Method and device for electromagnetic welding of moulded parts

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The Netherlands © 2000615 @ C PATENT²⁰ * 2?) Patent application: 2000 615 (5?) Int.CI .:

B29C65 / 36 (2006.01) H05B3 / 38 (2006.01) © Submitted: 26.04.2007

<td>Θ</td><td>Signed up: 28.10.2008 IE 2009/01</td><td> ©</td><td>Patent holder (s): Kok &amp; Van Engelen Composite Structures BV in The Hague.</td>

<td>Θ</td><td>Date: 28.10.2008</td><td> ©</td><td>Inventor (s):</td>

<td></td><td>Issued:</td><td></td><td>Martijn Jacques van Wijngaarden in Lisserbroek.</td>

<td></td><td>05.01.2009 IE 2009/01</td><td> ©</td><td>Authorized representative: Ir. H.Th. van den Heuvel cs in 5200 BN 's-Hertogenbosch.</td>

© Method and device for electromagnetic welding of molded parts.

^ 7) The invention relates to a method for electromagnetic welding of molded parts. In the method, a mold is provided, and at least two molded parts to be coupled are placed in the mold, at least one contact surface between the molded parts comprising a thermally activatable coupling agent and an induction-sensitive component. The coupling means is activated by heating the induction-sensitive component by means of an inductor. The inductor comprises an electrical conductor which, under alternating voltage, generates a substantially cylindrical electromagnetic field at least in a welding direction. The molded parts are compressed in the configuration determined by the mold, the molded parts being coupled by the thermally activated coupling means. The invention also relates to an assembly of molded parts obtained according to the method, an inductor for use in the method, an assembly thereof with an alternating current generator and an apparatus for electromagnetic welding.

NL C 2000 615

The contents of this patent correspond to the original filed description with claim (s) and any drawing (s).

The Netherlands Patent Center is an agency of the Ministry of Economic Affairs.

Method and device for electromagnetic welding of molded parts.

The invention relates to a method for electromagnetic welding of molded parts, an assembly of molded parts obtained according to this method, an inductor for use in the method, an assembly thereof with an alternating current generator and an apparatus for electromagnetic welding.

Various welding methods are already available for creating a continuous weld connection between molded parts, in particular thermoplastic molded parts. However, the presence of an electrically conductive component and / or fiber reinforcement hinders these methods when making a weld. When using a resistance wire, for example, a short circuit can occur between the resistance wire and the electrically conductive component. This resistance wire is fused between the thermoplastic molded parts during the welding process. This problem can be solved by electrically insulating the resistance wire from the conductive component in the thermoplastic. However, with this solution, in addition to the resistance wire, even more material is fused between the thermoplastic molded parts, which can be disadvantageous for the construction. In vibration welding, the fibers can be damaged by the movement. Ultrasonic welding is less suitable for continuous welding. Moreover, many of the available welding methods are not suitable for welding large and continuous welded joints. Particularly for high-quality applications where a high mechanical strength and load-bearing capacity of the welded joint is desired, in particular in aircraft construction, these known welding methods give products of insufficient quality.

It is an object of the invention to enable an improved method of welding shaped parts.

To this end, the invention provides a method for electromagnetic welding of shaped parts, comprising the processing steps: A) providing a mold, B) placing at least two molded parts to be coupled into the mold, at least one contact surface between the molded parts comprising a thermally activatable coupling agent and an induction-sensitive component, C) activating the coupling means by heating the induction-sensitive component by means of an inductor, the inductor comprising an electrical conductor which, under alternating voltage, generates a substantially cylindrical electromagnetic field at least in the welding direction at the level of the contact surface, D) compressing the molded parts in the configuration determined by the mold, the molded parts being thermally activated coupling agent. This method makes it possible to quickly and easily realize a high-quality welded connection between the molded parts, the product having a particularly good mechanical load-bearing capacity.

Preferably, one or more molded parts are made of a thermoplastic material that can be welded by fusion, but it is also conceivable to apply a thermoplastic material or a thermally activatable glue only as a thermal coupling agent on the contact surface between the molded parts.

Induction sensitive components usually contain an electrically conductive component such as a metal and / or carbon fiber. Preferably, the mold and other parts in the vicinity of the inductor which should not be heated are substantially free of induction sensitive components, with electrically insulating but thermally conductive components, such as ceramic material, being preferred to extract heat from the weld during welding. contact surface of the adjacent molded part. In such a mold, the electromagnetic field through the wall of the mold can then be applied at the desired position.

