Patent Description:
Composite materials are being used ever more extensively in many sectors, due to their excellent characteristics of mechanical strength and chemical resistance, combined with a particularly low specific weight. These characteristics allow the composite materials to be presented as excellent and often advantageous alternatives compared with other conventional structural materials, such as metals for example.

Amongst the most commonly used composite materials, reference can be made to those formed by reinforcement fibres embedded in a polymeric matrix, wherein the final characteristics of the material are determined by the type of fibres and the polymer used. The fibres can be long or short, continuous or discontinuous, and can be of organic or inorganic origin, such as, for example, glass fibres, carbon fibres or aramid fibres. The polymers used can be both thermoplastic and thermosetting, but typically they are thermosetting, such as, for example, epoxide resins.

The processes for production of articles made of composite materials based on fibres embedded in a thermosetting polymeric matrix in general include a phase of preparing a semi-finished product, which has the form of the final desired article, and a subsequent phase of hardening the semi-finished product, which leads to the cross-linking of the thermosetting polymeric matrix.

The step of preparing the semi-finished product is carried out starting from layers of composite material formed by fibres impregnated with a non-cross-linked polymer. These layers are known as "pre-impregnated" or "pre-preg" and are deposited manually on one another, optionally after having been cut out, on a mould unit which has the form of the article desired.

In this step of depositing the pre-impregnated layers, elements of other materials can advantageously be inserted in the semi-finished product, such as, for example, inserts made of metal or lightening material that are designed to be incorporated in the final article.

Once the semi-finished product has been prepared, it is subjected to temperature and pressure conditions which bring about the cross-linking of the polymer, so as to harden the semi-finished product and make its shape definitive.

In a first known production process, the semi-finished product is put into an autoclave, enclosed inside a bag which is connected to a vacuum pump.

The autoclave is closed, and the pressure and temperature inside it are brought to values suitable for causing the cross-linking of the polymer. At the same time, the inside of the bag is depressurised, so as to aspirate air and any gases which have developed in the cross-linking process.

This process makes it possible to obtain articles of optimum quality, which in particular have optimum characteristics of mechanical strength, and are free from surface defects.

However, this process requires very lengthy dwell times in the autoclave, typically of approximately a few hours, thus substantially increasing the production times and costs. This limits the use of this process to the production of articles which are reserved for relatively costly and niche markets.

In another known production process, the semi-finished product is placed in a heated press, where the pressure is exerted directly by a punch dropped directly onto the semi-finished product.

This second known process makes it possible to reduce the production times significantly, thus making more economically advantageous industrial production possible, which is suitable for more widely used articles.

However, this process has various disadvantages, including the poor surface quality and lower performance in terms of mechanical strength, which in some cases precludes the use thereof in highly technical fields, such as the aeronautical field for example.

In addition, this process is poorly suited to the production of articles made of composite material in which components of different materials are to be inserted, such as, for example, a honeycomb lightening structure, which components have a somewhat limited resistance to pressure.

It should be noted that, within the context of the present description and the appended claims, an element is made of "soft material" when it is easily deformable under the action of the forces and pressures to which it is subjected during the process according to the invention.

For example, if a first element made of soft material positioned above a second element is subjected to a certain pressure provided in the process according to the invention, it is deformed so as to adapt to the shape of the surface of the second element.

Document <CIT> discloses a process according to the preamble of claim <NUM> and an apparatus according to the preamble of claim <NUM> for producing an article made of a composite material.

The problem addressed by the present invention is that of providing a process for producing articles made of composite material functionally designed to at least partly overcome the limits described above with reference to the cited prior art.

Within the context of this problem, an objective of the invention is to provide a process which does not require particularly complicated and costly equipment.

This problem is solved and this objective is achieved by the present invention by means of a process for producing articles made of composite material carried out in accordance with the following claims.

Thus, in a first aspect, the invention is directed to a process according to claim <NUM> for producing an article made of composite material, comprising the steps of:.

The process then provides for closing the mould by means of a second unit of the mould that peripherally abuts the first unit of the mould or the separator element, so as to define a second chamber between the second unit and the separator element.

The first chamber is then depressurised and gas is introduced into the second chamber so as to push the separator element against the semi-finished product at a first predefined pressure.

