PET sheet production line and production method

Abstract

This application relates to a PET sheet production line and a process for producing PET sheets. This sheet production line comprises an extruder, a die, and a calender arranged in a processing direction. The extruder includes a cylinder and a twin-screw assembly located inside the cylinder. This twin-screw assembly consists of two meshing screws, the outside diameter of which decreases along the processing direction. The angle between the center axes of the two screws is between 10' and 1°20', and the two screws rotate about their respective center axes in opposite directions. This application improves the production line by taking into account the characteristics of PET, improving its plasticization, ensuring the quality of the PET sheets, increasing its production capacity, and reducing its energy consumption. Figure for the abstract: 1a

Description

Title of the invention: PET sheet production line and method for producing PET sheets

[0001] This application claims priority from Chinese patent applications filed with the China Patent Office on November 6, 2024, under application number 202422696756.6 and application number 202411572629.3, the entirety of which is incorporated by reference in this application. technical field

[0002] The present invention belongs to the technical field of the manufacture of sheets (or plates, or sheets), and relates in particular to a sheet production line and a process for producing PET sheets (PET plates). Technical context

[0003] PVC sheets, currently widely used on the market, have many drawbacks. First, they are polluting and can release harmful substances during their production and use, affecting indoor air quality. Second, their resistance to wear and heat is relatively low, making them prone to discoloration and deformation after prolonged use. Furthermore, their low recycling rate leads to resource waste and environmental pollution. Therefore, the search for more environmentally friendly and sustainable alternative materials has become a major necessity for industrial development.

[0004] Polyester (PET) is an emerging alternative material with superior performance. On the one hand, PET excels in environmental protection, with extremely low emissions and releases of harmful substances during production, thus meeting the requirements of modern green buildings. On the other hand, its resistance to wear and pressure is superior to that of traditional PVC, which extends its lifespan. PET also exhibits excellent heat resistance. Furthermore, its high recyclability reduces the environmental impact of production, thus aligning with the concept of sustainable development.

[0005] Despite the numerous advantages of PET, there are currently no high-capacity production lines suitable for PET sheets. This is mainly because the equipment and processes of existing high-capacity production lines are primarily designed for other types of materials. Due to the unique physical properties of PET, the mixing, extrusion, and molding processes used on these lines may not meet the requirements for plasticizing. PET, which results in inconsistent product quality and low yield. Furthermore, existing PET sheet production lines use a co-rotating twin-screw extruder, which requires high-speed operation but has low production capacity and high energy consumption. This makes high-capacity PET sheet production impossible, thus limiting its development and use. Summary of the invention

[0006] In order to remedy at least one drawback of the prior art, the present application proposes a sheet production line and a PET sheet production process.

[0007] In a first aspect, the present application relates to a sheet production line comprising an extruder, a die, and a calender arranged in a processing direction. The extruder comprises: - a cylinder; - a twin-screw assembly arranged inside the cylinder, this twin-screw assembly comprising two meshed screws, each having an outside diameter decreasing according to the direction of treatment and an angle between their central axes between 10' and 1°20', the two screws rotating around their respective central axes in opposite directions.

[0008] In some embodiments of the first aspect, the two screws have the same dimensions, the average of the maximum and minimum outside diameters of each screw is the median diameter, the length / median diameter ratio of each screw is between 25:1 and 35:1, and the length of each screw is between 3 and 7 m.

[0009] In certain embodiments of the first aspect, each screw comprises, in order according to the direction of processing, a preheating section, a first mixing section, a first exhaust section, a compression section, a second mixing section, a plasticizing section, a second exhaust section and an evacuation section.The preheating section mixes and heats the material in conjunction with the cylinder; the first and second mixing sections mix and finely grind the material; the cylinder is equipped with exhaust devices located at these two exhaust sections, connected to the inside of the cylinder to expel the exhaust gases generated during extrusion molding; the compression section applies pressure to compress the material; the plasticizing section exerts a shear force to melt the compressed material; and the discharge section removes the molten material from the extruder to the die.

[0010] In certain embodiments of the first aspect, the length of the preheating section represents 30% to 40% of the total length of the screw. The preheating section comprises, in the processing direction, a feed section, a first heating section, and a second heating section. The length of the feed section is greater than or equal to that of the first heating section, and the length of the first heating section is greater than that of the second heating section. The number of threads in the feed section, the first heating section, and the second heating section is identical, and the thread pitch and wing of these sections decrease in the axial direction of the screw.

[0011] In certain embodiments of the first aspect, the first and second mixing sections comprise several groups of agitator assemblies distributed axially at intervals on the outer surface of the screw, and each group of agitator assemblies comprises a plurality of tooth-like protrusions distributed along the circumference of the screw, the distribution and size of the agitator assemblies on the first mixing section and the second mixing section are the same, and the length of the second mixing section is greater than the length of the first mixing section.

[0012] In certain embodiments of the first aspect, the thread pitch of the compression section is less than that of the first exhaust section and greater than the spacing between the stirring assemblies of the first mixing section and second mixing section, and the dimension of the thread fin of the compression section in the axial direction of the screw is less than the length of the tooth-shaped protrusions of the first mixing section and second mixing section in the axial direction of the screw.

[0013] In certain embodiments of the first aspect, the plasticizing section comprises a reflux section and a stop section. A reflux groove is formed on at least a portion of the thread fins of the reflux section, the thread stroke of the stop section is less than that of the reflux section, and the length of the reflux section is greater than that of the stop section and less than twice the length of the stop section.

[0014] In certain embodiments of the first aspect, the dimensions of the pitch and the fin of the threads of the first and second escape sections are identical, and the dimensions of the pitch and the fin of the threads in the axial direction of the screw of the first and second escape sections are greater than those of the other sections of the screw, and the length of the second escape section is greater than that of the first escape section and less than twice its length.

[0015] In certain embodiments of the first aspect, three escape devices are provided, one escape device being provided at the level of the first escape section, and two escape devices being provided side by side in the direction of processing at the level of the second escape section.

[0016] In some embodiments of the first aspect, the calender comprises a frame and several sets of calendering rollers arranged on the frame along the processing direction, a sheet blank formed by the die enters the calender to be calendered, wherein a calendering roller located above a transport path of the sheet blank is the upper calendering roller, and a calendering roller located below the transport path of the sheet blank is the lower calendering roller, each set of calendering rollers comprises an upper calendering roller and a lower calendering roller arranged correspondingly above and below, or comprises only a single lower calendering roller, and a transport device is provided between at least two adjacent sets of calendering rollers;The transport device comprises a plurality of rollers (or support rollers) arranged between two adjacent sets of calendering rollers and a conveyor belt surrounding the plurality of rollers; upper end surfaces of the plurality of rollers are situated on the same plane such that the upper surface of the conveyor belt forms a plane, and a plane on which the upper surface of the conveyor belt is situated is tangent to the surface of the lower calendering roller in both adjacent sets of calendering rollers.

