290 3-D textile reinforcements in composite materials respect to time [2,3].These approaches,applied at the design stage,have permitted fill procedures to be developed by which resin-starved areas are eliminated through the use of accurately positioned vents and/or different resin injection points.These predictive approaches require greater knowl- edge of the properties of the reinforcements,particularly their permeabil- ity,which,depending upon the nature of the reinforcement constriction, may be different in the longitudinal and transverse directions.Modelling of isothermal flow of resin of constant viscosity through textile reinforcements with isotropic permeability is based on D'Arcy's equation(9.3): Q=-KA.ip u 8x [9.3] where K is the permeability,u the resin viscosity and op/ox the pressure drop per unit length. 5 For random mat non-woven reinforcements permeability is isotropic in- pooM plane while for other textile structures the permeability will be different in different directions depending upon the nature of the textile structure.This differential permeability will result in complex flow patterns in the tool, making flow prediction even more important,although the use of D'Arcy's equation then becomes an over-simplification. 着之5 The vast majority of the tows employed in woven,braided or knitted rein- forcements comprise low twist or untwisted continuous filament yarns.The pressure flow of the low viscosity resins can be assisted by capillary flow in the parallel channels between the filaments and control of the filling oper- ation must be exercised to ensure resin'racing'or'tracking'does not occur. If this is not controlled the resin flow front will race ahead (or fall behind) before rejoining the pressure flow front,leading to unimpregnated enclosed dry regions.Hence variations in fibre volume fraction will result. A variation of this process is to vacuum assist the resin into the tool.The cavity,with the reinforcement in situ,is evacuated and the resin is forced under pressure into the tool,thus wetting out the fibre.This approach is known as vacuum assisted resin injection (VARI)[4]. 9.4 Degassing One of the major difficulties associated with composite manufacture is that of void formation during impregnation and cure [5].When these become entrapped within the matrix,stress concentrations can be established within the matrix.These may originate: during mixing of the resin formulation; during the complex chemical reactions that take place during the cure of thermosetting resins,when volatile gases are released and become encapsulated in the crosslinked resin;
respect to time [2,3]. These approaches, applied at the design stage, have permitted fill procedures to be developed by which resin-starved areas are eliminated through the use of accurately positioned vents and/or different resin injection points. These predictive approaches require greater knowledge of the properties of the reinforcements, particularly their permeability, which, depending upon the nature of the reinforcement constriction, may be different in the longitudinal and transverse directions. Modelling of isothermal flow of resin of constant viscosity through textile reinforcements with isotropic permeability is based on D’Arcy’s equation (9.3): [9.3] where K is the permeability, m the resin viscosity and dp/dx the pressure drop per unit length. For random mat non-woven reinforcements permeability is isotropic inplane while for other textile structures the permeability will be different in different directions depending upon the nature of the textile structure. This differential permeability will result in complex flow patterns in the tool, making flow prediction even more important, although the use of D’Arcy’s equation then becomes an over-simplification. The vast majority of the tows employed in woven, braided or knitted reinforcements comprise low twist or untwisted continuous filament yarns. The pressure flow of the low viscosity resins can be assisted by capillary flow in the parallel channels between the filaments and control of the filling operation must be exercised to ensure resin ‘racing’ or ‘tracking’ does not occur. If this is not controlled the resin flow front will race ahead (or fall behind) before rejoining the pressure flow front, leading to unimpregnated enclosed dry regions. Hence variations in fibre volume fraction will result. A variation of this process is to vacuum assist the resin into the tool. The cavity, with the reinforcement in situ, is evacuated and the resin is forced under pressure into the tool, thus wetting out the fibre. This approach is known as vacuum assisted resin injection (VARI) [4]. 9.4 Degassing One of the major difficulties associated with composite manufacture is that of void formation during impregnation and cure [5]. When these become entrapped within the matrix, stress concentrations can be established within the matrix. These may originate: • during mixing of the resin formulation; • during the complex chemical reactions that take place during the cure of thermosetting resins, when volatile gases are released and become encapsulated in the crosslinked resin; Q KA p x =- ◊ m d d 290 3-D textile reinforcements in composite materials RIC9 7/10/99 8:32 PM Page 290 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9
