Description

Method for manufacturing a preform building element and oven

The invention relates to a method for manufacturing a preform building element used for building a rotor blade of a wind turbine , wherein a plurality of components is arranged at least partly overlappingly in a component stack on a surface of a carrier, wherein the stack comprises a plurality of sections with overlapping components between which a binding agent is arranged, wherein the sections of the stack comprise at least partly a di f ferent thickness and/or di f ferent types of components , wherein the component stack is heated using a heating means for activating the binding agent . Furthermore , the invention relates to an oven adapted to heat preform building material arranged on at least one carrier surface for producing preform building elements used for building a rotor blade of a wind turbine .

Preform building elements , which are also called preforms , are textile sub-products used in composite manufacturing processes to decrease manufacturing cycle times . In liquid composite molding processes like resin trans fer molding (RTM) , in particular vacuum assisted resin infusion (VARTM) , preform building elements may comprise a number of layers of a textile reinforcement material like glass fibers , carbon fibers and/or aramid fibers . The layers of the textile material are bonded together in order to form a semi-rigid structure from the textile layers to facilitate the laying-up of the textile material in the mold used for the resin inj ection to simpli fy and to shorten the preparation of the material prior to the resin infusion . Preform elements may consist of multiple components which are assembled within a preform casting mold, wherein the components may vary both in thickness and material composition, depending on the type of preform element to be manufactured . When used in the context of composite wind turbine blade manufacturing, preform building elements are required to have signi ficant variations in local layup, depending on which position in a wind turbine blade the preform building element will be integrated . For instance , areas near the root may require signi ficantly more layers than areas towards the tip . Furthermore , preform building elements may have decreasing number of layers towards the edges , in particular for forming a tapered profile towards the edge , which may be used as a scarf j oint to connect adj acently arranged preform elements .

Preforms can be formed by bonding textile layers together by a variety of methods including the usage of adhesives , stitching, tufting, breading, weaving, ultrasonic welding and/or by using solid or liquid binders . As binders , for instance a powdered thermoset or thermoplastic material may be used . The binder may be deposited, for instance evenly, upon the surface of the reinforcement textiles in a granular form and sintered in place for locally adhering the reinforcement textile layers for forming the preform building element .

The manufacturing of the preform building elements may comprise a heating phase and a subsequent cooling phase , when a binder is used . The component stack for forming the preform building element may be heated to a temperature at which the binder granules soften and the viscosity decreases to a point at which the binder can flow . The binder granules then begin to flow around the fibers on the outer surface of adj acent textile layers , driven by a combination of compaction forces and capillary action .

When the binder has flown to suf ficiently impregnate both surfaces of overlapping textile layers , the heating stops and the cooling phase begins . The temperature of the preform is then rapidly decreased until the viscosity of the binder has increased to a point at which it can no longer flow . Once cooled suf ficiently, the binder granules that have impregnated adj acent layers now hold the textiles together via a com- bination of mechanical and chemical coupling . The layers of textile now form a preform element which can be li fted and handled as a single structure . In addition to textile components , also rigid components for providing stif fening structures or the like may be used in a preform building element , depending on the intended position of the preform building element in the rotor blade to be manufactured . Also the rigid components may be adhered to the textile layers using a binder .

The heating and cooling processes require careful design as there are limited time and temperature process windows in which suf ficient bonding can be achieved . The binder must be held at an interdependent combination of temperature and heating time that allows for the binder to flow suf ficiently into adj acent textile layers . I f the binder is not heated to a high enough temperature , or heated for a too short period of time , respectively, then it will not suf ficiently impregnate the surfaces of the components , in particular of the textile layers . I f the binder is heated to a temperature that it is too high, or heated for too long, then it will be drawn into the textile layers by capillary action and away from the surfaces , resulting in poor interlaminar bonding between two or more overlapping components .

Textiles and core materials used in composite wind turbine blades typically have poor thermal conductivity . As a result of this , the outermost layers in a textile stack may reach thermal equilibrium with an applied heat source earlier than layers around the center point of the stack . This causes issues in the design of the preforming process for preform structures which have signi ficant local variations in their number of textile layers and/or in the thickness of the component used . For example , surface layer and areas which have few layers may reach the temperature where binder begins to flow more rapidly, while layers in the center of the thickest part of the structure may require signi ficantly more time . This may result in the preform having little adhesion between layers , for instance due to some sections having excessive binder migration while other sections have not had enough binder migration . An insuf ficient adhesion between the components of the preform building element makes it di f ficult to li ft and handle the preform building element and therefore af fects the usage of the preform element in the manufacturing of the rotor blade .

Additional di f ficulty in the process design may be caused by the required trade of between applied the temperature , preform layer adhesion, and cycle time . I f a high temperature is applied to the outside of the preform, then a greater thermal gradient will result in less time being required for the center of the thickest area to reach the minimum temperature for the adhesion to occur and therefore resulting in a lower cycle time . The layers on the outer edges of the preform will have been above the temperature of binder flow for a shorter time , however, the higher temperature may result in the binder migrating away from the textile surface more rapidly . Conversely, i f a low temperature is applied to the outside of the preform, then the lower thermic gradient results in more time being required for the center of the thickest area to reach the minimum temperature for adhesion to occur . However, this results in undesirable long cycle times . The layers on the outer edges of the preform to be manufactured will have been above the temperature of binder flow for a longer time . However, due to the lower temperature , the binder migration away from the textile surface is slower during this time .

