Method and device for electromagnetic welding of moulded parts

(54) Title: METHOD FOR ELECTROMAGNETIC WELDING OF MOLDED PARTS, A SET OF AT LEAST TWO MOLDED PARTS CONNECTED THROUGH AN ELECTROMAGNETIC WELDING, INDUCTOR, INDUCTOR SET AND ALTERNATING CHAIN ​​DEVICE AND MECHANICAL DEVICE.

(51) Int. Cl .: B29C 65/36; B29C 65/78; B29C 65/00; B29L 31/30 (52) CPC: B29C 65/3612, B29C 65/3668, B29C 65/7841, B29C 66/112, B29C 66/1122, B29C 66/131, B29C 66/349, B29C 66/43421, B29C 66/43441, B29C 66/8221, B29C 66/8242, B29C 66/836, B29C 66/863, B29C 65/3676, B29C 65/3696, B29C 66/71, B29C 66/8122, B29C 66/81811, B29K 2995/0008, B29L 2031/3076 (30) Unionist Priority: 26/04/2007 NL 200615 (73) Holder (s): KOK & VAN ENGELEN COMPOSITE STRUCTURES BV

(72) Inventor (s): MARTIJN JACQUES VAN WIJNGAARDEN (74) Attorney (s): MARTINEZ & MOURA BARRETO S / S LTDA.

(86) International Application: PCT NL2008050242 of 24/04/2008 (87) International Publication: WO

2008/133507 of 11/06/2008

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METHOD FOR ELECTROMAGNETIC WELDING OF MOLDED PARTS, SET OF AT LEAST TWO MOLDED PARTS CONNECTED THROUGH AN ELECTROMAGNETIC WELDING, INDUCTOR, INDUCTOR SET AND ALTERNATE CURRENT GENERATOR FOR ELECTRIC WELDING MOLDING

<td></td><td>THE</td><td>gift</td><td>invention</td><td>refers to</td><td>The</td><td>a method for</td>

<td>welding</td><td colspan="2">electromagnetic</td><td>of parts</td><td>molded,</td><td>one</td><td>set of parts</td>

<td>molded</td><td>obtained</td><td>according</td><td>like this</td><td>method, and</td><td>one</td><td>inductor for use</td>

in this method, a set of the same with an alternating current generator and an electromagnetic welding device.

Several welding methods are already available to create a continuous welding connection between molded parts, in particular thermoplastic molded parts. However, these methods are difficult when making a welded connection due to the presence of an electrically conductive component and / or a reinforcement fiber. When a resistance wire is used, for example, a short circuit can occur between the resistance wire and the electrically conductive component. This resistance wire is fused between the thermoplastic parts molded during the welding process.

This problem can be solved by the electrical insulation of the resistance wire from the conductive component in the thermoplastic. However, with this solution, in addition, more materials are melted between the molded thermoplastic parts, something that can adversely affect the construction. During vibration welding, fibers can be damaged by movement. Ultrasonic welding is less suitable for continuous welding. Many of the welding methods available are even more unsuitable for welding large, continuous welded connections. These welding methods already known

2/17 result in lower quality products, particularly for high-grade applications where high mechanical strength and load-bearing capacity of the welded connection is desirable, particularly in the aviation industry.

It is an object of the invention to provide an improved method for welding molded parts.

For this purpose the invention provides a method for electromagnetic welding of molded parts comprising the processing steps of: a) providing a mold; b) positioning at least two molded parts to engage the mold where at least one contact surface between the molded parts comprises a thermally activated coupling means and an induction sensitive component, c) activating the coupling means by heating the component sensitive to induction by means of an inductor, in which the inductor is located outside the mold, the inductor comprising an electrical conductor which at alternating voltage generates an electromagnetic field which is substantially cylindrical in at least the direction of the welding, and in which the inductor's electromagnetic field reaches the contact surface between the molded parts through a wall of the mold, d) pressing the molded parts together in the configuration defined by the mold, in which the molded parts are coupled by the thermally activated coupling means.

This method makes it possible to make a good quality welded connection between the molded parts in a quick and simple way, in which the product has a good mechanical load-bearing capacity. Because of the factor that the inductor does not come into contact with the mold or the molded parts, the chances of mechanical damage are minimized,

3/17 thus being particularly important for applications such as in the aviation area.

