Patent Description:
Wind power is becoming increasingly popular due to its clean and environmentally friendly production of energy. The rotor blades of modern wind turbines capture kinetic wind energy by using sophisticated blade design created to maximize efficiency. Turbine blades may today exceed <NUM> metres in length and <NUM> metres in width. The blades are typically made from a fibre-reinforced polymer material and comprise a pressure side shell half and a suction side shell half. The cross-sectional profile of a typical blade includes an airfoil for creating an air flow leading to a pressure difference between both sides. The resulting lift force generates torque for producing electricity.

The shell halves of wind turbine blades are usually manufactured using molds. First, a blade gel coat or primer is typically applied to the mold. Subsequently, fibre reinforcement and/or fabrics are placed into the mold followed by resin infusion. A vacuum is typically used to draw epoxy resin material into a mold. Alternatively, prepreg technology can be used, in which a fibre or fabric pre-impregnated with resin forms a homogenous material which can be introduced into the mold. Several other molding techniques are known for manufacturing wind turbine blades, including compression molding and resin transfer molding. The shell halves are assembled by being glued or bolted together substantially along a chord plane of the blade. The root region of each shell half typically has a circular cross section.

In vacuum assisted resin transfer molding (VARTM), glass fibre plies are placed in a mold with the correct orientation and subsequently resin is forced to flow through the fibres using a vacuum pump. This is usually followed by a curing cycle at atmospheric pressure.

A typical molding process includes bagging, resin infusion and subsequent curing. Bagging involves placing a vacuum foil on the fibre plies that have been laid up on the tool. The vacuum foil is used to press this part to the tool and to allow a vacuum to be drawn into the void formed by the bag and the tool such that the fibres of the part are infused with resin. Typical vacuum foils may be formed by one or more plastic sheets which are placed to cover the blade. Infusion comprises feeding resin under a vacuum to wet the laid out fibres to form a solid shell part. In subsequent curing, heating and subsequently cooling may be applied to harden the resin.

In particular when manufacturing large blades, the glass fibre layup at the root end becomes critical. Glass fibre material may slide down the almost vertical shell mold walls. The sliding of fibre material during manufacturing may lead to the formation of undesired wrinkles in the shell structure, which may present zones of structural weakness within the blade.

European Patent <CIT> discloses a method for manufacturing a wind turbine rotor blade comprising a trailing edge comprising fibre rovings. The method comprises the steps of laying up a number of layers comprising fibre material onto the inner surface of a first mold part, laying up a plurality of fibre rovings onto the number of layers at a position which forms the trailing edge of the blade, and casting the blade using Vacuum Assisted Resin Transfer Molding.

Similarly, <CIT> discloses a method involving layering of fibre material on a mold surface, includes laying rovings of said fibre material on the mold surface, or on a fibre material already laid on the mold surface, and applying vacuum to a space between the rovings and the mold surface. The rovings are rolled off a reel or laid out of a cassette.

<CIT> relates to a curved fibre mat including rovings being arranged side by side and being connected to one another in at least two connection areas by stitching. At least one of the rovings is continuous between the at least two connection areas and at least one of the rovings is discontinuous between the at least two connection areas.

<CIT> discloses a method of producing a composite shell structure comprising a reinforced fibre material embedded in a cured resin, i.e. a cured polymer material. The method comprises forming a preform by arranging a fibre material comprising a number of fibre layers on a preform forming surface, and bringing a mould part and the preform forming part together to an assembled position.

<CIT> relates to a unidirectional reinforcement comprising transversely arranged thin discrete flow passage forming means for ensuring good resin flow properties in a direction transverse to the direction of the unidirectional rovings. The thin discrete flow passage forming means are secured on the unidirectional rovings by means of additional yarns running on the thin discrete flow passage forming means and transverse thereto.

<CIT> discloses a method of manufacturing a wind turbine blade using prefabricated stacks of reinforcing material. The stacks comprise a plurality of plies of fibre material, joined together along a side edge to form a spine. The opposite edges of the stack are left unjoined so that the plies can separate and slide across one another.

Some of the afore-mentioned prior art approaches present challenges in handling the components such as fibre rovings, in particular in the root lay-up, as the slope of the mold for forming the wind turbine shell part can be quite steep. Thus, although fibre rovings may present an economical alternative to woven fibre mats or similar materials, their used is limited by these problems in layup and handling.

