Vacuum injection moulding for large structural applications

Transcription

1 Composites: Part A 34 (2003) Vacuum injection moulding for large structural applications W.D. Brouwer*, E.C.F.C. van Herpt, M. Labordus Centre of Lightweight Structures TUD-TNO, Kluyverweg 1, Delft 2629 HS, The Netherlands Abstract Resin Transfer Moulding (RTM) is a successful closed mould technology for small composite products in large series. For large products in smaller series (i.e. boat hulls or rotor blades) the Vacuum Assisted RTM is applied in order to save cost of moulds. Compared with RTM the VARTM technology is still not practised in many products today. The risk of failures in very large products is often considered too high. This paper describes ways to improve reliability and predictability of the VARTM technology in order to decrease development cost and to make the risk of failures during production as small as possible. First, appropriate methods to achieve void free processing of the composite material are described. Secondly, it is explained how the injection strategy can be optimised by using resin flow simulation software. Two examples of successful developments of large structural applications are described; a 20 m rotor blade and a 16 m long boat hull for a sailing yacht. q 2003 Published by Elsevier Science Ltd. Keywords: E. Resin transfer moulding (RTM); Vacuum injection process 1. Introduction The resin infusion processes are a group of composite processing techniques that enable the manufacture of large structures with high mechanical properties. It is a closed mould technique in which fibrous material is impregnated by a resin flow. Whereas, vacuum injection uses vacuum at the outlet side, resin transfer moulding (RTM) uses increased pressure at the inlet. Both techniques are often used for the following reasons [1]; * Corresponding author. Tel.: þ ; fax: þ address: r.brouwer@lr.tudelft.nl (W.D. Brouwer). Decreasing cost, compared to the traditional hand-layup or autoclaving the labour cost is reduced considerably. Increased mechanical properties, high fibre volumes up to 60% are possible while the composite material can be made void free. Environment and health. The closed mould technique prevents hazardous styrene emission. The potential mechanical properties allow the design and manufacture of primary structural components. At the same time, the requirements of such components are more demanding. A constant level of mechanical quality and X/03/$ - see front matter q 2003 Published by Elsevier Science Ltd. doi: /s x(03)

2 552 W.D. Brouwer et al. / Composites: Part A 34 (2003) reliability must be guaranteed. In order to assure this, the consistency of the composite compound must be perfect, or in other words, voids in the matrix of the composite must be avoided. A second technological challenge is the size of the product. Vacuum bagging makes use of the atmospheric pressure and consequently the possible size of the mould could be infinite. However, the applied resin with its viscosity and curing time will restrict this size. Also, in the case of more complex structures a well thought-out strategy is required to inject the resin and obtain a 100% impregnation. Below the latest technological developments within vacuum injection moulding are described. First the basic technology is explained and methods are discussed that enable the processing of a void free composite. Then two examples of industrial applications are discussed. The first example is a 20 m long rotor blade, where a short cycle time was an important design criterion. Secondly the manufacturing strategy of a 16 m boat hull is explained and it is shown how computer simulations can help with the design. 2. The resin injection technology RTM, Vacuum Assisted Resin Injection (VARI), SCRIMP and resin infusion are various names that describe a process, which is basically very simple: Dry reinforcement is placed in a mould, the mould is closed and resin flows into the mould and impregnates the reinforcement. The driving force for the flow of the resin is a pressure difference. To fully implement the resin injection technology one needs to understand the basic principle and the possible variations. When injecting a homogeneous material with a resin, the filling time of the injection of a rectangular strip can be calculated by using Darcy s Law (for simple line infusion): fill-time ¼ whl2 ð1þ 2kDP With w porosity of the reinforcement, k permeability of the reinforcement, h viscosity of the resin, l flow distance (length of the strip), DP applied pressure difference (constant during the injection). Eq. (1) clearly shows how different parameters influence the fill time and hence the cycle time of a product [1,3]. The parameters can be split in: Material properties Resin viscosity. The viscosity of the resin is usually limited to mpa s. The filling time is directly related to the viscosity (see Eq. (1)). Consequently there is not much to be gained in trying to optimise resin viscosity. Reinforcement porosity and permeability. The porosity of most reinforcements used is in between 0.5 and The permeability of reinforcements however, can differ greatly. This can be used to optimise fill time (also by applying additional highly permeable feeder materials). Product properties. Size, volume and shape of the structure (influencing flow distance). Process properties Pressure difference. Large products can only be manufactured cost effectively using a vacuum injection technique, since the use of pressure would require very stiff and very costly moulds. This implies that a maximum pressure difference of approximately 1 bar can be achieved. Injection strategy (influencing flow distance). 3. Towards void-free processing The quality of fibre composites is governed to a large extent by the process parameters and materials used in the manufacturing. One of the more important quality aspects is the void content in the finished part. This is due to the detrimental effect, which voids have on many properties, such as mechanical properties, dielectric properties, surface finish, etc. In other words, it is beneficial to keep the void content to a minimum. Two aspects of voids in composites have attracted most attention, namely, void formation and influence of voids on mechanical properties. Concerning void formation, the main causes are generally due to variations in permeability on a filament and filament bundle scale, outgassing of dissolved gas in the resin, evaporation of volatile components in the resin, shrinkage of the resin and leakage in connections and mould [2]. One way of reducing the void content is to use degassed resin. There are basically two advantages of using degassed resin: 1. greatly reduced risk of outgassing from the resin; 2. increased capability to dissolve bubbles in the resin that are formed during flow. Ad 1. The gas concentration at equilibrium generally increases linearly with respect to the absolute pressure

