Patent Description:
Composite structures are typically fabricated by curing a resin in a composite preform formed of multiple plies of reinforcing fibres. In one form, the plies are pre-impregnated with resin, each forming what is referred to as a prepreg ply, with the prepreg plies being stacked in a laminate form to form the composite preform. In another form, a stack of dry plies formed of woven or braided fibres and/or chopped strand mat are infused with resin under vacuum pressure prior to curing the resin.

To cure the resin, the resin impregnated composite preform is typically located within an oven or autoclave that is gradually heated to a suitable temperature for cure of the resin, and held at that temperature for a set period of time to ensure full curing of the resin. The resin curing process for thermosetting resins used in the fabrication of composite structures is an exothermic reaction. Heat energy is released during the curing process in order to form crosslinks. The energy released to form the crosslinks is generally absorbed by the surrounding environment. However, depending on the manner in which the resin is heated, the resin may release energy at a faster rate than the surrounding environment can absorb it. This may result in exothermic overheating of the composite preform, with the temperature of the composite preform exceeding the set cure temperature of the oven during the resin curing process.

<CIT>, in accordance with its Abstract, states a composite structure which is fabricated by staging at least a portion of an uncured, first composite component. The first composite component is assembled with a second composite component, and the staged portion of the first composite component is cocured with the second composite component.

<CIT>, in accordance with its Abstract, states that two pieces of composite material are simultaneously joined together throughout an interface between the two pieces, by induction heating the interface region with an induction coil placed, at least in part, adjacent to the bonding region, and forcing the composite material at the interface together while heating. The approach is particularly useful in joining pieces having "long interfaces" whose longitudinal dimension is substantially larger than its transverse dimension, for example the long interfaces between aircraft wing stiffeners and skins. The induction coil is configured so that, at any one longitudinal location along the interface, the primary current flowing therethrough does not flow in opposite directions in the portions of the coil overlying the interface, and preferably flows in substantially the same direction throughout the interface. Unheated "cold spots" in the interface being bonded, which would not bond properly, are thereby avoided. An electrically nonconductive but thermally conductive material may be placed between the induction coil and the composite material pieces to act as both a heat sink and a pressure-applying tool. Bonding may be enhanced by placing a susceptor made of the same materials as the composite materials being bonded but having a higher electrical conductivity, in the interface between the two composite material pieces prior to induction heating.

Document <CIT>, in accordance with its Abstract, states that, to provide a method of manufacturing a fiber-reinforced resin structure by which a resin passage enabling a large amount of resin to flow so as to further enhance impregnation efficiency, and a manufacturing device using this method, the method of manufacturing the fiber-reinforced resin structure is by vacuum-packing a fibrous base material laminated in a cavity and injecting a fluid resin into the vacuum pack. The outside of a first vacuum-packing sheet for forming the cavity is covered with a second vacuum-packing sheet. Next, the fibrous base material is vacuum-packed in the way that a resin passage molded component with an irregular part is superposed over the fibrous base material toward the sheet, between the sheet and the sheet. Thus a vacuum bag is structured in a double layer fashion. After that, the resin passage is formed inside the cavity by adjusting the pressure of the vacuum bag parts, then the fluid resin is injected into between the sheets and finally the fibrous base material is impregnated with the fluid resin to obtain the fiber-reinforced resin structure.

<FIG> depicts a typical temperature profile during the initial heating to cure temperature phase of the resin curing process, where the oven temperature is ramped up to the set cure temperature (about <NUM> in the example shown) and then held at the cure temperature. The temperature of the composite preform, measured at a set location on the upper surface of the composite preform with a thermocouple, initially lags the air temperature of the oven, but gradually increases toward and then exceeds the oven temperature, as a result of the increased heat energy generated through the exothermic cure of the resin. In the example shown, the local temperature of the upper surface of the composite preform reaches a peak of about <NUM>.

As a result of the potential for this exothermic overheating of the composite preform, the rate of increase of temperature of the oven, and maximum set cure temperature of the oven, needs to be controlled to avoid the curing composite structure from reaching excessive temperatures which may adversely impact the mechanical properties of the cured composite structure. In particular, localised portions of the composite preform may be subjected to excessive exothermic overheating. This may particularly be the case in thicker regions of the composite preform, such as where additional plies are utilised to form what is referred to as "padups". Padups are typically utilised in areas of a composite structure subject to increased local stresses, such as at metal fitting attachment points. Padups generally have a higher volume of resin per unit of preform surface area, given the increased thickness of preform and thus resin. Such thicker portions may thus be more susceptible to excessive exothermic heat build-up during exothermic curing of the resin, due to the increased volume of resin. Localised hotspots may also develop as a result of temperature inconsistencies within the oven.

This problem is typically managed by reducing the oven heating rate during the transition to cure of the resin, to ensure more gradual application of thermal energy during the time at which the resin is most likely to release excessive energy. This may, however, result in the cure profile being so slow that the composite structure is at risk of falling below minimum heating rate requirements for appropriate cure of the resin. A decrease in the heating rate also increases the total time required for cure of the composite structure, thereby reducing production rates.

The present disclosure is made bearing the above problem in mind.

The present disclosure is generally directed to a method and system for fabricating a composite structure. According to the present disclosure, as claimed herein, a heat sink is utilised to conduct heat away from a portion of a composite preform during cure. Such a heat sink is thus located in proximity to a portion of a composite preform that may otherwise be subject to excessive exothermic heating of resin during the curing cycle. In certain examples, the heat sink may be a passive heat sink formed of an endothermic material, such as a thermoplastic patch, which melts to absorb energy, whilst in other examples the heat sink may be an active heat sink, such as a thermoelectric cooling device.

