Patent Application
20170057183
The invention is in the fields of mechanical engineering and construction, especially mechanical construction, for example automotive engineering, aircraft construction, shipbuilding, machine construction, toy construction etc. It more particularly relates to processes of manufacturing articles of fiber-reinforced resin composite parts.
In the automotive, aviation and other industries, there has been a tendency to move away from steel constructions and to use lightweight material such as carbon fiber reinforced polymers instead.
Fiber reinforced composite parts for these industries often have to be manufactured with a high fiber content to satisfy the mechanical strength requirements they are subject to. Also, the fibers often need to be continuous fibers oriented in a well-defined manner. Manufacturing methods for these articles, such as Resin Transfer Molding (RTM) comprise placing a structure of the reinforcing material in a mold, injecting a liquid matrix and then causing the matrix material to harden. In variants, pre-impregnated structures of the articles may be used. Also, sometimes a vacuum is applied while injecting the matrix.
In reality, often when composite parts with a high fiber content are manufactured in this way, incomplete impregnation resulting in pores in the composite part is a problem. Such pores for some applications may be tolerated to a certain extent, but may constitute a cause of failure for other applications and/or if they are present in too high quantities.
To solve this problem, in addition to applying a vacuum it has also been proposed to apply sonic or ultrasonic vibrations to the mold during the matrix injection and prior to solidification of the matrix material so as to reduce the level of porosity. However, while this process may reduce the porosity to some extent, for manufacturing larger parts it imposes significant additional requirements on instrumentation and is complex and energy consuming. For this reason, it has become accepted only for niche applications.
Rather, often the composite parts are visually inspected after the manufacturing process. When defects are detected, either the part is disposed of, or a defective portion is cut out and replaced by a replacement piece. This turns out to be a rather uneconomical process.
It is an object of the invention to provide approaches that overcome disadvantages of the prior art and that provide a practical, efficient solution to the problem of insufficient impregnation of fibrous structures in fiber composite parts.
According to an aspect of the invention, a method of completing a fiber composite part is provided, the method comprising the steps of
In this text, the term “fiber composite part” generally refers to parts that comprise a structure of fibers, such as an arrangement of fiber bundles, a textile structure of fibers or any other structure of fibers, and a matrix material in which the fibers are embedded. Often, in such a structure, the fibers are so-called “continuous fibers”, i.e. fibers with lengths that may exceed 10 mm. The fibers will for example be carbon fibers. Such fiber composites are often referred to as “carbon fiber composites” or “carbon-fiber-reinforced polymers” or “carbon-fiber-reinforced plastics”; often also just as “carbon”. Other fibers than carbon fibers, such as glass aramide or high strength polyethylene (Dyneema) fibers, are not excluded; this includes the possibility of the fiber structure comprising fibers of different materials, such as carbon fibers and glass fibers.
The preparation may especially comprise or even consist of the matrix material. Alternatively, it may comprise an other hardenable material, such as an other thermosetting polymer. In these, the step of causing the material to solidify may simply comprise waiting until the material has hardened. In addition or as an alternative, the material of the preparation may be such that solidification is induced at least or accelerated by the mechanical vibration. Then, the step of causing the material to solidify may comprise keeping applying the mechanical vibration until the material has at least partly solidified.
As an alternative to a curable material, the preparation may comprise thermoplastic material. Then, the preparation may initially be in a flowable or non-flowable state. In the latter case, the mechanical vibration may be applied to the non-flowable preparation until at least a part thereof becomes flowable and is caused to impregnate the structure portion. Causing the material to solidify may then simply comprise allowing the thermoplastic material to cool after the vibration has stopped.
The method may further comprise pressing a tool by which the vibration is applied towards the surface portion while the vibration is applied.
The composite part during the step of applying the vibration may be placed against a support, especially a non-vibrating support.
A tool by which the vibration is applied may be a sonotrode coupled to a device for generating the vibration. Such a device may for example be a hand-held electrically powered device comprising appropriate means, such as a piezoelectric transducer, to generate the vibrations.
The mechanical vibration may be longitudinal vibration; the tool by which the vibration is applied may vibrate essentially perpendicular to the surface portion (and the tool is also pressed into the longitudinal direction); this does not exclude lateral forces in the tool, for example for moving the tool over the surface portion.
