TITLE
Method and mould system for net moulding of a co-cured, integrated
structure.
TECHNICAL FIELD
The present invention relates to a mould system and a method for net
moulding of a co-cured integrated structure comprising a skin provided with
stringers and rib feet. The structure can e.g. be used in the fuselage of
aircrafts or in boats.
BACKGROUND ART
When manufacturing integrated composite structures, such as skins
reinforced by rib feet also called "shear ties" and stringers see fig. 1, the rib
feet can either be connected or not connected with a composite layer against
the stringers.
If the rib foot is not connected with the stringer, it is desirable to have a gap
between the rib foot and the stringer in order to minimize the stress
concentration in that area. It is known today to manufacture aircraft skin
structures with a gap between the rib feet and the stringer.
The structures can e.g. be made of carbon-fibre-reinforced polymer or
carbon-fibre-reinforced plastic (CFRP or CRP or often simply carbon fibre).
CFRP or CRP is a very strong and light fibre-reinforced polymer which
contains carbon fibres. The polymer is most often epoxy, but other polymers,
such as phenolic and Bismaleimide are sometimes used.
In the known fabrication process prepregs are used to create the skin, the
stringers and the rib feet. Prepreg is the shortened name used as a reference
to composite fabrics that have been pre-impregnated with thermosetting
resin but not yet permanently cured. These usually take the form of thin
sheets consisting of a combination of a matrix (or resin) and fibre
reinforcement. The fibres can have one direction (unidirectional
reinforcement) or several directions (fabric reinforcement). If the fibres are
unidirectional the solid structure is obtained by placing a number of prepregs
on top of each other, with varying fibre direction. The role of the matrix is to
support the fibres and bond them together in the composite material. It also
keeps the fibres in their position and chosen orientation. The prepregs are
mostly stored in cooled areas since activation is most commonly done by
heat.
A structure part is created in two steps, lay-up processing and autoclave
processing. In the lay-up process the structure is formed. This is done by
applying a number of prepregs, resulting in a stack of prepreg, on a mould. A
vacuum bag is placed and sealed over the prepregs and the mould and all air
under the bag is evacuated. In the autoclave process the mould covered with
the stack of prepreg and the vacuum bag is put into and treated in an
autoclave. An autoclave is a pressure vessel which provides the curing
conditions for the composite where the application of vacuum, pressure, heat
up rate and cure temperature is controlled. In the autoclave a vacuum is still
maintained under the vacuum bag, and a heat and a high pressure is applied
to the composite structure.
In the existing fabrication process of aircraft structures, the skin, the stringers
and the rib feet are formed and cured separately. The cured stringers and rib
feet are bonded to the skin and trimmed to get the desired form comprising
the gap between the stringers and the rib feet.
The problem with the solution according to the prior art is that the trimming
and bonding operation is time consuming. There is thus a need for a faster
and more efficient way of manufacturing the reinforced structures with a gap
between the stringers and the rib feet.
DEFINITIONS
It shall be noted that through this application "longitudinal" refers to the
direction being parallel to the stringers and "transversal" refers to the
direction being parallel to the rib feet. The stringers are defined as the
longitudinal reinforcements and the rib feet are defined as the transversal
reinforcements of the structure.
SUMMARY
The object of the present invention is to provide an inventive method for net
moulding a structure for e.g. aircrafts, boats or other crafts comprising a skin
provided with at least two stringers and at least one rib foot and a gap
between the stringers and the at least one rib foot, where the previously
mentioned problems are avoided. This object is achieved by mould system
according to claim 1. The claim reveals a mould system for net moulding of a
co-cured structure comprising a skin provided with at least two stringers and
at least one rib foot and a gap between the stringers and the at least one rib
foot.
The mould system comprises a substantially rectangular frame made of a low
heat expanding material. The frame comprises a first longitudinal frame part,
a second longitudinal frame part, a first transversal frame part and a second
transversal frame part.
The mould also comprises at least one row unit wherein each row unit
comprises a row of at least two substantially rectangular boxes made of a
heat expanding material. Each box has a bottom, a first longitudinal side wall,
a second longitudinal side wall, a first transversal side wall and a second
transversal side wall. The at least two boxes in the row are positioned such
that the first transversal side wall of a first box is facing the second
transversal side wall of a second box.
