CA1320320C - Process for preparing stretched film and device for preparing the film - Google Patents

Abstract

Abstract of the Disclosure A process and device for preparing a stretched film by passing a tubular thermoplastic film into pairs of nip rolls having different peripheral speed and by inflating an obtained bubble with internal pressure under heating, wherein the pairs of nip rolls comprise at least three pairs of nip rolls designated with R1, R2 and R3, respectively, pressurized air is admitted into the stretched bubble situated between the rolls R1 and R2 through a slit-like air injecting nozzle provided in a gap between the pair of rolls of R2, and the bubble formed by the supplied pressurized air is inflated and stretched under heating. In this stage, if desired, the amount of injected pressurized air is controlled by manual control or automatic control by detecting the diameter of the bubble so as to keep the bubble's diameter constant. After stretching, the film is collapsed with a collapser device and is introduced to the nip rolls of R2, both side ends of the collapsed film is cut open with film cutting knives positioned between the pair of nip rolls of R2 to make two sheets of flat films, and then, the flat films are introduced to left and right guide rolls so as to avoid interference with the slit-like injecting nozzle and the cutting knives.
Finally, the films are drawn with the nip rolls of R3.
In the device of the present invention, the diameter and internal pressure of the bubble can be controled with manual or automatic control during the tubularly stretching, and particularly, the device can be suitably applied to tubularly stretching of pourus films.

Description

~32~32~

PROCESS FOR PREPARING STRETC~ED FILM ANn DEVICE
FOR PREPARIN~ THE FILM

BACKGROUND OF T~E INVENTrON
The present invention relates to a process for preparing stretched films and a device for preparing the films by heating and stretching a thermoplastic tubular film, and more particularly, to the process and device suitably employed in preparation of a tubular stretched film of which bubble portion is gas permeable.
Generally, a tubular biaxial stretched or oriented film is prepared by passing a tubular non-stretched film through two pairs of nip rolls arrangedwith a suitable distance, by heating the tubular non-stretched film to a predetermined temperature with a heater attached in a zone between the two pairs of nip rolls, by admitting pressurized air into the film to form a bubble and by drawing the film so as to pass through a group of collapser rolls with a pair of lower nip rolls.
The pressurized air is, in general case, admitted only at the time of start, and after a bubble ha~ing a predetermined size is formed, a fixed quantity of pressurized air is kept in the bubble by shutting the lower nip rolls, and the stretching is continued.
However, in the above-mentioned process for preparing biaxial stretched film where the air is trapped to keep the bubble, the bubble gradually becomes deflated since, during the stretching process, the pressurized air (small quantity) leaks out through the upper or lower nip roll portions. Therefore, opening angle of the group of collapser rolls is adjusted so that the lateral magnification is suitably kept without dropping.
However, in some cases, even though the opening angle of the collapser rolls is adjusted, the adjustment provides merely a change of stretching point and does not contribute to increase the transverse magnification.
Further, when the start-operation cannot provide the predetermined ~ize oE bubble, it is necessary to repeat ~3~320 the start-operation again, and the repetition is troublesome. Particularly, in a case where gas permeable film is used, though the upper and lower nip rolls are shut so as to exclose the pressurized air in the bubble, S the interior air leaks through the film face. Th2n, the bubble adjusted to the predetermined size becomes gradually deflated, and the biaxial stretching process cannot be continued.
As a measure against those problems, there have been conventionally proposed a process where pressurized air is continuously admitted in the bubble, for example, as disclosed in Japanese Examined Patent Publication No.
3727/1958, Japanese Unexamined Patent Publication No.
139965/1979, and the like.
For example, Japanese Examined P~tent Publication No. 3727/1958 etc. disclosed a device having lower nip rolls provided with continuous annular groove and a method to supply pressurized air through a tubular insert member by inserting the insert member into the groove. However, the film is easily deformed with frictional force when the tubular insert member becomes in contact with the film. Moreover, a portion of the film which is saved from contact and is pinched and drawn by the nip rolls receives transverse stretching according to the diameter of the tubular insert member. Therefore, uniformity of the film falls and the flatness of the film descends.
In other hand, the above-mentioned Japanese Unexamined Patent Publication No. 139965/1979 discloses a process where the film is cut at a position just upper the lower nip rolls and pressurized air is admitted through an air injectin~ nozzle inserted into the cut portion. However, the process has a disadvantage that puncture of the bubble happens easily, since the film is cut at an area where the film is transversely stretched. Particularly, in such a film that resistance to tear is poor, it is impossible to continue the stretching since the torn portion will widely extend.

