GB2024702A - Blown film process - Google Patents

Abstract

A blown film process in which optimum quality film is produced by automatically controlling either the frost line position on the the tubular on blown film or the temperature in an area between the frost line and the extrusion die head. A sensing device (30) monitors the temperature in a control area between a frost line (28) and an extrusion die head (14). The signal produced is fed to a controller (32) and to a temperature reading instrument (36). The controller (32) is set to continually compare the signal received from the sensing device (30) with a pre-determined control temperature for the control area. If there is any difference between these temperatures the controller (32) operates a butterfly valve (46) which either increases or decreases the rate of flow of air from a compressor (50) to a cooling ring (52), thereby correcting the temperature in the control area to the desired control temperature at which optimum quality film is produced. <IMAGE>

Description

SPECIFICATION Blown film process Blown film process as expressed herein, 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 is defined, a control temperature is determined and maintained for the control area through a variable cooling means, whereby improved film quality, and/or increased production rates are achieved. Alternately, the invention resides in defining a controlled frost line position on the tubular blown film, monitoring such position, and providing a variable cooling means to correct deviations therefrom, for improved results.

The ultimate properties and quality of blown film can be adversely affected particularly by cyclic variations in operating conditions, and by less than perfect regularity and consistency as regards the extruded resin.

For example, the temperature of the extrusion apparatus tends to have an inherent cyclic character. Cyclic conditions also occur 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 uniformity 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, when operating conditions are deteriorating, wrinkles appear in the film. Thus reliable control devices to predict an approaching deteriorating condition to give the operaor 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 certain 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 fluctuating operating conditions.

Accordingly, the blown film process of this invention achieves finer and more precise control over film quality and/or properties. More specifically, the process of this invention provides an automatic counter-balancing control to fluctuating and/or cyclic operating conditions 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 consistently produce better quality film.

The invention provides a blown film process involving the monitoring of a select control area of the film 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 condition whereby good and preferably optimum quality film is produced, as per any given set of companion operating conditions. A variable cooling source is regulated responsive to signals received from the monitoring device, to stabilize and maintain the control temperature or control position, as applies, as an essentially constant operating condition. This system control provides precise, automatic control over film properties.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.

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 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 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 die. The speed of the drive rolls is controlled to stretch or draw the 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 essentially the same as its extruded size). The area of stretching occurs essentially 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 crystallization effects near the frost line which tends to give a stable temperature reading, or a reading which is not adequately predictive of changing conditions for 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.

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 electical lead or connection 34, and also to a temperature reading or recording instrument 36, through an electrical lead or connection 38. The temperature reading instrument converts the signal to a dial reading, thus permitting the termperature 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 device40, through a pneumatic conduit or electrical line 42 connected therewith. The valve positioner is connected by a suitable linkage assembly 44 to operate and position a butterfly type valve 46. The butterfly valve is pivotally or rotatably 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 determination of a control or target temperature for the control area. This is most expendiently determine#d empirically, by arriving at a given set of operating conditions that produce optimum quality film. Upon determining a set of such 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 area.The controller is set to continually compare the signal received from the sensing device 30, with the control temperature, or equivalently a predetermined control signal. if the signal indicates that the temperature in the monitored area is rising, the controller 32 notes the difference and directs the valve positioner 40 to proportionately move the butterfly valve to permit increased airflowto cooling ring 52to reduce the temperature in the control area until it reaches the designated control 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 control 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 required, to regulate the supply of cooling air through supply line 48, to thereby maintain or stabililize the position of the frost line.

The control process taught herein is applicable broadly to the production of film from film forming synthetic resin materials, based on the blown film process(es). Representive examples of film forming materials which are typically produced by this process are, for example, polyethylene or known copolymers of ethylene and other copolymerizing agents such as propylene, acrylic acid, or ethyl acrylate, polypropylene or known copolymers thereof, film forming polyesters, polystyrene or known copolymers thereof, vinyls such as, for example, polyvinyl chloride SaranQR, 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 thermal-couples or thermistors of the feather sensor type, particularly applicable 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 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 signal and provide an output signal that is proportional to any deviation of the input signal relative to the control signal.

The valve positioning device 40 can be electrically or pneumatically driven, depending on the input signal, space available for same, or valve type. The butterfly valve 46 can be replaced by other regulating valve types, or other devices adapted to regulate the flow (or temperature) of the cooling gas or air supplied to the cooling ring 52. The blower can supply the required refrigerated or ambient gas or air for any given blown film process or selected resin. The cooling ring 52 is positioned in an area where it can most effectively influence the temperature of the film in the monitored area, or the height of the frost line. Preferably, the air ring is positioned adjacent 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 various types 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. 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 a revolving blown film process, in a like control procedure as that described above, essentially without modification.However, under certain 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 temperature in the monitored area(s), and/or to regulate a cooling change only atspecific 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 film properties or qualities explicitly mentioned above, the control temperature and/or control frost line position can also be determined to beneficially affect the more consistent attainment of film qualities such as relates to the properties of tear and impact strength, and film shrinkage characteristics.