In the method, the preferably thermoplastic molded parts are usually provided with an electrically conductive component, for example metal mesh, or this component is placed between the molded parts. In the electrically conductive component, Foucault currents or eddy currents are induced by a fluctuating electromagnetic field generated by an inductor, which is supplied with alternating current by a generator. Due to the Joule effect, these Foucault currents generate the heat needed to melt the thermoplastic material and / or to activate the coupling agent. By moving the inductor along the contact surface, the thermoplastic molded parts are joined together over their contact surface. For example, the inductor can be guided across the contact surface by a robot arm or linear guide to establish the connection.

The use of a substantially cylindrical electromagnetic field in the welding direction makes it possible to heat in a very controlled, even and targeted manner, so that overheating is avoided as much as possible. Overheating can lead to material degradation, causing unwanted structural weaknesses. Most prior art inductors utilize a multi-winding inductor which produces a toroidal electromagnetic field. By using such a known inductor with the direction of induction perpendicular to the inductor, a heating pattern is created in which a relatively cold zone is created in the middle. The cylindrical electromagnetic field, on the other hand, gives a much more favorable heating profile that allows uniform heating. In addition, a cylindrical electromagnetic field can be made very narrow up to a width of 10-20 mm. With toroidal fields, such a width in combination with the required heat-inducing power and penetration cannot be realized.

Before heating, the induction-sensitive component must be in thermal contact with the thermally activatable coupling agent. This can be done, for example, by mixing the induction-sensitive component and the coupling agent.

Compression of the molded parts can be done by means known from the prior art, such as pneumatic or hydraulic presses and rollers. Depending on the materials used, in particular the induction sensitive component and the distance of the inductor from that component, a suitable power and frequency can be determined. Among other things, the frequency determines the penetrating power of the electromagnetic field; the electrical power of the inductor determines the strength of the fluctuating electromagnetic field and thus the amount of heat generated in the induction-sensitive component.

It is advantageous if the thermally activatable coupling agent comprises a thermoplastic plastic. Thermoplastic plastics are easy to connect by fusion. In addition, it is easy to mix a thermoplastic plastic with an induction-sensitive component such as metal mesh or carbon fibers. Examples of particularly suitable thermoplastic plastics are polypropylene, polyamide, polyetherimide, polyetheretherketone and polyphenylene sulfide, although the method is in principle suitable for any thermoplastic.

Preferably, the induction heatable component contains carbon fibers and / or a metal. These materials are easy to heat by induction and, in addition to electrical conductivity, also have good thermal conductivity, so that the heat generated is well distributed. Carbon fibers incorporated in a thermoplastic plastic are preferred, because the carbon fibers also improve the material strength.

In a preferred embodiment, during step C), the inductor is moved relative to the contact surface along a path such that the coupling means is activated in a predetermined part of the contact surface. By moving the cylindrical electromagnetic field along the path, a very precisely determined connection on the contact surface is realized.

The inductor is preferably located outside the mold, the electromagnetic field of the inductor passing through a wall of the mold reaching the contact surface between the molded parts. Because the inductor does not come into contact with the mold or the molded parts, the risk of mechanical damage is minimized, which is particularly important for high-quality applications such as aviation.

The wall of the mold is preferably provided with a recess at the level of the contact surface, in other words above the welding position. Such a recess makes it possible to bring the inductor closer to the contact surface, which makes it possible to heat with greater precision, and which also requires less power.

A further preferred embodiment of the method according to the invention is characterized in that the electrically conductive component is electrically connected to an electrically conductive extension, which preferably extends beyond the surface of the assembly. Such an extension can in principle be made of any electrically conductive material, but is preferably made of metal, of carbon, or comprises an adjustable resistance. The Foucault currents or eddy currents induced in the contact plane are limited by the geometry of the molded parts. Edges, corners and holes in the molded parts influence the distribution of Foucault currents and thus also influence the heat generated. Such field disturbances can lead to heating of parts that do not need to become hot before the welding process. It is also possible that certain parts are difficult to heat. These problems can be solved by pushing the boundaries of the area where Foucault currents can occur at certain places in the thermoplastic molded parts. With this preferred variant, previously difficult-to-heat parts can nevertheless be heated and high temperatures in undesired places can be prevented.

The invention also provides an assembly of at least two molded parts connected by electromagnetic welding, obtainable by the method according to any one of the preceding claims. Such an assembly has a particularly good, regular weld on the contact surface between the molded parts with a high mechanical load capacity. By using the method according to the invention, the weld pool is preferably oval in the cross-section of the contact surface and, moreover, substantially continuous and even over almost the entire welding length. It is to be understood that an assembly in which the circumference of the welding bath in a cross-section at the contact surface has small deviations from the oval shape is also part of the invention. By substantially continuous is meant that the weld in the longitudinal direction thereof is not significantly interrupted.