A second pressure that is different from the first pressure is preferably exerted at least on one limited region of the semi-finished product by means of the second mould unit directly contacting the separator element.

The temperature inside the first chamber is therefore brought to a predefined value, so as to cross-link the polymeric matrix and obtain the article. The separator element is distinct from and independent of said first unit and said second unit;.

According to a second aspect, the present invention is directed to an apparatus according to claim <NUM> for producing an article made of composite material.

When the second unit of the mould is closed on the first mould unit, it comes into contact with the separator element and pushes on the semi-finished product at a pressure different from the first pressure maintained in the second chamber by the pressurisation device.

As a result of these features, the process according to the present invention and the corresponding apparatus make it possible to obtain an article made of composite material that has mechanical characteristics which are similar to, or even better than, those obtained using the process of cross-linking in an autoclave, but in a much shorter time, i.e. approximately <NUM>-<NUM> minutes compared with the approximately <NUM>-<NUM> minutes necessary for the process of cross-linking in an autoclave.

This advantage results in a direct reduction of the production costs, and in a substantial increase in the production capacity.

The provision of a soft separator element to be positioned above the semi-finished product makes it possible to define a first chamber which has a volume which is greatly reduced, and substantially equal to the volume of the semi-finished product. In fact, the action of the pressure exerted in the second chamber makes the separator element adhere to the semi-finished product on the side opposite the first mould unit. In addition, the pressure is exerted by the separator element contacting the entire semi-finished product substantially uniformly.

The second unit of the mould is also preferably shaped such that the second chamber which is formed between the unit and the separator element has a reduced volume.

As a result of the particularly reduced volumes of the first and second chambers (and of the masses which define them) compared with the volume (and the mass) of an autoclave, it is been found that the process according to the invention permits faster and more efficient control both of the development of the pressure and of the development of the temperature.

In addition, as a result of the feature whereby, in some limited regions, the second mould unit pushes on the separator unit and thus on the semi-finished product beneath, so as to impart a second pressure thereon which is different from the first pressure exerted on the separator element by the gas introduced into the second chamber, the semi-finished product can advantageously be subjected to different pressures at different regions thereof. By this means, the process becomes more flexible, and can be better adapted to the different functional and structural requirements of the article being produced.

In at least one of said aspects, the present invention can also have at least one of the preferred features described hereinafter.

The semi-finished product is preferably formed by superimposed layers of composite material based on pre-impregnated fibres in a polymeric matrix that is not cross-linked.

The fibres are preferably based on carbon or glass, and the matrix is based on epoxide resin.

The semi-finished product preferably comprises layers of pre-impregnated material based on both glass fibre and carbon fibre.

The semi-finished product preferably comprises a lightening element formed by a honeycomb structure, which element is inserted between the layers of pre-impregnated composite material.

By this means, it is possible to lighten the final article and reduce the production costs thereof.

The semi-finished product preferably comprises at least one reinforcement element, formed for example by a metallic component, which is inserted between the layers of pre-impregnated composite material.

By this means, it is possible to reinforce specific portions of the article and prepare the article in order to be optionally coupled to other articles by means of screws.

According to a preferred embodiment, the article made of composite material is a component of a seat for aeronautical use or automobile use, such as, for example, the body of a backrest or of a seat.

The separator element is preferably made of soft material, more preferably of silicon-based elastomer material which has such dimensions as to cover the entire semi-finished product and peripherally project therefrom so as to abut the first unit of the mould.

The separator element preferably has a thickness of between approximately <NUM> and approximately <NUM>.

The separator element can be made of other materials, provided that they are sufficiently flexible and resistant to the temperatures used in the process, and compatible with the polymeric material of which the semi-finished product is made.

The separator element is distinct from and independent of the first and the second mould units.

The surface of the separator element that is designed to contact the semi-finished product is preferably shaped so as to match the surface of the semi-finished product.

The first mould unit preferably also has a mould surface against which the semi-finished product abuts and which is shaped so as to match the form of the semi-finished product at its surface of contact with the first mould unit.

A plurality of channels are preferably made on the surface of the first mould unit, which channels permit efficient discharge of air and gas from the side of the semi-finished product abutting the first mould unit when the first chamber is depressurised.

These channels are preferably provided on the surface of the first mould unit, and have a very shallow depth, of approximately a few tenths of a millimetre. These channels preferably extend as far as the edge of the first mould unit where they are connected to a vacuum pump.

