DE4323590A1 - Three=dimensional lightweight sandwich structure - comprises thermoplastic foam core and fabric reinforced thermoplastic facings loosely stacked for moulding in hot tool. - Google Patents

Abstract

In mfg. a three-dimensional sandwich structure comprising a foamed thermoplastic core (2) and thermoplastic facing layers (3a,3b), a preform (10) consisting of the appropriate layers (2,3a,3b) loosely stacked together in the correct order is hot pressed in a tool to produce the final product. The moulding temp. is chosen to optimise the bond between the layers (2,3a,3b) without the need for additional adhesive. USE/ADVANTAGE - For prodn. of lightweight three-dimensional sandwich structures e.g. for aircraft construction. The process is simple and gives uniform mechanical properties over the whole structure without folding in the inner facing layer or the need for special tools to pre-shape the inner side of the component.

Description

Technical field

The present invention relates to the field of Lightweight construction using sandwich-like composite materials. they relates to methods for producing a three-dimensional molded composite component, which has a core layer made of a foamed first thermoplastic material comprises, on both sides with at least one each thermoplastic cover layer is integrally connected.

Such a method is e.g. B. from a prospectus of the company Schreiner Composites, Zoetermeer, NL, on a thermoplastic sandwich component with the brand name "Airsan", or known from EP-A1-0 431 669.

State of the art

In the field of lightweight materials, especially for use in aircraft construction are thermoplastic Sandwich materials have been developed that are characterized by high Ratios of rigidity or strength to weight excellent, are flame retardant and non-toxic, good thermal and  have acoustic insulation properties and are inexpensive to manufacture.

Composites of this type, as described in the publication mentioned at the outset, are produced in the form of flat plates which comprise a core layer made of a foamed thermoplastic material (for example 10 mm foam with a density of 90 kg / m ³ , which on glued on both sides with one or more cover layers made of fabric-reinforced thermoplastic material, that is to say they are bonded to one another, in which case the fabric reinforcement can consist, for example, of glass, aramid, carbon or similar fibers.

The finished glued sandwich panels form that Starting material from which by subsequent shaping (Bending, edges or the like) the desired three-dimensional Components such as half shells or angle profiles for use be manufactured in lightweight construction. Because of itself desired high rigidity of the sandwich panels are for molding, however, complicated hot-pressing processes required, which also leads to undesirable changes in the Lead sandwich structure: For example, around an edge in a It is not easy to insert the sandwich panel into the Edge area warmed up and then kinked because in the top layer lying on the inside of the edge when buckling uncontrollable shear stresses arise that lead to a Form wrinkles in the inner top layer. Such However, wrinkling is not acceptable because, on the one hand, in the Fold areas mechanically weakened the sandwich structure and on the other hand, the surface of the top layer no longer is.

To circumvent these problems, therefore, the prior art the sandwich panel in the edge area on the later Edge inside first by a specially shaped, heatable stamp notched and pre-bent and then kinked until the final edge angle is reached,  the inner top layer in the crease area is a single one raises defined fold that penetrates into the core layer. This leads to a massive disturbance of the core layer in the Edge area so that the edge itself is a weak point of the component. In addition, such Bending requires a complicated process sequence, the Parameters must be adhered to very precisely in order to achieve reproducible results.

Similar problems do not only occur when bending the panels on, but also in the Thermoforming process from the flat panels three-dimensional Generate moldings. Such transformations are made more difficult in that first the sandwich plate over all deforming area between special heating plates must be preheated over the entire surface, then in the actual press to be transferred and deformed there. In summary it can be said that the deformation of Sandwich panels is made difficult precisely by the properties which just characterize the sandwich panels as such.

Presentation of the invention

It is an object of the present invention to provide a method for the production of three-dimensionally shaped sandwich Specify components that are too mechanically perfect uniform components leads and particularly simple is to be carried out.

The task is in a method of the aforementioned Art solved in that the core layer and the at least two cover layers first in the appropriate order loosely piled up in a pile, and that then in a single hot pressing process the stack to the Composite component joined together and in the desired shape is brought.  

