FIELD OF THE INVENTION
The present invention relates to process for manufacturing co-cured composite structures and product.
The present invention more particularly provides a manufacturing process for primary structural composite aircraft components.
The present invention in particular relates to the development of Vacuum Enhanced Resin Infusion Technology (VERITy) process for manufacturing Co-cured Composite Structural components.
BACKGROUND OF THE INVENTION
Advanced composites, in particular, carbon fiber based polymer composites have become materials of choice for aerospace designers because of their inherent superior properties such as specific strength and specific stiffness. In the early years, the usage of composites was limited to the secondary and tertiary structures in most of the cases. The designers have gained the confidence to work with composites over decades. The primary structures of the aircraft are now being designed with composites based on their experience gained from the manufacture of secondary parts. The current generation of aircrafts have the primary structures such as wing, fin fuselages etc. Most of these structures are manufactured using T300 unidirectional carbon epoxy prepregs using autoclave moulding. This process offers the advantages of very good mechanical properties due to the high volume fraction of fibers, uniformly distribution of resin throughout the part and low void content.
However following are some of the limitations associated with the above process: Prepregs are shelf life items that require special storage facilities (storage at -18°C). Transportation costs are higher as the package temperatures need to be kept at -18°C during transportation.
Lay-up of prepregs requires clean room facilities where the temperature and humidity have to be maintained. As prepregs have a limited out life, the time for lay-up is limited.
Thus the above said limitations of the prepreg technology, limits the size of the component and makes this process costly. Hence, world wide, researchers are looking for low cost manufacturing techniques.
Today, with cost becoming a very significant factor, alternate manufacturing techniques wherein the cost can be reduced but having similar properties to prepreg systems are being explored. Liquid composite moulding is an area of research that is being addressed worldwide. For quite some time now, RTM products with less than 45% fibre volume fractions have been successfully made. However using the conventional RTM process primary aircraft structures could not be made. The reason for this is that fibre volume fraction as high as 60% is not possible as well as large structural components like an aircraft wing is not possible. The next improvement came in the form of the Vacuum Assisted Resin transfer Moulding process (VARTM). It has been sheen that a vacuum assistance provides a significant improvement in mechanical properties. This is due to the higher fibre volume fraction and lower void content. The vacuum helps removal of air from within the fiber bundles, which results in lower void content. However, there are some limitations with the VARTM process also. One of the major limitations of the VARTM process is to achieve fibre volume fraction of 60 + 2% and void content less than 1% in co-cured structures with thicknesses varying from 2mm to 20mm.
One of the challenges in developing large structures is to design the suitable infusion strategy. This takes a more complex turn when the structure happens to be co-cured with others substructures such as stringers, ribs, spars, gussets etc. One of the concerns in this process, especially when making large integrated co cured aircraft structures with varying thickness, is to design the infusion strategy to get the uniform distribution of the resin at each zone and completely wet out the reinforcement. In case of any dry patch, which is more than 25mm2 due to resin absence, the component will get rejected for its operational use thereby causing huge loss.
Vacuum assisted resin transfer molding (VARTM) has been used to produce a number of large fibre reinforced composite structures, such as boat hulls which incorporate materials such as fabric or mat, arranged in a single sided mould in a dry condition along with the desired resin feed line to form the desired finished part. The lay-up is then encapsulated in a vacuum bag and impregnated with resin under vacuum. The resin is allowed then to cure.
Various methods have been utilized to introduce and enhance the VARTM process as distribution of resin through the preform (the methods to distribute the resin, is to use of resin distribution medium, grooved core etc. which allow resin to flow from the outer to the inner layer of reinforcement fibre), different vacuum arrangement, infusion from mold side, controlled atmospheric pressure during infusion, removal of excess resin etc. One of the methods to distribute the resin is to use of resin distribution medium, which allow resin to flow from the outer to the inner layer of reinforcement fibre.
Reference may be made to US Patent Numbers 5316462, 5096651 and 4560523, wherein grooved foam as a core has been used in a closed mould resin injection process to facilitate resin flow. These grooved foams may not be a choice for structure where the weight is very crucial because the grooved area would get filled with resin hence the weight would go up.
Reference may be made to US patent No 5,052,906, wherein VARTM apparatus having multiple resin inlets and having a vacuum line source on the opposite side of the preform and further also having a distribution medium layer on the top and bottom of the perform is disclosed. This process does not confirm the uniform distribution of resin so as to get uniform fibre volume fraction and component manufactured with this process will have rough surface on both sides which is not desirable for the primary aircraft structural component.
