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1. WO2007090831 - PROCESS FOR MANUFACTURING A FUEL TANK AND TOOL FOR ITS IMPLEMENTATION

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

Process for manufacturing a fuel tank and tool for its implementation

The present invention relates to a process for manufacturing a fuel tank made of a plastic impermeable to gases and liquids.
The plastic fuel tanks used in the industry, in particular in the automotive industry for equipping vehicles, generally include one or more accessories that are associated therewith, such as aeration and venting valves, delivery tubes, various sensors and their connections, and fuel gauge and pumping modules.

These fuel tanks must meet increasingly drastic sealing and permeability requirements. The permitted emission limits have become so low that losses due to leaks and to permeability of the interfaces of the accessories with the tank assume a higher relative proportion in the total losses of the tank/accessories system.
It is known, from Patent Application WO 01/21428, to block off multilayer plastic tank openings, made for introducing an accessory into the tank or for fastening it thereto, with a sheet of multilayer structure compatible with that of the tank, which is welded to the external wall of the latter.
However, that document discloses that the sheets used have a rigidity close to that of the walls of the tank. As a result, these sheets cannot be easily used when the emergent part of an accessory is too bulky or the emergent surface of the accessory is irregular.
In addition, despite the bulge that can be produced near the edge of the sheet, so as to reduce locally the thickness of the plastic layers constituting the multilayer sheet, the level of fuel losses by emission at the point where the sheet is welded to the tank still often remains too high and sometimes even exceeds the very low limits imposed by the latest environmental standards.
Patent Application WO 03/035424 proposes a process for manufacturing a fuel tank in which a flexible film of multilayer structure, comprising at least one fuel barrier layer, is fastened to the tank by welding it to the outer surface of the latter, over the entire periphery of the film.
Such a process allows a tank to be manufactured in which the emissions at the interfaces between the tank and the accessory are further reduced. Since the film is fastened to the wall of the tank by being welded over its entire periphery, good impermeability is obtained.

However, the quality of the impermeability depends on the quality of the weld. The laser welding technique requires there to be contact between the surface of the tank and the film to be welded. However, the fuel tank generally has a non-plane and deformable surface so that intimate contact between the latter and the film is difficult to achieve. When a gap exists between the surfaces, the heating of the flexible film by the laser radiation becomes non-uniform and difficult to control.
The non-uniform heating may lead to "cold" zones and "burnt" zones. In the cold zones, the weld cannot generally withstand the mechanical tests of the quality procedure in force. In the burnt zones, the flexible film is burnt, or more precisely, the barrier layer of the film is impaired and becomes permeable to hydrocarbons. It is therefore important, in order to obtain a high-quality weld, to improve the contact between the flexible film and the outer surface of the fuel tank.
The object of the present invention is to propose a process for
manufacturing a fuel tank with better impermeability at the interfaces between tank and accessory. This objective is achieved by a process according to Claim 1. The invention therefore relates to a process for manufacturing a plastic fuel tank, the tank being provided with at least one accessory connected to the internal space of the tank via at least one orifice in the wall of this tank, said process comprising the steps consisting in supplying a flexible film and placing it on the tank so as to cover the interface between the tank and the accessory, and welding the film over its entire peripheral area to the wall of the tank.
According to an important aspect of the invention, before welding, a vacuum is created between the flexible film and the wall of the tank. Thanks to such a vacuum, the gaps between the flexible film and the wall of the tank are minimized, thus improving the contact between the surfaces. According to the invention, this vacuum is created using a vacuum tool having a suction chamber comprising an opening directed towards both the flexible film and the support, the opening in the suction chamber being peripheral and formed in such a way as to rest over the edge of the flexible film over its entire periphery, and at least one suction channel for sucking out the air from the suction chamber.
The intimate contact between these surfaces results in a more uniform weld, that is to say a weld in which the cold zones and the burnt zones are avoided. Thus, a higher-quality weld and greater impermeability at the interface between the tank and the accessory are obtained.

