IT201600105326A1 - METHOD AND APPARATUS FOR PRODUCING A CLOSED CABLE BODY IN COMPOSITE MATERIAL - Google Patents

Description

DESCRIPTION FOR INVENTION PATENT

Entitled: “Method and apparatus for producing a closed hollow body in composite material”.

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus for producing a hollow body, in particular a closed hollow body made of fiber-reinforced composite material, in particular by means of rapid impregnation obtained by injection or spreading or the so-called "squeeze casting" process in high resin pressure. and use of a removable core made of material capable of resisting this pressure and with a low melting temperature.

STATE OF THE ART

As part of the production of hollow bodies, it is known to use, inside molds, removable cores in sand or granulated conglomerates, wax, other sublimable or low-melting materials, to obtain molded objects with internal cavities.

With particular reference to the automotive and robotics sector, in recent years there has been a notable development in the technology of molding objects through the impregnation of carbon fiber composites due to the need to satisfy an increasingly growing demand for the production of high quantities of light and resistant automotive parts and movable arms trying to develop efficient and repeatable production processes at the same time.

In particular, the so-called HPRTM (High Pressure Resin Transfer Molding) technology has had a strong evolution, based on the impregnation of fiber by high pressure injection of a highly reactive resin in a mold kept closed with a strong pressing action, and also the so-called Gap Injection technology based on the impregnation of fiber by low pressure injection of a highly reactive resin into the cavity of a mold kept sealed but slightly open and subsequently closed with a strong pressing action.

The resin is dosed under high pressure and mixed by high turbulence of the jets with a mixing head applied to the mold cavity that reproduces the shape of the body to be molded, such as a structural piece. In this case, these are non-hollow shapes, having geometric sheet shapes, possibly three-dimensional and not excessively hollowed to avoid wrinkles induced in the fiber by sudden changes in shape.

In the molding cavity, layers of fiber, for example of carbon, woven and / or multidirectional and / or monodirectional and / or in the form of mat are positioned, superimposed and pressed by means of a press with a high pressing load.

During injection, the air present in the cavity and in the fiber fabric is evacuated from the closed mold and sealed by means of special sealing elements while the resin flows between the pressed weft of the fabric until it is completely filled and impregnated, discharging a small excess in the areas joint of the mold. In order for the resin to flow into the fabric weft, however, a pressure head must be established between the resin injection point, which the mixing head faces, and the sliding front of the resin impregnating the fabric, so that it can hydraulic creep resistance be overcome.

The pressure, at the injection point, rises progressively during the diffusion of the resin until it reaches very high values, typically up to 80 -100 bar, sometimes with peaks of up to 130 bar in the final instants of filling the cavity. Correspondingly, with the Gap injection the resin, mixed by turbulence induced by the high pressure, fills only a part of the mold cavity left slightly open (for example with the half-molds mutually spaced in the opening of 0.5 - 2 mm with respect to the final pressing position. Subsequently, the press closes the mold generating a high pressure on the resin and forcing the latter to flow and fill all the spaces inside the carbon fiber. A high final pressure is necessary on the one hand due to the high reactivity of the resin which over the years has increased to such an extent as to allow the extraction of a molded piece after about 100 seconds, compared to 5 - 6 minutes in the past, and on the other hand because with a high final peak of pressure, it is possible to compact the resin itself, obtaining composite pieces with very low percentage values in volume of resin, for example of about 40% - 45% of the total. Their percentages are now comparable with that obtained in an autoclave with pre-impregnated composite (so-called “Prep-preg”) for pieces intended for example for the aeronautical sector.

Therefore, in order to produce composite pieces with high structural characteristics using "HPRTM" or "Gap injection" technology, it is necessary to reach values of at least 80 bar of specific pressure on the surface of the piece in the process: there are no particular problems in this regard if the pieces to be made are substantially two-dimensional type, ie of the foil or plate-like type, therefore not hollow.

However, if the pieces to be produced must be hollow in such a way as to have greater mechanical stiffness and at the same time a very reduced weight and inertia, it is then necessary to print two half-shells which are then mutually welded or glued. Alternatively, it is necessary to make use of a soul. It is known the use of light cores intended to remain in the body permanently, or cores that are not removable, for example in balsa or polyurethane foam, which however can withstand pressures not exceeding about 15-20 bar. Alternatively, rather heavy cores of the removable type can be used, for example made of a rigid porous conglomerate material of the water-soluble type, which disintegrates in water to be evacuated through holes made in the molded hollow body. The use of this second type of removable core is also limited to applications with molding pressures not exceeding about 30 bar. It is also worth noting the possibility of using bladder elements filled with liquid to counteract the pressure of the injected resin; however in this case there are difficulties in extracting the bladder elements. Furthermore, the bladder elements have limits in the geometric precision that they manage to confer on the cavity of hollow objects, in particular they have difficulty in reaching any acute angles of the cavity to be defined and, given the thickness necessary for the resistance of the bladder itself, unfortunately the their extraction requires wide enough openings for their removal.

AIMS OF THE INVENTION

An object of the invention is to improve known systems for molding hollow bodies.

Another purpose is to provide a new method and equipment that allow to obtain, through higher molding pressures, closed hollow bodies made in a single piece, endowed with qualitatively superior mechanical characteristics and equipped with inserts or accessories integrally bound to the reinforced composite material. with fibers.

Another purpose of the invention is to provide a method and an apparatus that allow to obtain closed hollow objects with very short process times, to ensure high productivity, also reducing the energy consumption necessary for the molding process.

BRIEF DESCRIPTION OF THE INVENTION

These objects and further advantages are achieved by means of a method and an apparatus as defined in the claims.

In particular, in a first aspect of the invention a method is provided for producing a closed hollow body as defined in claim 1.