13~,~32~

SUMMARY OF THE INVENTION
The present invention was obtained as a result of study to eliminate the above-mentioned drawbacks.
That is, the present invention provides a process for preparing a stretched film by passing a tubular thermoplastic film into pairs of nip rolls having different peripheral speed and by inflating an obtained bubble with internal pressure under heating, in which pressurized air can be continuously supplied in the bubble through a slit~like nozzle, and the present invention provides a device employed in the process.
The pairs of nip rolls comprise at least three pairs of nip rolls designated with Rl, R2 and R3, respectively, and ratio of rl/r2/r3 in peripheral speed of the pairs of nip rolls Rl, R2 and R3 can be adjusted in a range of ~.1 through 0.8/1/1 through 1.1, where Rl is a pair of nip rolls situated at introducing side of non-stretched film, R2 is a pair of nip rolls situated at an end of collapser stage after tubularly stretching, and R3 is a pair of drawing nip rolls continuing after the end of collapser stage and the pair of nip rolls R2.
Pressurized air, if required, pressurized air controlled in flow rate in order to keep the diameter of bubble at a determined value with manual operation or with automatic operation through sencing diameter of the bubble, is admitted into the stretched bubble between the Rl and R2 through a slit-like air injecting nozzle provided in a gap between the pair of nip rolls R2.
After the bubble formed by the supplied air is inflated and stretched under heating, the film is collapsed with a collapser device and is introduced to the nip rolls R2, and both side ends of the collapsed film are cut open with film cutting knives positioned between the pair of nip rolls R2 to make two flat sheet films. After the flat sheet films are introduced to left and right guide rolls to avoid the slit-like injecting nozzle and the cutting knives, the films are drawn by the nip rolls of R3.

13~32~

According to the process and device of the present invention, the internal pressure of the bubble can be adjusted manually at times or automatically, and therefore, even if the film is gas permeable, the film can be suitably stretched in a tubular form.

