IT201900011751A1 - SYSTEM AND METHOD FOR CHECKING AND CORRECTING THE THICKNESS OF A PLASTIC FILM IN BLOWING FILM - Google Patents

Description

Description of the industrial invention entitled:

"SYSTEM AND METHOD FOR CHECKING AND CORRECTING THE THICKNESS OF A PLASTIC FILM IN BLOWING FILM"

Field of the invention

The present invention relates to a system for controlling and correcting the thickness of a plastic film in blow molding. In a further aspect, the present invention relates to a blow-molding head and a blow-molding plant equipped with a system for controlling and correcting the thickness of a plastic film.

Finally, the present invention relates to a method of controlling and correcting the thickness of a plastic film in the blow molding film.

Known art

In polymer technology, blow molding is a process used to obtain polymer films of reduced thickness, typically between 15 ÷ 500 µm, by means of blow molding systems. Such films can be used, for example, to produce films for packaging, plastic bags, or films for greenhouses.

The plastic material, or the polymer, in solid state (for example granules or powder) is initially fed into an extruder. A typical extruder used in the industrial field essentially consists of a cylinder inside which one or two worm screws rotate. A solid-state plastic material loading hopper is arranged in correspondence with the supply to the cylinder, while a "die" or "matrix" from which the molten plastic material comes out is placed at the exit of the cylinder.

The extruded product from the die is then blown by means of a suitable blow molding head defining a bubble conformation. The blown air allows to increase the diameter of the bubble and, at the same time, reduces its wall thickness, forming the tubular plastic film of the desired thickness. The tubular film then passes through a cooling ring and subsequently through a calibration cage, being pulled upwards by appropriate rollers, defining the stretching phase which aims to stabilize the geometry and morphology of the material.

Due to longitudinal stretching and blowing, the polymer melt is subjected to a biaxial elongation flow. The deformation in the two directions therefore orients the macromolecules both in the longitudinal and in the circumferential direction. These two actions take place within the so-called "frost line", while the deformation above the aforementioned section in which the polymer solidification occurs, or crystallization, are determined solely by the mechanical action on the tubular plastic film during handling. Downstream of the calibration stand suitable means for measuring the thickness of the tubular film are arranged. In particular, these measuring means are typically realized by means of a capacitive sensor arranged on a rotatable element around the tubular film. The rotatable capacitive sensor therefore defines a reading of the tubular film capable of analyzing the entire circumference in a short time. In particular, the capacitive sensor is made in such a way that the variation of the physical quantity to be detected produces the variation of one or more of the parameters relating to the dielectric constant, to the surface of the plates or to the distance between the plates, such as to produce a variation of the capacitance of a capacitor. It is also possible that the measurement of the thickness of the tubular film is carried out after flattening, or downstream of the main drive of the tubular film, with a sensor of the scanner type capable of transversely reading the information relating to the thickness. Following the measurements carried out, the measurement data are sent to appropriate control means that verify the compliance of the thickness with the established standards and modify the filming parameters of one or more parts of the blow molding plant.

In known blow-molding systems, the measurements carried out downstream of the calibration stand are carried out well above the freezing line, when the material constituting the tubular film is now stabilized. This allows the capacitive sensor to make a measurement of the actual thickness of the film in a position where further structural changes, i.e. changes to the geometry and morphology of the material, are no longer possible.

Therefore, the measuring means and the control means are separate and act on very distant portions of the tubular film. The thickness measurement with the capacitive sensor is, in fact, carried out well beyond the freezing line downstream of the film guide cage. The control system allows a feedback control but acting on portions of the tubular film upstream of the freezing line, therefore any error that is generated starting from the freezing line will be detected only much later with a considerable waste of material in case of errors. In particular, in case of need for corrections, the number of out-of-control meters of tubular film is directly related to the distance from the freezing line and to the extrusion speed. Furthermore, this distance does not allow to directly correlate the changes made to the thickness measurements detected, since it is necessary to continuously make changes to stabilize the tubular film.

The arrangement of the measuring means downstream of the film guide cage also increases the costs and time associated with the construction of the blow-molding film-making system, considerably increasing its complexity.

It would therefore be desirable to have a system for controlling and correcting the thickness in the blow-molding film, capable of minimizing the drawbacks described above. In particular, it would be desirable to have a system for controlling and correcting the thickness in the blow-molding film, capable of maximizing the effects of changes in the extrusion parameters, reducing the waste of extruded material.

Furthermore, it would be desirable to have a blow molding head and a blow molding plant capable of minimizing the drawbacks described above. In particular, it would be desirable to have a blow molding head and a blow molding plant with reduced manufacturing and assembly costs, capable of improving the thickness control of the extruded tubular film.

Finally, it would be desirable to have a method for controlling and correcting the thickness in the blow-molding film which is capable of minimizing the drawbacks described above. In particular, it would be desirable to have a method for controlling and correcting the thickness in the blow-molding film which is capable of obtaining a plastic film with optimal thickness control.

Summary of the invention

The object of the present invention is to provide a system for controlling and correcting the thickness of a plastic film in blow-molding and a method for controlling and correcting the thickness of a plastic film in blow-molding capable of minimizing the above problems.

In particular, the object of the present invention is to provide a system for controlling and correcting the thickness of a plastic film and a blow molding head equipped with thickness measuring means capable of measuring the thickness of the tubular film in correspondence with the frost line. , minimizing the waste of material in case of changes to the parameters of the blow molding head.

