MXPA99002951A - Method and installation for treating plastic tubes with bi-axial drawing - Google Patents

Abstract

The invention concerns a method for treating plastic tubes, in particular PVC tubes, obtained by extrusion, whereby a tube is brought to a molecular orientation temperature, higher than room temperature, and is subjected to a bi-axial drawing by radial expansion and axial elongation, to obtain a bi-oriented tube whose mechanical properties are improved. The tube (T) is cut into elements (4) of predetermined length immediately after being extruded and its temperature is higher than the molecular orientation temperature, each element constituting a blank (5);each blank (5) is placed in a chamber (E) to be cooled therein by a fluid (F) to a temperature close to the molecular orientation temperature, the dwell time of the blank in the chamber being greater than the time of its production by extrusion;and the blank (5) is then extracted from the chamber (E) to be subjected to a treatment enabling its bi-orientation.

Description

METHOD AND INSTALLATION FOR THE PROCESSING OF PLASTIC TUBES WITH BIAXIAL STRETCHING DESCRIPTIVE MEMORY The invention relates to a method for the processing of plastic tubes obtained by extrusion, said method is of the type according to which a plastic tube is raised to a temperature and molecular orientation greater than room temperature and is subjected to stretching biaxial by radial expansion and by axial elongation, to obtain a biorientation tube that has improved mechanical properties. Said method is known, particularly from GB-A-1, 432,539 or E.U.A.-4,340,344. The additional processing of the plastic tubes that this method requires allows a considerable increase in the prices of the finished products. However, the greater improvement in the mechanical characteristics of the products, in particular the increase in the mechanical resistance to internal pressure, leads to the use of said method in the products, in particular the tubes that are intended to transport the fluids under pressure , which must have a good mechanical resistance. According to the known method, tubes manufactured by extrusion are stored at room temperature, subsequently removed from storage and by heating they pass from room temperature to the molecular orientation temperature to undergo processing. The heating of a tube to be processed is relatively long and involves a high energy consumption, since it is necessary to cause the tube to change from the ambient temperature to the molecular orientation temperature. The result of the above is that the efficiency of the processing method is not very high and needs to be improved. The object of the invention is, above all, to provide a method for the processing of tubes made of plastic, especially PVC (polyvinyl chloride), as defined above, which allows greater efficiency, in particular by a reduction in time of processing and by a reduction in the energy consumption necessary for processing. It is desirable that the method makes it possible to improve the temperature profile in the wall of the tube for the purpose of biaxial stretching of the latter. It is desirable, in addition, that the processing method remain relatively simple and that it does not require a large amount of space, so that the cost of investments to carry out the method remains acceptable. According to the invention, a method for the processing of tubes made of plastic, especially PVC, obtained by extrusion, according to which a plastic tube passes at a molecular orientation temperature higher than room temperature and is subjected to the biaxial stretching by means of radial expansion and by axial elongation, to obtain a biorentating tube that has improved mechanical properties, is characterized because: - the tube is cut into two elements of specific length when it has just been extruded and when its temperature is higher than the molecular orientation temperature, such element constitutes a preform; Each preform is placed in a housing to be cooled by a fluid at a temperature close to the molecular orientation temperature, the residence time of the preform in the housing is greater than the time to produce the preform by extrusion, to allow correct fixing of the preform. temperature. - if said residence time is considered too long, it is possible that the preform cools considerably, but briefly, before it is introduced into the housing, by spraying or by a cold water bath. A thin layer of material can then be temporarily found at a temperature below the desired temperature for the biorientation operation. - the preform is subsequently removed from the housing to undergo processing that ensures bi-orientation. Preferably, after being extruded and before being cut, the tube is surface cooled by passing through a cooling fixture, to ensure a well-defined cut of the hot tube. The tube is advantageously cut by a cutting operation with a guillotine type blade device.