Therefore , the invention is based on the obj ective problem to provide an improved method for manufacturing a preform building element , which in particular reduces binder migration locally and therefore improves the adherence between the components forming the preform building element to be manufactured .

According to the invention, this problem is solved by a method as initially described, wherein during the heating, at least one heat input reduction means is used to reduce the heat input in the binding agent in at least one of the sections of the stack for reducing binding agent migration during the activation .

The plurality of components , from which the preform building element is manufactured, may be textile components like glass fiber, carbon fiber and/or aramid fiber layers . In addition, also rigid components like core elements , for instance made from balsawood or a polymer material , may be used . The components are arranged on a surface of the carrier in a component stack . The component stack comprises a plurality of components , which are arranged above each other, at which are at least partly overlapping . In addition, also one or more components may be arranged j uxtaposed on the surface of the carrier and/or as a layer of the stack, respectively . The components are at least partly overlapping, wherein in particular in between the overlapping components , a binding agent , in particular a thermoset or thermoplastic binder, is arranged to allow for an adherence of the overlapping components . The binder may be provided for instance as a powder, as pellets and/or as a liquid .

The stack of components comprises a plurality of sections , which di f fer in their thickness and/or in the type of components arranged within the section . Di f ferent types of components may be di f ferent types of textile layers , di f ferent core elements , components with di f ferent materials , components with varying thicknesses and/or of varying heat conductivity or the like . The thickness of a section refers to the height of the stack in the section and hence depend on the number of components arranged in the section and/or on their individual thicknesses .

For activation of the binding agent , the stack of components is heated using a heating means , for instance an oven or a furnace , in which the carrier with the component stack may be arranged . The heating means may also be a heating blanket placed on top of the stack and/or a heatable mold which heats the carrier on which the component stack is arranged . Also a combination of any of the aforementioned heating means is possible .

During the heating, at least one heat input reduction means is used to reduce the heat input in the binding agent in at least one of the sections of the stack . The reduction of the heat input in the stack may reduce the temperature and/or the temperature gradient in the section of the stack, so that the migration of the binding agent can be reduced . In particular, the heat input reduction means slows down the heating-up of the respective section, so that a di f ferent course of the temperature and/or of the temperature gradient over the cycle time can be obtained . It is in particular possible that by the heat input reduction means , a relatively even temperature distribution across the preform throughout the entire heating process is achieved so that all areas experience equal or approximately equal temperatures and duration of heating .

The binding agent migration may be driven by di f fusion into the components , in particular in the textile components . The binding agent migration may also include a di ffusion and/or a flow along the contacting surfaces between adj acently arranged components . The sections of the segment may di f fer in height of the stack, for instance due to di f ferent component thicknesses and/or due to di f ferent numbers of stacked components .

The carrier surface , on which the components are arranged, may be a flat surface or a curved surface , for instance a convex or concave surface . This allows for providing a mold shape for the preform building element , so that also curved preform building elements may be fabricated .

By the heat input reduction means , the heat input in di f ferent sections of the stack of components may be adj usted, depending on the structure of the stack and/or depending on the preform building element to be fabricated . In particular, the heat input reduction means is used to reduce the total heat energy being trans ferred into the key areas , or the sections of the stack in which the largest binding agent migration may occur . Advantageously, the adhesion between preform components , or layers , respectively can be maximi zed by preventing excessive binder migration away from the surface of the textile due to overheating of thin regions . In addition, the corresponding cycle times for the preform building element manufacture can be minimi zed by allowing for higher peak heat energy inputs in thick regions of the preform building element to be manufactured .

Advantageously, the heating means may have more or less a uni form heat input onto the entire stack, wherein the di f ferent heat input into the sections of the stack are adj usted by the heat input reduction means . This allows for the usage of comparatively simply heating means , like ovens or furnaces , which are not adapted for adj usting local heat input into the stack of components . In particular, the usage of a plurality of individually controllable heating means for heating di fferent sections can be avoided . This has the further advantage that the heating means may be used for a wide verity of di f ferent manufacturing processes .

Furthermore , the heating means may have a comparatively simple structure , so that the costs for the heating means may be reduced . This enables to use a plurality of heating means in parallel and hence to fabricate also a plurality of preform building elements in parallel , advantageously reducing the process time in the fabrication of the rotor blade . Furthermore , the ability to reduce the heat input in di f ferent sections of the stack of components reduces the limitation and the constraints in the design of the component stack, or of the preform building element to be fabricated, respectively . Consequently, less compromises have to be made in the design of the preform building elements and/or regarding the ef ficiency of the manufacturing process . The method enables to use a binding agent for locally attaching the components also for manufacture of preform elements with varying thicknesses and/or an uneven composition of components . This allows for making use of the advantages of the binding agent in these manufacturing processes , in particular with regard to the good scalability and the comparatively low time for application . Advantageously, less scalable and more time-consuming attachment methods like stitching, tufting, braiding, weaving, ultrasonic welding and the like may be omitted .