In a second aspect of the invention, an apparatus is provided for implementing the method according to claim 1, as defined in claim 12.

Thanks to the method and apparatus according to the invention, it is possible to achieve the intended purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further characteristics of the method and the apparatus will become clearer from the following description, with reference to the drawings, in which:

Figure 1 is a schematic view of an apparatus according to the invention, which shows a first version of the mold and a resin mixing and injection device, with its schematized control system;

Figures 2A and 2B show a second version of the mold included in the apparatus according to the invention;

Figures 3A and 3B are two different exemplary schematic perspective views of a hollow body made of fiber-reinforced composite material, in particular a hollow robotic arm equipped with inserts and made with the system according to the present invention;

Figures 4, 5, 6 are sections taken respectively along the IV-IV, V-V, VI-VI planes in Figure 3B;

Figure 7A shows, schematically and by way of example, a removable core in low melting material, equipped with inserts and a longitudinal distribution channel for the resin, which can be used with the system according to the invention to obtain the hollow body;

Figure 7B shows half of the removable core, cut lengthwise, shown in Figure 7A;

Figure 8A shows, schematically and by way of example, another version of a removable core in low melting material, similar to that of Figure 7A but equipped with transverse distribution channels for the resin;

Figure 8B shows half of the removable core, cut lengthwise, shown in Figure 8A;

Figures 9A and 9B are two perspective views of a mold for forming the removable core in low melting material that can be used with the system according to the invention to obtain the hollow body in fiber-reinforced composite material;

Figure 10 is a diagram relating to the choice of the low melting material, in particular of the melting temperature range, for the removable core according to the characteristics of the resin for molding the closed hollow body.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the attached figures, an apparatus 1 and a method for producing a closed hollow body 2 in fiber-reinforced composite material are described, in particular by impregnating resin with high pressure diffusion and using a removable core 3 made of low-grade material. dark. In particular, the low-melting material includes a eutectic metal alloy that must be chosen appropriately, based on certain mechanical properties and melting temperature, specified below.

The resin chosen for the impregnation can be epoxy, polyurethane, vinyl ester, unsaturated polyester, phenolic or other suitable type.

The closed hollow body 2, obtainable with the apparatus 1 and the method according to the invention, can be for example a structural element, in particular but in a non-limiting manner, an arm for a robot or an element intended for the realization of part of a vehicle, or in general any element that needs to have high mechanical resistance characteristics and at the same time a very low weight.

This closed hollow body 2 may comprise inserts in metal or other suitable material, which can be used for the interconnection between several parts.

The inserts can be: hinges, pulleys, shafts, pins, plates with threaded holes or reference and assembly pins, assembly nuts, plates for fixing to frames, connecting rods, circular crowns, plates equipped with numerous teeth to facilitate the transmission of stresses between composite and insert, or valves and containers in Aluminum, Bronze, Brass, or in plastic materials such as polyoxymethylene (POM) or polyetheretherketone (PEEK) etc.

With reference to schematic figures 9A and 9B, the apparatus 1 comprises a pair of half-molds 4A, 4B which together define a first forming cavity 4 for obtaining the core 3 in low melting material.

The apparatus 1 comprises a second forming cavity 8, of greater volume than the first cavity 4, intended to receive the core 3 - suitably wrapped with a fabric of reinforcing fibers 29 - and to receive the resin for high pressure molding. of the closed hollow body 2.

The aforementioned second forming cavity 8 is defined by a pair of half-molds 10A, 10B, which make up a mold 10, a first version of which is shown in Figure 1. A second version of the mold indicated with 10 '- configured to operate according to the “Gap injection” technology - is shown in Figures 2A, 2B.

In the first molding cavity 4 the low melting material is poured in the molten state to create - by casting molding - the removable core 3 intended to be used to obtain an internal cavity 5 of the final closed hollow body 2. In other words, the geometry of the core 3 is such as to reproduce the geometry of the cavity 5 that you want to define in the closed hollow body 2.

The aforementioned inserts or accessories 7 intended to permanently remain in the closed hollow body 2 are incorporated into the core 3 that is obtained.

For this purpose, the first forming cavity 4 comprises housing areas 33a, 33b (visible in Figures 9A, 9B) shaped for housing and positioning the aforementioned inserts 7a, 7b or accessories intended to form, with the low melting material cast, a single block, that is to say a core 3 incorporating the aforementioned inserts 7. In particular, in the first forming cavity 4, on an internal surface 32 thereof, raised portions or ribs 34 are formed (better visible in figure 9B) suitable for forming on the core 3 suitable resin distribution channels 6 shaped to guide and distribute the resin during the injection phase.

Two different versions of core 3 distribution channels 6 are better shown in Figures 7A to 8B.

The raised portions or ribs 34 extend linearly and have a section having a first base part having a rectangular, or square, or trapezoidal shape, and a second section part having a triangular shape, or a semicircle shape, or a shape bezel. The area of the total section of these ribs 34 can vary from 0.2 to 8 square millimeters.

Advantageously, the distribution channels 6 allow to obtain a more homogeneous and rapid diffusion of the resin that impregnates the layers of fiber 29 that are wrapped around the core 3.

In the aforementioned inserts 7a, 7b, such as for example metal bushes, flanges, pins, eyelets, small cavities and small channels 35 and / or suitable through openings 36 are obtained, in which the molten low-melting metal alloy penetrates during casting. In the step of molding the core 3, these channels or cavities 35 and through openings 36 have the function of receiving the melted eutectic low melting material which, solidifying, leads to a mutual fixing with undercut between the inserts 7a, 7b themselves and the remaining part of the core 3 consists of low melting eutectic material.

In particular, the inserts 7a, 7b are thus fixed to a section 17a, 17b of the core 3, as shown in 7A.

