Method and device for fabricating composite materials and bonded sheet items

[0001] This invention concerns a method and a device for use in connection with the fabrication of composite materials and bonded sheet items using the so-called “bagging technique” to increase the bonding pressure on specific parts of the fabricated products during fabrication.

[0002] The bagging technique is currently used very extensively to press the various layers of composite materials and bonded sheet components together during bonding and while the adhesive is curing. The basis of the technique is that the layers involved in the desired product are, once they have been coated with adhesive and brought together in the desired manner, sealed inside a plastic bag, from which the air is then extracted, thereby creating a pressure difference in the bag relative to the ambient air and thus pressing the layers together in an uniform manner and compacting the material. The pressure difference can then be increased by using an autoclave. In purely general terms, the bagging technique works very well for a very large number of different applications. It is used mainly in the fabrication of different types of laminates of metal, polymeric binders and fiber reinforcements, and in the fabrication of bonded multilayer sheet products. Problems arise with the technique mainly in connection with the fabrication of items whose adhesive bonds require higher pressures than those normally generated using the bagging technique, and which cannot be achieved even if an autoclaved is used. Problems may also arise in cases where the required autoclave is unavailable. In addition, it has been found difficult to use the bagging technique to produce interior corners with a sufficient degree of dimensional precision in cases where the interiors of the corners must, for reasons relating to molding and forming practice, face toward the bag.

[0003] Different variants of the bagging technique have been described in such a large number of different patents, books and articles that we can cite only a few to provide an idea of the state of the art.

[0004] The fabrication, using the bagging technique, of flat composite materials containing thermoplastic binders heated to their melting point while the starting material is kept under pressure in a vacuum bag is described in U.S. Pat. No. 4,915,896.

[0005] U.S. Pat. No. 5,242,523 describes the use of a partially modified bagging technique to fabricate complete panels by bonding finally-formed but not finally-cured stringers and support beams onto similarly finally-formed but not finally-cured cover sheets. The method described there is based on the use of an intermediate layer in the form of heat-resistant flexible rubber mats that are arranged to distribute pressure between the vacuum bag and the various panel components, which are disposed against one another in the desired manner.

[0006] Yet another way of using rubber material to distribute pressure between a vacuum bag and a plurality of different composite components that are to be bonded together and have external conformations that make it difficult to achieve the desired pressure distribution over all the critical surfaces using only one vacuum bag of standard design is described in U.S. Pat. No. 4,755,341. According to the method described there, the relevant composite components are covered with a free-flowing quantity of particulate silicon rubber that is in turn covered with a conventional vacuum bag that will thus load the silicon rubber particles, which will in turn transfer the pressure load to all the secondary surfaces of the composite components.

[0007] As was noted in the preamble, it is difficult to use the bagging technique to produce interior corners with high dimensional precision in those cases where the interiors of the corners must, for reasons related to manufacturing practice, face toward the bag. One way to solve this problem is described in U.S. Pat. No. 5,015,168. The method described there is based on arranging special calibration parts that define the inner dimensions of the corners in question on the outside of the vacuum bag, so that the relevant dimensions can be set precisely, using the bag as an intermediate layer. One major disadvantage of this method is that the special calibration parts must be capable of being placed in extremely well-defined final positions, which in turn entails a need for special fastening devices for the calibration parts, thus ultimately making for quite a complex arrangement.

[0008] The present invention now offers, in connection with the bagging method, both a general means of increasing the pressure on the components placed in the bag during curing and a means, proceeding from the same basic concept, of producing interior corners with a high degree of dimensional precision. The invention can be used either together with an autoclave treatment, whereupon it is possible to increase the pressure markedly, or to replace a current conventional autoclave treatment, since the autoclave treatment is often a bottleneck in the fabrication process, and the possibility of being able, at least under certain circumstances, to omit a previously necessary autoclave treatment is probably the most important gain offered by the invention.

[0009] The basic principle of the invention is now that we will create a larger surface to be loaded by the ambient air pressure than the material surface to be pressure-treated, and that the total pressure on the larger surface will be distributed over the surface to be pressure-treated. What is required to implement the invention is thus a movable tool part with a larger first surface that faces the interior of the bag, a second smaller surface that faces in the opposite direction and is brought into contact with the item which is to be pressure-treated, and a vacuum bag that must cover the entire tool part.

[0010] To summarize, the basis of the invention may thus be considered to be that the effect of the atmospheric pressure on a larger surface is distributed over the smaller surface of the product that one intends to fabricate. The increase in pressure to which the product can be subjected by this means during its fabrication will thus be proportional to the differences between these two areas. In order for it to be possible to implement the invention, both the larger and the smaller surface, which appropriately consist of the two confronting sides of a press die, must thus be arranged within the volume delimited by the bag used according to the basic method, and in which the pressure is lowered by pumping out air.