The essence of the invention is the manufacture of the Sandwich structure with the shape of the component in one summarize only one operation. So that becomes the problem bypassed a highly stable sandwich plate to have to deform in a complex manner. The stack from the initially loosely lying core and cover layers can change if the boundary conditions of the hot pressing process be chosen appropriately in the method according to the invention very easy to adapt to different shapes, then one to form a permanent composite structure. Because the individual Layers still relative to each other during the molding process are freely movable, can be caused by shear forces Warps and wrinkles in the outer layers, such as they occur in the prior art, can be safely avoided.

A first preferred embodiment of the invention The process is characterized in that the first thermoplastic material of the core layer, the material of the at least two thermoplastic cover layers and the Temperature during the hot pressing process that between the at least two cover layers and the Core layer without the use of additional adhesive or Adhesive gives a cohesive connection. this has the advantage that additional adhesive layers and thus associated process steps are not required. About that In addition, the stability (especially the peel strength) the composite structure through the direct cohesive Connection between layers increased.

This is a second preferred embodiment characterized in that the at least two cover layers each with a second thermoplastic Material impregnated fabric (prepreg), especially one Fiberglass fabrics are made up of the first and second thermoplastic material are substantially the same, and that as the first and second thermoplastic material Polyetherimide (PEI) is used. That way a very light and very easy way  Manufacture high-strength composite component, which in particular also the safety requirements in aircraft construction enough.

Another preferred embodiment of the The method according to the invention is characterized in that

• a) the hot pressing process in a hot press between two heatable forms is carried out;
• b) first the two forms to the required Temperatures are heated;
• c) the stack is then placed between the two molds;
• d) then the two forms during one predetermined pressing period are pressed together until the layer composite has formed; and
• e) the finished after the hot pressing process Composite component removed from the hot press and is quickly cooled outside the hot press.

This makes the hot pressing process particularly strong simplified and minimized in terms of time expenditure.

Further embodiments result from the dependent ones Claims.

Brief description of the drawings

The invention is based on Embodiments in connection with the drawings are explained in more detail. Show it

FIG. 1 shows in cross section the basic structure of a layer composite structure as is also the subject of the method according to the invention;

Figure 2 shows the folding problems in a finished sandwich panel occurring in the method according to the prior art.

Fig. 3a-c show various steps in the manufacture of a composite component according to a preferred embodiment of the method according to the invention; and

Fig. 4 shows the preferred temperature distribution during the hot pressing process and the resulting pore structure in the core layer of the component in the method according to the invention.

Ways of Carrying Out the Invention

The basic internal structure of a layer composite, as is also the subject of the invention, is shown in cross section in FIG. 1. The layer composite device 1 comprises essentially a made of a thermoplastic foam core layer 2, which is on its top and bottom each having a thermoplastic cover layer 3 a or 3 b integrally connected. Instead of a respective cover layer 3 a, b, however, may also include a plurality of superimposed cover layers are provided.

In the method, as is known from the prior art, a layered composite component 1 according to FIG. 1 is first produced in the form of a flat plate, which is then brought into the desired final component shape by corresponding shaping processes. If, for example, such a flat sandwich panel is to be folded at a certain point, the situation shown in FIG. 2 results: When the structure is bent at the bend 5 , the neutral fiber does not lie in the middle of the core layer 2 , but in the upper or outer cover layer 3 a because this, as a usually fabric-reinforced layer, allows practically no stretching. Accordingly, the core layer 2 and the lower or inner cover layer 3 in the direction b of the illustrated in Fig. 2 double arrows compressed. As a result of this compression, a kink zone 4 is formed in the core layer 2 and the lower cover layer poses wrinkles which, for. B. in the manner indicated in dashed lines in Fig. 2 at the kink zone 4 or elsewhere in the core layer 2 . In addition to unevenness on the surface of the component, this creates weak points in the composite structure, which can impair the functional reliability of the component.

The invention now takes a completely different approach (FIGS . 3a-c): Instead of first producing a flat sandwich panel and then deforming it, the shaping and production of the layer composite are carried out in a single hot pressing process, as is shown in FIGS . c method steps shown is exemplified. The hot pressing process preferably takes place in a device which comprises two molds 7 a and 7 b arranged on opposite heating plates 6 a and 6 b ( FIG. 3 a). The heating plates 6 a, b can be pressed together with the molds 7 a, b by a device (not shown), the pressing stroke preferably being limited by suitable stops 8 a, b in order to prevent the heated porous core layer 2 from being pressed further together.