The mould further comprises at least two connection plates connecting the at
least two boxes in the row of boxes along the first longitudinal side walls and
along the second longitudinal side walls by means of fastening means. Each
connection plate is adapted to be positioned on the first longitudinal side
walls or on the second longitudinal side walls of at least two adjacent boxes
in the row, and each connection plate is adapted to be connected to each of
the longitudinal side walls on which the connection plate is positioned. Each
connection plate is also adapted to extend along at least a part of each of the
first longitudinal side walls on which the connection plate is positioned.
A first longitudinal side of the at least one row unit is constituted by the first
longitudinal side walls of the boxes in the row unit and by the connection
plates being connected thereto and a second longitudinal side of the row unit
is constituted by the second longitudinal side walls of the boxes in the row
unit and of the connection plates being connected thereto. An external
surface of the first longitudinal side walls of the boxes and of the second
longitudinal side walls of the boxes and an external surface of the connection
plates in the at least one row unit have a form such that the first longitudinal
side and the second longitudinal side of the at least one row unit form flat
surfaces.
The advantage with the mould system is that a whole structure can be
formed and cured as one net moulded unit, which minimises the time for
trimming of the structure, as will be described in the method.
Said object is further achieved by a mould system according to first and a
second embodiment. This claim reveals a mould system wherein a flat
surface of a longitudinal side of the at least one row unit may be obtained by
at least one box in the row unit having a form such that an external surface of
at least one of the corresponding longitudinal side walls of the box is flat.
The advantage with the mould system according to this embodiment is that
the length of the connection plates is flexible and that one connection plate
can connect two or more boxes to each other.
Said object is further achieved by a mould system according to a third
embodiment. This claim reveals a mould system wherein a flat surface of a
longitudinal side of the at least one row unit may be obtained by at least one
box in the row unit having a form such that at least one of the corresponding
longitudinal side walls of the box has a recess in at least one of its respective
ends, wherein a depth of the recess is equal to a thickness of a connection
plate.
The advantage with the mould system according to this embodiment is that
the connection plates can have a specified length for connecting two boxes
to each other regardless of the longitudinal length of the boxes.
Said object is further achieved by the mould system wherein the boxes are
made of aluminium.
Said object is further achieved by the mould system wherein the frame is
made of invar.
Said object is further achieved by the mould system wherein the boxes are
provided with threaded holes for insertion of a tool for removal of the boxes.
Said object is further achieved by the mould system wherein the height of
each connection plate is substantially equal to or higher than the height of the
at least two boxes.
Said object is further achieved by a method for net moulding of a co-cured
structure comprising a skin provided with at least two stringers and at least
one rib foot and a gap between the stringers and the at least one rib foot. The
method comprises the following steps:
o providing at least two substantially rectangular boxes made of a
heat expanding material, wherein each box has a bottom, a first
longitudinal side wall, a second longitudinal side wall, a first
transversal side wall and a second transversal side wall,
o applying a first layer of prepregs onto an underside of the bottom
and on an external side of at least one of the first transversal side
wall and the second transversal side wall of each of the at least
two rectangular boxes thereby obtaining at least two boxes having
a bottom and at least one of the first transversal side wall and the
second transversal side wall covered with a first layer of prepregs,
o forming at least one longitudinal row unit by, for each row unit
> positioning at least two of said boxes covered with a first
layer of prepregs next to each other, with a covered
transversal side wall of a first box facing a covered
transversal side wall of a second box, thereby obtaining a
row of boxes,
> connecting the at least two boxes in the row of boxes along
the first longitudinal side walls and along the second
longitudinal side walls by means of at least two connection
plates and fastening means,
> forming a first longitudinal side of the at least one row unit
constituted by the first longitudinal side walls of the boxes in
the row unit and of the connection plates being connected
thereto, and forming a second longitudinal side of the row
unit constituted by the second longitudinal side walls of the
boxes in the row unit and of the connection plates being
connected thereto,
■ wherein an external surface of the first longitudinal
side walls and of the second longitudinal side walls of
the boxes and an external surface of the connection
plates in the at least one row unit have a form such
that an external surface of the first longitudinal sides
and an external surface of the second longitudinal
sides of the at least one row unit form flat surfaces,
> forming a bottom of the at least one row unit constituted by
the bottom of the boxes in the row unit,
o applying a second layer of prepregs onto an external surface of the
bottom and an external surface of the first longitudinal side and of
the second longitudinal side of the at least one row unit, thereby
obtaining at least one row unit having a bottom, the first
longitudinal side and the second longitudinal side covered with a
second layer of prepregs,
o forming of the skin by applying a third layer of prepregs onto a
plate made of a low heat expanding material,
o positioning the at least one covered row unit on the skin,
> when more than one row unit, positioning a first row unit
next to a second covered row unit with the first longitudinal
side of the first row unit being positioned next to the second
longitudinal side wall of the second row unit,
o providing a substantially rectangular frame made of a low heat
expanding material, wherein the frame comprises a first
longitudinal frame part, a second longitudinal frame part, a first
transversal frame part and a second transversal frame part,
o positioning the frame on the skin, such that it surrounds the at least
one covered row unit and such that the first longitudinal frame part
is positioned next to a first longitudinal side of at least one covered
row unit and that the second longitudinal frame part is positioned
next to a second longitudinal side of at least one covered row unit,
thereby obtaining a moulding assembly comprising the plate, the
frame, the at least one row unit, and the different layers of
prepregs,
loosening of the fastening means,
covering the moulding assembly with a vacuum bag for
conventional curing,
positioning the moulding assembly and the vacuum bag in
autoclave and exerting a pressure and applying a heat,
removing the frame, the connection plates and the boxes.
