Abstract:
A mold and method for vacuum assisted resin transfer molding of a fiber reinforced laminated structure are provided. The mold includes a first mold part and a second mold part. The first mold part defines a negative impression of the laminated structure, being structurally stable and forming a support for fiber reinforcement layers of the laminated structure. The second mold part connectable to the first mold part for closing the mold and defines together with the first mold part an enclosed space which can be evacuated. The mold further includes a flow duct for guiding a liquid polymer which is formed as a recess in the first mold part and/or a recess in the second mold part that is open towards the enclosed space and extends along a section of the periphery of the first mold part and/or the second mold part.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of Ser. No. 12/383,959 filed Mar. 31, 2009 now U.S. Pat. No. 7,980,840, which claims priority of European Patent Office application No. 08006813.3 EP filed Apr. 3, 2008. All of the applications are incorporated by reference herein in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a mould for a vacuum assisted resin transfer moulding of a fibre laminated structure, in particular of a wind turbine rotor blade, and to a method for vacuum assisted resin transfer moulding. 
     BACKGROUND OF INVENTION 
     Vacuum assisted resin transfer moulding (VARTM) of large structures has to date involved in the inclusion of conduits for resin distribution into the moulded structure. Examples therefore are disclosed in EP 1 310 351 B1, WO 2006/058540 A1, WO 2006/058541 A1, and WO 2007/038930 A1. 
     EP 1 310 351 B1 describes a mould for wind turbine rotor blades which comprises a lower mould part, an upper mould part, and a core. For forming a laminated rotor blade, fibre reinforcement layers are layered in the lower mould part and the core is put atop on the layers. Layer sections projecting over the lower mould are then wound around the core so as to overlap in the middle of the core and then the upper mould part is set atop on these layers. When the leading edge of the layer stack is formed a flow pipe is integrated into the stack of fibre reinforcement layers so as to allow for distributing resin to the layers. This flow pipe will be part of the laminated wind turbine rotor blade after curing the resin. 
     WO 2006/058540 A1 describes a vacuum infusion method by means of a semi-permeable membrane. In this method, a mould is applied with a mould cavity, in which a fibre insertion is placed which comprises a plurality of fibre layers and a distribution layer. The distribution layer allows for a higher rate of flow for a liquid polymer than the fibre layers. Inlet channels are located atop of the fibre insertion. The liquid polymer is directed via the inlet channels to the distribution layer after the mould has been closed by use of a vacuum bag. 
     WO 2006/058541 A1 describes a method and an apparatus for producing fibre composite mouldings by means of vacuum infusion. In the method, a mould with a mould cavity, a plurality of inlet channels communicating with the mould cavity, and a polymer source with liquid polymer is applied. A plurality of vacuum channels communicates with the mould cavity and a vacuum source. Fibre material is inserted into the mould cavity prior to the process of filling said mould, and where a reduced pressure is generated in the vacuum channels and thus in the mould cavity with the result that liquid polymer is drawn from the polymer source via the inlet channels into the mould cavity. One or more of the inlet channels can also communicate with the vacuum source and/or one or more of the vacuum channels can also communicate with a polymer source. 
     WO 2007/038930 A1 describes a method for producing a fibre reinforced product. In the method, one or more layers of reinforcing fibres are placed into the cavity of a mould. The layers are covered with a resin distribution member over which a number of resin inlets which are formed by pipes are placed. The resin distribution member can be removed after producing the laminate. Alternatively, the resin distribution member and the inlets can remain part of the laminate. If the resin distribution system, i.e. the inlet pipes and/or the resin distribution layer, remains in the laminated structure, it adds to weight without providing a substantial structural benefit. On the other hand, if the resin distribution layer is removed from the laminated structure, this is an elaborate process since the distribution layer is provided on a large fraction of the finished structure&#39;s surface area. 