The mechanical vibration may be ultrasonic vibration, for example vibration of a frequency between 15 KHz and 200 kHz, especially between 20 KHz and 60 kHz. For typical sizes of defects (for example with characteristic lateral dimensions of about 1-5 cm) and dimensions of composite parts for example for the automotive industry (car body parts), a power of around 100-200 W has turned out to be sufficient, although the power to be applied may vary strongly depending on the application.
In any embodiment, there exists the option of carrying out the method by a tool that comprises an automatic control of the pressing force. For example, the device may be configured to switch the vibrations on only if a certain minimal pressing force is applied, and/or to switch the vibrations off as soon as a certain maximum pressing force is achieved. Especially the latter may be beneficial for parts of which an undesired deformation must be avoided, such as certain car body parts.
To this end, according to a first option the capability of piezoelectric transducers to measure an applied pressure may be used. According to a second option, a special mechanism can be present in the device. For example, a unit that contains the transducer and to which the tool (sonotrode) is attached may be mounted slideable against a spring force within a casing. The device may be configured so that the vibrations can be switched on only if the unit is displaced by a certain minimal displacement and/or only if it is not displaced by more than a certain maximum displacement. To achieve this, means well-known in the art such as light barriers, sliding electrical contacts, position sensitive switches or other means may be used. Also a collapsible sleeve or similar of the kind described hereinafter may contain or operate a contact or switch or similar to control the pressing force.
By the vibrations, the flowable material of the preparation is caused to interpenetrate the structure of fibers, possible voids in the material are caused to evade. The vibrations cause small motions of the fibers themselves, and this helps to prevent spots from not being impregnated at all.
The vibration frequency can influence the manner in which the vibrations act. A lower frequency will lead to a longer wavelength. By adapting the wavelength to the dimensions of the part to be completed, the operator can have an influence on in which depth the effect of the vibrations is the strongest and on whether the energy is primarily absorbed in a ‘near field’ regime, in a ‘far field’ regime or in an intermediate regime.
A second effect, which may depend on the material of the preparation, is that a curing/hardening process may be accelerated by the mechanical vibration. In an example, a commercially available two-component epoxy adhesive was used as test material for the preparation to eliminate a defect having a size of about 1 cm (approximate diameter). The process took less than half a minute, and when the vibrations were turned off, not only the resin-free (so called “dry”) fiber material was, as far as susceptible to visible inspection, fully impregnated by the resin, but also the adhesive had already cured to an extent that it was not deformable any more. The curing process thus was faster than it would have been without the ultrasonic vibration by at least an order of magnitude. This may be attributed to the heating of the adhesive due to the vibration, and to the fact that at higher temperature the chemical reactions that are involved in the curing are faster. As a role of thumb, a temperature increase of about 10° C. reduces the setting time by 50%. In the experimental process, a short term, ultrasound induced temperature increase of about 100° C. was observed.
A further advantage of the invention is that also incompletely impregnated portions (defects) that reach through the composite part can be completed by being accessed from one side only. This is in contrast to vacuum infiltration for example. This makes possible that parts of profiles that so far could not be repaired can be completed by the method according to the invention.
A further advantage, especially if approaches for confining, pumping and/or applying the preparation through an interior channel described in more detail hereinafter, is that the method also works in situations where a prior art approach of just freely dosing a resin on a defect would fail because the resin would flow or drip away, for example if the surface portion to which the preparation is applied is not horizontal. Rather, the approaches according to embodiments of the invention also work if the surface portion is non-horizontal, for example even vertical or even overhead.
The various embodiments of the methods described herein may be applied in manufacturing processes, especially for repairing defects that arise due to insufficient impregnation during manufacturing. Further occurrences of insufficient impregnation, to which the invention is applicable, may comprise defects that have arisen due to later failures, for example after crashes that have caused local defects of the fiber structure, whereafter a defect is covered by a structure portion of fibers that initially is not impregnated or only partly impregnated (thus insufficiently impregnated) and thereafter treated by the method according to embodiments of the invention.
In a group of embodiments, the method may comprise the additional step of laterally confining a flow of the preparation while the mechanical vibration is applied. The term “lateral” in this refers to directions parallel to the surface portion.
