Abstract:
A device for improving the possibilities of process control in blown film manufacture is described. In blown film manufacture, the plastic material is extruded in the form of a tube from a slot die and is pulled off while maintaining an internal pressure in the tube as it cools during chemical formation of the film material. According to the invention, the film tube is flushed on all sides in this cooling area with an air stream that has been conditioned in terms of temperature and air purity, and this flushing air stream, in turn, is shielded against outside influence by a partition surrounding the area. Preferably, the flushing air stream is divided into a number of partial air streams.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to a method and a device for manufacturing a single or multilayer plastic film from a plastic raw material or a combination of several materials, said materials being extruded from a slot die and then being allowed to cool accompanied by chemical formation of the film material. 
     BACKGROUND OF THE INVENTION 
     Film production generally employs a blowing method or a casting method. The starting material is a plastic granulate that has graded properties, but has characteristics that differ from one batch to the next. In addition, the raw material can differ in temperature, moisture content, and aging. Differences in the material, especially the molecular structure, can result in different development of sensible heat in a worm conveyer screw of the extruder, which also influences the result. It is known that a conditioner located between the raw material silo and the extruder can adjust the state of the material suitably for the process. 
     The actual formation or crystallization then takes place after extrusion when the plastic is already in the film form, during the cooling and hardening of the film. 
     The quality of the film that is eventually obtained is evidenced by freedom from pinholes, in other words inclusions such as gel bodies, foreign matter, and burned plastic deposits, for example, as well as by the material and thickness homogeneity of the film and its uniform transparency, which has been shown to be an indicator of morphological structure. According to the prior art, when film sections that cannot be used are produced, they are destroyed and an attempt is made to adjust the parameters at the extruder. 
     However, correction of the factors that influence the quality of the film turns out to be rather complex. Accordingly, it has not yet been possible to perform continuous real-time control of the properties of such films. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method and a device for manufacturing a single or multilayer plastic film from a plastic raw material or a combination of several materials, said materials being extruded from a slot die and then being allowed to cool accompanied by chemical formation of the film material. During the cooling process, the extruded film passes through an air section for a certain distance. The device, in the case of blown film, has an extruder, or several extruders for a multilayer film, and a blow mold with an annular nozzle and air nozzles enclosed thereby for forming and maintaining an air bubble in the expelled tubular film. 
     The invention is intended to improve the possibilities of monitoring the process as the film is manufactured. According to the invention, this is accomplished by the fact that the film is treated by a stream of conditioned air in the vicinity of the air section through which the film material passes during the cooling process after the slot die. The air stream has been conditioned in terms of temperature and air purity. It is blown around the film on all sides, moving it in the same direction as the film cooled by it and shielding it against external influences by a partition or special flow-optimized encapsulation. Formation of the film material in this air section proceeds in a largely uncontrolled fashion. Both additional heat sources and a sudden lateral cold draft or similar asymmetric influences cause the film to shrink more rapidly on one of the two sides than on the other, with the result that the morphology is not even over the circumference and exhibits arbitrary properties in the extrusion direction such as nonuniform plane position, transparency, and bendability. In the case of blown films, these defects account for approximately one half of all the rejects. According to the invention, as a result of the flushing of this area of the air section according to the invention with a conditioned air stream which also includes undisturbed flow and the shielding of this area by encapsulation, the processes during cooling of the film material can be controlled and a possible influence is obtained that allows control with the control loop closed, for example, with constant monitoring of the film transparency. 
     Secondly, the method according to the invention provides a continuous measurement, as extensive as possible, of characteristic product properties of the cooled finished film, especially its transparency, which is scanned with an optical measuring system so that the product values of the cooled finished film are continuously measured and regulated in real time by controlling influential parameters along the film production line as a function of the measured values. These parameters can also affect only the conditioning of the raw material, but preferably this regulation is also combined with air flushing and encapsulation. As far as the handling of blown films is concerned, a device is especially preferred wherein the chamber adjoining the blow mold in which the extruded film is cooled is encapsulated by a protective housing that encloses this chamber snugly so that disturbing influences such as asymmetric heat radiation from the extruder or a sudden cold draft when the door is opened are kept away from the air stream flowing over the film. 
     The conditioning of the air flow is intended particularly for controlling the properties of temperature and air purity, especially freedom from particles and microorganisms as well as the moisture content. 