[0011] The invention also includes a variant that makes it possible to angle the pressure load laterally by means of a wedge-shaped element that is forced by a pressure surface in the main direction of the actuating pressure so as to simultaneously, as a result of its wedge action, force thereto adapted inner shaping elements laterally away from the direction of movement of the wedge. This latter variant of the invention is particularly well suited for use in the fabrication of high-precision interior corners, in that the movable inner shaping elements can be used to force treated material into tight abutment with its outer flanks against fixed outer shaping elements. The movable inner shaping elements will thus, according to this method, define the interior corners of the finished product, while the fixed outer shaping elements will define all of the outer corners of the product parts.

[0018] The general device for implementing the invention shown in FIG. 1 comprises a first tool part 1 in the form of a counter-die 1, a second tool part 2 that is movable relative to the counter-die 1 and exhibits a centrally arranged support 3, and two side pieces 4 and 5 that protrude on either side of the support piece, each of which terminates in a stop lug 6 and 7. The support piece 3 is adapted for the composite component or bonded component 8 whose various layers are to be joined together in that their intermediate binder layers will set under pressure between the tool parts. The sole purpose of the support lugs 6 and 7 is to ensure that the tool part 2 does not cant when a pressure P₁ is applied to the upper surface of the tool part 2. Because the present example pertains to a variant of the bagging method described above, this pressure is produced in that the entire tool setup is enclosed in a gas-tight bag 9, after which the air is pumped out of the bag 9. The combined area that will now be acted upon by the atmospheric pressure consists of the combined area corresponding to support surface 3 plus the surface of the two side pieces 4 and 5. The item 8 will then be loaded by the entire pressure P₁, which means that the compression of the item 8 will have been increased, since the area upon which the air pressure is acting, i.e. 4+5+3, considerably exceeds the area 3 in size.

[0019] If we assume that both parts 1 and 2 of the tool are of length L, then the following mathematical relation can be formulated based on the general designations specified for FIG. 2.
If b₁=4 b₂
and L₁=L₂
while
P₁×b₁×L₁=P₂×b₂×L₂
then
P₁×b₁=P₂×¼×b₁
which yields
P₂=4×P₁

[0020] FIGS. 3–5 illustrate a variant of the invention in which we utilize not only the option of increasing the load, i.e. the compression, but also extend the compression out laterally so that it also becomes possible to fabricate interior corners with very high precision. In this variant we use a first tool part 10 designed with a groove 11 conformed to the outer contours of a desired U-channel. The starting material 12 for the desired U-channel is then arranged in said groove. Two mirror-image-shaped shaping elements 13 and 14 are then arranged in this U-channel material 12. The outer sides 15 and 16 of the shaping elements that face the material 12 are intended to define the inner shape of the U-channel, while mutually opposing lateral edges 17 and 18 are arranged obliquely in relation to the dividing line between them. A wedge element 19 is then introduced down between the oblique lateral edges 17 and 18. The wedge element 19 is moreover fixedly connected to the pressure-loading area 20. The entire device is furthermore inserted into a bag 21 of the type discussed above, from which bag the air is then pumped out, whereupon the atmospheric pressure loads the area 20, driving the wedge 19 down between the shaping elements 13 and 14, whereupon they are pressed both down against the bottom of the groove 11 and out towards its lateral edges, whereupon the material 12 is thereby compressed and given its desired final conformation. As the figure shows, the shaping elements 13 and 14 also have their own loading areas 22 and 23, which are acted upon by the atmospheric pressure, and press the shaping elements down into the groove 11.

[0021] The following general relations may then be formulated for the device illustrated in FIG. 3. If the parts 13, 19 and 14 are assumed to comprise a unit, then the situation shown in FIG. 4 is applicable, where
F₃=P₃×b₃×L₃
while
F₄=P₄×b₄×L₃
whereupon L₃ refers to the length of the tool and the fabricated product, and an equation of equilibrium yields
F₃−F₄=0
P₃×b₃×I₃−P₄×b₄×L₃=0
P₄=P₃×b₃:b₄
whereupon, if
b₃=3×b₄, we obtain
P₄=P₃×3 b₄:b₄=3 P₃

[0022] However, we obtain the following relation if we instead assume that 19 is movable relative to 13 and 14:
A₅=b₃×L₃
F₅=P₅×A₅
F₆=P₆×A₆, which yields
F₅=2×F₆ cos α
F₆=F₅:2 cos α, which yields
F₆ ∞ when α90°