According to one aspect, the present disclosure provides a method of fabricating a composite structure, as defined in appended claim <NUM>. A composite preform having an upper surface and an opposing lower surface is provided. The upper and lower surfaces each define a preform major surface. A heat sink is located in proximity to one of the preform major surfaces so as to extend across only a portion of the composite preform. The heat sink thus does not extend across the entire preform major surface. A resin is cured in the composite preform to form the composite structure. The resin cures exothermically. During curing of the resin, heat is conducted away from the portion of the composite preform into the heat sink.

In certain examples, the heat sink is a passive heat sink formed of an endothermic material. The passive heat sink may comprise a thermoplastic patch. The patch melts during curing of the resin, absorbing heat during melting of the patch.

In alternative examples, the heat sink is an active heat sink. The active heat sink may comprise a thermoelectric cooling device. The thermoelectric cooling device is operated during curing of the resin to conduct heat away from the portion of the composite preform.

The method further comprises locating the lower surface of the composite preform on an upper tool surface of a tool. A first vacuum bagging film is placed over the composite preform and sealed relative to the tool surface to define a sealed first cavity between the first vacuum bagging film and the tool surface. The composite preform is located in the first cavity. At least partial vacuum pressure is applied to the first cavity during curing of the resin. The heat sink is located above the first vacuum bagging film in proximity to the upper surface of the composite preform.

In certain examples, the method further comprises placing a second vacuum bagging film over the first vacuum bagging film and heat sink and sealing the second vacuum bagging film relative to the tool surface to define a second cavity between the first and second vacuum bagging films. At least partial vacuum pressure is applied to the second cavity during curing of the resin. The heat sink is located in the second cavity.

The composite preform has a non-uniform thickness measured between the upper and lower surfaces, in which case the portion of the composite preform has a thickness greater than an average thickness of the composite preform.

In certain examples, the method may further comprise infusing the resin into the composite preform prior to curing of the resin.

In alternative examples, the composite preform may comprise a plurality of plies of prepreg composite material. In such a configuration, the resin is pre-impregnated in the prepreg composite material.

According to a second aspect, the present disclosure provides a system for fabricating a composite structure, as defined in appended claim <NUM>. The system includes a composite preform having an upper surface and an opposing lower surface. The upper and lower surfaces each define a preform major surface. The system further includes an exothermically curing resin, either to be infused into the composite preform, or pre-impregnated in the composite preform (when in the form of a prepreg composite material). A heat sink is located in proximity to one of the preform major surfaces so as to extend across only a portion of the composite preform. The heat sink thus does not extend across the entire preform major surface. A heat source is provided for heating the composite preform to cure the resin.

In certain examples, the heat sink is a passive heat sink formed of an endothermic material. The passive heat sink may comprise a thermoplastic patch. The thermoplastic patch has a melting temperature less than an exotherm peak temperature of the resin.

In alternative examples, the heat sink is an active heat sink. The heat sink may comprise a thermoelectric cooling device.

The system further comprises a tool having an upper tool surface. The lower surface of the composite preform is located on the tool surface. A first vacuum bagging film extends over the composite preform and is sealed relative to the tool surface to define a first cavity between the first vacuum bagging film and the tool surface. The composite preform is located in the first cavity. A vacuum source communicates with the first cavity. The heat sink is located above the first vacuum bagging film in proximity to the upper surface of the composite preform.

In certain examples, the system further comprises a second vacuum bagging film extending over the first vacuum bagging film and the heat sink. The second vacuum bagging film is sealed relative to the tool surface to define a second cavity between the first and second vacuum bagging films. The vacuum source communicates with the second cavity and the heat sink is located in the second cavity.

The composite preform has a non-uniform thickness measured between the upper and lower surfaces. The portion of the composite preform has a thickness greater than an average thickness of the composite preform.

In certain examples, the resin is to be infused into the composite preform.

In alternative examples, the composite preform comprises a plurality of plies of prepreg composite material containing the resin.

The features described above may be implemented independently in various examples of the present disclosure or may be combined in the other examples as will be appreciated by a person skilled in the art, within the scope of the appended claims.

Preferred examples of the present disclosure will now be described, by way of examples only, with reference to the accompanying drawings wherein:.

Methods and systems according to exemplary examples of the present disclosure will now be described in detail. In general, methods of fabricating a composite structure according to the present disclosure include providing a composite preform having opposing upper and lower surfaces that each define a preform major surface. A heat sink is located in proximity to one of the preform major surfaces (that is, the upper or lower surface) so as to extend across only a portion of the preform major surface. Thus the heat sink thus does not extend over the entire preform major surface. The heat sink may be a passive heat sink formed of an endothermic material, such as a thermoplastic patch that melts during the resin curing process, or an active heat sink, such as a thermoelectric cooling device. Resin in the composite preform is cured to form the composite structure, with the resin curing exothermically. During curing of the resin, heat is conducted away from the localised region of the preform major surface into the heat sink. The heat sink is thus applied to specific portions of the composite preform that would otherwise be subject to excessive exothermic heating during curing. The method may be applied to composite structure fabrication processes involving resin infusion of a dry preform, or alternatively may be applied to composite structure fabrication processes involving prepreg composite materials.

Now referring to <FIG> and <FIG> of the accompanying drawings, a system <NUM> for fabricating a composite structure according to a first example will now be described. The system <NUM> has a composite preform <NUM> having an upper surface <NUM> and an opposing lower surface <NUM>. The upper and lower surfaces <NUM>, <NUM> each define a preform major surface. In the first example, the composite preform <NUM> is a dry preform to be infused with an exothermically curing resin provided in a resin supply <NUM>. A heat sink, which in the first example is a passive heat sink formed of an endothermic material, is located in proximity to one of the preform major surfaces, and particularly the upper surface <NUM> in the first example. Endothermic materials are materials that absorb energy, typically by a phase change or chemical reaction, when exposed to heat. In the first example, the passive heat sink is in the form of a thermoplastic patch <NUM>. The thermoplastic patch <NUM> extends across only a portion <NUM> of the upper surface <NUM>. The thermoplastic patch <NUM> thus extends only over a region of the upper surface <NUM> having a smaller area than the total area of the upper surface <NUM> and does not extend across the entire upper surface <NUM>. The system further comprises a heat source, which may be in the form of an oven <NUM> (or autoclave), for heating the composite preform <NUM> to cure the resin, following resin infusion. The oven <NUM> may also be utilized to heat the composite preform <NUM> and resin supply <NUM> for resin infusion, as will be discussed below.

