Patent Description:
In order to obtain hollow components made of composite materials, a technique referred to as filament winding is known, according to which filaments of fibers impregnated with a resin are continuously wound around a winding mandrel which generally has a shape and size corresponding to those of the internal surface of the hollow component. The winding mandrel is generally made of a metal material, for example aluminum or steel.

Once the impregnated fibers have been wound around the winding mandrel, the assembly consisting of the winding mandrel and the fibers wound on the latter is heat treated to cure the resin and consolidate the shape of the hollow component. It is then necessary to remove the winding mandrel from the hollow component. In the case of hollow components having relatively complex shapes, producing winding mandrels which are formed by assembling several parts mechanically coupled to one another is known, so that it is possible to disassemble the winding mandrel to remove the winding mandrel from the hollow component. This known technique has the advantage of reusing the winding mandrel so as to use the same winding mandrel to sequentially produce a plurality of hollow components. However, due to defects or wear of the mechanical coupling systems, in some cases it is complex and difficult or even impossible to disassemble the winding mandrel. Moreover, in some cases, the winding mandrel disassembly and removal operations can impart excessive mechanical stresses to the hollow component. This stresses can damage the hollow component which, therefore, needs to be discarded after production. Moreover, in the production methods of the prior art, the prototyping costs, as well as the costs due to possible shape variations during the step of designing the hollow component, are relatively high. <CIT> discloses a method of fabricating a thermoplastic composite tubular structure that provides a mandrel of a soluble, expandable material.

It is a general object of the present invention to provide a method for producing a hollow component which is such as to solve or reduce as much as possible the drawbacks which affect the production methods of the prior art.

The aforesaid object, as well as other objects which will become apparent below, are achieved by a production method for producing a hollow component made of a composite material as defined in appended claim <NUM>. Preferred and advantageous embodiments of the aforesaid production method are defined in the dependent claims. The present invention also relates to a winding mandrel as generally defined in appended claim <NUM>.

The invention will be better understood from the following detailed description of particular embodiments thereof, given by way of example and not by way of limitation, with reference to the accompanying drawings briefly described in the following paragraphs.

<FIG> shows an exemplary and non-limiting embodiment of a method <NUM> for producing a hollow component <NUM> made of a composite material. In the present description, the aforesaid method <NUM> will also be referred to as production method <NUM> more briefly.

The hollow component <NUM> is, for example, but not exclusively, a component of a missile such as a fuselage, a tank, a booster of a missile, for example. According to an embodiment, the hollow component <NUM> is a canister of a missile. According to a further embodiment, the aforesaid hollow component is a component of an avionic or aerospace airframe or an automotive component. The aforesaid hollow component <NUM> can also be a portion of a more complex component which includes one or more further components to be assembled together to form the hollow component.

The hollow component <NUM> comprises a body, at least part of which is made of a composite material. The composite material is, or comprises, for example, carbon fibers wound and impregnated, or pre-impregnated, with a polymer matrix, for example a resin matrix, preferably an epoxy resin. The carbon fibers are conveniently continuous and wound carbon fibers arranged so as to form a fabric of carbon fibers.

The production method <NUM> comprises a step S1 of producing or providing a winding mandrel <NUM>, <NUM>, <NUM>, <NUM> having at least one water-soluble portion <NUM>, <NUM>. According to a particularly advantageous embodiment, the water-soluble portion <NUM> , <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is made of a water-soluble thermoplastic material. Said water-soluble material consists, for example, of an acrylic resin, for example, consisting of homopolymers, copolymers or methacrylic-styrene-butyl acrylate acid terpolymer. Preferably, said acrylic resin is additivated with a percentage by weight between <NUM>% and <NUM>% of titanium dioxide or triphenyl phosphate.

As shown in <FIG>, the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> has a mandrel axis Z. Such a mandrel axis Z is an axis about which the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is configured to be rotated during winding and/or an axis of a main longitudinal extension of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM>.

The at least one water-soluble portion <NUM>, <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> comprises one or more fluid passages. The water-soluble portion <NUM> has a body made as a three-dimensional mesh or grid in which a fluid, e.g., a liquid, can flow, in particular to dissolve the water-soluble portion <NUM>, <NUM>. According to a preferred embodiment, said one or more fluid passages extend between two end portions, opposite to each other, of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> which are axially spaced apart along the axis Z of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM>.

Step S1 of producing or making available the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> advantageously comprises an operation of producing the aforesaid water-soluble portion <NUM> by means of three-dimensional printing, or 3D printing. Thereby, it is possible to give the water-soluble portion <NUM>, <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> any desired, albeit complex, shape. According to a particularly advantageous embodiment, the aforesaid 3D printing is an FDM (Fused Deposition Modeling) printing.