Said object is further achieved by the method according wherein after the
step of positioning the at least one covered row unit on the skin the method
comprises the further step of:
o applying a fourth layer of prepregs onto an external surface of a
bottom and on an inner side wall of the first longitudinal frame
part and of the second longitudinal frame part.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in detail with reference to the
figures, wherein:
Figure 1. Figure 1 shows an integrated structure with a gap between the
stringers and the rib feet, created in accordance with the present
invention;
Figure 2 . Figure 2 shows an enlarged view of the gap between the
stringers and the rib feet.
Figure 3 . Figure 3A and 3B shows a side view of a number of boxes
covered with a first layer of prepreg according to the invention.
Figure 4 . Figure 4A and 4B shows a view seen from above of a number of
boxes covered with a first layer of prepreg according to the
invention.
Figure 5 . Figure 5 shows a view seen from above of a row unit according
to a first embodiment of the invention.
Figure 6 . Figure 6 shows a cross section view of the row unit of figure 5
seen from the cut A, according to the invention.
Figure 7 . Figure 7 shows a view seen from above of a row unit covered
with a second layer according to a first embodiment of the
invention.
Figure 8 . Figure 8 shows a cross section view of the row unit of figure 7
seen from the cut B, according to the invention.
Figure 9 . Figure 9 shows the mould system according to a first
embodiment of the invention seen from above.
Figure 10 . Figure 10 shows a cross section view of the mould system of
figure 9 seen from the cut C, according to the invention.
Figure 11. Figure 11 shows a cross section view of the mould system of
figure 9 seen from the cut D, according to the invention.
Figure 12. Figure 12 shows a view seen from above of a row unit according
to a second embodiment of the invention.
Figure 13 . Figures 13A and 13B show a view seen from above of a row unit
according to a third embodiment of the invention.
Figure 14. Figure 14 shows a perspective view of the mould system with
two row units.
Figure 15 . Figure 15 shows a flowchart of the moulding method according
to the present invention.
DETAILED DESCRIPTION
Figure 1 shows an integrated structure 1 which is manufactured by using the
mould system and the method of the present invention. The structure can be
used in e.g. aircrafts but it can be used also in other applications such as e.g.
boats or crafts. The structure comprises a skin 26. The skin 26 is reinforced
with longitudinal stringers 4 and transversal rib feet 2 , which projects from the
skin 26. The stringers 4 are substantially perpendicular to the rib feet 2 . The
rib feet 2 and the stringers 4 are spaced from each other by a gap 6 . This
gap 6 makes the structure more flexible and prevents the structure 1 from
breaking when exposed to high forces. Figure 2 shows an enlarged view of
the gap 6 between the rib feet 2 and the stringers 4 .