The fluid housing is advantageously formed by a hot water tank which, in the case of PVC processing, is at a temperature close to or equal to its boiling temperature at atmospheric pressure. It is desirable to ensure the best (and uniform) heat transfer between the fluid and an article of complex shape (the exterior and interior of a long tube). For this purpose, preferably, the preforms are agitated in the housing. Advantageously, the fluid is circulated in the housing, in particular in a direction parallel to the axis of the tube. The invention also relates to an installation for carrying out the method for processing plastic tubes as defined above. An installation for the processing of tubes made of plastic, especially PVC, according to the invention comprises an extruder for producing a tube and means for subjecting the tube to biaxial stretching by radial expansion and by axial elongation, to obtain a biorientation tube. which has improved mechanical properties, and is characterized in that it comprises: Cutting means at the exit of the extruder, to supply the tube in elements of specific length when it has been extruded and when its temperature is higher than the molecular orientation temperature, each element of cut constitutes a preform; A housing for receiving the preforms and cooling them by a fluid at a temperature close to the molecular orientation temperature, the housing is provided to ensure a residence time greater than the preform than the time to produce the preform by extrusion, to allow fixing correct temperature, and means for extracting the preform from the housing to cause it to pass to the means for carrying out the biaxial stretching. A cooling fixation can be provided at the exit of the extruder, to cool the tube superficially and allow a defined cut of the hot tube. The cutting means advantageously consist of a guillotine-type cutting blade device, in particular secured to the cooling fixture. The fluid housing can be formed by a hot water tank which, in the case of PVC processing, is at a temperature close to or equal to its boiling temperature at atmospheric pressure. Means are provided for circulating the fluid in the tank, in particular in the axial direction of the preforms. Preferably, the preforms are agitated in the housing. Means can be provided to fix in rotation the preform that will be extracted next to the tank. Advantageously, a roller conveyor or a band device is arranged at the outlet of the extruder, in alignment with the latter, to receive the tube. The speed of the conveyor is substantially equal to the exit velocity of the tube from the extruder, to reduce the stresses to a minimum in the outgoing pipe. The speed of the conveyor can be variable in this condition for the purpose of local over-thickening (or sub-thickening) of the tube preform. The fluid housing for setting the temperature of the preform is preferably placed on one side of the conveyor at a lower level. The pushing means is provided to transversely displace the preform and cause it to enter the fluid housing. In addition to the provisions written above, the invention consists of other provisions that will be described more explicitly below with respect to an exemplary embodiment which is described with reference to the accompanying drawings, but which in no way is limiting. Figure 1 of said drawings is a simplified diagrammatic top view of an installation for the manufacture of plastic tubes that carry out the processing method of the invention. Figure 2 is a simplified diagrammatic elevation view of the installation of Figure 1. Figure 3 is a cross section through a housing for correct temperature setting of the preforms, said housing is located next to a conveyor. Figure 4 is an elevation view of the device for cutting the tubes after extrusion.