Example 1 The invention as described is applied to a polyethylene "revolving tube type" blown film process having a 20 inch diameter die head. An Ircon Modlinee 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 temperature of about 2400F is established.An Icon~ proportional controller is employed, Instrument Series 3400, that receives continually the electrical output of the optical pyrometer and converts the same proportionally into a penumatic 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 1500 CFM (ft3/min) capacity blower unit is employed, and is operated at full capacity, subject to regulation only by the controlled position of the butterfly valve.Table I summarizes the comparative results between control and no control situations, wherein "Maximum Rate" refers to the maximum achievable rate of film production possible, but not practical for commercial runs, and "Maximum Good Production" is the maximum rate at which "good" film is produced based on acceptable standards of film flatness and uniformity of gauge profile. The latter figures are given in Ibs/hour.

TABLE i Description No Control Control Maximum Rate--ibs/hour 675 675 Maximum Good Production 550 650 Ibs/hour Example 2 The control process hereof is also tested in a still higher volume, polyethylene blown process or production 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. Significantly increased production capacity, as compared with the "no control situation", is also demonstrated in this test, with the results being tabulated below.

TABLE II Description No Control Control Maximum Rate--lbs/hour 1000 1000 Maximum Good Production 800 950 Ibs/hour According to one aspect of the present invention there is provided 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 crystallization of the heat-plastified film material; 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 temperature to establish a substantially constant temperature in the control are of the blown film.

According to a further aspect of the present invention there is provided a blown film process wherein film is produced by extruding a continuous tube of a film forming, heat-plastified, synthetic resinous material, and stretching or drawing the tubular film about a trapped air or gas bubble, which process comprises monitoring (a) the temperature of the tubular film in a control area extending between a frost line on 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 crystallization of the heat plastified film material or (b) the position of the frost line, comparing the monitored control area temperature or front line position with a pre-determined control temperature or control frost line position, and, if there is any difference between these temperatures or positions, respectively, adjusting the control area temperature or frost line position to said predetermined control temperature or control frost line position.

According to yet a further aspect of the present invention there is provided apparatus for use in a blown film process comprising means for extruding a continuous tube of a film forming, heat-plastified, synthetic resinous material, means for stretching or drawing the tubular film about a trapped air or gas bubble, means for monitoring the temperature of the tubular film in a control area extending between a frost line in the tubularfilm and an extrusion die head, means for comparing the monitored temperature with a pre-set control temperature; and means for controlling the temperature of the film about its circumference in the control area and above the extrusion die head in response to the monitored temperature to establish a substantially constant temperature in the control area of the blown film.

Claims (10)

1. A blown film process wherein film is produced by extruding a continous 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 crystallization of the heat-plastified film material; setting a control temperatureforthe 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 temperature to establish a substantially constant temperature in the control area of the blown film.

2. A process as claimed in claim 1 wherein said temperature controlling step comprises varying the rate of flow of a cooling gas or airthrough an air ring positioned in the control area above the extrusion die head.

3. A blown film process wherein film is produced by extruding a continuous tube of a film forming, heat-plastified, synthetic resinous material, and stretching or drawing the tubular film about a trapped air or gas bubble, which process comprises monitoring (a) the temperature of the tubular film in a control area extending between a frost line on 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 crystallization of the heat-plastified film material, or (b) the position of the frost line, comparing the monitored control area temperature or frost line position with a predetermined control temperature or control frost line position, and, if there is any difference between these temperatures or positions, respectively, adjusting the control area temperature or frost line position to said pre-determined control temperature or control frost line position.

4. A process as claimed in claim 3 wherein the control area temperature or frost line position is automatically adjusted by varying the rate of flow of a cooling gas or air to the control area.

5. A process as claimed in any one of the preceding claims wherein the synthetic resinous material is as specified hereinbefore.

6. A process as claimed in claim 1 substantially as hereinbefore described with reference to Example 1 or Example 2.

7. A process as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

8. A film produced by a process as claimed in any one of the preceding claims.

9. Apparatus for use in a blown film process comprising means for extruding a continuous tube of a film forming, heat-plastified, synthetic resinous material, means for stretching or drawing the tubular film about a trapped air or gas bubble, means for monitoring the temperature of the tubularfilm in a control area extending between a frost line in the tubular film and an extrusion die head, means for comparing the monitored temperature with a pre-set control temperature; and means for controlling the temperature of the film about its circumference in the control area and above the extrusion die head in response to the monitored temperature to establish a substantially constant temperature in the control area of the blown film.

10. Apparatus as claimed in claim 9 substantially as illustrated in the accompanying drawing.