Note: Descriptions are shown in the official language in which they were submitted.
~ 36 CA 3431
This invention relates to a blown tube process
and apparatus for producing biaxially oriented synthetic
thin gauge resin film such as polypropylene film, and
more particularly to a blown tube apparatus where the
extrucler die, rehea-t ovens, and hot air ring are caused to
rotate and oscillate in unison to provide a flatter, more
uniform polypropylene film~
In a ~ell-known biaxially oriented hlown tube
apparatus and process, polypropylene resin is fed through
an extruder die head to form a continuously moving cooled
and solidified tubular section or stalk which then passes ~ ?
through a reheat furnace to bring it up to a predetermined
~; orientation temperature. At the predetermined orienting
or softening temperaturel the stalk is insufflated or
expanded into a very large thin gauge cylindrical structure -
referred to as a blown tube or bubble. This cylindrical
structure i6 closed at one end by having it pass through
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a pair of nip rolls, and thereafter the double lay flat
biaxially oriented section emanating from the nip rolls is -~
slit and wound on storage or mill rolls.
One of the problems of such a process is that
an irregular section or a gauge non uniformity causes a
; continuing build up in the film in a mi]1 roll because
the non uniformity is continually wound on itself in each
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` turn. Large rolls so wound have differences in roll ; ;
diameters because of this cascading effect. These ;~
differences, because of the creep characteristics of the
film degrade the basic flatness eveness and straightness
of the film. When precision winding machines such as
capacitor roll winding machines are utilized to wind
capacitor rolls, this degradation causes skewering and
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~ wrinkling in the winding film which are extremely
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36 CA 3431
undesirable factors.
It has now been discovered, that the extruder
die and equipment utilized to increase or reduce the
temperatures in the process lending up to the point of
insufflation of the stalk must have their effects applied
or directed against the stalk in a manner to have an
effect on the uniformity of the gauge of polypropylene
stalk circumferentially, in order to provide a film with
optimum physical characteristics r particularly for
electrical capacitor use. In one preferred form of this
invention, the polypropylene stalk which only moves axially
passes through a reheat oven which coaxially surround ths
polypropylene stalk and i5 caused to rotate in unison with
a combination rotating die and mandrel. At the same -time,
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a hot air ring which is positioned near the shoulder of -~
the insufflated tube is also caused to simultaneously
rotate synchronously with the reheat ovens and the die
head~ After a period of rotation, these rotating units
are caused to reverse their rota~ion and a process is
~` 20 periodically repeated to provide a form of oscillation
movement.
This invention will be better understood when
taken in connection with the following description and
the drawings in which:
Fig. 1 is a schematic illustration of a blown
tube apparatus of this invention.
Fig. 2 is a schematic illustration of the
rotating die of this invention.
Fig. 3 is an enlarged schematic and cross
sectional view of the die- } t rotator of Fig. 2.
Fig. 4 is an enlarged schematic and cross
sectional view of the die and mandrel of Fig. 2.
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Fig. S is a partial schematic and cross
sectional illustration of the rotating reheat oven,
the rotating hot air ring of this invention, and a
bubble support system.
Fig. 6 is a schematic representation of the bubble
support system for a polypropylene bubble which does not ~ ;
have a rotating hot air ring.
Fig. 7 is a schematic and exaggerated illustration
of a polypropylene strip o~ the prior art with a gauge
non uniformity.
Fig. 8 is a schematic and exaggerated illustra~
tion of a polypropylene strip of this invention with a
gauge non uniformity.
A schematic illustration of an apparatus utilized
to provide polypropylene film by the blown tube process is
shown in Fig. 1 as appara~us 10. In apparatus lQ, polypropy~
~ lene resin is fed into hopper 11 from where it passes into
; an extruder 12 where the temperature is elevated sufficiently
to provide molten characteristics to the resin. The molten
2Q resin then passes upwardly through a feed duct 13 to a
combined rotating die and ma~drel unit 14 of this invention. ;~
-~ The molten resin emanates from the die and mandrel
unit as a thick walled tube section or stalk 15. In order to
provide the proper travel or axial movement of stalk 15
through die unit 14, a palr of tube advancer rolls 16 ~,~
are employed to provide an upward pull on the stalk 15.