Reference may also be made to US patent No 5316462, wherein a means of impregnating a fibrous preform by using a bag system with a resin distribution conduit and the
distribution medium formed as an integral part of the vacuum bag. This technique also does not ensure uniform resin distribution and moreover the top surface of the component may not have the desired contour since there is no caul plate used in this process. One more drawback of this process is that the integration of the distribution medium may affect the mechanical properties.
Reference may be made to US Pat No 7147818, 7189345 which discloses a VARTM process and apparatus to remove excess resin by connecting the vacuum line from resin source and operating after completion of infusion. This process does not confirm the uniform resin distribution and the prepreg equivalent quality of the component.
Reference may be made to US Pat No 7419627 which discloses a process of co-curing of prepreg and dry fabric. This process does not describe the co-curing of various substructures with the parent skin.
Reference may be made to US Pat No 7334782 which discloses the process wherein the resin tank is kept under vacuum and upon lowering the vacuum level for the resin tank the resin is infused. This process too does not confirm the quality approval by ultrasonic C-scan. However this process meets the requirement like fibre volume fraction with one side (tool side) finished surface.
Reference may be made to Seeman Composite (http://205.153.241.230/P2 Opportunity_Handbook/2_II_4.html) who has produced a variety of composite structures for Boeing using the Seeman Composite Resin Infusion Molding Process (SCRIMP) from flat panels for making mechanical tests coupons to complex demonstration wing structure with the intention to use SCRIMP for making aerospace parts. A problem experienced with this type of structure is they have lower than desired fibre volume fraction and concomitantly higher than desired finished thickness per ply. Above referred prior state of arts are associated with followings limitations:
1. Usage of unidirectional carbon fabric whose permeability is low compare to
bidirectional carbon fabric (which is used world wide for infusion process) is not
disclosed
2. Uniformity of vacuum level (atmospheric pressure) at resin inlet area and at vacuum outlet area, without applying vacuum at resin inlet, is not described
3. Infusion of pre determined quantity of resin is not disclosed
4. Part quality inspection using C-scan not reported
5. Quality and mechanical properties as good as Prepreg technology not reported
6. Pressure application at required time (depending on the resin system) during cure is not reported
7. Infusion of many substructures viz, spars, ribs, stringers, gussets etc. along with parent skin have not been disclosed
8. Primary Co-cured composite structures like aircraft wing have not been made using the above said technique.
9. Usage of top caul plate is not disclosed
10. Usage of vacuum strip which helps to get uniform vacuum not reported
The present invention Vacuum Enhanced Resin Infusion Technology (VERITy) has been developed to overcome the limitations of the VARTM process and make it a process suitable for manufacturing primary aircraft structures. This process can ensure fibre volume fraction of 60% + 2%, void content less then 1% and a uniform resin content throughout the part, using integrated tooling concepts, special bagging technique, appropriate bleeder technique and some hybridization with the autoclave moulding technology. Detailed di-electric studies have been carried out for the EPOLAM 2063 epoxy resin system in order to arrive at the pressure application window (the quantity and time of pressure application). Based on the di-electric studies, external pressure of lbar is applied when the resin viscosity approaches 900-1000Cps.
OBJECTIVE OF THE INVENTION
The main objective of the present invention is to provide process for manufacturing co-cured composite structures.
Another objective of the present invention is to provide a process using Vacuum Enhanced Resin Infusion Technology (VERITy) for manufacturing primary aircraft components.
Yet another objective of the present invention is to provide an economical process for manufacturing the composites structural parts.
Yet another objective of the present invention is to manufacture co-cured composite structural components having quality equivalent to prepreg components.