In this way it is possible to rigidly fix the inserts 7a, 7b to the core 3 which subsequently must be rigidly fixed in the cavity 5 of the hollow body 2. In particular, some inserts can be used for the precise positioning of the core 3 inside the hollow body 2 in order to obtain a perfectly centered cavity 5: in this way, once the core 3, wound with the fibers 29, is compressed by the half-molds 10A, 10B, it is possible to obtain thicknesses of the composite body 2 having close dimensional tolerances. Once the resin injection molding process has been completed, during the subsequent casting for the removal of the core 3, the aforementioned fixing cavities 35 are emptied of the molten material by gravity while the through openings 36 act as drainage channels for the evacuation to the outside of the liquid eutectic material deriving from the fusion of the removable core 3.

Once the molten core 3 has been drained from the outside, the desired internal cavity 5 remains defined in the closed hollow body 2, while the inserts 7a, 7b are permanently bound to the final closed hollow body 2 by means of the solidification under pressure of the resin matrix which impregnates the fiber 29 also in contact with the inserts 7a, 7b themselves.

Thanks to this configuration and operating mode, two advantages are achieved: the advantage of rigidly and precisely positioning the core 3 within the mold 10 by means of the inserts even when the composite is crushed by the pressing pressure and the considerable advantage of avoiding having to practice of the evacuation holes on the hollow object 2 obtained, thus guaranteeing much more performing mechanical characteristics of the piece (compared to a body with holes which has points of stress intensification) and also simplifying and speeding up the production process.

As an alternative to the above, it is possible to obtain the aforementioned 6 distribution channels for subsequent processing on the core 3.

Furthermore, thanks to the distribution channels 6 obtained on the core 3, instead of on the surface of the molding cavity of the hollow body 2, the following advantage is obtained: the resin protuberances deriving from the solidification of the resin that fills these channels 6, remain hidden inside the closed hollow body 2, thus providing an advantage from an aesthetic point of view, being invisible from the outside. Furthermore, these solidified protuberances can perform the function of internal reinforcement ribs.

The half-molds 10A, 10B, which define the second forming cavity 8, are suitably equipped with sealing gaskets 16, and are reciprocally movable from a closed molding position to an open position for the extraction of the molded hollow body 2 .

In the version of the mold 10 'of the Gap injection type, in addition to suitable sealing gaskets 16, suitable coupling and sealing portions 37 are also provided (Figure 2A) configured to contain the resin even when the two half-molds 10A' 10B 'are slightly spaced (0.5-2 mm) from each other, before the final stage of resin pressing.

The first molding cavity 4 for the realization of the core 3 is obtained by means of a further pair of half-molds 4A, 4B, distinct from the half-molds 10A, 10B or 10A ', 10B' mentioned above.

In the apparatus 1 there are means 13 for heating and thermostating both for the pair of half-molds 10A, 10B or 10A ', 10B and for the half-molds 4A and 4B. These heating and thermostating means 13 comprise suitable ducts for the circulation of the heating fluid (such as water or diathermic oil), and / or suitable electric candle elements for thermostating both the core 3 and the composite material which forms the closed hollow body 2 The ducts for the circulation of the heating fluid and the electric candle elements are shown in schematic form and indicated with the reference number 38.

The apparatus 1 comprises a high pressure mixing device 18 arranged to inject, in a gap 9 which is defined between the core 3 and the surface of the second forming cavity 8, the resin intended to impregnate the reinforcing fibers 29. In the interspace 9 the fiber 29 is interposed to form the composite shell; this fiber 29 can be already compressed due to the complete closure of the mold or partially compressed during the partial closure, in the case of the mold 10 'of a type suitable for Gap injection. A certain degree of vacuum is created in the mold 10 or 10 'by means of vacuum generating means 14, such as a suction pump and a suitable vacuum valve 15, connected to the forming cavity 8, to avoid the formation of air pockets. .

The vacuum generator means 14 operate to reduce the amount of air in the molding gap 9 to prevent residues of air trapped between the fibers and facilitate the resin injection and diffusion process.

The vacuum generating means 14, in particular, comprise a vacuum tank, suitable valve and suction elements 15, to generate, in the closed configuration of the two half-molds, the required degree of vacuum or depression necessary to avoid residues of air trapped between the fibers.

The channels obtained in the mold also facilitate the evacuation of air. The resin penetrates through the voids defined between adjacent fibers 29 or into the voids which result in the weft of the fabric.

In the half-molds 10A, 10B, or 10A ', 10B' one or more sensors 11 are provided to detect, in the molding gap 9, the pressure of the injected resin.

In addition, one or more presence sensors 12 are provided to detect the advancement front of the resin in the interspace 9.

The presence sensor (s) 12, in particular, comprise inductive sensors. An electronic control unit CU of a programmable type is operationally connected to the sensors 11, 12, the heating and thermostating means 13, the high pressure mixing device 18 and the vacuum generating means 14.

Thanks to the signal of the pressure sensor (s) 11, the electronic control unit CU adjusts the flow rate of the components of the reagent resin which is decreased, maintaining the stoichiometric ratio, with a step law, in particular with two or more steps. , to limit the resin injection pressure to a desired value.

The 12 inductive presence sensor / s, reached by the resin send a corresponding signal to the control unit CU which commands the interruption of the injection.

With reference to Figure 1, the scheme of the resin dosing and injection system is described, obtained by mixing two highly reactive components A, B.

For the sake of simplicity, the power supply and control circuit for only one of the two components will be described in more detail, for example component A, since the power supply and control circuit of component B is completely similar.

The high pressure mixing device 18 includes two injectors 21A, 21B for feeding the respective chemically reactive polymeric components A, B which are taken from respective tanks 19 and 20.