The invention also provides an inductor, suitable for use in a method according to any one of the claims, comprising an inductive part substantially made of an electrically conductive material and an induction segment provided with at least one electrically conductive supply conductor, at least one induction segment of the inductor is adapted to generate a substantially cylindrical electromagnetic field at least in a welding direction. For this purpose, the induction segment is preferably linear, the cross section of the electromagnetic field being influenced by the cross section of this induction segment. With such an inductor, a predetermined position can be heated simply and precisely and in a controlled manner. The electrically conductive material is preferably a metal such as copper.

For use in the method according to the invention, the inductor is connected to an alternating current generator, the alternating current generator being electrically connected to the electrical connecting means of the inductor. Usable frequencies are usually between 0.1-10 MHz. Preferably a frequency between 0.1 and 0.5 MHz is used, and more preferably a frequency between 0.15 and 0.4 MHz. At such a preferred frequency, an optimum balance between penetrating power of the electromagnetic field and heating rate is achieved.

Preferably, the induction segment is substantially linear. With such an induction segment, a cylindrical electromagnetic field can be realized in a simple manner. The electrical supply conductors can be curved if desired.

In a preferred embodiment, the induction segment has a substantially circular cross section. A circular cross-section provides in a simple manner a circular electromagnetic field at the location of the cross-section. The shape of the electromagnetic field at the location of the cross-section can be influenced by other design of the cross-section, for example triangular.

Preferably, the inductor has no windings. Such an inductor has a very compact design and is therefore suitable for very precisely determined induction.

It is advantageous if the inductor is substantially flat. This can be done, for example, by designing the winding-free inductor as an in-plane electrical conductor. Such a flat inductor is particularly compact and suitable for very precisely and uniformly applying an electromagnetic field at a specific position.

It is advantageous if the feed conductor has a larger cross-sectional area than the induction segment. As a result, an electromagnetic field generated by AC voltage is stronger at the location of the induction segment than in the supply conductor, so that the heating can be directed very precisely by induction.

In a preferred embodiment, the induction part is provided with at least one feed-through channel, adapted for feeding through a cooling medium. This allows the temperature of the induction part to be kept constant during use, which is also favorable for the electrical resistance of the inductor. The cooling medium is preferably a liquid, such as water, with a high heat capacity. The induction part can for instance be a metal tube bent into the desired shape through which the cooling medium is pumped, while an electromagnetic field is caused by the metal of the tube itself with an alternating voltage.

The invention also provides a device for electromagnetic welding of molded parts, comprising a mold for receiving at least two molded parts, such that a contact surface between the molded parts is created, an inductor adapted to generate a substantially cylindrical electromagnetic field at the location of the contact face, an alternating current generator connected to the inductor, and pressure means for compressing the molded parts in the configuration determined by the mold. The method according to the invention can be advantageously carried out in such an apparatus.

The invention will now be elucidated with reference to the following figures, without, however, being limited thereto. In the figures show:

Figures 1a and 1b the difference between electromagnetic welding with a toroidal field and a cylindrical field;

Figures 2a and 2b show an inductor according to the invention;

Figure 3 a welding device provided with an inductor according to the invention;

Figure 4 two molded parts which are coupled by the method according to the invention; and

Figure 5 shows a mold that can be used in electromagnetic welding according to the invention.

Figure 1a shows a cross-section of a multi-winding inductor 1, which produces a toroidal electromagnetic field 2 by supplying an alternating voltage of a frequency suitable for electromagnetic welding at a suitable power. In this electromagnetic field 2, a first molded part 3 and a second molded part 4 are brought into contact with each other. The molded parts are made of a thermoplastic plastic reinforced with carbon fibers. Under the influence of the electromagnetic field 2, heat is locally generated in the carbon fibers, whereby the thermoplastic plastic is heated above the melting point. By pressing with pressing means (not shown) it is possible to couple the thermally activated thermoplastic molded parts 3,4 to the contact surface 5, the coupling on the contact surface 5 becoming permanent after the molded parts 3,4 have cooled. The figure further shows the temperature diagram on heating on the contact surface, in which the relative temperature T is plotted against the location on the contact surface 5. The temperature diagram shows that the toroidal field 2 on the contact surface 5 causes irregular heating, wherein in the center of the contact surface 5 in the welding direction A at right angles to the windings of the inductor creates a relatively cold zone 6. This relatively cold zone makes uniform heating impossible, which results in a relatively high number of irregularities in the coupled assembly between the molded parts, which imply a reduced mechanical load capacity. The irregularities may include, for example, thermally degraded parts of the molded parts due to local overheating and locally incomplete bonding of the molded parts.