In the configuration of the first example depicted in <FIG> and <FIG>, the system is of a single vacuum bag composite layup configuration. In this configuration, the system <NUM> has a tool <NUM> having an upper tool surface <NUM>, with the lower surface <NUM> of the composite preform <NUM> being located on the tool surface <NUM>. A vacuum bagging film <NUM> extends over the composite preform <NUM> and is sealed relative to the tool surface <NUM> to define a sealed first cavity <NUM> between the vacuum bagging film <NUM> and the tool surface <NUM>. The composite preform <NUM> is located in the first cavity <NUM>. In the first example, the thermoplastic patch <NUM> is located in proximity to the upper surface <NUM> of the composite preform <NUM>, particularly being located directly on top of the vacuum bagging film <NUM>.

The tool <NUM> may be formed of any of various structural materials, including mild steel, stainless steel, invar or a carbon composite material that will maintain its form at elevated temperatures associated with curing, so as to provide a geometrically stable tool surface <NUM> though the resin curing process. The tool surface <NUM> may be substantially flat for the production of composite structures having a substantially flat lower surface, such as wing or fuselage skin panels, or otherwise shaped as desired so as to provide a shaped surface of a nonplanar composite structure.

The composite preform <NUM> may take any form suitable for resin infusion and as dictated by the geometric and structural requirements of the laminated composite structure to be fabricated. The composite preform <NUM> may comprise a layup of multiple plies of reinforcing material, each formed of woven or braided fibres and/or chopped strand mat. The preform plies may be formed of any of various reinforcing fibres, such as carbon, graphite, glass, aromatic polyamide or any other suitable material for fabricating a resin reinforced laminated composite structure. The plies may form a dry preform, without any resin, or alternatively the preform may have some pre-existing resin content prior to the resin infusion process. The composite preform <NUM> is located on the tool surface <NUM> with the lower surface <NUM> of the preform <NUM> oriented on the tool surface <NUM> such that the lower surface of the resulting cured composite structure will match the form of the tool surface <NUM>. The composite preform <NUM> located on the tool surface <NUM> has a laterally extending downstream edge <NUM>, an opposing laterally extending upstream edge <NUM> and opposing longitudinally extending side edges <NUM>. In the context of the present specification, upstream and downstream sides of the composite preform <NUM> are identified with reference to the direction of flow of resin, as will be further described. The preform <NUM> may take any desired shape corresponding to the shape of the laminated composite structure to be formed.

The composite preform may have a uniform thickness or, alternatively as depicted in the example of <FIG> and <FIG>, the composite preform <NUM> may have a non-uniform thickness as measured between the upper and lower surfaces <NUM>, <NUM>. Particularly, in the arrangement depicted, the portion <NUM> of the composite preform <NUM> has a thickness greater than an average thickness of the composite preform <NUM>. In the first example, the thicker portion <NUM> of the composite preform <NUM> is in the form of a padup, having an increased thickness as a result of being provided with additional plies of reinforcing material to provide local structural reinforcement. Padups, and other portions of composite preforms having a locally increased thickness, generally have a higher volume of resin per unit of preform surface area once resin infused, given the increased thickness of preform and thus resin. Such thicker portions may thus be more susceptible to excessive exothermic heat build-up during exothermic curing of the resin, due to the increased volume of resin. Other portions of the composite preform <NUM> that do not have an increased thickness may also be subject to exothermic heat build up, such as resulting from uneven temperatures within the oven <NUM>, and thus suitable for application of the thermoplastic patch <NUM>.

The resin supply <NUM> communicates with the first cavity <NUM> through one or more resin infusion inlets <NUM> extending through the tool <NUM> on the upstream side of the composite preform <NUM>, via one or more resin supply pipes <NUM>. The resin supply pipes <NUM> are typically formed of copper. A first vacuum source <NUM> communicates with the first cavity <NUM> through one or more vacuum outlets <NUM> extending through the tool <NUM> on a downstream side of the composite preform <NUM>, via one or more vacuum outlet pipes <NUM>, which are also typically formed of copper. Rather than communicating the resin supply <NUM> and first vacuum source <NUM> with the first cavity <NUM> via the resin infusion inlet <NUM> and vacuum outlet <NUM> extending through the tool <NUM>, it is also envisaged that the resin supply <NUM> and first vacuum source <NUM> may communicate with the first cavity <NUM> through the vacuum bagging film <NUM>. In such a configuration, apertures may be formed in the vacuum bagging film <NUM> and communicated with the resin supply <NUM> and first vacuum source <NUM>, sealing around the apertures. In the example depicted, the resin supply <NUM> also communicates with a second vacuum source <NUM> via a second vacuum pipe <NUM>.