Resin impregnation and prediction of fabric properties 291 during filling of the cavity as described above; owing to the complex nature of the textile reinforcement,since air can become entrapped in the interstices of the fabric structure.This can be particularly evident when coarse yarns(or tows)are used or in complex 3-D braided or woven structures and may be most prevalent at the solid tool/composite interface. During the formulation stage of the resin system,mixing is necessary to ensure that the hardeners,the crosslinking agents or any other additives are uniformly distributed and dispersed.The agitation during this formulation draws air into the uncured polymer along with the air already absorbed within the low viscosity fluid.As indicated above,these 'volatiles'are potential problem areas and must be eliminated in high-performance composites. After rigorous mixing,the resin mixture is degassed,under full vacuum, giving a deaerated fluid ready for application to the reinforcement.This deaeration can be assisted by heating the resin,to reduce its viscosity, although great care must be exercised to ensure that crosslinking is not initiated. 9.5 Preimpregnation One of the limitations of producing high-performance composite materials lies in the difficulty of achieving uniformity of fibre/resin distribution with low void content.Instead of relying on the pressure flow to force the resin throughout the reinforcement,dip coating and lick roll technology are used to apply a controlled and uniform amount of uncured resin to the rein- 8 forcement.The resin bath contains both the base matrix resin and the hard- eners in a partially cured resin system.The rolls of prepreg'are wrapped in release film and can be stored under refrigerated conditions for a period of time before the shelf-life of the product expires (normally 90 days at -18C for aerospace quality materials).Adoption of this route ensures uni- formity of resin distribution in the reinforcement and eliminates the need for the processor to handle resin systems but does require that low- temperature storage facilities are available on the production site. 9.6 Vacuum bagging The vacuum bagging system is used for producing non-critical components. Plies of thawed out and conditioned prepreg are cut into the appropriate shape either by hand or by an automated process such as a Gerber cutter system.Plies are placed in a precise order and orientation on a tool surface. The lay-up sequence and orientation of the plies is critical to the perfor- mance of the composite.A layer of release film is laid on top of the ply lay-
• during filling of the cavity as described above; • owing to the complex nature of the textile reinforcement, since air can become entrapped in the interstices of the fabric structure. This can be particularly evident when coarse yarns (or tows) are used or in complex 3-D braided or woven structures and may be most prevalent at the solid tool/composite interface. During the formulation stage of the resin system, mixing is necessary to ensure that the hardeners, the crosslinking agents or any other additives are uniformly distributed and dispersed. The agitation during this formulation draws air into the uncured polymer along with the air already absorbed within the low viscosity fluid. As indicated above, these ‘volatiles’ are potential problem areas and must be eliminated in high-performance composites. After rigorous mixing, the resin mixture is degassed, under full vacuum, giving a deaerated fluid ready for application to the reinforcement. This deaeration can be assisted by heating the resin, to reduce its viscosity, although great care must be exercised to ensure that crosslinking is not initiated. 9.5 Preimpregnation One of the limitations of producing high-performance composite materials lies in the difficulty of achieving uniformity of fibre/resin distribution with low void content. Instead of relying on the pressure flow to force the resin throughout the reinforcement, dip coating and lick roll technology are used to apply a controlled and uniform amount of uncured resin to the reinforcement. The resin bath contains both the base matrix resin and the hardeners in a partially cured resin system. The rolls of ‘prepreg’ are wrapped in release film and can be stored under refrigerated conditions for a period of time before the shelf-life of the product expires (normally 90 days at -18 °C for aerospace quality materials). Adoption of this route ensures uniformity of resin distribution