According to the invention, the expression "cover the interface between the tank and the component" in fact means to cover both the periphery of the accessory and the tank wall surrounding the latter.
In the context of the invention, the term "fuel tank" is understood to mean any type of tank capable of storing a liquid and/or gaseous fuel under various pressure and temperature conditions. More particularly intended are tanks of the type of those encountered in motor vehicles. The term "motor vehicle" is understood to also include cars, motorcycles and trucks.
The term "plastic" is understood to mean any material comprising at least one synthetic resin polymer.
All types of plastic may be suitable. Very suitable plastics belong to the category of thermoplastics.
The term "thermoplastic" is understood to mean any thermoplastic polymer, including thermoplastic elastomers, and also blends thereof. The term "polymer" is understood to mean both homopolymers and copolymers
(especially binary or ternary copolymers). Examples of such copolymers are, non-limitingly: random copolymers, linear block copolymers, other block copolymers and graft copolymers.
Any type of thermoplastic polymer or copolymer whose melting point is below the decomposition temperature is suitable. Synthetic thermoplastics that have a melting range spread over at least 10 degrees Celsius are particularly suitable. Examples of such materials include those that exhibit poly dispersion in their molecular weight.
In particular, polyolefins, polyvinyl halides, thermoplastic polyesters, polyketones, polyamides and copolymers thereof may be used. A blend of polymers or copolymers may also be used, likewise a blend of polymeric materials with inorganic, organic and/or natural fillers such as, for example, but not limitingly: carbon, salts and other inorganic derivatives, natural fibres and polymeric fibres. It is also possible to use multilayer structures consisting of stacked layers, bonded together, comprising at least one of the polymers or copolymers described above.
Polyolefins have given good results. Among polyolefins, high-density polyethylene (HDPE) is preferred.
The invention relates to a fuel tank provided with at least one accessory located at least partly on the outside of the tank. The term "accessory" is understood to mean any member in general through which liquid or gas passes, or which is in contact with liquid or gas and which fulfils a particular function specific to the fuel system of which the tank forms part, including the function of transferring liquid and/or gas between two other members.
Examples of such accessories include, non-limitingly, the following accessories:
- a container for containing any chemical or physical composition, especially a vapour absorption canister;
- a liquid or gas gauge;
- an electrical connection terminating in a liquid or gas gauge; a liquid or gas pump;
- a safety valve for controllably shutting off the tank in certain particular situations;
- a drainable vessel for collecting liquid;
- an electrical connection for powering the motor for a liquid or gas pump; - a liquid line terminating in a device for supplying any device, especially an engine; and
- a liquid/vapour separation device.
It is also possible to use any combination of at least two accessories, optionally in the presence of several examples of the same accessory.
The accessory according to the invention may be made of any material.

