ES2852702B2 - PLASTIC FILM MANUFACTURING PROCEDURE AND EQUIPMENT TO CARRY OUT SAID PROCEDURE - Google Patents

Description

DESCRIPTION

PLASTIC FILM MANUFACTURING PROCEDURE AND

EQUIPMENT TO CARRY OUT SAID PROCEDURE

OBJECT OF THE INVENTION

The invention, as expressed in the title of this specification, refers to a process for manufacturing plastic film and equipment to carry out said process, providing, to the function for which it is intended, advantages and characteristics that they are described in detail later and that suppose an improvement of the current state of the technique.

The object of the present invention lies in a process for manufacturing plastic film (or film), in particular a process for manufacturing plastic film of the type known as circular row that is based on the formation of a bubble and the use of water for the cooling process of the material, which essentially distinguishes itself in an improved cooling system and in that it allows the calibration of the bubble. Specifically, it is distinguished, on the one hand, in that the cooling by water occurs both on the inner face and on the outer face of the bubble, preferably through the use of a tank where said bubble is submerged and some blades that cut it, with which the sealing function to prevent air from escaping from the bubble is exerted by the surface of the water level itself. On the other hand, the method of the present invention is also distinguished by the inclusion of a gauge that allows adjusting the diameter of the bubble to the width of the desired film. A second aspect of the invention relates to equipment that, among other features, comprises said tank and said gauge to carry out carry out this manufacturing process.

FIELD OF APPLICATION OF THE INVENTION

The field of application of the present invention falls within the sector of the industry dedicated to the manufacture of plastic products, focusing particularly on the field of manufacturing blown plastic film, for example for use in packaging.

BACKGROUND OF THE INVENTION

As is known, in the transformation processes of plastic materials we can distinguish, at least, two well-differentiated phases. The first is the melting phase of the plastic material, where the plastic grain (raw material) is subjected to temperatures above its melting point until a molten plastic mass is obtained. The second phase is responsible for giving the desired shape and cooling the plastic until it solidifies. The method object of the invention provides improvements in said second phase while the process remains the same in the first.

These processes are capable of industrializing the manufacture of a wide range of products for countless applications. Within the plastic containers and packaging market, flexible packaging stands out, which commonly consists of making bags from film.

At present, several processes coexist for the manufacture of plastic film .

One can distinguish between two families of processes, flat die and circular die, the latter being the one that concerns the present invention.

The most widespread circular die process is blown film extrusion, a scheme of which can be seen in figure 1.

In this process, the raw material in the form of plastic grain is introduced into a hopper and falls by gravity to an extruder. The extruder screw rotates and its blades push towards a head and the plastic melts due to the effect of temperature, forming the plastic mass. The dough advances through the head and exits through an outlet row or nozzle, which is located in the upper part of the head and practically in contact with an air ring. From the row the bubble is formed, which has a cylindrical shape and traps a certain amount of air inside it. This air entrapment is possible because nip rolls, located at the top of the bubble, act as a sealant. Some guide plates have the function of shaping the cylindrical bubble into a flat bubble as it approaches the draft rollers. At this point, film has already been obtained, which is in the form of a flattened cylinder, or in other words, a double film.

The film manufacturing speed will depend on the rotational speed of the draft rollers, and the thickness of the film on the relationship between the speed of the draft rollers and the output of the extruder.

Downstream, the manufacturing lines can treat the film differently depending on its final application. One way would be to subject the film to a winding process.

It should be noted that the solidification of the plastic mass is carried out by air cooling. This air comes from an air ring, which cools the bubble on the outside, while the air trapped inside the bubble cools the inside. On modern manufacturing lines They install systems that allow the renewal of trapped air to reduce its temperature and thus reinforce cooling.

Due to its low thermal conductivity and its low heat capacity, the rate of cooling of the plastic mass by air is relatively slow. In industrial processes where a fast cooling rate is required, the cooling element used is usually water. The properties of water, with its high conductivity and thermal capacity, facilitate a much higher thermal shock than air.

Some properties of the film are affected by the rate of cooling in its transformation process. Thus, a film produced by a rapid cooling process usually has better optical and mechanical properties (gloss, transparency, puncture resistance, thermoformability).

