CA1098271A - Blown film process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A blown film process wherein the temperature of the film is continually monitored in a select control or target area that is remote from the film frost line;
a control or "target temperature" is determined based empirically ox otherwise on a given set of companion operating conditions; and a variable film cooling source or supply is regulated to establish the control tempera-ture as an essentially constant or non-variable operating condition. The process is characterized by an essentially stable frost line position and to the extent movement of the frost line can be optically or otherwise observed or sensed, and an appropriate signal derived therefrom, the process can be alternately practiced based on moni-toring the position of the frost line, defining a control or "target" frost line position, and regulating a film cooling source responsive to deviances or movement from the control position.

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Description

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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 .
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 16,763B-F
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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 16,763B-F -3-, ::

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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 16,763B-F -3a-, . ~
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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~ : ;
~: ' 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 16,763B-F

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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.
- ' 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.
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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 16,763B-F

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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~
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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 16,763B-F
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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 16,763B F

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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 16,763B F
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Claims (2)

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

1. A blown film process wherein film is produced by extruding a continuous 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 film and an extrusion die head, the control area being remote from and spaced sufficiently from the frost line so as to be less influenced by crystal-lization 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 temperature of the film about its circumference in the control area and above the extrusion die head in response to the monitored tempera-ture to establish a substantially constant temperature in the control area of the blown film.

2. The process of Claim 1, wherein said tempera-ture controlling step comprises varying the rate of flow of a cooling gas or air through an air ring positioned in the control area above the extrusion die head.

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