However, for preference it is mainly based on a plastic, and in particular on a plastic whose dimensional stability is hardly affected by contact with the liquids and gases liable to be contained in the tank, to which the plastic is barely permeable. Polyacetals, polyamides, polyesters and polyvinyl halides give good results. Plastics that are very suitable are polyacetals, in particular POM
(polyoxymethylene) .
Particularly preferably, the accessory is at least partly made of injection-moulded plastic, that is to say it has been formed by a moulding technique in which the material is injected under pressure into a mould.
The accessory may be based on a material having a low fuel permeability, or made of polyethylene. Preferably, when the component is based on polyethylene, it is treated so as to reduce its permeability. The treatment consists for example in sulphonation or fluorination of the component.
The accessory may be fastened to the tank in any appropriate manner. For example, it may be fastened by clip-fastening (preferably with an intermediate seal to ensure sealing), or by welding. In the latter case, however, this means that the part to be welded has to be compatible with the constituent material of the tank. In the case of a component mainly made of POM intended for an HDPE tank, the component may, for example, be made a two-material component (for example by injection overmoulding HDPE onto POM).
The accessory is located at least partly on the outside of the tank. The accessory may be located completely outside the tank. One example is a module mounted on one wall of the tank and fulfilling a particular function involving the presence of fuel. The accessory may also be located only partly outside the tank. In this case, it passes through the wall of the latter and is provided with an interface with this wall which is sealed relative to gases and liquids.
The accessory is covered with a protective device that improves the impermeability of the tank/accessory assembly. In other words, the presence of the protective device allows the fuel losses at the interface between the accessory and the tank to be significantly reduced. This protective device is a flexible film, preferably of multilayer structure, that is to say a laminated structure resulting from the stacking of several layers each comprising at least one thermoplastic material, the thickness and the nature of the layers of which are such that a flexible film is substantially more flexible than the wall of the tank. Preferably, the flexible film is easily deformable by a simple manual action.
The flexible film has a surface of various types. For example, it may constitute a convex surface having a shape bounded by more or less circular or elliptical curves, or on the contrary, having sharp angles, such as rectangular or polygonal shapes. The flexible film may also have a non-convex surface that includes at least one convex surface located on the inside of the surface of the flexible film. In each of these convex surfaces included within the non-convex surface of the flexible film, the multilayer structure is interrupted so as to leave an enclosed space in the non-convex surface.
According to the invention, the flexible film may be a continuous film or it may include an opening (generally a central opening) for an element projecting from the accessory, extending to the outside of the tank, such as, for example, a pipe. This is in particular the case when the accessory is a valve connected to a venting line. The way in which such films are used forms the subject of a co-pending application in the name of the Applicant. These films are commonly referred to as "donut"-shaped films.
According to the invention, the flexible film advantageously includes at least one fuel barrier layer. The term "fuel barrier layer" is understood to mean a layer impermeable to gaseous and liquid fuels. The barrier layer generally comprises a barrier resin. Any known barrier resin may be present in the barrier layer, provided that it is effective with respect to the fluid fuels liable to be in contact with the tank and/or the accessory, particularly hydrocarbons, and provided that it is compatible with the technique for manufacturing the structure of the multilayer film.
Among possible resins, mention may be made, non-limitingly, of polyamides or copolyamides and random ethylene/vinylalcohol copolymers. A blend of various barrier resins is also possible. Very good results have been obtained with a barrier layer comprising a barrier resin made of a random ethylene/vinylalcohol copolymer.
The flexible film may have been obtained by any known technique resulting in the manufacture of a thin and flexible multilayer structure. One possible technique is the technique of extruding a multilayer film through a sheet die. Another possible technique is the compression moulding of a multilayer sheet.
The flexible film covering the accessory of the tank is, according to the invention, fastened to the tank (or near it, around the component) by welding. The flexible film may, for example, form a pocket trapping an accessory, said pocket being welded to the tank.
Welding of the flexible film is localized on the outer surface of the tank, over the entire outer peripheral area of the film.
If the film is of the "donut" type as mentioned above, the film is also preferably welded, over the entire border area around the opening is in the film, to the surface of the accessory. Such a process allows the film to be fastened by welding, not only in its peripheral area, but also in its border area around the opening.
In this variant, the Applicant has surprisingly found that it is sufficient to create a vacuum at the periphery of the film and that it is unnecessary to do so around the perimeter of the opening, in particular when the film flanks the accessory. This allows the leakage path to be reduced. This path may be further reduced by modifying the geometry of the film so that the opening is in a recess (concave relief) of the film (i.e. so that the film is in staircase form and that the last step of the staircase - that terminating in the opening - is slightly descending, so as to naturally seal by bearing on the accessory).