Optionally, after activation of the binding agent , a cooling of the components may occur . Therefore , the heat input of the heating means may be switched of f . In addition or alternatively, the carrier may be removed from the heating means or vice versa . It is also possible that a cooling means is used and/or that the carrier is placed in a cooling means for providing an active cooling reducing the time until the component stack is consolidated and a preform building element is manufactured and ready for further transport and/or processing .

Preferably, the components are arranged underneath a vacuum foil or enclosed in a vacuum bag . The vacuum may be established in particular prior to the heating, in particular prior to an arrangement of the carrier in a heating means like an oven . By providing a vacuum, the relative positions of the components in the stack may be fixated so that the fabrication of a preform building element with the desired geometry and/or structure is facilitated . Furthermore , damages and/or misalignment between the component due to a transport of the carrier or the like may be prevented .

In an embodiment , the heat input reduction means is used to reduce the heat input in an edge area of the component stack . In particular in the edge areas of the component stack, sections with a reduced height of the stack may be present . For example , these sections may be used for providing tapered edge areas of the preform building element to be manufactured, wherein the tapered edge areas may be used afterwards as scarf j oints for connecting adj acently arranged preform building elements during the manufacture of the rotor blade .

In addition or as an alternative to the edge areas , also center areas of the preform building element may have di f ferent thicknesses and/or di f ferent types of components . This may be the case for instance in sections , in which solid or rigid core components are arranged and which di f fer therefore from neighboring sections comprising only textile layers as components . In particular rigid core components may have a di f ferent heat conductivity and/or di f ferent requirements regarding the adhesion of the activated binding agent than textile components like fiber mats or layers .

In a preferred embodiment , at least one heat shield is used as heat input reduction means , wherein the heat shield is arranged on and/or above of at least one of the sections of the component stack . In particular, the heat shield is arranged in such manner that at least a section of the overlapping components , into which the heat input shall be reduced, is arranged between a surface of the carrier and the heat shield .

Preferably, at least one cover element , at least one reflective foil and/or at least one foam layer is used as heat shield, wherein at least one of the sections of the component stack is embedded between the carrier surface and the heat shield . The cover element may comprise a high thermal resistivity and/or a high heat capacity so that the cover element directly arranged on the section, into which the heat input shall be reduced, reduces the temperature and/or the temperature gradient in the covered section . The cover element may be arranged directly on the section, into which the heat input is to be reduced . The cover element may also be reflective , or it may comprise at least one reflecting surface , re- spectively, so that at least of a part of the heat may be reflected from the cover element and therefore does not heat up the cover element and/or the covered section of the component stack . When a vacuum bag or vacuum foil is used, it is possible that the cover element is arranged inside or outside the vacuum bag, or on top of or underneath the vacuum foil , respectively .

Using an insulating material with a low thermal conductivity as cover element and/or as foam layer, results in a reduction of the heat energy which is trans ferred to the covered section of the preform element . This also increases the time , which is required to reach the thermal equilibrium and therefore the heat input in the binder arranged in the covered section may be ef ficiently reduced . It is possible that all edge barriers , or all peripheral areas , respectively of the component stack are covered by cover elements , hence that one or more cover elements are arranged on the entire outer circumference of the component stack . The cover elements may be provided as separate parts , which can be placed on the one or more sections of the component stack using handles or the like .

It is also possible that as cover elements , further components of the same type of components which are used for fabricating the preform building element are used . These extra components are not adhered with a binder and removed after consolidation of the binder . The additional material may not be thermal insulating, but it may comprise a high speci fic heat capacity to store a lot of thermal energy, so that they heat up slowly . This again protects the covered section of the preform building element located underneath the cover element from being overheated, as the section will experience a lower temperature and thereby the binding agent in this section will not melt away . For instance , cover elements of balsawood may be used . In addition or alternatively, a reflective foil may be used as heat shield . A reflective foil can be used to reduce heat trans fer into desired sections through reducing radiative heat trans fer . For instance , within a typical oven used as heating means , the heat energy is trans ferred to the component stack both via the flow of heated air in a convective heat trans fer and via radiated heat from the surfaces within the oven and hence as radiative heat trans fer . One or more reflective foils can be placed directly on the surface of the stack to reduce the total heat energy being absorbed . The use of a reflective foil or a low emissivity foil , respectively, is ef fective as it allows for simple , accurate, and flexible shielding of targeted sections from radiated heat . The regions covered can easily be adj usted, allowing for fine turning of the heating process . The reflective foil may be placed on either or both sides of the component stack and optionally placed in a vacuum bag or underneath a vacuum foil . Once the vacuum foil/bag is removed, the foil will likewise be removed .

The heat shield may also be a foam which may be applied for instance by hand or automatically, in particular to the edges of the component stack . The foam may be applied on top of the respective section, in particular outside of a vacuum bag, or on the outer side of a vacuum foil , respectively .

Preferably, one or more shielding elements , in particular shielding panels and/or shielding curtains , are used as heat shield, wherein the shielding elements are arranged in a distance above at least one of the sections of the component stack . The shielding elements may be arranged above sections that require less heat energy to reach the target temperature . The purpose of the shielding elements is to reduce the heat trans fer via convective and radiative heat trans fer . The shielding elements may be arranged parallel or under an angle to the edge of the carrier, or the surface of the carrier, respectively . By the shielding elements , one or more sections of the component stack are shadowed at least partly from heat which is trans ferred to the component stack from above . The shielding elements may have the form of shielding panels or shielding flaps . Also the usage of shielding curtains is possible . The shielding elements may be attached to the heating means , for instance to the inner wall of an oven, which is used as heating means . In particular, the position of the shielding elements may be adj ustable so that the area of coverage can be varied between di f ferent preform building elements to be fabricated .