In the hot pressing process according to the invention, the heating plates 6 a and 6 b are first preheated to the desired temperatures T1 and T2, which are then largely adopted by the molds 7 a and 7 b. The molds 7 a, b thus provide the heat that is necessary for deforming and gluing the cover layers and the core layer. A loose stack 10 is formed from the initially separate cover layers 3 a, b and the core layer 2 in the correct order, which is placed between the heated molds 7 a, b. In order to prevent the outer layers 3 a, b from sticking to the hot molds 7 a, b, it is advantageous to use separating foils (not shown in FIG. 3 a) between the outer layers 3 a, b and the corresponding molds 7 a, b Kapton, Teflon or a similar suitable material. The two separating foils can expediently form an insertion pocket into which the stack 10 is inserted and thereby secured against slipping.

After the stack 10 has been placed between the molds 7 a, b, the actual hot pressing process begins, in which the two molds 7 a, b are slowly pressed together. Due to the heat emanating from the molds 7 a, b, the cover layers 3 a, b are first heated and thus made deformable. The temperatures T1 and T2 must be chosen so that the cover layers 3 a, b become soft and sticky, but without melting. The heat then gradually passes over to the core layer 2 , which also becomes deformable as a result and, together with the softened cover layers 3 a, b, adapts to the surface profile of the molds 7 a, b. The temperature is higher on the outer surfaces of the core layer 2 than in its interior, so that the outer surfaces as well as the cover layers 3 a, b can become sticky, while the porous foam body of the core layer 2 remains largely stable on the inside. In this way, core layer 2 and cover layers 3 a, b deform under the influence of heat, are pressed against one another and, at the same time, adhere to their interfaces ( FIG. 3 b). Instead of direct gluing, which is expedient in particular for the same materials for the core and cover layers, additional adhesive layers (for example from a hot glue) can be provided between core layer 2 and cover layers 3 a, b.

As soon as the shaping and gluing process is completed, the layered composite component is immediately removed from the hot-pressing apparatus and quickly cooled outside the apparatus ( FIG. 3c). The component remains dimensionally stable if the interior of the core layer 2 has not been heated significantly above a lower temperature necessary for the deformation during the hot pressing process. The temperatures T1 and T2 as well as the pressing time depend on the thickness of the layer composite, on the selected material, but also on the desired component shape. The temperatures T1 and T2 are usually the same, but can also be selected differently if the particularity of the component shape requires it.

A desirable curve A of the temperature distribution during the hot pressing process over the spatial coordinate x perpendicular to the surface of the layer composite is shown schematically in FIG. 4. The temperature drops from relatively high values in the area of the cover layers 3 a, b, which are necessary for the gluing, to a considerably lower value in the interior of the core layer 2 , which is just sufficient to allow the foam body to be deformed. In accordance with this temperature gradient, a size distribution of the pores 9 in the foam body is established in the core layer 2 , in which small pores are predominantly present in the edge area, while the pore diameters increase towards the center. However, such a pore distribution is particularly favorable with regard to the stability of the finished composite component.

As starting materials for the thus produced layered composite components especially foams of polyetherimide (PEI) have proven effective for the core layer 2, whereas the outer layers 3 a, b preferably made of pre-impregnated with PEI glass fiber fabric, so-called prepregs PEI exist. The direct bonding of these layers then creates an integral layer composite with a (because of the similarity of the materials) almost seamless transition between the individual layers, which results in particularly favorable mechanical properties.

When using PEI materials, temperatures T1, T2 for the heating plates 6 a, b or molds 7 a, b have proven to be favorable, which are constant and are in the range between 180 ° C. and 290 ° C. The pressing time changes in a way counter to the heating temperature and is in the range between 10 s and 500 s. It has turned out to be particularly favorable with regard to the strength that can be achieved if the temperatures (T1, T2) of the heating plates 6 a, b are kept at about 190 ° C. during the hot pressing process and the pressing time is between 300 s and 500 s.