BRIEF ~ESCRIPTION OF THE DR~WINGS
Fig. 1 is a diagrammatic elevational view showing an embodiment of the device of the present invention and illustrating a process for heating, inflating and stretching a tubular non-s~retched film, Fig. 2 is a diagrammatic side elevational view showing the device of Fig. 1 where film cutting devices are provided in a gap of a slit-like air injecting device;
Fig. 3 is a diagrammatic front view showing an embodiment of an air injecting nozzle device 14 and cutting knife devices in the device of the present invention, where the film cutting knife devices are provided in a gap of a slit-like air injecting nozzle device;
Fig. 4 is a diagrammatic side elevational view of the devices shown in Fig. 3;
Fig. 5 is a diagrammatic side view of the device of Fig. l, where film cutting knife devices are provided at both outer sides of the nip rolls with respect to the slit-like air injecting nozzle device, Fig. 6 is a diagrammatic front view of an embodiment of an air injecting nozzle device 14 and cutting knife devices in the present invention, where the film cutting knife devices are provided at both side en~s of the nip rolls with respect to the slit-like air injecting no%zle device; and Fig. 7 is a diagrammatic plan view of an embodiment of a photoelectric switch in a cutting knife portion.
It is to be understood that the above-menti.oned Figs. 1 through 7 show only embodiments of the present ~3~20 invention, and the present invention is not limited to those embodiments. Of course, various changes and modifications thereof can be made in the invention as far as it has the function of the present invention.
Each reference number in the drawings means the member or a part of the member mentioned hereinafter.
Rl: first nip rolls R2: second nip rolls R3: third nip rolls 1: a non-stretched tubular film 2: a heater 3: a stretched bubble 4: a group of collapser rolls 5,5': a cutting knife 6: an air injecting nozzle 7: a stretched flat film 8, 8': a guide roll 9, 9': a cutting knife holder 10, 10': an air-sealing bar 11, 11': a motoz for actuating a knife holder 12: an air inlet pipe 13, 13': a screwed rotary shaft 14: an air injecting nozzle device 15, 15': a bubble diameter detector 16, 16': a screwed movable member 17, 17': a bearing 18, 18': a bar for supporting a cutting knife holder 19, 19': a bracket 20, 23: an opitical fiber 21, 22: a photoelectric switch 24: a bracket for attaching a photoelectric switch 8 25: an output controller 26: an automatic air supplying valve 27: an air supplying blower DETAILED DESCRIPTION
As shown in the attached drawings ~Figs. 1 ~32~32~
~;
through 7), the device of the present invention is a device for producing stretched films by passing a tubular thermoplastic film into pairs of nip rolls having different peripheral speed and by inflating the film with S internal pressure under heating.
The pairs of nip rolls comprise at least three pairs of nip rolls designated with Rl, R2 and R3, respectively. Ratio rl/r2/r3 of peripheral speeds of the pairs of nip-rollers Rl, R2 and ~3 is adjustable in a range of 0.1 through 0.8/1/1 through 1-1, where Rl is a pair of nip rolls situated at an introducing position of the tubular non-stretched film, R2 is a pair of nip-rolls situated at an end of collapse stage after tubularly stretching, and R3 is a pair of drawing nip rolls continuing after the end of collapsing stage and the pair of nip rolls R2.
A heater for heating the introduced tubular film between the Rl and R2 is provided, and a device for collapsing the stretched film is provided between the Rl and R2. A gap of rolls of the nip rolls R2 is adjustable, and a slit-like air injecting nozzle device for injecting air into the bubble is provided in the gap. Two film cutting knife devices movable in a longitudinal direction of the nip rolls of R2 are provided in the gap in order to cut both side ends of the collapsed film, and at least one pair of guide rollers 8,8' for guiding the cut films 7,7' to make each film's path capable of getting out of the slit-like nozzle device and the cutting knife devices for each flat film 7,7'.
The above-mentioned air injecting nozzle device 14 comprises an air injecting nozzle 6 and an air inlet pipe 12, and the nozzle 6 supplies pressurized air to the bubble 3 through the gap of the nip rolls R2.
Though flow rate and pressure of the injected air can be manually regulated in an optional value, it is preferable to employ a bubble diameter detector 15, 15', an output controller 25, an automatic air supplying valve ~32~2~