Figures 3 - 11 aim at describing the manufacturing method of the integrated
structure 1 and the mould system that is used for the manufacturing method
according to a first embodiment of the present invention. Figures 3 and 4
show the first step of creating the rib feet. This step is denoted by step 1 in
figure 15 . Figure 3A shows a side view of four boxes 8 and figure 4A shows
the four boxes 8 from above. Figure 3B show a side view of two boxes 8 and
figure 4A shows the two boxes 8 from above. The boxes 8 are hollow with
the opening upwards. Each box 8 comprises a bottom 30 and four side walls,
a first longitudinal side wall 46A, a second longitudinal side wall 46B, a first
transversal side wall 44A and a second transversal side wall 44B. The boxes
8 are made of a heat expanding material, like e.g. aluminium or steel. A
number of prepregs forming a first layer 10 is applied on the boxes. The first
layer 10 is applied on the underside of the bottom 30 of the boxes 8 and on
the external side of at least one of the transversal side walls 44A and 44B of
the boxes 8 , thereby forming boxes that have a prepreg covered bottom
surface 30 and at least one prepreg covered transversal side wall 44A or 44B.
In all figures a number of prepregs positioned one upon the other and
forming a thick layer (stack of prepreg) is illustrated by a thick, black line. In
all figures there is a small clearance between the boxes 8 and the layers.
This clearance only serves for illustration purpose in order to make the
figures clearer. There is no clearance in the actual embodiments of the
invention.
The second step of the manufacturing method is to create at least one row
18 of boxes. This step is denoted by step 2 in figure 15 . A row 18 of boxes 8
is formed by positioning a number of covered boxes 8 next to each other, just
as in figures 3A, 3B, 4A and 4B, so that the covered transversal side wall
44A of a first box 8 and the covered transversal side wall 44B of a second
box 8 are facing each other. The first layers 10 covering the first transversal
side wall 44A of the first box and the second transversal side wall 44B of the
second box 8 will later form one rib foot. The number of boxes 8 in one row
18 depends on the desired number of rib feet 2 . Two boxes 8 are necessary
in order to obtain one rib foot, three boxes 8 makes two rib feet and so on.
(Compare figures 3A and 4A with figures 3B and 4B.) The row 18 of boxes 8
has two transversal sides 47A and 47B (constituted by the covered/non
covered transversal side wall 44A of a first end box and the second
covered/non covered transversal side wall 44B of a second end box of the
row), a bottom 49 (constituted by the covered bottom 30 of the boxes in the
row ) , and two longitudinal sides, a first longitudinal side 45A and a second
longitudinal side 45B (constituted by the longitudinal side walls 46A and 46B
of the boxes 8).
The third step is shown in figures 5 and 6 . This step is denoted by step 3 in
figure 15 . The aim of this step is to connect the boxes to each other in order
to obtain a rigid row unit 20. This is performed by positioning two long
connection plates 16 in the form of straight rulers on each of the respective
longitudinal sides 45A and 45B of the row and connects each connection
plate 16 to each of the boxes 8 by means of fastening means 14. The
fastening means14 can e.g. be screws or bolts. In addition to making a rigid
row unit 20 the connection plates 16 creates the gap 6 between the stringers
4 and the rib feet 2 . Without the connection plates 16 the stringers 4 would
abut against the rib feet 2 . Therefore the connection plates 16 are very
fundamental in the present invention. The row unit 20 has two transversal
sides, a first transversal side 47A and a second transversal side 47B (the
same sides as for the row of boxes), a bottom 49 (the same bottom as for the
row of boxes), and two longitudinal sides, a first longitudinal side 48A and a
second longitudinal side 48B. The first longitudinal side 48A of the row unit
20 is constituted by the first longitudinal side walls 46A of the boxes 8 and of
the connection plate 16 connecting the boxes 8 along these longitudinal side
walls 46A. The second longitudinal side 48B of the row unit 20 is constituted
by the longitudinal side walls 46B of the boxes 8 and of the connection plate
16 connecting the boxes along these longitudinal side walls 46B.The
connection plates 16 are made of a heat expanding material such as
aluminium or steel. According to this embodiment the length of the
connection plates 16 is substantially equal to the length of the row unit 20.
The height of the connection plates 16 is substantially equal to the height of
the boxes 8 and the thickness of the connection plates 16 is substantially
equal to the width of the desired gap 6 . The connection plates 16 in this
embodiment are positioned on and cover each of the longitudinal sides 45A
and 45B of the row 18 of boxes 8 . They connect the boxes 8 to each other,
and thereby create a rigid row unit 20. This has the advantage that several
boxes 8 can be handled as one unit, which makes it easier to apply additional
prepregs. Depending on the desired number of stringers 4 one or several
rigid row units 20 can be created.