Finally, an object of the present invention is to provide a blow molding plant with a reduced production and assembly cost, capable of ensuring effective control of the thickness measurement.

The aforementioned purposes are achieved by a system for controlling and correcting the thickness of a plastic film in the blowing film, in accordance with the attached claims.

The control and correction system for the thickness of a plastic film in blow molding includes:

- a cooling ring of a plastic film, defined by a plurality of cooling sectors and provided with an inlet portion and an outlet portion of the plastic film;

- means for measuring the thickness of the plastic film;

- control means operatively connected to the thickness measuring means;

the system for controlling and correcting the thickness of a plastic film in blow molding is characterized by the fact that the thickness measuring means comprise at least one non-contact temperature sensor arranged in correspondence with the outlet portion of the cooling ring to detect continuous thickness of the plastic film,

in which the control means are adapted to automatically control the cooling ring as a function of the thickness identified by the thickness measuring means by selective actuation of one or more of the cooling sectors of the cooling ring.

The measuring means arranged at the exit portion of the cooling ring allow to obtain a measurement of the thickness of the film at the frost line, minimizing the waste of material in the event of modifications. In particular, the use of a non-contact temperature sensor allows the indirect measurement of the thickness without any contact with the material even before the formation of the frost line.

Preferably, the thickness measuring means comprise a contactless temperature sensor movable along the outlet portion.

Even more preferably, the system for controlling and correcting the thickness of a plastic film in the blowing film comprises a rotating ring operatively connected and concentric to the cooling ring, in which the non-contact temperature sensor is arranged on the rolling ring.

In this way, it is possible to check the thickness of the entire circumference by detecting any systematic errors.

Preferably, the thickness measuring means comprise a plurality of non-contact temperature sensors fixed with respect to the cooling ring and equally spaced along the circumference of the cooling ring at the outlet portion.

Even more preferably, the thickness measuring means include four non-contact temperature sensors fixed with respect to the cooling ring and spaced in succession at an angle equal to 90 ° along the circumference of the cooling ring at the outlet portion.

In this way, it is possible to check the thickness of the entire circumference by detecting any random errors.

Preferably, the non-contact temperature sensors are of the infrared type and the thickness measuring means are adapted to define an infrared image of the plastic film,

in which the control means are designed to automatically control the cooling ring according to the thickness identified by the thickness measuring means by means of the infrared image and / or the infrared wavelength. The analysis of the image or infrared wavelength allows to infer, or to indirectly determine, the thickness of the tubular film.

Preferably, the non-contact temperature sensors are of the infrared type and the thickness measuring means are adapted to define a temperature matrix of the plastic film,

in which the control means are adapted to automatically control the cooling ring as a function of the thickness identified by the thickness measuring means by means of the temperature matrix.

The identification of the temperature in a certain area of the tubular film, or the temperature variation over time, allows to infer, or to indirectly determine, the thickness of the tubular film.

The aforementioned purposes are further achieved by a blow molding head, in accordance with the attached claims.

The blow molding head comprises a die of a plastic film, defined by a plurality of heating sectors, and

the blow molding head is characterized in that it comprises a system for controlling and correcting the thickness of a plastic film in the blowing film according to one or more of the attached claims, and

in which the cooling ring is arranged downstream of the supply chain.

The aforementioned purposes are further achieved by a blow-molding film-making plant, in accordance with the attached claims.

The blow molding film plant includes:

- a blow molding head for making a tubular plastic film;

- one or more extruders arranged upstream of the blow molding head to feed the extruded plastic material;

- a sizing cage arranged downstream of the blow molding head;

the blow molding plant is characterized by the fact that the blow molding head is according to one or more of the attached claims, in which the calibration cage is arranged downstream of the control and correction system for the thickness of a plastic film, And

in which at least one non-contact temperature sensor is arranged at the exit portion of the cooling ring to continuously detect the thickness of the plastic film upstream of the calibration cage. The realization of the measuring means in correspondence with the cooling ring and, in particular, upstream of the calibration cage and, in particular, upstream of the freezing line, allows to minimize the costs of construction and assembly of the system. Furthermore, this arrangement allows to optimize the modifications in the extrusion, cooling and stretching phases, reducing the waste of material to a minimum.

Finally, the aforementioned purposes are further achieved by a method of controlling and correcting the thickness of a plastic film in the blowing film, in accordance with the attached claims.

The method of controlling and correcting the thickness of a plastic film in blow molding includes the following steps:

- feeding at least one solid plastic material into at least one extruder for making the molten plastic material;

- feeding the melted plastic material to a blow molding head for making a tubular plastic film;

- feeding the tubular plastic film to a calibration cage arranged downstream of the filming head by blowing;

the method of controlling and correcting the thickness of a plastic film in blow molding is characterized by the fact that it comprises the following steps: - identification of the thickness of the plastic film by means of a control and correction system for the thickness of a plastic film in blow molding comprising a cooling ring and at least one non-contact temperature sensor arranged in correspondence with an outlet portion of the cooling ring, in which said system for controlling and correcting the thickness of a plastic film is arranged between the filming head for blow molding and the calibration cage to continuously detect the thickness of the extruded plastic film upstream of the calibration cage. - automatic control of the cooling ring according to the thickness identified by selective activation of one or more of the cooling sectors of the cooling ring.