A flow path <NUM> extends from the resin supply <NUM>, through the first cavity <NUM>, the composite preform <NUM> and to the first vacuum source <NUM>. An upstream portion of the flow path <NUM> comprises the resin supply pipe(s) <NUM> and resin infusion inlet <NUM> extending through the tool <NUM>. A mid portion of the flow path <NUM>, defined by the first cavity <NUM>, is formed by the composite preform <NUM> and various layers of layup materials located beneath the vacuum bagging film <NUM>. The layup materials include a permeable peel ply <NUM> located directly on, and extending over, the entirety of the composite preform <NUM>, beyond each of the edges <NUM>, <NUM>, <NUM> of the composite preform <NUM>, with a downstream portion <NUM> of the peel ply <NUM> extending downstream of the downstream edge <NUM> of the composite preform <NUM>. A layer <NUM> of permeable flow media is placed over the peel ply <NUM> and extends beyond the upstream edge of the peel ply <NUM> to beyond the resin infusion inlet(s) <NUM>. The layer <NUM> of permeable flow media extends to beyond the downstream edge <NUM> of the composite preform <NUM> but does not cover the entirety of the downstream portion <NUM> of the peel ply <NUM>. The peel ply <NUM> serves to prevent the layer <NUM> of permeable flow media from sticking to the composite preform <NUM> and also provides a path for infusion of resin into the composite preform <NUM>, both along the upstream edge <NUM> of the composite preform <NUM> and through the upper surface <NUM> of the composite preform <NUM>. The peel ply <NUM> also allows volatiles given off during curing of the resin to be drawn away from the composite preform <NUM>. The peel ply <NUM> also constitutes a permeable flow media, and may suitably be in the form of a PTFE coated fibreglass fabric, such as Release Ease® <NUM>, available from AirTech International Inc, or any other permeable peel ply material. The layer <NUM> of permeable flow media provides a passage for the resin through the first cavity <NUM> along the top of the composite preform <NUM>, along with a path for the escape of volatiles from the first cavity <NUM>. The layer <NUM> of permeable flow media may suitably be in the form of a nylon mesh material, such as Plastinet®<NUM> also available from AirTech International Inc, or any other highly permeable media enabling passage of resin therethrough.

A downstream portion of the flow path <NUM> comprises a further strip <NUM> of permeable flow media, the vacuum outlet(s) <NUM> and vacuum pipe(s) <NUM>. The strip <NUM> of permeable flow media extends across the downstream edge of the downstream portion <NUM> of the peel ply <NUM> and extends further downstream across the vacuum outlet(s) <NUM>. The strip <NUM> of permeable flow media is typically formed of the same material as the layer <NUM> of permeable flow media. A gap is located between the layer <NUM> and strip <NUM> of permeable flow media.

The vacuum bagging film <NUM> extends over the entire layup formed by the composite preform <NUM>, peel ply <NUM> and layer <NUM> and strip <NUM> of permeable flow media. Any of various vacuum bagging film materials may be utilized, including but not limited to Airtech WL7400 or SL800 vacuum bagging films available from Airtech International Inc. The vacuum bagging film <NUM> is sealed relative to the tool surface <NUM> about the periphery of the vacuum bagging film <NUM> by way of strips <NUM> of sealing tape, which may conveniently be in the form of a mastic sealant tape, such as GS-<NUM>-<NUM> sealant tape available from AirTech International Inc.

As may be best appreciated from <FIG>, the vacuum bagging film <NUM> defines the upper boundary of the resin flow path <NUM>. In the gap located between the layer <NUM> and strip <NUM> of permeable flow media, the vacuum bagging film <NUM> restricts the thickness of the flow path <NUM> between the tool upper surface <NUM> and vacuum bagging film <NUM> to the downstream portion <NUM> of the peel ply <NUM>, which is typically of a reduced permeability as compared to the layer <NUM> of permeable flow media. All downstream flow of resin is thus restricted through the downstream portion <NUM> of the peel ply <NUM>, which defines a permeable resin flow control choke.

The thermoplastic patch <NUM> is configured to melt when the thicker portion <NUM> of the composite preform <NUM> is subject to excessive exothermic heat build-up during exothermic cure of the resin <NUM>. In particular, the thermoplastic patch <NUM> may be selected such that it has a melting temperature exceeding the temperature at which the resin <NUM> is infused (which may be at an elevated temperature, typically of the order of <NUM> to <NUM>), and equal to or less than the peak exotherm temperature of the resin <NUM> that would otherwise be experienced during cure without the thermoplastic patch <NUM> (as may be determined by a trial cure cycle without the thermoplastic patch <NUM>). Different epoxy resins may release heat energy as a result of crosslinking during the cure process at different temperatures, depending on the resin selected. For example, one epoxy resin commonly used in resin infusion applications, HexFlow® RTM6, available from Hexcel Corporation, can begin releasing energy from about <NUM> during heating to cure temperature. For certain cure profiles, with a typical cure temperature of <NUM>, HexFlow® RTM6 may have a peak exotherm temperature of about <NUM> to <NUM>. Selecting a thermoplastic material for the thermoplastic patch <NUM> with a melting temperature less than the peak exotherm temperature, and particularly lying in the range at which the resin releases energy faster than it can be absorbed by the surrounding environment (about <NUM><NUM>C to <NUM><NUM>C for the example depicted in <FIG>) may be appropriate for use with typical epoxy resins.

