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Autoclave forming of thermoplastic composite parts
Journal of Materials Processing Technology 143–144 (2003) 266–269
Autoclave forming of thermoplastic composite parts I. Fernández∗ , F. Blas, M. Frövel INTA Composites Department, Ctra. Ajalvir, km. 4 Torrejón de Ardoz, Madrid 28850, Spain
Abstract Thermoplastic advanced composites are of great interest for meeting the needs of new challenges in the aerospace industry. Thermoplastics offer a number of important advantages over thermosets like better toughness and damage tolerance, rapid fabrication cycle and the possibility of assembling substructures by welding. Progress has been made by INTA in setting up the autoclave forming technologies for the manufacturing of thermoplastic composite parts. The standard autoclaving and vacuum bagging techniques, which are well developed for the production of thermoset composite components, have been adapted to the high-temperature processing of thermoplastics. Some demonstrators based on actual aeronautical skin substructures have been fabricated. Present paper summarises the results obtained in this work. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Composites; Thermoplastic matrix; Autoclave forming
1. Introduction Thermoplastic advanced composites are of great interest for meeting the needs of new challenges in the aerospace industry. Thermoplastics offer a number of important advantages over thermosets like better toughness and damage tolerance, rapid fabrication cycle and the possibility of assembling substructures by welding [1]. In the last few years a new interest on thermoplastic composites is arising in the European aeronautic industry. Some Airbus parts are being now manufactured in thermoplastics and new programs like the A380 and the A400M allow expecting an important market for these new materials [2,3]. Several manufacturing techniques are employed for thermoplastic advanced composites. One of these manufacturing techniques is autoclave forming. Autoclave forming is quite suitable for manufacturing large and complex parts. It also allows co-consolidation and manufacturing parts with local reinforcement in only one forming cycle [4]. It follows a description of the work done by the INTA Composites Department in autoclave forming of thermoplastic composites. This work is enclosed inside a collaboration program between the Spanish aeronautical industry, represented by EADS-CASA and two R&D centres, INTA and the Polytechnical University of Madrid. The main aim of
∗ Corresponding author. Tel.: +34-915292178; fax: +34-915201274. E-mail address: [email protected] (I. Fern´andez).
this collaboration program is setting up the manufacturing technologies related to thermoplastic advanced composites [5].
2. Autoclave forming The essence of thermoplastic composites processing differs radically from thermoset composites processing. While the latter involves a chemical reaction, the former only involves physical changes in the matrix. Nevertheless, in the tuning up of thermoplastics autoclave forming technologies advantage has been taken of the large experience of INTA Composites Department in thermosets processing. Autoclave forming of thermoplastic composites requires the use of high processing temperatures, generally above 300 ◦ C. As a result, a high-temperature autoclave is needed. One of the autoclaves in INTA facilities was found suitable for thermoplastics forming being able to operate up to 650 ◦ C and up to 70 bar. Its main dimensions are 3.5 m in length and 1.5 m in diameter (see Fig. 1). 2.1. Materials The thermoplastic composites employed in this work are based on a polyphenyl sulphide (PPS) matrix. The use of this PPS matrix is quite interesting because of its resistance to most of the chemical solvents as a result of its semicrystalline structure. Two different kinds of reinforcement fibres are used, CETEX® T300 carbon fabric and CETEX®
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00309-1
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
Fig. 1. High-temperature autoclave (650 ◦ C and 70 bar) in INTA Composites Department facilities.
E-glass fabric. Materials designations are CD0286/8538 for the carbon/PPS composite and SS0303/8538 for the glass/PPS composite. The composite is supplied from Ten Cate Advanced Composites in a form called semipreg. In the semipreg the thermoplastic resin is laminated to the reinforcement without really impregnating the fibres (see Fig. 2). The semipreg consolidation process is therefore similar to a resin film infusion process. Two types of semipreg are employed: single-sided and double-sided semipreg. In the last one the resin is laminated to both sides of the reinforcement whilst in the first one the resin is laminated to only one side of the reinforcement. 2.2. Steps in manufacturing As already mentioned, advantage has been taken of the large experience of INTA Composites Department in thermosets processing for developing thermoplastics forming techniques. In fact, similar manufacturing techniques have been used mainly consisting in semipreg lay-up and vac-
Fig. 2. PPS/carbon and PPS/glass fabric semipreg.