26 and an air supplying blower 27 in order to change amount of the supplied air so as to keep the diameter of the bubble to a predetermined value.
The above-mentioned cutting knife devices comprise, as shown in Figs. 3 and 6, rotary shafts 13, 13' combined with motors 11, 11' and having left-handed and right-handed screws thereon, knife holders 9, 9' provided on movable members 16, 16' capable of axially moving due to female screws engaged with the male screws of the rotary shafts 13, 13', and cutting knives 5,5' fixed on the knife holders 9,9'.
Further, in case of Fig. 3, as shown in Fig. 4, air sealin~ bars 10, 10' are provided in the knife devices so that air is not leak out of the gap between the nozzle portion and the knife holders. In case of Fig. 6, bars 18, 18' for supporting the cutting knife holders are included in the cuttiny knife devices.
The nip rolls Rl have a function to seal the pressurized air injected through the air inje~ting nozzle from the R2 portion of the tubular film to prevent upward leakage, and are driven at a peripheral speed of 0.1 through 0.8 times of the peripheral speed of the nip-rolls R2. ~he ratio of the peripheral speeds is to be determined according to the required longitudinal stretching magnification. For example, if four times longitudinal magnification is reguired, the peripheral speed is to be adjusted to 0.25 times of that of the nip rolls R2.
The gap between the pair of rolls of the nip rolls R2 can be adjusted in an extent where an air injecting nozzle 6 is arranged between the gap, for example in an extent from 1 through 10 mm. However, in the case of Fig. 6, any size of nozzle can be employed so far as the bubble is supplied with sufficient amount of pressurized air through the nozzle.
The nip rolls R3 have a function to draw the two films cut with the cutting knives 5, and are driven with 1 through 1.1 in peripheral ~peed ratio with re3pect 13~20 to that of the nip rolls R2. Fundamentally, the nip rolls R2 and the nip rolls R3 can have the same peripheral speed. ~owever, when the films are drawn~ in some cases, the Eilm become loosened due to load of guide rollers 8,8' or the like. Such problem can be eliminated by increasing the peripheral speed of the nip rolls R3 than that of the nip rolls R2 in order to make tension in the films.
If required, two pairs of nip rolls R3 can be employed so that each pair can draw each cut flat film 7,7'.
The slit-like air injecting nozzle 6 is set in a gap a of the nip rolls R~ so that the nozzle end is situated on a plane (or the adjacent position) which includes the axes of two rolls of the nip rolls R2. The nozzle 6 has a function to admit pressurized air into the stretched bubble in accordance with amount of ai} leak from the cut portions or the like in order to keep the bubble in a predetermined size. The nozzle 6 can be freely adjusted to upward or downward with respect to the nip rolls' position.
Further, the width of the slit-like air injecting nozzle 6 ~in the direction of the length of the rolls) can be suitably selected. ~owever, in case of Fig. 3 where the film cutting knife devices are provided in the gap or opening of the nozzle 6, the width is generally set to an extent equal to the length of nip rolls or somewhat larger. In case of Fig. 6 where the cutting device is arranged at the outside of the nozzle, the width of the nozzle in the direction of len~th of rolls is set to such extent that the knives can be entered in the width of the collapsed non-stretched tubular film to be stretched when the distance between the knives is adjusted in the narrowest state.
As a means to move the knives, for example, there can be employed a mechanism comprising a rotary shaft 13 having a left-handed screw portion and a right-handed screw portion on each hal~ of the shaft, a pair of ~2~
g knie holders 9 each having female screw portion engageable with the corresponding male screw, and a pair of knives held on the knife holder 9. The pair of knives 5 can be symmetrically moved along the rotary shaft 13 by rotating the shaft 13 with drive of electric motor 11.
Of course, the rotary shaft 13 can be separated at the center thereof into the left and right halves 13, 13' each jointed with an electric motor 11, 11' respectively, in order to drive the cutting knives independently each other.
On the other hand, as shown in Fig. 6, when the width of the nozzle 6 is narrower than the width of the non-stretched tubular film in a collapsed state, the knives 5, S' and the knife holders 9, 9' are situated at both out sides of the nozzle 6 and are fixed on a frame of the stretching machine or the like by means of brackets 19, 19'. In the above-mentioned cutting device, the knives S, 5' can be moved in the axial direction of the nip rolls by rotating the rotary shaft with the electric motor 11 or the like. Then, even though the width of the tubular film in a collapsed state changes, both side ends of the film can be cut with the knives.
In this case, each knife~holder 9, 9' is mounted on a supporting bar or rod 18, 18' so that the holder 9, 9' is not rotated together with the rotation of the rotary shaft 13. In this case, bearings 17, 17' are preferably employed so that the holders 9, 9' can smoothly move in the longitudinal direction of the nip rolls R2.
When the positions of the knife holders are automatically adjustable by mounting a photoelectric switch through brackets 24, 24', the cutting width of the both side ends of the collapsed film can be automatically adjusted. The photoelectric switch in the embodiment comprises a photo-ejector 21 for ejecting ray guided by an optical fiber 20, a photo-receiver 22 for receiving the ray, an optical fiber 23 and a relay.
When the width of the Eilm becomes narrower and ~32~