Finally, figure 5 is a left side view with reference to figure 4. With reference to the drawings, in particular to figures 1 and 2, an installation 1 carrying out the method of the invention can be observed. The installation comprises an extruder 2, shown diagrammatically, which is supplied with thermoplastic granules, especially with PVC granules, from a feeder tank H shown diagrammatically. As is well known, the double effects of heating and pressure inside the extruder make it possible to agglomerate the plastic granules and obtain, at the outlet of the extruder flange 3, a preform of tube or tube T, whose wall is relatively smooth and has a temperature relatively high With respect to PVC, the temperature of the tube wall at the outlet of the extruder is of the order of 180 ° C to 200 ° C. In a conventional installation (not shown), the extruder is followed by a unit (not shown) which ensures the progressive cooling of the tube T which becomes rigid in proportion to said cooling. At the outlet of said cooling unit, the tube, which has not undergone the assurance process having biorientation, is supplied in portions of specific length and stored at room temperature. The portions prepared in this way are subsequently selected again to undergo bi-orientation processing after preheating to the molecular orientation temperature. The cutting of the tube at the outlet of the cooling unit is carried out relatively easily because the tube is rigid. The length of the cooling unit depends on a plurality of parameters, in particular the thickness of the tube emerging from the extruder. As a non-limiting indication, the thickness scale of the preforms of the tube T can be from a few millimeters up to 40 or 50 mm. In the case of tubes having a thick wall, for example of the order of 20 mm at the outlet of the extruder 2, the length of the cooling unit will be longer and will allow a high investment. According to the invention, to overcome said drawbacks, the tube T is cut into four elements of specific length when it has been extruded and when its temperature is higher than the molecular orientation temperature, each element cut in this way constitutes a preform 5. Experience has shown that said cutting can be done correctly, in particular, by cutting with guillotine 6 which will be described in more detail with reference to figures 4 and 5. In addition, a cooling fastener 7 of small length, in particular smaller than 0.50 m., Is advantageously provided at the outlet of the extruder and ensures surface cooling of the tube to place the outer layer of the tube and ensure a defined guillotine cut. The tube emerging from the extruder 2 and the cooling attachment 7 is received in a roller conveyor 8 disposed in the axial exhaustion of the extruder 2. The speed of the conveyor 8 is substantially equal to the exit velocity of the extruder tube T, to minimize the stretching or compression forces on the outgoing tube. It should be noted that it is possible to locally overdense the preform of the tube T by reducing the speed of the conveyor 8 at the moment when the material constituting the desired zone passes the cooling attachment 7. Conversely, the preform of the tube can be subengrose locally when accelerating the conveyor 8 at the right time. The surface cooling of the tube, initiated in the cooling fixture 7, can continue above the conveyor 8 by means of a sprinkler preform (not shown) that sprays water in the preform 5. As regards the PVC, the temperature of the surface can be around 70 ° C. Each preform 5 is subsequently placed in a housing E arranged on one side of the conveyor 8, to be cooled by a fluid F which is substantially at the molecular orientation temperature of the plastic of the preform. In the case of tubes made of PVC, the molecular orientation temperature which is on the scale of approximately 90 ° C to 110 ° C, an advantageous solution involves the use of hot water as a cooling fluid at a temperature close to its boiling temperature at atmospheric pressure, it is say 100 ° C. In an exemplary embodiment, the tank water was found at a temperature between 95 ° C and 98 ° C. The water in the tank, although close to its boiling temperature, acts as a cooling fluid, since the preform that emerges from the extruder is at a temperature of approximately 180 ° C, in the case of PVC, although only the surface area has cooled. Thermal self-regulation occurs in tank 9 through the evaporation of water. The residence time of the preform 5 in the tank 9 is sufficient to allow the correct fixing of temperature, that is, the homogenization of the preform temperature, essentially in the longitudinal direction and the peripheral direction. Reference will be made below to the temperature in the tube thickness, or radial direction. The residence time is longer than the time needed to produce the preform by extrusion; the residence time can be of the order of 20 to 30 minutes for the PVC pipes, whose thickness at the outlet of the extruder is of the order of 20 mm. The time to produce a preform by the extruder is in the order of a few minutes, for example 3 to 5 minutes. Accordingly, the tank 9 is designed to store a sufficient number of preforms and is adapted to the extrusion processing time and to the time for subsequent biorientation processing. The temperature setting is carried out in this way with a tank 9 of reduced length. Advantageously, the hot water circulates inside the tank by means of the pumps 10, 11 and pipes 12 connecting the longitudinal end zones of the tank to the inlet and to the supply of each pump. The assembly as a whole is arranged so that the water circulation in the tank 9 is carried out in a general direction parallel to the length of the tank and to the axis of the preforms 5 immersed in said tank. The pump 11, located on the right side of the tank 9 according to the drawings, is equipped, in its supply, by an injector 13 located to send the water jet to the preform 5a, which is the oldest in the tank 9 and which is the one that is extracted immediately afterwards from the tank. According to Figure 3, said preform 5a is located at the left end of the juxtaposition of the preforms 5. Furthermore, in order to further improve the homogenization of the temperature of the preform 5a, the means 14 for controlling said preform 5a are provided in rotation. Said means 14 may consist of 2 rollers which are parallel to the axis of the preform and one of which is a control roller. The means for controlling the rotation 14 are supported by the arms 15 in the shape of an inverted gooseneck. Said arms 15 are distributed in a sufficient number in the longitudinal direction of the preform 5a and are secured to a support 16 that can be raised relative to the frame of the tank 9 by means of 1 or more plugs 17, in such a way that they extract the preform 5a of the tank 9. It should be noted that the temperature of the preform 5a is homogenized essentially in the longitudinal direction in the peripheral direction. In the radial direction (thickness) of the preform, it is not necessary for the temperature to be homogeneous. Since the immersion time is relatively short (20 to 30 minutes), the temperature of the PVC in the center (half in the thickness of the preform) is at an intermediate value between the extrusion temperature (approximately 200 ° C) and the water temperature (approximately 100 ° C).