In an embodiment , a heating means providing a hot airflow streaming above the components is used as the heating means and at least one air flow separation means is used as the heat input reduction means , wherein the air flow separation means separates at least a part of the volume above the component stack for locally reducing the air flow speed above at least one of the sections of the component stack .

The air flow separation means may be used to physically divide the sections , in which the heat input should be reduced, from the other sections of the component stack . By the air flow separation means , the volume above the component stack is separated in a plurality of hori zontally adj acent subvolumes . This reduces the flow speed of hot air, which is blown above the component stack in the divided volume . The reduced flow speed of the air causes also a reduced heat trans fer to the surface of the stack so that also the heat input in one or more sections may be reduced . Consequently, the air flow separation means compartmentali zes the volume above the stack for creation of isolated heating zones of the component stack .

Preferably, a plate-like separation element arranged above the components and/or an air curtain arrangement providing at least one air stream towards at least one of the sections of the component stack, in particular to an edge of the section is used as air flow separation means .

One or more plate-like separation elements may be used as protective curtain shields , which may be arranged for instance perpendicular or at an angle to the surface of the carrier, or to the component stack, respectively . For instance , the convection coef ficient from the air into the section of the component stack may be signi ficantly reduced by producing a laminar air flow by the plate-like separation elements , wherein the laminar air flow signi ficantly reduces the heat input compared to a turbulent air flow in these sections . Due to the reduced heat trans fer into at least one of the sections of the component stack, the sections heat up slower and experience therefore a lower temperature so that the binding agent migration in these sections can be reduced . The plate-like separation elements therefore reduce the heat trans fer in the sections and not the temperature of the section, which will be the same in all portions of the stack when only plate-like separation elements are used .

In addition or alternatively, an air curtain arrangement may be used as the heat input reduction means , wherein the air curtain arrangement provides an air stream towards at least one of the sections of the preform stack, in particular towards an edge of the section . The air stream works as an air curtain . Therefore , the air curtain arrangement may be used to create separate heating zones within a heating means like an oven . The at least one air stream, which forms the air curtain, may be created by a plurality of j ets , through which air is blown to create a curtain that divides the volume above the component stack into di f ferent sub-volumes above di f ferent sections of the stack that require di f ferent levels of heat energy .

By such an air curtain, for instance the speed of hot air circulating within each region may be adj usted to change the heat trans fer coef ficient from the hot air to the preform . By having high levels of air flow over thick areas of the preform and low levels of air flow over thinner regions of the preform, a relatively uni form heating may be achieved .

Air curtains allow for more flexibility in the creation of separate heating zones than physical dividers like the platelike separation elements . For instance , an air curtain can be easily repositioned to adj ust the si ze of heating zones through changing the angle of the air j et . Furthermore , air curtains work at variable distances and hence do not require modi fication as physical dividers do when dimensions of components and/or preform building elements change . Advantageously, this facilitates the manufacture of di f ferent types of preform building elements using the same setup .

In an embodiment , a cooling arrangement is used as the heat input reduction means , wherein the cooling arrangement is arranged above at least one of the sections of the component stack, underneath at least one of the sections of the component stack, and/or in the vicinity of the carrier surface .

The cooling arrangement may provide an active cooling to prevent a rapid heating of thinner sections of the preform stack, for instance within an oven used as heating means . The cooling means removes heat from at least one of the sections and therefore reduces the net heat input into the sections .

The cooling arrangement may comprise one or more cooling ducts , which may be arranged on at least one of the sections of the component stack and/or underneath of the carrier, in particular opposite to the at least one section to which the heat input shall be reduced . In addition or alternatively, the cooling arrangement may comprise one or more openings in a housing of the heating means , which may be for instance open to remove heat from the edge regions of the preform component stack . In addition or alternatively, the cooling arrangement may have one or more fans which allow to vent cooler air over the sections of the component stack to either reduce the rate at which the heat or to cool regions which have already achieved the target heating temperature for the required duration of time .

In particular, the cooling arrangement may be used to cool at least one section of the component stack once a target heat input and/or a target heating temperature of the section has been reached to prevent this section from overheating . Therefore , a control unit may be provided, which activates the cooling arrangement , for instance by providing a cooling flow through a cooling duct , by opening one or more of the openings in the housing and/or by activating and/or adj usting an air flow and/or an air temperature of a cool air provided by a cooling means of the cooling arrangement . The cooled section of the component stack may be considered to have a shorter heating cycle time than an uncooled section, ef fectively reducing the heat input in the binding agent in these sections .

In particular, the cooling arrangement may be used with further heat input reduction means . In particular the cooling arrangement may be used in a setup in which compartmentali zed and isolated heating zones , for instance by using the separation elements , the air curtain and/or the shielding elements , are used .

