The present invention relates to a method for manufacturing a tank, in particular a fuel- tank for a commercial motor-vehicle, comprising a hollow body of elongate shape, which is arranged horizontal during operation and is manufactured in a mould with a plastic material by means of the rotational moulding technique.

The tanks made with the known methods of the type specified above have in general good characteristics in that they are strong, light and low-cost.

On the other hand, large-size tanks, i.e. those which have a large capacity, for example greater than 200 litres, manufactured using these methods, during use pose numerous problems as regards form stability and dimensions. In order to overcome this drawback, it is possible to consider increasing the wall thickness of these large-capacity tanks. However, this increase in the wall thickness is in any case an unsatisfactory solution since it results in an increase in weight and cost, against a relative improvement of the form stability and dimensions. An object of the present invention is therefore to propose a method for manufacturing a tank for liquids, in particular a fuel-tank for a commercial motor-vehicle, which is able to overcome the aforementioned drawbacks of the methods according to the prior art.

This object, along with other objects, is achieved according to the invention by a method having the characteristics which are defined in the attached Claim 1.

Further characteristic features and advantages of the invention will become clear from the following description provided purely by way of a non-limiting example, with reference to the attached drawings in which:

Figure 1 is a perspective view of a tank manufactured in accordance with the present invention;

Figure 2 is another perspective view of the tank according to Figure 1 , shown in the condition connected to a pair of support brackets;

Figure 3 is a partial, axially sectioned view of the tank shown in Figure 1 ;

Figure 4 is partially exploded, perspective view similar to that shown in Figure 3;

Figure 5 is a partially exploded perspective view which shows part of the stabilizing structure included in a tank manufactured in accordance with the present invention; and

Figure 6 is a partial cross-sectioned view which shows part of a mould used for manufacture of a tank according to the present invention, with a stabilizing structure pre- positioned inside it. In Figures 1 to 4, 1 denotes overall a tank for liquids, manufactured in accordance with the present invention. This tank 1 is for example a tank for the fuel, typically diesel fuel, of a commercial motor- vehicle. The tank 1 comprises a hollow body 2 with an elongate, essentially parallelepiped shape, which is intended to be arranged in a horizontal position during operation.

The body 2 is manufactured with a plastic material, for example polyethylene, by means of the rotational moulding technique, using a mould such as that partially and schematically shown in Figure 6 and indicated there by M. In the embodiment shown (see in particular Figures 1 , 2 and 4), the body 2 has at the top an opening 3 for introducing and mounting an electric level sensor of the type known per se, intended during use to provide electric signals indicating the level of the liquid contained inside this body 2. On the opposite side to the opening 3, the body 2 has a through-hole 4 for mounting a bleeding device or pipe, likewise of the type known per se (Figure 4).

In Figures 1 and 2, 5 denotes a further opening in the body 2 intended to receive a filling nozzle, also of the type known per se and not shown.

In the example of embodiment shown in the drawings (see Figure 2), the tank 1 is intended during use to be fastened to a supporting structure including a pair of essentially L-shaped brackets L by means of retaining belts 6 tightened between the opposite ends of the brackets B (see Figure 2).

Correspondingly, the body 2 of the tank 1 has a pair of essentially transverse grooves 7 inside which the retaining belts 6 engage (see Figures 1 and 2).

With reference in particular to Figures 1, 3 and 4, in the region of the middle transverse surfaces of the grooves 7 the lateral walls of the body 2 have a plurality of recessed formations 8, which are angularly spaced from each other.

With reference to Figures 3 and 4, the body 2 is provided internally with a stabilizing structure denoted overall by 9.

In the embodiment shown the stabilizing structure 9 comprises a longitudinal rod 10 which extends conveniently along the axis A-A of the tank, between the two opposite end walls 2a, 2b of the body 2.

The rod 10 conveniently is made using a material which is essentially heat-stable at the melting temperature of the plastic material from which the body 2 is made. This rod may be for example made of metallic material, such as iron, steel or aluminium or plastic material, such as a polyamide.