These properties of the film are due to the fact that, thanks to the rapid cooling of the film, the degree of crystallinity of the plastic is low, maintaining an amorphous structure in the solid state.

When the final application of the film requires these properties, it can be manufactured on water quench lines, in English Water Quench blown film. This process, with similarities to the already explained blown film, can be seen in figure 2.

Specifically, in this case, until the exit of the head, the process is identical to that of blowing or Blown Film previously described, although the arrangement of the head is reversed, leaving the plastic downwards. In the head there is usually a cooling ring similar to the blown film air ring, also facing downwards. the bubble is the same to that of a blown film but with the direction of flow reversed (from top to bottom).

The main difference is that, in said cooling ring, there is a new element, a cooling ring ( Water Calibrator) that puts a layer of cold water in contact with the outer face of the bubble. However, the diameter of this water cooling ring cannot be changed dynamically, so it acts as a limiter on the diameter of the film bubble. Thus, in this technology the diameter of the bubble is fixed and therefore so is the width of the resulting film. The procedure to be able to change the width of the film requires a change of tools to be carried out with the line stopped, which can last several hours depending on the manufacturer.

The water cooling ring (Water Calibrator) consists of a ring that surrounds the bubble. Internally it has a water overflow, (the overflow works as such because the internal wall is lower than the external one). The bubble in downward flow passes through the interior of the annulus in a tight measure. The thin layer of overflowing water acts as a coolant when it comes into contact with the bubble. The system is not sealed, there is a certain flow of water that escapes down the bubble and a recirculation system that replenishes the water in the calibrator.

Another limitation of this system is that a vacuum lip is usually necessary to adhere the bubble to the refrigerant. This lip, incorporated in the ring itself, causes tensions in the film that are not compatible with small thicknesses. For this reason, the films manufactured in the water cooling or Water Quench process must have a minimum thickness that varies according to the manufacturer of the machine and the polymers to be processed.

On the other hand, in the manufacture of high-thickness films, above 200 microns, the Water Quench technology is also problematic as the plastic mass coming out of the head supports the total weight of the bubble. This weight exerts a downward tension that can stretch the bubble in the area where the material is still above its melting temperature. When this happens the process becomes unstable and unable to maintain thickness.

In circular die film extrusion processes, where it is obtained from the collapse or flattening of a bubble with a cylindrical section, an undesired phenomenon arises that originates from the asymmetry of this collapse.

This phenomenon, with various names in the sector, but which in this document will adopt the Anglo-Saxon name bagginess (slack) as it is the most internationally accepted, causes problems in the use of the film in subsequent industrial processes such as printing, lamination, cutting, shaping of bags, application of labels, etc...

Bagginess manifests itself in the film in the form of zones, normally longitudinal stripes, where the tension of the film is lower than in other stripes, that is, subjecting the film to a certain tension, it is not shown equally throughout the width, showing fringes where undulations are observed.

The explanation for this phenomenon is found in the fact that during the film manufacturing process one or several areas have been subjected to unequal stretching caused by a geometric problem that is currently not completely resolved. Poor collapsing is a frequent cause of Bagginess generation.

The objective of the present invention is, therefore, the development of a new procedure for the manufacture of plastic film that allows solving the drawbacks and limitations of the currently existing procedures by using water both for cooling the interior and exterior of the film. material, allowing it to be carried out more quickly than only with air or with air from the inside and water from the outside and, in addition, including a calibrator that makes it possible to adjust the diameter of the bubble to the width of the film without requiring a stop for tool change. The invention also provides relevant modifications in the collapse to solve the geometric problems of current systems that negatively affect the flatness of the film.

EXPLANATION OF THE INVENTION

The plastic film manufacturing procedure and the equipment to carry out said procedure that the invention proposes are configured as the ideal solution to the aforementioned objective, with the characterizing details that make it possible and that distinguish them conveniently collected in the final claims accompanying this description.

Specifically, what the invention proposes, as noted above, is a process for manufacturing plastic film, in particular a process for manufacturing plastic film with a composition based on thermoplastic polymers, of the type known as circular row, which It is based on the formation of a film bubble and the use of water to cool the material, which, unlike known procedures, which either use air to cool the material bubble both inside and outside, or use air to cool it internally and water to cool it externally, contemplates the use of water for cooling both on the inside and on the outside of the film bubble, so that a symmetrical and faster cooling is favored, and in which, in addition, the use of a calibrator is also optionally contemplated to dynamically adjust the diameter of the bubble to the desired or used film width, a second aspect of the invention being equipment comprising said tank and optionally said calibrator for carrying out said manufacturing process.