In the fourth step of the manufacturing method a second layer 11 of prepregs
is applied on the rigid row units 20. This step is denoted by step 4 in figure 15 .
For each row unit 20 the second layer 11 is applied on the bottom 49 of the
row unit 20 (onto the first layer 10 covering the bottom surface 30 of the
boxes 8) and on the first and the second longitudinal sides 48A and 48B of
the rigid row unit 20. Since the connection plates 16 are positioned on the
longitudinal sides 48A and 48B of the row unit 20, this means that the second
layer 11 is applied on the bottom 49 of the row unit 20 and on each of the
connection plates 16. The result of this step is that the bottom 49 of the row
unit 20 and the first and second longitudinal sides 48A and 48B of the row
unit 20 are covered by prepregs. Figures 7-8 illustrate how this step is
performed.
The fifths step in the manufacturing procedure is to create the skin 26. This
step is denoted by step 5 in figure 15 . The skin 26 is created by placing a
number of prepregs on the surface of a plate 40 made of a low heat
expanding material. The plate 40 can have a plain surface but the surface
can also be curved. The form of the plate 40 depends on the desired form of
the skin 26. The prepregs forms a third layer 50 constituting the skin 26. This
third layer is shown in figures 10 and 11.
In the sixth step the rigid row units 20 covered with a second layer 11 are put
onto the third layer 50 constituting the skin 26. This step is denoted by step 6
in figure 15 . If there are several rigid row units 20, the rigid row units 20 are
positioned on the third layer 50 such that the first longitudinal side 48A of a
first row unit 20 is positioned next to the second longitudinal side 48B of a
second row unit 20. This is illustrated in figure 9 . The figure also illustrates
how the gaps 6 are obtained by means of the connection plates 16.
In addition to the mentioned boxes 8 , the connection plates 16 and the
fastening means 14, the mould system comprises a substantially rectangular
frame 12 made of a low heat expanding material. Figure 9 shows the frame
12, the rigid row units 20 and the two first layers 10 and 11 seen from above.
The frame 12 comprises two longitudinal frame parts, a first longitudinal
frame part 54A and a second longitudinal frame part 54B, a first transversal
frame part 56A and a second transversal frame part 56B, wherein each
frame part 54A, 54B, 56A and 56B have a bottom 60 and at least one side
wall 62 (see figures 10 and 11) . The at least one side wall 62 has
approximately the same height as the longitudinal side walls 46A and 46B of
the boxes 8 but it can also be higher than the longitudinal side walls 46A and
46B of the boxes 8 . The frame 12 can either be made as one unit or it can
comprise of several parts that are connected to each other to make one unit.
In step 7 a fourth layer 52 of prepregs is applied on the underside of the
bottom 60 and on the inner side of the side wall 62 of at least one of the first
longitudinal frame part 54A and the second longitudinal frame part 54B. This
step is denoted by step 7 in figure 15 . The result of the step is that the
underside of the bottom 60 and the inner side of the side wall 62 of at least
one of the longitudinal frame parts 54A and 54B are covered by prepregs.
The step can be omitted, which is shown in figure 15 by the dotted lines.
However, this leads to thinner end stringers 4 .
In step 8 the frame 12 is positioned on the third layer 50 forming the skin.
This step is denoted by step 8 in figure 15 . The frame 12 surrounds the row
units 20 so that the prepreg covered longitudinal frame part 54A is positioned
next to the prepreg covered longitudinal side 48A of a first end row unit 20
and the prepreg covered longitudinal frame part 54B is positioned next to the
prepreg covered longitudinal side 48B of a second end row unit 20.
Figure 10 shows a cross section view seen from a cut C and figure 11 shows
a cross section view seen from a cut D in figure 9 . As seen from figure 10 ,
the stringers 4 are formed by the second layer 11 covering a longitudinal side
48A of a first row unit 20 and by the second layer 11 covering a longitudinal
side 48B of second row unit 20 being positioned next to the first row unit 20.
Alternatively the stringers 4 are formed by the second layer 11 of one end
row unit 20 and the fourth layer 52 on the inner side of the side wall 62 of a
longitudinal frame part 54A or 54B. It is important that the external surfaces
of the longitudinal sides 48A and 48B of the row units 20 is flat. If not, the
stringers 4 will not be straight.