[0017] In certain embodiments of the first aspect, the transport device further comprises a roller mounting element, the two ends of each roller being respectively mounted on the frame via the roller mounting element, the roller mounting element having a mounting groove, and the two ends of each roller being respectively having a mounting shaft, the mounting shaft being disposed in the mounting groove and supported by a bottom of this groove, so as to mount the roller on the frame.

[0018] In some embodiments of the first aspect, a bolt hole is formed in the roller mounting element and communicates with the bottom of the mounting groove, a setting bolt is threaded into the bolt hole, and one end of the setting bolt extends from the bottom of the mounting groove into this groove and supports the roller.

[0019] In certain embodiments of the first aspect, a bolt hole is formed in the roller mounting element and communicates with the bottom of the mounting groove, a setting bolt is threaded into the bolt hole, and an end The adjusting bolt extends from the bottom of the mounting groove into that groove and supports the roller.

[0020] In certain embodiments of the first aspect, the transport device further comprises two guide rollers, installed between the drive roller and the plurality of rollers, located between the two lower calendering rollers of two sets of adjacent calendering rollers, and the conveyor belt is wound around the plurality of rollers and guided towards the drive roller by the two guide rollers on opposite sides.

[0021] In certain embodiments of the first aspect, at least one calendering roller set among the plurality of calendering roller sets comprises only one lower calendering roller, and a first heating device provided above the calendering roller set comprises only one lower calendering roller for heating the sheet.

[0022] In some embodiments of the first aspect, a displacement sensor for measuring the lifting distance of the upper calendering roller is provided for each upper calendering roller, and a second drive motor for controlling the lifting and lowering of the upper calendering roller is provided on the frame, the displacement sensor is connected to the second drive motor and is configured to send a signal to the second drive motor to raise and lower the upper calendering roller to a defined position.

[0023] In some embodiments of the first aspect, a cooling support, a drawing machine and a cutting device are provided at the rear of the calender, and a second cooking device is provided at the front of the drawing machine, the second cooking device being configured to heat a PET sheet before cutting.

[0024] In some embodiments of the first aspect, the production line has a production capacity greater than or equal to 1600 kg / h.

[0025] A second aspect of the present application relates to a process for producing PET sheets, using the sheet production line described in any one of the first aspects, comprising the following steps: - mixture of a PET mixture and auxiliary materials with calcium powder in a ratio of 1:2 to 5; - the mixed material is fed into an extruder for extrusion molding. The two screws of the extruder rotate in opposite directions at a speed of 10 to 40 rpm; - the plasticized material from the extruder enters a die for molding; - the molded sheet blank enters a calender for calendering; - the sheet blanks coming out of the calender are cut to form PET sheets.

[0026] In certain embodiments of the second aspect, the extruder screw comprises a preheating section, a first mixing section, a first exhaust section, a compression section, a second mixing section, a plasticizing section, a second exhaust section and a discharge section successively along the processing direction; the temperature of the extruder in the preheating section is 210 to 260 °C, the temperature from the first mixing section to the plasticizing section is 230 to 280 °C, the temperature in the second exhaust section and the discharge section is 190 to 250 °C, and the temperature decreases from the second exhaust section to the discharge section; the temperature of the mold is 220 to 260 °C; and the temperature of the calendering roller in the calender is 180 to 250 °C.

[0027] Compared to the prior art, the advantages and positive effects of the present application are as follows:

[0028] (1) The sheet production line, according to at least one embodiment of The present application improves the structure of the extruder screw according to the characteristics of PET, which makes it possible to obtain a shear force suitable for the plasticization of PET, to improve the effect and efficiency of the plasticization, and to reduce the energy consumption of the extrusion process;

[0029] (2) The sheet production line, according to at least one embodiment of The present application is equipped with a conveying device between adjacent sets of calendering rollers. This conveying device uses an adjustable inclined conveyor belt to transport the sheet blanks, which effectively prevents deformation of the surface of the sheet blanks and ensures the flatness of the final sheet;

[0030] (3) At least one embodiment of the present invention relates to a line PET sheet production line equipped with heating devices in the calender and upstream of the tensile testing machine, adapted to the changing properties of the PET sheet material during the production process. Thus, the sheet / plate can be heated according to actual needs, avoiding the problem of cracking due to excessive brittleness during the calendering and cutting processes;

[0031] (4) The process for producing PET sheets, described in at least one mode of The implementation of the present invention uses an improved production line and increases the proportion of calcium powder in the material to obtain anisotropic extrusion (counter-rotation of the two screws) at low speed. Simultaneously, the temperature of the mold (or die) and that of the calender roll are limited to an appropriate range depending on the characteristics of PET, thus ensuring the quality of the finished PET sheet, improving production capacity and reducing energy consumption. Brief description of the figures

[0032] The drawings described herein are intended to better understand and form an integral part of this application. The embodiment examples and their descriptions are intended to explain this application and do not limit it. In the drawings:

[0033] Fig. 1a is a side view of the sheet production line according to an embodiment of the present application;

[0034] Fig. 1b is a top view of the sheet production line according to an embodiment of the present application;

[0035] Figure [Fig. 2] is a schematic view of the extruder structure according to a mode of fulfillment of this request;

[0036] Fig. 3 is a schematic view of the structure of the twin-screw assembly according to one embodiment of the present application;

[0037] Fig. 4a is a schematic view of the distribution of each section along the axial direction of the screw in the embodiment of the present application;

[0038] The [Fig.4b] is a cross-sectional view along section AA of the [Fig.4a];

[0039] The [Fig.4c] is a cross-sectional view along section BB of the [Fig.4a];

[0040] Figure 5a is a first schematic view of the structure of the grille according to a method of carrying out this request;

[0041] Fig. 5b is a second schematic view of the structure of the grille according to one embodiment of the present application;

[0042] Fig. 5c is a side view of the grille according to one embodiment of the present application;

[0043] [Fig.6] is an enlarged partial view of part C of [Fig.5c];

[0044] Figure 7 is a first schematic view of the conveyor according to a mode of fulfillment of this request;

[0045] Fig. 8 is a second schematic view of the conveyor according to one embodiment of the present application;

[0046] Figure 9a is a schematic view of the conveyor according to one embodiment of the present application, with the conveyor belt removed, illustrating the structure of the entire roller assembly;

[0047] Figure 9b is a partial enlarged view of part D of Figure 9a, the whole of roller assembly being partially cut out to reveal internal bolt holes;

[0048] Fig. 10 is a schematic view of the side of the grille equipped with a displacement sensor according to an embodiment of the present application. Detailed description of the invention

[0049] The following, in conjunction with the accompanying drawings, provides a clear and complete description of the technical solutions of the embodiments of this application. Of course, the embodiments described represent only a part of the embodiments of this application, and not all of them. All other embodiments derived from the embodiments of this application by persons skilled in the art without inventive effort are covered by the protection of this application.