Thus, the method object of the invention, to the common stages of melting and extruding the material and forming the bubble, introduces, as an essential novelty, the cooling of the internal part of the bubble by means of water, for example by submersion bubble in a tank of water.

The arrangement of the head and air ring is similar to the Water Quench system and its flow from top to bottom. However, the bubble is not completely filled with air but is mixed, part air and part water.

The volume occupied by the air extends from the head assembly to the water level in the tank, instead of extending to the draft rollers as it would in existing systems. With this, the main difference is that the sealing function to prevent air from escaping from the bubble is not performed by the draft rollers by collapsing the bubble, but by the surface of the water level itself. This is possible thanks to the fact that the bubble is opened with cutting means, such as blades, provided for this purpose inside the tank and this opening allows the water from the tank to enter until it reaches, through communicating vessels, approximately the same level as in the the outside of the bubble.

Water cooling, both on the inside and on the outside, favors a faster cooling of the material than in the process of prior art Water Quench, also preserves the symmetry of the Blown film process, which is also an advantage to achieve films with better flatness, due to the absence of tensions that cause undulations caused by asymmetric cooling. In addition, the higher cooling rate that is achieved with the method of the invention also has an advantageous effect on the properties of the film.

In a preferred embodiment, the method object of the invention uses a dynamically variable internal gauge. This is an element that does not exist in current processes and makes it possible, by combining it with the air pressure regulation of the bubble, to adjust the diameter to the width of the film that you want to use. This is a great advantage over the Water Quench process. For example, if it is desired to obtain a film of greater width with the procedure of the invention, the internal air pressure must be adjusted as would be done in the existing Water Quench systems, the difference lies in the possibility of regulating the diameter of the calibrator, that while in the Water Quench systems this is fixed, in the invention it is variable. The reason why the gauges in the Water Quench systems have a fixed diameter and not a variable one is that a hitherto unsolvable technical problem arises, which is to achieve an element that expands its diameter while maintaining a perfect and continuous circumference ring to guarantee a homogeneous cooling water thickness throughout. In the method object of the invention, the requirement that the circumferential ring be continuous does not exist because in reality the gauge only acts as a guide, without exercising the function of generating the thin layer of cooling water.

The elements of the calibrator likely to come into contact with the bubble are preferably submerged in water. Preferably, the calibrator is completely submerged in water, thus reducing the probability of causing friction that could deteriorate the surface of the film, but if necessary optionally the expandable elements of the variable internal calibrator can be equipped with a padding system by water pressure. This system consists of projecting a certain water pressure from inside the elements of the calibrator, through holes. This flow of water prevents direct contact of the film with the solid surfaces of the calibrator.

Finally, it should be noted that circular row film manufacturing processes preferably require a rotation of the shooting roller station to randomly distribute the thickness of the film. This rotation prevents a point of thickness from always accumulating in the same area of the coil.

In the method object of the invention, instead, either a rotation of the complete water tank has been provided for, by including a mechanical system suitable for it or the inclusion of a system that rotates the internal mechanism but not the tank.

In short, the advantages provided by the method of the invention are evident:

- While in order to change the width of the film in a Water Quench procedure line it is necessary to stop manufacturing and change tools, with the procedure and equipment object of the invention, thanks to the existence of the variable internal gauge, the width is changed without having to stop manufacturing. This results in more productivity, more flexibility and less waste.

- While with the Water Quench procedure there is a limitation regarding the minimum thickness of the film to be manufactured, with the procedure and the equipment object of the invention, thanks to the elimination of friction Between the bubble and the water cooler, thinner films can be made. An estimate would be a minimum of 60 microns for a film with the Water Quench procedure and 30 microns for a film with the new procedure, although it will also depend on the raw materials used.