3 W.D. Brouwer et al. / Composites: Part A 34 (2003) (Henry s law), where the gas concentration at absolute vacuum is zero. Thus, for manufacturing methods where the mould is evacuated (vacuum injection, RTM, etc.), it is likely that the resin will become over-saturated with gas when exposed to a lower absolute pressure. For an epoxy resin, it can be measured that the equilibrium gas concentration of nitrogen at atmospheric pressure is approx. 1.7% by volume. These 1.7% of dissolved gas may not seem like a considerable amount of gas, but at a vacuum pressure of 20 mbar, which is commonly used in vacuum injection, this would expand 50 times if brought out of solution. Ad 2. If bubbles are formed during flow due to ill impregnation, and the resin is not yet saturated, the gas from the bubble can dissolve in the resin. The amount of gas, that can dissolve, depends on the saturation level of the resin, the diffusion speed of the gas molecules in solution in relation to the gel-time of the resin, the absolute pressure and of the characteristics of the gas-resin system. The standard procedure to degas resin is simply to expose the resin to partial vacuum. The idea is to make use of the fact that the gas solubility decreases as the pressure is reduced (Henry s law), where at absolute vacuum the gas solubility is zero. So, if the pressure is decreased, at a certain moment, the resin will become over-saturated and gas should come out of solution. But the dissolved gas is dispersed as molecules and not as bubbles. Therefore, gas will only come out of solution if bubbles or bubble nuclei are already present in the resin. What in fact will happen when the pressure is reduced, is that the bubbles, which have been whipped in during mixing of the resin, will increase in size (Gas Law). With increasing size, the rising speed of the bubbles also increases (Archimedes Law). This will result in a foaming resin, suggesting that the resin is being degassed. In fact, the resin is mainly de-bubbled! Of course, some of the dissolved gas will indeed diffuse into these rising bubbles which resulted from mixing or pouring the resin in a different container thereby entrapping air in scratches or imperfections in the container. If no bubbles or bubble nucleation sites have been added, the standard degassing procedure will not cause any outgassing at all. So, standard degassing is highly questionable if performed by simply reducing the pressure. If no bubble nucleation sites or bubbles are present there will not be any outgassing. Two other methods for degassing the resin are more effective: Adding nucleation material. Vacuum injection experiments with different fibre reinforcement materials have shown quite a different void content and void distribution. The main cause of voids with these experiments was outgassing of the resin. Apparently, some reinforcement materials (like Unifiloe) exhibit better bubble nucleation properties than other materials, and therefore will result in laminates with a higher void content. This material contains bubble nuclei as entrapped air in cavities. Fig. 1. A current LM wind turbine with rotor blades of approx. 20 m long. The difference in gas concentration between the entrapped air and the dissolved gas causes gas molecules to diffuse from the resin into the entrapped air. So the trick is to bring the resin into contact with such a nucleation material at reduced pressure prior to the actual injection. This leads to a much more effective degassing procedure and a better laminate quality during and after injection. Materials that have been proven to be very effective are Scotch Brite and Unifilo glass mat. Sparging. A container is filled with resin. The pressure in this container is reduced to a pressure below the injection pressure to be used during the vacuum injection process. At the bottom, air is fed into this container. The air is forced through a very fine filter, thus creating many small bubbles. These bubbles rise through the resin. At the reduced pressure, the resin will be over-saturated with air (or components of air). The difference in gas concentration between the air bubble and the dissolved gas causes gas molecules to diffuse from the resin into the bubble. This process continues until a new equilibrium situation is reached, e.g. the resin is saturated (but no longer oversaturated) with air. If degassed resin (either by adding a nucleation material during degassing or by sparging) is used during the vacuum Fig. 2. A worker inspecting the root of the rotor blade.

4 554 W.D. Brouwer et al. / Composites: Part A 34 (2003) Example 1. The 20 m rotor blade Fig. 3. The root of the rotor blade just before the vacuum injection process starts. injection process, and the injection pressure is higher than the degassing pressure, there is no risk of outgassing of the resin. There will even be the possibility of dissolving some bubbles, which have been formed during the flow of resin, entrapping air in fibre bundles. The competition between composite rotor blade manufacturers force them to highly optimise engineering and production procedures. The time-to-market of new types of rotor blades is very short. Every year the blade s length is increased by several meters reacting on the market demands for higher energy output. Current LM Glassfibre Holland rotor blades are shown in Fig. 1. The turbine has three rotor blades with a length of approximately 20 m. Fig. 2 shows a worker inspecting the joint of the root to the axle. The rotor blades shown in Figs. 1 and 2 are made of two shells consisting of 1200 kg vinyl-ester resin, 1300 kg of glass fibre reinforcement (approx. 75% unidirectional and 25% ^ 458 fabrics) and PVC foam core. The shells are bonded together with adhesives. Optimisation of the production processes is necessary due to required short cycle times and the protection of workshop personnel and the environment; the allowed emission of styrene from vinyl-ester resins (conventionally Fig. 4. One rotor blade halve during vacuum injection. Fig. 5. Cross-section of the wind turbine sandwich.

5 W.D. Brouwer et al. / Composites: Part A 34 (2003) Fig. 6. The injection strategy and position of the injection channels. processed with open mould techniques) has recently been decreased in the Netherlands. So, sound and fast production is the driving force for innovative production methods like vacuum injection. Two years ago LM and Centre of Lightweight Structures TUD-TNO co-operated in a project that aimed an industrial implementation of the vacuum injection technique into the workshops of the composites processing company [4]. Key issue in this implementation was the choice of the right processing strategy in combination with the materials used. For a product of this size this determines whether a reliable and quick manufacture can be assured. Fig. 3 shows the root of the rotor blade (the part nears to the axle) just before the vacuum injection process is started. The glass fibre reinforcement (the white material along the edges) lies in the mould; it is covered successively with a release fabric (not visible), a flow enhancing knitted fabric, a runner and a nylon film. The film is sealed on the mould flanges with tacky tape. A vacuum is applied between mould and film, so the complete lay-up is compressed in the mould. A liquid vinyl-ester resin will be flowing from a resin container though the runner in the length of the mould. The resin then flows through the flow enhancing knitted fabric into the reinforcement while impregnating the fibre bundles completely. This process is shown in Fig. 4 in which the two flow fronts are visible on both sides of the runner. In the left bottom corner the sealing is visible. The filling of the mould takes approximately 1 h. After the cure of the resin the rotor blade halve can be released from the mould. Fig. 5 shows a detail of the sandwich part in the rotor blade. The sandwich structure is left in the rotor blade of Fig. 4, between the flow front and the left mould edge. So, the resin is just entering the foam core. The core consists of small square blocks of PVC foam on a fabric back. Consequently, small channels are present between the blocks in which the resin can flow rapidly. These channels thus support fast mould filling. 5. Example 2. Injection strategies for a boat structure The main part of a boat will be discussed in detail: the hull-shell. The Contest 55 hull is both a long (16.4 m), wide (4.5 m) and high (2.5 m) product to inject in one shot. The height results in a hydrostatic pressure difference which results in a lower driving force for the injection. There are three basic variations possible on the edge injection strategy to distinguish: Fig. 7. Part of the hull with dry spot formation at the location of the keel.

6 556 W.D. Brouwer et al. / Composites: Part A 34 (2003) Fig. 8. Part of the hull without dry spot formation. 1. Edge injection downward 2. Edge injection upward 3. Edge injection sideways The injection downward is not preferred for two reasons. Firstly, bubbles in the resin and laminate will be entrapped more easily and secondly, there is higher risk on the occurrence of dry spots due to racetracking of the resin through highly permeable runner channels. The injection upward can only be carried out with sequential injection channels (otherwise injection time will be too long). This however, would result in about 10 sequential injection channels each of which need to be controlled and opened at the right time. Based on the requirement to have a simple process there was a clear preference for an injection with a grid of injection channels in which the majority of the channels would allow for a sideways injection. This would allow for one single resin inlet port (and consequently also one resin tank level to control). This grid would consist of a main injection channel running from the stern to the bow (via the keel) and branches of channels from the main channel in the keel to the flange Fig. 10. Disturbance of resin flow-front due to a runner channel perpendicular to the flow-front. (with the deck). The overall injection channel pattern is shown in Fig. 6 (from the Dutch mould filling simulation software RTM-worx, described in Ref [5]). Based on the overall chosen injection strategy we must verify that there are no local details, which can cause the formation of dry spots or else, disturb the injection. The Balsa core was sufficiently permeable. Therefore, there was no additional feeder material required on all laminates containing Balsa. However, the laminate used in the keel would have a low overall permeability. In addition there is a substantial thickness change. This could cause dry spots on the keel, which were clearly shown using flow-front simulations. Figs. 7 and 8 show the simulations done without (Fig. 7) and with (Fig. 8) additional feeder material on top of the reinforcement. A small scale experiment was carried out to validate this simulation result. A small part of the hull laminate was injected on a flat plate. This part consisted of the keel laminate (partly with and partly without additional feeder material) and the balsa sandwich laminate shown in Fig. 9. It was injected using a main injection Fig. 9. Test injection set up and resulting flow front. Fig. 11. No disturbance of resin flow-front due to a runner channel parallel to the flow-front.

7 W.D. Brouwer et al. / Composites: Part A 34 (2003) Fig. 12. Half way the injection process, white areas are dry glass, dark brown is already impregnated. Fig. 13. Release from the mould. channel at the edge of the keel laminate and an injection channel branch in the middle of the laminate. The carefully registered flow-front proved the occurrence of a dry spot on the part without the feeder material and the absence of a dry spot on the part with the additional feeder material. To determine the injection strategy of a part with many edges one must consider that every edge can lead to a highly permeable channel, causing resin racetracking. The least and most favourable situation will occur when the resin flow-front is progressing perpendicular and, respectively, parallel to the runner channel [6]. This is shown in Figs. 10 and 11. When the resin flow-front is perpendicular with the runner channel the flow-front will be disturbed and the resin will flow much faster through the runner channel (shown in Fig. 10). In Fig. 11 it can be clearly seen that a runner channel, parallel to the flow-front will not cause disturbances of the resin flow front. When using simulation software to identify the filling pattern of a complex structure one must realise that runner channels can occur. This should be included in the simulation of the filling pattern. Below the actual injection of the boat hull and the release from the mould are shown (Fig. 12). The injection of the complete hull takes about 2 h. Then, after curing, the hull is released from the mould. This important moment is shown in Fig. 13. More on boat hull manufacture can be found in Refs. [7,8]. 6. Conclusions Large structural composite components can be manufactured successfully with the Vacuum Assisted RTM technique. It is a clean and closed mould technique that allows the manufacture of complex composites structures, with high quality (low void content) and high fibre volume

8 558 W.D. Brouwer et al. / Composites: Part A 34 (2003) content. The feasibility of an injection of very large structures with the VA-RTM process is mainly determined by the following aspects: (1) the geometry of the product, (2) the materials used in the product, (3) injection tooling and materials, (4) the injection strategy. The first two cannot be changed a lot because it is prescribed by the structural design. But with the right tooling and strategy it has been proven that very large and complex structures can be manufactured in a one shot vacuum infusion process. A resin flow simulation program, such as RTM-worx, is an essential tool within the strategy design. Acknowledgements The research and development presented in Chapter 5 was funded by the Netherlands Ministry of Economical Affairs of the Netherlands (project code SMO97106). Much of the practical work has been carried out by dedicated employees of Conyplex under the supervision of Han van Aalst and Cor van Zantvliet. This work was not possible without the close co-operation with the Netherlands shipyards Friesland Polyester and Standfast Yachts, resin supplier Reichhold and CVK. References [1] Hoebergen JA, Holmberg. Vacuum infusion. ASM handbook, vol. 21, composites; [2] Labordus M. Voids and bubbles during vacuum infusion. Tech pap: Soc Manufact Engrs 2001; EM [3] Labordus M, Verheus A. EUCLID RTP 3.15 Resin Transfer Moulding (RTM) for aerospace structural component in-plane permeability tests. EUC-DOC CLC. [4] Hoebergen. Process control of vacuum injection techniques for large parts in small series. Final Technical Report of VITCON, EC-Craft Project BRST-CT [5] Koorevaar. Simulation of injection moulding. SAMPE 2002 Proceedings, Paris; [6] Gebart BR, Gudmundson P, Lundemo CY. An evaluation of Alternative Injection Strategies in RTM. 47th Annual Conference, Composites Institute, The Society of Plastics Industry; [7] Martens H. Sterker door Infuus-Conyplex Bouwt met Vacuüminjectie. Waterkampioen 15; Issued by ANWB Netherlands. [8] Hoebergen. The practical application of the vacuum injection technique in boat building. 15th International Symposium on Yacht design and construction, Amsterdam; November p