The rod 10 may also be conveniently made with a core of rigid, metallic or plastic or other material, pre-coated with a layer of the same plastic material, for example polyethylene, from which the tank is then made.

In the example of embodiment shown the stabilizing structure 9 also comprises a pair of internal transverse bulkheads 1 1 which extend from the rod 10, radially outwards, in respective longitudinally spaced positions.

In this example of embodiment, each bulkhead 1 1 extends between and is gripped by the recessed formations 8 in the body 2 of the tank. Each bulkhead 11 comprises an essentially circular, peripheral, annular formation 12 including a substantially cylindrical portion 12a which is connected to an outwardly flared portion 12b directed towards the centre of the body 2 of the tank 1. Passages 13 are defined between the periphery 12 of the internal bulkheads 11 and the lateral walls of the body 2 and in particular between pairs of adjacent recessed formations 8 (Figures 3 and 4), through which passages the three regions into which the inside of the tank 2 is divided up by the bulkheads 1 1 are able to communicate with each other. In the embodiment shown, each bulkhead 1 1 comprises a central hub 11a, which extends around the rod 10 and has, projecting from it, a shaped transverse wall l ib, the periphery of which is connected to the peripheral annular band 12. This wall 1 lb has a configuration consisting of two series of essentially triangular, angularly spaced segments, said series of segments being axially staggered relative to each other and connected together. This configuration is able to give the bulkheads 11 a significant structural rigidity.

Whatever their configuration, the walls l ib of the bulkheads 1 1 may be provided with through-openings which are designed to allow a braked flow of liquid from one side to the other thereof, in order to dampen the fluctuating movements of the liquid and the associated swashing noise, in particular when the motor vehicle on which the tank is used decelerates or accelerate suddenly. These openings may have, for example, a circular form or any other suitable form.

The bulkheads 11 are also made with a material which is substantially heat-stable at the melting temperature of the plastic material with which the body 2 of tank is made. Therefore, the bulkheads 11 may for example be made with a metallic material or with a high-melting plastic material, such as a polyamide.

The bulkheads 1 1 may be conveniently provided with a surface coating made using the same plastic material from which the tank is then made.

The ends of the rod 10 are conveniently fastened in a liquid-tight manner to the end walls 2a and 2b of the tank body 2, for example in the manner which will now be described with reference in particular to Figures 3 to 5.

In the embodiment shown, the central zone of the end walls 2a and 2b of the body 2 are provided with respective openings 2c and 2d inside which the ends of the rod 10 emerge and are accessible.

Respective sleeves 15, integral with the body 2, are overmoulded during the rotational moulding process around the axial end portions of this rod 10.

The end portions of the rods 10 protrude slightly outwards and respective O-ring seals 16 are mounted around them so as to bear against the ends of the sleeves 5 which are directed outwards. A disk 17, for example made of metallic material, is then applied against the transverse end faces of the rod 10, being fixed to the rod 10 and to the body 2 by means of screws 18. The arrangement is such that each disk 17 compresses the corresponding sealing ring 16.

Each disk 17 may then be conveniently covered and hidden by means of a lid 19, for aesthetic purposes and in order to hide the screws 18, said lid being made for example of plastic material and being snap-engaged or otherwise connected to the corresponding end opening of the body 2.

Obviously, other solutions which are known or fall within the common knowledge of the persons skilled in the art may be used in order to fasten the ends of the rod 10 to the end walls 2a and 2b of the tank body 2.

During use, the structure 9 formed by the rod 10 and the associated transverse bulkheads 11 provides the tank 1 with a notable form and dimensional stability. The tendency of the tank to swell or to expand is thus also effectively offset.

Figure 6 shows in schematic form, part of a mould M which can be used for manufacturing, by means of rotational moulding, the tank described above with reference to Figures 1 to 5. In this figure the stabilization structure 9 pre-positioned inside the mould 6 is shown.