267
Fig. 3. Double-sided semipreg plies automatically cut.
uum bag processing. It follows a description of the different manufacturing stages that have been followed. The semipreg plies are manually or automatically cut depending on the complexity of the design and the number of plies that need to be cut. In both cases a knife cutter is employed obtaining satisfactory results. The cutting machine (see Fig. 3) allows directly working with common design files such as CATIA or AUTOCAD files. After cutting the plies the semipreg is laid up. As the thermoplastics have no tack, the pre-cut plies are spot welded together by locally melting the thermoplastic matrix. For this operation the double-sided semipreg is preferred although it is also possible to spot weld the single-sided semipreg plies. If the part has a certain degree of complexity it can help to preform the already laid-up semipreg. The preform is done at a temperature between the softening temperature and the first crystallisation temperature of the matrix. As the softening temperature is quite low for this kind of materials, 90 ◦ C for PPS, preforming is straightforward. Fig. 4 shows a preform obtained for a leading edge skin panel. The high-temperature vacuum bag is composed of the same elements as a conventional low-temperature vacuum
Fig. 4. Preform for a leading edge skin panel made from PPS/carbon fibre semipreg.
268
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
bag and it is also constructed in a similar way. The main difference between both the cases is the nature of the ancillary materials employed. High-temperature auxiliary materials, in particular polyimide bagging and high-temperature sealants, are more difficult to work with. Also the risk of failure of this kind of vacuum bag is bigger than usual and special care must be taken during its construction and also during the processing cycle. Once the high-temperature vacuum bag is constructed, the material is heated above the resin melting temperature and then consolidated at a pressure between 6 and 10 bar. As no chemical reaction takes place during this process, typical autoclave-cycle times for thermoplastic composites are much shorter than the ones for thermoset composites processing. For the parts manufactured in this work cycle times were about 90 min. When working with semicrystalline thermoplastic resins it has to be considered that the cooling rate from the consolidating temperature is a critical parameter of the forming cycle. The cooling rate has to be tailored in such a way that an acceptable degree of crystallinity is obtained in the absence of microcracking. Differential scanning calorimetry and ultrasonic inspection are employed for inspecting the resulting parts. 2.3. Results Progress has been made in the setting up of the autoclave forming technology by means of different kinds of demonstrators based on real aeronautical substructures. One of those demonstrators was a small-sized (110 mm × 220 mm) double-hat stiffened panel (see Fig. 5). The semipreg plies for the stiffeners and panel were laid up at the same time and consolidated in only one autoclave cycle. As it was mentioned before, co-consolidation is one of the interesting possibilities offered by autoclave forming. It reduces the number of processing cycles needed from three— consolidating and forming the stiffeners, consolidating the panel, bonding or welding the stiffeners to the panel to only one—consolidating all parts together in a unique fabrication step. Moreover, co-consolidation allows obtaining a high-quality joint between the stiffeners and the panel.
Fig. 5. Double-hat stiffened panel demonstrator obtained by autoclave co-consolidation techniques.
Fig. 6. Leading edge skin demonstrator based on current metallic parts.
Work has also been done in leading edge skins. The first step was an 800 mm long leading edge skin based on current metallic parts. The design of the thermoplastic part was extremely simplified in order to get in touch with the problems arising in the fabrication of this kind of parts. For this purpose the ply-sequence was simplified and there were no local reinforcing layers in this leading edge skin. Fig. 6 shows the result obtained after setting up the processing techniques for this specific part. A second step was the manufacturing of a leading edge skin based on current design for a thermoset composite leading edge skin in the A340-600 elevator. The thermoset original part is reinforced with a complex architecture of position plies. A similar design for thermoplastic composite was proposed. As already mentioned, one of the advantages of autoclave forming is that it allows manufacturing locally reinforced parts in only one fabrication cycle. Fig. 7 shows two different demonstrators for this specific part. Fig. 7a shows a leading edge with simplified stacking sequence and
Fig. 7. PPS/carbon fibre demonstrators for the A340-600 elevator leading edge skin: (a) simplified part with no local reinforcement; (b) locally reinforced skin.
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
no local reinforcement. Fig. 7b shows the locally reinforced part.
269
skins with local reinforcement, have been obtained by autoclave forming. Further effort is needed in order to minimise fabrication costs.
3. Conclusions References INTA Composites Department has settled down the first steps for manufacturing high-performance thermoplastic matrix composite parts. Work has been done in skin manufacturing by autoclave forming. Very promising results have been obtained in the fabrication of skins demonstrators based on current aeronautical substructures. Integrated structures, i.e. hat stiffened panels, have been obtained by autoclave co-consolidation. Complex parts, like
[1] A. Offringa, J. Adv. Mater. July (1997) 2. [2] D. Myers, A. Offringa, A. Buitenhuis, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001. [3] J. Pora, J. Hinrichsen, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001. [4] S. Olson, Compos. Manuf. (1991) 95. [5] I. Fernández, M. Frövel, L. Rubio, J. Diaz, I. Gonzalez, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001.