The process of the invention also provides advantages for the manufacture of high-thickness films (thicknesses greater than 200 microns). In current Water Quench blown film lines, the molten plastic coming out of the die is subjected to a tension produced by the same weight of the bubble, from the die to the pull rollers. This tension compromises the stability of the bubble and makes it impossible for the process to maintain the desired high thickness. However, in the invention we must account for only part of that weight, since a good part of the bubble is submerged in water and in this medium there is no resulting weight due to the fact that the density of the plastic polymers is usually lower than that of the medium. For this reason the bubble stability is better thanks to the invention, especially in high thickness films where the bubble is heavier.

Another advantage is energy efficiency. With the process of the invention it is not necessary to cool the process water to 6-18°C, which is usually recommended by the manufacturers of Water Quench process lines. The process of the invention foresees working at process water temperatures of around 25°C, and this temperature is considered within the reach of refrigerators based on cooling towers that are known to be more efficient than other compressor systems. The temperature can be higher because, by cooling the film on both sides instead of one, the same cooling speed is achieved as with a lower process water temperature.

The necessary height of the warehouse: With the procedure and equipment of the invention, a height from floor to ceiling lower than that required by a Water Quench procedure and equipment. While in a 1,200 millimeter wide Water Quench line, the necessary height of the ship is around 11 meters, it is estimated that with the invention this height can be 6 meters.

Symmetric cooling: With the Water Quench procedure, the inner face of the film is cooled with air and the outer face with water. Due to the different thermal conductivities of air and water, the outer face will have a higher cooling rate. Instead, with the method and equipment of the invention, both sides are cooled with water. This fact makes the cooling rates on both sides of the film similar. The cooling rate of the film affects aspects related to the optical and mechanical properties due to the degrees of crystallinity. The symmetry in the cooling speeds on both sides avoids tensions and ripples in the film.

In short, the plastic film manufacturing process that is the object of the invention consists of a circular row-type process that, comprising:

- a bubble formation stage, in which the plastic mass is extruded by a circular die head that forms a film bubble

- a stage of cooling the film bubble by introducing a liquid refrigerant inside the film bubble, such that the internal part of the film bubble is cooled by liquid.

- a stage for forming the flat tubular film, in which rollers flatten the film bubble in the form of a double film, and

Preferably, the introduction of liquid refrigerant in the film bubble It is produced by immersing at least a part of the film bubble in a coolant tank and through cuts, preferably blades, that are made in the film bubble. In this embodiment, a plastic film is obtained that is cooled on both sides, internal and external, of the bubble by liquid.

In a preferred embodiment, the process comprises one or more of the following steps.

- a melting and extrusion stage, in which the raw material in the form of plastic grain is introduced into a hopper and falls by gravity to an extruder whose screw pushes it towards a head and the plastic melts due to the effect of temperature, forming the mass plastic;

- a stage of air cooling of the external part of the bubble

- an adjustment stage of the internal air pressure in the bubble, inflates it and determines its diameter;

- a stage of modification of the diameter of the bubble and therefore the width of the film through a dynamically variable diameter gauge provided inside the bubble.

- a collapse stage in which the cylindrical bubble becomes flat. To do this, the system has pass rollers and a draft calender, being flattened in the form of a double film;

- a stage that avoids, through the rotation of the tank or its internal elements, the accumulation of differences in thickness of the plastic film at the same point of the width of the collection reel.

- a drying stage subsequent to the exit of the collapsed bubble from the water tank;

- a final treatment step, for example a winding process;

The stage that prevents the accumulation of differences in thickness of the plastic film at the same point of the width of the collection reel preferably comprises a rotational movement with respect to a vertical axis of all the elements inside the tank, including the tank.

In an alternative embodiment, the stage that prevents the accumulation of differences in thickness of the plastic film at the same point of the width of the collection reel preferably comprises a rotational movement with respect to a vertical axis of all the elements inside the tank, maintaining the tank (11) not rotated.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of plans is attached to this description, as an integral part of it, in which for illustrative and non-limiting, the following has been represented:

Figure number 1.- Shows a side elevation view of a scheme of the equipment, according to the current technique, to carry out a procedure for manufacturing plastic film by bubble with air cooling, showing the main parts and elements that it comprises. , especially the head arrangement with upward flow and the air ring for inner and outer cooling of the bubble;

Figure number 2.- Shows a side elevation view of a scheme of the equipment, according to the current technique, to carry out a procedure for manufacturing plastic film by bubble with water cooling, appreciating the inverted arrangement of the head and the water coolant on the outside of the bubble;

Figures number 3 and 4.- Show respective front and side elevation views, respectively, of a very schematic first representation of the equipment to carry out the plastic film manufacturing procedure object of the invention, generically appreciating the main characteristic parts and elements that it comprises; Y

Figures number 5 and 6.- Show two other views, front and side elevation, of another embodiment of the equipment object of the invention in which a roller system is included for threading and dragging the bubble film and a rotating platform for rotate the tank.