<FIG> provides a graph depicting the melting profile of various thermoplastic materials, as sourced from http://us. com/dam/LabDiv/Campaigns/MPE2013/performance_plastics/download/thermal_an alysis_of_thermoplastics. The graph of <FIG> depicts the heat flux of each thermoplastic material plotted against temperature through the melting process, with the trough in each graph representing the endothermic melting process. When these thermoplastic materials are heated to a temperature corresponding to the endothermic trough and melting, the melting phase change comprises an endothermic reaction, allowing the thermoplastic patch <NUM> to act as a heat sink by absorbing heat. When the exothermic curing of the resin results in excessive heat energy being released by resin within the portion <NUM> of the composite preform <NUM>, heat transfers from the upper surface <NUM> of the composite preform <NUM> at the portion <NUM> to the lower surface of the thermoplastic patch <NUM>, due to its proximity. The thermoplastic patch <NUM> should thus be located in sufficient proximity to the upper surface <NUM> (or lower surface <NUM>) of the composite preform <NUM> to allow this heat transfer. When the thermoplastic patch <NUM> is heated to a temperature corresponding to the endothermic trough around the melting temperature of the thermoplastic material, the thermoplastic patch <NUM> absorbs heat energy as it melts, thereby conducting heat away from the portion <NUM> of the composite preform <NUM>. For an epoxy resin having an exotherm peak temperature of around <NUM> to <NUM>, and a rate of heat energy release exceeding the rate of energy absorption by the environment over the range of around <NUM> to <NUM><NUM>C, polyoxymethylene (POM), which has an endothermic trough and melting temperature at about <NUM> to <NUM><NUM>C may be suitable for the thermoplastic patch <NUM>]. Other thermoplastic materials with endothermic troughs and melting at lower temperatures may also be suitable, including high density polyoxyethylene (HPPE) which has an endothermic trough and melting temperature within the range of about <NUM><NUM>C to <NUM><NUM>C, or polypropylene (PP) which has an endothermic trough and melting temperature within the range of about <NUM><NUM>C to <NUM><NUM>C. Other thermoplastic materials (or combinations of multiple thermoplastics) may alternatively be selected to suit particular applications and resin. Apart from epoxy resins, other suitable thermoset resins used for resin infusion that also exhibit a positive heat of reaction during curing (and therefore possible exothermic behaviour) may include bismaleimide, benzoxazine, polyimide, cyanate esters and polyamide-imide resins.

The thermoplastic patch <NUM> may be formed with any size or shape to suit the specific application, and will generally be sized and shaped to generally correspond to the upper surface of the portion <NUM> of the composite preform <NUM> that is susceptible to experiencing excessive exothermic heating as a result of local exothermic behaviour of the resin during cure. For any given portion <NUM>, a single thermoplastic patch <NUM> may be applied or, alternatively, multiple smaller thermoplastic patches <NUM> may be applied. It is also envisaged that thermoplastic patches <NUM> may be applied to multiple portions across the upper surface <NUM> of the composite preform <NUM> as appropriate where multiple portions may be susceptible to excessive exothermic heating. The size and location of portions <NUM> for application of one or more thermoplastic patches <NUM> may be determined through simple assessment and identification of regions of the composite preform <NUM> that have a higher resin content due to locally increased thickness, or through trial and error in processing sample composite preforms without the use of any heat sink, identifying, by way of thermocouple, areas of local overheating. Alternatively, computational modelling may be utilized to identify regions that will likely be susceptible to excessive exothermic heating.

In use, once the system <NUM> has been assembled as discussed above, the resin supply <NUM> is heated to bring the resin to a suitable resin infusion temperature. Typically the entire system is heated within the oven <NUM> that is also used for subsequent curing. The temperature for resin infusion will be dependent upon the resin system utilised, and will typically be selected to provide a suitable viscosity enabling the resin to flow through the resin flow path <NUM>. For epoxy resins, a suitable infusion temperature may be in the range of <NUM> to <NUM><NUM>C.

At least partial vacuum pressure is applied to the downstream end of the first cavity <NUM>, via the first vacuum source <NUM> and vacuum outlet(s) <NUM>. A smaller partial vacuum (i.e., a higher absolute pressure) may also be applied to the resin supply <NUM>, by way of a second vacuum source <NUM> connected to a second vacuum pipe <NUM>, as shown in <FIG>. Where partial vacuum is applied to the resin supply <NUM> by the second vacuum source <NUM>, a pressure differential may be maintained between the first vacuum source <NUM> and second vacuum source <NUM> such that the absolute pressure at the vacuum outlet(s) <NUM> applied by the first vacuum source <NUM> is lower than the absolute pressure at the resin supply <NUM>. In one example, a full vacuum (<NUM> mbar / <NUM> kPa absolute pressure) may be applied by the first vacuum source <NUM> and a higher pressure / lower vacuum of <NUM> to <NUM> mbar absolute pressure (<NUM> to <NUM> kPa) may be applied to the second vacuum source <NUM>, thereby providing a pressure differential of the same amount driving resin from the resin supply <NUM> through the resin flow path <NUM>. Full vacuum pressure may also be applied to the resin supply <NUM> by the second vacuum source <NUM> prior to resin infusion to degas the resin.

Maintaining at least partial vacuum on the resin supply ensures at least a partial vacuum is maintained throughout the first cavity <NUM>. Atmospheric pressure acting on the composite preform <NUM> through the vacuum bagging film <NUM>, the layer <NUM> of permeable flow media and the peel ply <NUM> acts to consolidate the composite preform <NUM>. Resin moves through the first cavity <NUM> along a wave front, through the layer <NUM> of permeable flow media, which will generally have a greater permeability than both the peel ply <NUM> and the composite preform <NUM>, thus forming the path of least resistance. Resin passing through the layer <NUM> of permeable flow media will infuse down through the less permeable peel ply <NUM> and into the preform <NUM>. Some resin will also flow laterally through the upstream edge <NUM> of the composite preform <NUM> and, to a lesser degree, through the opposing side edges <NUM> of the composite preform <NUM>. Having the downstream edge <NUM> of the layer <NUM> of permeable flow media finish short of both the strip <NUM> of permeable flow media and the downstream portion <NUM> of the peel ply <NUM> prevents resin bypassing the preform <NUM> and simply being drawn through the layer <NUM> of permeable flow media directly into the vacuum outlet(s) <NUM>. The rate of advance of the resin wave front is inhibited by forcing the resin to pass downstream longitudinally through a permeable resin flow control choke defined by the downstream portion <NUM> of the peel ply <NUM> once it passes the downstream edge <NUM> of the composite preform <NUM> and the downstream edge of the layer <NUM> of permeable flow media.