It is possible that a cooling duct , which may be used for cooling a section of the component stack, is also used as a heating means by providing a hot fluid flow to the cooling duct . This allows for controlled heating in the areas in the vicinity of the cooling duct , for instance in the adj acent portion of the carrier surface . This increases the flexibility of the production setups since also preform building elements with thicker edges and hence an increase requirement for heat exposure may be manufactured .

An oven according to the invention is adapted to heat preform building material arranged on at least one carrier surface for producing preform building elements used for building a rotor blade of a wind turbine and comprises a housing with an interior adapted to receive at least one carrier with a plurality of components arranged on a surface of a carrier in a receiving volume in the interior, wherein the oven comprises a heating means for heating the interior and at least one heat input reduction means for locally reducing the heat input of the heating means in a section of the receiving volume .

Preferably, the heat input reduction means is a heat shield comprising one or more shielding elements , in particular shielding panels and/or shielding curtains , wherein the shielding elements are arranged directly adj acent to the receiving volume and/or in a distance above the receiving volume .

In an embodiment , the shielding elements are pivotably and/or slidably attached to the housing . The shielding elements may be relocated and/or adj usted by pivoting and/or sliding, so that the oven may be adapted to di f ferent manufacturing processes for di f ferent preform building elements . In particular, the shielding elements are removably attached to the oven, so that they also may be removed from the interior of the oven i f they are not needed in a certain production process .

Preferably, the heating means is adapted to provide a hot air flow streaming through an air stream volume above the receiving volume , wherein the heat input reduction means comprises at least one air flow separation means , wherein the air flow separation means is adapted to locally reduce the air flow speed at least above a section of the receiving volume .

Preferably, the air flow separation means is a plate-like separation element arranged above the components and/or an air curtain arrangement with a plurality of j ets , wherein at least one air stream directed to a section of the receiving volume is generatable through the j ets . The j ets of the air curtain arrangement may be adj ustable for adj usting the direction of the generated air stream .

In an embodiment , the oven comprises a cooling arrangement , wherein the cooling arrangement is arranged above at least one section of the receiving volume , underneath at least one section of the receiving volume , and/or in a wall of the housing .

The cooling arrangement may comprise one or more cooling ducts , one or more air providing means and/or one or more openings in the wall of the housing .

All details and advantages described in relation to the method according to the invention apply correspondingly to the oven according to the invention and vice versa .

Other obj ects and features of the present invention will become apparent from the following detailed description considered in conj unction with the accompanying drawings . The drawings , however, are only principle sketches designed solely for the purpose of illustration and do not limit the invention . The drawings show :

Fig . 1 components for manufacturing a preform building element arranged on a carrier,

Fig . 2 the carrier arranged inside an oven,

Fig . 3 an edge region of a component stack,

Fig . 4 a first embodiment of cover elements used as a heat shield and arranged on the component stack in an embodiment of a method according to the invention,

Fig . 5 a side view of the cover elements , Fig. 6 a second embodiment of cover elements used as a heat shield,

Fig. 7 a foam used as heat shield on the component stack,

Fig. 8 a reflective foil used as a heat shield on the component stack,

Fig. 9 a cut view of the reflective foil,

Fig. 10 a section of a first embodiment of an oven according to the invention,

Fig. 11 a cut view of the section of fig. 10,

Fig. 12 a section of a second embodiment of an oven according to the invention,

Fig. 13 a cut view of the section of fig. 12,

Fig. 14 a section of a third embodiment of an oven according to the invention,

Fig. 15 a cut view of the section of fig. 14,

Fig. 16 a section of a fifth embodiment of an oven according to the invention,

Fig. 17 a cut view of a section of the oven according to fig. 16,

Fig. 18 a section of a sixth embodiment of an oven according to the invention, and

Fig. 19 a cut view of a section of the oven according to f ig . 18. In fig . 1 , an arrangement 1 used in a method for manufacturing a preform building element is shown . The arrangement 1 comprises a carrier 2 , on which a plurality of components is arranged at least partly overlapping the in a component stack 3 on a surface 4 of the carrier 2 . The carrier 2 with the component stack 3 is arranged into an oven 5 , from which a section of a housing 6 is depicted schematically . Prior to the arrangement in the oven 5 , the carrier 2 may be arranged outside the oven, for instance to allow for the arrangement of the components on the surface 4 .

The carrier 2 comprises a concave top surface 4 , on which the component stack 3 is arranged . The carrier 2 may be slid into the housing 6 of the oven 5 for instance using a pair of rails 8 . The top surface 4 of the carrier 2 may also be flat , convex or of any other shape required for forming a preform building element to be fabricated from the component stack 3 .

In fig . 2 , a view on a face side of the oven 5 is shown . Inside the housing 6 , the carrier 2 with the component stack 3 is arranged . The component stack 3 comprises a plurality of components , which are arranged at least partly overlappingly on the surface 4 of the carrier 2 . The stack 3 comprises a plurality of sections with overlapping components between which a binding agent is arranged . For activation of this binding agent , the oven 5 is used as a heating means for heating the component stack 3 . Therefore , the oven 5 may comprise one or more heating arrangements 9 .

In fig . 3 , a detailed view of an edge area of the stack 3 is shown . The stack 3 comprises a plurality of textile components 10 , which are arranged in such manner that they are at least partly overlapping . The textile components 10 may be layers of a fiber material like glass fibers , carbon fibers and/or aramid fibers . In between the components 10 , the binding agent is distributed . The binding agent may be for instance a powder binder comprising a thermoplastic of thermo- set material , which is used for locally adhering the textile components 10 together .