In embodiments, laterally confining the preparation comprises providing the preparation in a receptacle and pressing the tool, by which the vibrations are applied, towards the surface portion while the preparation or at least a portion thereof is within the receptacle.
Such a receptacle may especially be open towards a distal side, i.e. a side facing towards the surface portion. It may be entirely open towards the distal side, or may comprise openings facing the distal side or have open pores open towards the distal side.
In a first sub-group of these embodiments, laterally confining comprises providing the preparation surrounded by a non-flowable sleeve, which during the step of applying the vibration is pressed against the surface portion surrounding at least a part of the structure portion (defect). Such a non-flowable sleeve is an example of a receptacle in which the preparation is held during the step of applying.
Such a sleeve may be provided to be collapsible. According to an alternative, the tool by which the vibration is applied is slideable in longitudinal directions within the sleeve, the sleeve laterally and tightly surrounding tool.
In a second sub-group of these embodiments, the preparation is provided in a structure with pores open at least towards the surface portion.
Such structure may for example be an open porous, deformable structure serving as the receptacle or as part thereof, such as a sponge-like structure or a corresponding structure with an anisotropic pore structure.
By way of example, such an open porous deformable structure may comprise an open porous foam of a suitable material, such as polyurethane, with a porosity of for example at least 75%, especially between 80% and 95%. There exist for example foams of this kind with a high porosity and with a trabecular structure yielding a high permeability for a contained flowable substance.
An open porous structure may be optionally provided with a lateral closed coating to keep material of the preparation from being pressed out of the deformable structure laterally.
In addition or as an alternative, such an open porous structure may be provided such as to have an anisotropic porosity, such that it is permeable in longitudinal directions (out-of-plane directions) but not permeable or permeable to a substantially lower extent in lateral directions (in-plane directions).
In addition or as yet another alternative, the receptacle may be a cushion filled by the preparation, the cushion having pores on the underside, i.e. the side that faces the surface portion while the tool applies the pressure and the vibrations from the other side. Optionally, such a cushion may further be filled by an open porous foam in the pores of which the preparation is kept.
In embodiments, the mechanical vibration is coupled into the preparation by a sonotrode equipped for acting as a sonic pump on the preparation. In this text, the definition of “sonic pump” includes ultrasonic pumps, i.e. the pump effect may apply for both, vibrations in audible frequency ranges and vibrations in ultrasonic frequencies.
For acting as a sonic pump, it is not sufficient if the sonotrode is subject to mere axial vibrations (up-and-down vibrations in the proximodistal directions) of a flat distal end face. Rather, the pumping effects in the up and down directions of such mere axial vibrations of a flat distal end face tend to compensate each other, and in addition it could be observed that this kind of vibration tends to atomize the preparation instead of efficiently driving it into the structure of fibers.
Thus, to be capable of acting as sonic pump on the preparation, the sonotrode is designed so that its distal end surface is different from a flat surface that is merely subject to axial vibrations. Rather, one of or a combination of two designs A and B may apply:
For design A., the sonotrode may for example be essentially cylindrical but with a neck portion (for example formed by a circumferential groove) close to its distal end. The portion distally of the neck then acts as the wing portion, and the neck itself forms the shaft. Alternatively, the sonotrode may consist of an elongate shaft with the wing portion attached to its distal end.
In accordance with a possibility for design B, the sonotrode may for example according to a first option be designed with a peripheral ridge, which may extend continuously, without interruption, along the full circumference or alternatively be interrupted. The interruptions then form channels connecting an interior inside of the peripheral ridge with an exterior.
It has surprisingly been found that the sonotrode with an interrupted peripheral ridge exhibits an efficient pumping effect, and that the impregnation is especially efficient with this embodiment. Especially, it has been observed that hardenable material that was initially displaced laterally of the sonotrode is pumped in through the channels and into the structure of fibers. In this, the sonotrode can be merely pressed against the fiber composite part, without any need to displace it laterally or for axial back and forth movements with respect to the surface.
In accordance with a further possibility, the distal end of the sonotrode may be essentially flat but comprise a plurality of pockets formed by indentations. Also in this further possibility, there exists the option of having channels between the pockets and the outside. In addition or as an alternative, optional channels may exist between the pockets.