[0050] The reference to "embodyments" in this application means that the specific features, structures, or elements described in connection with the embodiments may be included in at least one embodiment of this application. The presence of such expressions in various places in the description does not necessarily refer to the same embodiment, nor does it constitute independent or mutually exclusive alternative embodiments. Those skilled in the art understand, both explicitly and implicitly, that the embodiments described in this application may be combined with other embodiments without contradiction.

[0051] In the description of this application, it is understood that the terms "upper," "lower," "front," "rear," "horizontal," "top," "bottom," "inside," and "outside," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships illustrated in the accompanying drawings and are solely intended to facilitate and simplify the description of this application. They do not indicate or imply that the devices or components mentioned must have a specific orientation, construction, or operation. Therefore, they shall not be interpreted as limitations of this application. The terms "first" and "second" are used for descriptive purposes only and shall not be interpreted as indicating or implying relative importance or implicitly specifying the number of technical features shown.

[0052] In the description of this application, it should be noted that, unless otherwise indicated or restricted, the terms "mounted," "connected," and "connect" are to be interpreted broadly. For example, they may refer to fixed, removable, or integral connections; direct connections, indirect connections via an intermediate medium, or internal connections between two components. A person skilled in the art will understand the specific meaning of these terms in this application.

[0053] In a first aspect, the present application relates to a sheet production line particularly suited to the production of PET sheets. It is understood that Other types of sheets can also be produced depending on the actual conditions. As illustrated in figures 1a and 1b, in some embodiments the production line includes a feeding device 1, an extruder 2, a die 3, a calender 4, a cooling support 5, a drawing machine 7, a cutting device 8, etc., along a processing direction.

[0054] The material is mixed by the feeding device 1 and then introduced into the extruder 2 for processing. The extruder 2 used in this application is a twin-screw extruder. The material is conveyed forward into the extruder 2 by the rotation of the screw. During this movement, the material is heated, sheared, and compressed by the screw, which mixes, melts, and plasticizes it, thus preparing it for subsequent sheet forming.

[0055] Different materials exhibit different characteristics during processing. For example, PET material has higher hardness and strength, making it more brittle and rigid, and its glass transition temperature is higher, requiring a higher processing temperature, but it is also environmentally friendly. When using an existing extruder to extrude PET material, the plasticizing process is difficult to control, and problems such as insufficient or excessive plasticizing, significant current fluctuations, unstable product quality, and low production capacity are easily encountered. To address the aforementioned problems, this application first improves the production line of extruder 2 to enhance the plasticizing effect of PET materials, thereby improving the quality of subsequent sheet forming.

[0056] As illustrated in Figures 2 and 3, the extruder 2 comprises a cylinder 21 and a twin-screw assembly located inside the cylinder 21. This twin-screw assembly comprises two meshing screws 22. The outside diameter of each screw 22 gradually decreases in the direction of processing, and the angle α between their central axes is between 10' and 1°20'. The two screws 22 rotate in opposite directions: one clockwise and the other counterclockwise. Figure 3 is a simplified structural diagram of the twin-screw assembly. To better illustrate the inclination of the screws 22, their threads are not shown in Figure 3. As illustrated in Figure 3, the two screws 22 have a slight taper close to that of a flat screw (i.e., a cylindrical screw with a constant outside diameter). This taper is very small compared to the length of the screws 22. The angle a between the central axes of the two screws 22 is between 10' and 1°20'.The final value of a can be determined based on the material composition and processing conditions. For example, a can also be 30', 35', 40', 1°, 1°10', etc. Optionally, a is between 10' and 1°.

[0057] In certain embodiments, the two screws 22 have the same dimensions. The median diameter is the average of the maximum and minimum outside diameters of each screw 22. The length-to-median-diameter ratio of each screw 22 is between 25:1 and 35:1, and its length is between 3 and 7 meters. In this embodiment, the taper and length of the screws 22 are limited. Compared to existing tapered screws, this screw 22 has a greater length and a lower taper. This optimizes the combination of the shear force exerted by the screw 22 and the duration of its action on the PET, resulting in more complete plasticization of the PET, better plasticization results, and the achievement of the desired transformation state. For example, the ratio of the length to the median diameter of each screw 22 can be 26:1, 27:1, 28:1, 29:1, 30:1, 31:1, 32:1, 33:1, 34:1, etc.and the length can be 3.5 m, 4 m, 4.5 m, 5 m, 5.5 m, 6 m, 6.5 m, etc. Based on the length of the screw and the ratio of the length to the median diameter, the value of the outside diameter of the feed end (maximum outside diameter) and the value of the outside diameter of the output end (minimum outside diameter) of the screw 22 can be determined, thus determining the outside dimensions of the screw 22.

[0058] The twin-screw assembly used in extruder 2 is an innovative screw structure, intermediate between a flat twin-screw and a tapered twin-screw. Existing tapered twin-screws exhibit significant taper, with the screw diameter varying considerably from the infeed end to the outfeed end. For example, the maximum diameter is approximately twice the minimum diameter. They are relatively short, with a length-to-diameter ratio between 22:1 and 28:1. As the material advances inside the screw, the space becomes increasingly smaller and the pressure on the material increases. Although this structure offers good plasticizing capabilities, overplasticization or mushyness can easily occur when processing PET materials if it is not properly controlled.Furthermore, pressure control is necessary during the production process, and poor control will lead to fluctuations and insufficient stability. Existing flat twin screws have identical outside diameters and no taper. Compared to tapered twin screws, their plasticizing capacity is lower. Therefore, it is necessary to increase the screw length to improve the plasticizing effect. The length-to-diameter ratio of existing flat twin screws is generally greater than 30:1, and some can reach 38:1 or even more than 40:1. The absence of taper in flat twin screws helps prevent pressure variations and material fluctuations during propulsion. The production process is thus relatively easy to control and offers good stability. However, for processing PET, plasticizing is insufficient, and the PET cannot achieve a [specific plasticizing effect - likely a specific ... Optimal plasticized state. The twin-screw assembly designed for this application is based on the two existing screw structures mentioned previously. By maintaining and reducing the appropriate taper of the tapered twin-screw, its tendency towards overplasticization is avoided. The advantage of the tapered twin-screw, which is easy to plasticize, is controlled within a suitable range by maintaining adequate pressure. Consequently, the reduced taper significantly decreases the plasticizing capacity. The required degree of plasticization is achieved by increasing the screw length. Compared to existing tapered screws, the lower taper and greater length of the twin-screw provide a shear force suitable for plasticizing PET, allowing for more complete plasticization without overplasticization. The process is easier to control during production and offers good stability.It combines the advantages of the double conical screw and the double flat screw.