A layer of fabric is preferably arranged between the separator element and the semi-finished product, which layer, by acting as an aerator, assists the discharge of air and gas from the side of the semi-finished product in contact with the separator element when the first chamber is depressurised.

The first chamber is preferably brought to an absolute pressure lower than approximately <NUM> bar (or, in other words, to a negative pressure of at least <NUM> bar), preferably between approximately <NUM> and <NUM> bar absolute.

The apparatus preferably comprises a heating device, which is designed to bring the temperature inside the first chamber to a predefined value.

More preferably, the heating device comprises a plurality of ducts arranged in the first mould unit below the surface which is in contact with the semi-finished product, through which ducts a fluid is made to flow at a controlled temperature.

The second mould unit is preferably connected to a line for introducing gas, for example compressed air, which line is designed to introduce gas inside the second chamber at a first predefined pressure.

The second chamber is preferably brought to a first pressure of between approximately <NUM> and <NUM> bar, preferably between approximately <NUM> and <NUM> bar. The second chamber is preferably closed.

The second pressure is preferably between <NUM> and <NUM> bar, preferably between <NUM> and <NUM> bar.

This possibility is particularly advantageous, firstly when the semi-finished product has regions which cannot be subjected to excessively high pressures, for example portions of a semi-finished product comprising an insert made of lightening material, such as a honeycomb structure, and secondly when the semi-finished product has highly curved regions, such as grooves, corner regions or ribs, which require relatively high pressures in order to obtain an optimum surface finish quality.

The features and advantages of the invention will become more apparent from the detailed description of a preferred embodiment thereof, illustrated by way of non-limiting example and with reference to the appended drawings, in which:.

In the <FIG> indicates as a whole an apparatus for producing an article made of composite material that operates in accordance with the process of the present invention.

The article made of composite material can be any type of object, but in the preferred example described in detail here it is a component of a seat, in particular a backrest. The seat is designed to be used in the aeronautical sector.

The article is obtained from a semi-finished product <NUM>, composed of a plurality of layers based on pre-impregnated fibres in a polymeric matrix having an epoxide base which is not cross-linked.

In particular, some pre-impregnated layers are layers based on glass fibres, and others are pre-impregnated layers based on carbon fibres, arranged on one another in an appropriate order.

The semi-finished product <NUM> also comprises a lightening element <NUM>, formed by a honeycomb structure, which is inserted between the layers of pre-impregnated composite material, as well as a pair of reinforcement elements, formed by respective metal bars <NUM>, which are inserted between the layers of pre-impregnated composite material, and which receive some screws for securing the seat. In particular, the lightening element <NUM> is placed at a median region <NUM> of the semi-finished product, which region is almost flat or has reduced curvature, whereas the metal bars <NUM> are arranged at the sides of the semi-finished product <NUM>.

Regions <NUM> which are highly curved (i.e. having a reduced radius of curvature, in particular less than <NUM>, preferably less than <NUM>), which regions correspond for example to the seats for positioning the metal bars <NUM>, are also defined on the semi-finished product <NUM>.

The apparatus <NUM> comprises a mould <NUM>, made of metal material, in turn comprising a first mould unit <NUM>, known as the "matrix", and a second mould unit <NUM>, known as the "punch", which can be moved by means of an oleodynamic movement system (which is in itself conventional, and is therefore not shown in the appended figures) from and towards the first mould unit <NUM>, so as to close the semi-finished product <NUM> inside the mould <NUM>.

The first mould unit <NUM> receives the semi-finished product <NUM> on a mould surface <NUM> thereof which faces upwards and is shaped in such a way as to match the form of the surface of the semi-finished product with which it comes into contact.

The mould surface <NUM> is surrounded by an edge <NUM>, and a plurality of channels 13a pass through said surface, which channels have a depth of a few tenths of a millimetre, for example <NUM>, extending as far as the edge <NUM>. The first mould unit <NUM> is also connected to a depressurisation device <NUM>, comprising for example a vacuum pump and communicating with the channels 13a provided on the mould surface <NUM>.

The apparatus <NUM> also comprises a separator element <NUM> which is designed to be positioned above the semi-finished product <NUM> once the semi-finished product has been deposited on the mould surface <NUM>.