SUMMARY OF THE INVENTION
Accordingly, present invention provides process for manufacturing Co-cured composite structures and the said process comprising the steps of:
i. placing a non-porous release film on the mould;
ii. placing a peel ply over the said non porous release film;
iii. placing a unidirectional carbon fabric (reinforcement layer),
dry cu-mesh and glass cloth as per the designed lay-up
sequence; iv. preforming of reinforcement layer at temperature ranging
between 77-83°C under vacuum; v. placing the peel ply again over the preformed reinforcement
layer; vi. laying up of porous release film on above said peel ply; vii. laying up of resin distribution medium on above said porous
release film; viii. placing the caul plate on the above said resin distribution
medium; ix. positioning of resin infusion lines; x. positioning of breather cloth, special bleeder cloth and vacuum
strips; xi. positioning of vacuum lines; xii. sealing of set-up; xiii. vacuum bagging of entire set-up; xiv. positioning the vacuum valves inside the vacuum bag; xv. vacuum sealing of the bag; xvi. connecting vacuum lines, resin trap and vacuum pump;
xvii. connecting resin inlet hoses, resin inlet valve and resin
container containing resin mixture; xviii. heating the set-up at 47-53°C for 50-70 minutes in an
autoclave; xix. mixing of EPOLAM 2063 resin and EPOLAM 2063 hardener
in predetermined quantities; xx. degassing of resin mixture and simultaneously heating to 42-
47°C; xxi. infusing the predetermined quantity of mixed warm resin into
the perform; xxii. bleeding of 1 % to 2% of infused resin mixture at vacuum end; xxiii. curing of infused preform in an autoclave to chosen cure
cycle; xxiv. subjecting the set up to external pressure of 0.8-1.2 bar when
the resin viscosity is about 900-1000Cps; xxv. cooling of set-up to room temperature; xxvi. removal of cured part from mould; xxvii. post curing of the part in an autoclave 177-183°C according to
chosen polymer cycle; xxviii. inspecting the part by ultrasonic c-scan for quality qualification, wherein the improvement lies in using caul plate containing carbon fabric, silicone rubber and epoxy resin in specific proportions so as to provide enough flexibility to take the top contour of the component as well as stiff enough to transfer the external pressure to the perform; positioning of bleeder cloth and continuous vacuum strip at the vacuum end near the component edge lifted by maximum height of 0.3mm in reference to the bottom surface of mould to provide uniform vacuum suction so as to get uniform resin distribution during resin infusion; sealing of set-up using uncured silicone rubber and modeling clay to avoid leakage of resin, race tracking of resin and to avoid resin reach or resin starve zone; degassing and simultaneous heating of resin mixture to 45°C, infusion of part having thickness variation for 1.36mm to 20mm.

In an embodiment of the present invention, curing of infused preform in an autoclave involves following steps:
a. heating the infused perform to 62-68°C and dwell for 170-190
minutes;
b. heating the component as obtained in step (a) to 77-83°C and
dwell for 27-33 minutes;
c. applying 0.8-1.2 bar pressure;
d. further dwelling of component at 77-83°C for 200-220 minutes,
e. running vacuum for further 27-33 minutes and
f. subjecting the perform set-up to external pressure when the
viscosity of resin is nearly 900-1 OOOCps to achieve fibre volume
fraction of 60 + 2% and void content less than 1%.
In an embodiment of present invention, non porous release film used is Teflon coated
polymer.
In another embodiment of present invention, peel ply used is Nylon polymer.
In yet another embodiment of present invention, resin mixture is a mixture of EPOLAM
2063 resin and EPOLAM 2063 hardener in the ratio of 100:107 by weight.
In yet another embodiment of present invention, vacuum of 760mm of hg with maximum
30mm drop is applied.
In yet another embodiment of present invention, a single side bleeding is provided for
controlling the uniformity of vacuum in the set-up.
In still another embodiment of present invention, bleed of small quantity of resin mixture
at vacuum end is allowed for removing any trapped air.
In yet another embodiment of present invention, mould is selected from the group
consisting of glass fibre composite mould and rigid carbon fibre composites (CFC) mould.
In yet another embodiment of present invention, resin distribution medium used is
polyethylene knitted mesh.
In still another embodiment of present invention, caul plate is made out of carbon fabric,
silicon rubber and epoxy resin.
In still another embodiment of present invention, caul plate in the master region is made
out of carbon fabric and epoxy resin.In still another embodiment of present invention, caul
plate in the slave region is made out of carbon fabric and silicon rubber.
In yet another embodiment of present invention, sealing of edges of preform and any joints
of caul plate is done by silicone rubber or modeling clay or combination thereof.
In still another embodiment of present invention, at least one vacuum valve is provided per
square meter of bagged area.
In yet another embodiment of present invention, minimum of one vacuum valve is
provided at vacuum outlet of set-up.
In yet another embodiment of present invention, degassing of mixed resin is carried out by
applying the vacuum on the resin bowl.
In still another embodiment of present invention, better compaction of preform, low void
content is achieved by applying external pressure of 1 bar.
In yet another embodiment of present invention, Co-cured composite structures
manufactured by using the said process, having fiber volume fraction within 58 to 62
percent, void content less than 1 percent, c- scan attenuation level between 3 to 25 dB
depending on the thicknesses, tensile strength between 1400 to 1500 Mpa, compressive
strength between 900 to 1000 Mpa and flexural strength in between 1300 to 1500 Mpa.