It has been observed that the if the sonotrode is designed to act as a sonic pump in the discussed manner according to option A or B, a lateral confinement is achieved, and additional confinement means such as a receptacle of the hereinbefore discussed kind not necessary any more (though they may still be used).
Also, it has been observed that option A works better for large area sonotrodes, whereas option B is suitable for large as well as for smaller areas. Especially, it has been observed that for option A it is advantageous if the distal end face area has a diameter of at least 20 mmm especially at least 40 mm or at least 50 mm. In examples, it was especially favorable if the distal end face area (disc area) of a sonotrode according to option A was between 50 mm and 80 mm.
For option B, the positive effect can be observed independent of the sonotrode size and, within certain boundaries, also independent of a curvature of the distal end face. Especially, a curvature can be in one direction (for treatment of edges) or double-curved (for corners), in both cases convex or concave. Especially, since in industrial manufacturing processes defects may occur predominantly at same places, the sonotrode size and curvature may be adapted to the respective place. This will enhance the effectively of the process in such situations.
In a group of embodiments, the preparation may in for example be applied to the surface portion in a manner that the whole defect (i.e. the entire surface portion in which the structure of fibers is exposed) is covered by the preparation.
In a first sub-group of this group, the tool, by which the mechanical vibration is applied, laterally extends over the whole defect. Then, the defect is repaired in a one-step procedure.
In a second sub-group of this group, the tool is moved laterally over the defect in a pressing-iron-like manner or iteratively at different positions.
In an other group of embodiments, the preparation is applied to a part of the defect only, and after application of the vibrations at one place, an other preparation is applied to an other part, etc., until the whole defect is amended.
In both groups, the method may comprise choosing one receptacle of different available receptacles, depending on the size and shape of the defect, prior to the step of applying the preparation.
In an even further group of embodiments, at least a portion of the preparation is applied while the vibrations are applied, for example continuously. For example the sonotrode, by which the vibrations are applied, may be chosen to comprise an interior channel leading from a site accessible from proximally to its distal end, and the preparation is applied through the channel in an interior of the sonotrode, for example while the sonotrode is pressed against the fiber composite part. In this, a tube (or other vessel through which the preparation is applied) may be vibrationally de-coupled from the sonotrode so that the preparation does not absorb substantial amounts of vibration energy until it is in contact with the structure of fibers. Such de-coupling can be achieved by one or a combination of:
In all embodiments, the preparation may be provided pre-made, for example already within a receptacle. In this, the preparation may for example be stored at a cooled place and taken out therefrom only shortly before being applied. If the preparation comprises a resin (for example consists of a resin), alternatively, it may be prepared immediately prior to being applied, especially if it has a tendency to harden at room temperature without any energy input. To this end, suitable tools, for example a dosing two-component mixer and applicator may be provided.
In particular, in all embodiments, the preparation may especially initially be provided in a frozen, non-flowable state.
For example if the preparation is provided in a receptacle, such as an open porous, deformable structure or a cushion-like structure, the preparation may be frozen in a pre-made state immediately after having been mixed and then inserted in the receptacle. By this, the preparation can be kept in the pre-made state considerably longer than if it for example was kept at room temperature.
In another example, the preparation can be kept in a pre-made state as a frozen drop, for example of a desired shape into which it is cast an in which it is then frozen. In this alternative example, the fact that the preparation is applied in a frozen state contributes to an at least initial lateral confinement.
In many embodiments in which the preparation is of a curable material, the mechanical vibration is applied to the preparation that is initially separate from the fiber composite part until material of the preparation has impregnated the previously insufficiently impregnated structure portion and then until this material has cured at least to an extent that it is not flowable any more.
In a variant of these embodiments, the mechanical vibration is applied in a two-step procedure. In a first step, the vibration is applied in the manner described hereinbefore, for example with lateral confinement and/or pumping, until the material has impregnated the fibers. However, the vibration is stopped while the material is still flowable at least to some extent. Then, a separation element, especially a flexible, flat element such as a foil or a membrane or a sheet is placed on the impregnated structure portion. Alternatively, a separation element may be formed by a distal end of the sonotrode used for the second vibration applying step, for example a non-stick coating of for example PTFE (known as Teflon), Fluoro-Choloro-Polyolefines, amorphous carbon etc. Thereafter, in the second vibration applying step the vibration is applied via the separation element until the material is cured and not flowable any more. This second step may especially serve for smoothing out the surface of the completed fiber composite part. After the second vibration applying step, the separation element may be removed.