According to a possible embodiment of the method <NUM>, during the step S1 of realizing the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> it is possible to produce or provide a winding mandrel <NUM>, <NUM>, <NUM>, <NUM> which is entirely water soluble. In alternative and advantageous embodiments, however, it is possible to provide that during step S1 of producing the winding mandrel <NUM>, <NUM>, <NUM>, <NUM>, only one or more portions <NUM>, <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> are produced from a water-soluble material while one or more remaining portions <NUM>, <NUM> are produced from a water-insoluble material. Such portions of water-insoluble material <NUM>, <NUM> are made, for example, of homopolymers, copolymers or terpolymers, such as Acrylonitrile-Butadiene-Styrene (ABS), Polyetherketoneketone (PEKK), Acrylonitrile-Styrene-Acrylate (ASA), Nylon, polyetherimide, polypropylene, carbon fiber reinforced polyamides, polycarbonate, polylactic acid, polyphenylsulfone, or metal or metal alloys, for example. The water-insoluble portions <NUM>, <NUM> can be produced, for example, through additive techniques, such as 3D printing of polymers, metal or metal alloys, or by means of traditional production processes, such as casting or milling from solid.

In the example in <FIG>, a water-insoluble portion <NUM> is made integral with one or more water-soluble portions <NUM>, <NUM> during the same printing process of the water-insoluble portion <NUM> or by mechanically inserting the water-insoluble portion <NUM> into the water soluble portion <NUM>, <NUM> or vice versa. In the particular example shown in <FIG>, a central water-insoluble portion <NUM> is engaged into at least one water-soluble portion <NUM>, <NUM>, in particular into a plurality of water-soluble portions <NUM> axially spaced apart from one another along the mandrel axis Z. The water-soluble portions <NUM> axially spaced apart from one another can be mutually separated or can be joined by a tubular water-soluble portion <NUM> in which the water-insoluble portion <NUM> is arranged. Preferably, the axially spaced water-soluble portions <NUM> are radially protruding portions, for example radially protruding with respect to the central water-insoluble portion <NUM>. In the particular example shown in <FIG>, the winding mandrel particularly comprises, without introducing any limitation, three water-soluble portions <NUM>. In the shown example, each of the water-soluble portions <NUM> is a plate-like ring.

Moreover, in the particular example shown in <FIG>, one or more water-insoluble portions <NUM>, which are, for example, shells or half-shells, are arranged radially external with respect to the at least one water-soluble portion <NUM>. The external water-insoluble portions <NUM> can be made integral with the water-soluble portions <NUM>, <NUM> during the carbon fiber winding process or they can be previously coupled to one another.

The water-insoluble portions <NUM>, <NUM> are, for example, portions of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> intended to be then removed from the hollow component <NUM> to be conveniently reused in the production method <NUM> to produce further hollow components <NUM>, or they are water-insoluble portions <NUM> which are structural or connection elements which are intended to be integrated into the hollow component <NUM>.

For example, with reference to <FIG>, in the example shown, the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> comprises one or more water-soluble portions <NUM>, for example, three water-soluble crowns <NUM>, and at least one water-insoluble portion <NUM>, <NUM>. The at least one water-insoluble portion <NUM>, <NUM> comprises, for example, a first water-insoluble portion <NUM> adapted and configured to be then removed, for example a central core, or a radially inner core, which is removable from the hollow component <NUM>. Alternatively or additionally, the at least one water-insoluble portion <NUM>, <NUM> comprises, for example, a second portion <NUM>, which is a structural insert, for example, such as an internal reinforcing structural insert, adapted and configured to be integrated into the hollow component <NUM>. In the particular example shown in <FIG>, the second water-insoluble portion <NUM> is a tubular reinforcing insert. Alternatively or in addition, the at least one water-insoluble portion <NUM>, <NUM> comprises, for example, a connection portion, for example a joint or a coupling flange, adapted to mechanically couple other hollow components <NUM>, produced according to the production method <NUM> or in general further components which form part of a complex product which includes the hollow component <NUM>, to the hollow component <NUM>. The water-insoluble connection portion can, for example, allow mechanically coupling the hollow component <NUM> to one or more further components, for example by means of a mechanical coupling of the direct type, for example by interlocking or screwing, or by means of an indirect mechanical coupling, in which, for example, coupling elements are used, such as screws, pins, rivets, etc., for example.

Moreover, an embodiment in which the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> comprises, in addition to at least one water-soluble portion <NUM>, at least a first water-insoluble portion <NUM> and at least a second water-insoluble portion <NUM>, is particularly advantageous. The at least one water-soluble portion <NUM> acts as a fixing element between the first water-insoluble portion <NUM> and the second water-insoluble portion <NUM> making them integral with each other. Once the at least one water-soluble portion <NUM>, <NUM> has dissolved, the first water-insoluble portion <NUM> can be extracted from the hollow component <NUM> since the first water-insoluble portion <NUM>, <NUM> is no longer fixed to the second water-insoluble portion <NUM>. The second water-insoluble portion <NUM> is instead intended to be integrated into the hollow component <NUM>, i.e., to remain in the hollow component <NUM> to form an integral part of the latter.