     When blown film is produced, the foil tube, until it is flattened and wound, has an air section of several meters to travel, for example 10 meters in a large system. According to one special and preferred embodiment of the invention, the air flow along this air section is divided into partial air streams each of which envelops the film, especially the film tube, along a partial stretch and each of which is produced in succession in the flow direction. At the beginning of the next partial air stream, the previous air stream is drawn off again or even deflected outward and carried away, recycled, for example. The partial air streams can therefore be adjusted to meet the local requirements of the tube, for example turbulent in the lowermost portion and uniformly laminar on all sides in the upper sections. 
     As an example of the dimensioning of such a capsule housing, where the annular nozzle has a diameter of d, the resultant foil tube to be cooled has a diameter of approximately 4d and the surrounding cylindrical capsule housing then advantageously has a diameter of 12d. In smaller systems, a narrower air stream chamber is sufficient while in larger systems of course the air stream chamber is also larger and additional parts of the installation can also be accommodated therein, such as gratings on which personnel walk. 
     The regulation of the characteristics of the film produced, especially its transparency, in the area downstream from the slot die also influences the conditioning properties of the air in the air stream and the strength and speed of the air stream. As an alternative or in addition, the regulation also influences the conditioning and transport of the raw material, especially the extrusion pressure, temperature, and moisture content, upon which the viscosity of the plastic melt in turn depends. 
     Further details, advantages, and improvements on the invention follow from the description below of preferred embodiments with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a system for manufacturing blown films according to the invention. 
     FIG. 2 is a section through a portion of the system according to a modified design. 
     FIG. 3 is a schematic view of a portion of the system in a modified design. 
     FIG. 4 is a regulating diagram for the method according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the system according to FIG. 1, plastic granulate is conducted from a silo  1  through a measuring and conditioning device  2  and a device  3  for input metering through an extruder  4  to a blow mold  5 . In extruder  4 , the material is melted and moves forward under pressure to an annular nozzle  11  in the blow mold which initially expels the material in an initially viscous form in an annular shape and produces a standing tube  12  by a pulling system and by blowing. This tube is pulled upward constantly and simultaneously cooled and solidified. After traveling a sufficient distance in air  13 , the solidification process is sufficiently advanced that the tube is flattened by rollers  14  and conducted further to a winding station  15 . 
     The annular nozzle  11 , used even for multilayer films, is surrounded in known fashion, not shown in detail in the drawing, by a cooling ring and has a Venturi profile that produces a pressure distribution that pulls the wall of the tube outward and produces an increase in diameter relative to the diameter of annular nozzle  11 , as well as stabilizing the tube. Within annular nozzle  11  is an internal cooling system with an air blower nozzle  21  and an air suction nozzle  22  that keep tube  12  in an inflated column-shaped state. The air inside of the tube must be constantly replaced to carry away heat. Nozzles  21  and  22  are connected by corresponding lines to an air delivery and conditioning device  23  that keeps the internal pressure in tube  12  and the state of the air present therein within certain value ranges. 
     To the extent described so far, the system corresponds to the prior art. 
     Tube  12 ′, folded flat after leaving the gap between rollers  14 , passes through a test station  25  that constantly monitors, using different techniques (especially a capacity test and a transparency test performed with a scanning camera), the characteristics and the homogeneity of the manufactured film and delivers the corresponding measured values as signals over a line  26  to a data processing system (DP system  27 ). DP system  27  also receives, over a line  28 , measured values that indicate the state of the plastic granulate and, after processing the values sent to it, sends commands over a line  29  to the measuring and conditioning device  2  for conditioning, for example pre-drying, of plastic granulate. DP system  27  further sends commands over a line  30  to device  3  for input metering and sends commands over a line  31  to extruder  4  which, depending on these commands, determines the temperature of the molten  25  plastic that is delivered to annular nozzle  11 . DP system  27  influences air delivery and conditioning device  23  through data transmitted over a line  32  which is intended especially to keep the internal pressure of tube  12 , as well as the other states of the air contained in tube  12 , within limits. 