Autoclave forming of thermoplastic composite parts I. Fernández∗ , F. Blas, M. Frövel INTA Composites Department, Ctra. Ajalvir, km. 4 Torrejón de Ardoz, Madrid 28850, Spain
Abstract Thermoplastic advanced composites are of great interest for meeting the needs of new challenges in the aerospace industry. Thermoplastics offer a number of important advantages over thermosets like better toughness and damage tolerance, rapid fabrication cycle and the possibility of assembling substructures by welding. Progress has been made by INTA in setting up the autoclave forming technologies for the manufacturing of thermoplastic composite parts. The standard autoclaving and vacuum bagging techniques, which are well developed for the production of thermoset composite components, have been adapted to the high-temperature processing of thermoplastics. Some demonstrators based on actual aeronautical skin substructures have been fabricated. Present paper summarises the results obtained in this work. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Composites; Thermoplastic matrix; Autoclave forming
1. Introduction Thermoplastic advanced composites are of great interest for meeting the needs of new challenges in the aerospace industry. Thermoplastics offer a number of important advantages over thermosets like better toughness and damage tolerance, rapid fabrication cycle and the possibility of assembling substructures by welding [1]. In the last few years a new interest on thermoplastic composites is arising in the European aeronautic industry. Some Airbus parts are being now manufactured in thermoplastics and new programs like the A380 and the A400M allow expecting an important market for these new materials [2,3]. Several manufacturing techniques are employed for thermoplastic advanced composites. One of these manufacturing techniques is autoclave forming. Autoclave forming is quite suitable for manufacturing large and complex parts. It also allows co-consolidation and manufacturing parts with local reinforcement in only one forming cycle [4]. It follows a description of the work done by the INTA Composites Department in autoclave forming of thermoplastic composites. This work is enclosed inside a collaboration program between the Spanish aeronautical industry, represented by EADS-CASA and two R&D centres, INTA and the Polytechnical University of Madrid. The main aim of
∗ Corresponding author. Tel.: +34-915292178; fax: +34-915201274. E-mail address: [email protected] (I. Fern´andez).
this collaboration program is setting up the manufacturing technologies related to thermoplastic advanced composites [5].
2. Autoclave forming The essence of thermoplastic composites processing differs radically from thermoset composites processing. While the latter involves a chemical reaction, the former only involves physical changes in the matrix. Nevertheless, in the tuning up of thermoplastics autoclave forming technologies advantage has been taken of the large experience of INTA Composites Department in thermosets processing. Autoclave forming of thermoplastic composites requires the use of high processing temperatures, generally above 300 ◦ C. As a result, a high-temperature autoclave is needed. One of the autoclaves in INTA facilities was found suitable for thermoplastics forming being able to operate up to 650 ◦ C and up to 70 bar. Its main dimensions are 3.5 m in length and 1.5 m in diameter (see Fig. 1). 2.1. Materials The thermoplastic composites employed in this work are based on a polyphenyl sulphide (PPS) matrix. The use of this PPS matrix is quite interesting because of its resistance to most of the chemical solvents as a result of its semicrystalline structure. Two different kinds of reinforcement fibres are used, CETEX® T300 carbon fabric and CETEX®
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00309-1
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
Fig. 1. High-temperature autoclave (650 ◦ C and 70 bar) in INTA Composites Department facilities.
E-glass fabric. Materials designations are CD0286/8538 for the carbon/PPS composite and SS0303/8538 for the glass/PPS composite. The composite is supplied from Ten Cate Advanced Composites in a form called semipreg. In the semipreg the thermoplastic resin is laminated to the reinforcement without really impregnating the fibres (see Fig. 2). The semipreg consolidation process is therefore similar to a resin film infusion process. Two types of semipreg are employed: single-sided and double-sided semipreg. In the last one the resin is laminated to both sides of the reinforcement whilst in the first one the resin is laminated to only one side of the reinforcement. 2.2. Steps in manufacturing As already mentioned, advantage has been taken of the large experience of INTA Composites Department in thermosets processing for developing thermoplastics forming techniques. In fact, similar manufacturing techniques have been used mainly consisting in semipreg lay-up and vac-
Fig. 2. PPS/carbon and PPS/glass fabric semipreg.