DETAILED DESCRIPTION OF THE INVENTION
The present invention, Vacuum Enhanced Resin Infusion Technology (VERITy), provides a manufacturing process for composite structural components using unidirectional carbon fabric and appropriate resin system. The process can be adopted for aerospace as well as for non aerospace applications. The improved Vacuum Enhanced Resin Infusion Technology and the apparatus for carrying out this process of the present invention provides a product having improved dimensional tolerance, increased fibre volume fraction, uniformity of vacuum every where in the bagged area, quality compatible to prepreg technology, less void content, usage of innovative tooling concepts, controlled bleed out of resin etc. The application of external pressure of 1 bar (760mm of Hg) gives better compaction and it also helps to bleed the excess resin, thereby optimum fibre volume fraction is achieved. This process is a novel combination of VARTM and Autoclave moulding technique.
In the VERITy process, release film, peel plies, fabric, distribution medium, caul plate, resin inlets, vacuum outlets, vacuum strips, breather, bleeder are positioned on the mould (as mentioned in Figure 1). Edges are sealed in such a way that there is only one way for resin to bleed out, which is on vacuum side. At vacuum side a vacuum strip is placed. The assembly under vacuum is heated up in the autoclave. The resin (EPOLAM 2063 resin) is mixed with hardener (EPOLAM 2063 hardener) and degassed prior to infusion. The resin inlet tube of the setup is connected to the resin mixture container and then the resin mixture is infused into the preform. After completion of infusion, curing process begins. During cure the external pressure is applied and further cure is carried out.
Present invention is a preferred method for making aerospace-grade composites parts where optimum fibre volume fraction, void content and thickness are very important in obtaining composites having the optimum specific strength and specific modulus (i.e., load carrying capability/specific gravity)
The present invention describes the preform operation to achieve better compaction of dry reinforcement and to help in assembling of many other substructures to the parent skin prior to infusion, which allows production of parts with higher percentage of fibre volume fraction and better dimension tolerance than the prior art methods discussed earlier.
In the present invention, resin flows into the dry preform to achieve higher fibre volume fraction by controlling the quantity of resin to be infused in each zone.
The present invention uses a caul plate which is made out of carbon fabric, RTV silicone rubber and tooling resin system consisting of LY 5210 epoxy resin and K24 hardener. This caul plate is made in the principle of master and slave basis. In a single caul plate, in the master region epoxy resin and carbon fabric is used and in the slave region silicone rubber is used with carbon fabric. This caul plate helps to retain the shape of stringer, ribs etc.,
and same time it is used to transfer the pressure on to the preform. This caul plate also helps in proper consolidation of stringer radius and ribs and spar radius junctions.
The present invention employs a vacuum strip and a bleeder cloth. This bleeder cloth does not absorb much resin but allows good air flow at vacuum end to achieve uniform vacuum through the preform. The vacuum strips along with bleeder cloth confirm no blockage of vacuum during infusion and for the finial curing process. The above said method helps to remove small quantity of resin so that any volatile or air is removed. The vacuum strip further helps in the uniform flow of resin into the dry preform.
One of the techniques of the present invention is sealing of all the edges and other openings from where resin can leak. This sealing helps to retain the resin in the preform and avoids resin starvation area in the final part. The above said sealing further helps to get uniform resin content through out the part.
In the present invention, predetermined quantity of resin is infused to keep minimum resin bleed. It helps to minimize the cost and wastage. The minimum resin bleed confirms no blockage of vacuum line which otherwise would block during further curing process. In addition some other vacuum ports are provided on the other area of the component (quantity of vacuum ports are calculated as 1 vacuum port/1 sq m of bagged area) which is opened during the application of external pressure and kept opened till final cure. This helps to get uniform vacuum pressure to achieve better compaction of preform.
The present invention provides a process to manufacture the structural composite part where the thickness is varying from 1.36mm to 20mm. One of challenging tasks is to infuse the resin in 20mm zone where the flow resistance is very high because of low permeability. Moreover the resin which is infused has a tendency to move in low resistance zone i.e., the thinner zone than high resistance zone i.e., thicker zone.
The present invention also discloses the time and quantity of external pressure to be applied after completion of infusion during curing process. This pressure is crucial in the
process because the pressure is required to achieve better compaction in the areas where the thickness is more then 10mm. Experiments shows that with external pressure the laminate has a better attenuation level in C-scan report and lower void content, which is the principle requirement for airworthy components.