Description of the figures

These and further characteristics and advantages of the present invention will become evident from the description of the preferred embodiment, illustrated by way of non-limiting example in the attached figures, in which: - Figure 1 is a perspective view of a first embodiment of the system for controlling and correcting the thickness of a plastic film, in accordance with the present invention;

- Figure 2 is a top plan view of the control and correction system of the thickness of a plastic film of Figure 1;

Figure 3 is a side plan view of a preferred embodiment of the blow molding head comprising the system for controlling and correcting the thickness of a plastic film of Figure 1, in accordance with the present invention;

- Figure 4 is a top plan view of a second embodiment of the system for controlling and correcting the thickness of a plastic film, in accordance with the present invention.

Detailed description of the invention

The production process of blow molding films, so-called blown film, consists, as known, in the extrusion of a polymer melt through an annular hole. The extruded product is blown by means of a suitable blow molding head defining a bubble conformation which is, therefore, inflated by means of air blown inside defining a tube which is subsequently stretched. The blown air allows to increase the diameter of the bubble and, at the same time, reduces its wall thickness, forming the tubular plastic film of the desired thickness.

With reference to Figures 1-3, a first preferred embodiment of the system 100 for controlling and correcting the thickness of a tubular plastic film 10 in the blow-molding film is illustrated, in accordance with the present invention. With reference to the same Figures 1-3, a first preferred embodiment of the system 100 for controlling and correcting the thickness of a tubular plastic film 10 in the blow-molding film is illustrated, as well as of the blow-film film head 1 comprising the aforementioned system 100 for controlling and correcting the thickness of a plastic film 10 in the blowing film, in accordance with the present invention.

In the following, the same numbering will be maintained in the description of further embodiments, if reference is made to the same elements. The blow-molding head 1 comprises a spinneret 11 of a tubular plastic film 10 and a system 100 for controlling and correcting the thickness of a tubular plastic film 10 in the blowing-forming film, provided with a cooling ring 21 for the tubular plastic film 10 . The die and the cooling ring 21 are arranged in succession, or the cooling ring 21 is arranged downstream of the die 11.

The spinneret 11 is preferably made up of one or more superimposed circular elements and is provided with a plurality of distinct heating sectors, not illustrated in detail. The heating sectors identify a plurality of circular sectors which integrally define the circumference of the aforementioned spinneret 11, in which each circular sector preferably has the same size. Each heating sector is independent, ie it can be operated independently with respect to the other sectors in such a way as to precisely identify the portion of tubular plastic film 10 to be heated. The die could also be made according to a different configuration without modifying the inventive concept of the present invention.

The cooling ring 21 is made of a circular element and is provided with a plurality of distinct cooling sectors, not illustrated in detail. The cooling sectors identify a plurality of circular sectors which integrally define the circumference of the aforementioned cooling ring 21, in which each circular sector preferably has the same size. Each cooling sector is independent, ie it can be operated independently with respect to the other sectors in such a way as to precisely identify the portion of tubular plastic film 10 to be cooled. It is possible to identify two distinct portions of the cooling ring 21, preferably as a function of the positioning with respect to the spinneret 11. In particular, an inlet portion 121 and an outlet portion 221, respectively adjacent and distant from the spinneret 11, in which the portion 121 inlet is suitable for the inlet of the tubular plastic film 10 to be cooled while the outlet portion 221 is suitable for the outlet of the tubular plastic film 10 cooled by the cooling ring 21. The tubular plastic film 10 then passes through the cooling ring 21, from the inlet portion 121 to the outlet portion 221, and subsequently through a calibration cage, (not shown) being pulled upwards by suitable rollers, defining in thus the "stretching phase" whose purpose is to stabilize the geometry and morphology of the material.

Unlike known solutions, the system 100 for controlling and correcting the thickness of a tubular plastic film 10 according to the present invention further comprises means 31 for measuring the thickness of the tubular plastic film 10. In particular, the thickness measuring means 31 allow the detection of the thickness profile of the tubular plastic film 10, as described in detail below.

In the first embodiment, as illustrated in Figures 1-3, the system 100 for controlling and correcting the thickness of a tubular plastic film 10 comprises a rotatable ring 51 operatively connected and concentric to the aforementioned cooling ring 21. The measuring means 31 are, therefore, arranged on the rotatable ring 51 to be moved along the circumference of the cooling ring 21, or along the circumference of the tubular plastic film 10 to be measured. The measuring means include at least one non-contact temperature sensor, for example any wavelength sensor, including a sensor with infrared or even different from infrared wavelength. Similarly, the non-contact temperature sensor can be defined, for example by an image analysis sensor. In particular, in the preferred embodiment described therein, the thickness measuring means 31 comprise a single sensor 131 of the infrared type arranged at the outlet portion of the cooling ring 21 to continuously detect the thickness of the film 10 tubular plastic. This infrared sensor 131 is further arranged on the aforementioned rotatable ring 51 and, therefore, can be moved along the outlet portion of the cooling ring 21, around the circumference of the tubular plastic film 10 in formation. In this way, it is possible to control the thickness of the entire circumference with a single sensor, reducing both construction and computational costs. In addition, the arrangement of the single rotatable sensor allows you to define a mobile control profile, or a control area that defines a spiral with respect to the tubular plastic film 10. This makes it possible to detect any systematic errors since, during rotation around the circumference, the tubular plastic film 10 continues its axial sliding pulled by the rollers arranged downstream.