267
Fig. 3. Double-sided semipreg plies automatically cut.
uum bag processing. It follows a description of the different manufacturing stages that have been followed. The semipreg plies are manually or automatically cut depending on the complexity of the design and the number of plies that need to be cut. In both cases a knife cutter is employed obtaining satisfactory results. The cutting machine (see Fig. 3) allows directly working with common design files such as CATIA or AUTOCAD files. After cutting the plies the semipreg is laid up. As the thermoplastics have no tack, the pre-cut plies are spot welded together by locally melting the thermoplastic matrix. For this operation the double-sided semipreg is preferred although it is also possible to spot weld the single-sided semipreg plies. If the part has a certain degree of complexity it can help to preform the already laid-up semipreg. The preform is done at a temperature between the softening temperature and the first crystallisation temperature of the matrix. As the softening temperature is quite low for this kind of materials, 90 ◦ C for PPS, preforming is straightforward. Fig. 4 shows a preform obtained for a leading edge skin panel. The high-temperature vacuum bag is composed of the same elements as a conventional low-temperature vacuum
Fig. 4. Preform for a leading edge skin panel made from PPS/carbon fibre semipreg.
268
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
bag and it is also constructed in a similar way. The main difference between both the cases is the nature of the ancillary materials employed. High-temperature auxiliary materials, in particular polyimide bagging and high-temperature sealants, are more difficult to work with. Also the risk of failure of this kind of vacuum bag is bigger than usual and special care must be taken during its construction and also during the processing cycle. Once the high-temperature vacuum bag is constructed, the material is heated above the resin melting temperature and then consolidated at a pressure between 6 and 10 bar. As no chemical reaction takes place during this process, typical autoclave-cycle times for thermoplastic composites are much shorter than the ones for thermoset composites processing. For the parts manufactured in this work cycle times were about 90 min. When working with semicrystalline thermoplastic resins it has to be considered that the cooling rate from the consolidating temperature is a critical parameter of the forming cycle. The cooling rate has to be tailored in such a way that an acceptable degree of crystallinity is obtained in the absence of microcracking. Differential scanning calorimetry and ultrasonic inspection are employed for inspecting the resulting parts. 2.3. Results Progress has been made in the setting up of the autoclave forming technology by means of different kinds of demonstrators based on real aeronautical substructures. One of those demonstrators was a small-sized (110 mm × 220 mm) double-hat stiffened panel (see Fig. 5). The semipreg plies for the stiffeners and panel were laid up at the same time and consolidated in only one autoclave cycle. As it was mentioned before, co-consolidation is one of the interesting possibilities offered by autoclave forming. It reduces the number of processing cycles needed from three— consolidating and forming the stiffeners, consolidating the panel, bonding or welding the stiffeners to the panel to only one—consolidating all parts together in a unique fabrication step. Moreover, co-consolidation allows obtaining a high-quality joint between the stiffeners and the panel.
Fig. 5. Double-hat stiffened panel demonstrator obtained by autoclave co-consolidation techniques.
Fig. 6. Leading edge skin demonstrator based on current metallic parts.
Work has also been done in leading edge skins. The first step was an 800 mm long leading edge skin based on current metallic parts. The design of the thermoplastic part was extremely simplified in order to get in touch with the problems arising in the fabrication of this kind of parts. For this purpose the ply-sequence was simplified and there were no local reinforcing layers in this leading edge skin. Fig. 6 shows the result obtained after setting up the processing techniques for this specific part. A second step was the manufacturing of a leading edge skin based on current design for a thermoset composite leading edge skin in the A340-600 elevator. The thermoset original part is reinforced with a complex architecture of position plies. A similar design for thermoplastic composite was proposed. As already mentioned, one of the advantages of autoclave forming is that it allows manufacturing locally reinforced parts in only one fabrication cycle. Fig. 7 shows two different demonstrators for this specific part. Fig. 7a shows a leading edge with simplified stacking sequence and
Fig. 7. PPS/carbon fibre demonstrators for the A340-600 elevator leading edge skin: (a) simplified part with no local reinforcement; (b) locally reinforced skin.
I. Fern´andez et al. / Journal of Materials Processing Technology 143–144 (2003) 266–269
no local reinforcement. Fig. 7b shows the locally reinforced part.
269
skins with local reinforcement, have been obtained by autoclave forming. Further effort is needed in order to minimise fabrication costs.
3. Conclusions References INTA Composites Department has settled down the first steps for manufacturing high-performance thermoplastic matrix composite parts. Work has been done in skin manufacturing by autoclave forming. Very promising results have been obtained in the fabrication of skins demonstrators based on current aeronautical substructures. Integrated structures, i.e. hat stiffened panels, have been obtained by autoclave co-consolidation. Complex parts, like
[1] A. Offringa, J. Adv. Mater. July (1997) 2. [2] D. Myers, A. Offringa, A. Buitenhuis, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001. [3] J. Pora, J. Hinrichsen, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001. [4] S. Olson, Compos. Manuf. (1991) 95. [5] I. Fernández, M. Frövel, L. Rubio, J. Diaz, I. Gonzalez, in: Proceedings of the 22nd SAMPE Europe International Conference, Paris, 2001.