The above contributes to improve the processing of subsequent biaxial stretching. In addition, the means P, merely indicated in figures 1 and 2, are provided to displace the preform 5a, raised from the tank, in the longitudinal direction and to introduce it in a unit 18, where it will undergo radial expansion and axial elongation which lead to biorientation and an improvement in the mechanical properties of the material. An elevator 19 is provided between the hot water tank 9 and the unit 18. The elevator 19 is equipped with a guide 20 which, in the high position, supports the preform during its transfer from the arms 15 to the unit 18. Preferably, the preforms 5 stored in the tank 9, with the exception of the preform 5a to be removed, are supported by the transverse arms 21 mounted rotatably about the axis 22 parallel to the longitudinal direction of the preforms. The arms 21 are activated in an alternating reciprocating movement about the axis 22 by means of a control system 23 comprising a motor 24 controlling an off-center 25 which is connected to one end of the arm 21 by means of a connecting rod 26. As can be seen in figure 3, the conveyor 8, which receives the tube at the outlet of the extruder 2, is placed parallel to a long side of the tank 9 in the upper region of said tank. A transverse pushing device 27 is provided to transfer the preform 5 into the tank 9, where it falls as a result of gravity.

An inclined fin V is provided (Figure 3) to allow passage of the preform 5, while preventing it from falling into the space between the tank 9 and the conveyor 8. The cuts 6 with guillotine are illustrated in more detail in the figures 4 and 5. Said cuts comprise a blade 28, the cutting edge to which it is oblique and which can be displaced vertically by means of a plug 29. In contrast to the diagrammatic illustration in figures 1 and 2, said cutting blade 26 is located at the outlet of the cooling fixture 7, whose distal end of the guillotine 6 is equipped with a flange 30 to be secured at the outlet of the extruder. The cooling attachment 7 comprises a double cylindrical housing defining an annular chamber 31, in which the cooling water circulates, the plastic tube passes to the internal cylindrical housing 32 of the cooling attachment 7. Accordingly, the Installation functions are as follows. The extruder 2, supplied with granules of raw material or powder by means of the feeder device H, supplies at the outlet a soft-walled tube T which, on passing through the cooling attachment 7, cools on its surface. The tube T is received in the conveyor 8. When the length of the tube reaches a predetermined value (for example, 6 meters), the cutting device 6 of the guillotine cuts an element 4 that continues its journey in the conveyor 8 until it is in the position to expel towards the tank 9.

The pushing device 27 is then actuated to push the element 4 / preform 5 towards the tank 9 in transverse relation to the conveyor 8. The last preform 5 that has entered the tank 9 is located on the right according to figure 3. Bliss The preform will remain for a period equal to a multiple of the time required for the extrusion of an element and will move progressively in the transverse direction towards the rollers 14. When the preform reaches 5a in the rollers 14, it is fixed in rotation around its axis and a stream of water coming from the injector 13 passes along its axis. The preform 5a is subsequently raised by the arms 15 and pushed longitudinally by the device P to pass through the guide 20 and enter the processing unit 18 which is aligned with the preform 5a raised by the arms 15. The method e installation of the invention allow online processing of the tubes, that is, the biorientation processing can be carried out consecutively to the extruder. The intermediate tank 9, by storing several preforms, makes it possible to set the temperature correctly, by ensuring a reduced overall side. The energy saving is appreciable, although the preform, which is at a temperature higher than the molecular orientation temperature, is directly in this last temperature, without cooling to room temperature, which may need subsequent preheating.

The installation of the extrusion is simplified, as it does not have a unit to ensure santial cooling or a device to draw the tube. The invention has been described with reference to the PVC example, but it is clear that it is applied to other plastics capable of undergoing a similar processing, for example, polyethylene or polypropylene. It is feasible to regulate the fluid temperature of the housing E in accordance with the plastic. The fluid used in housing E may, of course, be different from water. It is possible, for example, to complete or replace the action of water by air or by oil.