According to further embodiments, not shown, the infrared sensor can rotate with respect to the cooling ring by means of different solutions, for example a fifth wheel arranged spaced from the outlet portion of the aforementioned cooling ring. Furthermore, the number of infrared sensors used could be different, i.e. more than one. Finally, it is possible to provide a number of rotatable rings, or solutions for the rotation of the sensors of the infrared type, greater than one, by arranging each sensor in succession with respect to the sliding direction of the tubular plastic film. In this case, the rotation speed of each infrared type sensor could be the same or the speeds of one or more of the above mentioned infrared type sensors could be different, depending on the control profile to be obtained.

The measuring means 31 arranged at the outlet portion of the cooling ring 21 therefore allow to obtain a measurement of the thickness of the tubular plastic film 10 in correspondence with the frost line. In particular, the use of an infrared sensor allows the indirect measurement of the thickness without any contact with the material even before the formation of the frost line. The aforementioned thickness measuring means 31 are suitable for defining an infrared image of the tubular plastic film 10, used by the control means, described in greater detail below, to automatically control the cooling ring 21 as a function of the thickness identified by means of the infrared image and / or the infrared wavelength. The analysis of the image or infrared wavelength allows to infer, or to indirectly determine, the thickness of the tubular film. In particular, it is possible to correlate the type of image or the infrared wavelength to the thickness of the portion of the tubular plastic film 10 to which the aforementioned image refers. In addition, or alternatively, the thickness measuring means 31 are adapted to define a temperature matrix of the plastic film 10, used by the aforementioned control means to automatically control the cooling ring 21 as a function of the thickness identified by the matrix. of temperature. The temperature matrix corresponds to the transduction of the detected infrared image, or its components, into numerical temperature values (according to a predetermined scale). The identification of the temperature in a certain area of the tubular plastic film 10, or the temperature variation over time, allows to infer, or to indirectly determine, the thickness of the tubular plastic film 10. In particular, it is possible to correlate the identified temperature matrix to the thickness of the portion of the tubular plastic film 10 to which the aforesaid matrix refers. Finally, in addition or alternatively, the thickness measuring means 31 are suitable for identifying the freezing line of the tubular plastic film 10, used by the aforementioned control means to automatically modify the extrusion parameters of the film-forming head 1 by blowing in a function of the thickness identified by the thickness measuring means 31 by means of the frost line. The identification of the frost line can be carried out automatically or set appropriately by an operator. The measuring means 31 can therefore operate in correspondence with the frost line, or above or below it, so as to minimize the waste of material in correcting errors.

Therefore, the system 100 for controlling and correcting the thickness of a tubular plastic film 10 according to the first embodiment comprises suitable control means, not shown, operatively connected to the thickness measuring means 31. It is also possible that the control means are operatively connected to the thickness measuring means without necessarily being integral parts of the blow-molding head, for example, they can be parts of the blow-molding system or only a dedicated component of the blow-molding system. thickness control and correction. Such control means are, for example, realized by means of an industrial PLC or other computerized means.

The aforementioned control means are adapted to automatically control the cooling ring 21 as a function of the thickness identified by the thickness measuring means 31. In particular, the control of the cooling ring 21 is obtained by selective actuation of one or more of the cooling sectors of the same cooling ring 21. The selective actuation is controlled according to the thickness profile identified by the means 31 for measuring the thickness and / or any comparisons with one or more predefined thickness profiles. Therefore, the control means allow each cooling sector of the cooling ring 21 to be turned on or off independently, depending on the data assumed by the thickness measuring means 31. The selectivity of the cooling sectors also allows to control the film-forming head 1 by blow molding as a function of the data on the thickness inferred by the data detected by the thickness measuring means 31, in such a way as to modify the extrusion parameters and obtain a greater or lesser modification of the structure of the extruded tubular plastic film 10 and, consequently, control over the desired thickness.

The thickness control can also be carried out using a methodology similar to that of a control chart. It is possible to make a control with respect to zero, where zero represents the desired thickness value or, consequently, the desired temperature or infrared wavelength value. It is also possible to check against two limit values, where each limit value corresponds to a lower limit value and an upper limit value or, consequently, to corresponding temperature or infrared wavelength values. Finally, it is possible to check the thickness profile determined, where a non-linear optimization of the aforementioned thickness profile allows one or more heating sectors and / or one or more cooling sectors to be activated simultaneously to obtain the desired thickness.

The present invention also relates to a blow molding plant, not shown, and comprising the blow molding head 1 described above for making a tubular plastic film 10.

In particular, the blow-molding filming system comprises the aforementioned blowing filming head 1 and at least one extruder arranged upstream of the same for feeding the extruded plastic material. It is also possible that the blow molding plant includes more than one extruder or multi-component extruders.

Finally, the blow molding system includes a calibration cage arranged downstream of the blow molding head 1.