Rolls 16 neck the stalk 15 so as to close off the stalk
volumes below and above the rolls 16 around a central
air pipe (not shown).
Stalk 15 then passes from the advancer rolls
16 into a rotating ~t oven unit 17 which may be a combina-
tion of one or more stacked ovens. In the oven 17 the
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36 CA 3431
the temperature o-f the stalk 15 i5 raised to a predetermined
level where the plasticity of the polypropylene is in
the desired orientation range of the insufflation step.
Stalk 15, after passing through the oven 17 is caused
to move through a rotating hot air unit 18. Hot air
riny unit 18 provides currents of hot air to be directed
against the stalk 15 to stabilize and equalize the
temperature at a constant and predetermined orientation
level above that in the reheat oven. At this point, air
is introduced into stalk from the central air pipe (not
shown) to expandthe 5talk into the bubble 19, as
illustrated. `~
After the stalk 15 has been expanded to a
bubble form 19, it passes through a cold air ring 20
which cools the bubble to provide a fixed level of ~ ;
solidification of the polypropylene. Bubble 19 then
passes upwardly through a collapse frame roller assembly
21 which tapers the bubble towards a line section through
a pair of nip rolls 21a. The nip rolls 21a pinch off the
bubble 19 to retain a given amount of alr pressure there-
withinn In this process the film is biaxially oriented.
It is oriented in the cross machine direction (CMD) because
the expanding stalk or bubble provides lateral stretching. ;~
At the same time, the nip rolls provide an axial stretching
of the bubble for orientation in the machine direction (MD).
The polypropylene which emanates from the nip rolls
emanates as a dual lay flat biaxially oriented sheet joined
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at the edges. At this point the polypropylene film
runs through a slitting and winding assembly 22 wherein the
polypropylene strip is slit to provide a pair of strips
each of which are then wound on supply rolls 23, as
illustrated.
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An important feature of -this invention is a
rotating die mandrel unit 14, which is more particularly
illus-trated in Figs. 2 and 3. Referring now to Fig. 2,
the rotating die mandrel unit comprises a ro-tating assembly
24 which includes a motor and gear box unit 25, and a
chain drive 26, which rotates the rotator 27. Rotator 27
is more particularly described with respect to Fig. 3.
;; Referring now to Fig. 3, rotator assembly 27
is illustrated as comprising a stationary apparatus 29
and a rotary apparatus 30. Rotary apparatus 30 with its
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-~; sprocket drive 31 is mounted for rotation on stationary
apparatus 29 by means of bearings 31, and is effectively
sealed by means of seals 32. As the molten resin proceeds
from the extruder 12 through duct 13, it enters the
rotating die and mandrel assembly 28 as illustrated in
Fig. 4.
Referring now to Fig. 4, a rotating die and
mandrel assembly 28 comprises a die member 34 having a
cylindrical orifice 35 therein through which the molten
resin from duct 13 is caused to enter. The molten resin ;
~; which emanates from die member 34 passes over a cylindrical
-~- quench mandrel 36~ Quench mandrel 36 contains a cooling
- medium, as known in the art, for example in U. S. patent
4,008,022 Carrico dated February 15, 1977 assigned to
the same assignee as the present invention, and reduces
the temperature of the molten polymer emanating from
the die 13 to cause it to solidify and to progressively
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move upwardly as a tube. In the present invention the
mandrel 36 is connected to the die member 34. By means
of the chain drive 26, the die 34 and the mandrel 36
are caused to rotate in unison so that the polypropylene
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stalk slips at the interface between the stalks and the
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~ 36 CA 3431
mandrel 36. sy this means, gauge irregularities are
circumferentially distributed by the rotator of the die
39. Slippage of the stalk 15 permits the use of a
stationary tube ad~ancer (roll assembly 16) thus reducing
the complexity. It furthermore avoids the need for a
rotating bubhle.
