Patent Application
20080277049
The invention relates to a method for adhering components to a composite molding. The invention also relates to a device for producing the composite molding.
In the industry, and in the aircraft construction industry in particular, methods are often applied whereby components such as fuselage parts, wing profiles and stiffeners are adhered to a composite molding using for example a thermosetting adhesive. To this end, the components to be adhered are provided with an adhesive layer and are then connected to each other. The bond between the components is obtained by curing the thermosetting bonding means, by bringing it to a first increased temperature. It is important inter alia to remove as much air as possible that may be present between the components and/or in the adhesive layer as this makes it possible to achieve the desired good bond between the components. A known method thus comprises retaining the components in a vacuum-sealed film, whereby the pressure required to expel the air is exerted by applying a vacuum pressure in the space between film and components. An improved known method is implemented in a pressure vessel or autoclave, the inside volume of which can be controlled in terms of both temperature and pressure. The processes of temperature treatment required for adhering the components (this treatment generally comprises curing the thermosetting adhesive), de-aeration and pressurizing of the components are thus combined in a single device.
Although moldings with good mechanical properties can be obtained with the existing method, it is particularly time-consuming. Due to the high risks involved in applying moldings in the aircraft industry, it is very important to ensure that the adhesive layers are kept at the first temperature for a sufficient period of time during the known method, to guarantee the effective curing thereof. A typical autoclave cycle generally lasts for several hours. This time is needed to bring the composite molding (components and adhesive layers) to the desired first temperature, and then to cool it down again to a temperature at which it can be removed from the autoclave. The known method also generally requires high investment in autoclaves and peripheral equipment. The larger the dimensions of the moldings to be produced, the higher the investment. In a growing number of areas of application, there is a clear trend toward larger moldings.
The object of this invention is to provide a more efficient method for adhering components to a composite molding, whereby the molding also demonstrates good mechanical properties.
The method according to the invention for adhering components to a composite molding is thereto characterized as referred to in claim 1. More particularly, the method comprises stacking the components provided with an appropriate bonding means, positioning a heating means, provided with at least one cavity, over at least a section of the stack, and passing a medium brought to a first temperature through the at least one cavity of the heating means, whereby the stack is brought at least partially to the first temperature, and the components are interconnected to the molding. By means of the measures referred to in claim 1, it is inter alia achieved that the average cycle time for producing the molding can be significantly shorter, thus making the method according to the invention more efficient than the known method. Because the heating means can be positioned in the direct vicinity of the adhesive layers to be heated, the molding is indeed only brought to the desired temperature where this is required. In the known autoclave method, the entire inside space of the autoclave must be brought to the desired first temperature, which is time-consuming. A further advantage of the method according to the invention is that it only requires a little energy, because only those sections of a device appropriate for the method that are necessary to form the molding are heated up. Furthermore, it is possible with the method according to the invention, where so desired, to achieve a very rapid heating and/or cooling rate, because a mass of the medium can already be brought to the first temperature before being passed through the heating means.
Although in the method according to the invention, it is in principle possible to apply any medium that can be pumped relatively simply, a liquid is preferably passed through the cavity (cavities). Liquids are simple to pump and can convey a large quantity of heat. Appropriate liquids can for example be water, oil and/or other liquids that can be brought to the first temperature.
In a preferred embodiment of the method according to the invention, the heating means is designed such that when positioning it on the stack, it substantially takes on the form of the surface of the stack. This is advantageous for the transfer of heat between heating means and stack. It should be noted that the heating means according to the invention cannot be of such a stiffness that it is able to substantially deform the stack, as would be the case for example with a pressure plate provided with heating channels.
In a further preferred embodiment of the method according to the invention, a heating means is applied in the form of a flexible membrane, provided with an inlet and outlet for the medium, preferably a liquid. Such a membrane is simple to affix and can be used more than once if desired. By preferably selecting a very extensible and elastic material for the membrane material, the heating means can be affixed to the stack with a good fit, thus enabling an effective transfer of heat from heating means to molding even with moldings having a complex form. According to the invention, the heatable membrane can for example comprise a heat-resistant, elastic matrix, such as a silicon rubber or modified silicon rubber. Most preferably, this is a natural rubber or elastomer.
