METHOD AND COEXTRUSION HEAD FOR PRODUCING REINFORCED FLEXIBLE TUBES, AND THE TUBES OBTAINED
The present invention relates to a method and operating head for producing reinforced flexible tubes, and the flexible tubes obtained. Each of the stated aspects conforms to the introduction to the respective independent claim. The technology for producing tubes of thermoplastic material incorporating a helical core of a different material is well known and based on the coextrusion of two or more materials within a coextrusion head to obtain a web, followed by helically hot-winding the coextruded web on a rotating mandrel. Examples of this method are at least partly described in the documents DE 3202854A, .US 4870535A, US 3963856, DE 3145702A and US 4303457A, in the international publications WO 01/0430A2 of 18 January 2001 and WO 9825064A of 11 June 1998, in US 4870535 of 26 September 1989, in GB 2073360A of 14 October 1981 and in DE 19854585 of 24 June 1999.
In detail and' with reference to the state of the art, reinforced flexible tubes are known possessing a helical core of non-stabilized rigid PVC, a core cladding formed from materials chosen from the group consisting of polyamides, polyesters, polyurethanes, polyketones, polyethylene copolymers and nitrile rubber, and a tubular wall of PVC plasticized by plasticizing agents, two components (i.e. core and cladding) being totally immersed in this wall. The purpose of- the cladding is to form a barrier, i.e. to prevent migration of the plasticizer towards the rigid material of the helical core which, if contaminated, becomes fragile.
The state of the art also describes a head for coextruding the three said components, to obtain, at the outlet of a die connected to the coextrusion head, a continuous web of required cross-section to be helically hot-wound on the rotating mandrel. The known coextrusion head comprises (for the core material) a central conduit, at an intermediate point of which the coextrusion with the core cladding material takes place, with coextrusion of the third material taking place only at the outlet of the central conduit, to incorporate the previously coextruded product .
The known method comprises (relative to the coextrusion of the three layers) a first coextrusion with two of the materials representative of the core and of the relative cladding, and a final coextrusion in which the third material incorporates the previously coextruded two. The method is completed by forming a web from the triple coextrudate by drawing to a predetermined cross-section and helically hot-winding this formed web with its turns in mutual contact on a rotating mandrel to obtain the required tube, which is then cooled. A main object of the present invention is to provide an extrusion head specifically intended for obtaining inventive flexible tubes, which is compact, reliable and of simple construction.
The said object is attained by an extrusion head in accordance with the technical teachings of claim 1.
A further but determining object of the present invention is to provide a reinforced flexible tube different from current flexible tubes and is distinguished by being able to use, for the flexible part other than the core, materials which are of lesser quality or more economical
(for example salvaged PVC) while at the same time presenting internal and external surfaces which make it suitable for satisfying particular utilization requirements . A further object of the present invention is to provide a reinforced flexible tube which is structurally simple, easily produced and of low cost.
These and further objects (which will be more apparent from the detailed description specifically pertaining to tubes) are attained by a flexible tube in accordance with the technical teachings of the relative accompanying claim, either taken alone or in its possible combinations with the sub-claims which depend thereon. Another object of the present invention is to provide a method for producing flexible tubes of the invention which enables their economical and substantially continuous production while not involving substantial modifications to the known methods, but in fact integrates them.
The aforesaid object is achieved by a method in accordance with the technical teachings of the accompanying relative main claim. Further characteristics and advantages of the invention will be apparent from the description of a preferred but non-exclusive embodiment of the inventive coextrusion head, illustrated by way of non-limiting example in the accompanying drawings, in which: Figure 1 is a longitudinal section through the coextrusion head of the present invention;
Figure 2 is a longitudinal section through the head of
Figure 1 taken from a different angle;
Figure 3 is a front section through a detail of the coextrusion head;
Figure 4 is a view on the section line 4-4 of the detail of
Figure 3;
Figure 5 is a view of the detail of Figure 3 from above;
Figure 6 is a schematic cross-section through the triple- layer extruded bead before being drawn;
Figure 7 is a view of a longitudinal half-section through the spiral tube, formed by winding the triple layer extruded bead of Figure 6, in which the deformation of the turns can be seen due to the forces generated during the winding of the tube;
Figure 8 is a longitudinal section through the constituent coextrudate of the tube before its winding to form the tube but following the drawing process; and
Figure 9 is a longitudinal section through two turns of the coextrudate, just deposited on the mandrel but not yet deformed. With initial reference to Figures 1-5, these show a coextrusion head indicated overall by 1 , for producing the coextrudate of Figure 6 and the tube of Figure 7. It comprises a support part 2 in which three conduits indicated respectively by 3, 4 and 5 are provided to feed into the head a first, a second and respectively a third component to be extruded. The three conduits are hydraulically connected to known feed units for the components . A holed spacer 6 for supporting a nosepiece carrier 7 is fixed to the support part 2. Three inlet holes 3a, 4a and 5a are provided in correspondence with the conduits of the support part 2. The nosepiece carrier 7 is provided with a central passage hole 9 for a nosepiece 11 and with an aperture 4b in correspondence with the hole 4a. The central hole 9 presents a step 10, which together with the dowel 8 enables said nosepiece 11 to be fixed to the nosepiece carrier 7.