A preferred embodiment of the method is characterized by the fact that the inductor comprises a linear induction segment which generates an electromagnetic field which is substantially cylindrical in at least one welding direction and that the inductor is positioned in such a way that the induction segment substantially runs parallel to the mold wall. In this way the contact surface can be heated in a highly selective manner, in which a precise and accurate welded connection is obtained.

One or more molded parts are preferably manufactured from a thermoplastic material which can be welded by melting, although it is also possible to design an arrangement of thermoplastic material or a thermally activated adhesive only on the contact surface between the molded parts as a thermal coupling means.

Accordingly, induction sensitive components comprise an electrically conductive component such as a metal and / or a carbon fiber. The mold and the other components in the vicinity of the inductor, which preferably do not need heating and substantially free of induction sensitive components, in which thermally conductive components, but still electrically insulated such as ceramic material, are recommended for the extraction of heat from the contact surface of the adjacent molded part during welding. In such a mold the electromagnetic field can then be applied in the desired position through the mold wall.

In the method, the preferably molded thermoplastic parts are generally provided with an electrically conductive component, for example, metallic straw or this

4/17 component is arranged between the molded parts. Eddy currents or eddy currents are induced in the electrically conductive component by means of a floating electromagnetic field which is generated by means of an inductor fed with alternating current through a generator. Due to the Joule effect, these eddy currents generate the heat required to melt the thermoplastic material and / or to activate the coupling medium. By moving the inductor along the contact surface, the thermoplastic molded parts are mutually connected over their contact surface. The inductor can be guided over the contact surface, for example, by means of a robot arm or a linear guide in order to make the connection.

The use of a substantially cylindrical electromagnetic field in the direction of the welding allows a well controlled, uniform and targeted heating in such a way that it is prevented from overheating as far as possible. Overheating can result in material degradation, and hence, cause an unwanted weakening when building. Most prior art inductors make use of an inductor with a plurality of windings, something that produces a torus-shaped electromagnetic field. By using such a known inductor with the direction at right angles with respect to the inductor as an induction direction, a heating pattern is created in which a relatively cold zone occurs in the center. On the other hand, the cylindrical electromagnetic field produces a much more favorable heating profile which allows uniform heating. In addition, a cylindrical electromagnetic field can be made narrowly with a width of up to 10 - 20 mm. In fields with a torus shape, such a width cannot be achieved in combination with the

5/17 induced heat power and penetration required.

For the purpose of heating the induction sensitive component must be in thermal contact with the thermally activated coupling medium. For example, this is possible by mixing the induction sensitive component and the coupling medium.

Because of the factor that the inductor is located outside the mold and the electromagnetic field of the inductor reaches the contact surface between the molded parts through a mold wall, the molded parts can be placed under pressure through the mold during welding. This is a great advantage. Generally, existing methods only apply pressure after the induction heating of the molded parts for proper coupling. Compression of molded parts can occur using means known in the prior art such as pneumatic and hydraulic presses and bearings. Preferably, pressure is applied to the mold on the non-inducing side of the mold, in other words, on that side of the mold where the inductor is not located.

Preferably, the mold wall is provided with a recess at the position of the contact surface, in other words above the welding position. Such a recess makes it possible to move the inductor closer to the contact surface, hence, this can be done with greater precision and accuracy and where less power is also required.

In order to put pressure on the molded parts for a coupling as high as possible in the location of the welding position, it is advantageous to make the width of the recess as small as possible and preferably in such a way that it barely exceeds the width of the inductor. With

6/17 In order to obtain the highest possible pressure, the wall is manufactured in the recess position from a material with high rigidity.

Depending on the material used, in particular the induction sensitive component and the inductor distance from this component, a suitable power and frequency can be determined. The frequency determines inter alia the penetrating power of the electromagnetic field; the electrical power of the inductor determines the resistance of the floating electromagnetic field and hence, the degree of heat generated in the induction sensitive component.