If the prepregs mentioned, which generally contain larger pores, are used as the top layers 3 a, b, it may be advantageous, in order to improve the surface quality, to first pre-compress the prepregs in an upstream hot pressing process.

It should also be ensured that in a uniaxial pressing process, as shown in Fig. 3a, b, the normals of the mold surfaces (in this case: the edge sides) do not deviate too much from the pressing direction in order to shift the to prevent individual layers 2 and 3 a, b against each other. In this context, however, it is also conceivable to replace one of the molds 7 a, b with a pressurized membrane in order to obtain a largely hydrostatic pressure distribution even in the case of complicated molds.

Overall, the invention results in a fast, flexible and inexpensive process for the production of mechanically very stable composite components.

Label list

1 layer composite component
2 core layer
3 a, b top layer
4 kink zone
5 kink
6 a, b heating plate
7 a, b form
8 a, b stop
9 pore
10 stacks
A temperature distribution curve
T, T1, T2 temperature
x location coordinate

Claims (11)

1. A method for producing a three-dimensionally formed film composite component (1) comprising a core layer (2) made of a foamed first thermoplastic material on both sides, each with at least one thermoplastic cover layer (3 a, b) is cohesively connected, characterized characterized in that the core layer ( 2 ) and the at least two cover layers ( 3 a, b) are first loosely piled up in the appropriate order to form a stack ( 10 ), and then in a single hot pressing process the stack ( 10 ) to form the layer composite Component ( 1 ) assembled and brought into the desired shape. 2. The method according to claim 1, characterized in that the first thermoplastic material of the core layer ( 2 ), the material of the at least two thermoplastic outer layers ( 3 a, b) and the temperature during the hot pressing process are chosen so that between the at least two Cover layers ( 3 a, b) and the core layer ( 2 ) results in a material connection without the use of additional adhesives or adhesives. 3. The method according to any one of claims 1 and 2, characterized in that the at least two cover layers ( 3 a, b) each consist of a fabric impregnated with a second thermoplastic material (prepreg), in particular a glass fiber fabric. 4. The method according to claim 3, characterized in that the first and second thermoplastic material in the are essentially the same.   5. The method according to claim 4, characterized in that as the first and second thermoplastic material Polyetherimide (PEI) is used. 6. The method according to any one of claims 1 to 5, characterized in that

• a) the hot pressing process is carried out in a hot press between two heatable molds ( 7 a, b);
• b) the two molds ( 7 a, b) are first heated to the required temperatures (T1, T2);
• c) the stack ( 10 ) is then placed between the two molds ( 7 a, b);
• d) the two molds ( 7 a, b) are then pressed onto one another for a predetermined pressing period until the layer composite has formed; and
• e) after the end of the hot pressing process, the finished composite composite component ( 1 ) is removed from the hot press and rapidly cooled outside the hot press.

7. The method according to claim 6, characterized in that to prevent sticking of the cover layers ( 3 a, b) to the molds ( 7 a, b) during the hot pressing process between the cover layers ( 3 a, b) and the molds ( 7 a, b) separating films, in particular made of Kapton or Teflon, are arranged. 8. The method according to any one of claims 6 and 7, characterized in that a stop ( 8 a, b) is provided between the molds ( 7 a, b), which specifies a minimum distance between the molds ( 7 a, b) during hot pressing. 9. The method according to claim 3, characterized in that before the hot pressing process the at least two cover layers ( 3 a, b) are hot pre-compressed to reduce the proportion of pores. 10. The method according to claim 6, characterized in that

• a) the two molds ( 7 a, b) are each arranged on a heating plate ( 6 a, b);
• b) as the core layer (2) and polyether imide prepregs as cover layers (3 a, b are used) polyetherimide foam;
• c) the temperatures (T1, T2) of the heating plates ( 6 a, b) are kept at fixed values in the range from 180 ° C. to 290 ° C. during the hot pressing process; and
• d) the pressing time is between 10 s and 500 s depending on the temperature.

11. The method according to claim 10, characterized in that the temperatures (T1, T2) of the heating plates ( 6 a, b) are kept at about 190 ° C during the hot pressing process and the pressing time is between 300 s and 500 s.