Note: Descriptions are shown in the official language in which they were submitted.
~32~32~
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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
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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.
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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
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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
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~;
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
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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
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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
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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
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-- 11 --
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~
12
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 ~
- 13
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
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- 14
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~
- 15
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.