Unlike the known type of blow-film-forming systems, the blow-film-forming system according to the present invention comprises a system 100 for controlling and correcting the thickness of a tubular plastic film 10 in the blow-forming film, provided with at least one sensor of the type infrared devices arranged in correspondence with the outlet portion of the cooling ring 21, in particular upstream of the calibration cage, in such a way as to allow the thickness of the tubular plastic film 10 to be continuously detected in correspondence with the frost line. Therefore, the calibration stand is arranged downstream of the system 100 for controlling and correcting the thickness of a tubular plastic film 10. This allows to minimize the costs of construction and assembly of the system, as well as to optimize the changes in the extrusion, cooling and ironing phase, minimizing the waste of material.

Additional elements that are part of the blow molding system can be, for example, the handling rollers and a clamping calender which respectively allow the movement of the tubular plastic film 10 as well as the closure of the formed bubble.

The following describes the use of the blow molding system and, in particular, the system 100 for controlling and correcting the thickness of a tubular plastic film 10 as well as the relative head 1 for blowing film in accordance with the present invention. The use is described by means of a method of control and correction of the thickness in the film by blowing, in accordance with the present invention. By means of the steps of the aforementioned method, a continuous production process of the tubular plastic film 10 is obtained with a single operation, where the adjustment of the circumference and thickness of the tubular plastic film 10 takes place through the control of the cooling ring 21. The thickness control and correction method comprises at least the steps described below.

Feeding phases of at least one solid plastic material into at least one extruder for the production of the molten plastic material.

The solid state plastic material, for example the polymer in granules or powder, is initially fed into an extruder, through a hopper for loading the solid state plastic material. A "matrix" is placed at the exit of the extruder cylinder, within which the flow of plastic melt is forced to enter with a constant speed, defining an annular-shaped die to form a continuous "tube" of plastic fluid.

Feeding step of the melted plastic material to a blow molding head 1 for making a tubular plastic film 10.

Leaving the annular matrix, a polymeric tube is formed, while the flowing plastic melt is cooled by the blowing of air along the tubular plastic film 10. The product extruded from the spinneret is then blown by means of the blow molding head 1 defining a "bubble conformation". Initially, the polymer tube created is inflated by an internal pressure, due to the blowing of air coming from an orifice of the annular matrix, slightly higher than atmospheric pressure. This creates the bubble conformation. The blown air allows to increase the diameter of the bubble and, at the same time, reduces its wall thickness, forming the tubular plastic film 10 of the desired thickness.

Feeding step of the tubular plastic film 10 to a calibration cage arranged downstream of the filming head 1 by blowing.

The tubular plastic film 10 first passes through a heating ring 11, then through the system 100 for controlling and correcting the thickness of a tubular plastic film 10 and, in particular, through the cooling ring 21. Subsequently, the tubular plastic film 10 passes through a calibration cage, being pulled upwards by the handling rollers, defining the stretching phase which has the purpose of stabilizing the geometry and morphology of the material. The internal pressure and the axial draft contribute to the bi-axial extension of the tubular plastic film 10.

During the process, the tubular plastic film 10 cools and solidifies, so that it can be folded between the movement rollers which pull it. Cooling is carried out by forced convection by blowing air through the cooling ring 21. In steady state conditions, the amount of air inside the bubble is constant and small leaks are compensated for by air that is blown inside so that the pressure remains constant. Due to the stretching in the longitudinal direction and the blowing, the polymer melt is subjected to a biaxial elongation flow in the two directions of "Machine Direction" (MD) and "Transverse Direction" (TD). As soon as it leaves the die, the polymer melt is already partially oriented in MD due to the forced passage through the die and undergoes a further stretching in MD due to the movement of the drive with the handling rollers.

The MD orientation effect can be evaluated through the value of the "Take Up Ratio" (TUR), that is, through the ratio between the towing speed (Vt) and the output speed of the molten polymer (Vu). The TD orientation effect, on the other hand, is obtained through the "inflation" which causes the melted polymer to stretch in a direction almost perpendicular to the direction of extrusion. The inflation value is generally indicated by the “Blow Up Ratio” (BUR), and is expressed by the ratio between the diameter of the bubble (Db) and the diameter of the die (Df). The "overall stretch ratio" (RS) is therefore given by the influence of the individual contributions of the BUR and the TUR and can be evaluated through the ratio between the thickness of the film in the die (ie the width of the air gap) and the thickness of the film after cooling (SF).

It should be emphasized that the air cannot escape from the bubble, as this is pinched by the calender located downstream of the calibration cage. In this way the tubular plastic film 10 is thus stretched in the circumferential or transverse direction. At the end of the extrusion process, the wrapping of the tubular plastic film 10 causes elongation in the longitudinal direction and produces the stretching. The deformation in the two directions therefore orients the macromolecules both in the longitudinal and in the circumferential direction. These two actions take place up to a defined height of the frost line, close to the cooling ring 21, while the deformation of the bubble beyond the aforementioned frost line and beyond crystallization, certainly starting from the calibration cage, is negligible and determined. by the mechanical action alone on the tubular plastic film 10 during handling as described in greater detail below. In this area, the speed of the tubular plastic film 10 varies, passing from that of extrusion to that of stretching, and the radius of the same tubular plastic film 10 passes from that of the die to the final one of the tubular plastic film 10.

The solidified tubular plastic film 10 is finally pulled upwards and flattened into a double-layer sheet by the handling rollers and the pinch calender which close the bubble and form an airtight seal in the final part.