For this possible second vibration applying step, the same sonotrode as in the first step may be used, or a different sonotrode may be used. Especially, if for a pumping effect in the first step a sonotrode with a structured distal surface is used, then for the second vibration applying step a sonotrode with a smooth distal surface may be applied. Also lateral dimensions may vary between a first and second sonotrode; for example the second sonotrode may be chosen to have a larger lateral extension.
In a special group of embodiments, a sonotrode having a main body and a replaceable foot portion is used. A foot portion may be mechanically connected to the main body by any suitable reversible fastening means, such as a snap connection, a bayonet fitting, a press fit, a thread, any other connection or a combination of these connections.
The foot portion may be of a same material as the main body or may be of a different material, for example of a low-cost material if the foot portion is designed to be disposable. Especially, the main body may be metallic, and the foot portion may be of a same or of a different metal, or it may be of a plastic. The latter may especially be advantageous for the second vibration application step in cases where a two-step procedure of the mentioned kind is carried out because it makes possible that the sonotrode foot portion is provided with an anti-stick coating or consists of a polymer that does not melt under the conditions during the process and to which the preparation material cannot adhere, such as PTFE or a Fluoro-Choloro-Polyolefine polymer.
The concept of the replaceable foot portion may be used in order to have different sonotrode shapes and/or dimensions for different fiber composite parts and different shapes and sizes of ‘defects’ (insufficiently impregnated portions). In addition or as an alternative, the concept may be used for the above-mentioned two-step procedure, for example if the first step is carried out by a sonotrode with a distal structure and/or with a smaller sonotrode distal end coupling area than the second step.
In any embodiment, there is the option of vacuum assisting. To this end, the support, against which the composite part is positioned during the step of applying the vibrations, is chosen to have a plurality of holes to which an underpressure is applied, so that air remaining in the defect is drawn out.
The invention also concerns a method of manufacturing a fiber composite part, the method comprising the steps of placing a structure of fibers in a mold, thereafter injecting a resin in the mold, hardening the resin, and completing the fiber composite part by the method described hereinbefore.
The invention further contains a kit of parts for carrying out the method of completing, the kit of parts comprising raw material of a hardenable preparation, such as components of a multiple-component resin, a plurality of receptacles for laterally confining a flow of the penetration, and a vibration application tool capable of applying the vibrations to the preparation when the same is within the receptacle.
Especially, such a kit of parts may comprise receptacles and possibly also tools of different dimensions to adapt to different shapes and sizes of defects.
In the following, ways to carry out the invention and embodiments are described referring to drawings. The drawings are schematical. In the drawings, same reference numerals refer to same or analogous elements. The drawings show:
A defect 4 is constituted by a portion of the part where the structure 2 is insufficiently impregnated by the resin 3 so that the fibers are exposed, and possibly the fibers are not even wetted. Such a defect may be through-going or, as in
A sonotrode 6 is used to apply the vibrations while being pressed towards the surface 7. This causes the flowable curable material to interpenetrate the fibers and may additionally accelerate the curing process.
As in all subsequent embodiments, an optional intermediate protecting layer (not shown), for example of a textile material or a material impervious to the preparation 10 (e.g. PTFE or Silicon films or coated papers and textiles, well known in manufacturing of fiber composite materials), may be used between the sonotrode and the preparation—alternatively, the sonotrode may be coated with such a non-adhering material (PTFE, poly-fluoro-chloro polyolefines, a-CH amorphous carbon or diamond like carbon).
The embodiment of
In the depicted example, the sleeve is attached to the sonotrode 6 along a circumferential region 6.1 thereof so as to close off the contained volume of the preparation 10 towards the upper (proximal) side.
For the process, the preparation 10 and the arrangement that comprises the sonotrode 6 and the sleeve 12 are placed on the surface 7, with the distal ends of the sleeve in contact with the surface 7. Then the sonotrode 6 is pressed towards the surface 7 while mechanical vibration is coupled into the sonotrode.