[0059] In one embodiment of the present application, the twin-screw assembly is based on an existing 92 mm tapered twin-screw, which has a maximum diameter of 188 mm and a minimum diameter of 92 mm. The average of the maximum and minimum diameters of the existing tapered twin-screw is (188 + 92) / 2 = 140. Alternatively, in another embodiment of the present application, the minimum and maximum diameters of the twin-screw assembly are 190 mm and 145 mm, respectively, and the median length-to-diameter ratio is 33:1. The length of the screw is then (190 + 145) / 2 * 33 = 5,527.5 mm. The length is therefore 5,527 mm, and this type of twin-screw assembly is called a 168 mm lightweight tapered flat twin-screw.

[0060] Furthermore, the current PET extrusion process is limited by the screw structure. The two screws rotate in the same direction and must operate at high speeds, resulting in high energy consumption and low production capacity. By improving the screw structure, the extruder 2 can adopt a counter-rotating system, where the two screws 22 rotate in opposite directions. Compared to the current unidirectional (same-direction) system, the speed can be reduced, for example, from 25 to 40 rpm, thereby reducing energy consumption while increasing production capacity.

[0061] In certain embodiments, as illustrated in [Fig. 4a], each screw 22 comprises, in the processing direction, a preheating section 221, a first mixing section 222, a first exhaust section 223, a compression section 224, a second mixing section 225, a plasticizing section 226, a second exhaust section 227, and a discharge section 228. The material is initially in a solid, granular, or powdered state when it is introduced into the extruder 2. Heating rings 25 are provided on the cylinder 21 of the extruder 2, corresponding to each section of the screw 22, in order to control the internal temperature of each section of the cylinder 21. In this embodiment, Thanks to improvements in the screw's structure, the screw's functional sections, from the feed end to the discharge end, have been reorganized with two mixing sections and two exhaust sections. This ensures more thorough mixing and discharge of the PET material before it enters the plasticizing section.

[0062] The functions of each section of the screw 22 are achieved through the design of the screw's fins (ridges) and the temperature control of the cylinder 21. It is understood that this application focuses on adjusting the distribution of functional sections on the screw according to the improved screw structure, rather than on the specific implementation of the functions of each section. A person skilled in the art can implement solutions to achieve the corresponding functions of each section by referring to existing technologies. Any omission in this application shall not be considered insufficient disclosure.

[0063] The cylinder 21 continuously heats the material in the preheating section 221. This preheating section 221 of the screw 22 continuously agitates the material, ensuring uniform heating and efficient preheating. The first and second mixing sections 222 and 225 serve to mix and finely pulverize the material, resulting in a smaller and more evenly distributed dispersed phase. During the rotation of the screw 22, the material is subjected to shear forces between the screw 22 and the cylinder 21, as well as between the different flow layers of the material, which allows it to be mixed. During the processing of the materials, air, water vapor, and low molecular weight volatile compounds carried along with the material are expelled. If they are not expelled in time, defects such as bubbles and voids will form in the product, leading to a decrease in its quality. As illustrated in [Fig.[2] Cylinder 21 is equipped with exhaust devices 24 located at positions corresponding to the first and second exhaust sections 223 and 227. These devices communicate with the interior of cylinder 21 and allow the exhaust gases generated during the extrusion process to be evacuated. Screw 22 has a large thread pitch in the first exhaust section 223 and the second exhaust section 227, which loosens the material, increases the contact area between the material and the air, and facilitates exhaust. Compression section 224 compresses the material, thus increasing its density. Screw 22 has a smaller pitch and a shallower thread in compression section 224, which reduces the space occupied by the material and ensures the compression function.After preheating and compression, the material enters the plasticizing section 226 for further heating and shearing, which melts it completely and forms a uniform viscous flow, thus achieving the desired plasticizing effect. The plasticized material is... smoothly conveyed forward through discharge section 228, exiting extruder 2 and entering die 3.

[0064] In certain embodiments, as illustrated in Figures 4a and 4b, the first and second mixing sections 222 and 225 comprise several groups of agitator assemblies 23 spaced axially on the outer surface of the screw 22. Each group of agitator assemblies 23 includes several tooth-like protrusions 231 distributed around the circumference of the screw 22. The spacing between adjacent agitator assemblies 23 is 20 to 35 mm, and the length of each tooth-like protrusion 231 along the axial direction of the screw 22 is 20 to 35 mm. The distribution and size of the agitator assemblies 23 in the first and second mixing sections 222 and 225 are identical, with the length of the second mixing section 225 being greater than that of the first mixing section 222.In this embodiment, the design of the agitator assemblies 23 in the mixing section of the screw 22 disrupts the flow of the material, thereby increasing its lateral mixing. The two-stage mixing process prior to plasticization ensures more thorough mixing of the material, thus preparing it for plasticization.

[0065] It is evident that, with the exception of the first and second mixing sections 222 and 225, the external surfaces of the other sections of the screw 22 are all provided with helical threads, and the corresponding functions are achieved by designing the thread pitch, the thread lead, the screw fins (ridges), etc. of the helical threads.

[0066] In some embodiments, the length of the preheating section 221 represents 30% to 40% of the total length of the screw 22. The thread pitch of the preheating section 221 is 50 to 65 mm. In the processing direction, the preheating section 221 comprises, in order, a feed section 2211, a first heating section 2212, and a second heating section 2213. The length of the feed section 2211 is greater than or equal to that of the first heating section 2212, and the length of the first heating section 2212 is greater than that of the second heating section 2213. The number of threads in the feed section 2211, the first heating section 2212, and the second heating section 2213 is identical, and the dimensions of the screw pitch and fin decrease in the axial direction of the screw 22.In this embodiment, the long length of the preheating section 221 ensures complete heating of the material before mixing. Dividing the preheating section 221 into three functional sections with progressively decreasing screw pitch and vane dimensions allows for more complete heating of the material and a smoother transition to the mixing section.