A flow rate QA of component A contained in the storage tank 19 is fed to the injector nozzle 21A via a delivery line 22 comprising a filter 23, a metering pump P1 operatively connected to an electric motor M1, and a flow transducer 24A located downstream of pump P1. The delivery line 22 to the high-pressure mixing head 18 can be further connected to the storage tank 19 by means of a first recirculation line 25 for component A at a pressure lower than that with which the same component must be injected into the head 18 to mixing with the other component B. Respective heat exchangers 28A, 28B are provided along the delivery lines and are arranged to bring the respective components A and B to the suitable temperature before mixing.

A sectioning valve 26 is provided for the recirculation circuit 25, while 27 indicates a second component A recirculation line towards tank 19.

The above applies exactly to the flow rate QB of component B contained in the tank 20, controlled by the flow transducer 24B, fed in turn to a respective nozzle 21B of the mixing device 18 by means of the dosing pump P2 operatively connected to its own electric motor M2. .

The control unit CU calculates and controls, with the aid of the aforementioned sensors 11 and 12, the flow rates QA and QB with the due reaction ratio between the two reagent resins A and B.

The CU control unit acts on the M1, M2 motors of the P1, P2 dosing pumps, to control in real time the flow rates of components A, B.

In particular, the control unit CU is operatively connected to the motors M1 and M2 of the metering pumps P1, P2 and includes inside it a memory, or memories 30A, 30B programmed with software suitable for generating the QRA, QRB signals of a reference flow rate respectively for the two reagent resins A and B to be fed respectively to the nozzle 21A, 21B of the high pressure mixing device 18.

The control unit CU operates by comparing the QRA, QRB signals of reference flow, contained in memories 30A, 30B, with the actual flow signals QA, QB detected on the respective two reagent resins A and B.

In the case of the resin reagent A, the QRA signal of the reference flow rate generated by the memory 30A of the control unit CU, is compared in closed loop, in a first node N1, with a flow signal QA of the component A fed to the nozzle 21A of the mixing head 18; the flow rate signal QA detected by the flow transducer 24A, is operated back to the control unit CU, for example by means of a pulse / frequency converter 31A.

A signal E1 deriving from the comparison in N1 between the reference signal QRA and the flow rate signal QA adduced to the nozzle 21A, is sent to a control circuit of the electric motor M1.

As previously mentioned, what has been said for component A also applies to component B; in Figure 1, QB indicates the flow rate measured by the flow transducer 24B, QRB indicates the reference flow rate signal generated by the memory 30B.

As already described, the electronic control unit CU regulates the flow rate of the resin which is decreased with a step law, in particular with two or more steps.

The operation of the apparatus 1 is now described and therefore the method according to the invention.

The eutectic low melting metal alloy, in accordance with the present method, is carefully chosen in order to satisfy certain conditions which are clarified below with the aid of the diagram of Figure 10. In the diagram of Figure 10 they are highlighted, along the axis of the temperatures: an operative molding field A (RTM, resintransfer Molding), chosen, with a certain safety margin below the melting temperature TmeltCored of the eutectic alloy, and an operative field C of melting-evacuation of the core, and subsequent further resin hardening, both defined with a certain margin above the melting temperature TmeltCor of the eutectic alloy. Therefore, between the fields A and the fields C, which are mutually spaced, a safety window B is interposed which indicates the difference in temperature between the molding phase of the hollow object 2 and the subsequent melting and evacuation phase of the core 3 with simultaneous post-hardening treatment of the resin.

The eutectic alloy, in addition to having a good mechanical resistance to the stresses induced by the closing pressure of the mold and by that of the injected resins, must have a melting temperature TmeltCore with a strict tolerance, i.e. it must have a rather narrow window of temperature values in to melt. Otherwise, with too wide melting temperature windows, there would not be adequate process repeatability but rather unpredictable final results on the product obtained.

Using, for the injection, an epoxy resin having a maximum temperature TpostCureMAX that is bearable without being damaged and to which the composite is subjected to a post-hardening treatment, it is necessary to adopt a eutectic metal alloy that is physically and geometrically stable during the entire phase of injection and distribution of the resin, and that, subsequently, it can be easily melted and drained from the hollow body 2 at a temperature having a Tevac value. Coretal to guarantee the complete and rapid evacuation of the core 3 without exceeding the aforementioned maximum temperature TpostCureMAX unbearable by the resin.

Basically, in accordance with the method of the invention, the following relationship must be satisfied,

TmeltCoreXB <Tevac.Core <TpostCureMAX

where TmeltCore indicates the melting temperature of the eutectic alloy. When core 3 is removed, the alloy is heated to a Tevac temperature. Core-evacuation melting core which, to ensure complete, effective and rapid drainage of the alloy itself, is higher than the melting temperature TmeltCore by an amount XB which can vary between about 1 ° C and the maximum post-curing temperature of the resin, depending on the specific geometry of the body 2 and core 3 and the characteristics of the materials that compose them. Furthermore, as described further on, Tevac.Core is chosen in such a way as to operate at the same time as the casting also a post-hardening of the resin, thus obtaining synergistically both an energy saving and an improvement in the mechanical characteristics of the piece obtained.

For the realization of core 3, proceed as follows.

In the first molding cavity 4, once the aforementioned inserts 7 have been positioned, the eutectic metal alloy is cast in the molten state, chosen, as already mentioned above, according to the resin used.

To avoid problems of bubble formation, shrinkage and gluing, the first forming cavity 4 is sprinkled with a release powder, and then brought to a forming temperature Tform close to the melting point of the eutectic alloy. In particular, the forming temperature (Tform) is slightly higher than the melting temperature of the core 3, to allow its rapid extraction at the end of the forming operation.