It is advantageous if the thermally activated coupling means comprises a thermoplastic plastic. Thermoplastic plastics can be coupled in a simple melting way. In addition, it is easy to mix a thermoplastic plastic with an induction sensitive component such as metal straw or carbon fiber. Examples of particularly suitable thermoplastic plastics are polypropylene, polyamide, polyether imide, polyether ether ketone and polyphenylene sulfide, although the method is in principle suitable for any thermoplastic.

Preferably, the component that is heated by means of induction comprises carbon fibers and / or a metal. These materials can be readily heated by means of induction and also have electrical conduction, a good thermal conduction, through which the heat generated is well distributed. Carbon fibers embedded in a thermoplastic plastic are recommended because carbon fibers also improve the material's strength.

In another preferred embodiment of the method the component

7/17 induction heated comprises ferromagnetic particles and are, for example, described in the international patent application published under No. WO 0185827 and have the additional advantage that when they reach their so-called Curie temperature, they lose their magnetic dipoles, hence they are no longer heated. This can form a protection against the overheating factor.

In the method according to the invention it is possible for both the method and the inductor to be stationary. For example, this may be suitable for coupling a relatively small portion of the contact surface of the molded parts. In a preferred embodiment, the inductor is moved along a path relative to the contact surface during processing step C) in such a way that the coupling means is activated on a predetermined part of the contact surface. A very precise and exact connection is made on the contact surface when the cylindrical electromagnetic field moves along the path. It is also possible to keep the inductor stationary and move the mold with the molded parts.

An additionally preferred embodiment of the method according to the invention has the characteristic that the electrically conductive component is electrically connected to an electrically conductive extension piece, which preferably extends outside the surface of the assembly. Such an extension piece can, in principle, be manufactured from any electrically conductive material, but it is preferably manufactured from metal, from carbon, or comprises an adjustable resistor. Eddy currents or eddy currents induced on the contact surface are connected via the

8/17 geometry of molded parts. Edges, corners and holes in the molded parts influence the distribution of eddy currents and therefore also influence the generated heat. Such field interruptions can result in the heating of components which do not need to be heated for the welding process. Conversely, it is also possible that certain parts are difficult to heat. These problems can be solved by repositioning the limits of the area where eddy currents can begin to occur in certain locations of the molded thermoplastic parts. With this preferred variant, parts which were previously difficult to heat can otherwise be heated and high temperatures in the unwanted locations can be prevented.

The invention also provides a set of at least two molded parts connected by means of an electromagnetic welding obtained by means of the method according to any one of the following claims. Such a set has a particularly good and regular weld on the contact surface between the molded parts with a high mechanical load-bearing capacity. By applying the method according to the invention, the casting bath is preferably oval in cross-section on the contact surface and additionally substantially continuous and uniform over substantially the entire length of the weld. It will be apparent that the assembly in which the periphery of the welding bath has small variations from the oval shape in a cross section at the position of the contact surface, similarly, forms part of the invention. It must be understood that substantially continuous means that the weld has no interruptions that are necessary to mention in its longitudinal direction.

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The invention also provides an inductor, evidently suitable for use in a method according to any one of the claims, comprising an induction part which is substantially manufactured from an electrically conductive material and an induction segment provided with at least one electrically conductive power conductor, in which at least one inductor segment of the inductor is adapted to generate an electromagnetic field which is substantially cylindrical in at least one welding direction. For this purpose, the induction segment preferably takes a linear shape, in which the cross section of the electromagnetic field is influenced by the cross section of this induction segment. With such an inductor it is possible in a simple and precise way to heat a predetermined position in a uniform and controlled manner. The electrically conductive material is preferably a metal such as copper. It should be understood that, within the scope of this application, a linear induction segment means an induction segment with a length which is at least twice, and preferably at least ten times the dimension of the linear cross section (the diameter for a section circular cross section).

For application in the method according to the invention, the inductor is connected to an alternating current generator, in which the alternating current generator is electrically connected to the electrical connection medium of the inductor. Usable frequencies generally range between 0.1 - 10 MHz. A frequency between 0.1 and 0.5 MHz is preferably used, and more preferably a frequency between 0.15 and 0.4 MHz. At such a preferred frequency an optimized balance is achieved between the penetrating power of the electromagnetic field and the rate of heating.

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Preferably the induction segment is substantially linear. With such an induction segment, a cylindrical electromagnetic field can be realized in a simple way. If desired, the power supply conductors can be bent.