the inside photoelectric switch which has been intercepted by the film becomes to receive the ray, the switch of the motors 11, ll' turns to "ON" to rotate the shafts 13, so that the knife holders move inwardly.
On the contrary, when the width of the film becomes wider and the outside photoelectric switch which has received the ray becomes to be intercepted by the film, the switch of the motor ll, ll' turns to ON to rotate the rotary shaft 13, 13' in the direction opposite to the above-mentioned. Then, the knife holders are moved outwardly.
That is to say, the knife holders are moved so that the side ends of the collapsed film are always positioned in an area between the inside and outside photoelectric switches without deviating from the area of width of the cutting knife. Then, the cutting is continued automatically.
In case of Fig. 3 where the slit-like air injecting nozzle 6 is wider than the width of the collapsed film at the nip rolls R2, there is no stretched bubble to be inflated at the area out of the cutting knives 5, 5'. ~hen, pressurized air is uselessly exhausted to the outside air. Such useless exhaust out of the bubble can be reduced by providing cylindrical air sealing bars or air ejection control bars 10 each having a diameter larger than the nozzle gap a of the slit-like nozzle 6 and having a length almost half of the width of the nozzle 6. The bars 10 are located at the left and right outside positions of the knife-holders 9, 9' and under the slit portion of the inside of the slit-like nozzle 6.
In a preferable case, the air ejection control bars 10, 10' are connected with the knife holders 9, 9' so that the control bars 10, 10' can be moved together with the knife holders 9, 9'.
Further, by employing bubble diameter detectors 15, 15' to automatically detect the width oE the collapsed tubular film 3 to be cut and by providing a ~3~320 mechanism to move the cutting knives 5 in relation to the detected width of the film, another automatic adjusting device is obtained. In addition, by interlocking the flow rate of the supplied air with the stretched bubble's diameter detected by the above-mentioned detector 15, 15', the range of the fluctuation of the diameter of the bubble can be further reduced.
Amount of the supplied pressurized air to be supplied is automatically regulated in accordance with the diameter of the bubble detected by the bubble-diameter detector 15, 15'. The bubble diameter detector 15, 15' is a device to measure a distance from the bubble by utilizing measuring ~ensor which emits visible ray, infrared ray or lazer beam, detects the reflected ray and measures the distance from the reflecting body. The bubble diameter detectors 15, 15' can be set not only under the heater 2 but also on the outside of the heater.
The detector 15, 15' can detect the distance by receiving the reflected ray passing through a hole formed in the heater 2. In the latter case, distances between the detector and the bubble can be detected at the middle positions of the heater.
An output controller 25 is a device for calculating the diameter of the bubble on the basis of the distance obtained by two or more distance sensors Ibubble-diameter detectors 15, 15') and for regulating the opening degree of an air supplying valve 26 in responce to the difference between the predetermined bubble diameter and the detected diameter so that the detected diameter accords with the predetermined bubble diameter.
The air supplying valve 26 regulates the flow rate of the pressurized air supplied through a blower 27 by changing the valve-opening degree in response to signals from the output controller 25.
Further, when another bubble diameter detector is provided just above the cut portion and the detected diameter of the lower portion of the bubble i9 lnputted ~ ~2~2~

to the output controller 25, an air supply control where air i5 supplied in accordance with the difference between the diameters of the upper portion and the lower portion of the bubble or in accordance with air lealc at the cut portion can be carried out.
In a case where the fluctuation of the bubble's diameter is small, a pair of fixed knives, preferably having long edges, can be employed.
The group of collapser rolls 4 has a function to gradually collapse the stretched bubble film 3 and to introduce the film to the nip rolls R2. The guide rolls 8, 8' have a Function to smoothly introduce the cut-opened flat films 7, 7' toward the drawing nip rolls R3 so that the films are not in contact with the slit-like air injecting nozzle 6 and the cutting knife devices.
The non-stretched film l is heated by a ring-like heater 2 provided around the tubular stretched bubble, and is inflated by the air supplied through the nozzle 6.
Further, in a case where the film is stretched in a longitudinal direction mainly due to difference between peripheral speeds of nip rolls Rl and R2, the film is biaxial stretched as a result thereof. Of course, monoaxial stretching can be carried out by setting the bubble's diameter to accord with the non-streched tubular film when interior pressure is applied.
~ ereinafter, referring to FigsO 1 through 7, more concrete examples of the device and process for preparing stretched films of the present invention are explained. However, the present invention is not limited to those embodiments, and various changes and modifications can be employed without departing the scope and extent of the present invention.