It is therefore an object of the present invention to overcome one or more of the above-discussed drawbacks of the known methods.

It is another object of the present invention to provide a method for molding a shell part of a wind turbine blade that is simple and cost-effective.

In particular, it is an object of the present invention to provide a method for molding a shell part of a wind turbine blade that is associated with improved stability and easier handling of the components used in said method.

In a first aspect, the present invention relates to a method of molding a shell half of a wind turbine blade, the blade having a profiled contour including a pressure side and a suction side, and a leading edge and a trailing edge with a chord having a chord length extending therebetween, the wind turbine blade extending in a spanwise direction between a root end and a tip end, said method comprising:.

By mixing and joining together the fibre rovings with a binding agent, the resulting preformed sheet is both cost-effective and easy to handle in the subsequent shell part molding step. The method of the present invention was found to result in material cost savings of up to <NUM>-<NUM> % owing to the simple and efficient use of fibre rovings instead of more costly materials, such as woven fibre mats. It has also been found by the present inventors, that the inventive method results in a reduced tendency of the fibre rovings to produce irregularities in the shell part laminate, since the individual roving will not be able to "stand up" or fold in a vertical direction. The latter is a problem of prior art fibre mats.

Without being bound by theory, it is believed that the binding agent is also beneficial in creating channels for the subsequent resin infusion by providing spacings between the rovings. This facilitates resin infusion of the preformed sheets after the sheets have been arranged in the blade mold.

The preformed sheet of the present invention was found to be sufficiently flexible and able to follow the necessary movements and form adaptions during manufacturing of the blade. In particular, such preformed sheet can be advantageously used for lay-up in parts of the blade molds, which are too steep for some of the afore-mentioned prior art approaches using fibre rovings. Since the fibre rovings are mixed and joined together by the binding agent, the risk of slipping, displacement or other unwanted movement is greatly diminished.

The shell part molded by the method of the present invention is a shell half. Usually, the tip end of the mold cavity will correspond to the tip end of the blade to be manufactured. Likewise, the root end of the mold cavity will usually correspond to the root end of the blade.

At least two or more preformed sheets are arranged in the mold cavity. In other embodiments, at least three, such as at least four, or at least five preformed sheets are arranged in the mold cavity. The preformed sheets may be arranged side-by-side in the mold cavity. Typically, each preformed sheet has a length, a width and a thickness, as well as two longitudinally extending lateral edges in between a top surface and an opposing bottom surface. Also, each preformed sheet will usually have a front edge and an opposing back edge. The top surface and bottom surface of the sheet may be constituted by a top fibre mat and a bottom fibre mat, respectively.

In a preferred embodiment, the rovings comprise glass fibres or consist of glass fibres. In other embodiments, the rovings may comprise, or consists of, glass fibres, carbon fibres, aramid fibres, basalt fibres, natural fibres or mixtures thereof.

In one embodiment, the rovings are arranged side-by-side within the preformed sheets. In other embodiments, the rovings are arranged unidirectionally within the preformed sheets. Usually, the mixture between fibre rovings and binding agent is such that at least <NUM>%, more preferably at least <NUM>%, most preferably at least <NUM>% of the surface of the fibre rovings is contacted with the binding agent.

The resin for injecting the one or more preformed sheets may be an epoxy, a polyester, a vinyl ester or another suitable thermoplastic or duroplastic material.

In a preferred embodiment, each preformed sheet further comprises at least one fabric, such as a top fibre mat and/or a bottom fibre mat. The fibre rovings may be arranged on top and/or below such fabric.

In a preferred embodiment, the binding agent is present in an amount of <NUM>-<NUM> wt% relative to the weight of the fibre rovings. Preferably, the binding agent is present in an amount of <NUM>-<NUM> wt%, preferably <NUM>-<NUM> wt%, more preferably <NUM>-<NUM> wt%, relative to the weight of the fibre rovings. The binding agent may also comprise two or more different substances, as long as the total binding agent is present in an amount of <NUM>-<NUM> wt% relative to the weight of the fibre rovings.

It was found that the comparatively low amount of binding agent of <NUM>-<NUM> wt% relative to the weight of the fibre rovings provides improved flexibility as contrasted to known fibre preforms for manufacturing wind turbine blades. It was also found that this amount of binding agent results in sufficient stability for handling during the blade molding process.