Generally speaking, the forming temperature Tform is related to the melting temperature of the eutectic alloy according to the relationship:

Tform = TmeltCore + XC,

wherein XC has a value between about 1 ° C and about 15 ° C.

The mold in which the first forming cavity 4 is obtained has suitable holes with extraction pins and a sealing gasket to avoid excessive burrs or uncontrolled losses of the molten alloy.

The molten metal alloy is then poured or injected into the first forming cavity 4.

As mentioned above, the molten alloy creeps into the through openings 36 obtained in the inserts 7 and solidifying establishes the mechanical connection with undercut between the inserts 7 themselves and the remaining eutectic alloy part of the core 3.

Once formed, the core 3 is removed, cleaned of the detaching substance and any eutectic alloy burrs are removed.

An assembly is thus obtained which can be used to obtain, in a subsequent reactive resin injection molding step, the internal cavity of the closed hollow body to be obtained. This assembly is therefore composed of

- a core 3 molded by casting a low-melting metal alloy, e

- inserts 7a, 7b or accessories solidly constrained to sections 17a, 17b of the core 3 by means of molten alloy beads penetrated, during the previous casting molding, into special channels 35 and through openings 36 in undercut which are provided on the inserts 7a, 7b ) or accessories.

Coupling points are defined on the inserts 7a, 7b or accessories and on the core 3 to allow the assembly to be clamped onto a support of a machine tool to carry out the required removal operations. The stationary placement on the machine tool makes it possible to correctly work by removing material, for example on the inserts 7 or accessories themselves, and / or on the composite material that will be applied to this assembly at the end of the injection molding of the reactive resin impregnating the fibers 29, as described below.

Once the correct position of the inserts and the absence of surface bubbles or under the skin have been verified, on the surface of the core 3 incorporating the inserts 7, a layer or more layers is applied by wrapping or depositing alternate layers of carbon fibers 29 oriented according to certain directions or according to multidirectional configurations.

On the core 3, suitable recesses or protrusions in the shape of strips or shaped pins can be obtained both to facilitate the winding of the layer or strips of fibers 29 and to provide grip and locking areas for them in a correct position.

For this purpose, hollow areas are provided on the internal surfaces of the first forming cavity 4, and preferably also on the areas intended for mutual contact and closing or joining of the two half-molds 4A, 4B, which allow formation on the core 3, during the molding, of the protrusions intended for the gripping of the fibers 29 which are wound on the removable core 3. The aforementioned hollow areas can extend in a wavy or knurled way, or they have a shape of comb prongs and are shaped so that the gripping protrusions to be defined on the core 3 have heights up to a value corresponding to the thickness of the layer of fibers. 29 which are subsequently wrapped on the core 3.

As an alternative to winding the fiber onto the core 3, the carbon fibers 29 are applied first in the forming half-cavity and, once the core 3 has been inserted into the latter, other fibers are applied. of carbon 29 on the core 3 before closing the forming cavity 8.

The core 3, before being closed in the second forming cavity 8, can be previously heated, to prevent its excessively low temperature from altering the correct flow of the injected resin.

The two half-molds 10A, 10B are closed under pressure or, in the case of the half-molds 10A ', 10B' (gap injection), they are approached and kept partially open up to a mutual distance D ("Gap") of 0.5 - 2 mm from the closed position under pressing to start the injection; the control unit CU activates the vacuum generator 14 to reach the appropriate depression inside the interspace 9, which can take values between 400 and 5 absolute millibars.

The CU control unit activates one or more opening-closing valves of the channel through which the vacuum is generated: these valves, on command, close pneumatically or hydraulically the inlet of the channel from which the vacuum is applied as soon as it reaches it the resin face.

The control unit CU controls the heating and thermostating means 13 so that the two half-molds 10A, 10B are preheated to a suitable molding temperature Tmold. In particular, the molding temperature Tmold must be between a minimum molding temperature TmoldMIN and a maximum molding temperature TmoldMAX which must be lower than the melting temperature TmeltCored of the euctectic alloy. In particular, the following relationship is applied:

TmoldMAX = TmeltCore– XA,

in which XA has a value equal to or greater than 1 ° C depending on the geometry and characteristics of the materials of the core 3 and the resin.

Once the injection phase has started, the pressure sensor 11 and the resin presence sensor 12, in communication with the control unit UC, ensure the correct filling operation of the molding gap 9. Once a 10 or 10 'pressure reference value in the mold has been detected, the CU control unit decreases the flow rate in steps. Once filled, the unit detects a pressure peak between 30 and 300 bar and places the mixing device 18 in the recirculation mode of components A, B, thus ending the injection phase.

In the version with "Gap injection" type 10 'mold, the control device measures the total quantity injected and the presence of the resin in suitable positions by means of the presence sensors 12 distributed in the mold, and places the mixing device 18 in the mode of recirculation by subsequently controlling the complete closing under pressure of the two half-molds 10A ', 10B', by means of the press.

After the due hardening time ("curing"), we proceed with the opening of the half-molds 10A, 10B and the extraction of the assembly 2 'obtained, consisting of the core 3 with inserts 7 and the molded superstructure in reinforced composite, precursor of the closed hollow body 2.

At this point, once the assembly 2 'has cooled, it is possible to take advantage of the high strength and mechanical stiffness of the core assembly 3, inserts 7 and body in fiber-reinforced composite (polymerized), to facilitate its positioning on certain machine tools for any mechanical machining, for example by removal, of areas of the inserts 7 or of composite parts.

The clamping of the core 3 on a machine tool support takes place using the attachment points provided on the inserts 7 and possibly also on the core 3.