Once the composite preform <NUM> has been fully resin infused, the resin infused composite preform <NUM> may then be cured by gradually elevating the temperature of the oven <NUM> to a temperature suitable for curing of the resin. For typical epoxy resins, curing temperatures of the order of <NUM><NUM>C to <NUM><NUM>C will be typical. Full vacuum is typically maintained on the first vacuum source <NUM> during the curing process, to ensure the resin infused composite preform <NUM> remains consolidated and to assist in curing of the resin. If the curing resin within the portion <NUM> of the composite preform <NUM> starts to exhibit exothermic behaviour and locally overheat the portion <NUM>, also heating the thermoplastic patch <NUM> to a temperature exceeding its melting temperature, the thermoplastic patch <NUM> will melt, thereby conducting heat away from the portion <NUM> of the composite preform <NUM>, with the heat being absorbed through the endothermic reaction of the melting process. As a result, the oven <NUM> may be heated to curing temperature more rapidly, and potentially to a higher curing temperature for quicker cure, than otherwise available without the use of the thermoplastic patch <NUM>, with a decreased risk of overheating the portion <NUM> of the composite preform <NUM>. More rapid process cycle times, whilst maintaining reliable cure, may thus be obtained.

Vacuum bagging films typically exhibit some air permeability, particularly at elevated temperatures associated with resin infusion and/or resin cure. Accordingly, one potential deficiency of the single vacuum bag configuration of the resin infusion system <NUM> of the first example is that, with vacuum applied during the curing process, air may permeate through the vacuum bagging film <NUM> and into the composite preform <NUM>, potentially resulting in porosity and resin starvation within the cured composite laminate. A double bag resin infusion system is thus envisaged in an effort to minimise or avoid such air permeation, by providing a second vacuum bagging film covering the first vacuum bagging film and applying vacuum pressure to the second vacuum bagging film during both the resin infusion and curing stages of operation.

<FIG> depicts a schematic cross-sectional view (corresponding to <FIG>) of such a double-bag configuration of the system <NUM> of the first example, fabricating a system <NUM> according to a second example. The system <NUM> of the second example is substantially identical to the system <NUM> of the first example, with the addition of a second vacuum bagging film <NUM> and associated breather layer <NUM>. Accordingly, features of the system <NUM> of the second example that are identical to features of the system <NUM> of the first example are provided with identical reference numerals and will not be further discussed.

In the system <NUM> of the second example as depicted in <FIG>, the composite preform <NUM> and associated consumable layers, such as the layer <NUM> and strip <NUM> of permeable flow media, peel ply <NUM> and first vacuum bagging film <NUM>, are first assembled in the same manner as described above in relation to the system <NUM> of the first example. The thermoplastic patch <NUM> is also located on the first vacuum bagging film <NUM> in the manner described above in relation to the first example. A breather layer <NUM>, typically being a highly permeable fabric formed of fibreglass or the like is then located over, and fully covering, the first vacuum bagging film <NUM> and the thermoplastic patch <NUM>. A suitable breather layer is a breather cloth formed of a high film non-woven polyester material, such as Airweave® N10, available from Airtech International Inc. The breather layer <NUM> extends over a vacuum groove <NUM> that extends around the perimeter of the tool surface <NUM> and is connected to the first vacuum source <NUM> (or a separate third vacuum source) by way of a third vacuum pipe <NUM>. The second vacuum bagging film <NUM> is then located to cover the entire breather layer <NUM> and is sealed relative to the tool surface <NUM> by way of further strips <NUM> of sealing tape, forming a sealed second cavity <NUM> between the first and second vacuum bagging films <NUM>, <NUM>. The composite preform <NUM> is resin infused and subsequently cured using the same process as described above in relation to the system <NUM> of the first example, with at least a partial vacuum being applied to the second cavity <NUM> between the first and second vacuum bagging films <NUM>, <NUM> by the first vacuum source <NUM> (or separate third vacuum source) throughout resin infusion and curing. The second vacuum bagging film <NUM> and associated vacuum applied to the second cavity <NUM> protects against any minor leaks associated with the first vacuum bag <NUM>, with the vacuum applied evacuating any air permeating through the second vacuum bagging film <NUM> toward the composite preform <NUM> through the breather layer <NUM>, rather than allowing it to permeate through to the composite preform <NUM>.

<FIG> depicts a schematic cross-sectional view of another double-bag configuration, forming a system <NUM> according to a third example, not in accordance with the present claims. The system <NUM> of the third example is substantially identical to the system <NUM> of the second example, apart from the location and arrangement of the thermoplastic patch <NUM>. Accordingly, features of the system <NUM> of the second example that are identical to features of the systems <NUM>, <NUM> of the first and second examples are provided with identical reference numerals and will not be further discussed.

Rather than locating the thermoplastic patch <NUM> on the first vacuum bagging film <NUM>, as is the case with the second example, in the system <NUM> of the third example, the thermoplastic patch <NUM> is located beneath the first vacuum bagging film <NUM>, on top of the layer <NUM> of permeable flow media. Such an arrangement brings the thermoplastic patch <NUM> in slightly closer proximity to the upper surface <NUM> of the composite preform <NUM>, slightly enhancing heat transfer between the composite preform <NUM> and thermoplastic patch <NUM>. To avoid the possibility of the thermoplastic patch <NUM> from mixing and adhering to the composite preform <NUM> upon melting, the thermoplastic patch <NUM> may conveniently be housed within a pocket <NUM> of non-porous release film, such as Airtech A7250, available from Airtech International Inc. The thermoplastic patch <NUM> may alternatively be otherwise isolated from the composite preform <NUM> to avoid contact with melted thermoplastic material. The system <NUM> of the third example is operated in the same manner as the system <NUM> of the second example to form a composite structure.