The separator element <NUM> is made of a silicon-based elastomeric material, and has dimensions so as to cover the entire semi-finished product <NUM> and peripherally project therefrom, so as to abut the first mould unit <NUM> at the edge <NUM> thereof.

A groove <NUM> is preferably made in the edge <NUM>, which groove extends so as to surround the mould surface <NUM> and in which a corresponding edge <NUM> of the separator element <NUM> is received.

The separator element <NUM> has an average thickness of approximately <NUM>, whereas at its edge <NUM> it has a thickness which is increased by approximately <NUM>.

The separator element <NUM> is also shaped so as to match the shape of the semi-finished product <NUM> at the respective contact surfaces.

A layer of fabric <NUM> is preferably arranged between the separator element <NUM> and the semi-finished product <NUM>, which layer acts as an aerator element in order to assist the passage of air or gas between the surfaces of the separator element <NUM> and the semi-finished product <NUM>.

The separator element <NUM> is separate and distinct from the first and the second units <NUM> and <NUM>, and is nevertheless designed to be arranged therebetween.

In fact, when the second mould unit <NUM> is closed on the first mould unit <NUM>, it abuts the separator element <NUM> at least at the edge <NUM> thereof.

By this means, when the second mould unit <NUM> is closed on the first mould unit <NUM>, the space between the two mould units <NUM> and <NUM> is subdivided by the separator element <NUM> into a first closed chamber <NUM>, which is defined between the separator element and the first mould unit <NUM>, and can contain the semi-finished product <NUM>, and into a second closed chamber <NUM>, which is defined between the separator element <NUM> and the second mould unit <NUM>. When the second mould unit <NUM> is closed on the first mould unit <NUM>, the separator element <NUM> is in general slightly spaced, for example by a distance of between <NUM> and <NUM>, from the second mould unit <NUM>, with the exception of its edge <NUM> and, as explained in greater detail hereinafter, a first region <NUM> thereof, in which the second mould unit <NUM> is directly in contact with the separator element <NUM>.

On the side opposite the second mould unit <NUM>, the edge <NUM> of the separator element <NUM> is in contact with the first mould unit <NUM>, whereas the first region <NUM> is in contact with the highly curved regions <NUM> of the semi-finished product <NUM>.

In order to ensure the hermetic sealing of the mould <NUM> when the second mould unit <NUM> is closed on the first mould unit <NUM>, a seal 12a is fitted on the perimeter edge of the second mould unit <NUM>.

The second mould unit <NUM> is connected to a pressurisation device <NUM>, which is designed to introduce a gas, typically air, inside the second chamber <NUM>. As a result of the pressurisation device <NUM>, the pressure inside the second chamber <NUM> can be brought to predefined values at a controlled speed.

In addition, the apparatus <NUM> comprises a device for heating the mould <NUM>, which device is associated both with the first mould unit <NUM> and the second mould unit <NUM>.

In particular, the heating device comprises a plurality of ducts <NUM> which pass through the first mould unit <NUM> below the mould surface <NUM>, and along which a liquid is made to pass at a predefined and controlled temperature, for example water. In addition, the heating device comprises an electrical resistor which is associated with the second mould unit <NUM>, in order to regulate the temperature thereof.

As a result of the heating device, the temperature inside the first chamber <NUM> can be brought to predefined values at a controlled speed, both in the heating phase and in the cooling phase.

The apparatus <NUM> also comprises a control unit <NUM> which is designed to control its various components according to the phases of the production process specified hereinafter.

First of all, the semi-finished product <NUM> is prepared by means of depositing and overlapping layers of fibres, which may also be of a different type such as carbon fibres and glass fibres, and which are pre-impregnated in epoxide resin which is not cross-linked.

The preparation of the semi-finished product includes inserting both the metal bars <NUM> and the lightening element <NUM> at a suitable position.

The semi-finished product <NUM> is then made to abut the mould surface <NUM> of the first mould unit <NUM> (<FIG>).

The layer of fabric <NUM> and thus the separator element <NUM> is positioned above the semi-finished product <NUM>, so as to position the edge <NUM> thereof in the groove <NUM> provided on the edge <NUM> of the first mould unit <NUM> (<FIG>).

The mould is closed by lowering and clamping the second mould unit <NUM> on the first mould unit <NUM> (<FIG>).