The kinematics of the operation is particularly complicated in the area between the die and the freezing line, where the polymer melt undergoes deformation. In this area the material is subjected to the biaxial elongation flow. In this regard, the manufacturing method according to the present invention further comprises the step of identifying the thickness of the plastic film 10 by means of a system 100 for controlling and correcting the thickness of a tubular plastic film 10 in the blow molding process comprising a cooling ring 21 and at least one sensor 131 of the infrared type arranged in correspondence with the outlet portion 221 of the cooling ring 21, said system 100 being arranged between the blow molding head 1 and the calibration cage to continuously detect the thickness of the film 10 extruded tubular plastic upstream of the calibration cage.

The thickness measuring means 31, by means of the infrared sensor 131, are adapted to define the thickness profile of the tubular plastic film 10. The movement of the rotatable ring 51 allows, at the same time, the rotation of the infrared sensor 131, arranged on the same rotatable ring 51, around the tubular plastic film 10. The continuous rotation of the rotatable ring 51 allows the infrared sensor 131 to scan the tubular plastic film 10 continuously by analyzing the entire circumference during handling by means of the movement rollers.

The measuring means 31 arranged at the outlet portion of the cooling ring 21 allow to obtain a measurement of the thickness of the tubular plastic film 10 in correspondence with the frost line, minimizing the waste of material in the event of modifications. In particular, the use of an infrared sensor 131 allows the indirect measurement of the thickness without any contact with the material even before the formation of the frost line. The thickness measuring means 31 and, in particular, the sensor 131 of the infrared type are adapted to define an infrared image of the tubular plastic film 10 subjected to continuous measurement. The thickness measuring means 31 are adapted to define, in addition or alternatively, a temperature matrix of the same tubular plastic film 10. The analysis of the image or the infrared wavelength allows to infer, or to indirectly determine, the thickness of the tubular plastic film 10, without any contact with the same plastic film 10. Similarly, the identification of the temperature in a certain area of the tubular plastic film 10, or the temperature variation over time, allows to infer, or to indirectly determine, the thickness of the tubular plastic film 10.

The use of the infrared sensor 131 also allows to identify the frost line of the tubular plastic film 10, according to the isothermal curve set, for example equal to 96.5 ° C, or detected by machine learning.

The manufacturing method finally includes the step of automatic control of the cooling ring 21, according to the thickness identified in the previous step of identifying the thickness of the tubular plastic film 10. Following the measurements carried out, the measurement data are sent to the control means which verify the compliance of the thickness with respect to the established standards and modify the filming parameters of one or more parts of the blow-molding plant, or of the system. 100 for checking and correcting the thickness of a tubular plastic film 10 and, optionally, of the film-forming head 1 by blowing. The automatic control phase is carried out by selective actuation of one or more of the cooling sectors of the cooling ring 21. In particular, the control means are adapted to automatically control the cooling ring 21 as a function of the thickness identified by the means 31 for measuring the thickness by means of the infrared image and / or the infrared wavelength. Similarly, the control means can automatically control the cooling ring 21 as a function of the thickness identified by the thickness measuring means 31 by means of the temperature matrix.

The aforesaid control means can also automatically modify the extrusion parameters of the blow molding head 1 as a function of the thickness identified by the thickness measuring means 31 by means of the frost line. The identification of the frost line can be carried out automatically or set appropriately by an operator by identifying the isothermal curve. The measuring means 31 can therefore operate in correspondence with the frost line, or above or below it, so as to minimize the waste of material in correcting errors.

As in known blow-molding systems, further means for measuring the thickness of the tubular film can be further arranged downstream of the calibration stand, for example provided with at least one capacitive sensor. The measurements carried out downstream of the calibration cage are carried out well above the freezing line, when the material constituting the tubular plastic film 10 is now stabilized. This allows the capacitive sensor to make a measurement of the actual thickness of the film in a position where further structural changes, i.e. changes to the geometry and morphology of the material, are no longer possible. Therefore, the control means can also be operatively connected to the further measuring means arranged downstream of the calibration stand to verify that the modifications made to the cooling ring 21 as a function of the thickness measurements carried out and of the values communicated by the means 31 of thickness measurement.

Figure 4 illustrates a system 200 for controlling and correcting the thickness of a tubular plastic film 10 in the blowing film, according to a second embodiment. The aforementioned system 200 for controlling and correcting the thickness of a tubular plastic film 10 comprises the cooling ring 21 provided with the measuring means 32 in a second embodiment, in accordance with the present invention. In this second embodiment, the system 200 for controlling and correcting the thickness of a tubular plastic film 10 comprises a plurality of sensors of the fixed and equally spaced infrared type along the circumference of the cooling ring 21 at the outlet portion. In particular, the thickness measuring means 32 comprise four sensors 132, 232, 332, 432 of the fixed infrared type and spaced in succession at an angle equal to 90 ° along the circumference of the cooling ring 21 at the exit. In this way, it is possible to check the thickness of the entire circumference by detecting any random errors. The aforesaid sensors 132, 232, 332, 432 of the fixed infrared type are arranged on a fixed ring 52 operatively connected and concentric to the aforesaid cooling ring 21, at the outlet portion of the cooling ring 21, and are capable of to monitor a portion of the tubular plastic film 10 corresponding to a quarter of the circumference to continuously detect the thickness of the tubular plastic film 10. In this way, it is possible to control the thickness of the entire circumference without any movement by the sensors 132, 232, 332, 432, defining a fixed control profile, i.e. a control area corresponding to a cross section of the plastic film 10 tubular, perpendicular to the direction of movement, allowing the detection of any random errors.