Example 1 Before starting operation, a non-stretched tubular film made of linear low density polyethylene (MI=2.0) wherein 35 % by weight of low molecular weight 3 2 ~

polyethylene telephthalete (IV=0.25~ fine particles were disparged, was passed through nip rolls Rl, a group of collapser rolls 4 and nip rolls R~. Then, independently operating the electric motors 11, 11' each connected to rotary shafts 13, 13' separated with each other r cutting knives 5, 5' were adjusted or had been adjusted to a width of the collapsed non-stretched tubular film 1. The tubular film was cut by so adjusted cutting knives 5, 5' into two sheets of flat films 7, 7', and the flat films were drawn by nip rolls R3 through guide rolls 8, 8'.
Next, in operation stage, the non-stretched tubular film 1 was heated by a heater 2, a bubble 3 was formed by supplying pressurized air into the tube from an air injecting nozzle 6. Then, nip rolls Rl and R2 located upper and lower positions were driven, the temperature of the heater 2 was regulated, a ratio of peripheral speeds of the nip roll Rl to the nip roll R2 is set to 0.31 to make longitudinal magnification 3.2 times, and fur~her, by regulating the pressure of the 2~ air, the transverse magnification was set to 2.5 times, so that the simaltaneous biaxial stretching was carried out to obtain gas permeable biaxial stretched film 3.
When the stretched film regulated as mentioned above was collapsed and reached the nip rolls 2, the width of the collapsed portion gradually became larger. However, the stretching and cutting open smoothly continued since the positions of the cutting knives 5 were gradually extended by driving the electric motors 11, 11'.
Thereafter, the diameter of the stretched film 3 was kept to an almost constant level by regulating supplied pressurized air. After the two sheets of stretched flat films 7, 7' cut open at the location of the nip rolls R2 were drawn by the nip rolls R3 through the guide rolls 8~ 8', both flat films were wound by two winders, respectively. The obtained biaxial stretched films 3 were gas perpeable films and had good flatness.

Example 2 ~32~32~

The same non-stretched tubular film 1 as shown in Example 1 was prepared and was passed through the rolls in the same manner as in Example 1. Then, the non-stretched tubular film 1 was heated with a heater 2, and bubble-diameter detectors 15, 15', an output controller 25, an air supplying valve 26, photoelectric switches of cutting positions and relays were operated. A switch of the heater was switched on, and about one minute after, the switches of the driving motors of nip rolls Rl, R2 and R3 were switched on to start stretching. While a bubble is gradually inflated, opening degree of the air supplying valve was gradually decreased, and about five minutes after, the bubble's diameter accorded with a predetermined value and became stable. During the above-mentioned operation, the cutting knives 5, 5' were extended due to the operation of the photoelectric switches and the relays in relation to inflation of the bubble. Then frequency of switching of the photoelectric switches was reduced and became stable.
The stretched flat films 7, 7' cut with the knives 5, 5' situated at the position of the nip rolls R2 were wound on the drawing winder in the same manner as in ~xample 1.
Though the obtained biaxial stretched film 3 was gas permeable, the motion of the valve was stabl~ dueto the automatic air supply control, and the cut and wound flat film had a uniform width and a good flatness.
The device for preparing tubular stretched film of the present invention can be continuously operated for long time with constant stretching magnification without stoppage of operation, since pressurized air is supplied through a nozzle of a slit-like air injecting device during the operation, and therefore, the bubble can be filled with pressurized air corresponding to the leakage. Further, even in a case where a porous film is prepared by stretching a gas permeable tubular film or a film which becomes gas permeable when stretched, the gas permeable stretched eilm can be produced stably for long ~2~3~

time, since the amount of air corresponding to the leakage of the interior air through the film can be suitably supplled to always keep the bubble's di~meter constant.