~` The rotating reheat oven uni~ 17 and hot air
ring 18 are best described with respect to E'ig. 5.
Referring now to Fig. 5, there is illustrated a partial
~- 10 and schematic view of a combined hot air ring 18 and
~:'''A re~,ef,;~
e~a~ oven unit 17. Reheat oven unit 17 includes a motor
~; drive assembly 37 which rotates the furnace assembly
through a ring gear 38. The internal su~face of the ovens
(not shown), include appropriate electricallradiant heaters
;~ arranged peripherally about the interior surface which
are connected by means of electrical cables 39 to a ;
, source of power 40 (not shown). These heaters operate
in a temperature range of 700-1200F and provide heating
circumferentially of the stalk. Coiled electrical cables
. .
;~ 20 39 are sufficiently long to permit at least 360 of rotating `~
of the reheat oven 17 and are suspended on trolley means.
Reheat oven 17 may thus be defined as a circumferential
:::
or ring oven. As the stalk 15 passes through the reheat
oven, its temperature is raised to a predetermined level
which is ordinarily in the range necessary for insufflation.
The temperature in the rehat ovens must be sufficiently
high that all points of the stalk are brought up close to
- the required temperature. The stalk temperature is then
raised by a small increment to the orientation level,
within the hot air ring structure 18.
The hot air ring structure 18 of this invention ;
is also best described with respect to Fig. 5. Referring
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36 CA 3431
~ain to Fig. 5, there is illustrated a partial and
cross sectional view of a rotating hot ring structure 18.
Air ring structure 18 is supported on a frame or platforrn
41 separate from the reheat oveh 17 for independent
rotation. ~Iot air for the air ring structure 18 is supplied
from a source 42 (not shown) through conduit 43 into an
annular chamber 44. Fitted into the annular chamber is
a hot air lip assembly 45~ Lip assembly 45 forms a
closure wall for chamber 44 and is rotationally mounted
on chamber 44 through bearing means 46 and 47. The lip
assembly 45 is driven by means of a chain drive 48 from
a motor source of power 49 (Fig. 1). Air from annular
chamber 44 passes through a series of flow distributors
50 and a specially shaped orifice 51 to be directed against
stalk 15. ~he temperature of the air in the hot air ring
18 is from about 250F to ~50F and the flow rate is from ;~
70-130 cub. ft. per minute with a stalk speed of from
15 to 8~ ft. per minute.
In addition to the rotating die 14, reheat oven
17, and hot air ring 18, an improved bubble support system
52 is employed. This support system 52 is also fixed
to -the rotating lip assembly 45 as is illustrated in Fig.
1. Referring again to Fig. 1, the bubble support system
52 is illustrated as being interposed between the hot air
ring assembly 18 and the shoulder section 53 of the bubble
19. The function fo the bubble support system is not
only to lend some stability and support to the bubble
during the operation of the process and apparatus, but
also to provide initial support during the start-up
process when the bubble is first formed. A prior art
support system is illustrated schematically in Fig. 6.
Referring now to Fig. 6, a section of the bubble
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~ ~$ ~ 36-CA-3431
19 is shown as extending from the ho-t air ring 18 to the
bub~le shoulder 52. Directly above the hot air rlng there
is a series of cylindrical roll means 5~ in circumferential
and -transverse relationship to bubble 19. These rollers
are usually of a non-metallic or soft material such as
TeflonTM which lightly engage the bubble and are ro-tated by
frictional engagement with the bubble. Usually four
such rolls 5~ are employed. It has been discovered that
the optimum advantages gained by the rotary apparatuses of
this invention were being detrimentally affected by the
~; channelling and localizing of hot air flow from hot air
ring 18 around the rolls.
