the right half showing the electric power (watt) consumed of the electric heater as the auxiliary heating means~ and the left half showing the amount ~liters/minute) of air at 25C. blown as a cooling medium. me straight lines 34, 35 and 36 in Figure 3 correspond respectively to the holes 34, 35 and 36 in Figure l. As will be seen from Figure 3, the efficiency of adjustment is the highest in the hole 36, and then in the holes 34 and 35 in decreasing order. These differences in ad~usting efficiency seem to be ascribable to differences in conditions such ;~ as the shape of the holes or the positions of the holes. These conditions ¦-~

I also have to do with the range over which the adjustment ls effective, that is to say, the degree of precision of adjustment. Thus, the hole 36 is .~ suitable for adjustment of thickness unevenness occurring at short inter-vals, and the hole 35, for adjusting thickness unevenness occurring at long ';'', . .

intervals (rough adJustment). ~hen the rough adjustment by the hole 35 alone is insufficient, it can be per~ormed also by the main heating means 33.
It can be seen from Flgure 3 that the blowing of air at room tem- ;
perature can reduce the thickness of the film locally, that the cooling of air blow at a rate of 10 liters/minute is comparable to the heating by a 30 watt-heater, and that the effect of adjustment is obtained substantially ; proportional to the degrees of heating and cooling. `-Figure 4 roughly shows the response characteristics of thickness unevenness adjustment which was performed using the hole 36 in the extruding ;~
of films under the same operating conditions as shown above. The axis of ordinates in Figure 4 shows the ratio of change in film thickness after the ~;
end of the adjustment, in which the amount of change in film thickness at a time when the effect of the adjustment becomes saturated is taken as 1.
The curve 10 shows the results which were obtained when air at 25C. was blown at a rate of 10 liters/minute from a copper pipe having an lnside diameter of 2 mm into hole 36 through a passageway formed by hole 41 and grooves 42 of the cartridge heater 40. The curve 11 shows the results which were obtained when the electric power of the electric heater 43 within the cartridge heater 40 was changed to 30 watt. The curve 12 shows the results which were obtained when only one of 30 watt electric heaters inserted in a number of holes 36 was switched off for suspending electricity supply to i ~
allow the film to cool spontaneously in accordance with a procedure taken ~ `
for reducing film thickness in a conventional method.
` ;, The adjusting methoa in accordance with the conventional method :
i' involves a combination of heating adjustment shown by the curve ll and cool- ;
l ing adjustment shown by the curve 12. However, as shown in Figure 4, the ¦ heating adjustment greatly differs in response characteristics from the . 'l ~.
cooling adjustment. In contrast, since the present invention uses a com-bination of the heating adjustment shown by the curve ll and the cooling adjustment shown by the curve 10, the adjustment is performed rapiclly, and :: ' no difference exists in response time.
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Figure 5 shows the outline of a model test for investigal;ing the '.: ` ~ ' , ,: , r limit of precise adJustment of thickness unevenness in the widthwise direc~
tion of film. In Figure 5, A, B, C, D and E represent 5 adJoining holes arranged in the widthwise direction of the die shown in Figure 1 at inter-vals of 15 mm. The curve 13 shows a pattern of thickness unevenness of film generated by the test.
Changes (R%) of film thickness in the process of this invention and the conventional method were comparatively tested, and the results are shown in Table 1. Operating conditions other than those shown in Table l , were the same as hereinbefore indicated.
Table l A B C ~e condit LonsE _ ~ -; Test 1 30 W 30 W O W 30 T~ 30 W about 3.5% ;
. _ _ i 10 Test 2 15 W 15 W 5 1/~1n. 15 W 15 W ¦ 1.5% - ; ;

It can be seen from the results shown in Table 1 of a test in which 1 the adJustment was performed at the position of hole C that the adJusting method of test 2 in which positive cooling was performed is far superior to ~
the adjusting method of test 1 in which cooling was done spontaneously, and ~;
can permit the adjustment of minute thickness unevenness which occurs in the widthwise direction of film.
According to one embodiment of the process of this invention, a , molten polymer maintained uniformly at a substantially constant temperature by a main heating means 33 is extruded into a film form while operating cartridge heaters 40 inserted in number of holes 35 such that a heating 1 action is counterbalanced with a cooling action, and when a portion having thickness unevenness in the widthwise direction is detected, the balance `
between heating and cooling is destroyed (in other words, one of the heating action and the cooling action is reduced or made ineffective) on that car~
tridge heater 4O which is at a position corresponding to the detected portion thereby to ad~ust the extruded film to uniform thickness. One example of the results of R% measurement in accordance with this embodiment are shown in ~ Table 2.