     In addition, DP system  27  transmits commands over a line  33  to an air delivery and conditioning device  36 . The function of DP system  27  in this case is not limited to controlling the supply of raw material to annular nozzle  11 , but also includes control of the processes in the wall of tube  12 , namely the cooling and chemical formation that occur there. In the system according to FIG. 1, conditions are created in the vicinity of tube  12  that are uniform and adapted to the chemical processes in the respective portions of tube  12 . Air section  13  is divided into individual partial air sections  38 , each of which, in an annular arrangement around tube  12 , has a row of blowing nozzles  39  at its lower end and a row of suction nozzles  40  or baffle plates  41  at its upper end. The nozzle arrangements are each connected with the air delivery and conditioning device  36  that appropriately adjusts the conditions of the air streams in the partial air sections, in other words the air throughput and the flow rate, air temperature, and air moisture content, etc.. The air blowing nozzles  39  blow air in a direction essentially parallel to the extrusion direction of plastic annular nozzle  11  and are located coaxially to the axis of plastic annular nozzle  11 . A first arrangement of air blowing nozzles  39  is located in the vicinity of and surrounding plastic annular nozzle  11 . Each arrangement of air suction nozzles  40  is located at a distance from one of the arrangements of air blowing nozzles  39  and surrounding the axis of plastic annular nozzle  11 . The arrangements of air blowing nozzles  39  and the arrangements of air suction nozzles  40  are arranged pairwise with respect to each other wherein an arrangement of air blowing nozzles  39  abuts an arrangement of air suction nozzles  40 . To guide the air streams in the partial air sections  38  and to shield them against external influences, which can be very strong and disturbing especially in large holes, tube  12  and air sections  13 ,  38 , as well as the arrangements of nozzles  39 ,  40  and diaphragms  41 , are sealed by a cylindrical housing  45  and encapsulated. The processes in the tube wall therefore take place in a completely undisturbed environment. 
     As a result of the division of air section  13  into partial air sections  38 , each of which can be controlled separately in terms of its partial air flow, as a function of material and other parameters, the appropriate conditions can be provided and produced for each partial section. Chemical formation can therefore be controlled at any time to produce complete homogeneity but also can be optimized in terms of the film characteristics. In an improvement on the method, the entire manufacturing process can be divided into individual method steps with separate monitoring of the steps in the various sections, which are viewed as separate chambers. This is shown in FIG.  4 . The individual method steps are separated in order to prevent mutual influence. Each method step is encapsulated in an enclosed process chamber so that the process can proceed undisturbed by external and mutual influences. In-line measuring systems allow significant measured values that can be assigned to the individual influential parameters. The goal in this case is that in-line measured product quality data, evaluated regulating behavior, stable process data, and relevant evaluated material data can be fed through a DP system to a control and regulating system, with the desired quality being obtained through the measured product quality. The corresponding chambers of the process sections are shown in FIG. 4 in detail wherein: 
     Chamber I comprises an extrusion chamber. Constant extrusion conditions produced by a conditioned environment, monitored material state, and metering, allow evaluation of the regulating behavior on the basis of stability and a DP system, as well as the attribution of deviations in an in-line measured quality criterion to an influential parameter. It is possible to determine heat losses from the difference between the energy applied and the heat loss. The difference is proportional to the energy content (enthalpy) of the plastic melt. 
     Chamber II comprises a process chamber. The process is stabilized and measurable here. Measurement data such as the temperature above the height of the bubble and microdust contamination of the individual defined sections are significant and can be attributed to other process procedures. 
     Chamber III comprises a measurement chamber. On the basis of the processes that previously proceeded in a stable manner, measurement data such as optical measurement of transparency, thickness distribution via infrared systems, and inclusions in the film can be gathered. This data also functions as predictors and can be assigned to other process data by the DP system. 
     Chamber IV comprises a recycling chamber. Under conditioned conditions, deliberate after-treatment is possible, for example tempering or restructuring of the mechanical shape, printing, etc. 
     Chamber V comprises a winding chamber. At this point, changes in the regulating behavior during winding such as web tension and winding quality can be traced back to process steps. 
     The system shown is therefore an intelligent quality feedback process control (IQFP) system. 
     FIG. 2 shows an even more extensive differentiation of the method and the device. In the section made through a partial air section, the blowing nozzles  39  can be seen at the bottom. In between are arrangements of ray sources  49  that emit beams  50  at an acute solid angle and are located opposite receiving grids  51  that receive beams  50  and deliver a signal that indicates the shadow of tube  12 . Such arrangements of ray sources  49 /receiving grids  51  are installed at different levels in housing  45 . The results of the receiving grids  51  are transmitted to regulator  27  which immediately recognizes possible deviations in the travel of the tube wall. In this design the different blowing nozzles  39  of a common partial section  38  can be controlled separately, at least as far as throughput rate and air flow rate are concerned, so that asymmetries can immediately be controlled. 
     FIG. 3 shows schematically a design simplified by comparison with the system in FIG. 1 for a small system. Here, a single set of blowing nozzles  39  is located in housing  45  and produce an essentially cylindrical air curtain  55  that extends from the bottom to the top and surrounds tube  12 . The air curtain covers the entire air section  13  and escapes at the top through outlet openings  42  from housing  45 . The air for air curtain  55  comes from the air delivery and conditioning device  23  and has a state optimized for the formation of the wall of tube  12 .