Air ducts and liquid conduits, such as for example cooling liquid conduits, are still commonly fabricated from textile-armored rubber pipes and rubber hoses. Pipes and hoses of thermoplastics present in contrast thereto a significant reduction of costs and weight and additionally possess an improved recycling potential. However, comparable flexural flexibility such as achieved with rubber pipes is required for the thermoplastic pipes in this material substitution.
Especially, the higher material stiffness of the relevant thermoplastics can be compensated by providing the pipe with a corrugated structure. This geometry, which is known simply as corrugated pipe, exists in different embodiments. But the flexural flexibility achieved in this way deteriorates the tensile stiffness, i.e. a higher extension of the pipe has to be accepted for a tensile load.
From DE 44 32 584 C1 (Ems-Inventa AG), there is known a corrugated pipe, wherein the corrugations extend over all radial sectors in the longitudinal direction. EP 0 671 582 A1, DE 43 21 575 C1 as well as GB 12 09 569 describe further known corrugated pipes. These known constructions have external contours which ensure a corrugated line in all radial sectors on the pipe surface area.
A corrugated pipe which has corrugations formed in zones facing each other and being otherwise free of corrugations is known from GB 12 20 975.
In contrast thereto, EP 863 351 B1 (Ems-Inventa AG) describes a pressurizable one-lumic corrugated pipe having a high flexural flexibility and high bursting pressure strength, which does not require additional reinforcing means and has a low extension in pressurizing. The external contours of the corrugated pipe described in EP 863 351 B1 are formed so that, in two zones on the pipe surface area approximately facing each other, a corrugation of the pipe can be abandoned completely. The zones free of corrugation are continuous in the longitudinal direction and can have in principle the shape of any curve. Preferably, they are arranged continuously straight, helically or sectionally straight in varying angular positions. However, the geometry described in EP 863 351 B1 is very costly to produce; and thus great quantities of corrugated pipes are today still produced with a conventional, symmetric, circular corrugated pipe profile.
The production of the aforementioned polymer hose lines or tubings is performed by coextrusion of a polymer pipe or by the so-called conex method, and then formation of the corrugations by blow or vacuum molding. Alternatively, these conduits can be produced by coextrusion blow molding. In principle, such production takes place as a continuous hollow body blow molding. The cylindrical portion of the pipe die extends into the closed region of a circulating molding chain. The plastic hose is pressed by inner overpressure or by a vacuum to the profiled circulating molding followers so that the desired corrugated pipe profile is achieved. By the further passage up to the opening of the mold closure, the plastic hose has to be cooled down sufficiently so that it leaves the circulating mold followers in a dimensionally stable state. For example, EP 368 096 A1 (Lupke) describes a device for producing plastic pipes having spirally corrugated sections.
EP 754 898 B1 (Ems-Inventa AG) describes an uncorrugated three-layer flexible cooling liquid conduit having high hydrolysis and burst pressure resistance, which consists of an outer layer of polyamide 12, hereinafter sometimes PA12, an inner layer of cross-linked HDPE and an intermediate layer of grafted maleic anhydride HDPE. Such intermediate layer serves exclusively for the purposes of coupling so that no delamination of the layers occurs. Of course, the need for such intermediate layer make the production of this three-layer tubing more expensive.
U.S. Pat. No. 5,191,916-A describes pipes having a spiral geometry, the helixes of which are reinforced with metal insertions. This reinforcement is necessary for freely laid subterranean tubings to provide strength against high outer and inner pressures. However, the production of such pipes is very costly and thus the resultant pipes are very expensive.
WO 97/39270 (Valeo) describes a multilayer plastic conduit for fluids having a polyamide outer layer, an inner layer of polyethylene and an intermediate bonding layer to prevent delamination. The conduit described in WO 97/39270 has a continuously corrugated wall more particularly a spirally corrugated wall. As a result, the screws or helixes on the outside of the pipe are broader than those on the inside. Further, the screws or helixes on the outside are flattened.
EP 486 962 A1 describes a corrugated pipe having spiral geometry which is provided at the ends on the outside with threads and which extends conically on the inside towards the ends. Therefore, the subject matter of the EP 486 962 is a flexible sleeve for use in the field of protective coverings for cables to ensure the pushing-through of cables in a better way.
EP 574 449 A1 describes a corrugated pipe having corrugations in the longitudinal and the transversal directions to the pipe axis, wherein the latter can also extend spirally. A special feature of the EP 574 449 A1 is that the thickness of the corrugated wall on the inside, i.e. on the inner bend, is reduced. A narrower bending radius is thus achieved at mild material stressing, wherein stiffness and compression resistance are maintained. However, this construction does not solve the problem endemic among plastic pipes of elongating when hot, i.e. the problems to reduce the extension in the hot state is not solved by this construction.
EP 671 582 B1 (Ems-Inventa AG) describes cooling liquid conduits of several polymer layers having polymers which are compatible with one another on the contact surfaces of the layers, wherein single adjacent corrugations of a corrugated section are connected to each other by at least one rib on the inner contour of the corrugations, and the webs are formed by a bead-like deformation of the pipe wall, and wherein successive webs are displaced continuously in the longitudinal direction of the conduit about a defined angle, and the webs are oriented parallel to the longitudinal axis of the conduit. Also, this construction does not solve the problem to reduce the extension in hot state.
DE 43 21 575 C1 (Rasmussen) describes a corrugated pipe of thermoplastic or a thermoplastic elastomer, wherein the corrugations are formed eccentrically in such a way that the inside of the wall portion is free of corrugations and it has at each site a circular cylindrical cross-section. In this manner liquids are said to be prevented from accumulating in the corrugated pipe in bottom portions of the depressions. This construction is very expensive to produce; and also it does not solve the problem of extension in the hot state.
DE 44 28 236 C1 (Rasmussen) describes a three-layer corrugated pipe which is intended for use as cooling liquid hose in automobiles. The inner layer of the multi-layer tubing according to DE 44 28 236 C1 consists of a plastic layer comprising a modified thermoplastic elastomer having a fully cross-linked rubber phase. A layer of polyamide and a further layer of a thermoplastic elastomer connect thereto, which comprises a copolymer of a functionalized polyolefin as main component and a polyamide as a compatibilizing agent. However, no details are given in relation to the selection of the polyamide material and those skilled in the art know that no tightly connection is present depending on the selection of the polyamide material, because incompatibilities occur, for example for polyamide 6 with polyamide 12; therefore, enablement is lacking. Also, the pipes described in DE 44 28 336 C1 do not solve the problem of reducing extension in the hot state.
FR 28 02 274 (Nobel Plastiques) describes a cooling liquid conduit for the field of automobiles which is formed by a layer of thermoplastic elastomer material and a thermoplastic layer of a blend of polyamide and polypropylene. Additionally, an outer protective layer of impact-resistant modified polyamide which encloses the layer of the polyamide-polypropylene blend is provided. The protective layer consists of impact-resistant modified polyamide.