Note: Descriptions are shown in the official language in which they were submitted.
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BLO~N FILM PROCESS
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Blown film process as expressed h0rein, refers
to the manufacture of films or sheets from a synthetic
resinous material. The film is produced by continuous
extrusion of a tube of heat plastified resin, stretching
or drawing the tube about a trapped air or gas bubble, and
simultaneously cooling the tube such as by external or
internal cooling means.
The invention particularly relates to such a
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process wherein a control area i5 defined, a control tem- ~
10 perature is detérmined and maintained for the control area ~i;
~through a variable cooling means, whereby improved film
quality, and/or increased production rates are achieved.
Alternately, the invention resides in defininq a con~
trolled frost line position on the tubular blown film,
monitoring such position, and providing a variable cooling
means to correct deviations theréfrom, for improved results.
The ultimate properties and quality of bl-own
film can be adversely affected particularly by cyclic
variations in operating conditionsj and by less than per-
ect regular~ity and consistency as regards the extrudedresin. For example, the temperature of the extruslon
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apparatus tends to have an inherent cyclic character.
Cyclic conditions also occux in conventional film cooling
systems whether based on a refrigerated source or if
taken from ambient air. Additionally, film extruders
frequently require a filtering system that will gradually
clog up with impurities~ thus inducing a variable effect,
particularly a change in melt index, in the resin passing
through the filter. The resin itself may not be entirely
consistent in quality, and may have a changing melt index
value and/or melt temperature.
Operating inconsistencies and material variables
of the above type can produce film of a poor quality in
the sense of poor film flatness (i.e. appearance of wrinkles
in the film) and poor uniformiky of the gauge profile of
the film. The gauge profile can be somewhat controlled
by thickness measuring devices and systems, which give
the operator some indication and warning when the gauge
profile is deteriorating so -that the appropriate control
corrections can be made. However, ~hen operating condi-
tions are de~eriorating, wrinkles appear in the ~ilm.Thus reliable control devices to predict an approaching
deteriorating condition to give the operator adequate
warning to make appropriate corrections have not yet been
developed.
Even with close attention by a skilled operator,
it has been difficult to control film quality above cer-
tain ceiling production rates. Even when operating within
a production range considered manageable by a skilled
operator, the film quality can be less than desired and
less than prescribed by specification tolerances, due to
imprecise and inadequate control over cyclic and/or fluc-
tuating operating conditions.
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Accordingly, the blown film process of this
lnvention achieves finer and more precise control over
film quality and/or prop~rties. More specifically, the
process of this invention provides an automatic counter-
balancing control to fluctuating and/or cyclic operatingconditions to permit a significant increase in the rate
of extrusion while retaining an acceptable level of
quality in the film produced, and/or which would consis-
tently-produce better quality film.
The invention provides a blown film process
involving the monitoring of a select control area of the
filml or alternately, the monitoring of the position of
the film frost line. A control temperature or control
frost line position is determined which reflects the con-
dition whereby good and preferably optimum quality film
is produced, as per any given set of companion operating
conditi n3. A variable cooling source is regulated res-
ponsive to signals received from the monitoring device,
to stabilize and maintain the control temperature or con-
trol position, as applies, as an essentially constant
operating condition. This system control provides precise, ~-
automatic control over film propexties. It is particularly
advantageous in providing prompt corrective response to the
approaching condition of loss of film flatness, for which
there has been inadequate warning or predicting systems
in the past.
~ he invention particularly resides in a blown
film pxocess wherein film is produced by extruding a con-
tinuous tube of a film forming, heat-plastified, synthetic
resinous material, stretching or drawing the tubular film
about a trapped air or gas bubble, comprising the steps of:
monitoring the temperature of the tubular film in a control
area extending between a frost line in the tubular ~ilm and
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an extrusion die head, the control area being remote from
and spaced sufficiently from the frost line so as to be
l~ss influence~ by crystallization of the heat plastified
film material; cooling -the film about its circumference
between the control area and extrusion die head~ setting a
control temperature for the control area; comparing the
monitored temperature with the control temperature; and
controlling the tempera~ure of the film about its circum-
ference in the control area and above the extrusion dle
head in response to the monitored temperature to establish
a substantially constant temperature in the control area
of the blown film.