The novelty of the present invention lies in providing an economical process for manufacturing primary aircraft structures which gives the uniform resin distribution throughout the component where the thickness is varying from 1.36mm to 20mm, co-curing of skin with sub structure like ribs, spars, stringers, gussets etc and part quality is equivalent to those made by prepreg technology.
The above said novelties of present invention have been achieved by introducing following inventive steps in the manufacturing process:
1. Development of an integrated caul plate containing carbon fabric, RTV silicone rubber and tooling epoxy resin system in specific proportions.
2. Positioning of vacuum strip at the vacuum end, near the component edge lifted by maximum height of 0.1 to 0.3mm in reference to the bottom surface of mould to provide uniform vacuum suction during resin infusion.
3. Sealing of set-up using uncured silicone rubber and modeling clay to avoid leakage of resin and race tracking of resin.
4. Infusion of pre determined quantity of resin keeping the resin viscosity in the range of200-300Cps.
5. Infusion of the parts where the thickness is varying from 1.36mm to 20mm.
6. Applying the external pressure of lbar on the set-up, after infusion, when the viscosity of resin is nearly 900-1 OOOCps.
7. Special bagging technique using COREMAT VMS 160A is used to prevent the excess resin bleed. As shown in figure 1 the COREMAT VMS 160A (10) is placed at the vacuum end. This material does not absorb much resin but allows good air flow. Therefore the resin resides within the component. After placement of COREMAT the edge is sealed to prevent the resin bleeding except at vacuum end.
In the present invention, reinforcement is held in a tool cavity, infused with a resin system and a differential pressure (difference of atmospheric pressure and vacuum) is maintained so as to completely wet out the fibres. The schematic of the process is shown in Figure. 1.
Figure 1 represents the schematic view of the present invention. In this the mould (1) is a rigid mould which gives the required contour to the part and on which the non porous release film (2) is placed. On the above said non porous release film (2) a peel ply (3) which is required for secondary operations e.g. drilling, bonding, painting etc. is placed. On the above said peel ply (3) carbon UD fabric (4) is places as per required dimensions and orientations. On the above said carbon fabric (4) a peel ply (3) is placed. On the above said peel ply (3) a porous release film (5) which is required for easy removal of the caul plate (7) is placed. On the above said porous release film (5) resin distribution medium (6) is placed. On the above said resin distribution medium (6) caul plate (7) is kept. Then the resin infusion lines (8) are positioned on to the assembly. A breather cloth (9) is placed on top of the caul plate (7). A special bleeder cloth, COREMAT VMS 160A, (10) is positioned at vacuum end as shown in the figure 1. A vacuum strip (11) is placed at the end of the laminate at vacuum end. The setup is sealed using uncured silicone rubber and modeling clay (13). The vacuum valves (12) are placed at respective location. Then the setup is vacuum bagged (14). The above said vacuum bag edges is sealed using uncured silicone rubber (15). The vacuum lines (16) are connected to a resin trap (17) and the resin trap is connected to a vacuum pump (18). The resin inlet hoses (19) are connected to resin inlet valve (20) and the valve is connected to the resin container in which warm resin mixture (21) is kept. The resin inlet valve (20) is opened and infusion process starts. The resin distribution medium (6) helps to distribute the resin uniformly on top. The resin mixture (21) travels through the porous release film (5) and then goes into the dry preform of carbon UD fabric (4). During infusion, the sealing (13) does not allow race tracking and leaking any where else other than bleeding of small quantity of resin at vacuum end to remove any trapped air. The vacuum strip (11) helps to get uniform vacuum suction therefore resin mixture (21) travels uniformly and this vacuum strip (11) also ensure uniform vacuum suction until cure is completed. The special bleeder cloth (10) absorbs
very little quantity of resin mixture (21) therefore resin mixture (21) stays within the laminate and ensures resin content in the component as planned. The resin trap is used to safe guard the vacuum pump in case the resin mixture (21) bleeds and goes to the vacuum lines (16). The vacuum pump (18) is used to apply vacuum continuously on the setup.
DESCRIPTION OF THE DRAWING
Fig.l shows schematic of the VERITy process.
Fig.2 shows the C-scan results of 2.04 mm (with 0° orientations) and 9.18mm (quasi-
isotropic) laminate. Fig.3 shows the geometry of the wing test box. Fig.4 shows the photograph of co-cured top skin. Fig. 5a, 5b and 5c shows the c-scan plot for the co-cured top skin.
EXAMPLES
The present invention will be more specifically explained by following examples. However, the scope of the present invention is not limited to the scope of these examples below.