According to the invention, at least one medium brought to the first temperature must be pumped through the cavity (cavities) of the heating medium. The first temperature is selected such that the components can interconnect at this temperature by keeping at least the adhesive layers at this temperature for an appropriate period of time. If the bonding means between the components to be adhered comprises a thermosetting polymer, the first temperature is preferably selected such that it is at least equal to the curing temperature of the thermosetting polymer. If the bonding means comprises a semi-crystalline thermoplastic polymer, the first temperature is preferably selected such that it is higher than the melting temperature of the thermoplastic polymer. If the bonding means comprises an amorphous thermoplastic polymer, the first temperature is preferably selected such that it is at least equal to the softening temperature of the thermoplastic polymer.
It turned out that with the method according to the invention, the time required to adhere the components can be significantly shorter than is the case with the known method. To further reduce this time, in addition to passing the medium brought to the first temperature through at least one cavity of the heating means, it is advantageous to pass a medium brought to a second temperature through the heating means. The second temperature is selected such that the components interconnect at this temperature and the composite molding can be stored and/or loaded. If the bonding means between the components to be adhered comprises a thermosetting polymer, the second temperature is preferably selected such that it is lower than the curing temperature of the thermosetting polymer. If the bonding means comprises a semi-crystalline thermoplastic polymer, the second temperature is preferably selected such that it is lower than the melting temperature of the thermoplastic polymer. If the bonding means comprises an amorphous thermoplastic polymer, the second temperature is preferably selected such that it is lower than the softening temperature of the thermoplastic polymer. The medium brought to a second temperature can comprise the same medium as the medium brought to the first temperature, but this is not a prerequisite. In other words, it is possible to apply different media (for example a liquid and a gas, or several liquids having different properties) in the method according to the invention. The medium brought to a second temperature can be passed through before, and/or during and/or after the medium brought to the first temperature has been passed through. If media brought to different temperatures are passed through the cavity (cavities) at approximately the same time, the average temperature thereof while being passed through will be between the first and second temperature. Such an embodiment makes it possible to control the temperature of the heating means in an almost continuous fashion. For instance, it is possible to change over very gradually from the first to the second temperature by mixing the flow of the medium brought to the first temperature with a flow of medium brought to the second temperature, whereby the flow rate of the latter medium is gradually increased. The reverse, namely changing over very rapidly from the first to the second temperature, is also possible, in turn making it possible, when applying thermoplastic polymers, to control the crystallinity thereof. It should be noted that the person skilled in the art has various options available in this respect. The characteristics of the method according to the invention make it possible to obtain a sensitive means of controlling temperature, which furthermore requires relatively little energy.
In a further preferred embodiment, the method according to the invention is characterized in that a medium brought to a first temperature is passed through a first cavity, and a medium brought to another temperature is passed through another cavity of the heating means, whereby the corresponding sections of the laminate are brought to the first, or respectively the other, temperature.
This can be highly advantageous, for example if another polymer is applied as an adhesive material or bonding means in different parts of the molding to be produced, thus requiring this polymer for example to be cured at another temperature. It is also possible with this preferred method to switch rapidly from a higher temperature to a lower (cool) temperature.
In the method according to the invention, pressure is preferably exerted on the molding for at least some of its production time. More preferably, the pressure is exerted by retaining the assembly of stack and heating means between a substrate and a flexible retaining body, between which a vacuum pressure is then applied. By applying an at least partial vacuum, not only is an effective evacuation of air achieved in the adhesive layers, but also the contact between the stack and adjacent heatable membrane is improved, thus facilitating the heat transfer from membrane to stack. The stack is also effectively held in position by the vacuum. For the same reasons, it can be advantageous where necessary to apply an overpressure to the outside of the assembly of stack, heating means and flexible body in the method according to the invention.