The nosepiece 11 presents a central channel 3b in correspondence with the hole 3a of the nosepiece carrier 7, and carries a nozzle 12 fixed in known manner to the end of the nosepiece 11. The head also comprises a hollow member 13 for housing a conveyor member 14 which is also axially hollow and is fixed to the hollow member 13 by a step and a second dowel 16, similar to the fixing between the nosepiece 11 and the nosepiece carrier 7. The conveyor member 14, shown in greater detail in Figures 3-5, presents a central hole 17 with its axis A coinciding with that of the nosepiece 11 and hence of the nozzle 12, and an elbow conduit 18 communicating with a groove 19, the geometry of which is described hereinafter and which forms an important . part of the present invention. The central hole 17 houses the nosepiece 11, between the outer surface of the nosepiece and the surface of the central hole 17 there being formed an annular conduit 20 communicating with the conduit 4 via a passage 21 which itself • communicates with the aperture 4b and through which the second component arrives to form the filler of the coextrudate of Figure 6.
The groove 19 is advantageously formed by spiral milling of the surface of the conveyor member 14, the cylindrical surface S downstream of this groove (to the left in Figure 4) being subsequently cylindrically milled in a manner eccentric to the axis A, and practically on a circumference C centered at Bl (Figure 3) . Hence the base D of the groove 19, well evident in proximity to the elbow conduit 18, is in a position opposite the opening of the conduit 18, practically removed by said milling operation. When the conveyor member 14 is mounted, it forms with the cylindrical wall of the hollow member 13 a passage or conduit (22a, b) for the third component, said passage having its passage opening continuously widening radially in moving away from the elbow conduit 18 to obtain, as its final result, a uniform flow rate of the component in such a manner as to reduce the pressure drops and make uniform the pressure of the fluid fed into the gap (22a, b) between the cavity in the hollow member 13 and the outer surface of the conveyor member 14. As can be seen from Figure 1, the passage gap 22a in the upper part of the figure, between the inner surface of the cavity in the hollow member 13 and the outer surface of the conveyor member 14, is very narrow, whereas the said gap in the lower part 22b is wider. This compensates the pressure drops which occur in feeding the fluid into the gap and, besides allowing it to be properly filled, it makes the flow rate of the fluid fed into it uniform.
The gap in question extends conically towards the central hole 17 and opens into it at the end of the annular conduit 20, i.e. before the opening of the nozzle- 12, with the result that the materials originating from the channel 20 meet the gap (22a, b) before they encounter the material leaving the nozzle 12. Downstream of the hollow member 13 there is fixed a conventional conveyor/die assembly 23 presenting a substantially converging cavity, favouring bonding between the various coextruded layers. A die block 25 gives the final shape to the coextruded bead, in this case it giving the bead the shape of Figure 9.
The operation of the coextrusion head according to the invention is apparent from that described and illustrated, and is substantially as follows.
The components which are to form the spiral tube at the end of the process are injected respectively into the first conduit 3, the second conduit 4 and the third conduit 5. In particular into the first conduit 3 there is fed a first component which at the end of the forming process will constitute the rigid part of the tube, i.e. the helical core (31, 300) . This material can vary according to the final characteristics of the tube to be formed and will be discussed in detail hereinafter. In the example the rigid core is rigid PVC. If the rigid material were instead, for example, a metal, the conduit 3b would assume a different shape, but in any event conventional and tubular. The second component (32, 200) is fed into- the second conduit 4 to constitute at the end of the forming process the preponderant elastic part of the spiral tube, it being substantially virgin plasticized PVC or regenerated plasticized PVC. In the illustrated example regenerated plasticized PVC is injected into the conduit 4. The component to form the inner and outer cladding of the tube is fed under pressure into the third conduit 5, and gives the tube its special characteristics. Thus electroconductive PU, virgin PVC, polyurethane-PVC, nitrile-PVC, oil-repellent PVC, abrasion resistant PU or any other material able to give the final spiral tube a specific physical characteristic can be injected. For example nitrile-PVC is injected into the third conduit 5, to give the tube a specific resistance to oily substances. The material injected into the first conduit 3, in this case rigid PVC, is simply conveyed along the axis of the nosepiece 11, while the elastic material injected into the conduit 4, in this case plasticized regenerated PVC, wraps it only after being wrapped in its turn by the cladding material.
The cladding material, fed. into the conduit 5 (for example nitrile-PVC) , traverses the curved conduit 18 and is distributed within the groove 19 and then, uniformly, within the section between the hollow member 13 and conveyor member 14 (i.e. within the gap 22a, b) . Distribution is particularly uniform by virtue of the fact that the discharge passage has an increasing cross-section as the distance from the insertion point increases. Because of this the pressure drop decreases as the distance from the curved conduit 18 increases. At the exit from the conical point of the gap 22a, b there is pinch region 26, the purpose of which is to uniformly increase the exit pressure, to ensure the total filling of the upstream sections. The regenerated PVC layer of the elastic filler part (32, 300) which moves within the conduit 20 encounters, in a region upstream of the outlet of the nozzle 12, the thin layer of cladding material (33, 600), "'' to form • a first coextrudate, said first coextrudate encountering the exiting rigid PVC (31, 300) at the outlet of the nozzle 12 to form a triple coextrudate 30 (Figure 6) .