Advantageously, the inserts 7a, 7b themselves and the core 3 can be equipped with external protrusions or coupling areas that can act as locking and unlocking points that can be grasped precisely by automatic pliers and clamping devices; in this way it is possible to move the pieces automatically with high precision, speed and repeatability without the danger of damaging the surfaces and it is possible to clamp the pieces rigidly to the tables of the machine tools for the related mechanical processing.

Core 3 present in assembly 2 'provides great stiffness and resistance to deformation, clamping and vibrations caused by machining on machine tools. In addition, the metal core 3 favors the dissipation of the heat generated during processing.

Once any mechanical processing has been completed, the assembly of the hollow body 2 ', still containing the core and positioned on a special template support, is introduced into the oven which heats it by air, or induction, to the melting-evacuation temperature. Soul Tevac, discussed above, to initiate the removal of the soul 3.

The molten eutectic alloy flows through the through openings 36 obtained in the inserts 7, and flows freeing the cavity 5 which is defined in the closed hollow body 2 and leaving the inserts 7 which are an integral part of it well bound to the latter. The molten eutectic alloy is collected to be used again to form another core. Any rotation or movement of assembly 2 'facilitates the evacuation of the alloy even from areas or pockets of stagnation.

As can be seen from the above, an important peculiarity of this method is the careful choice of the combination of materials (eutectic alloy and resin) combined with the definition of advantageous process temperatures. In particular, the careful choice of the eutectic material according to the resin used, and the correct choice of the forming temperature Tform of the core 3, the molding temperature Tmold of the resin, and the melting-evacuation temperature Tevac. process speed as well as energy consumption and the quality of the closed hollow body 2 obtained.

By way of example, some process parameter values and some materials suitable for use in the process described can be provided. For example, using an epoxy resin, suitable for HPRTM technology, having a maximum tolerable TpostCureMAX temperature of 155 ° C, it is possible to operate with a TmoldMIN minimum molding temperature of 100 ° C, a maximum molding temperature TmoldMAX of about 120-135 ° C . In this case it is possible to use a eutectic metal alloy in Bismuth-Tin, having a melting temperature TmeltCore of 137 ° C.

The melting and evacuation of core 3 can be carried out at a temperature ranging from

vac. Between the minimum post-curing temperature and the maximum temperature TpostCure-

MAX of post-curing tolerable by the resin equal to 155 ° C: for example the temperature Tevac.Core can be around 145 ° C.

By using instead a reagent resin for HPRTM technology having a maximum tolerable TpostCureMAX of 110 ° C, another eutectic alloy will be chosen, according to the relationships described above, it will be possible to operate with a molding temperature TmoldMINminimum of 80 ° C, a molding temperature TmoldMAXmassima of about 95 ° C, and a Tevac temperature. Melting-evacuation rate of about 100 ° C.

Eutectic alloys can be:

- a tin-zinc alloy,

- a Bismuth-Lead alloy,

- a Bismuth-Lead-Tin alloy,

- a Bismuth-Lead-Tin-Cadmium-Indium alloy,

- a Bismuth-Tin-Indium alloy,

- a MCP 69 alloy containing Bismuth-Lead-Tin-Cadmium

- a MCP 137 alloy containing Bismuth-Tin

or other suitable alloys.

From what has been described, it is evident that the new method and apparatus 1 make it possible to obtain closed hollow bodies 2 made in a single piece with mechanical performance and machinability a on superior machine tools, using very high molding pressures so far never reached in molding systems. hollow bodies in one piece.

The 2 closed hollow objects obtained can have very thin reinforced composite walls with a high fiber density, therefore very light but equally resistant to external torsion, traction and compression stresses.

It also achieves both the aim of obtaining closed hollow objects in very short process times, with high productivity, and the aim of also reducing the energy consumption necessary for the molding process.

Thanks to the choice of specific eutectic alloys, in relation to the resin used, it is possible to obtain very rigid and compact cores 3, with a modulus of elasticity close to that of aluminum.

Another advantage deriving from the use of cores 3 in low-melting metal is given by the possibility of inserting sensors embedded in the core to analyze certain process parameters, useful for guaranteeing the repetitiveness of the mechanical characteristics of the molded pieces, especially for series productions.

The sensors, for example piezoelectric sensors to detect pressure and temperature, or inductive or capacitive ones to detect the passage of the front of the resin or the extent of the polymerization of the latter, can be inserted in the core 3 in order to detect the process parameters such as pressure, temperature, degree of polymerization. Sensors resistant to high temperatures, and having very small dimensions, can be adopted, so as to allow their recovery once the core 3 in which or on which they are positioned has been melted.

Thanks to the fact that eutectic alloys do not have marked softening characteristics before the melting temperature, it is possible to benefit from a constancy of geometry and consistency and mechanical resistance to pressures. The eutectic alloy therefore behaves, even near the melting point, like a solid metal and undergoes the transition from solid to liquid phase without affecting the process or the geometry of the piece to be obtained. Furthermore, the eutectic alloy does not bind with the carbon of the fibers or with the metal inserts (aluminum, steel, bronze, brass) which have decidedly higher melting points such as not to trigger any phenomenon of contact welding.

From what has been described and illustrated above, it is evident that the method and the apparatus 1 according to the invention achieve the predetermined purposes of speeding up (with consequent advantages on the production level) and improving the mechanical and precision characteristics of the closed hollow bodies in reinforced composite .

What has been said and shown in the attached drawings has been provided by way of illustration of the innovative features of the apparatus 1 and other modifications may be made to the apparatus 1, or parts of it, without thereby departing from the claims.

In practice, the materials, insofar as they are compatible with the specific use and with the respective individual elements for which they are intended, can be suitably chosen according to the required requirements and according to the state of the art available.

It is possible to configure and size the apparatus 1, with possible variations and / or additions to what is described above and illustrated in the attached drawings.