PREFERRED EMBODIMENT OF THE INVENTION

In view of the aforementioned figures, and according to the numbering adopted, it can be seen in them, in addition to the examples of the prior art with cooling only by air or Blown film procedure (figure 1) and by air and water or procedure Water Quench (figure 2), a non-limiting example of embodiment of the equipment to carry out the plastic film manufacturing process of the invention, which includes what is described in detail below.

Thus, according to Figure 1, it can be seen how, in equipment for the manufacture of plastic film, according to the prior art, by the procedure blown film, the raw material, in the form of grain, is introduced into a hopper (2) and falls by gravity to an extruder (3) whose screw pushes it towards a head (4) where the molten plastic, due to the effect of temperature , forms a mass (1) of plastic.

After said head (4), the material passes through a cooling ring (5) based on air, which is applied internally and externally, and inflates forming the film bubble (10) that is sealed by pull rollers ( 6) located superiorly.

In this process, guide blades (7) are provided that flatten the bubble (10) before passing through the shooting rollers (6), after which it is flattened in the form of a double film or flattened film (100).

Finally, by means of guide rollers (8), the flattened film (100) is led to the final stage, for example for coil formation (not shown).

Referring to Figure 2, it can be seen how, in a Water Quench type procedure, also according to the prior art, equipment is contemplated that also comprises a hopper (2) for introducing the material into an extruder (3) from which it is pushes the molten material (1) to a head (4) that passes through a cooling ring (5) based on air that is applied internally and inflates forming the bubble (10) of film that is sealed by pull rollers ( 6), in this case located below, as well as a liquid coolant (9), normally water, which is applied externally to the bubble (10).

As in the previous case, guide blades (7) are provided that flatten the bubble (10) before passing through the pull rollers (6), after which it is flattened in the form of a double film or flattened film ( 100) and, by means of guide rollers (8), it is driven to the final stage, for example, for forming a coil (not shown).

And, according to figures 3 and 4, it can be seen how, with the process of the invention, the equipment used, similarly to what is already known, comprises a hopper (2) for introducing the material into an extruder (3) from which the molten material (1) is pushed to a head (4) that passes through a cooling ring (5) based on air that is applied internally and inflates forming the bubble (10) of film, as well as a liquid refrigerant (9), preferably water, which is applied externally to the bubble (10), with some pull rollers (6), located below, after which the bubble (10) is flattened in the form of a double film or flattened film (100 ) which, by means of guide rollers (8), is driven to its final stage, for example for the formation of a coil (not shown).

And, from said already known configuration, the equipment of the invention is distinguished by the fact that the liquid refrigerant (9) is also applied inside the bubble by means of a tank (11), where the bubble is at least partially submerged. (10) of film, and that includes cutting means (12), preferably blades, such that they break the bubble (10) of film causing said refrigerant (9) to penetrate inside it and the air rises to the level ( N) of the tank (11). In addition, it also comprises a calibrating device (13) that allows the diameter of the bubble (10) to be adjusted and that is located inside said bubble (10).

Furthermore, preferably, the rollers (6) and the guide rollers (8) are integral with the tank (11) and because they comprise mechanical means for rotating the tank (11), the rollers (6) and the rollers about a vertical axis. guide (8). The mechanical means for causing rotation are preferably a rotating platform (14).

In an alternative embodiment (not shown), the rollers (6) and the The guide rollers (8) are rotatable with respect to the tank (11) and because they comprise mechanical means for rotating the rollers (6) and the guide rollers (8) with respect to a vertical axis.

Looking at figures 5 and 6, it can be seen how, in a preferred embodiment, the roller system for forming the film after the bubble (10), in addition to the pull rollers (6) or calender pressure rollers, It comprises pre-optimal flattening rollers (61), guide or threading rollers (8) in which the flattened film (100) is dried by means of dryers (19), a pass roller (81), an oblique reversing roller ( 82), with which the position of the film is rotated, two vertical rollers (83) and an external vertical roller (84) that is installed outside the platform (14) in which the tank (11) is incorporated with the rest of the rollers mentioned.