At the inlet to the conveyor/die assembly 23, 24 the extrudate 30 is as indicated in Figure 6, hence substantially with a circular core 31 of rigid PVC, a first core cladding 32 of regenerated PVC, constituting the preponderant part, and a second thin nitrile-PVC cladding 33 constituting the characterising' part of the tube. The final bonding of the three components takes place within the conveyor/die assembly 23. The extrudate 30 is then shaped by the die block 25 into the final shape, which in this case is that shown in Figure 8.
The bead of extrudate is then hot-wound on a rotating mandrel in accordance with the known art to form the tube, which is then cooled.
Analyzing in greater detail the tube formed by this head, it being for example particularly economical and suitable for use in transporting water, this comprises (Figure 7) : " - a helical core 300 in the interior of the tube and formed, for example, of rigid PVC; a filler part 200 of engineering polymer, in this case regenerated plastic PVC, intended to constitute the 1,1 preponderant part of the tube and in which the helical core
300 is embedded; a cladding 600 on the tube filler part 200, which is of higher quality (and hence more costly) PVC, and in particular virgin PVC.
As a result of the hot-winding of the coextrudate of Figure 8 on a rotating mandrel in accordance with the known art, the spiral tube 100 is formed, as shown in axial half- section in Figure 7. Substantially, a continuous strip of coextrudate becomes hot-deposited on the rotating mandrel. The rotation of the mandrel causes, at each revolution, that spiral turn of the coextrudate which has just been deposited to come into lateral contact with that previously wound on the mandrel. This contact of the lateral portions 900, 1000 (the sides of the spiral turns, Figure 9) and the fact that deposition takes place hot enables perfect bonding of the coextrudate to be achieved along the sides of the turns. The inner wall 700 and the outer wall 800 of the tube are hence formed. In particular, the adjacent sides of the turns 900, 1000 which come into contact and are hence bonded during deposition of the coextrudate pertain to the virgin PVC cladding; in this respect, this virgin PVC cladding completely wraps and isolates the filler part 200 of lesser quality, inexpensive and non-characterising material. This enables the sides of the spiral turns to bond together in an optimal manner, and which would not occur if lesser quality materials were to come into contact, such as that with which the filler part 200 is formed. As is well known, regenerated plastic PVC does not bond together properly. A spiral tube such as that illustrated is particularly advantageous because there is only virgin PVC visible on the outside, and in contact with the liquid or fluid on the inside, even though most of the tube is of regenerated plastic PVC, with considerable advantage in terms of costs, economy and environmental impact.
In a different embodiment the cladding 600 of the filler part 200 of the tube 100 is of enriched PVC, which gives the tube special resistance characteristics. In this case the tube is wear resistant and in particular abrasion resistant, hence the cladding 600 is a PVC-polyurethane mixture. The preferred polyurethane percentage is around 40% by weight, but can vary from 3% to 90%. In particular cases the cladding can be of substantially pure polyurethane . A spiral tube such as that illustrated in this example is particularly advantageous because there is only PVC-PU visible on the outside and in contact with the liquid or fluid on the inside, even though most of the tube is of regenerated plastic PVC, with considerable advantage in terms of costs, economy and environmental impact.
A further advantage is that the quality material cladding, in this case wear-resistant, is on both the inner surface and outer surface of the tube. This considerably improves the resistance characteristics of the tube compared with traditional tubes, and in particular with wear resistant tubes . In an alternative embodiment a part of the filler 200 is of virgin PVC, to make the wear resistant tube of enhanced quality but still more economical than one formed only of PVC-PU. In yet another embodiment regarding an oil resistant industrial tube, the cladding 600 is formed of nitrile-PVC. The nitrile percentage is about 20%, but can vary from 3 to 30%. The addition of nitrile enables the cladding to resist oils. It is evident that also in this embodiment the filler part 200. can be formed of regenerated plastic PVC or virgin plastic PVC according to requirements.
In a different embodiment regarding an electroconductive industrial tube, the cladding 600 is formed of PVC-PU, i.e. antistatic. The addition of polyurethane enables the electroconductive inner wall to be in electrical transmission contact with the outer wall of the tube, also electroconductive, and hence discharge to earth any static electricity which may be generated inside the tube due to friction between the fluid flowing through it and its inner wall.
Again in this embodiment the filler part 200 can evidently be of regenerated plastic PVC or virgin PVC according to requirements .
The tube formed in this manner is particularly advantageous because it combines the flexibility deriving from its construction with perfect antistatic characteristics, and is hence very useful for transporting fluids with suspended particles or flammable liquids.
Various embodiments have been illustrated, however others can be conceived using the same inventive concept. In particular the thin cladding part 600, which gives the tube its special characteristics, can be formed of any material in addition to those listed, according to the particular characteristics to be given to the industrial tube.