It is also envisaged, not in accordance with the present claims, that the system <NUM> of the first example may be modified to locate the thermoplastic patch <NUM> beneath the vacuum bagging film <NUM>, preferably within a pocket of release film or similar, in a similar manner. The thermoplastic patch <NUM> could alternatively be located on the exterior of either the single-bag system <NUM> of the first example, on the vacuum bagging film <NUM>, or on the exterior of the double-bag system <NUM>, <NUM> of either of the second or third examples, on the second vacuum bagging film <NUM>. It is, however, preferred to located the thermoplastic patch <NUM> in greater proximity to the surface of the composite preform <NUM> to enhance heat transfer between the composite preform <NUM> and the thermoplastic patch <NUM>. It is still further envisaged that, in the various examples described, the thermoplastic patch <NUM> may be located in proximity to the lower surface <NUM> of the composite preform <NUM> rather than upper surface <NUM>. The thermoplastic patch <NUM> could be located on the lower surface of the tool <NUM>, particularly when a relatively thin tool is utilized. Alternatively, the thermoplastic patch <NUM> could be located between the tool surface <NUM> and the lower surface <NUM> of the composite preform <NUM>, typically within a pocket of non-porous release film. Such a location may, however make an undesirable imprint on the lower surface <NUM> of the composite preform <NUM>.

<FIG> depicts a schematic cross-sectional view of a further double-bag configuration that is identical to the configuration of the system <NUM> of the second example, apart from the configuration of heat sink. Accordingly, features of the system <NUM> of the fourth example that are identical to features of the systems of the first through third examples are provided with identical reference numerals and will not be further discussed.

In the system <NUM> of the fourth example, rather than utilising a passive heat sink, such as a thermoplastic patch, an active heat sink is utilised. In the depicted example, the active heat sink is in the form of a thermoelectric cooling device <NUM>. Thermoelectric cooling devices, otherwise commonly referred to as Peltier devices or Peltier heat pumps, generate a heat flux between two adjacent plates of differing materials upon application of a voltage cross the plates. Commonly available thermoelectric devices with maximum operating temperatures exceeding the exotherm peak temperature of the resin being utilized would be particularly suitable. Such a thermoelectric cooling device may be utilised with the cooling side of the thermoelectric device located in proximity to the upper surface <NUM> of the composite preform, thereby conducting heat away from the thickened portion <NUM> of the composite preform <NUM> during curing.

In the arrangement depicted in <FIG>, the thermoelectric cooling device <NUM> is located on the first vacuum bag <NUM>, within the second cavity <NUM> between the first and second vacuum bags <NUM>, <NUM>. A pair of wires <NUM>, that are used to apply the voltage to the thermoelectric cooling device <NUM>, may pass along the second cavity <NUM> and through to the exterior of the system <NUM> through the sealing tape <NUM> sealing the second vacuum bag <NUM> to the tool surface <NUM>. Alternatively, the wires <NUM> could extend through an aperture formed in the second vacuum bag <NUM>, with the aperture appropriately sealed.

The system <NUM> of the fourth example is operated in the same manner as the system <NUM> of the second example to form a composite structure except that, rather than conducting heat away from the portion <NUM> of the composite preform <NUM> passively with the thermoplastic patch <NUM>, heat is conducted away actively by operating the thermoelectric cooling device <NUM> during the cure cycle.

Rather than locating the thermoelectric cooling device <NUM> on the first vacuum bagging film <NUM>, the thermoelectric cooling device <NUM> could alternatively be located on the exterior of the system <NUM>, on the second vacuum bagging film <NUM>, although it is preferred to located the thermoelectric cooling device <NUM> in greater proximity to the localised region <NUM> of the composite preform <NUM>. It is also envisaged that the thermoelectric cooling device <NUM> could be located below the first vacuum bagging film <NUM>, within a pocket of release film, in a similar manner to the arrangement of the thermoplastic patch <NUM> in the system <NUM> of the third example, or in proximity to the lower surface <NUM> of the composite preform <NUM>.

Rather than utilizing a thermoelectric cooling device, other forms of active heat sink are also envisaged. One such alternative may be a radiator and coolant arrangement, in which a series of tubes are arranged in proximity to the upper surface <NUM> of the composite preform <NUM> at the thickened portion <NUM> with coolant pumped through the tubes to conduct heat away from the portion <NUM>.

<FIG> depicts a schematic cross-sectional view of a system <NUM> for fabricating a composite structure according to a fifth example. The system <NUM> involves fabricating a composite structure from prepreg composite material, rather than resin infusing a dry composite preform as part of the process, as is the case with the first through fourth examples. Features of the system <NUM> of the fifth example that are identical or equivalent to features of the first through fourth examples described above are provided with like reference numerals.

The composite preform <NUM> comprises a pluarality of plies of prepreg composite material. With resin being pre-impregnated in the prepreg composite material there is no need for a resin infusion process, as is the case with the first through fourth examples. The composite preform <NUM> has an upper surface <NUM> and an opposing lower surface <NUM>. The upper and lower surfaces <NUM>, <NUM> each define a preform major surface. A heat sink, which may be a passive heat sink formed of an endothermic material, such as a thermoplastic patch <NUM> as described above, or an active heat sink such as a thermoelectric cooling device, is located in proximity to one of the preform major surfaces, and particularly the upper surface <NUM> in the depicted example. The thermoplastic patch <NUM> extends across only a portion <NUM> of the composite preform <NUM>. As with the portion <NUM> of the first through fourth examples, the portion <NUM> may be a portion of the composite preform <NUM> that is susceptible to excessive exothermic heating during resin cure, such as a thicker portion of the composite preform <NUM> forming a padup. The system <NUM> further comprises a heat source, which may again be in the form of an oven (not depicted) for heating the composite preform <NUM> to cure the pre-impregnated resin within the composite preform <NUM>.