in the reinforcement and eliminates the need for the processor to handle resin systems but does require that lowtemperature storage facilities are available on the production site. 9.6 Vacuum bagging The vacuum bagging system is used for producing non-critical components. Plies of thawed out and conditioned prepreg are cut into the appropriate shape either by hand or by an automated process such as a Gerber® cutter system. Plies are placed in a precise order and orientation on a tool surface. The lay-up sequence and orientation of the plies is critical to the performance of the composite. A layer of release film is laid on top of the ply layResin impregnation and prediction of fabric properties 291 RIC9 7/10/99 8:32 PM Page 291 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9
292 3-D textile reinforcements in composite materials Ply lay-up Breather cloth Bagging film Release film Vacuum port Seal Seal Tool base plate 9.2 Vacuum bagging process for the production of composites. up to prevent the resinous stack of plies from adhering to the fibrous breather cloth.This cloth is used to absorb any excess resin and distributes the applied pressure evenly over the lay-up.The complete assembly is enclosed in a sealed bag or the bagging layer is sealed to the surface of the tool surround beyond the boundaries of the component as shown in Fig. 9.2.A vacuum connector is inserted into this bagging film so that the ply stack can be consolidated under approximately one atmosphere of vacuum. This complete assembly,while still under vacuum,is placed in an oven at an elevated temperature to cure the resin system. 周网 While this route uses prepreg material,which should ensure an even dis- tribution of resin throughout the reinforcement,it is only operated at a maximum pressure of approximately 1 bar to consolidate the plies into a homogeneous'layer.This low pressure is insufficient to compact the layers adequately to produce a high performance component with high fibre volume fraction and low void content. 9.7 Autoclave For high-performance composites,high fibre volume and low void contents are essential.It is also important that distribution of both fibre and resin is uniform throughout the component.This is achieved by taking the vacuum bagging process one stage further.As previously described,prepregs in the form of unidirectional tows or woven fabrics impregnated with a partially cured resin system are used.The process follows the stages outlined in Fig. 9.3. A number of the steps in this process are similar to those used in the vacuum bagging process.In these steps care must be exercised both from the point of view of health and safety and to ensure that the lay-up is con- tamination free.Such contamination can seriously impair the performance of the composite component.A clean room is required and protective cloth-
up to prevent the resinous stack of plies from adhering to the fibrous breather cloth. This cloth is used to absorb any excess resin and distributes the applied pressure evenly over the lay-up. The complete assembly is enclosed in a sealed bag or the bagging layer is sealed to the surface of the tool surround beyond the boundaries of the component as shown in Fig. 9.2. A vacuum connector is inserted into this bagging film so that the ply stack can be consolidated under approximately one atmosphere of vacuum. This complete assembly, while still under vacuum, is placed in an oven at an elevated temperature to cure the resin system. While this route uses prepreg material, which should ensure an even distribution of resin throughout the reinforcement, it is only operated at a maximum pressure of approximately 1 bar to consolidate the plies into a ‘homogeneous’ layer. This low pressure is insufficient to compact the layers adequately to produce a high performance component with high fibre volume fraction and low void content. 9.7 Autoclave For high-performance composites, high fibre volume and low void contents are essential. It is also important that distribution of both fibre and resin is uniform throughout the component. This is achieved by taking the vacuum bagging process one stage further. As previously described, prepregs in the form of unidirectional tows or woven fabrics impregnated with a partially cured resin system are used. The process follows the stages outlined in Fig. 9.3. A number of the steps in this process are similar to those used in the vacuum bagging process. In these steps care must be exercised both from the point of view of health and safety and to ensure that the lay-up is contamination free. Such contamination can seriously impair the performance of the composite component.A clean room is required and protective cloth- 292 3-D textile reinforcements in composite materials 9.2 Vacuum bagging process for the production of composites. RIC9 7/10/99 8:32 PM Page 292 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9