EXAMPLE 1
MANUFACTURING OF LAMINATES OF DIFFERENT THICKNESSES
Laminate of different thicknesses from 1.02mm to 19.36mm have been made successfully using VERITy method. To manufacture the laminate unidirectional (UD) carbon fabric and EPOLAM 2063 resin system are used. For 1.02mm thick laminate, 6 layer of UD carbon fabric has been used, similarly for 19.36mm thick laminate, 118 layers of UD carbon fabric has been used.
A non porous release film is placed on to the mould followed by peel ply. UD carbon fabric is cut as per designed orientation and dimensions and placed over the peel ply as per designed lay-up sequence. A peel ply fabric is placed on to the dry preform followed by porous release film, resin distribution medium (polyethylene knitted mesh) and caul plate. Resin infusion channel is placed at one end of the laminate and vacuum line at the other
end of the laminate. The assembly is sealed and vacuum bagged. Full vacuum is ensured and the setup is placed in an autoclave for preheating at 50°C for 1 hour. The required amount of resin mixture is calculated on the basis of 32% of the final laminate weight. EPOLAM 2063 resin and EPOLAM 2063 hardener is mixed in the ratio of 100:107 by weight and degassed for 20minutes. Simultaneously the resin mixture is heated to 45°C to get required viscosity range for infusion. After preheating of setup, the mixed resin is then infused in to the laminate. After completion of infusion, the infusion tube is sealed and further curing process started in autoclave as per chosen polymer cure cycle. After curing, the laminate is demoulded and post cured in an autoclave as per chosen polymer cure cycle. The laminate quality inspection is done using ultrasonic c-scan. Some of these laminates are used for mechanical testing for material characterization.
Table 1 show the mechanical test results conducted on specimens manufactured by the present invention and prepreg technology. The test results show there is similarity in the mechanical properties measured. However there is reduction in the inter laminar shear strength which is mainly resin dependent property. Figure 2 shows the C-scan results of 2.04mm (with 0° orientations) and 9.18mm (quasi-isotropic) laminate manufactured by present invented method.
Table 1: Few Mechanical test results of the test coupons manufactured by present invention and prepreg technology are given below (Table Removed)

EXAMPLE 2
MANUFACTURING OF CO-CURED BOTTOM SKIN FOR WING TEST BOX
The co-cured bottom skin, with different substructures e.g., 2 spars, 4 ribs, 6 stringers and 38 gussets, has been made successfully using VERITy method. This part has the thickness variation from 2mm to 18.36mm. To manufacture this box carbon UD fabric and EPOLAM 2063 epoxy resin system is used. A glass mould is used as a base mould to develop this co-cured bottom skin. The glass mould is used to generate the flow front data for further use. A non porous release film is placed on the mould. A peel ply cloth is then place over the non porous release film. Reinforcement layer as per designed lay-up sequence is placed on top of the peel ply. The other substructure is then placed on top of the skin the whole assembly is preformed together. A peel ply is then placed on top of preform assembly followed by porous release film, resin distribution medium (Polyethylene knitted mesh) and the caul plate. Infusion ports are positioned at designed infusion lines. The respective areas are sealed using uncured silicone rubber and modeling clay. The vacuum strip, bleeder cloth and breather cloth are placed subsequently. Sequential infusion strategy has been used to infuse the part. Three infusion lines for infusion and two vacuum lines for vacuum purpose have been used. The whole assembly is kept in an autoclave for pre heating at 50°C for 1 hour. The warm degassed resin mixture (EPOLAM 2063 resin and EPOLAM 2063 hardener) is then placed at respective places and infusion lines are connected to the respective bowls containing the quantity of resin mixture needed. The first line of infusion is opened and required quantity of resin is infused. After completion of first infusion subsequently second and third infusions are carried out. The first line of infusion took 25minutes, whereas the second line of infusion
took 45minutes and the third line of infusion took 50 minutes. After completion of infusion, curing process is started as per the specific cure cycle. The component is demoulded and post cure after curing. The component is then c-scaned for quality check. The dimensions has been checked critically and found to be satisfactorily. This box has been tested for static loads for various load cases and passed successfully. Figure 3 shows the geometry of the co-cured box.