It is thus apparent that the aforesaid water-soluble portion <NUM>, <NUM> acting as a fixing element, once the fibers have been wound on the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> and consolidated, prevents the extraction of the first water-insoluble portion <NUM> from the hollow component <NUM> since it structurally connects the first water-insoluble portion <NUM> to the second water-insoluble portion <NUM>. Once the water-soluble portion <NUM>, <NUM> has dissolved in water, or in an aqueous solution, the mechanical connection between the first water-insoluble portion <NUM> and the second water-insoluble portion <NUM> fails.

The production method <NUM> further comprises a step S2 of winding filaments or fibers around the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> produced during step S1. Preferably, the winding step S2 comprises a step of impregnating said filaments or fibers into, or with, a polymer matrix, for example into a resin. Said resin is preferably a thermosetting epoxy resin. According to an embodiment, the winding step S2 is carried out continuously, for example by rotating the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> about a rotation axis, or winding axis Z, and by drawing the fibers or filaments from one or more feed rolls by dragging. Preferably, during the winding step S2, the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is rotated about a rotation axis and during the rotation, the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is also simultaneously translated along the rotation axis, alternately in two opposite directions. For example, it is possible to engage a rotating shaft inside the winding mandrel <NUM>, <NUM>, <NUM>, <NUM>, for example in a central hole <NUM>, so as to make the rotating shaft and the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> rotationally integral with each other. The rotating shaft can be rototranslated, and hence the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is thus rototranslated together with the rotating shaft. In the example in <FIG>, the central hole <NUM> is defined in the central water-insoluble portion <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM>.

The production method <NUM> further comprises a heat treatment step S3 of the assembly consisting of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> and the hollow component <NUM> to consolidate the shape of the hollow component <NUM>. Said consolidation occurs due to the consolidation of the polymer matrix.

The heat treatment step S3 preferably comprises a first progressive heating sub-step which is linear or instep from an initial temperature to a final temperature, a second intermediate heat treatment sub-step at a constant temperature, for example equal to, or approximately equal to, <NUM>, and a third natural cooling sub-step, for example for cooling down to room temperature. For example, the first sub-step has a duration equal to, or approximately equal to, <NUM> minutes, the second sub-step has a duration equal to, or approximately equal to, <NUM> minutes, and the third sub-step has a duration equal to, or approximately equal to, <NUM> minutes. It should be apparent that, in any case, the temperatures and durations of the sub-steps of the heat treatment step S3 essentially depend on the resin used as a matrix of the composite material. Moreover, it should be apparent that, during the heat treatment step S3, the temperatures of such a step must always be lower than the glass transition temperature of the material or materials of which the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> is made.

The production method <NUM> further comprises a step S4 of dissolving the at least one water-soluble portion <NUM>, <NUM> of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> by immersing the assembly consisting of the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> and the hollow component <NUM> in water or in an aqueous solution. In order speed up the dissolution of the water-soluble portion <NUM>, <NUM> of the winding mandrel, it is advantageous that, during the dissolution step S4, the water or the aqueous solution has a temperature between <NUM> and <NUM> and/or that a small amount of sodium carbonate, for example a few grams per liter, is added.

If the winding mandrel <NUM>, <NUM>, <NUM>, <NUM> has at least a first water-insoluble portion <NUM> which is not intended to be integrated into the hollow component <NUM>, the production method <NUM> further comprises a step S5 of extracting said first water-insoluble portion <NUM> from the hollow component <NUM>.

From the above, it is apparent that a production method <NUM> for producing a hollow component <NUM> of the type described above allows fully achieving the preset objects in terms of overcoming the drawbacks of the prior art. In fact, with the aforesaid production method <NUM> it is possible to produce a hollow component <NUM>, even having relatively complex shapes, in a reliable, economical and rapid manner.

Claim 1:
A method (<NUM>) for producing a hollow component (<NUM>) made of a composite material, comprising the steps of:
- producing or providing (S1) a winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>) having at least one water-soluble portion (<NUM>, <NUM>);
- winding (S2) filaments or fibers of the composite material around the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>) to form an assembly comprising the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>) and the hollow component wound around the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>);
- subjecting the assembly consisting of the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>) and the hollow component (<NUM>) to a heat treatment (S3) to consolidate the shape of the hollow component (<NUM>);
- dissolving (S4) the at least one water-soluble portion (<NUM>, <NUM>) of the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>), by immersing the assembly consisting of the winding mandrel (<NUM>, <NUM>, <NUM>, <NUM>) and the hollow component (<NUM>) in water or in an aqueous solution;
characterized in that said at least one water-soluble portion (<NUM>, <NUM>) has a body made as a mesh or a three-dimensional grid in which a fluid can flow.