Referring particularly to the drawing which
illustrates a preferred embodiment of the invention, a
film forming synthetic resinous material is introduced
into a heated extruder 10 through a hopper 12, from whence
it is eventually expelled in a heat plastified condition
to a die head 14 by way of a connecting conduit 16. The
resin emerges from the die in the form of a continuous
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-tubular film or tubular envelope 18. The tube is stretched
or drawn about a trapped bubble that is maintained and
replenished by a regulated pressure line 20 that introduces
controlled amounts of air or gas to the interior of the
5 tube. A collapsing rack 22, and cooperating nip rolls 24 -~
and 26, eventually collapse and flatten the tube at an area
remote from the die head. This process will also typically
include drive rolls (not shown) located beyond nip rolls
24 and 26, to provide a pulling -force to advance the tube
from the dieO rrhe speed of the drive rolls is controlled
to stretch or draw thé tube longitudinally, and this
speed factor, together with other controlling factors,
will determine the circumferential size of this tube (i.e.
whether it is distended, drawn-down, or maintained essen~
tially the same as its extruded size). The area of stretching
occurs essential~y between the die head and the film frost
line shown at 28. Above the frost line, the film has
advanced to a solidified or semi-solidified condition.
A temperature sensing device 30 is focused to
read and continually monitor the temperature of the film
in a control or target area that extends below frost line
28, and above die head 14. The area of the film directly
adjacent the frost line is not as good a predictive or
control area. This is presumably because of film crystal-
lization effects near the frost line which tends to givea stable temperature reading, or a reading which is not
adequatély predictive of changing conditions ~or which
the control system is designed to correct automatically.
A good predictive control area, however, will exist at an
area remote from and spaced sufficiently downwardly from
the frost line so as to be less influenced (or non-influenced)
by crystallization effects occurring at and in the near
vicinity of the frost line.
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The signal produced by the temperature sensing
device, or the output of this device, is fed to a controller
or controlling means 32 -through an electrical lead or con-
nection 34, and also to a temperature reading or recording
S instrument 36, through an electrical lead or connection
38~ The temperature reading instrument converts the signal ::
to a dial readingl thus permitting the temperature in the
control area to be determined numerically at any given time
in the operation.
The output of the controller is adapted to operate
an air or electric motor or valve positioner device 40,
through a pneuma-tic conduit or electrical line 42 connected
therewith. The valve positioner is connected by a suitable
linkage assembly 44 to operate and position a butterfly
lS type valve 46. The butterfly valve is pivotally or rota-
tably positioned in a cooling air or gas supply line 48
which delivers air from a blower or compressor unit 50, -
: . to a cooling ring 52 that is disposed about the lower extreme
~or tube 18, just above die head 14~ : ;
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The system depends on the selection or determina-
tion of a control or target temperature ~or the control
area. This is most expediently determined empirically~, ~y
arriving at a given set of operati.ng conditions that pro-
duce optimum quality film. Upon determining a set of such
?5 conditions~ the temperature in the control area is read
: and established as the control temperature. The dial reading
while not essential in operating the controls, permits the
operator to observe and record, if desired, the temperature
in the control areaO The controller is set to continuall~ :
compare the signal received from the sensing device 30,
with the control temperature, or equivalently.a predetermined
control signa~. If the signal indicates that the tempera- -
ture in the monitored area is rising, the controller 32 notes
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the difference and directs the valve positioner 40 to
proportionately move the butterfly valve to permit increased
air flow to cooling ring 52 to reduce the temperature in
the control area until it reaches the designated cont~ol
temperature~ Alternately, when a drop in temperature
in the control area is sensed, the controller responds by
regulating the butterfly valve 46 to decrease the flow or
output of cooling air to the cooling ring 52. Necessarily
the null position, that is, the position the valve assumes
when reading a stable temperature condition, is at a point
between the extreme open and extreme closed positions of
the butterfly valve.
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The invention may also be practiced by utilizing
the height or position of the frost line as the control
indicator. The monitoring device would be modified to
- optically or otherwise read the frost line height and
produce signals indicating deviances therefrom. The con-
trol frost line position can be determined as before, ~ -
that is, by operating empirically to define a given set
of conditions under which quality film is produced, and
defining the control position as that at which the frost
line resides under such conditions. The signal provided
by the monitoring device would be fed to the controller
and compared with a control signal. Corrective action is
then taken, as re~uired, to re~ulate the supply of cooling
air through supply line 4~, to thereby maintain or stabili~
lize the position of the frost line.
.
The control process taught herein is applicable
broadly to the production of film from film forming syn-
thetic resin rnaterials~ bas~d on the blown film process~es).Representative exarnples of film forming materials which
are typically produced by t`his process are, for example,
polyethylene or ~nown copolymers of ethylene and other
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copolymerizlns agents such as propylene, acrylic acid,
or ethyl acrylate, polypropylene or known copolymers
thereof, film forminy polyesters, polystyrene or known
copolymers thereoE, vinyls such as, for example, poly-
vinyl chloride Saran~, or film forming polyamides.