In a variant, it would be possible to not attach the sleeve to the sonotrode but keep the sonotrode shiftable in a piston-like manner within the sleeve. In this variant, it is advantageous to tightly fit the sleeve to the sonotrode. As a further possibility, such a loose fitting, flexible sleeve would allow to cover the defect and move the sonotrode inside the sleeve. The latter would allow to minimize the emission of resin fumes during the impregnation and curing process, especially if combined with a fume suction unit providing a slight pressure drop inside the sleeve.
The embodiment of
In
The example of
The variant of
Due to this, the receptacle does, in contrast to the previous embodiments, not have any coating.
In
The receptacles of the preparations shown in the previous figures may be available in different shapes and sizes so that an operator may choose a suitable receptacle, depending on the size and shape of the defect.
For preparing the preparation from a two-component material, for example a two-compartment syringe (one compartment per component) with a mixing head may be used. The mixing head may for example be disposable. It is possible to provide such a mixing head with an interface directly adapted to the used compartment, or to make the compartment one-piece with the mixing head. For example, the mixing head could directly interface with a coating 32 of the previously described kind.
An example of a material that can be transformed from a solid state to a flowable state by mechanical energy is Poly(methyl methacrylate) (PMMA).
There may be situations where the defect 4 is too large for a single preparation. Then the above-described process—in any one of the shown embodiments—is applied firstly for one section of the defect and then is repeated at an other section until the full defect is repaired.
The vibration generating unit comprises a unit contact 56, and the casing comprises a casing contact 57. The device is configured so that the vibrations can only be switched on if the contacts 56, 57 contact each other (additionally, it may optionally be required, that the operator operates a manual switch (not shown). The contacts 56, 57 are arranged so that the contact each other only when the vibration generating unit 53 is displaced relative to the casing 52 by a certain minimal displacement, and when the vibration generating unit is not displaced too far. Therefore, the device will only operate if the sonotrode 6 is pressed against the preparation/the surface by a certain minimal pressing force and if the pressing force does not exceed a certain upper limit.
A similar principle could be applied by other means, such as light barriers, toggle switches etc. It would also be possible make an arrangement by which the device only defines an upper limit or only defines a lower limit for the pressing force.
In the arrangement of
When the sonotrode is subject to vibrations in axial directions (the axis in this is the proximodistal axis 20) about which the sonotrode may but does not need to be rotationally symmetric, this will cause bending vibrations of the wing portion 63 (see arrows). It has been observed that this causes an advantageous pumping effect on the preparation, which instead of being sprayed into various directions, as can be the case for plain sonotrodes without any lateral confinement, is efficiently confined and pumped into the structure of fibers 2 of the defect 4.
The fact that the bending vibrations of the wing portion 63 do cause a confinement could be verified by using the sonotrode of
It was observed that the sonotrode designs of
A first possible explanation for this improved behavior is a simple partial confinement of the preparation by the peripheral ridge 15 as shown in
However, especially, and somewhat surprisingly, designs like the one of
A possible explanation for this effect is that due to the deviation from a plain sonotrode design due to the ridge 15, vibration modes different from purely axial (longitudinal) vibrations are excited in the sonotrode. Possible vibrations may include Bessel vibrations on the distal end side. Especially, such possible additional vibration modes will cause the volume 67 to be non-constant but to be subject to a breathing effect. This is very schematically (and exaggeratedly) illustrated in
As illustrated in
With respect to
The two-step process may be, depending on parameters like the sonotrode design, the preparation composition, the size of the defect and others, advantageous in situations where the fiber composite part after the process needs to have a smooth surface.
In a first step, shown in
The sonotrode 6 used in the second step may be the sonotrode also used in the first step. Alternatively, a different sonotrode may be used in the second step, or the sonotrode may be provided with a different replaceable foot for the second step. An exchange of the sonotrode or a foot portion thereof for the second step may especially be advantageous in embodiments in which the sonotrode has a non-smooth distal end face, for example by having a ridge of the hereinbefore described kind.
Foot portions with different dimensions and shapes of distal end faces may exist.
| Number | Date | Country | Kind |
|---|---|---|---|
| 668/14 | May 2014 | CH | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CH2015/000072 | 5/4/2015 | WO | 00 |