Before covering the now prepared moulding assembly 64, i.e. the frame 12,
the plate 40, the row units 20 and the layers 10 , 11, 50, 52 of prepregs, with
a vacuum bag and placing the moulding assembly in an autoclave for curing
of the structure, the fastening means 14 are loosened. The reason for this is
that the heat expanding boxes 8 shall be able to move and float freely within
the rigid frame 12 during the hardening process. In this way the different
layers 11 and 12 and 50 and 52 are forced towards the rigid frame 12 by the
expansion of the heat expanding boxes 8 and the heat expanding connection
plates 16, and remaining air between the prepregs can be pressed out,
making the structure 1 solid and hard. These steps are denoted by steps 9-
11 in figure 15 .
The last step is to remove the frame 12, the connection plates 16 and the
boxes 8 . The frame 12 can be removed easily but the connection plates 16
and the boxes 8 are more difficult to remove. Due to their expansion in the
autoclave they are now pressed towards the carbon-fibre structure 1. In order
to facilitate the removal, the boxes 8 can be provided with threaded holes 32
shown in figure 14 for insertion of a tool for removal of the boxes 8 .
According to the first embodiment the connection plates 16 are in the form of
rulers and their length is substantially equal to the length of the row units 20.
According to a second embodiment the connection plates 16 are designed as
shorter rulers and a single connection plate 16 does not cover the entire
longitudinal side 48A and 48B of a row unit 20. This has the advantage that
the connection plates 16 are more flexible. Instead of positioning a
connection plate 16 on each longitudinal side 45A and 45B of a row of boxes
8 as in the first embodiment and connect all boxes 8 in the row 18 of boxes 8
to the two connection plates 16, several connection plates 16 connect the
boxes 8 in a row of boxes 8 along the respective longitudinal sides 45A and
45B. Each connection plate 16 is adapted to connect one of the respective
longitudinal sides 45A, 45B of two or more boxes 8 in a row 18 of boxes 8 ,
but not necessarily of all boxes 8 in the row 18 . This embodiment is shown in
figure 12. As can be seen in the figure six connection elements 16 are used
for fixing four boxes 8 to each other in one row unit 20. Three connection
elements 16 have the total length which is substantially equal to the length of
the row unit 20. According to this embodiment the four boxes 8 in figure 12
could also have been connected by e.g. four connection elements 16, where
two connection elements 16 connect three adjacent boxes 8 to each other on
each of the longitudinal side 45A and 45B of the row 18 of boxes 8 , and two
connection elements 16 connect said three boxes 8 to the fourth box 8 on
each longitudinal side 45A and 45B of the row 18 of boxes 8 . In figure 12
there is a clearance between the connection plates 16. This clearance is only
for illustration. In the invention the connection plates 16 abut against each
other.
Just as in the first embodiment the row unit 20 has two transversal sides, a
first transversal side 47A and a second transversal side 47B (the same sides
as for the row of boxes), a bottom (the same bottom as for the row of boxes),
and two longitudinal sides 48A and 48B. The longitudinal sides 48A and 48B
of the row unit 20 are, just like in the first embodiment, constituted by the
connection plates 16 and the longitudinal side walls 46A and 46B of the
boxes 8 .
According to a further embodiment each connection plate 16 only connects
two adjacent boxes 8 to each other. Each of the boxes 8 in a row unit 20 has
a recess 58 in the ends of at least one of the first and second longitudinal
side walls 46A and 46B. Each recess 58 is adapted to receive a part of a
connection plate 16. The recesses can be seen in figure 13B. A perspective
view if the mould system according to this embodiment is shown in figure 14.
As mentioned before it is important that the external surfaces of the
longitudinal sides 48A and 48B of the row units 20 is flat. If not, the stringers
4 will not be straight. By making the depth of the recesses 58 equal to the
thickness of the connection plates 16 the external surface of the connection
plates 16 aligns with the external surface of the boxes 8 when the connection
plates 16 are connected to the boxes 8 .
The invention is not limited to the described embodiments. The embodiments
can be combined and/or further developed without limiting the scope of the
invention. The number of boxes 8 in a row unit can be varied according to the
desired number of rib feet 2 . The number of row units 20 can be varied
according to the desired number of stringers. The three embodiments can be
combined so that each row unit 20 can be provided with connection plates 16
according to one embodiment on one longitudinal side 48A and according to
another embodiment on the other longitudinal side 48B. The embodiments
can even be combined within one longitudinal side 48A or 48B as seen in
figure 13A.