Monitoring devices applicable for use in this
invention would be heat sensing devices such as, for
example, optical pyrometers or radiation thermometers,
or t~lermal-couples or thermistors of the feather sensor
type, particularly applicabl~ for very thin and delicate
webs of synthetic resinous materials~ When the control
is based on a controlled frost line position, a haze meter
can be employed to read the position of the frost line,
~ and to produce or generate a signal for regulating the
: lS supply of cooling air from the source.
The controller 32 is adapted to compare the input
signal from the monitoring device 30 with a control slgnal
and provide an output signal that:is proportional to any
deviation of the input signal relative to :the controI :~
20 ~ signal~
.
The valve positioning device 40 can be electri-
; .~ cally X pneumatically driven, depending on the input signal,
~ space available for same, or valve type. The butterfly
. vaXve 46 can be replaced by other regulating valve types t
25~ or other devices adapted to regulate the flow (or tempera-
ture) of the cooling gas or air supplied to the cooli~g~
ring 52. The blower can supply the required refrigerated:
or ambient gas or air for any given blown film process or
~elected resin. The cooling ring 52 is positioned in an
-area where it can most effectively influence the tempera-
ture of the ilm in.the monitored area, or the;height of
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the frost line. Preferably, the air ring is positioned
ad~acent to the die head 14 as shown in the drawing. Other
cooling devices can be substituted for the cooling ring 52
or employed together therewith (i~e., of the varlous types
5 known to the art, such as internally positioned cooling
devices).
Certain of the known blown film processes include
operating modes that may necessitate some modification
hereof in order to apply these teachings to such a process~
10 For example, a revolving die head, or a revolving take-up
assembly, or the like (i.e., such as to continually revolve
tube 18), is oftentimes employed in the blown film process
for certain resins and to produce certain end products.
The process described above can and has been applied to
15 a revolving blown film process, in a like control procedure
as that described above, essentially without modification. ~ ~
However, under cer-tain conditions, it may be desirable to ?
read or monitor several control areas about a revolving
tube, and/or to employ an integrator to average the tem- ~-
~i~ 20 perature in the monitored area(s), and/or to regulate a
cooling change only at specific intervals, such as after
each complete revolution of the film~ as may be found
desirable or advantageous in any specific film line.
In addition to controlling the ~ilm properties
or qualities explicitly mentioned above, the control tem-
perature and/or control frost line position can also be
determined to beneficially affect the more consistent
attainment of film qual-ties such as relates to the pro-
perties of tear and impact strength, and film shrinkage
characteristics.
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Exa ple 1
The invention as described is applied to a
polyethylene "revolving tube type" blown film process
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having a 20 inch diameter die head. An Ircon Modline~
non~contacting optical pyrometer (radiation thermometer),
"Instrument Series 3400~' is used as the instrument to sense
and monitor the temperature of the film~ A control area
is defined that is at least 3 inches below the frost line,
and most optimally is about 9 inches below the frost line
and at least 6 inches above the die head. A control tem-
perature of about 240F is establishedO An Ircon~ propor-
tional controller is employed, Instrument Series 3400,
that receives continually the electrical output of the
optical pyrometer and converts the same proportionally
into a pneumatic output that controls an air piston motor
having an integral butterfly valve. The latter unit or
assembly is available under the trade designation "Valtek
Vector One Butterfly Valve". An approximately 15~0 CFM
(ft3/min) capacity blower unit is employed, and is operated
at full capacity, subject to regulation only by the con-
trolled position of the butterfly valve. Table I sumrnarizes
the comparative results between control and no control
20 situations, wherein "Maximum Rate" refers to the maximum~ ;
- achievable rate of film production possible, but not prac-
tical for commercial runs, and "Maximum Good Production"
is the maximum rate at which "good'l film is produced based
on acceptable standards of film flatness and lmiformity
of gauge profile. The latter figures are given in lbs/hour.
TABLE I
No ControlControl
~aximum Rate~-lbs/hour 675 675
Maximum Good Production 550 650
30 lbs/hour
Exarnple 2
The control process hereof is also tested in a
still higher volume, polyethylene blown process or production
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line, employing a 30 inch diameter die, the process being
also of the revolving tube type. The control process and
the apparatus for accomplishing the same, is essentially
-the same as described supra. The control temperature and
control area is near the same as with Example 1. Signi~
ficantly increased production capacity, as compared with
the "no control situation", is also demonstrated in this
test, with the results being tabulated below.
TABLE II
10 DescriptionNo Control Control
Maximum Rate--lbs/hour 1000 1000 ~-
Maximum Good Production 800 950
lbs/hour
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