     SUMMARY OF INVENTION 
     It is therefore a first objective of the present invention to provide an advantageous means for producing fibre reinforced laminated structures. It is a second objective of the present invention to provide an advantageous method for vacuum assisted resin transfer moulding of a fibre reinforced laminated structure. 
     The first objective is solved by a mould for vacuum assisted resin transfer moulding of a fibre reinforced laminated structure. The second objective is solved by a method for vacuum assisted resin transfer moulding. The depending claims contain further developments of the invention. 
     According to the invention, a mould for vacuum assisted resin transfer moulding of a fibre reinforced laminated structure is provided. The mould comprises at least a first mould part and a second mould part where the first mould part defines a negative impression of the laminated structure, is structurally stable, and forms a support for fibre reinforcement layers of the laminated structure. The second mould part is connectable to the first mould part for closing the mould and defining together with the first mould part an enclosed space which can be evacuated. According to the invention, at least one flow duct for guiding a liquid polymer is formed as a recess in the first mould part and/or in the second mould part. The recess is open towards the enclosed space and extends along a section of the periphery of the first mould part and/or the second mould part. 
     In the inventive mould, the flow duct is located outside the actual volume of the finished fibre reinforced laminate structure but is part of the volume which will be evacuated. When forming the fibre reinforced laminated structure the flow duct is kept free of any material until the injection of resin starts. Thus, when the injection of the resin is started, the resin can run freely through the flow duct from an inlet port through which the flow duct is connected to a resin reservoir. As the flow duct is filled with resin, the slower process of resin flowing into the fibre and core materials of the laminated structure starts. Once the resin has cured and the mould has been removed the surplus resin which is the reminder of the resin in the flow duct can be removed by mechanical means. By using a flow duct which is part of the mould rather than of the moulded structure lay-up time for the layer stack, weight, costs and complexity of the laminated structure can be reduced. 
     The first mould part can, in particular, define the negative impression of a laminated structure having an elongated shape with a leading edge and a trailing edge. In this case, a first flow duct may be present in the first mould part and/or in the second mould part at a location where the leading edge is to be fowled. Furthermore, a second flow duct may be present in the first mould part and/or in the second mould part at a location where the trailing edge is to be formed. The first and second flow ducts allow for introducing resin from the leading edge, as well as from the trailing edge into the fibre- and core materials of the layer stack in the mould. The use of the first and second flow ducts allow for reducing the infusion time making the use of cheaper, faster setting resins possible. The flow front is then essentially a combination of two more or less straight lines, parallel to the leading edge and the trailing edge, respectively. In combination with non-parallel leading and trailing edges this ensures that the flow fronts coalesce in a systematic, controlled manner, thereby minimising the risk of entrapping air. 
     The laminated structure with the elongated shape can, in particular, be a laminated wind turbine rotor blade. 
     In a first implementation of the inventive mould the second mould part is also structurally stable. This helps to prevent wrinkles in the finished laminated structure. Moreover, in this implementation a first recess can be formed in the first mould part while a second recess is formed in the second mould part. The first recess and the second recess are then located such in the respective first and second mould parts that parts of their open sides are aligned to each other when the mould is closed. In other words, when the mould is closed the first recess and the second recess together form the flow duct for the liquid polymer to be introduced into the stack of fibre- and core materials. 
     Like the first mould part, the second mould part can also define a negative impression of the laminated structure. Then, a mould core is present in addition to the first and second mould parts. The outer dimensions of the mould core are smaller than the inner dimensions of the first and second mould parts. The mould core may optionally be surrounded by an inflatable member. By using first and second mould parts which both define negative impressions of the laminated structure and the mould core, a whole closed structure can be formed in one moulding process. For example, if a wind turbine rotor blade is to be formed as the laminated structure the whole shell of the rotor blade can be formed in one moulding process. This overcomes the necessity of glue joints at the trailing edge and the leading edge of the rotor blade since the fibre layers can be dimensioned such that they can be wound around the core so as to overlap, for example, in the centre of the upper mould part. When forming the laminated structure the inflatable member can be used to press the stack of reinforcement- and core layers on the first and second mould parts. Inflating the inflatable member can be automatically done by evacuating the space between the first and second mould parts on the one side and the core on the other side. If the inflatable member has an internal pressure or a flow connection to air pressure reservoir, e.g. inside the mould core, the inflation will be self-affected by evacuating the space between the core and the first and second mould parts. 