Claims (8)

WEAT WE CLAIM IS:

1. A process for preparing a stretched film by passing a tubular thermoplastic film into pairs of nip rolls having different peripheral speed and by inflating an obtained bubble with internal pressure under heating, wherein:
said pairs of nip rolls comprises at least three pairs of nip rolls designated with R1, R2 and R3, respectively;
a ratio r1/r2/r3 of peripheral speed of said pairs of nip rolls R1, R2 and R3 is set in a range of 0.1 through 0.8/1/1 through 1.1;
where R1 is a pair of nip rolls situated at introducing side of non-stretched film, R2 is a pair of nip rolls situated at end of collapse stage after tubularly stretching, and R3 is a pair of drawing nip rolls continuing after said end of collapse stage;
pressurized air is admitted into the stretched bubble situated between said Rl and R2 through a slit-like nozzle inserted between said pair of rolls of R2;
the bubble formed by the supplied pressurized air is inflated and stretched to one or more times in size in a radial direction under heating;
then said film is collapsed with a collapser device and is introduced to said nip rolls of R2;
both side ends of said collapsed film is cut open with film cutting knives positioned between said pair of nip rolls of R2 to make two sheets of flat films; and said flat films are introduced to left and right guide rolls so as to avoid interference with said slit-like injecting nozzle and said cutting knives; and said films are drawn with said nip rolls of R3.

2. The process for preparing a stretched film of Claim 1, wherein:
said tubular thermoplastic film is a gas-permeable film or a film which becomes gas permeable when stretched.

3. A process for preparing stretched films, comprising:
a step for introducing a tubular thermoplastic film into a first pair of rolls and a second pair of rolls arranged with a gap therebetween;
a step for forming a bubble of the film at a section between the first and second rolls by injecting a gas in the section of the film through the gap of the second pair of rolls, and by heating the section;
a step for collapsing lower part of the bubble so that the tubular film is introduced in the gap of the second pair of rolls;
a step for cutting both sides of the collapsed tubular film to obtain two flattened sheets; and a step for drawing the sheets by means of a third pair of rolls.

4. A device for preparing stretched films by passing a tubular thermoplastic film into nip rolls having different circumferential speed and by inflating the film with internal pressure under heating, wherein;
said pairs of nip rolls comprise at least three pairs of nip rolls designated with R1, R2 and R3, respectively;
a ratio r1/r2/r3 of peripheral speed of said pairs of nip rolls R1, R2 and R3 is set in a range of 0.1 through 0.8/1/1 through 1.1;
where R1 is a pair of nip rolls situated at introducing position of said tubular non-stretched film, R2 is a pair of nip rolls situated at end of collapsing stage after tubularly stretching, and R3 is a pair of drawing nip rolls continuing after said end of collapsing stage;
a heater for heating said introduced tubular film is provided between said R1 and R2;
a collapser device for collapsing said stretched film are provided between said R1 and R2;
a slit-like nozzle device for injecting pressurized air into said bubble is provided between said gap;
two film cutting knife devices movable in a longitudinal direction of said nip rollers of R2 are provided between said gap in order to cut both side ends of said collapsed film; and at least one guide roll for guiding said cut film to make a film's path capable of getting out of said slit-like nozzle device and said cutting knife devices for each flat film.

5. The device of Claim 4 wherein, said film cutting knife devices are provided in an opening of said slit-like air injecting nozzle.

6. The device of Claim 4 wherein, said film cutting knife devices are situated at both outer sides of said slit-like air injecting device in the direction of said nip rolls.

7. The device of Claim 4, further comprising an automatic control device including means for detecting a diameter of said bubble, and means for automatically changing those flow rate and pressure of the pressurized air in order to eliminate the flucturations of the diameter of the bubble.

8. A device for preparing stretched films comprising:
a first pair of rolls abutting with each other;
a second pair of rolls having a gap therebetween;
a heater situated between the first and second pairs of rolls;
a means for injecting a gas through the gap between the second pairs of rolls into the tubular film at a section between the first and second pairs of rolls to form a bubble at the section;
a pair of cutting device situated in the gap of the second pairs of rolls, for cutting both sides of the collapsed film; and a third pair of rolls to draw the cut films.