In a preferred embodiment, the binding agent is a thermoplastic binding agent. Typically, the fibre rovings are at least partially joined together by means of the binding agent by thermal bonding. In a preferred embodiment, the binding agent is a binding powder, such as a thermoplastic binding powder.

According to another embodiment, the melting point of the binding agent is between <NUM>° and <NUM>, preferably between <NUM> and <NUM>, such as between <NUM> and <NUM>, or between <NUM> and <NUM>.

According to another embodiment, the preformed sheets have an elastic modulus (Young's modulus) of between <NUM> and <NUM> GPa, preferably <NUM>-<NUM> GPa, such as between <NUM>-<NUM> GPa or between <NUM>-<NUM> GPa. Preformed sheets with such elasticity were found to be particularly well suited for a blade manufacturing process according to the present invention.

According to one embodiment, the binding agent is a thermoplastic binding agent. According to another embodiment, the binding agent comprises a polyester, preferably a bisphenolic polyester. An example of such binding agent is a polyester marketed under the name NEOXIL <NUM>. Examples include NEOXIL <NUM> PMX, NEOXIL <NUM> KS <NUM> and NEOXIL <NUM> HF 2B, all manufactured by DSM Composite Resins AG. Preferably, the binding agent is a polyester, preferably a bisphenolic polyester. In other embodiments, the binding agent is a hotmelt adhesive or based on a prepreg resin.

In one embodiment, the preformed sheets are arranged in the mold cavity such that the longitudinal axes of the preformed sheets are aligned substantially parallel to each other.

According to another embodiment, the preformed sheets are arranged in the mold cavity such that a longitudinally extending lateral edge of at least one preformed sheet abuts a longitudinally extending lateral edge of an adjacent preformed sheet. The indivdual sheets will then usually be connected to the adjacent sheets by the resin infusion step. If more than two preformed sheets are arranged in this embodiment, for some sheets, both lateral edges will abut respective lateral edges of adjacent sheets on either side.

Alternatively, the preformed sheets may be arranged in the mold cavity such that a longitudinally extending lateral edge of at least one preformed sheet overlaps with an adjacent preformed sheet. Again, the indivdual sheets will then usually be connected to the adjacent overlapping sheets by the resin infusion step.

According to another embodiment, each preformed sheet has a length, width and thickness, wherein its length-width ratio is at least <NUM>:<NUM>. In some embodiments, the length-width ratio is at least <NUM>:<NUM>, such as at least <NUM>:<NUM> or at least <NUM>:<NUM>, such as at least <NUM>:<NUM> or at least <NUM>:<NUM>.

According to another embodiment, each of the preformed sheets further comprises a top fibre mat and a bottom fibre mat in between which the fibre rovings are arranged. For the fibre mats, a woven mat of fibre material may be used.

In one embodiment, the one or more preformed sheets are placed onto a fibre material, a blade gel coat and/or a primer, already laid on the surface of the mold cavity.

According to another embodiment, the method further comprises a step of laying up a vacuum foil onto the preformed sheets as the topmost layer prior to resin injection. Preferably, the blade part is molded using Vacuum Assisted Resin Transfer Molding.

Typically, the mold cavity comprises a substantially semi-circular cross section at its root end. The preformed sheets will usually be arranged such in the mold cavity that they extend from the root end towards the tip end of the mold cavity.

According to another embodiment, the length of each preformed sheet is at least <NUM>, preferably at least <NUM>, such as at least <NUM>, or at least <NUM>. In another embodiment, the length of a preformed sheet extends along the full length of the blade part.

In a preferred embodiment, the thickness of at least one preformed sheet decreases from its front edge to its back edge as seen in its longitudinal direction. Such a tapered shape is advantageous in that it provides a gradual transition between a relatively high wall thickness in the root region and a usually lower wall thickness in the transition region and the airfoil region of the blade.

According to another embodiment, the preformed sheets are arranged in the mold cavity such that the angle between the horizontal plane and a line that is tangential to the vertex of a curved bottom surface of a preformed sheet is different for each preformed sheet.

According to another embodiment, at least one preformed sheet is arranged in the mold cavity such that the angle between the horizontal plane and a line that is tangential to the vertex of a curved bottom surface of said preformed sheet is more than <NUM>°, preferably more than <NUM>°, such as more than <NUM>°. Thus, the preformed sheet of the present invention is advantageous in that it can be laid along a curved path or a steep mold cavity surface without producing wrinkles or creases, and without any significant sliding of the fibre rovings in the mold.