In a preferred embodiment the induction segment has a substantially circular cross section. A circular cross section produces a circular electromagnetic field at the position of the cross section in a simple way. The shape of the electromagnetic field in the cross-sectional position can be influenced by a different design from that of the cross-section, for example, triangular.

Preferably, the inductor has no bearing at all.

Such an inductor can be incorporated in a very compact manner and is therefore suitable for a specific, precise and exact induction.

It is advantageous if the inductor is substantially flat. This is possible, for example, by incorporating the bearing-free inductor as an electrical conductor resting on a plane. Such a flat inductor is exceptionally compact and suitable for the application of an electromagnetic field in a given position in a very precise and even manner.

It is advantageous that the supply conductor has a larger cross-sectional area than that of the induction segment.

An electromagnetic field generated by an alternating current is here much stronger in the position of the induction segment than in the supply conductor, in which heating by means of induction can be precisely and exactly desired. The ratio of the linear cross-sectional dimension of the supply conductor and

11/17 of the linear cross-section dimension (the diameter for a circular cross-section) of the induction segment is for this purpose preferably chosen between 1 and 20, and more preferably between 1.2 and 10. The ratio of the length of the segment of induction induction and the linear cross-sectional dimension of the induction segment is even more preferable chosen for this purpose between 2 and 100, and more preferably between 5 and 50.

In a further preferred embodiment, the induction part is provided with at least one supply channel for the passage of a cooling medium. Here, the temperature of the induction part can be kept constant during use, which is also favorable for the electrical resistance of the inductor. Preferably, the cooling medium is a liquid such as water, with a high heating capacity. The induction part can, for example, be a metal tube curved to a desired shape, through which the cooling medium is pumped while an electromagnetic field is generated through the metal of the tube itself with an alternating voltage.

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

The invention will now be described with reference to the

12/17 figures to follow, but without being limited to them. In the figures:

Figures la and lb show the difference between an electromagnetic weld with a torus-shaped field and a cylindrical field;

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

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

Figure 4 shows two molded parts coupled by the method according to the invention; and Figure 5 shows a mold which can be used in electromagnetic welding according to the invention.

Figure 1 shows a cross section of an inductor 1 with a plurality of bearings which cause an electromagnetic field in the form of a torus 2 by applying an alternating current with a frequency suitable for electromagnetic welding at a suitable power. A first molded part 3 and a second molded part 4 are placed in mutual contact in this electromagnetic field 2. The molded parts are manufactured from a carbon fiber reinforced thermoplastic. The heat develops locally in the carbon fibers under the influence of the electromagnetic field 2 in which the thermoplastic plastic is heated up to above the melting point.

By means of the pressure carried out by the pressure medium (not shown), it is possible to couple the molded and thermally activated thermoplastic parts 3, 4 on the contact surface 5, in which the coupling on the contact surface becomes permanent after the parts have cooled. molded 3, 4.

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Additionally, the figure shows the temperature diagram on the contact surface during heating in which the relative temperature T is guided against the position on the contact surface 5. The temperature diagram shows that the torus-shaped field 2 causes a heating irregular on the contact surface 5, in which a relatively cold zone 6 occurs in the center of the contact surface 5 in the direction of welding A at right angles to the inductor bearings.

Due to this relatively cold zone, uniform heating is impossible, something that results in the set coupled between the molded parts with a relatively high number of irregularities, something that implies a reduced load bearing mechanical capacity. The irregularities can, for example, comprise parts of the thermally degraded molded parts by means of local overheating and by means of incomplete local mutual adhesion of the molded parts.

Figure 2 shows an electromagnetic weld according to the invention. A cross section shows how a linear part of a bearing-free inductor 10 causes a substantially cylindrical electromagnetic field 11 under the influence of an alternating voltage. A first molded part 12 and a second molded part 13 of a material comparable to parts 3, 4 of figure 1a are welded together electromagnetically on the contact surface 14 under the influence of this field and this optional pressure medium. The linear part of the bearing-free inductor 10 is here directed in parallel to the contact surface 14.