Different embodiments of the measuring means are also possible, with particular reference to the number of infrared sensors used and to the mutual spacing between them.

Therefore, according to a second embodiment, not shown, the blow molding plant according to the present invention comprises the system 200 for controlling and correcting the thickness of a tubular plastic film 10, as illustrated in Figure 4, as well as the head of blow molding comprising the aforementioned system 200 for controlling and correcting the thickness of a tubular plastic film 10. The blow molding plant comprises at least the aforementioned system 200 for controlling and correcting the thickness of a tubular plastic film 10, the aforementioned blow molding head and, as previously described for the first embodiment, at least one extruder arranged upstream and a calibration cage arranged downstream of the blow molding head.

Unlike the first embodiment, the blow-molding film plant according to the second embodiment therefore comprises a plurality of sensors of the fixed infrared type, i.e. four sensors 132, 232, 332, 432, equally spaced along the circumference of the cooling ring 21 at the outlet portion, in particular spaced in succession by an angle equal to 90 °. As described for the first embodiment, the aforementioned sensors 132, 232, 332, 432 of the infrared type are arranged upstream of the calibration cage, in such a way as to allow continuous detection of the thickness of the tubular plastic film 10 in correspondence with of the frost line, monitoring a portion of the tubular plastic film 10 corresponding to a quarter of the circumference.

The additional characteristics of the implant of the second embodiment correspond, mutatis mutandis, to the characteristics of the implant according to the first embodiment, therefore they will not be further described below. Similarly, the use of the blow molding plant of the second embodiment corresponds, mutatis mutandis, to the use already described above for the plant according to the first embodiment, therefore only the following will be further described below. changes with respect to the thickness control and correction method described above.

By means of the steps of the aforementioned method, a continuous production process of the tubular plastic film 10 is obtained with a single operation, where the adjustment of the circumference and thickness of the tubular plastic film 10 takes place through the control of the cooling ring 21. The means 32 for measuring the thickness, by means of the sensors 132, 232, 332, 432 of the infrared type, are able to define the thickness profile of the tubular plastic film 10. The fixed arrangement of the aforementioned sensors 132, 232, 332, 432 allows you to scan the tubular plastic film 10 continuously by analyzing the entire circumference at each sliding instant of the aforementioned tubular plastic film 10 during movement through the movement rollers.

The measuring means 32 arranged at the outlet portion of the cooling ring 21 allow to obtain a measurement of the thickness of the tubular plastic film 10 at the frost line, minimizing the waste of material in the event of modifications. In particular, the use of infrared sensors allows the indirect measurement of the thickness without any contact with the material even before the formation of the frost line. As previously described for the first embodiment, the thickness measuring means 32 are adapted to define an infrared image and, in addition or alternatively, a temperature matrix of the tubular plastic film 10. The analysis of the image or the infrared wavelength allows to infer, or to indirectly determine, the thickness of the tubular plastic film 10, without any contact with the same tubular plastic film 10. Similarly, the identification of the temperature in a certain area of the tubular plastic film 10, or the temperature variation over time, allows to infer, or to indirectly determine, the thickness of the tubular plastic film 10. The use of sensors 132, 232, 332, 432 of the infrared type also allows to identify the frost line of the tubular plastic film 10, as described above.

The method of controlling and correcting the thickness finally includes the phase of automatic control of the cooling ring 21, according to the thickness identified in the previous phase of identifying the thickness of the tubular plastic film 10.

Following the measurements carried out, the measurement data are sent to the control means which verify the compliance of the thickness with respect to the established standards and modify the filming parameters of one or more parts of the blow-molding plant, or of the head. of film-forming by blowing and of the system 200 for controlling and correcting the thickness of a tubular plastic film 10. The automatic control phase is carried out by selective actuation of one or more of the cooling sectors of the cooling ring 21. In particular, the control means are adapted to automatically control the cooling ring 21 as a function of the thickness identified by the means 32 for measuring the thickness by means of the infrared image and / or the infrared wavelength. Similarly, the control means can automatically control the cooling ring 21 as a function of the thickness identified by the thickness measuring means 32 by means of the temperature matrix.

The aforesaid control means can also automatically modify the extrusion parameters of the blow-molding head as a function of the thickness identified by the thickness measuring means 32 by means of the frost line. The identification of the frost line can be carried out automatically or set appropriately by an operator, as described above. The measuring means 32 can therefore operate in correspondence with the frost line, or above or below it, so as to minimize the waste of material in correcting errors.

Therefore, the thickness control and correction system, the blow-molding head, and the relative blow-molding system, according to the present invention are capable of minimizing the aforementioned problems. In particular, they are able to maximize the effects of changes in extrusion parameters by reducing the waste of extruded material. The blow molding system according to the present invention has a reduced manufacturing and assembly cost and at the same time allows to improve the control of the thickness of the extruded tubular film.

In particular, the blow molding head according to the present invention is provided with thickness measuring means capable of measuring the thickness of the tubular film in correspondence with the frost line, minimizing the waste of material in the event of changes to the parameters of the head. of film by blowing.