According to another embodiment, the preformed sheets are arranged in the mold cavity in a region between the root end of the mold cavity and the position of maximum chord length, as seen in the longitudinal direction of the mold cavity.

In another aspect, the present invention relates to a shell part of a wind turbine blade obtainable by the method of the present invention.

In yet another aspect, the present invention relates to a preformed sheet for use in a method according to the present invention, the preformed sheet comprising a mixture of fibre rovings and a binding agent, wherein the fibre rovings are at least partially joined together by means of the binding agent, and wherein the binding agent is present in an amount of <NUM>-<NUM> wt% relative to the weight of the fibre rovings.

The different embodiments and features of the preformed sheet, as described above in the context of the method of molding a shell part, equally apply to the preformed sheet as such and may be combined accordingly. In particular, the thickness of the preformed sheet of the present invention may decrease from a front edge to a back edge as seen in its longitudinal direction.

Typically, the preformed sheets have an at least partly curved top surface and/or an at least partly curved bottom surface. When laying the preformed sheet into a curved mold cavity, the bottom surface of the preformed sheet may advantageously adapt the form of the curved mold cavity surface. In other embodiments, the preformed sheets have a planar top surface and/or a planar bottom surface.

In another aspect, the present invention relates to a method of manufacturing a preformed sheet according to any of the preceding claims comprising the steps of contacting fibre rovings with a binding agent, and subsequently heating the fibre rovings and the binding agent for forming the preformed sheet.

The step of contacting the fibre rovings with a binding agent may be accomplished by using a bath containing the binding agent and pulling the fibre rovings through the bath. Subsequently the fibre rovings and binding agent can be laid up in the preform mold, for example after having been pulled through a nozzle or the like, for ensuring unidirectional orientation of the fibre rovings. Alternatively, the step of contacting the fibre rovings with a binding agent may be accomplished during the roving manufacturing process, such as during sizing.

In one embodiment, the mixture of the fibre rovings and the binding agent is laid in a preform mold, followed by heating the laid up fibre rovings and binding agent for forming the preformed sheet. Advantageously, the preform molds do not need to be vacuum tight, which is beneficial in terms of cost reduction. Also, typically no extreme precision is required for manufacturing the preformed sheets (cm range rather than mm range). In some embodiments, the fibre rovings are laid on a bottom fibre mat and subsequently covered by a top fibre mat. Thus, the fibre rovings are sandwiched between two fibre mats. For the fibre mats, a woven mat of fibre material may be used.

The fibre rovings can advantageously be laid back and forth around a pin or similar device. In one embodiment, the rovings are arranged side by side in the preform mold.

Preferably, the mold cavity of the preform mold is substantially horizontal, i.e. only minimally curved. In one embodiment, the preform mold has a curved mold cavity, wherein the ratio of the width (Wm) and the maximum height (H) of the curved mold cavity is <NUM>:<NUM> or more, such as <NUM>:<NUM> or more, or <NUM>:<NUM> or more. Such a mold of minimal curvature may advantageously allow for the lay-up of fibre rovings, which would be more challenging in a more curved, steeper mold cavity. Thus, the resulting shell part may include a higher amount of fibre rovings, replacing some of the previously needed fibre mats. The same approach would not be feasible directly in the blade mold because of the steep slopes of its mold cavity, in particular at the circular root end thereof.

As used herein, the term "sheet" denotes a laminar element having a width and length substantially greater than the thickness thereof.

As used herein, the term "horizontal" refers to an orientation parallel to the ground upon which the mold that used in the method of the present invention is placed.

The term "substantially parallel", as used herein, refers to the respective longitudinal axes of two adjacent preformed sheets not intersecting at an angle greater than <NUM>°, preferably not greater than <NUM>°, most preferred not greater than <NUM>°.

As used herein, the term "wt%" means weight percent. The term "relative to the weight of the fibre rovings" means a percentage that is calculated by dividing the weight of an agent, such as a binding agent, by the weight of the fibre rovings. As an example, a value of <NUM> wt% relative to the weight of the fibre rovings corresponds to <NUM> of binding agent per kilogram of fibre rovings.

The skilled reader will understand that the elastic modulus, also known as Young's modulus, defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material. Thus, the elastic modulus is a measure of the stiffness of a material. The elastic modulus can be determined by the cantilever beam test, as is well known in the art.