Claims (17)

NOVELTY OF THE INVENTION CLAIMS

1. - The method for processing tubes made of plastic, especially PVC, obtained by extrusion, according to which a tube is at a molecular orientation temperature higher than room temperature and is subjected to biaxial stretching by radial expansion and by axial elongation, to obtain a bio-erosion tube having improved mechanical properties, characterized in that: the tube (T) is cut in element (4) of specific length when it has just been extruded, when its temperature is higher than the orientation temperature molecular; each element constituting a preform (5); each preform (5) is placed in a housing (E) to be cooled by a fluid (F) at a temperature close to the orientation temperature, the residence time of the preform in the housing is greater than the time to produce the preform by extrusion, to allow the correct establishment of temperature, and the preform (5a) is subsequently removed from the housing E to undergo the processing that ensures the biorentation.

2. The method according to claim 1, further characterized in that the tube (T), just after being extruded and before being cut, is cooled superficially while passing through a cooling fixation (7), the length of the which is less than 0.50 m.

3. - The method according to claim 1 or 2 further characterized in that the tube is cut by a cutting operation with a guillotine time device (6) with a blade (28).

4. The method according to one of the preceding claims, further characterized in that the housing (E) of the fluid is formed by a hot water tank (9).

5. The method according to claim 4 for the processing of PVC tubes, further characterized in that the water temperature is close to 100aC, located in particular within the range of 95aC to 989aC.

6. The method according to claim 4 or 5 further characterized in that the preforms (5) are agitated in the housing (E).

7. The method according to one of the preceding claims, further characterized in that the fluid is circulated in the housing, in particular in a direction parallel to the axis of the tube.

8. The installation for the processing of tubes made of plastic, especially PVC, to carry out a method according to one of claims 1 to 7, comprising an extruder (2) for producing a tube (T) and means ( 18) for subjecting the tube to a biaxial stretching by radial expansion and by axial elongation, to obtain a biorientation tube having improved mechanical properties, further characterized in that it comprises: cutting means (6) at the outlet of the extruder (2), to supply the tube in elements (4) of specific length when it has just been extruded and when its temperature is higher than the molecular orientation temperature, each cutting element constitutes a preform (5); a housing (E) for receiving the preforms (5) and cooling them by a fluid (F) at a temperature close to the molecular orientation temperature, the housing (E) is provided to ensure a residence time of the preform which is greater than the time to produce the preform by extrusion to allow correct temperature setting, and means (15, 17) to extract the preform from the housing to cause it to pass to the means (18) to carry out the biaxial stretching.

9. The installation according to claim 8, further characterized in that the cooling attachment (7) is provided at the outlet of the extruder (2), to cool the tube (T) surface and allow a defined cut of the tube still hot .

10. The installation according to claim 9, further characterized in that the cutting means (6) consist of a cutting blade device (28) of the guillotine type, secured in particular to the cooling attachment (7).

11. The installation according to one of claims 8 to 10, further characterized in that the fluid housing (E) is formed by the hot water tank (9) which, in the case of PVC processing, is a temperature close to or equal to its boiling temperature at atmospheric pressure.

12. - The installation according to claim 11, further characterized in that the means (10, 11, 12, 13) are provided to circulate the f in the tank, in particular in the axial direction of the preforms.

13. The installation according to claim 12, further characterized means (11, 12, 13) for circulating the f in the tank are arranged to inject a stream of f along the axis of the preform (5a) which is then extracted from the tank (9).

14. The installation according to claim 11, further characterized in that the tank (9) comprises means (14) to be fixed in rotation about the axis of the preform (5a) which is then removed from the tank (9).

15. The installation according to claim 11, further characterized in that the preforms (5) are supported in the tank (9) by the transverse arms (21) mounted rotatably about an axis (22) parallel to the longitudinal direction of the preforms, and because the arms (21) can be activated in an alternate oscillating movement about the axis (22).

16. The installation according to claim 8, further characterized in that it comprises a roller conveyor (8) disposed at the outlet of the extruder, in alignment with the latter, to receive the tube (T), and because the housing (E) ) of the f to set the temperature of the preform is placed on one side of the conveyor (8).

17. The installation according to claim 16, further characterized in that the speed of the conveyor (8) is substantially equal to the exit speed of the tube (T) from the extruder (2), and because it can be variable around said condition to locally over-prune (or subengrose) the tube preform (T).