By this means, the first chamber <NUM> and the second chamber <NUM> are defined. The substantially hermetic closure of the two chambers is obtained by the pressure of the second mould unit <NUM> on the seal 12a and on the edge <NUM> of the separator element <NUM>.

In addition, the second mould unit <NUM> also pushes directly on the highly curved region <NUM> of the semi-finished product <NUM>, by means of the first region <NUM> of the separator element <NUM>.

At this point, the depressurisation device <NUM> is actuated, such that any air and gas present in the first chamber <NUM> is discharged through the channels provided in the mould surface <NUM> and the layer of fabric <NUM> (<FIG>). In particular, the first chamber <NUM> is brought to an absolute pressure value of approximately <NUM> bar, i.e. to a depressurisation value of approximately <NUM> bar.

Subsequently, the pressurisation device <NUM> is actuated, which device introduces air into the second chamber <NUM>, at a controlled speed and until the inside of the second chamber <NUM> is brought to a first pressure at a value of approximately <NUM> bar (<FIG>).

The separator element <NUM> is therefore pushed by the first pressure against the surface of the semi-finished product <NUM> and, since it is made of soft material, is deformed so as to adhere to the semi-finished product <NUM> and transmit the first pressure uniformly thereto.

The first pressure is such that it does not damage the lightening element <NUM>. On the other hand, the regions <NUM> of the semi-finished product <NUM> are subjected to a second pressure exerted by the direct contact of the second mould unit <NUM> clamped on the first region <NUM> of the separator element <NUM>. The second pressure is substantially independent from the first pressure, and is advantageously greater than the first pressure, at values of approximately <NUM>-<NUM> bar.

By this means, the highly curved regions <NUM> of the semi-finished product <NUM> can be subjected to a pressure which is sufficiently high to obtain optimum results of surface finish and resistance to delamination.

The mould <NUM> is then brought to the desired temperature in order to bring about the cross-linking of the epoxide polymeric matrix by means of actuating the heating device which is associated both with the first mould unit <NUM> and the second mould unit <NUM>.

During this phase, the depressurisation device <NUM> is kept operating, so as to aspirate any gases developed by the cross-linking reaction, or any air which has infiltrated into the first chamber <NUM>.

The mould <NUM> is maintained in the pressure and temperature conditions for the time necessary to complete the cross-linking of the polymeric matrix, after which the mould is cooled, for example by making cold water flow through the ducts <NUM> and switching off the electric heater associated with the second mould unit <NUM>, and the mould is then opened in order to permit extraction of the hardened semi-finished product.

The process lasts for approximately <NUM>-<NUM> minutes, and makes it possible to obtain an article with optimum mechanical and surface finish characteristics, which are altogether comparable to (if not better than) those obtained by means of a process of cross-linking in an autoclave.

Claim 1:
Process for producing an article made of a composite material, comprising:
- providing a semi-finished product (<NUM>) comprising at least one layer of fibres embedded in a polymeric matrix which is not cross-linked;
- positioning said semi-finished product in a first unit (<NUM>) of a mould (<NUM>);
- positioning a separator element (<NUM>) above said semi-finished product (<NUM>) such that said separator element peripherally abuts said first unit (<NUM>) of said mould so as to define a first chamber (<NUM>) between said separator element (<NUM>) and said first unit (<NUM>), in which chamber said semi-finished product is contained,
- closing said mould (<NUM>) by means of a second unit (<NUM>) of said mould that peripherally abuts said first unit (<NUM>) of said mould or said separator element so as to define a second chamber (<NUM>) between said second unit (<NUM>) and said separator element (<NUM>),
- depressurising said first chamber (<NUM>),
- introducing gas into said second chamber (<NUM>) so as to push said separator element (<NUM>) against said semi-finished product (<NUM>) at a first predefined pressure,
- bringing the temperature inside said first chamber (<NUM>) to a predefined value so as to cross-link said polymeric matrix and obtain said article,
wherein a second pressure that is different from said first pressure is exerted on at least one limited region (<NUM>) of said semi-finished product by means of said second unit (<NUM>) of said mould directly contacting said separator element (<NUM>) so as to subject different regions of said semi-finished product to differentiated pressures,
characterised in that said separator element (<NUM>) is distinct from and independent of said first unit (<NUM>) and said second unit (<NUM>).