This platform (14), preferably, is a rotating platform, driven by a motor (18) to ensure the rotation of the assembly.

In addition, as shown in figure 6, preferably, the calibrator (13) of the equipment comprises expansion elements (13a) and a cold air lip (13b), as well as a cold water supply element (17) to the inside of the bubble (10).

On the other hand, said figure shows the existence of a cold air supply tube (15) and a hot air extraction tube (16) that run through the calibrator (13) inside the film bubble. (10) filled with air. The conduction of these tubes for cooling the air inside the bubble of film (10) could also start from the extrusion head (4) towards the interior of the bubble of film (10) filled with air.

Having sufficiently described the nature of the present invention, as well as the way of putting it into practice, it is not considered necessary to do more his explanation is extensive so that any expert in the field understands its scope and the advantages derived from it.

Claims (9)

1. - Plastic film manufacturing procedure, comprising: - a bubble formation stage (10), in which the plastic mass (1) is extruded by a circular die head (4) that forms a film bubble (10) - a stage of cooling the film bubble (10) by immersing at least a part of the film bubble (10) in a tank (11) of liquid refrigerant (9) and the entry of said liquid refrigerant (9 ) inside the film bubble (10) through cuts made in the film bubble (10) by means of cutting means (12), and a stage for forming the flat tubular film (100), in which rollers (6, 61) flatten the film bubble (10) in the form of a double film, and is characterized in that the stage for forming the flat tubular film (100) is performed by one or two pairs of rollers (6, 61). 2. - Method for manufacturing plastic film, according to claim 1, characterized in that the diameter of the bubble (10) is modifiable through a calibrator (13) of variable diameter provided inside the bubble (10). 3. - Process for manufacturing plastic film, according to any of the preceding claims, characterized in that it comprises a stage of rotation (such as rotation) about a vertical axis of the tank (11), of the rollers (6) that flatten the film bubble (10) and the guide rollers (8) to avoid the accumulation of material in the same point of the collection reel. 4. - Plastic film manufacturing procedure, according to any of claims 1 to 2, characterized in that it comprises a stage of rotation about a vertical axis of the rollers (6) that flatten the film bubble (10) keeping the tank (11) without rotating, to avoid the accumulation of material in the same take-up spool point. 5. Equipment for carrying out a plastic film manufacturing process, according to the process described in any of claims 1 to 4, comprising - a circular die head (4) through which the plastic mass (1) is extruded forming a film bubble (10) - a coolant, and - some rollers (6), located below the head (4), after which the bubble (10) is flattened in the form of a double film (100) - a tank (11), in which a liquid refrigerant (9) is incorporated and the film bubble (10) is submerged, at least partially, and - some cutting means (12) located inside the tank (11) and before the rollers (6) that break the film bubble (10) causing said coolant (9) to penetrate inside the film bubble (10 until the level (N) of the coolant (9) in the tank (11). - guide rollers (8), it is characterized in that the rollers (6) for flattening the bubble (10) in the form of the double film (100) consist of one or two pairs of rollers (6, 61). 6. Equipment for carrying out a plastic film manufacturing process, according to claim 5, characterized in that the rollers (6) to flatten the bubble (10) in the form of a double film (100) consist of pull rollers (6) or calender pressure rollers and optimal flattening rollers (61) located above the pull rollers (6) and inside the bubble (10) 7. - Equipment for carrying out a method of manufacturing plastic film, according to any of claims 5 to 6, characterized in that it comprises a calibrating device (13) to regulate the diameter of the film bubble (10) located in the inside the film bubble (10). 8. - Equipment for carrying out a plastic film manufacturing process, according to any of claims 5 to 7, characterized in that the rollers (6) and the guide rollers (8) are integral with the tank (11) and that it comprises mechanical means for rotating about a vertical axis the tank (11), the rollers (6) and the guide rollers (8). 9. - Equipment for carrying out a method of manufacturing plastic film, according to any of claims 5 to 7, characterized in that the rollers (6) and the guide rollers (8) are rotating with respect to the tank (11).