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Conditions for the cartridge heal ers R%

A B C D E
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Test 1 30 W 30 W 30 W 30 W 30 W
10 10 20 ~0 10 ~.8 1/min. 1/min. 1/min. 1/min. l~mIn.
____ _ ~ _____ ____~_ _ Test 2 30 W 30 W O W 30 W 30 W 2.1 In Table 2, test 1 refers to the process of this invention in which the amount of air blown into hole C is increased to cool the film, and test

2 refers to a conventional method in which air cooling is not performed but the amount of heat is changed. R~ represents the ratio of reduction in film thickness.
According to another embodiment of the process of this invention, a number of heat transfer medium passageways are provided at substantially equal intervals along thè entire width of the die near its die slit, and the ; 10 thickness unevenness of film can be ad~usted by changing the temperature of the heat transfer medium within the passageways, instead of inserting car-tridge heaters 40 in a number of holes 35 provided along the entire width of ;-~
the die and performing heating by electric heaters and cooling b~ air held at room temperature. In this embodiment, when a portion with thickness unevenness is detected in the extruded film, a heat transfer medium at a higher or lower temperature is passed through that passageway which is sit-uated at a position corresponding to the detected portion, and by a heat exchange action~ the temperature of the molten polymer immediately before extrusion is rendered higher or lower. For this reason, the amount of the molten polymer extruded per unit time from the die slit can be rendered '- ! .
¦ uniform along the entire width of the die.
One example of the apparatus for performing the process in accor-dance with this embodiment is shown in Figure 6. In Figure 6, the reference numeral 31 represents a die; 32, a die slit defined by a pair of die lips 30; 33, a main heating meansj 35, a heat transfer medium passage~a~; P, a molten polymer. The main heating means 33 is provided so as to cover the upper part of the die at a position sufficiently remote from the die slit ~'`' ~"

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32. The molten polymer P is maintained at a predetermined temperature, ~; ;
flows down between the d$e lips 30, and is extruded from the die slit 32.
The molten polymer P is subJected to heat exchange by a heat transfer medilIm -which passes through the passageway 35 provided within the body of the die , lip in proximity to the inside surfaces of the die lips 30 (the surfaces over which the molten polymer flows down, that is, the die land), and con ;
sequently, the amount of the molten polymer extruded changes locally, and a ~
film of uniform thickness over the entire die width can be extruded. i'~ ~ -The heat transfer medium passageways 35 are provided at sub~
stantially equal intervals within the body of the die lip 30 so that they are near the die land and substantially parallel to it. This serves to increase the heat exchange effect. The heat transfer medium may, for exam-ple, be a gas or liquid medium such as air, vapors or silicone oils. The - `
`j~ temperature of the heat transfer medium can be ad~usted as desired to a temperature equal to, or hl~her or lower than, the preset temperature by means of a temperature ad~usting heater 51. A circulating means 52 such as a pump can be provided so that the heat transfer medium can be circulated. ~ `~
The following Examples and Comparative Example illustrate the ' advantages of this invention by experiments using a film-forming apparatus .''~i ..' 20 for large-sized polyester films. `
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Using a film extruding die having a die width oP 60 mm, and a die slit clearance of 1.5 mm and equipped with a number of holes 35 of the type ~.J
shown in Figure 1 along the entire width of the die, the standard tempera-ture was pre-set at 275 C., and ordinary polyethylene terephthalate was `;~
melt-extruded at a temperature of 285 C. at a rate of 200 Kg/hour. The extruded sheet was stretched at a stretch ratio of 1:3.6 both in the lon-.`!
gitudinal and transverse directions and heat-set to form a biaxially stretch-ed film having a thickness of 12 microns.