The flexible film may be welded in a variety of ways. All types of welding that are compatible with the plastics to be assembled are suitable. Preferably, the nature of the welding is of the infrared or laser radiation welding type, the latter being most particularly preferred. In this case, the flexible film may
advantageously be welded to a peripheral zone larger than with other types of welding. Such a "wide" zone may be obtained by successive, parallel and partially overlapping scans. Thus, it is possible in practice to make 1 to 6 passes each 4 to 5 mm in width so as to produce a weld whose width may vary from 4 to 18 mm.
According to the invention , the vacuum is created by using a vacuum tool on the periphery of the flexible film, this vacuum tool (which will be described in detail later) comprising a suction chamber placed over the entire periphery of the flexible film and at least one suction channel for removing the air from the suction chamber. Thus, the suction chamber is capable of sucking out the air beneath the flexible film so as to create an intimate contact between the flexible film and the wall of the tank and the accessory respectively. The air is removed from the suction chamber via at least one suction channel connected to a suction pump.
Preferably, the vacuum is created by applying a relative vacuum within a range from -800 to -60 mbar. Such a vacuum ensures that there is intimate contact between the flexible film and the support on which it rests.
Advantageously, pressure is exerted on the weld region after and/or before welding, in particular when the latter takes place by laser radiation. Exerting pressure on the weld region, before and/or after welding (but in particular after), makes it possible for the pressure of the flexible film on the tank to be locally increased so as to improve the intimate contact between the flexible film and the wall of the fuel tank.
The pressure may be exerted by means of a pressure device, such as a rotary wheel or a sliding pad. In the case of a thick film, a sliding pad is generally preferred, the sliding principle of which replaces the rotary system of a wheel, which is too fragile under high pressure. The term "sliding pad" is understood to mean a kind of shoe provided with an opening through which the laser beam can effect the welding (for example by a principle similar to that of a sewing machine). This tool has the advantage of applying continuous uniform pressure all around the weld zone while it is being produced. When a rotary wheel is used however, this can apply pressure only at a single point (just before or just after the weld/laser beam).
Locally increasing the pressure of the flexible film on the tank ensures better melting of the materials, especially in the case of a fuel tank with an irregular surface. It also ensures that the flexible film conforms better to the geometry of the tank and/or of the accessory, especially in the case of a fuel tank with concave and/or convex shapes over at least part of the welding path.
Preferably, the pressure exerted lies within a range from 6 to 18 bar.
According to the invention, the vacuum tool (for creating a vacuum between a support and a flexible film placed on the support) comprises a suction chamber comprising an opening directed towards both the flexible film and the support, the opening in the suction chamber being peripheral (i.e. capable of overlapping/flanking the edge of the flexible film over its entire periphery), and at least one suction channel for sucking out the air from the suction chamber. Preferably, the tool comprises several (at least 3, or 6 or even 8) suction channels, preferably distributed uniformly on the periphery of the film.
According to a preferred embodiment, the vacuum tool comprises, at the opening in the suction chamber, a peripheral seal, a first portion of which is intended to bear on the flexible film and a second portion of which is intended to bear on the support, the suction chamber being placed between the first portion and the second portion. When the suction tool is in its working position, that is to say, placed on the flexible film, the first portion of the peripheral seal rests on the flexible film, while the second portion of the peripheral seal rests on the support. The edge of the flexible film is thus positioned between the first and the second portions and is consequently in communication with the suction chamber placed between the two portions. Turning on a suction pump, connected to the suction channel, sucks out the air from the suction chamber and, in turn, from the space between the flexible film and the support. Thus, the air is removed from beneath the flexible film. The vacuum thus created ensures that the flexible film comes into intimate contact with the surface of the support.
Advantageously, the peripheral seal is a lipped seal and is arranged so as to form an enlarged suction chamber close to the opening in the suction chamber. By this is meant either an upside-down "U"-shaped seal moulded directly, or a relatively wide circular seal in which a groove is machined, with the groove width being greater than that of the suction channel (see Figure 1). Thanks to the enlarged chamber, the precision in positioning the vacuum tool is less critical. Furthermore, better suction of the air beneath the flexible film is obtained.
Alternatively (to a single machined lipped seal), the peripheral seal could in fact consist of two concentric circular seals.
The peripheral seal is preferably a seal made of deformable material, for example an elastomer, preferably an expanded elastomer such as a cellular rubber. This is because, owing to the additional thickness of the film in the internal region (on the accessory side), the fact of making the seal deformable avoids having to machine one of the legs of the "U", as in Figure 1 appended to the present application (the leg on the accessory side then being more
compressed than the leg on the tank side).
Preferably, the process according to the invention uses at least one pressure device placed so as to exert pressure on the film, in the weld zone, before and/or after welding (in particular when the latter takes place by laser radiation). This pressure device may, for example, be a rotary wheel or a sliding pad (see above). The pressure device allows pressure to be exerted on that region of the film to be welded, so as in this way to further improve the intimate contact between the flexible film and the support.
The pressure device preferably includes a pressure regulator. With such a pressure regulator, the pressure exerted by the pressure device on the flexible film can be controlled. Preferably the pressure is kept constant over the entire periphery of the flexible film. This pressure is preferably between 6 and 18 bar.