Channelling and localizing of this hot air has
been significantly reduced by the ball arrangement illus~
trated in Fig. 5. Referring again to Fig. 5, a frame
assembly 55 is shown as attached to the lip asser~ly ~5 of
~;~ hot air ring 18. A plurality of circumferentially mounted
brackets 56 support a circular ring or axle 57 which
` surrounds the stalk 15. Mounted for rotation on the
ring is a series of about 30 Teflon balls 58 of about 1.0
inch diameter. The balls 58 are rotated by being in
frictional engagement with tube 19 and because of their
number and shape, they provide better distribution and
equalization of the hot air from the hot air ring 18.
An improvement in film gauge is obtained through the use
of this rotating air ring.
The rotating systern of this invention comprises ;~
three units i.e., (1) die and mandrel unit, (2) reheat
oven unit of one or more ovens, and a (3) hot air ring
unit (including the bubble support) which are preferably
operated in unison and are synchronized with respect
to speed and origin. The rotational speed of each of
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- ` 36 CA 3431
the three uni-ts is adjusted to the same value a-t a common
point in their rotation. The units the rotate at the same
;~ speed for 360 oE travel. At that point a suitable control
such as a microswitch for each unit is tripped and the
unit is caused to move 360 in the reverse direction ~ -
wherein a further microswitch is tripped and the rotational
cycle is then repeated.
The electrical circuit which interconnects
~; the three units is designed so that in the vent one of
the units trips its mocroswitch prior to the tripping
of the other switches, there will be a delay in operation
until the other microswitches are tripped. Then all
-~ three units~ béi~ the reverse cycle simultaneously. This
arrangement provides sufficient although not precise
synchronization.
It has been discovered that operation, i.e.,
oscillation of the rotary units of this invention without
synchronization unexpectedly actually reinforces film
gauge irregularities at various positions about the periphery
of the bubble 19. Howev~r, oscillation even without
synchronization provides more uniformity than would be
- the case if none of the units oscillated. There are,
however, various combinations of oscillations which may be
used in the practice of this invention. Preferably a 360
max travel is employed to obtain better results.
In one practice of this invention a rotational
speed of the rotating units may fall in the range of 4 to
~- 15 minutes per revolution. A speed of 6 minutes per
:'
revolution gave good results with polypropylene film and
approximates 5 revolutions of the entire bubble for one
bubble.
The practice of this invention provides enhanced ~
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~ 36 CA 3431
control over the many variables in the bubble process of
manufacturing bia~ially oriented polypropylene film and
improves the final product, particularly with respect
to roll uniformity and flatness of the film as well as
with respect to bag and camber. Thick and thin spots in a
film strip as a result of gauge variations are very dettri-
mental to electrical grade films used in capacitors. The
improved process steps of this invention are directed to the
rotation or oscillation of the noted three units to perform
their simultaneous heat treating and smoothing functions
circumferential of the moving stalk. Initially the die
and mandrel rotates while the stalk does not rotate, but ;
moves axially from the die. Then the reheat ovens apply
a uniform exposure to heat because of the rotating reheat
ovens. And finally, a hot air ring provides uniform drafts
of hot air circumferentially of the axially moving stalk.
All units must be carefully coaxially aligned to prevent
hot spots.
The practitioner has a choice of changing the
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many process variables such as flow rate, resin temperature,
die opening, etc., to change the quality of the final fil~m.
owever at the point where this invention starts these
variables have been controlled to a very precise degree, and
yet the minor variations which occur and which are indeed
~- small when viewed alone, build up rapidly on a more or less
standard roll which may weight about 500 to 1000 lbs., be
4 to 7 feet wide and contain at least 60,000 ft. of film.
In some bubble processes of the non biaxially oriented kind,
it has been known to rotate a die head or even the nip rolls
and other upper components. The marked improvements of the
present invention are obtained by the rotation of the ~;
; temperature related components of the process, i.e., die
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36 CA 3431
and cooling mandrel, reheat oven and hot air riny, and ;
their rotation and oscillation at predetermined rates.