<FIG> illustrates a conventional modern upwind wind turbine according to the so-called "Danish concept" with a tower <NUM>, a nacelle <NUM> and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub <NUM> and three blades <NUM> extending radially from the hub <NUM>, each having a blade root <NUM> nearest the hub and a blade tip <NUM> furthest from the hub <NUM>. The rotor has a radius denoted R.

<FIG> shows a schematic view of a first embodiment of a wind turbine blade <NUM> according to the invention. The wind turbine blade <NUM> has the shape of a conventional wind turbine blade and comprises a root region <NUM> closest to the hub, a profiled or an airfoil region <NUM> furthest away from the hub and a transition region <NUM> between the root region <NUM> and the airfoil region <NUM>.

<FIG> and <FIG> depict parameters which are used to explain the geometry of the wind turbine blade according to the invention.

<FIG> shows a schematic view of an airfoil profile <NUM> of a typical blade of a wind turbine depicted with the various parameters, which are typically used to define the geometrical shape of an airfoil. The airfoil profile <NUM> has a pressure side <NUM> and a suction side <NUM>, which during use - i.e. during rotation of the rotor - normally face towards the windward (or upwind) side and the leeward (or downwind) side, respectively. The airfoil <NUM> has a chord <NUM> with a chord length c extending between a leading edge <NUM> and a trailing edge <NUM> of the blade. The airfoil <NUM> has a thickness t, which is defined as the distance between the pressure side <NUM> and the suction side <NUM>. The thickness t of the airfoil varies along the chord <NUM>. The deviation from a symmetrical profile is given by a camber line <NUM>, which is a median line through the airfoil profile <NUM>. The median line can be found by drawing inscribed circles from the leading edge <NUM> to the trailing edge <NUM>. The median line follows the centres of these inscribed circles and the deviation or distance from the chord <NUM> is called the camber f. The asymmetry can also be defined by use of parameters called the upper camber (or suction side camber) and lower camber (or pressure side camber), which are defined as the distances from the chord <NUM> and the suction side <NUM> and pressure side <NUM>, respectively.

Airfoil profiles are often characterised by the following parameters: the chord length c, the maximum camber f, the position df of the maximum camber f, the maximum airfoil thickness t, which is the largest diameter of the inscribed circles along the median camber line <NUM>, the position dt of the maximum thickness t, and a nose radius (not shown). These parameters are typically defined as ratios to the chord length c. Thus, a local relative blade thickness t/c is given as the ratio between the local maximum thickness t and the local chord length c. Further, the position dp of the maximum pressure side camber may be used as a design parameter, and of course also the position of the maximum suction side camber.

<FIG> shows other geometric parameters of the blade. The blade has a total blade length L. As shown in <FIG>, the root end is located at position r = <NUM>, and the tip end located at r = L. The shoulder <NUM> of the blade is located at a position r = Lw, and has a shoulder width W, which equals the chord length at the shoulder <NUM>. The diameter of the root is defined as D. The curvature of the trailing edge of the blade in the transition region may be defined by two parameters, viz. a minimum outer curvature radius ro and a minimum inner curvature radius ri, which are defined as the minimum curvature radius of the trailing edge, seen from the outside (or behind the trailing edge), and the minimum curvature radius, seen from the inside (or in front of the trailing edge), respectively. Further, the blade is provided with a prebend, which is defined as Δy, which corresponds to the out of plane deflection from a pitch axis <NUM> of the blade.

<FIG> illustrates a mold <NUM> comprising a mold cavity <NUM> for molding a shell part of a wind turbine blade. The mold cavity has a root end <NUM> and an opposing tip end <NUM> corresponding to the respective root and tip ends of the blade to be manufactured. In the embodiment shown in <FIG>, three preformed sheets 72a, 72b, 72c are arranged in the mold cavity <NUM> for subsequent infusion with a resin to mold the shell part, e.g. by Vacuum Assisted Resin Transfer Molding. The respective longitudinal axes 74a, 74b, 74c of the preformed sheets 72a, 72b, 72c are arranged such that they are aligned substantially parallel to each other. A different embodiment of the method is illustrated in <FIG>. Here, only one preformed sheet <NUM> is arranged in the mold cavity <NUM>, acting as a main laminate extending along substantially the entire blade length.