Claims (22)

CLAIMS 1. Method for producing a closed hollow body (2) in composite material obtained from fiber-reinforced reactive resin (29), comprising the steps of: - a1) provide, by means of two half-molds (4A, 4B), a first molding cavity (4) for molding a removable core (3) intended to be used to obtain an internal cavity (5) of said closed hollow body (2), and positioning in said first forming cavity (4) one or more inserts (7) or accessories intended to remain incorporated in said final closed hollow body (2), - a2) provide, on the internal surfaces of said first cavity (4), raised portions (34) shaped to obtain, on said core (3), channels (6) for the preferential feeding and distribution of said reactive resin during the 'injection, - a3) prepare, on the internal surfaces of said half-molds (4A, 4B), hollow areas to obtain protrusions on said core (3) for the gripping of the fibers (29) during winding on said removable core (3), - b) pouring or injecting into said first forming cavity (4) a low melting material having a melting temperature (TmeltCore) so as to mold by casting said removable core (3) to which said inserts (7) or accessories remain bound , in which said one or more inserts (7) or accessories have melting temperatures higher than said temperature (TmeltCore) and comprise cavities and channels (35) and through openings (36), into which said low melting material in the molten state penetrates during the and is received so as to establish an undercut fastening between said one or more inserts (7a, 7b) or accessories and a section (17a, 17b) of said core (3), - c) define, through a pair of half-molds (10A, 10B; 10A ', 10B'), a second molding cavity (8), having a larger volume than said first molding cavity (4), - d) positioning in said second forming cavity (8) said core (3) provided with said inserts (7) or accessories and to which a reinforcing fiber material (29) is wound, and using surfaces of the inserts (7) and protuberances of said core (3) for the firm positioning of said core (3), equipped with said inserts (7a, 7b) inside said second forming cavity (8); - e) thermostating said pair of half-molds (10A, 10B; 10A ', 10B') at a temperature (Tmold) between a minimum molding temperature value (TmoldMIN) and a maximum molding temperature value (TmoldMAX) less than said melting temperature (TmeltCore), where TmoldMAX = TmeltCore– XA, wherein XA has a value equal to or greater than 1 ° C as a function of geometric parameters and characteristics of the materials of said core (3) and said hollow body (2) closed, - f) injecting or distributing through a mechanical or high pressure type mixing device (18), in a gap (9) defined between core (3) and surface of said second forming cavity (8), a resin intended to impregnate said material (29) in reinforcing fibers so as to obtain a precursor assembly (2 ') of said closed hollow body (2), incorporating said inserts (7) and still containing said core (3) inside, and pressing the reacting resin and said material (29) into reinforcing fibers by reaching a peak pressure of the reacting resin in the step of filling said second forming cavity (8) or by completing the closure of said pair of half-molds (10A ', 10B') subjected to a clamping pressure, and allow said resin to harden at a hardening temperature (Tcure), - g) subjecting said assembly (2 ') to a core melting-evacuation temperature (Tevac.Core) to melt and evacuate said removable core (3) through said through openings (36), and at the same time carry out a post-hardening of said resin , said melting-evacuation-core temperature (Tevac.Core) being chosen so as to satisfy the following relationship: TmeltCoreXB <Tevac.Core <TpostCureMAX where XB has a minimum value of 1 ° C depending on the geometry and characteristics of said resin and said low-melting material, and TpostCureMAX is the maximum temperature that can be tolerated by said resin without being damaged. Method according to claim 1, wherein said raised portions (34) extend linearly and have a section having a first base part having a rectangular, or square, or trapezoidal shape, and a second section part having a triangular shape , or a semicircle shape, or a bezel shape, the total section having an area from 0.2 to 8 square millimeters, and in which said hollow areas extend in a wavy or knurled way, or have a comb prong shape and are shaped so that said gripping protrusions have heights up to a value corresponding to the thickness of the layer of fibers (29) which are subsequently wound on said core (3). 3. Method according to claim 1 or 2, in which during said injection and / or distribution phase f) it is expected to generate a certain degree of vacuum inside said interspace (9), between 400 and 4 absolute millibars. Method according to one of the preceding claims, in which said injection and / or distribution step f) takes place at a pressure ranging from 1 to 130 absolute bar. Method according to one of the preceding claims, wherein said low melting material comprises a metal alloy selected from a group comprising: - a tin-zinc alloy, - a Bismuth-Lead alloy, - a Bismuth-Lead-Tin alloy, - a Bismuth-Lead-Tin-Cadmium-Indium alloy, - a Bismuth-Tin-Indium alloy, - MCP 69 alloy containing Bismuth-Lead-Tin-Cadmium, - MCP 137 alloy containing Bismuth-Tin, Method according to one of the preceding claims, in which in said step a1) it is provided to configure an internal surface (32) of said first forming cavity (4) with rib portions (34) intended to obtain, during said step b) , on said core (3) distribution channels (6) adapted to guide and distribute the resin in the injection and / or distribution step f). Method according to one of the preceding claims in which the step of forming the core (3) takes place by bringing said first forming cavity (4) to a forming temperature (Tform) slightly higher than the melting temperature of the core (3 ), to allow correct filling by said low melting material and to allow a subsequent faster extraction of the core (3) just formed after cooling, said forming temperature (Tform) satisfying the following relationship: Tform = TmeltCore + XC, where XC has a value between 1 ° C and 15 ° C. Method according to one of the preceding claims, wherein said resin is selected from a group comprising: epoxy resins, polyurethane resins, vinyl ester, unsaturated polyesters, phenolic resins. Method according to one of the preceding claims, in which metered quantities of a first (A) and a second (B) chemically reactive component are fed, with constant reaction ratio, to a high pressure mixing device (18), having injection nozzles (21A, 21B) for the respective individual reagent components (A, B), in which each injection nozzle (21A, 21B) is operably connected to a storage tank (19, 20) of the respective reagent resin (A , B) by means of a corresponding metering pump (P1, P2) controlled by an electronic control unit (CU) of the programmable type, and in which it is provided, through said electronic unit (CU), to control both the pressure inside said forming gap (9) by means of one or more pressure sensors (11), and the advancement of said resin by means of a resin-presence sensor (12). 