The invention is not limited to the specific flowchart presented, but includes
all variations within the scope of the present claims. The internal sequence of
steps for arriving at the structure can of course be varied according to the
situation.
Reference signs mentioned in the claims should not be seen as limiting the
extent of the matter protected by the claims, and their sole function is to
make claims easier to understand.
As will be realised, the invention is capable of modification in various obvious
respects, all without departing from the scope of the appended claims.
Accordingly, the drawings and the description thereto are to be regarded as
illustrative in nature, and not restrictive.

CLAIMS
1. A mould system for net moulding of a co-cured structure comprising a
skin (26) provided with at least two stringers (4) and at least one rib
foot (2) and a gap (6) between the stringers (4) and the at least one rib
foot (2), the mould system comprising:
o a substantially rectangular frame (12) made of a low heat
expanding material, the frame (12) comprising a first longitudinal
frame part (54A), a second longitudinal frame part (54B), a first
transversal frame part (56A) and a second transversal frame part
(56B)
o at least one row unit (20), wherein each row unit (20) comprises
> a row ( 18) of at least two substantially rectangular boxes (8)
made of a heat expanding material,
■ wherein each box (8) has a bottom (30), a first
longitudinal side wall (46A), a second longitudinal
side wall (46B), a first transversal side wall (44A) and
a second transversal side wall (44B), and
■ wherein the at least two boxes (8) in the row ( 18) are
positioned such that the first transversal side wall
(44A) of a first box is facing the second transversal
side wall (44B) of a second box,
> at least two connection plates (16) connecting the at least
two boxes (8) in the row (18) of boxes (8) along the first
longitudinal side walls (46A) and along the second
longitudinal side walls (46B) by means of fastening means
(14),
■ wherein each connection plate (16) is adapted to be
positioned on the first longitudinal side walls (46A) or
on the second longitudinal side walls (46B) of at least
two adjacent boxes (8) in the row ( 18), and
■ wherein each connection plate (16) is adapted to be
connected to each of the longitudinal side walls (46A;
46B) on which the connection plate (16) is positioned,
and
■ wherein each connection plate (16) is adapted to
extend along at least a part of each of the first
longitudinal side walls (46A) or (46B) on which the
connection plate (16) is positioned,
> wherein a first longitudinal side (48A) of the at least one row
unit (20) is constituted by the first longitudinal side walls
(46A) of the boxes (8) in the row unit (20) and by the
connection plates (16) being connected thereto and a
second longitudinal side (48B) of the row unit (20) is
constituted by the second longitudinal side walls (46B) of
the boxes (8) in the row unit (20) and of the connection
plates (16) being connected thereto, and
> wherein an external surface of the first longitudinal side
walls (46A) of the boxes (8) and of the second longitudinal
side walls (46B) of the boxes (8) and an external surface of
the connection plates (16) in the at least one row unit (20)
have a form such that the first longitudinal side (48A) and
the second longitudinal side (48B) of the at least one row
unit (20) form flat surfaces.
The mould system of claim 1, wherein a flat surface of a longitudinal
side (48A or 48B) of the at least one row unit (20) is obtained by at
least one box (8) in the row unit (20) having a form such that an
external surface of at least one of the corresponding longitudinal side
walls (46A or 46B) of the box (8) is flat.
The mould system of claim 1, wherein a flat surface of a longitudinal
side (48A or 48B) of the at least one row unit (20) is obtained by at
least one box (8) in the row unit (20) having a form such that at least
one of the corresponding longitudinal side walls (46A or 46B) of the
box (8) has a recess (58) in at least one of its respective ends,
wherein a depth of the recess (58) is equal to a thickness of a
connection plate (16).
The mould system according to any of the claims 1-3, wherein the
boxes (8) are made of aluminium.
The mould system according to any of the claims 1-4, wherein the
frame (12) is made of invar.
The mould system according to any of the claims 1-5, wherein the
boxes (8) are provided with threaded holes (32) for insertion of a tool
for removal of the boxes (8).
The mould system according to any of the claims 1-6 wherein the
height of each connection plate (16) is substantially equal to or higher
than the height of the longitudinal side walls (46 A, 46B) of the at least
two boxes (8).