As best seen in the root end front view of <FIG>, the preformed sheets 72a, 72b, 72c are arranged such that a longitudinally extending lateral edge 76a of each preformed sheet abuts a longitudinally extending lateral edge 76b of an adjacent preformed sheet (as exemplified for sheets 72a and 72b). The sheets 72a, 72b, 72c are then joined together in the subsequent resin infusion step, optionally after laying a vacuum foil <NUM> as the topmost layer.

In an alternative embodiment shown in the root end front view of <FIG>, the preformed sheets 72a, 72b, 72c are arranged such in the mold <NUM> that a longitudinally extending lateral edge 76a, 76b of each preformed sheet 72a, 72b, 72c overlaps with an adjacent preformed sheet. Again, the sheets 72a, 72b, 72c are subsequently joined together in by resin infusion and curing (vacuum foil not shown). <FIG> also shows that at least the preformed sheet 74c is arranged in the mold cavity such that the angle α between the horizontal plane <NUM> and a line <NUM> that is tangential to the vertex of a curved surface of the sheet exceeds <NUM>°.

<FIG> illustrates a possible manufacturing method for a preformed sheet <NUM> of the present invention, in particular one that can be used as main laminate. Herein, the fibre rovings <NUM> and binding agent are sandwiched between a number of fabrics, or top and bottom mats <NUM>, <NUM>, heated in a heating station <NUM>, and subsequently laminated in a lamination station <NUM> to produce the preformed sheet <NUM>.

<FIG> shows a schematic drawing of another embodiment of a preformed sheet <NUM> as molded in a substantially horizontally oriented preform mold <NUM>. The sheet <NUM> has a length Ls, a thickness Ts, and a width Ws. It is formed by sandwiching a plurality of fibre rovings <NUM> in between a bottom fibre mat <NUM> and a top fibre mat <NUM>. This could be done by laying a mixture of fibre rovings <NUM> and a thermoplastic binding agent on top of the bottom fibre mat <NUM>, covering the rovings <NUM> with the top fibre mat <NUM>, followed by heating to form the preformed sheet. As best seen in <FIG> and <FIG>, the preformed sheet <NUM> has a longitudinally extending lateral edge <NUM> extending in between a top surface <NUM> and a bottom surface <NUM> of the sheet <NUM>.

The cross-sectional view of <FIG> shows some dimensions of the preform mold <NUM>. It has a curved mold cavity <NUM> with a width Wm and a maximum height H, wherein the width Wm and maximum height H have a ratio of <NUM>:<NUM> or more.

<FIG> shows another embodiment of a preformed sheet <NUM> according to the present invention. Here, the thickness of the preformed sheet <NUM> decreases from its front edge <NUM> to its back edge <NUM> as seen in its longitudinal direction. Typically, the front edge <NUM> with the higher thickness will be located at the root end of the blade mold cavity when laying the preformed sheets, while the back edge <NUM> with the lower thickness will be closer to the tip end of the mold cavity.

Claim 1:
A method of molding a shell half of a wind turbine blade, the blade (<NUM>) having a profiled contour including a pressure side and a suction side, and a leading edge (<NUM>) and a trailing edge (<NUM>) with a chord having a chord length extending therebetween, the wind turbine blade (<NUM>) extending in a spanwise direction between a root end (<NUM>) and a tip end (<NUM>), said method comprising:
- providing a mold (<NUM>) comprising a mold cavity (<NUM>) with a root end (<NUM>) and an opposing tip end (<NUM>),
- arranging three or more preformed sheets (72a, 72b, 72c) in the mold cavity (<NUM>), wherein each preformed sheet comprises a mixture of fibre rovings (<NUM>) and a binding agent, wherein the fibre rovings are at least partially joined together by means of the binding agent, and
- injecting the three or more preformed sheets (72a, 72b, 72c) with a resin to mold the shell half,
wherein the preformed sheets (72a, 72b, 72c) are arranged in the mold cavity (<NUM>) such that both longitudinally extending lateral edges of at least one preformed sheet (72b) abut respective longitudinally extending lateral edges of adjacent preformed sheets (72a, 72c) on either side, and/or wherein the preformed sheets are arranged in the mold cavity (<NUM>) such that both longitudinally extending lateral edges of at least one preformed sheet (72b) overlap with respective adjacent preformed sheets (72a, 72c) on either side.