[0067] In some embodiments, the thread pitch of the compression section 224 is smaller than that of the first exhaust section 223, and larger than the spacing between the agitator assemblies 23 of the first and second mixing sections 222 and 225. The thread pitch dimension of the compression section 224 in the axial direction of the screw 22 is smaller than the length of the tooth-like protrusions 231 of the first mixing section 222 and the second mixing section 225 in the axial direction of the screw 22. The small pitch and the small size of the screw fins (ridges) of the compression section 224 can improve the extrusion of the material, thus enabling tighter compression.

[0068] In some embodiments, the dimensions of the pitch and the fins (crests) of the first and second exhaust sections 223 and 227 are identical to and larger than those of the other sections of the screw 22. The length of the second exhaust section 227 is greater than that of the first exhaust section 223, but less than twice that of the first exhaust section 223. The large pitch and the large size of the fin (crest) of the exhaust section increase the residence time of the material and reduce the pressure, which helps the gas in the material to be evacuated from the cylinder 21. In addition, two exhaust sections, one at the rear of the first mixing section 222 and the other at the rear of the plasticizing section 226, promote the plasticizing of the PET.The greater length of the second exhaust section 227 facilitates the complete evacuation of gases from the plasticized material, thus reducing defects such as bubbles in the product.

[0069] In certain embodiments, three exhaust devices 24 are provided: one exhaust device 24 is located at the first exhaust section 223, and two exhaust devices 24 are located side by side in the direction of processing at the second exhaust section 227. The exhaust devices 24 may utilize any conventional device for venting the extruder. For example, reference may be made to Chinese patent application No. 202422531684.X, filed with the Patent Office of the People's Republic of China on October 18, 2024, the entire contents of which are incorporated by reference into this application.

[0070] In some embodiments, the plasticizing section 226 includes a reflux section 2261 and a stop (material blocking) section 2262. As illustrated in [Fig. 4c], a reflux groove 2263 is formed on at least a portion of the screw vanes in the reflux section 2261. The thread stroke of the stop section 2262 is less than that of the reflux section 2261, and the length of the reflux section 2261 is greater than that of the stop section 2262 but less than twice its length. The reflux section 2261 can cause some of the material to reflux back into the screw 22. During reflux, the material entering the screw The material 22 at different levels and at different times can be perfectly mixed, which increases the residence time of the material in the plasticizing section 226, promotes heat transfer, further enhances the plasticizing of the material, and ensures that the material reaches a good plasticized state. The thread stroke of the reflux section 2261 can be from 80 to 100 mm (e.g., 90 mm), while that of the stop section 2262 is smaller, for example, from 40 to 55 mm (e.g., 50 mm). The axial propulsion speed of the material in the stop section 2262 is relatively slow, and the pressure can be adjusted to prevent excessive reflux. This allows the material exiting the reflux section 2261 to be propelled in an orderly and stable manner in the predetermined direction to the stop section 2262, thus improving the stability of the plasticizing process.Furthermore, the design of the lengths of the reflux section 2261 and stop section 2262 allows for a balance between efficiency and plasticizing effect, thus enabling complete material processing and efficient extrusion.

[0071] In certain embodiments, a first transition section 229 is provided between the first mixing section 222 and the first exhaust section 223, and a second transition section 2210 is provided between the second mixing section 225 and the plasticizing section 226. The pitch of the first transition section 229 is greater than the spacing of the stirring elements 23 in the first mixing section 222 and less than that of the first exhaust section 223. The pitch of the second transition section 2210 is greater than the spacing of the stirring elements 23 in the second mixing section 225 and greater than that of the plasticizing section 226.

[0072] In certain embodiments, the discharge section 228 comprises a first discharge section 2281 and a second discharge section 2282, located at the rear end of the second discharge section 227, in the processing direction. The pitch of the first discharge section 2281 is greater than that of the second discharge section 2282, and the number of threads in this section is less than that of the second discharge section 2282. The length of the first discharge section 2281 is more than twice that of the second discharge section 2282. The design of the first and second discharge sections 2281 and 2282 of the discharge section 228 allows for efficient material transport and stable pressure, ensuring smooth material flow and preventing pressure fluctuations that could lead to product quality problems.

[0073] The present application also improves the calender 4 of the sheet production line, making it more suitable for the production of PET sheets. As illustrated in Figures 5a to 5c, the calender 4 comprises a frame 41 and several sets of calendering rollers 42 arranged along the frame 41. The sheet blanks The sheets formed by the die 3 enter the calender 4 to be calendered. The calendering rollers located above the conveying path of the sheet blanks are the upper rollers 421, and those located below are the lower rollers 422. Each set of calendering rollers 42 comprises an upper roller 421 and a lower roller 422, positioned respectively above and below the sheet blanks. Alternatively, each set of calendering rollers 42 may comprise only one lower roller 422. A conveyor 43 is provided between at least two adjacent sets of calendering rollers 42. (See Figures 6 to 8): the conveyor 43 comprises a plurality of rollers 431 (or support rollers) arranged between two adjacent sets of calendering rollers 42, and a conveyor belt 432 wound around these rollers 431.The upper surfaces of the rollers 431 are coplanar, so that the upper surface of the conveyor belt 432 forms a plane. The upper surface of the conveyor belt 432 is tangent to the surfaces of the lower calendering rollers 422 of the two adjacent sets of calendering rollers 42.

[0074] In the embodiment above, one or more transport devices 43 may be provided, depending on the distance between adjacent sets of calendering rollers 42. For example, if the distance between two adjacent sets of calendering rollers 42 is significant, a transport device 43 may be provided. If the distance between two adjacent sets of calendering rollers 42 is small, no transport device 43 is provided. The transport device 43 is in the form of a conveyor belt 432 wound around the rollers 431. The upper surface of the conveyor belt 432, supported above the rollers 431, is flat. Due to the high rigidity of the PET sheets (plates), the calender 4 maintains a relatively high temperature during calendering, keeping them in a relatively flexible and easily deformable state.During the transport of PET sheets (plates) between the calendering rollers 42, the flat conveyor belt 432 provides a flat support, preventing any corrugated deformation and ensuring the flatness of the PET sheet.

[0075] The upper surface of the conveyor belt 432 is tangential to the surfaces of the lower calendering rollers 422 of the two adjacent sets of calendering rollers 42, thus ensuring a smooth connection between the conveyor belt 432 and the rollers at both ends. The plates unloaded from one set of calendering rollers 42 remain flat during their transport over the conveyor belt 432 and are then conveyed to the next set of calendering rollers 42, thus guaranteeing their flatness. The upper surface of the conveyor belt 432 can be horizontal or inclined, depending on the height of the lower calendering rollers 422 at its front and rear ends. The inclination the upper surface of the conveyor belt 432 is determined by the arrangement height of the plurality of rollers 431.

[0076] In certain embodiments, as illustrated in Figures 9a and 9b, the conveyor 43 also includes roller mounting elements 433. Each roller 431 is mounted on the frame 41 by means of the roller mounting elements 433 at its two ends. These roller mounting elements 433 are provided with mounting grooves 4331. Each roller 431 is provided with mounting shafts 4311 at its two ends. These mounting shafts 4311 are arranged in the mounting grooves 4331 for mounting the roller 431 onto the frame 41. The roller mounting elements 433 may be sheet or block structures, through which the mounting grooves 4331 pass. These grooves may be open upwards or in the form of a hole.

[0077] In certain embodiments, as illustrated in Figures 9a and 9b, a bolt hole 4332 is defined in the roller mounting element 433 and communicates with the bottom of the mounting groove 4331. An adjusting bolt 434 is screwed into the bolt hole 4332. Its end extends from the bottom of the mounting groove 4331 towards this slot and supports the mounting shaft 4311 of the roller 431. To adjust the height of the roller 431 and adjust the flatness and inclination of the upper surface of the conveyor belt 432, it is simply a matter of turning the adjusting bolt 434 to adjust the height at which its end protrudes from the bottom of the mounting groove 4331. This operation is simple and easy.

[0078] In certain embodiments, as illustrated in Figures 5c to 8, the conveying device 43 further comprises a first drive motor 435 and drive rollers 436 mounted on the frame 41. The conveyor belt 432 is wound around several rollers 431 and drive rollers 436. The output shaft of the first drive motor 435 is connected to the drive rollers 436. The first drive motor 435 drives the drive rollers 436, thereby driving the conveyor belt 432. In this embodiment, the first drive motor 435 drives only one drive roller 436 to ensure the transport of the entire conveying device 43, thus eliminating the need to drive each roller 431 separately. This improves drive efficiency while ensuring stable transport of the slabs. The first drive motor 435 can be a servomotor, for example.

[0079] In some embodiments, the transport device 43 also includes two guide rollers 437 mounted between the drive roller 436 and the rollers 431. These two guide rollers 437 are located between the two lower calendering rollers 422, in two adjacent groups of calendering rollers 42. The conveyor belt 432 wraps around the rollers 431 and is guided towards the The drive rollers 436 are driven by two guide rollers 437 located on either side. In this embodiment, the drive rollers 436 are mounted on the frame 41, near the lower part. The guide rollers 437 allow adjustment of the direction of the conveyor belt 432 to prevent any interference between it and the calendering rollers.

[0080] In some embodiments, as illustrated in Figures 5a to 5c, at least one of the calendering roller groups 42 comprises only a single lower calendering roller 422. A first heating device 44 is located above the calendering roller group 42, which comprises only a single lower roller 422, to heat the sheet / plate. This first heating device 44 may be an oven or a heating device such as a heat lamp. Its length may be comparable to that of the calendering rollers, thus covering the entire width of the sheet / plate and ensuring uniform heating. Due to their high rigidity, PET materials can become brittle when the temperature drops during processing. To prevent the PET sheet / plate from becoming brittle and causing defects such as cracks during calendering, the first heating device 44 is installed in the calender 4.It can be heated as needed depending on the changing properties of the PET sheet during calendering.

[0081] In some embodiments, a lift 45 is installed on the first baking device 44. This lift 45 is mounted on the frame 41. Its activation allows adjustment of the distance between the oven and the sheet / plate, thus modulating the heating effect. Since the first baking device 44 is relatively long, two lifts 45 can be installed at each end. A connecting rod 46 links the two lifts 45 to ensure synchronous movement. The lifts can be operated using any conventional structure capable of performing a lifting function.

[0082] In some embodiments, the frame 41 of the grille 4 is mounted on a floor rail 9 to facilitate adjustment of the position of the grille 4 on the production line.

[0083] In certain embodiments, as illustrated in [Fig. 10], a displacement sensor 47 is provided for each upper calendering roller 421 to measure its upward or downward stroke. A second drive motor 48 is mounted on the frame 41 to control the raising and lowering of the upper calendering rollers 421. The displacement sensor 47 is connected to the second drive motor 48 and configured to send it a signal to raise or lower the upper calendering rollers 421 to a defined position, thus allowing precise adjustment of the gap between the calendering rollers 42.

[0084] In certain embodiments, as illustrated in Figures 1a and 1b, a cooling support 5, a drawing machine 7 and a cutting device 8 are located at the rear of the calender 4. A second curing unit 6 is also located at the front of the drawing machine 7. This second curing unit 6 is used to heat the PET sheet before cutting. Since the PET sheet can become brittle after passing through the cooling support 5, heating it before cutting is necessary to prevent defects such as cracks and ensure optimal cutting. The specific configuration of the second curing unit 6 is referenced above for the first curing unit 44 and will not be described in further detail here.

[0085] In certain embodiments, a laminating unit (not shown) can be installed at the rear of the calender 4, if necessary, to laminate (for example, deposit a film onto) the PET sheet. This unit can be equipped with one or more sets of laminating rollers, with reference, if necessary, to the prior art.

[0086] The sheet production line proposed in this embodiment of the application mainly improves the extruder 2 and the calender 4, taking into account the characteristics of PET materials. This production line can achieve a high production capacity of over 1,600 kg / h, and the resulting PET sheet / plate is of stable quality, with few defects and high yield.

[0087] A second aspect of the present application relates to a process for producing PET sheets, produced using the sheet production line described in one of the first aspects above, comprising the following steps: - Mixture of a mixture of PET material and auxiliary materials with calcium powder in a ratio of 1:2 to 5; - The mixed material is introduced into an extruder 2 for extrusion molding. The two screws 22 of the extruder 2 rotate in opposite directions at a speed of 10 to 40 rpm; - The plasticized material from extruder 2 enters a die 3 for molding; - The molded sheet blank enters a calender 4 for calendering; - The sheet blank exiting the calender 4 is cut to form PET sheets.

[0088] The PET sheet production process described above is based on an improved production line. Increasing the calcium powder mixing ratio allows for adjusting the material's condition in extruder 2. The two screws 22 of extruder 2 can operate at low speed in reverse, which improves the plasticizing effect, reduces energy consumption, and increases production. Molding and calendering are carried out while the material is in a well-plasticized state, thus improving the molding and calendering effect of the sheet and ensuring the quality of the finished PET sheet.

[0089] In the above embodiment, the mixture of PET, auxiliary materials, and calcium powder can be mixed in a ratio of 1:3 to 5, more specifically 1:4. The auxiliary materials can include, for example, a plasticizer, a heat stabilizer, a lubricant, and a flame retardant, or a combination of two or more of these, the specific ratio being determined as required. The speed of the screw 22 during extrusion molding can be 15 rpm, 18 rpm, 20 rpm, 25 rpm, 28 rpm, 30 rpm, 35 rpm, etc., and can be determined by those skilled in the art according to the actual requirements.

[0090] In some embodiments, the temperature of the extruder 2 is between 190 and 280 °C. More specifically, the extruder 2 is equipped with a cylinder sleeve 211 corresponding to each section of the screw 22, and a heating ring 25 is provided on the cylinder sleeve 211 to heat the cylinder 21, the temperature of the extruder 2 in the preheating section 221 being from 210 to 260 °C (for example, it can be 220 °C, 230 °C, 240 °C, 250 °C, etc.), the temperature of the first mixing section 222 to the plasticizing section 226 being from 230 to 280 °C (for example, it can be 240 °C, 250 °C, 260 °C, 270 °C, etc.), and the temperature in the second exhaust section 227 and the discharge section 228 being from 190 to 250 °C (for example, it can be 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, etc.), then the temperature decreases from the second exhaust section 227 to the evacuation section 228.Extruder 2 sets the corresponding temperatures for each functional section of screw 22, allowing each section of screw 22 to optimize its function at the corresponding temperature, thus ensuring mixing and plasticization of the entire material.

[0091] In some embodiments, when the material extruded by the extruder 2 enters the die 3 for molding, the temperature of the latter is between 220 °C and 260 °C, for example 230 °C, 240 °C, 250 °C, etc., and a person skilled in the art can adjust this temperature according to the condition of the extruded material.

[0092] In some embodiments, the temperature of the calendering rollers of the calender 4 is between 180 °C and 250 °C, for example 190 °C, 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, etc., and a person skilled in the art can adjust this temperature according to the condition of the plate formed.

[0093] Depending on the characteristics of the PET during processing, the temperatures of the die 3 and the calendering rollers 4 are limited to appropriate ranges. This allows the temperature to be adjusted according to the condition of the PET sheet during production, thus improving the quality of the finished sheet, reducing defects, and further increasing the capacity of the production line. In some embodiments, the calender 4 is equipped with a first heating device 44 to heat the sheet / plate during the calendering process. The temperature of this The temperature of the first heating device 44 is between 180 °C and 220 °C. The temperature of the first heating device 44 can be 190 °C, 200 °C, 210 °C, etc. After heating the sheet by the first heating device 44, the temperature increases, the brittleness decreases, and the risk of cracking during calendering is reduced.

[0094] Finally, it should be noted that the various embodiments of this description are described progressively. Each embodiment emphasizes the differences compared to the other embodiments, and the similarities between the embodiments can be translated in the references.

[0095] The above embodiments are merely an illustration of the technical solutions of this application and are not intended to limit them. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications to the specific embodiments of this application or the substitution of certain technical features may be made without departing from the spirit of the technical solutions of this application and are deemed to be covered by the scope of the technical solutions claimed in this application.

Claims

Demands

1. Sheet production line, comprising an extruder (2), a die (3) and a calender (4) arranged in a processing direction, characterized in that the extruder (2) comprises: - a cylinder (21); - a twin-screw assembly disposed in the cylinder (21), comprising two meshed screws (22), the outside diameter of each screw (22) decreasing in the processing direction, and the angle between the central axes of the two screws (22) being between 10' and 1°20', the two screws (22) rotating about their respective central axes and in opposite directions.

2. Sheet production line according to claim 1, characterized in that the two screws (22) have the same size, the average of the maximum and minimum values ​​of the outside diameter of each screw (22) being the median diameter, the length / median diameter ratio of each screw (22) being between 25:1 and 35:1, and the length of each screw (22) being between 3 and 7 m.

3. Sheet production line according to claim 3, characterized in that each screw (22) comprises a preheating section (221), a first mixing section (222), a first exhaust section (223), a compression section (224), a second mixing section (225), a plasticizing section (226), a second exhaust section (227) and an evacuation section (228) arranged successively in the direction of processing; the preheating section (221) being configured to mix the material and heat it in cooperation with the cylinder (21); the first mixing section (222) and the second mixing section (225) being configured to mix and grind the material, the cylinder (21) being equipped with exhaust devices (24) located respectively at the first exhaust section (223) and second exhaust section (227);these exhaust devices (24) being connected to the inside of the cylinder (21) and configured to evacuate exhaust gases generated during extrusion molding; the compression section (224) being configured to apply pressure to the material in order to compress it; the plasticizing section (226) being configured to apply a shear force to; bring the compressed material to a molten state; the discharge section (228) being configured to discharge the molten material from the extruder (2) to the die (3).

4. Sheet production line according to claim 3, characterized in that a length of the preheating section (221) represents 30 to 40% of the total length of the screw (22); the preheating section (221) comprises a feed section (2211), a first heating section (2212) and a second heating section (2213) in the processing direction; the length of the feed section (2211) is greater than or equal to the length of the first heating section (2212), and the length of the first heating section (2212) is greater than the length of the second heating section (2213); the number of threads in the feed section (2211), the first heating section (2212) and the second heating section (2213) is identical, and the pitch and fin of the threads in these sections decrease in the axial direction of the screw (22).

5. Sheet production line according to claim 3, characterized in that the first mixing section (222) and the second mixing section (225) comprise several groups of agitator assemblies (23) distributed axially at intervals on the outer surface of the screw (22), and each group of agitator assemblies (23) comprises a plurality of tooth-shaped protrusions (231) distributed along the circumference of the screw (22), the distribution and size of the agitator assemblies (23) on the first mixing section (222) and the second mixing section (225) are the same, and the length of the second mixing section (225) is greater than the length of the first mixing section (222).