Method for net moulding of a co-cured structure ( 1 ) comprising a skin
(26) provided with at least two stringers (4) and at least one rib foot (2)
and a gap (6) between the stringers (4) and the at least one rib foot (2),
the method comprising the following steps:
o providing at least two substantially rectangular boxes (8) made of a
heat expanding material, wherein each box (8) has a bottom (30),
a first longitudinal side wall (46A) a second longitudinal side wall
(46B), a first transversal side wall (44A) and a second transversal
side wall (44B),
o applying a first layer ( 10) of prepregs onto an underside of the
bottom (30) and on an external side of at least one of the first
transversal side wall (44A) and the second transversal side wall
(44B) of each of the at least two rectangular boxes (8), thereby
obtaining at least two boxes (8) having a bottom (30) and at least
one of the first transversal side wall (44A) and the second
transversal side wall (44B) covered with a first layer (10) of
prepregs,
forming at least one longitudinal row unit (20) by, for each row unit
(20)
> positioning at least two of said boxes (8) covered with a first
layer ( 10) of prepregs next to each other, with a covered
transversal side wall (44A) of a first box facing a covered
transversal side wall (44B) of a second box, thereby
obtaining a row (18) of boxes (8),
> connecting the at least two boxes (8) in the row ( 18) of
boxes (8) along the first longitudinal side walls (46A) and
along the second longitudinal side walls (46B) by means of
at least two connection plates (16) and fastening means
(14),
> forming a first longitudinal side (48A) of the at least one row
unit (20) constituted by the first longitudinal side walls (46A)
of the boxes (8) in the row unit (20) and of the connection
plates (16) being connected thereto, and forming a second
longitudinal side (48B) of the row unit (20) constituted by the
second longitudinal side walls (46B) of the boxes (8) in the
row unit (20) and of the connection plates (16) being
connected thereto,
■ wherein an external surface of the first longitudinal
side walls (46A) and of the second longitudinal side
walls (46B) of the boxes (8) and an external surface
of the connection plates (16) in the at least one row
unit (20) have a form such that an external surface of
the first longitudinal sides (48A) and an external
surface of the second longitudinal sides (48B) of the
at least one row unit (20) form flat surfaces,
> forming a bottom (49) of the at least one row unit (20)
constituted by the bottom (30) of the boxes (8) in the row
unit (20),
applying a second layer ( 1 1) of prepregs onto an external surface
of the bottom (49) and an external surface of the first longitudinal
side (48A) and of the second longitudinal side (48B) of the at least
one row unit (20), thereby obtaining at least one row unit (20)
having a bottom (49), the first longitudinal side (48A) and the
second longitudinal side (48B) covered with a second layer ( 11) of
prepregs,
forming of the skin (26) by applying a third layer (50) of prepregs
onto a plate (40) made of a low heat expanding material,
positioning the at least one covered row unit (20) on the skin (26),
> when more than one row unit (20), positioning a first row
unit (20) next to a second covered row unit (20) with the first
longitudinal side (48A) of the first row unit (20) being
positioned next to the second longitudinal side wall (48B) of
the second row unit (20),
providing a substantially rectangular frame (12) made of a low heat
expanding material, wherein the frame comprises a first
longitudinal frame part (54A), a second longitudinal frame part,
(54B), a first transversal frame part (56A) and a second transversal
frame part (56B),
positioning the frame (12) on the skin (26), such that it surrounds
the at least one covered row unit (20) and such that the first
longitudinal frame part (54A) is positioned next to a first
longitudinal side (48A) of at least one covered row unit (20) and
that the second longitudinal frame part (54B) is positioned next to a
second longitudinal side (48B) of at least one covered row unit (20),
thereby obtaining a moulding assembly (64) comprising the plate
(40), the frame (12), the at least one row unit (20), and the different
layers ( 10 ; 11; 50) of prepregs,
o loosening of the fastening means (14),
o covering the moulding assembly (64) with a vacuum bag for
conventional curing,
o positioning the moulding assembly (64) and the vacuum bag in an
autoclave and exerting a pressure and applying a heat,
o removing the frame (12), the connection plates (16) and the boxes
(8).
9 . The method according to claim 7 , wherein after the step of positioning
the at least one covered row unit (20) on the skin (26) the method
comprises the further step of:
o applying a fourth layer (52) of prepregs onto an external surface
of a bottom (60) and on an inner side wall (62) of the first
longitudinal frame part (54A) and of the second longitudinal
frame part (54B).