Claims (10)

CLAIMS 1. System (100; 200) for controlling and correcting the thickness of a tubular plastic film (10) in the film by blowing comprising: - a cooling ring (21) for the said tubular plastic film (10), defined by a plurality of cooling sectors and provided with an inlet portion (121) and an outlet portion (221) of the said film (10) tubular plastic; - means (31; 32) for measuring the thickness of said tubular plastic film (10); - control means operatively connected to said thickness measuring means (31; 32); the said system (100; 200) for controlling and correcting the thickness of a tubular plastic film (10) in the blowing film is characterized in that the said thickness measuring means (31; 32) comprise at least one sensor (131; 132, 232, 332, 432) temperature without contact arranged in correspondence with said outlet portion (221) of said cooling ring (21) to continuously detect the thickness of said tubular plastic film (10), in which said control means are adapted to automatically control said cooling ring (21) as a function of the thickness detected by said thickness measuring means (31; 32) by selective actuation of one or more of said cooling sectors of the said cooling ring (21). System (100) for controlling and correcting the thickness of a tubular plastic film (10) in the blow-molding film according to claim 1, in which said thickness measuring means (31) comprise a temperature sensor (131) without movable contact along said outlet portion (221). 3. System (100) for controlling and correcting the thickness of a tubular plastic film (10) in the blow-molding film according to claim 2, comprising a rotatable ring (51) operatively connected and concentric to said cooling ring (21), in wherein said contactless temperature sensor (131) is disposed on said rotatable ring (51). System (200) for controlling and correcting the thickness of a tubular plastic film (10) in the blow-molding film according to one or more of claims 1 to 3, in which said thickness measuring means (32) comprise a plurality non-contact temperature sensors (132, 232, 332, 432) fixed with respect to said cooling ring (21) and equally spaced along the circumference of said cooling ring (21) at said outlet portion (221). System (200) for controlling and correcting the thickness of a tubular plastic film (10) in the blow-molding film according to claim 4, in which said thickness measuring means (32) comprise four sensors (132, 232, 332 , 432) of temperature without contact fixed with respect to said cooling ring (21) and spaced in succession by an angle equal to 90 ° along the circumference of said cooling ring (21) at said outlet portion (221). System (100; 200) for controlling and correcting the thickness of a tubular plastic film (10) in the blowing film according to one or more of claims 1 to 5, wherein said sensors (131; 132, 232, 332 , 432) of temperature without contact are of the infrared type and in which the said means (31; 32) for measuring the thickness are adapted to define an infrared image of the said tubular plastic film (10), in which the said control means are adapted to automatically control the said cooling ring (21) as a function of the thickness identified by the said means (31; 32) for measuring the thickness by means of the said infrared image and / or the length of infrared wave. System (100; 200) for controlling and correcting the thickness of a tubular plastic film (10) in the blowing film according to one or more of claims 1 to 6, wherein said sensors (131; 132, 232, 332 , 432) of temperature without contact are of the infrared type and in which the said means (31; 32) for measuring the thickness are adapted to define a temperature matrix of the said tubular plastic film (10), wherein said control means are adapted to automatically control said cooling ring (21) as a function of the thickness identified by said thickness measuring means (31; 32) by means of said temperature matrix. 8. Filming head (1) by blow molding comprising a die (11) of a tubular plastic film (10), defined by a plurality of heating sectors, and characterized in that it comprises a control system (100; 200) and correction of the thickness of a tubular plastic film (10) in the blowing film according to one or more of claims 1 to 7, and wherein said cooling ring (21) is arranged downstream of said spinneret (11). 9. Blow molding film plant comprising: - a blow molding head (1) for making a tubular plastic film (10); - one or more extruders arranged upstream of said head (1) for blowing film to feed the extruded plastic material; - a calibration cage arranged downstream of said blow molding head (1); the said blow molding plant is characterized by the fact that the said blow molding head (1) is according to claim 8, in which the said calibration cage is arranged downstream of the said system (100) for controlling and correcting the thickness of a tubular plastic film (10), and in which at least one non-contact temperature sensor (131; 132, 232, 332, 432) is arranged at the outlet portion (221) of said cooling ring (21) to continuously detect the thickness of said tubular plastic film (10) upstream of said calibration cage. 10. Method for controlling and correcting the thickness of a tubular plastic film (10) in the blow molding process comprising the following steps: - feeding at least one solid plastic material into at least one extruder for making the molten plastic material; - feeding the melted plastic material to a film-forming head (1) by blowing for the production of a tubular plastic film (10); - feeding the said tubular plastic film (10) to a calibration cage arranged downstream of the said film-forming head (1) by blowing; the said method of controlling and correcting the thickness in the blow-molding film is characterized by the fact that it comprises the following steps: - identification of the thickness of said tubular plastic film (10) by means of a system (100; 200) for controlling and correcting the thickness of a tubular plastic film (10) in the blowing film comprising a cooling ring (21) and at least one sensor temperature without contact arranged at an outlet portion (221) of said cooling ring (21), in which said thickness control and correction system (100; 200) is arranged between said head (1) blowing film and said calibration cage for continuously detecting the thickness of said extruded tubular plastic film (10) upstream of said calibration cage; - automatic control of said cooling ring (21) as a function of said thickness identified by selective actuation of one or more of the cooling sectors of said cooling ring (21).