A particularly appropriate method according to the invention is characterized in that the pressure is exerted by retaining the assembly of stack and heating means between the substrate and a stiff retaining body, and the medium, in this case the liquid, is passed through the heating means subjected to such a pressure that the heating means expands and is pressed against the retaining body. The retaining body can for example be connected to a section of the substrate. This variant does not require any separate pressurizing device and is therefore more efficient. The pressure on the stack is indeed autonomously established by the hydrostatic pressure present in the heating means. The method according to the invention can be advantageously applied for producing a molding made of a fiber-reinforced material. Fiber-reinforced materials, also referred to in the industry as composites, are obtained by impregnating reinforcing fibers with an appropriate matrix material to create single-layer semi-finished products (“prepregs”) or multi-layer laminates. In this way, the matrix material acts as a means of bonding the reinforcing fibers and prepregs to each other, and can be both heat-curable (thermosetting) and heat-meltable (thermoplastic). When producing moldings made of fiber-reinforced material, generally several layers of fiber-reinforced material (as set out in claim 1, each layer corresponds to a component) are stacked on top of each other, after which this stack is molded into a molding in a molding tool while being subjected to temperature and possibly pressure. To ensure inter alia that the various layers of the composite effectively bond to each other and as much air as possible that may be present in and/or between these layers is removed, the known methods preferably apply molding tools that can exert a pressure on the molding while it is being produced. For instance, a molding can be molded in a temperature-controlled compression press. Another frequently applied method comprises retaining the laminate in a vacuum-sealed film, whereby the pressure required to expel the air is exerted by applying a vacuum pressure in the space between film and laminate. This known method is generally implemented in a pressure vessel or autoclave, in which the temperature can be controlled and an additional pressure can be exerted on the laminate. However, the known method has the same disadvantages as referred to above, including low efficiency. For instance, the known autoclave process typically lasts for 3 to 5 hours. The method according to the invention makes it possible to obtain a molding made of fiber-reinforced material in 1 to 1.5 hours.
It is also possible to use the method according to the invention to connect several moldings to each other. In such cases, the substrate comprises a second molding and the laminate brought to the first temperature is held against the second molding for an appropriate period of time, thus interconnecting the laminate and the second molding. In a preferred embodiment, the second molding is provided with an adhesive layer at the applicable sections thereof before the laminate is affixed to this second molding.
The method according to the invention can in principle be applied to the production of moldings made of any fiber-reinforced material. The method is also advantageous in that it can be applied for fiber-reinforced materials with a thermosetting as well as a thermoplastic matrix. In a preferred embodiment of the method, at least one of the fiber-reinforced material layers comprises a thermoplastic polymer, whereby after the medium brought to the first temperature has passed through the at least one cavity of the heating means, a medium brought to a second temperature is passed through the heating means, with the first, or respectively second, temperature being higher, or respectively lower, than the melting temperature of the thermoplastic polymer. This makes it possible to bring the thermoplastic polymer relatively rapidly to below its melting temperature, which is in turn advantageous for the cycle time. Examples of thermoplastic polymers that are appropriate for the method according to the invention are polyamides, polyimides, polyethersulphones, polyetheretherketone, polyurethane, polyethylene, polypropylene, polyphenylene sulphides, polyamide-imides, acrylonitrile butadiene styrene (ABS), styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend (PPO), thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, as well as mixtures and copolymers of one or more of the above polymers.
In yet another preferred embodiment, the method according to the invention is characterized in that at least one of the fiber-reinforced material layers comprises a thermosetting polymer, and in that the first temperature is higher than the curing temperature of the thermosetting polymer. Also when using thermosetting matrix materials, it is possible to apply a medium brought to a second or further temperature if desired. The second temperature is in this case preferably lower than the curing temperature of the thermosetting polymer and can for example temporarily keep the viscosity of the matrix at a low level, to improve impregnation and interconnection of the layers, or to enable more air to be discharged. Thermosetting polymers that are appropriate for use include for example epoxies, unsaturated polyester resins, melamine/formaldehyde resins, phenol formaldehyde resins, polyurethane, etcetera.
When reference is made in this application to a first and second temperature, it is understood to mean an average temperature. It should be noted that when reference is made for instance to bringing the laminate to a first temperature, this means that the laminate has on average assumed the first temperature, subject to local variations. The same applies for the second and further temperatures.