Other particularities and features of the invention will become apparent from the description of an advantageous embodiment presented below, by way of illustration, and with reference to the appended drawings, which show:
- Figure 1 : a schematic sectional view of part of a vacuum tool according to
one embodiment of the invention; and
- Figure 2: a perspective view of a vacuum tool according to one embodiment of the invention.
Figure 1 shows a radial end of a vacuum tool 10 used to improve the contact between a flexible film 12 and a wall of a fuel tank 14. The vacuum tool 10 comprises a suction chamber 16 comprising an opening 18 directed towards both the flexible film 12 and the tank 14. The opening 18 in the suction chamber 16 is peripheral and formed so as to flank the edge 20 of the flexible film 12 over its entire periphery. The vacuum tool 10 further comprises at least one suction channel 22 for removing the air from the suction chamber 16 (Figure 1 illustrating just a section through such a channel).
The opening 18 in the suction chamber 16 is provided with a peripheral seal 24 directed towards both the flexible film and the tank 14. When the vacuum tool 10 is in its working position, as shown in Figure 1, a first portion 26 of the peripheral seal 24 rests on the flexible film 12, while a second portion 28 of the peripheral seal 24 rests on the tank 14. The edge 20 of the flexible film 12 is thus positioned between the first and second portions 26, 28 of the peripheral seal 24. Thus the edge 20 of the flexible film 12 communicates with the suction chamber 16 placed between the two portions 26, 28.
When a suction pump (not shown) connected to the suction channel 22 is turned on, the air in the suction chamber 16 is sucked out, as is, in turn, the space between the flexible film 12 and the tank 14. Thus, the air is removed from beneath the flexible film 12. The vacuum thus created ensures that the flexible film 12 comes into intimate contact with the wall of the tank 14.
Preferably, the flexible film 12 is a three-layer circular multilayer film with a total thickness of 0.3 mm to 0.6 mm, comprising an EVOH barrier layer sandwiched between two HDPE layers, that HDPE layer facing the tank 14 being filled with 0.25% by weight of carbon black, having been welded by scanning diode laser radiation used in pulsed mode (laser of the FAP, YAG type with a wavelength of 809 nm and a power of 35 W).
Figure 2 shows a vacuum tool 30 comprising a support ring 32 to which a vacuum ring 36 is connected by means of several bars 34. The vacuum tool 30 may further include a central gripping element 38 capable of taking hold of the flexible film and depositing it both on the tank and on the component. For this purpose, the gripping component 38 may comprise a plurality of suckers 40 connected to a vacuum means, e.g. a vacuum pump (not shown). According to another embodiment, it is possible to devise a vacuum tool with a vacuum ring 36 that also provides the function of supporting/placing the film in the device. Thus, it is unnecessary to provide the support ring 32 and the bars 34 of the embodiment illustrated.
The vacuum ring 36 includes a peripheral seal 24 having an inner first portion 26 and an outer second portion 28. The first and second portions 26, 28 are spaced radially so as to form a suction chamber 16. The first and second portions 26, 28 may be formed by two separate seal elements with the suction chamber 16 placed between said two elements over a lower part and a vacuum pump placed between the two over an upper part. Alternatively, the first and second portions 26, 28 may form part of a unitary sealing element in the form of an upside-down "U" comprising two branches and a base. The suction chamber 16 is formed between the two branches of the sealing element and a plurality of openings are machined in the base in order to connect the suction chamber 16 to the vacuum pump.
The peripheral seal 24 is designed in such a way that the suction chamber 16 has an inside diameter smaller than the diameter of the flexible film and an outside diameter larger than the diameter of the flexible film. Thus, the peripheral edge 20 of the flexible film is, when the suction tool is in its working position, placed between the two portions 26, 28 and is in communication with the suction chamber 16. Preferably, the peripheral edge 20 of the flexible film is placed in the middle of the suction chamber 16, that is to say equidistant from the two portions 26, 28.
The first portion 26 may have a slightly shorter length than the second portion 28. This difference preferably corresponds to the thickness of the flexible film, thus increasing the effectiveness of the vacuum.
Alternatively (and preferably) the "U" is symmetrical (as already mentioned above), hence greater pressure/better bearing on the edge of the film. Legend of the figures:
10 Vacuum tool
12 Flexible film
14 Wall of a fuel tank
16 Suction chamber
18 Opening
20 Edge
22 Suction channel
24 Peripheral seal
26 First portion of the peripheral seal
28 Second portion of the peripheral seal
30 Vacuum tool
32 Support ring
34 Bar
36 Vacuum ring
38 Gripping element
40 Sucker