The binding agent is heated, or activated, respectively, using the oven 5 so that the binding agent migrates locally into the textile components 10 . After the migration, when the binding agent is cooled for hardening again, the components 10 of the stack 3 are locally adhered to each other . The stack of components 3 then forms a preform building element which may be used for building a rotor blade of a wind turbine . In addition or alternatively to at least some of the textile components 10 , also rigid core components may be used as components for fabricating the preform building element . Also rigid core components , like panels of balsawood or the like , are adhered to adj acent components using the binding agent .

As can be seen from fig . 3 , the edge area 7 of the stack 3 comprises a di f ferent thickness than a center area 11 . The borderline between the edge area 7 and the center area 11 is schematically indicated by the dashed line 12 . Due to the reduced thickness in the edge area 7 , the edge area 7 is a section 13 of the component stack 3 which comprises a lower thickness than the center area 11 .

Additionally or alternatively two sections 13 with di f ferent thicknesses , the stack 3 of the components 10 may also comprise sections 13 with di f ferent types of components 10 , in particular sections with rigid core components and sections without rigid core components . These sections 13 may not necessarily be at the edge area 7 of the component stack 3 but may also be present within the middle section 11 .

Sections 13 with di f ferent thicknesses and/or di f ferent types of components may require di f ferent heat input to avoid excessive binding agent migration during the heating of the component stack 3 . Therefore , in a method for manufacturing of a preform building element according to the invention, at least one heat input reduction means 14 is used to reduce the heat input in the binding agent in at least one of the sections 13 of the stack 3 for reducing binding agent migration during the activation of the binding agent .

In fig . 4 , a first embodiment of the method is shown . In this embodiment , cover elements 15 are used as heat reducing means 14 . The cover elements 15 serve as a heat shield 16 and are arranged on the edge area 7 of the stack 3 to cover the edge area 7 of the stack 3 . In particular, the entire edge area 7 of the component stack 3 along the circumference of the stack 3 is covered . The center area 11 , in which the stack 3 comprises for instance a larger thickness than in the edge area 7 , remains uncovered since no cover elements 15 are arranged . The edge area 7 is embedded in between the carrier surface 4 and the cover elements 15 .

Each of the cover elements 15 comprises a tilted shape . The cover elements 15 may be an insulating material like expanded polystyrene or the like . The cover elements 15 are therefore thermally insulating and have a low thermal conductivity . This results in a reduction in the heat energy which is trans ferred to the edge area 7 of the component stack 3 . The cover elements 15 may each comprise one or more handles 38 for facilitating their arrangement . The components 10 of the component stack 3 may be arranged underneath a vacuum foil 17 or in other words , the components 10 of the component stack 3 may be covered by a vacuum foil 17 . Alternatively, the components 10 may be enclosed in a vacuum bag . The cover elements 15 may be arranged on top of the vacuum foil 17 , or the vacuum bag, respectively, so that they do not af fect the arrangement of the components and/or the application of the vacuum underneath the vacuum foil 17 , or in the vacuum bag, respectively .

In fig . 5 , a cut view of a section of the arrangement depicted in fig . 4 is shown . The cover elements 15 are used to cover a section 13 of the stack 3 , namely the edge area 7 . By using the cover elements 15 , the heat input from a heating means , for instance from the heating arrangement 9 of the oven 5 as depicted in fig . 2 , into the edge area 7 is reduced .

Correspondingly, the time which is required to reach the thermal equilibrium, is increased . By providing the cover elements 15 as a heat shield 16 , a more even temperature distribution across the component stack 3 through the entire heating process may be obtained . Advantageously, all areas , or sections 13 , respectively, of the component stack 3 experience equal or approximately equal temperatures , for instance within an interval of 10 K around a target temperature , and duration of the heating .

As it is depicted in fig . 6 , it is also possible that the cover elements 15 are placed underneath the vacuum foil 17 , or inside the vacuum bag, respectively . As an alternative to a thermally insulating material , also a material with a high speci fic heat capacity, which is able to store a lot of thermal energy, may be used for the cover elements 15 .

The cover elements 15 may for instance consist of balsawood as it would normally be used in the manufacture of the preform building elements , or of the rotor blade of the wind turbine , respectively . By providing cover element 15 consisting of or comprising a material with a high heat capacity, the edge area 7 or another section 13 of the stack 3 may be protected from the heat input from a heating means like the oven 5 . The section 13 underneath the cover element 15 heats up to a lower temperature compared to the center area 11 , which remains uncovered, so that the binder migration in the covered section 13 is reduced . The ef fect is equal to the addition of the cover elements 15 comprising from a thermally insulating material , but the underlined physical phenomena is di f ferent for a material comprising a higher speci fic heat capacity . In fig . 7 , a further embodiment of a heat shield 16 used as a heat input reduction means 14 is shown . In this embodiment , a foam layer 18 is used as a heat shield 16 , wherein the foam layer 18 is applied to the edge area 7 of the component stack 3 . The foam layer may be applied prior to the heating either manually or automatically by a foam distribution means on one or more sections 13 of the component stack 3 .