The practice of this invention also provides
a trouble indicator in the film making process. For
example, in the prior process the major if not solely
mechanically adjustable item was the die which was
adjusted to provide a thicker or more unifrom thickness
final film. When an irregularity occurrred in -the final
film the die was usually adjusted to accommodate the
irregularity wherever else it occurred in the system, usually
in an unknown location. If, fox example, the irregularity
occurred in the reheat oven, oscillation of the oven units
could very well magnify -the problem or create a more
serious problem with the other variables in the system. By
the same token, oscillation of one variable not contributing
to the problem could also magnify the problem if it were
occurring later in -the cycle. sy the present invention,
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the operation is provided with selection menas whereby he
may change one or more variables to flnd the variabIe which
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is causing a problem.
Many variations of the mounting and drivi~g means
. ,
of this invention will become apparent to those skilled in
the art~ All units may be separately mounted or mounted
~
for integral rotation. Several intermediate combinations are
possible. Drive mean~s may follow the same format.
In prior biaxial film processes where a gauge
non uniformity appeared, it became rolled up on itself
over thousands of feet of film in a roll until a signifi-
cant diameter difference is noted. After storage for only
- 30 a short period of time, the creep characteristics of the
film cause a set in the film such that upon unrolling
there is a distinct uneveness in the film. When such a
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~ 36 CA 3431
roll is placed in a winding machiner there are various
detrimental results, notably wrinkling of the film in
the winding machine because the irregularity causes variation
in ra-te of feed of the film from the lonyer or smaller roll
diameter embodying the non uniformity. Where the film has
a camber, because for example of a ridge in the roll diameter
at one end of a roll, the straight feed on a winding
machine causes wrinkling of the film being taken off -the roll.
The present invention is a unique defect
handling and distributor arrangement. To some degree,
because of its temperature effects it tends to ameliorate
some film irregularities. To a greater extent, it
distributes the irregularities throughout the film strip -
or bubble surface. It operates in one respect to remove
; the registration effect of film irregularities in a roll
and to distribute them laterally over a roll.
This distribution becomes even more important in
film metallizing processes where a film web is passed
through an evaucated and heated chamber wherein aluminum
is vacuum deposited on the film. Wrinkling and bagginess
~ in the film is exceedingly difficult ~o handle in such
- a process. The film of the present invention lays flat
under these extreme conditions because of its unique non
uniformity distribution structure.
For example, referring now to Fig. 7, there is ~ ;
schematically illustrated a partly unrolled section 59
of the film of the prior art. In the film section 59,
the line 61 denotes a line of a non uniformity, for example
an area of a thicker film. This area 61 is axial, i.e.,
in the machine direction and is wound up on itself. In
Fig. 8 illus-trating the film section 60 of this invention,
the same non uniformity now numbered 62 is distributed
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~ 36 CA 3431
laterally or, in effect, in both the machine direction
: and cross machine direc-tion first in one CMD and then in
the opposite C~D. Other irregularities are distributed
laterally depending on the rotation and oscillation.
Oscillation of 180 provides a dis-tribution in CMD over
one half the film width. A metallized layer 63 on this
film strip represents an improved electrode and dielectric ~ .
; for capacitor use. It is characterized by having linear
or series irregularities, non uniformity, or defects
foll.owing a bias distribution pattern which has significant
~; lateral or diagonal directions extending over about one ~-
; half or more of the film width in one CMD before reversing
to provide a diagonal path in an opposite CMD. This
film strip i.s much more stable when passing through a
vacuum metallizer due to its reduced bagginess and camber ~:~
while being unwound, passed through the vacuum chamber
. in an unsupported state and then being rewound. ~ ~:
~; While this invention has been disclosed wit~
respect to particular embodiments thereof, numerous
modifications may be made by those skilled in the art
;~ without departing from its true spirit and scope.
; Therefore, it is intended that the appended claims
cover all such modifications and variations which
come within the true spirit and scope of the present
invention.
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