10. Method according to one of the preceding claims, and further comprising step h) of clamping on a support for machine tool, by means of hooking points provided on said inserts (7) and said core (3), said assembly (2 ') made rigid by the presence of said core (3) inside it, and carry out a processing with removal of material on said assembly (2 '). 11. Assembly adapted to obtain, during the forming phase, the internal cavity of a closed hollow body (2), said assembly comprising: - a core (3) obtained by casting a low-melting metal alloy in a first molding cavity (4), and - inserts (7a, 7b) or accessories having undercut channels (35) and through openings (36), said inserts (7a, 7b) or accessories being solidly constrained to sections (17a, 17b) of said core (3) by means of molten alloy beads penetrated, during said casting molding, into said channels (35) and through openings (36) undercut, - on said inserts (7a, 7b) or accessories and on said core (3) being obtained hooking points suitable to allow the clamping of said assembly on a support of a machine tool to carry out machining by removal once said assembly is molded the reinforced composite material generating said hollow body (2). 12. Apparatus for producing a closed hollow body (2) of fiber-reinforced reactive resin composite material (29), comprising: - a first molding cavity (4) designed to receive a low-melting material for molding a removable core (3) intended to be used to obtain an internal cavity (5) of said closed hollow body (2), - said first forming cavity (4) comprising housing areas (33) for one or more inserts (7a, 7b) or accessories intended to remain incorporated in said final closed hollow body (2), - a pair of half-molds (10A, 10B; 10A ', 10B') which mutually define, in closed configuration, a second forming cavity (8) having a larger volume than said first forming cavity (4) and suitable for rigidly receiving and supporting said core (3) equipped with said inserts (7) and wrapped in a reinforcing fiber material (29) through suitable positioning and abutment surfaces, - heating and thermostating means (13) for said pair of half-molds (10A, 10B; 10A ', 10B'), - a mechanical or high pressure mixing device (18) for injecting, in a gap (9) defined between core (3) and surface of said second forming cavity (8), a reactive resin intended to impregnate said material ( 29) in reinforcing fibers, - pressure sensor means (11) and presence sensor means (12) for detecting the pressure and the advancement of said resin in said cavity (9), - an electronic control unit (CU) of the programmable type operatively connected to said pressure sensor means (11), to said presence sensor means (12), to said heating and thermostating means (13) and to said device (18) mechanical or high pressure mixing to control the temperature of said pair of half-molds (10A, 10B; 10A ', 10B'), to control the injection and distribution of the resin during the molding process of said body closed cable (2) and to command the closing of the vacuum valves when the resin front arrives. 13. Apparatus according to claim 12, further comprising sealing means (16) for sealing the two half-molds (10A, 10B; 10A ', 10B') in the completely closed position. 14. Apparatus according to claim 12 or 13, further comprising sealing coupling means (37) obtained on said half-molds (10A ', 10B'), and configured to seal the injected resin both when said half-molds ( 10A ', 10B') are completely closed to compact the resin with the fiber material (29) and, both when said half-molds (10A ', 10B') are mutually positioned, in a configuration not yet completely closed, at a distance (D ) opening up to 2 mm. 15. Apparatus according to any one of claims 12 to 14, wherein said electronic unit (CU) is configured to step down, according to set pressure thresholds, the injection flow rates of the reagent components (A, B) of said resin, and to stop the injection when a final pressure peak between 30 and 210 bar is reached or when a desired quantity of injected resin is reached. Apparatus according to any one of claims 12 to 15, further comprising vacuum generating means (14) configured to reduce the pressure in said forming gap (9). 17. Apparatus according to any of claims 12 to 16, comprising a closing valve (15) operated pneumatically or hydraulically to close the vacuum application channel upon arrival of the resin front. 18. Apparatus according to any one of claims 12 to 17, wherein rib portions (34) suitable for forming channels (6) on said core (3) for preferential feeding are formed in said first forming cavity (4) and distribution of the reactive resin in said interspace (9) during the injection step, said rib portions (34) presenting a section having a first base part having a rectangular, square or trapezoidal shape, and a second section part having a triangular shape, or a semicircle shape, or a bezel shape, the total section having an area from 0.2 to 8 square millimeters. 19. Apparatus according to one of claims 12 to 18, wherein said first forming cavity (4) is defined by a further pair of half-molds (4a; 4b) distinct from said pair of half-molds (10A, 10B; 10A ', 10B '). 20. Apparatus according to one of claims 12 to 19, wherein hollow areas are obtained on the internal surfaces of said first forming cavity (4) and on areas of mutual contact and closure, suitable for forming on said core (3), by way of the penetration of molten metal alloy, protrusions for the gripping of the fibers (29), said hollow areas extending in a wavy or knurled manner, or having a shape of comb prongs and being shaped so that said gripping protrusions have heights up to to a value corresponding to the thickness of the layer of fibers (29) to be applied on said core (3). 21. Apparatus according to one of claims 12 to 20, in which said half-molds (10A, 10B; 10A ', 10B') are internally provided with suitable seats for the stop positioning of said inserts (7a, 7b) and of said core (3) during resin pressing and injection. 22. Apparatus according to one of claims 12 to 21, wherein said heating and thermostating means (13, 38) comprise ducts for the circulation of a fluid and electric candle elements for thermostating said core (3) and material composite forming said closed hollow body (2).