EXAMPLE 3
MANUFACTURING OF TOP SKIN FOR THE CIVIL AIRCRAFT WING
The co-cured top skin for the SARAS aircraft wing has been manufactured by the present invented method. The size of the wing is 5.6m length x 1.7m width at root and 0 .7m width at tip. The thickness is varying from 1.36mm to 8.16mm. This co-cured skin consists of parent skin, 12 hat type stringers and 23 'T' type stringers. This part is made using carbon UD of the top skin. A non porous release film is first place on to the mould. A fabric with EPOLAM 2063 epoxy resin system. A rigid CFC base mould (carbon fibre composite) has been used for lay-up. The above said mould gives the required contour to the top surface peel ply cloth is then placed on to the above said non porous release film. The reinforcement layer (carbon UD fabric), dry cu-mesh (for lightning protection) and glass cloth (for galvanic corrosion at certain location) is placed as per designed lay-up sequence. The whole assembly is preformed under vacuum. The stringers are preformed on the stringer preforming tool. After preforming, the layer drops are cut accordingly. These stringers are then placed on the parent skin. The whole assembly is preformed once again under vacuum. After preforming, a peel ply cloth is placed on top of preformed assembly. A porous release film is then placed on top of above said peel ply. A resin distribution medium is then placed on top of the non porous release film. The caul plate is then placed on top of the above said resin distribution medium. This caul place has an opening wherever the resin port is to be placed. The resin infusion accessories are positioned on top of the caul plate. A total of 3 infusion lines and 26 ports have been designed to infuse the co-cured top skin. The vacuum strips are placed at vacuum end. All the areas have been sealed using uncured silicone rubber and modeling clay. A breather cloth is placed on top
of the caul plate. A bleeder cloth is placed at vacuum side. Finally the vacuum bag is made for the whole setup and the setup is vacuum sealed. The whole setup is then placed in an autoclave for preheating at 50°C for 1 hour. This ensures the uniform temperature distribution throughout the mould and helps to maintain the resin viscosity during infusion. The warm resin mixture of required quantity after degassing is collected in different bowls. All the bowls are placed at respective location on weighing balance. The first line of infusion tube is inserted into the bowls and the resin inlet valve is opened. After the required amount of resin is infused the valve is closed. After closing of all the first lines of infusion, the second line of infusion tubes are inserted into the resin bowls. The second lines of infusion are started and when the required quantity of resin is infused the lines are closed and third lines of infusion are opened. After completion of the infusion all the resin inlet valves are closed and resin bowls are removed. The infusion tubes are sealed and the curing process begins. By starting the heating cycle in the autoclave when the resin viscosity approaches to 950 Cps, lbar external pressure is applied and curing of infused component is carried out as follows:
a. heating the component to 65°C and dwell for 3 hours.
b. heating the component to 80 C and dwell for 0.5 hours,
c. applying 1 bar pressure,
d. further dwelling of component at 80°C for 6V2 hours,
e. running vacuum for further 0.5 hour and
f. subjecting the perform set-up to external pressure when the
viscosity of resin is nearly 900-1 OOOCps to achieve fibre volume
fraction of 60 + 2% and void content less than 1%.
After completion of curing the component is demoulded. The co-cured top skin is then post cured according to chosen polymer cure cycle. After post curing the top skin is send for ultrasonic c-scan for quality checkup. Along with the above said co-cured top skin a traveler coupon is also made which has been subjected to mechanical tests and met the requirement. The skin is scaned in three segments. Figure 4 shows the photograph of co-cured top skin and figure 5a, 5b and 5c shows the c-scan plot for the co-cured top skin.
ADVANTAGES OF THE INVENTION
Present invention provides a manufacturing method to manufacture a large and complex
co-cured composite structural component
Present invention provides a manufacturing method by which airworthy primary structural
component can be manufactured.
Present invention provides a manufacturing method wherein a product quality is
equivalent to those of made by prepreg technology.
Present invention provides a economical manufacturing method by which aerospace
component like aircraft wing can be manufactured by <12% lower in cost compare to
prepreg technology.
Present invention provides an innovative tooling concept by which uniform distribution of
pressure, component dimensions and contour can be controlled.
Present invention provides a manufacturing method whereas the composite component can
have higher fibre volume fraction and lower void content.
Present invention provides an innovative cure cycle and bagging technique to achieve
minimum resin bleed, better compaction of layers etc.