The associated temperature diagram shows that the cold zone 5 of the torus-shaped field 2 in figure 1 a is absent when a cylindrical electromagnetic field is used, when and where

14/17 much more uniform heating is possible on the contact surface 14. In the product obtained the two molded parts 12, 13 are mutually coupled by means of the contact surface 14, in which significantly smaller irregularities occur than in the coupling obtained under conditions comparable to a torus-shaped field. This results in the fact that a product obtained under the influence of the cylindrical electromagnetic field has a better mechanical load-bearing capacity than the product obtained with a torus-shaped electromagnetic field. The electromagnetic field 11 can, in addition, be precisely and exactly directed in the indicated A direction.

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, in which an electromagnetic field 23 with a profile which is cylindrical at least in the direction of induction A is generated when an alternating voltage is applied (Fig. 2b).

The inductor 20 is hollow inside, in which a feed channel 24 is formed for a cooling medium such as water, through which it can be fed during use. The cross-sectional area of ​​the supply conductors 21 is larger than that of the induction segment, in which the cylindrical electromagnetic field 23 is more concentrated and has a greater power over a short distance than the non-cylindrical electromagnetic field (not shown) , something that results around the other parts of the inductor 20. Here it is possible to aim the power of the electromagnetic field 23 very precisely and

15/17 exactly in one position for electromagnetic welding, for example, of two adjacent molded parts. Inductor 20 comprises a plane-free inductor in which the inductor is exceptionally compact.

Figure 3 shows a welding device 30 provided with an inductor 31 similar to inductor 20 of figures 2a and 2b according to the invention. The inductor 31 can be guided along a pre-programmed path by means of a six-axis industrial robot 32 in order to achieve a desired welding. In this case, the molded parts for welding (shown in Fig. 4) are fixed and pressed together in a mold 33 manufactured for this purpose. The mold 33 is provided with a recess 34 through which the inductor can be moved close to the molded parts for welding (Fig. 4). The mold 33 is shown in greater detail in Figure 5. The inductor is connected to an alternating current generator 35 arranged on the robot 32 for the purpose of generating the electromagnetic field.

Figure 4 shows two molded parts 40, 41 coupled by means of the method according to the invention. Both molded parts 40, 41 are manufactured from carbon fiber reinforced thermoplastic plastic, in which carbon fibers also serve as an induction sensitive component to heat the thermoplastic plastic for the purpose of 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. Using a cylindrical electromagnetic field, inductor 43, similar to that of the aforementioned figures 1b, 2a, 2b and 3, the molded parts 40 and 41 are heated in the situation where they were placed together to a temperature which is high enough

16/17 to thermally activate the thermoplastic plastic (or optionally a thermally activated adhesive applied to the contact surface 42, 42 '). Here the inductor is moved over the contact surface in the B direction of the longitudinal axis of the cylindrical electromagnetic field without making any physical contact. During heating and / or optionally a short time thereafter, the thermally activated surfaces can be pressed together by means of pressure (not shown) in such a way as to thus make a connection between the molded parts (40, 41). This connection has a particularly high mechanical load-bearing capacity. The molded parts may, for example, consist of carbon fibers reinforced with polyphenylene sulfide, for example, with a material with a thickness of 1 - 3 mm. A shearing power higher than 30 MPa can be achieved in a simple way using the method according to the invention. The width of the welding carried out can be particularly small, for example, 10 mm, using the cylindrical electromagnetic field according to the invention.

Figure 5 shows a mold 50 which can be used in electromagnetic welding according to the invention. The mold 50 comprises several electrically non-conductive parts (51, 52) among which the molded parts of, for example, Figure 4 can be pressed together. In the position of the intended contact surface for the coupling, a part of material 53 is arranged which is not electrically conductive, but which is thermally conductive such as a ceramic material, which serves to distribute and discard the heat generated from of molded parts when applying an electromagnetic field and also to prevent deformation of the

17/17 thermoplastic material. A recess 54 is arranged in the mold 50, through which the inductor can be placed as close as possible (0.5 - 2 cm from the welding zone) in the vicinity of the contact surface between the molded parts without coming into contact with it . In order to enhance mechanically and in order to discharge heat from the mold, the mold is provided on the outside with an external metal layer 54. The parts in the vicinity of the inductor are preferably manufactured from non-electrically conductive materials such as wood or plastic.

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