As with the first through fourth examples, the composite preform <NUM> is located on the upper tool surface <NUM> of a tool <NUM> and a vacuum bagging film <NUM> is again sealed relative to the tool surface <NUM> to define a sealed first cavity <NUM> between the vacuum bagging film <NUM> and the tool surface <NUM>, with the composite preform <NUM> located in the first cavity <NUM>. Various layers of layup materials are located beneath the vacuum bagging film <NUM>. Specifically, a permeable peel ply <NUM> is located directly on, and extends over, the entirety of the composite preform <NUM>. A bleeder layer <NUM> is located on, and extends over, the permeable peel ply <NUM>. The bleeder layer <NUM> acts to absorb any excess resin bleeding from the composite preform <NUM> when compacted under atmospheric pressure, and also provides a path for the escape of volatiles during the curing procedure. A release film <NUM> is located on and extends over the bleeder layer <NUM> so as to retain excess resin within the bleeder layer <NUM> and prevent the same from sticking to the breather layer <NUM> that is located on, and extends over, the release film <NUM>. The breather layer <NUM> extends beyond the composite preform <NUM>, peel ply <NUM>, bleeder layer <NUM> and release film <NUM> to beyond a vacuum outlet <NUM> which communicates with a first vacuum source <NUM> by way of a vacuum pipe <NUM>. The vacuum bagging film <NUM> is sealed to the tool surface <NUM> by way of strips <NUM> of sealing tape, as with the first through fourth examples. In the arrangement depicted, the thermoplastic patch <NUM> is located on the release film <NUM>, which is generally non-porous. The release film <NUM> will thus prevent the thermoplastic patch <NUM>, when melted, from mixing with and sticking to the composite preform <NUM>. It is also envisaged, however, that the thermoplastic patch <NUM> may be located beneath the release film <NUM>, located within a pocket of release film, in a similar manner to the third example, with the release film pocket acting to prevent the thermoplastic patch <NUM> from mixing with or sticking to the composite preform <NUM> upon melting. It is still further envisaged that the thermoplastic patch <NUM> might be located on top of the vacuum bagging film <NUM>, or in proximity to the lower surface <NUM> of the composite preform <NUM>.

In use, once the composite preform <NUM> and layup materials have been assembled as discussed above, at least partial vacuum pressure, typically full vacuum pressure (<NUM> mBar/<NUM> kPa absolute pressure) is applied to the first cavity <NUM> by the first vacuum source <NUM>, thereby allowing atmospheric pressure (or greater via the aid of an autoclave) acting on the exterior of the vacuum bagging film <NUM> to act on the composite preform <NUM> and compact the same. The composite preform <NUM> may then be cured by gradually elevating the temperature of the oven or autoclave to a temperature suitable for curing of the resin. For typical epoxy resins, curing temperatures of the order of <NUM> to <NUM> will again be typical. Full vacuum is typically maintained on the first vacuum source <NUM> during the curing process, to ensure the resin infused composite preform <NUM> remains consolidated and to assist in curing of the resin. If the curing resin within the portion <NUM> of the composite preform <NUM> starts to exhibit exothermic behaviour and locally overheat the portion <NUM>, also heating the thermoplastic patch <NUM> to a temperature exceeding its melting temperature, the thermoplastic patch <NUM> will melt, thereby conducting heat away from the portion <NUM> of the composite preform <NUM>, with the heat being absorbed through the endothermic reaction of the melting process. As a result, the oven may be heated to curing temperature more rapidly, and potentially to a higher curing temperature for quicker cure, than otherwise available without the use of the thermoplastic patch <NUM>, in a similar manner to that described above in relation to the first through fourth examples. In place of the thermoplastic patch <NUM>, the use of an active heat sink such as a thermoelectric cooling device as described above in relation to the fourth example is also envisaged.

A general method of resin infusing the composite preform as discussed above is depicted in general terms in the flow diagram of <FIG>. At block <NUM>, a composite preform having an upper surface and an opposing lower surface is provided. The upper and lower surfaces each defining a preform major surface. A block <NUM>, a heat sink is located in proximity to one of the preform major surfaces so as to extend across only a portion of the composite preform. At block <NUM>, a resin is exothermically cured in the composite preform to form the composite structure. At block <NUM>, during curing of the resin, heat is conducted away from the portion of the composite preform into the heat sink.

Persons skilled in the art will appreciate that the specific examples described above are merely examples of the present disclosure. Persons skilled in the art will appreciate that the various features described in relation to different examples may be used in combination or as alternatives within the scope of the appended claims.

Claim 1:
A method of fabricating a composite structure, said method comprising:
providing a composite preform (<NUM>) having an upper surface (<NUM>) and an opposing lower surface (<NUM>), said upper and lower surfaces (<NUM>, <NUM>) each defining a preform major surface;
locating a heat sink (<NUM>) in proximity to one of said preform major surfaces so as to extend across only a portion (<NUM>) of said composite preform (<NUM>);
curing a resin in said composite preform (<NUM>) to form the composite structure, said resin curing exothermically; and
during curing of said resin, conducting heat away from said portion (<NUM>) of said composite preform (<NUM>) into said heat sink (<NUM>),
wherein the method further comprises:
locating said lower surface (<NUM>) of said composite preform (<NUM>) on an upper tool surface (<NUM>) of a tool (<NUM>);
placing a first vacuum bagging film (<NUM>) over said composite preform (<NUM>) and sealing said first vacuum bagging film (<NUM>) relative to said upper tool surface (<NUM>) to define a sealed first cavity (<NUM>) between said first vacuum bagging film (<NUM>) and said upper tool surface (<NUM>), said composite preform (<NUM>) being located in said first cavity (<NUM>); and
applying at least partial vacuum pressure to said first cavity (<NUM>) during curing of said resin;
and wherein said heat sink (<NUM>) is located above said first vacuum bagging film (<NUM>) in proximity to said upper surface (<NUM>) of said composite preform (<NUM>).