The wall of the cavity inside the mould M has in particular protrusions 20 which project towards the axis X-X of the mould and coincide in the embodiment shown with the axis A- A of the rod 10, so as to form in the body 2 of the tank, the recessed formations 8 described above.

In general, in the pre-positioned condition inside the mould M, the transverse bulkheads 11 of the stabilizing structure may be axially located opposite the protrusions 20 from which they are radially spaced by an amount corresponding to the desired thickness of the tank body 2, plus a predetermined amount in order to take account of shrinkage of the material of the body 2.

Once moulding has been completed, the mould M in a manner known per se is opened and the body 2, together with the stabilizing structure 9 gripped inside it, is extracted. During the following hours, the shrinkage of the plastic material forming the body 2 ensures firm clamping, in a primarily radial direction, of the bulkheads 11 between the recessed formations 8 of the body 2.

Alternatively, as shown in Figure 6, the bulkheads 11 may be mounted on the rod 10 so as to be able to be displaced longitudinally along said rod, for example by means of a threaded-type connection. In this case, as shown in Figure 6, before starting moulding 1 1 , the bulkheads 11 are arranged, relative to the rod 10, in an initial position where their peripheral edges 12 are longitudinally and radially spaced slightly from the protrusions 20 which are intended to form the recessed formations 8 of the tank body 2, so that they do not hinder the flow of the liquid plastic material on the wall of the mould during the course of rotational moulding.

Then, after moulding and before shrinkage of the material (for example polyethylene) which forms the wall of the body 2 has finished, the bulkheads 1 1 are displaced relative to the rod 10 into a final position where they are located opposite the associated recessed formations 8 in the body 2 of the tank, such that, as a result of the subsequent shrinking of the body 2, the periphery of the bulkheads 1 1 is clamped radially by said recessed formations 8.

As already mentioned previously, in another embodiment the at least one transverse bulkhead 11 and optionally also the rod 10 are made with a high-melting material, for example sheet steel or aluminium, and are provided beforehand with a coating made of the same plastic material, for example polyethylene, with which the tank body 2 is made. During manufacture of this body 2 by means of rotational moulding, said coating is softened owing to heating of the mould, without however becoming liquid, since the bulkhead or bulkheads 1 1 and the rod 10 are situated relatively further away from the heat source, and without trickling owing to the continuous rotation of the mould. The softened coating of the bulkhead or bulkheads 11 , and optionally of the rod 10, allows the incorporation of any loose particles of said plastic material inside the mould, which thus are unified with this coating. In this way it is ensured that these particles may not subsequently become detached from the bulkheads 11 and/or the rod 10 and cause dangerous blockages. A variation of the mode of implementation of the method according to the invention described above will now be described.

In order to form the body 2 of a tank using the rotational moulding technique, the mould into which the plastic material has been introduced is moved and heated such that this plastic material melts and may be "spread" over the walls of the mould. The mould is then cooled in order to allow the gradual solidification of the body formed inside it.

According to the variation, after the external layer of the wall of the body 2 has solidified, and before also the internal layer of this wall solidifies, the body is subjected to localized deformations in the region of these recessed formations 8, which are thus clamped against the corresponding peripheral portions of the at least one bulkhead 1 1 , and the innermost layer of the material of this formations 8, still in the plastic state, melts with the plastic coating material of the bulkhead or the bullheads 11 , which is also still in the plastic state, forming a kind of weld.

These localized deformations are formed in a shaping device in which the body 2 is placed after being extracted from the mould, while the internal layer of the wall of said body is still at least partly in the plastic state.

Alternatively, the aforementioned localized deformations of the body 2 are formed in the same mould used for rotational moulding of this body, the mould in this case being provided with moving parts able to perform said deformation.

Obviously, without altering the principle of the invention, the embodiments and the constructional details may be greatly varied with respect to that described and illustrated purely by way of a non-limiting example, without thereby departing from the scope of the invention as defined in the accompanying claims.