Figure 2 shows electromagnetic welding according to the invention. A cross-section shows how a linear part of a winding inductor 10 produces a substantially cylindrical electromagnetic field 11 under the influence of AC voltage. A first molded part 12 and a second molded part 13 of a material comparable to the parts 3,4 of figure 1a are electromagnetically welded together on a contact surface 14 under the influence of this field and possible pressing means. The linear part of the winding-free inductor 10 is oriented parallel to the contact surface 14. The associated temperature diagram shows that the cold zone 5 of the toroidal field 2 in Fig. 1a is absent when using a cylindrical electromagnetic field, allowing much more uniform heating over the contact surface 14. In the product obtained, the two molded parts 12, 14 are mutually coupled by means of the contact surface 14, whereby significantly fewer irregularities occur than with coupling with a toroidal field obtained under comparable conditions. As a result, the product obtained under the influence of the cylindrical electromagnetic field has a better mechanical loadability than the product obtained with a toroidal electromagnetic field. In addition, the electromagnetic field 11 can be directed very precisely in the indicated direction A.

Figures 2a and 2b show an inductor 20 according to the invention. Inductor 20 is made of copper, which is a good electrical and thermal conductor. The supply conductors 21 can be connected to an alternating current generator. The supply conductors 21 connect to a linear induction segment 22. The induction segment 22 has a circular diameter, so that when an alternating voltage is applied an electromagnetic field 23 is generated with a profile which is at least cylindrical in the direction of induction A (Fig. 2b). On the inside, inductor 20 is hollow, forming a feed-through channel 24 for a cooling medium such as water that can be passed during use. The cross-sectional area of ​​supply lines 21 is larger than that of the induction segment, making the cylindrical electromagnetic field 23 more concentrated and at a short distance has greater strength than the non-cylindrical electromagnetic field (not shown) surrounding other parts of the inductor 20 is created. This makes it possible to position the power of the electromagnetic field 23 very precisely at an electromagnetically weldable position of, for example, two adjacent molded parts. The inductor 20 comprises a winding inductor, which is located in a plane, which makes the inductor very compact.

Figure 3 shows a welding device 30 provided with an inductor 31, comparable to the inductor 20 of figures 2a and 2b according to the invention. By means of an industrial six-axis robot 32, the inductor 31 can be guided along a preprogrammed path to achieve a desired weld. In this case, molded parts (shown in Fig. 4)) to be welded are fixed and compressed in a mold 33 prepared therefor. The mold 33 is provided with a recess 34 through which the inductor can be moved near the molded parts to be welded (fig. 4). The mold 33 can be seen in more detail in Figure 5. To generate the electromagnetic field, the inductor is connected to an AC generator 35 mounted on the robot 32.

Figure 4 shows two molded parts 40, 41 which are coupled by the method according to the invention. Both molded parts 40, 41 are made of a thermoplastic plastic reinforced with carbon fibers, the carbon fibers also serving as an induction-sensitive component for heating the thermoplastic plastic for welding. The first molded part 40 is a flat part, the second molded part 41 has a folded edge 42 which forms the intended contact surface for coupling between the molded parts. The inductor 43, similar to that of the previous figures 1b, 2a, 2b and 3, heats the molded parts 40, 41 in the assembled state by means of a cylindrical electromagnetic field to a temperature high enough to heat the thermoplastic plastic (or possibly a thermally activatable adhesive applied to the contact surface 42,42 ') to activate thermally. The inductor is moved in the direction B of the longitudinal axis of the cylindrical electromagnetic field over the contact surface without making physical contact. During heating and / or possibly shortly thereafter, the thermally activated surfaces can be compressed by non-illustrated pressure means in order to establish a connection between the molded parts (40, 41). This connection has a particularly high mechanical load capacity. The molded parts can for instance consist of carbon fiber-reinforced polyphenylene sulphide, for example with a material thickness of 1-3 mm. With the aid of the method according to the invention, shear strengths higher than 30 MPa can be easily realized. By making use of the cylindrical electromagnetic field according to the invention, the width of the realized weld can be very small, for instance 10 mm.

Figure 5 shows a mold 50 that can be used in the electromagnetic welding according to the invention. The mold 50 comprises some electrically non-conductive parts (51, 52) between which the molded parts of, for example, figure 4 can be pressed together. A non-electrically conductive, but thermally conductive material part 53, such as a ceramic material, which serves to distribute and dissipate the generated heat from the mold parts during application of an electromagnetic field is provided at the location of the intended contact surface to be coupled and also prevent deformation of the thermoplastic material. A recess 54 is provided in the mold 50, so that the inductor can be brought as close as possible (0.5-2 cm from the welding zone) to the contact surface between the molded parts without coming into contact with it. For mechanical reinforcement and to dissipate heat from the mold, the mold is provided on the outside with a metal outer layer 54. The parts in the vicinity of the inductor are preferably made of non-electrically conductive materials such as wood or plastic.