6. Sheet production line according to claim 5, characterized in that the thread pitch of the compression section (224) is less than that of the first exhaust section (223) and greater than the spacing between the agitator assemblies (23) of the first mixing section (222) and second mixing section (225), and the dimension of the thread fin of the compression section (224) in the axial direction of the screw (22) is less than the length of the tooth-shaped protrusions (231) of the first mixing section (222) and second mixing section (225) in the axial direction of the screw (22).

7. Sheet production line according to claim 3, characterized in that the plasticizing section (226) comprises a reflux section (2261) and a stop section (2262), a reflux groove (2263) is formed on at least a portion of the thread fins of the reflux section (2261), the thread stroke of the stop section (2262) is less than that of the reflux section (2261), and the length of the reflux section (2261) is greater than that of the stop section (2262) and less than twice the length of the stop section (2262).

8. Sheet production line according to claim 3, characterized in that the dimensions of the pitch and the fin of the threads of the first escape section (223) and second escape section (227) are identical, and that the dimensions of the pitch and the fin of the threads in the axial direction of the screw (22) of the first escape section (223) and second escape section (227) are greater than those of the other sections of the screw (22), and in that the length of the second escape section (227) is greater than that of the first escape section (223) and less than twice its length.

9. Sheet production line according to claim 8, characterized in that three exhaust devices (24) are provided: one exhaust device (24) is provided at the first exhaust section (223) and two exhaust devices (24) are provided side by side in the processing direction at the second exhaust section (227).

10. A sheet production line according to claim 1, characterized in that the calender (4) comprises a frame (41) and several sets of calendering rollers (42) arranged on the frame (41) along the processing direction, a sheet blank formed by the die (3) enters the calender (4) for calendering, wherein a calendering roller located above a conveying path of the sheet blank is the upper calendering roller (421), and a calendering roller located below the conveying path of the sheet blank is the lower calendering roller (422), each set of calendering rollers (42) comprising an upper calendering roller (421) and a lower calendering roller (422) arranged correspondingly above and below, or comprising only a single roller lower calendering roller (422), and a transport device (43) is provided between at least two adjacent sets of calendering rollers (42); the transport device (43) comprises a plurality of rollers (431) arranged between the two adjacent sets of calendering rollers (42) and a conveyor belt (432) surrounding the plurality of rollers (431); upper end surfaces of the plurality of rollers (431) are located on the same plane such that the upper surface of the conveyor belt (432) forms a plane, and a plane on which the upper surface of the conveyor belt (432) is located is tangent to the surface of the lower calendering roller (422) in the two adjacent sets of calendering rollers (42).

11. Sheet production line according to claim 10, characterized in that the transport device (43) further comprises a roller mounting element (433), the two ends of each roller (431) being respectively mounted on the frame (41) via the roller mounting element (433), the roller mounting element (433) having a mounting groove (4331), and the two ends of each roller (431) having respectively a mounting shaft (4311), the mounting shaft (4311) being disposed in the mounting groove (4331) and supported by a bottom of this groove (4331), so as to mount the roller (431) on the frame (41).

12. Plate production line according to claim 11, characterized in that a bolt hole (4332) is formed in the roller mounting element (433) and communicates with the bottom of the mounting groove (4331), a setting bolt (434) is threaded into the bolt hole (4332), and one end of the setting bolt (434) extends from the bottom of the mounting groove (4331) into this groove and supports the roller (431).

13. A sheet production line according to claim 10, characterized in that the conveying device (43) further comprises a first drive motor (435) and a drive roller (436) mounted on the frame (41), the conveyor belt (432) surrounds the plurality of rollers (431) and the drive roller (436), the output shaft of the first drive motor (435) is connected to the drive roller (436), and the first drive motor (435) drives the drive roller (436) rotating, thus driving the conveyor belt (432) in rotation.

14. Sheet production line according to claim 13, characterized in that the transport device (43) further comprises two guide rollers (437), installed between the drive roller (436) and the plurality of rollers (431), located between the two lower calendering rollers (422) of two sets of adjacent calendering rollers (42), and the conveyor belt (432) is wound around the plurality of rollers (431) and guided towards the drive roller (436) by the two guide rollers (437) on opposite sides.

15. Sheet production line according to claim 10, characterized in that at least one set of calendering rollers (42) among the plurality of sets of calendering rollers (42) comprises only one lower calendering roller (422), and that a first heating device (44) is provided above the set of calendering rollers (42) comprising only one lower calendering roller (422) for heating the sheet.

16. Sheet production line according to claim 10, characterized in that a displacement sensor (47) for measuring the lifting distance of the upper calendering roller (421) is provided for each upper calendering roller (421), and a second drive motor (48) for controlling the lifting and lowering of the upper calendering roller (421) is provided on the frame (41), the displacement sensor (47) is connected to the second drive motor (48) and is configured to send a signal to the second drive motor (48) to raise and lower the upper calendering roller (421) to a defined position.

17. Sheet production line according to claim 1, characterized in that a cooling support (5), a drawing machine (7) and a cutting device (8) are provided at the rear of the calender (4), and a second cooking device (6) is provided at the front of the drawing machine (7), the second cooking device (6) being configured to heat a PET sheet before cutting.

18. Sheet production line according to any one of claims 1 to 17, characterized in that its production capacity is greater than or equal to 1600 kg / h.

19. A method for producing PET sheets, carried out using the sheet production line according to any one of claims 1 to 18, characterized in that it comprises the following steps: - mixture of a PET mixture and auxiliary materials with calcium powder in a ratio of 1:2 to 5; - the mixed material is fed into an extruder (2) for extrusion molding; the two screws (22) of the extruder (2) rotate in opposite directions at a speed of 10 to 40 rpm; - the plasticized material from the extruder (2) enters a die (3) for molding; - the molded sheet blank enters a calender (4) for calendering; - The sheet blanks coming out of the calender (4) are cut to form PET sheets.

20. A method for producing PET sheets according to claim 19, characterized in that the screw (22) of the extruder (2) comprises a preheating section (221), a first mixing section (222), a first exhaust section (223), a compression section (224), a second mixing section (225), a plasticizing section (226), a second exhaust section (227) and a discharge section (228) successive along the processing direction; the temperature of the extruder (2) in the preheating section (221) is from 210 to 260 °C, the temperature from the first mixing section (222) to the plasticizing section (226) is from 230 to 280 °C, the temperature in the second exhaust section (227) and the discharge section (228) is from 190 to 250 °C, and the temperature decreases from the second exhaust section (227) to the discharge section (228); the temperature of the mold (3) is from 220 to 260 °C;and the temperature of the calendering roller in the calender (4) is 180 to 250 °C.;

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