Reinforcing fibers that are appropriate for use in the fiber-reinforced materials include for example glass fibers, carbon fibers, metal fibers, drawn thermoplastic polymer fibers, such as aramid fibers, PBO fibers (Zylon®), M5® fibers, and ultrahigh molecular weight polyethylene or polypropylene fibers, as well as natural fibers such as flax, wood and hemp fibers, and/or combinations of the above fibers. It is also possible to use commingled and/or intermingled rovings. Such rovings comprise a reinforcing fiber and a thermoplastic polymer in fiber form.
The fiber-reinforced plastic layer preferably comprises substantially continuous fibers that extend in at least two almost orthogonal directions (isotropic woven fabric). In another preferred embodiment, the fibrous layer and/or the fiber-reinforced plastic layer comprise substantially continuous fibers that mainly extend in one direction (UD tissue). The specific type of woven fabric selected depends inter alia on the desired mechanical properties and the deformability of the woven fabric. The selected embodiment of the method according to the invention can be a significant factor in this respect.
In a particular preferred embodiment, the method according to the invention is characterized in that the laminate comprises at least one metal layer. Such laminates are also known as fibrous metal laminates and comprise one or more metal layers and intermediary fiber-reinforced plastic layers. The laminates referred to are obtainable by connecting a number of metal layers and intermediary fiber-reinforced plastic layers to each other by means of heating under pressure and then cooling them. The fiber-reinforced plastics applied in the fibrous metal laminates are light and strong and comprise reinforcing fibers embedded in a polymer. The polymer also acts as a bonding means between the various layers.
Fibrous metal laminates have good specific mechanical properties (properties per unit of density). Metals that are particularly appropriate to use include light metals, in particular aluminum alloys, such as aluminum copper and/or aluminum zinc alloys, or titanium alloys. In other respects, the method according to the invention is not restricted to producing moldings based on laminates using these metals, so that if desired steel can be used for example or another appropriate structural metal.
It turned out that by applying the method according to the invention, the bond between the components, and/or impregnation of the laminate are improved without it being necessary for example to work under a pulsating pressure. Where necessary it can be advantageous, if the medium, preferably the liquid, is brought on average to the first temperature and then passed through the at least one cavity of the heating means under a pulsating temperature. Pulsating in this respect is understood to mean both continuously pulsating (like a wave) and discontinuously pulsating (in the sense of peaks). Such a preferred embodiment can lead to better properties and a shorter process cycle. By affixing the concave and flexible membrane on the stack according to the invention and for example pressing it tightly against the stack by means of vacuum pressure, a profound contact is created between membrane and stack, enabling heat to be rapidly exchanged between the medium pumped through the membrane and the stack. This rapid heat exchange makes it possible to apply a pulsating temperature, which also actually affects the bonding means.
The invention also relates to a device for adhering components to a composite molding, said device at least comprising a liquid source and means for bringing the liquid to a first temperature; and heating means that can be affixed on the molding, provided with at least one cavity and connection means connecting thereto for feeding and draining the liquid, and pumping means for passing the liquid through the at least one cavity. Further particular embodiments of the device according to the invention are referred to in claims 15 to 23.
When reference is made in this application to heating means, this must also be understood to mean cooling means. The term heating must therefore be broadly interpreted in this application, as it can also refer to cooling. The method and device according to the invention will emerge from the accompanying figures, in which:
With reference to
In practice, a laminate 10 made of fiber-reinforced material layers is affixed on the substrate 1. The heating means 11, preferably in the form of a concave flexible membrane, is then affixed over at least a section of the laminate 10. The membrane is connected to the pipes 8 via connection means 13. If desired, auxiliary materials 14 are used when affixing the fiber-reinforced material 10 on the forming mold 1. Such auxiliary materials for producing fiber-reinforced moldings are known to the person skilled in the art, and are applied for example to give the molding a rough surface (“peel ply”), to collect excess polymer (“bleeder”), or to be able to discharge air easily (“breather”). In the variant shown in
With reference to
With reference to
| Number | Date | Country | Kind |
|---|---|---|---|
| 1030029 | Sep 2005 | NL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NL2006/050235 | 9/25/2006 | WO | 00 | 7/18/2008 |