claim
1. A process for manufacturing Co-cured composite structures and the said process comprising the steps of:
i. placing a non-porous release film on the mould;
ii. placing a peel ply over the said non porous release film;
iii. placing a unidirectional carbon fabric (reinforcement layer),
dry cu-mesh and glass cloth as per the designed lay-up
sequence; iv. preforming of reinforcement layer at temperature ranging
between 77-83°C under vacuum; v. placing the peel ply again over the preformed reinforcement
layer; vi. laying up of porous release film on above said peel ply; vii. laying up of resin distribution medium on above said porous
release film; viii. placing the caul plate on the above said resin distribution
medium; ix. positioning of resin infusion lines; x. positioning of breather cloth, special bleeder cloth and vacuum
strips; xi. positioning of vacuum lines; xii. sealing of set-up; xiii. vacuum bagging of entire set-up; xiv. positioning the vacuum valves inside the vacuum bag; xv. vacuum sealing of the bag;
xvi. connecting vacuum lines, resin trap and vacuum pump; xvii. connecting resin inlet hoses, resin inlet valve and resin
container containing resin mixture; xviii. heating the set-up at 47-53°C for 50-70 minutes in an
autoclave;
xix. mixing of EPOLAM 2063 resin and EPOLAM 2063 hardener
in predetermined quantities; xx. degassing of resin mixture and simultaneously heating to 42-
47°C; xxi. infusing the predetermined quantity of mixed warm resin into
the perform; xxii. bleeding of 1% to 2% of infused resin mixture at vacuum end; xxiii. curing of infused preform in an autoclave to chosen cure
cycle; xxiv. subjecting the set up to external pressure of 0.8-1.2 bar when
the resin viscosity is about 900-1000Cps; xxv. cooling of set-up to room temperature; xxvi. removal of cured part from mould; xxvii. post curing of the part in an autoclave 177-183°C according to
chosen polymer cycle; xxviii. inspecting the part by ultrasonic c-scan for quality
qualification. 2. A process as claimed in step (xxiii) of claim 1, wherein curing of infused preform in an autoclave comprising the steps of:
a. heating the infused perform to 62-68°C and dwell for 170-190
minutes;
b. heating the component as obtained in step (a) to 77-83°C and
dwell for 27-33 minutes;
c. applying 0.8-1.2 bar pressure;
d. further dwelling of component at 77-83°C for 200-220
minutes,
e. running vacuum for further 27-33 minutes and
f. subjecting the perform set-up to external pressure when the
viscosity of resin is nearly 900-1 OOOCps to achieve fibre
volume fraction of 60 ± 2% and void content less than 1%.
3. A process as claimed in step (i) of claim 1, wherein non porous release film used is Teflon coated polymer.
4. A process as claimed in step (i) of claim 1, wherein peel ply used is Nylon polymer.
5. A process as claimed in step (i) of claim 1, wherein resin mixture is a mixture of EPOLAM 2063 resin and EPOLAM 2063 hardener in the ratio of 100:107 by weight.
6. A process as claimed in claim 1, wherein vacuum of 760mm of Hg with maximum 30mm drop is applied.
7. A process as claimed in step (i) of claim 1, wherein mould is selected from the group consisting of glass fibre composite mould and rigid carbon fibre composites (CFC) mould.
8. A process as claimed in step (vii) of claim 1, wherein resin distribution medium used is polyethylene knitted mesh.
9. A process as claimed in step (viii) of claim 1, wherein caul plate is made out of carbon fabric, silicon rubber and epoxy resin.
10. A process as claimed in step (viii) of claim 1, wherein caul plate in the master region is made out of carbon fabric and epoxy resin.
11. A process as claimed in step (viii) of claim 1, wherein caul plate in the slave region is made out of carbon fabric and silicon rubber.
12. A process as claimed in step (xv) of claim 1, wherein sealing of edges of preform and any joints of caul plate is done by silicone rubber or modeling clay or combination thereof.
13. A process as claimed in step (xiv) of claim 1, wherein at least one vacuum valve is provided per square meter of bagged area.
14. A process as claimed in step (xiv) of claim 1, wherein minimum of one vacuum valve is provided at vacuum outlet of set-up.
15. A process as claimed in step (xx) of claim 1, wherein degassing of mixed resin is carried out by applying the vacuum on the resin bowl.
16. A process as claimed in step (xxiii) of claim 1, wherein better compaction of preform, low void content is achieved by applying external pressure of 1 bar.
17. A process as claimed in claim 1, wherein Co-cured composite structures manufactured by using the said process, having fiber volume fraction within 58 to 62 percent, void content less than 1 percent, c- scan attenuation level between 3 to 25 dB depending on the thicknesses, tensile strength between 1400 to 1500 Mpa, compressive strength between 900 to 1000 Mpa and flexural strength in between 1300 to 1500 Mpa.
18. A process for manufacturing Co-cured composite structures substantially as herein described with reference to the examples and drawings accompanying this specification.