The thickness of the film was adjusted by operating a cartridge heater inserted in a hole which was situated at a position corresponding to :, that portion of the film whose thickness unevenness had been detected by a .~.
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~-ray thickness tester. The cartridge heater was of the type shown in Fig~
ure 2 and had an outside diameter of 15 mm, a length of 90 mm, a hole (41) -~
diameter of 1 mm and a groove (43) depth of 2 mm It included an electric heater 43 (lO0 V, 50 W) as an auxiliary heating means. As cooling means, air at 25C. was blown by a nozzle into the hole 41 and the grooves 42.
j The time required from the initiation of the thickness adJustment ;~
; until thickness unevenness was corrected to afford a satisfactory film was ~;
20 to 40 minutes. The average thickness unevenness which was finally reached after operating for one month was 1.5 to 2% as measured by a ~-ray thickness tester which evaluated the rolled product as a whole.
For film thickness adjustment, 5 blow nozzles and 8 heaters had to be operated. The degree of the required adjustment was 7 liters/min. at most for the cooling means, and 12 watt at most for the auxiliary heating ` means.
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Example 2 `~ ~
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;~l Using the same extrusion die as used in E~ample 1, molten poly~
ethylene terephthalate having an intrinsic viscosity C~ of o.60 was melt-extruded at a rate of 200 Kg/hour while maintaining it at a temperature of ~ ~-, ~,, i 280C., and biaxially stretched at a ratio of 1:3.6 in both the longitudinal , 1~ 20 and transverse directions by a known method, and heat-set to form a film .. . . ..
having a thickness of 12 microns.
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A heating electric power of 30 watt and a cooling air at 25 C.
(lO liters/minute) were supplied to each of the cartridge heaters so that in each of the holes in whioh the cartridge heaters were inserted, the heating ~;
'l .
. action was counterbalanced with the cooling actlon to provide thermal equi~
librium. The supply of one of the electric power and the air was reduced according to the thickness unevenness of the film detec~ed by a ~-ray thick~
~ ness tester. ~hus, the thickness of the film was ad~usted by the desired '~ amounts at the desired positions. ~ ;
~ 30 The time required from the initiation of the thickness adjustment i until a film having satisfactorily uniform thickness was obtained was 20 to ~, 40 minutes. After continuing the film production for 5 days,;the average ~-~
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~O~ 'Z ,, thickness unevenness of the film produced was 1.8~ as measured by a r-ray ;~-thickness tester. This shows that the product obtained was of very good `~
quality. ~;-Example 3 Uslng a film extrusion die having a die width of 760 mm and a die ~ slit clearance of 1.5 mm and equipped with a number of heat transfer medium ;
; passageways 35 of the time shown in Figure 6 along the entire width of the -- die~ the standard temperature was set at 270 C., and ordinary polyethylene terephthalate was extruded at a temperature of 285C. and at an eXtrusiQn ; :, ,.:
rate of 200 Kg/hour. The extruded sheet was stretched by a known method at a stretch ratio of 1:3.6 both in the longitudinal and transverse directions, and heat-treated to form a biaxially stretched film having a thickness of 12 microns.
During this time, air at 270C. as a heat transfer medium was blown into each of the heat transfer medium passageways at a rate of 20 liters/minute, and the temperature of the blowing air was either elevated or lowered only in that heat transfer medium passageway which was situated at a position corresponding to a portion with thickness unevenness detected by a ray thickness tester, thereby to adJust the thickness of the film. The time required from the initiation of the thickness adjustment until the thickness unevenness of the film was adJusted to provide film with satis- ~;
factory thickness uniformity was 20 to 40 minutes. After continuing the film production for one month, the average thickness unevenness of the film produced was 1.5 to 2% by means of a ~-ray thickness tester which evaluated the rolled product as a whole.
Com~arative Example The procedure of Example 1 was repeated except that cartridge heaters were inserted in all of the holes 35, and an electric power of 25W
was supplied to all of the heaters from the start of the film production, and the thickness of the extruded film was ad~usted by changing the electric power of bhe heaters according to changes in the readings of a ~-ray thick-ness tester. The operator who performed the adjustment was the same person :. -- 19 --:'' ~

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as one who did it in Example l.
The time required from the initiation of the thickne6s adjustment until a first product was obtained was 2 to 3 hours. When the fllm produc~
tion was continued for 5 days, the average thickness unevenness of the film produced during this time was 2 to 4% as measured by a ~-ray thickness tester. The results was evidently inferior to those obtained in the previous Examples. `~
During the thickness adjustment, a high electric power of 40 to 50 `-watt had to be supplied to each of the heaters in the 5 adjoining holes. The lO portions of the film which corresponded to these holes became larger in ;
"
;` thickness, and measurement by a ~-ray thickness tester showed that thickness unevenness of about 1.5 to 2.2% newly occurred. The new thickness unevenness , could not be corrected.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a polymeric film free from thickness unevenness along its entire width, which comprises continuously extruding a molten polymer into a film form from a slit of a die while maintaining the molten polymer within the die at a substantially con-stant temperature, continuously measuring the thickness of the extruded film along its entire width by means of a thickness tester, and when a portion having a larger thickness than the desired thickness is detected, locally cooling that portion near the die slit which corresponds to the detected portion, and when a portion having a smaller thickness than the desired thickness is detected, locally heating that portion near the die slit which cor-responds to the detected portion, thereby to equalize the amount of the molten polymer extruded from the die slit along the entire width of the die.

2. An apparatus for producing a polymeric film free from thickness unevenness along its entire width, compris-ing a pair of die lips defining a slit of a die, a main heating means for maintaining a molten polymer within the die at a substantially constant temperature, auxiliary heating means for locally elevating the temperature of the molten polymer immediately before extrusion through the die slit, and cooling means for locally lowering the temperature of the molten polymer immediately before extrusion through the die slit, said main heating means being disposed sufficiently remote from the die slit to maintain the temperature of a molten polymer within the die substantially constant, and said auxiliary heating means and said cooling means being disposed in a plurality of holes provided along the entire width of the die in at least one row at substantially equal intervals and being operable independently from each other.

3. The apparatus of claim 2 wherein said auxiliary heating means and cooling means are cartridge heaters having an electric heater included therein, a hole at their center and grooves notched at their periphery.