In fig . 8 , a further embodiment of a heat shield 16 used as a heat input reduction means 14 is shown . In this embodiment , a reflective foil 19 is used as heat shield 16 . The reflective foil 19 is arranged underneath the vacuum foil 17 , which is at least partly transparent . The reflective foil 19 may be for instance aluminium foil , which allows for simple , accurate and flexible shielding of one or more sections 13 of the stack 3 .

As is schematically depicted in fig . 9 , the heat , indicated by the arrows 20 , is partly reflected from the reflective foil 19 in the edge area 7 of the component stack 3 . Therefore , the reflective foil 19 reduces the heat input into the edge area 7 and therefore reduces the binding agent migration in the edge area 7 .

In a typical oven 5 , the heat energy is trans ferred to the preform stack 3 both as a flow of heated air, or in a convective heat trans fer, respectively, and as radiated heat from the surfaces of the housing 6 of the oven and/or from the heating device 9 in a radiative heat trans fer . The reflective foil 19 may have a low emissivity to reduce the local heat energy being absorbed in the edge area 7 . The reflective foil 19 may be placed for instance underneath the vacuum foil 17 or on top of the vacuum foil 17 .

The at least one section 13 , which is covered by the reflective foil 19 , may be easily adj usted allowing for fine tuning of the heating process . It is also possible that the flexible foil 19 , or an additional flexible foil 19 , respectively, is placed underneath the edge area 7 of the stack 3 , and hence in between the stack 3 and the surface 4 of the carrier 2 . Also the further reflective foil 19 may be placed in a vacuum bag or outside of the vacuum bag, respectively .

In fig . 10 , a further embodiment of a method for manufacturing a preform building element is shown . The method uses a first embodiment of an oven 5 according to the invention . For reducing the heat input in the edge area 7 of the component stack 3 , a heat shield 16 is used as heat input reduction means 14 . In this embodiment , the heat shield 16 is provided as a shielding element 21 . As shielding element 21 , a shielding panel 22 is used . The shielding panel 22 is arranged in a distance above at least one of the sections 13 of the component stack 3 , or the edge area 7 of the component stack 3 , respectively . The shielding panel 22 is attached to an inner wall 42 of the housing 6 of the oven 5 by a hinge 23 , so that it might be pivoted as indicated by the arrow 24 . The shielding element reduces the heat trans fer via convective and radiative heat trans fer, as schematically indicated by the arrows 20 in fig . 11 . By varying the angle between the shielding panel and the surface 4 of the carrier 2 , or the wall 6 of the housing, respectively, the width of the covered area may be adapted to di f ferently large edge areas 7 of di f ferent types of preform building elements to be fabricated .

The shielding means 21 may be provided in a wide variety of shapes , for instance as shielding panels or, as it is depicted in fig . 12 , as shielding curtains 25 . The shielding curtains 25 are attached to the inner wall of the housing 6 by a slidable coupling using a plurality of rings 26 and a plurality of rods 27 , on which the rings 26 are hanging . Also the rods 27 may be attached to the inner wall of the housing 6 by a hinge 23 , so that the rods 27 and the shielding curtains 25 may be pivoted towards the wall , when a further heat protection of the edge area 7 of the component stack 3 is not required . As can be seen from fig . 13 , the shielding curtains are slidable along the rods 27 , so that the portion, which is covered by the shielding curtains 25 may be adj usted to cover di f ferently wide edge areas 7 and/or other sections 13 of the component stack 3 . Advantageously, this allows the adaption of the shielding curtains 25 to di f ferent preform building elements to be manufactured .

The carrier 2 is arranged inside a receiving volume 39 of the oven, wherein the shielding elements 21 , in particular the shielding panels 22 and/or the shielding curtains 25 are arranged directly adj acent to the receiving volume 39 and/or in a distance above the receiving volume . This allows for covering a section 13 of the component stack 3 , when the carrier with the corresponding component stack 3 is received in the receiving volume 39 of the oven 5 .

In fig . 14 , a further embodiment of a method for manufacturing a preform building element for building a rotor blade of a wind turbine using a further embodiment of an oven 5 is depicted . The oven 5 is adapted to provide a hot air flow streaming through an air stream volume 40 above the receiving volume 39 , in which the carrier 2 and the component stack 3 are arranged .

As heat input reduction means 14 , a plurality of air flow separation means 41 are used, which are each provided as a plate-like separation element 28 . The separation elements 28 are arranged above the receiving volume 39 to hori zontally separate at least a part of the air stream volume 40 for locally reducing the air flow speed at least above the section of the receiving volume corresponding to a section 13 of the component stack 3 . The hot air may be blown through the interior of the housing 6 of the oven 5 as indicated by the arrows 29 . The plate-like separation elements 28 separate the air flow volume 40 above the top surface of the component stack 3 . Dividing the air flow volume 40 into a plurality of subvolumes in between two adj acently arranged separation elements 28 may cause in particular a laminar air flow instead of a turbulent air flow, in particular in the limited portion of the air flow volume 40 above the edge section 7 , which is shielded at each side from the remainder of air flow volume 40 above the middle section 11 by the separation element 28 . In regions of laminar air flow, less heat is trans ferred to the component stack 3 since the convection coef ficient from the air onto the preform components of the stack 3 is signi ficantly reduced .

