54 3-D textile reinforcements in composite materials development goals.A detailed description of the various textile technol- ogies can be found in Chapter 1. Impregnating the complex shaped 3-D fibre structures applies for suit- able infiltration and consolidation techniques in order to obtain high and constant fibre volume fractions with low void content during the composite manufacturing process. In general,the manufacturing methods used in both aerospace and non- aerospace applications are quite different.While autoclave prepreg tech- nology is the most important technique for aerospace components,injection moulding and pressing techniques(SMC,GMT)are used for high-volume applications.The reason is that the autoclave prepreg technique results in large fibre volume fractions(typically 60%)and high performance.On the other hand,there is a penalty in the form of extremely high cycle times, typically lasting several hours.SMC,GMT and injection moulding tech- niques allow cycle times of less than one minute.On the other hand,fibre volume fractions are relatively poor (typically 30%). In combination with textile preforms,the RTM process (resin transfer moulding)is of special interest.In this process,a resin is pressed under vacuum into a closed mould where the fibre preform is fixed.The achiev- able fibre volume fractions amount to more than 50%,while cycle times of less than 10 minutes can be realized with appropriate resin systems. Although the density of the 3-D fibre structures can be very high,the impregnation speed is more or less higher compared with conventional 2-D structures.The reason for this effect is that the additional fibres in thickness direction form 'flow channels'which support resin transfer through the thickness. The continuous pultrusion process is of greatest interest for the impreg- 8 nation of profile-shaped fibre preforms with a constant cross-section.Inter- esting developments are performed,especially in combination with 3-D braiding. The most important impregnation techniques are summarized in Fig.2.7. In general,thermoplastic matrix composites offer a high potential for realizing short cycle times because no chemical reaction has to take place in the mould,and quick hot-forming techniques,comparable to the press- ing of metal sheets,can be applied.On the other hand,thermoplastics gen- erally require higher temperatures and pressure,and thus more expensive tooling and higher energy consumption.This is especially true of PEEK, the only thermoplastic matrix material for aerospace structural components with a melting point of 400C. The use of hybrid structures,consisting of reinforcing fibres and thermo- plastic fibres,is of special interest in combination with textile technologies. According to the level of fibre mixture,the process is called commingling, or co-weaving (or co-braiding).In the commingling process,the com-development goals. A detailed description of the various textile technol￾ogies can be found in Chapter 1. Impregnating the complex shaped 3-D fibre structures applies for suit￾able infiltration and consolidation techniques in order to obtain high and constant fibre volume fractions with low void content during the composite manufacturing process. In general, the manufacturing methods used in both aerospace and non￾aerospace applications are quite different. While autoclave prepreg tech￾nology is the most important technique for aerospace components, injection moulding and pressing techniques (SMC, GMT) are used for high-volume applications. The reason is that the autoclave prepreg technique results in large fibre volume fractions (typically 60%) and high performance. On the other hand, there is a penalty in the form of extremely high cycle times, typically lasting several hours. SMC, GMT and injection moulding tech￾niques allow cycle times of less than one minute. On the other hand, fibre volume fractions are relatively poor (typically 30%). In combination with textile preforms, the RTM process (resin transfer moulding) is of special interest. In this process, a resin is pressed under vacuum into a closed mould where the fibre preform is fixed. The achiev￾able fibre volume fractions amount to more than 50%, while cycle times of less than 10 minutes can be realized with appropriate resin systems. Although the density of the 3-D fibre structures can be very high, the impregnation speed is more or less higher compared with conventional 2-D structures. The reason for this effect is that the additional fibres in thickness direction form ‘flow channels’ which support resin transfer through the thickness. The continuous pultrusion process is of greatest interest for the impreg￾nation of profile-shaped fibre preforms with a constant cross-section. Inter￾esting developments are performed, especially in combination with 3-D braiding. The most important impregnation techniques are summarized in Fig. 2.7. In general, thermoplastic matrix composites offer a high potential for realizing short cycle times because no chemical reaction has to take place in the mould, and quick hot-forming techniques, comparable to the press￾ing of metal sheets, can be applied. On the other hand, thermoplastics gen￾erally require higher temperatures and pressure, and thus more expensive tooling and higher energy consumption. This is especially true of PEEK, the only thermoplastic matrix material for aerospace structural components with a melting point of 400 °C. The use of hybrid structures, consisting of reinforcing fibres and thermo￾plastic fibres, is of special interest in combination with textile technologies. According to the level of fibre mixture, the process is called commingling, or co-weaving (or co-braiding). In the commingling process, the com- 54 3-D textile reinforcements in composite materials RIC2 7/10/99 7:25 PM Page 54 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:29:45 AM IP Address: 158.132.122.9