     In a second implementation of the inventive mould the second mould part is a vacuum bag. In this case, the at least one flow duct is formed as a recess in the first mould part. By using a vacuum bag a mould core becomes unnecessary so that the number of mould parts can be reduced as compared to the first implementation. However, forming a laminated structure in which both ends of the fibre- and core layers of the layer stack overlap so as to form, e.g., a closed shell of a wind turbine rotor blade, is not possible so that glue joints will be necessary. 
     The inventive method for vacuum assisted resin transfer moulding of a fibre reinforced laminated structure uses an inventive mould. In the method, the at least one flow duct is kept free of any material until the resin is injected by injection means. The resin then runs in the at least one flow duct from an inlet port to the far end of the mould. When the resin has set, the mould parts are removed and the surplus resin in the at least one flow duct may be removed mechanically. 
     If at least two flow ducts are present, a resin reservoir may be connected to the at least two flow ducts in such a way that the resin is injected into the at least two flow ducts simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, properties and advantages of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings. 
         FIG. 1  shows a first embodiment of the inventive mould. 
         FIG. 2  shows a detail of  FIG. 1   
         FIG. 3  shows the detail of  FIG. 2  in a top view 
         FIG. 4  shows a second embodiment of the inventive mould. 
         FIGS. 5 to 9  show a third embodiment of the inventive mould and its use for making a fibre reinforced laminated wind turbine rotor blade. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The first embodiment of the inventive mould is shown in  FIG. 1 . The figure shows a mould for a shell part of a fibre reinforced laminate wind turbine rotor blade. The shell part to be formed by the mould forms the pressure side of the later rotor blade, i.e. the side which shows substantially towards the direction from which the wind comes during operation of the wind turbine. A similar mould (not shown) will be used for forming the suction side shell part. The wind turbine rotor blade is then formed by joining the pressure side shell and the suction side shell by glue joints. 
     The mould  1  shown in  FIG. 1  comprises a first mould part  2  which is made from a rigid and structurally stable material. The mould part  2  comprises an inner surface  4  which defines a negative impression of the wind turbine rotor blade&#39;s pressure side shell part. The surface  4  of the mould part  2  may be provided with a suitable release agent which allows for removing the pressure side shell part from the mould after the moulding process. 
     The mould  1  further comprises a vacuum bag  6  which may be fixed in a pressure tight manner to edges  8 ,  10  of the mould part  2 . The vacuum bag  6  and the mould part  2  together define an enclosed space which can be evacuated through suitably placed vacuum channels (not shown in the figure). 
     The rigid mould part  2  is equipped with recesses  14 ,  16  which form flow ducts after evacuating the enclosed space. The flow ducts  14 ,  16  can be provided with spacer elements  18  which prevent the vacuum bag  6  from being sucked into the flow ducts  14 ,  16 . The spacer elements  18  can be seen in  FIGS. 2 and 3  which show an enlarged view of the flow duct  14  in a sectional view and in a top view, respectively. 
     For vacuum assisted resin transfer moulding of a wind turbine rotor blade as a fibre reinforced laminated structure a layer stack  12  is laid in the mould part  2 . Then, the vacuum bag  6 , which constitutes a second mould part in the present embodiment, is fixed airtight to the edges  8 ,  10  of the mould part  2 , and the enclosed space between the mould part  2  and the vacuum bag  6  is evacuated. This leads to pressing the vacuum bag  6  against the layer stack  12  by means of the air pressure outside of the mould  1 . Then, a liquid polymer is introduced into the flow ducts formed by the recesses  14 ,  16  from where it flows into the layer stack  12 . The reservoir of the liquid polymer is connected to both flow ducts so that it can be injected into both flow ducts  14 ,  16 , simultaneously. As the flow ducts become filled with the liquid polymer, the slower process of liquid polymer flowing into the layer stack  12  starts. The flow front of the resin is essentially a combination of two more or less straight lines, which run parallel to the leading and trailing edge of the shell part to be formed. When combined with non-parallel leading and trailing edges, this ensures that the flow front coalesce in a symmetric, controlled manner, thereby minimising the risk of entrapment of air. 
     When the layer stack  12  is fully wetted by the liquid polymer the liquid polymer is cured, for example by using heat or ultraviolet light. In the present case, heat is used which is applied by a heat transfer system  20  that may, e.g., be realised by heating filaments integrated into the mould part  2 . After the resin is cured the mould will be removed again leaving surplus liquid polymer at the leading and trailing edges of the shell part. This surplus polymer is then removed by mechanical means. 
     The layer stack in the present embodiment can comprise one or more layers of fibre reinforced material onto which a core material is provided which may be made, e.g., from balsa wood or PVC foam. 
     Although the mould part  2  in  FIG. 1  is realised as a single mould element it may as well be realised by a number of mould elements which are joined together to form the mould part  2 . 
     A second embodiment of the inventive mould will be described with respect to  FIG. 4 . The mould  21  comprises a first mould part  2  and a second mould part  22  which are both made from a structurally stable, rigid material. The first mould part  2  substantially corresponds to the mould part  2  of the first embodiment and will therefore not be described again. Moreover, the elements of the first mould part  2  as well as the layer stack  12  shown in  FIG. 4  are designated with the same reference numerals as in  FIG. 1 . 
     The second embodiment differs from the first embodiment in that a second structurally stable, rigid mould part  22  is used instead of a vacuum bag  6  for closing the mould to form an enclosed space which can be evacuated. The second mould part  22  comprises a surface  23  that defines a positive impression of the wind turbine rotor blade&#39;s pressure side shell and edges  24 ,  26  which allow for joining the first mould part  2  and the second mould part  22  airtight so that an enclosed space is formed between both mould parts. Like the surface  4  of the first mould part  2  the surface  23  of the second mould part  22  may be provided with a suitable release agent for simplifying dismantling the finished shell part from the mould. The second mould part  22  further comprises a first recess  28  and a second recess  30  which are dimensioned and located such in the second mould part  22  that open sides of them are aligned with the recesses  14 ,  16  of the first mould part  2  so as to form flow ducts for the liquid polymer. Furthermore, the second mould part  22  is equipped with one or more suitably located vacuum pipes  32  for evacuating the enclosed space between the first and second mould parts  2 ,  22 . 
     The process of vacuum assisted resin transfer moulding of the shell part is substantially the same as in the first embodiment and will therefore not be described again. 
     A third embodiment of the inventive mould and its use will be described with respect to  FIGS. 5 to 9 . 
       FIG. 5  shows a first mould part of the mould  101  according to the third embodiment which corresponds to the first mould parts of the first and second embodiments except for the fact that no recess  16  is present in the mould part&#39;s edge  110  that is located where the wind turbine rotor blade&#39;s trailing edge will be formed. The remaining parts of the first mould part  102  of the third embodiment correspond to the respective parts of the mould part  2  of the first and second embodiments and will therefore not be described again. Elements of the mould part  102  which correspond to the mould part  2  are designated with the reference numerals which are increased by 100 with respect to those used for the mould part  2 . 
     A layer stack  112  is formed in the first mould part  102  (see  FIG. 6 ) by first putting one or more layers of fibre reinforcement material in the mould, then putting a core material atop of these layers and then again one or more layers of fibre reinforcement material atop of the core material. The layers of fibre reinforcement material are dimensioned such that their end sections  122 ,  124  project over the edges  108 ,  110  of the mould part  102 . At the edge  110  where the trailing edge of the wind turbine rotor blade is to be formed, a special core part  126  is put atop of the layers of fibre reinforcement material. To aid distribution of the liquid polymer during the moulding process, the layer stack may be equipped with transfer passages  128 ,  130  extending through the layer stack. 
     In the next step, as it is shown in  FIG. 7 , a mould core  132  is set onto the layer stack  112 . In the present embodiment, the mould core  132  consists of a first and a second mould core part  134 ,  136  between which a shear web  138  is located. The shear web may consist of a layer stack with the same layer structure than the layer stack  112  in the first mould part  102 . During the vacuum assisted resin transfer moulding process the shear web will be firmly joined to the shell of the wind turbine rotor blade to increase its stability. Although only one shear web  138  is present in the shown embodiment, two or more shear webs could be present as well. 
     The mould core parts  134 ,  136  are made from a structurally stable material like, for example, wood. Moreover, the structurally stable material may be surrounded by a flexible member like, for example, foam rubber. In addition, each core part  134 ,  136  is surrounded by a flexible membrane the outer surface of which may be treated with a suitable release agent. When the space between the mould parts and the mould core will be evacuated after closing the mould, the air pressure still present between the structurally stable material of the mould core parts  134 ,  136  and the flexible membrane will lead to an inflation of the membrane so that the layer stack surrounding the mould core parts  134 ,  136  is pressed against the respective mould parts. 
     In a next step, as it is shown in  FIG. 8 , the top fibre reinforcement layers, i.e. those layers atop of the core material of the layer stack  112 , are wound around the mould core  128  such that both ends of the layers overlap. Then, a core material is put atop of these layers and then the lower fibre reinforcement layers of the layer stack  112 , i.e. those which are below the core material of this stack, are wound around the core material to form a layer stack  140 , thereby overlapping in their end sections. In this way the whole shell of a wind turbine rotor blade can be made in form of a single laminate structure without the need of glue joints at the leading and trailing edges of the rotor blade. 
     In a last step, as it is shown in  FIG. 9 , the second mould part  142  of the mould  101  is set atop of the layer stack  140 . The second mould part  142  comprises edges  144 ,  146  by which the second mould part  142  can be joined airtight with the first mould part  102 . Suitable vacuum channels (not shown) can be used for evacuating the space enclosed between the mould parts  102 ,  142 . The second mould part further comprises a first recess  148  and a second recess  150 . The location of the first recess  148  and its dimension are chosen such that an open side of the recess  148  is aligned with an open side of the recess  114  in the first mould part  102  when both mould parts are joined together. Hence, the recess  114  in the first mould part  102  and the recess  148  in the second mould part  142  form together a flow duct for liquid polymer at the leading edge of the wind turbine rotor blade to be formed which is open towards the space enclosed between the mould parts. In addition, the recess  150  in the second mould part  142  forms a further flow duct for liquid polymer which is located at the trailing edge of the wind turbine rotor blade to be formed. 
     After evacuating the enclosed space between the mould parts  102 ,  142  a liquid polymer is injected into the flow ducts which then flows into the layer stacks  112 ,  140  and the shear web  138 . When the layer stacks  112 ,  140  and the shear web  138  are completely wetted by the liquid polymer, the liquid polymer is cured by applying heat via heat transfer systems  120 ,  152  present in the first and second mould parts  102 ,  142 . Like in the other embodiments, the heat transfer system may be realised in form of heating filaments integrated into the mould parts  102 ,  142 . When the liquid polymer is cured the mould parts  102 ,  142  and the mould core parts  134 ,  136  are removed. After removing the mould from the wind turbine rotor blade surplus polymer material remains at the leading edge and the trailing edge of the wind turbine rotor blade. This surplus polymer material is then removed mechanically. 
     The inventive mould allows for forming fibre reinforced laminated structures by using a vacuum assisted resin transfer moulding process without leaving flow pipes in the finished structure. Moreover, removing the surplus resin material is easier and less elaborate than removing flow pipes used in the state of the art moulding processes.