Also the separation element 28 may be slidably coupled to the housing 6 , for instance by providing a plurality of ori fices 30 in the separation element 28 , which engage in rods 27 protruding from the inner wall of the housing 6 . This allows for sliding the separation elements 28 along the rods 27 , as it is schematically indicated by the arrow 31 .

In fig . 16 , a further embodiment of a method using a further embodiment of the oven 5 is shown . In this embodiment , the oven 5 comprises an air curtain arrangement 32 , wherein the air curtain arrangement 32 comprises a plurality of j ets 33 . The air curtain arrangement 32 is used as an air flow separation means 41 and therefore as heat input reduction means 14 .

Through the j ets 33 , at least one air stream directed to a section of the receiving volume is generatable . In this embodiment , the air stream produced by the air curtain arrangement 32 is directed on the edge section 7 of the component stack 3 , as schematically indicated by the arrows 34 . It is also possible that the air curtain arrangement 32 comprises further j ets 33 , which are directed for instance on the middle part 11 or on further sections 13 of the component stack 3 , as schematically indicated in dashed lines in fig 17 showing the cut view corresponding to fig . 16 . Through the jets 33, air can be blown to create curtains within the oven that divide the sections 13 of the component stack 3 that require different levels of heat energy. Therefore, the air can be blown in particular on the edge of a section 13 and/or on the boundary between two adjacent sections 13. The speed of air circulating above each section 13 of the stack 3 can be adjusted to change the heat transfer coefficient from the hot air created from the oven 5 to the component stack 3. By having high levels of air flow over thick areas of the component stack 3 and low levels of air flow over thinner sections 13 of the component stack 3, a relatively uniform heating of the stack 3 can be achieved. The jets 33 may be adjustable to adjust the direction of the air flow, or the position of the air curtains created by the air curtain arrangement 32, respectively. This allows for adapting the oven 5 for the manufacture different types of preform building elements.

In fig. 18, a further embodiment of a method using a further embodiment of an oven 5 is shown. In this embodiment, the oven comprises a cooling arrangement 35, which is provided as a cooling duct 36. The cooling arrangement 35 is arranged underneath the carrier 2, in particular so that the cooling duct is in direct contact with the lower surface of the carrier 2. The cooling arrangement 35, or the meandering cooling duct, respectively, is abutting the lower side of the carrier 2 adjacently to the edge area 7 of the stack 3. The cooling arrangement 35 may be used to cool the edge area 7 to reduce the net heat input into the edge area 7, since further heat is removed from the edge area 7 due to the cooling arrangement 35.

In addition, the cooling arrangement 35 may comprise openings 37 in the housing 6 of the oven 5. The openings 37 may be opened for instance automatically, when the edge area 7 of the component stack 3 has been exposed to a sufficient temperature for a sufficient duration, or to a sufficient heat input, respectively. By opening a cover of the openings 37, the temperature in the edge area of the component stack 3 can be reduced . Additionally or alternatively, the cooling arrangement 35 may comprise fans or j ets , which allow for directing an air stream of cool air on the edge area 7 , or on other sections 13 of the component stack 3 , respectively .

The cooling duct 37 may also be used for providing a heated liquid to the edge area 7 , or through other sections 13 of the stack 3 , respectively, during the manufacture of the preform elements . This allows for adapting the manufacturing process also for the fabrication of preform building elements , which for instance comprise thicker edge regions and/or edge regions with di f ferent types of components in the stack 3 .

The cooling arrangement 35 may be used to cool at least one section 13 of the component stack 3 once a target heat input and/or a target heating temperature of the section 13 has been reached to prevent this section from overheating . Therefore , a control unit (not depicted) may be provided, which activates the cooling arrangement 35 , for instance by providing a cooling flow through the cooling duct 36, by opening one or more of the openings 37 in the housing and/or by activating and/or adj usting an air flow and/or an air temperature of a cool air stream provided by a cooling means of the cooling arrangement 35 . The cooled section 13 of the component stack 3 may be considered to have a shorter heating cycle time than an uncooled section 13 , ef fectively reducing the heat input in the binding agent in the cooled section 13 .

As can be seen from fig . 19 , the cooling means 37 may be combined with other types of heat input reduction means . Exemplarily, a shielding element 21 provided as a shielding panel 22 is shown . The cooling through the cooling arrangement 35 is more ef ficient when it is combined with other means for reducing the heat input in the edge area 7 , since the amount of heat , which has to be removed using the cooling arrange- ment 35 , is reduced, when the heat is partly shielded or not trans ferred to the component stack 3 , respectively .

It is possible that in the method for manufacturing a preform building element , more than one embodiment of a heat input reduction means 14 is used . The aforementioned embodiments of heat input reduction means 14 may be used in any combination .

Also an oven 5 according to the invention may comprise di fferent embodiments of a heat input reduction means 14 . An oven 5 may comprise one pair of rails 8 or a plurality of pairs of rails 8 , so that more than one carrier 2 may be inserted in the oven 5 at the same time . In this case , the heat input reduction means 14 may be provided adj acently to each inserted carrier 2 , or to each receiving volume 39 , respectively, in particular when the oven 5 heats the component stack 3 using hot air as previously described .

Although the present invention has been described in detail with reference to the preferred embodiment , the present invention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention .