NL1030304C2 - Electromagnetic welding method for thermoplastic parts, comprises pressing parts against die while guiding inductor so that Foucault currents are induced in conductive component - Google Patents

Abstract

The parts (1, 2) or the space in between them includes an electrically conductive component. The parts are pressed against a rigid die (4) having the same shape as the intended end assembly. An inductor (3) is guided along a path covering at least a portion of the die surface so that Foucault currents are induced in the conductive component, causing the parts to be heated in the region of this path.

Description

Method and device for electromagnetic welding of molded parts from thermoplastic material.

The invention relates to a method for electromagnetic welding of molded parts 5 from thermoplastic material, in order to establish a continuous connection between the thermoplastic molded parts.

In known methods, the thermoplastic molded parts are usually provided with an electrically conductive component, for example metal mesh, or this component is arranged between the molded parts. In the electrically conductive component, Foucault currents or eddy currents are induced by a magnetic field generated by an inductor which is supplied with alternating current through a generator. Due to the Joule effect, these Foucault currents generate the heat needed to melt the thermoplastic material. By moving the inductor, the thermoplastic molded parts are connected to each other over their contact surface.

Various welding methods are already available for creating a continuous weld connection between thermoplastic molded parts. However, the presence of an electrically conductive component and / or fiber reinforcement hinder these methods when making a weld connection. When a resistance wire is used, for example, a short circuit may 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 isolating the resistance wire from the conductive component in the thermoplastic. With this solution, however, in addition to the resistance wire, even more material is fused between the thermoplastic molded parts, which can be disadvantageous for the construction. With 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 welding connections.

From DE 10000347 a method is known in which fiber-reinforced thermoplastic molded parts are welded electromagnetically with a mobile system. The molded parts are thereby pressed by pressure rollers which press the weld through the inductor after heating and cool it down. With this method there is no mold material between the inductor and the thermoplastic moldings.

US 005710414 describes a ceramic mold with integrated inductors for welding thermoplastic moldings. A bellows presses the molded parts from the inside against the ceramic mold. Since the inductors are integrated in the mold, the inductors must be large and long enough to heat the entire weld length simultaneously. To achieve this, a very large generator is needed to generate sufficient high-frequency alternating current.

US005833799 describes a welding skate, with integrated inductor, which glides over the 10 thermoplastic moldings to be welded. The skate is guided along the mold parts with a guide, the skate also exerting pressure on the mold parts. This pressure is needed to keep the layers of carbon fibers in the thermoplastic molded parts pressed together during the welding process. By first heating the thermoplastic moldings with an inductor and then pressing with pressure roller or skate, the material has not been under pressure for some time. Moreover, after the pressure roller or skate has passed, the thermoplastic must be sufficiently cooled to prevent delamination after all.

WO 96/20823 describes a method for the inductive welding of thermoplastic molded parts, wherein use is made of a flexible hydraulic mold in which a magnetic coil is moved. The flexible hydraulic mold ensures a homogeneous pressure distribution over the molded parts to be connected.

US 5,240,542 also describes a method for the inductive welding of thermoplastic molded parts, wherein use is made of a specially shaped coil which covers a larger surface area than the surfaces to be joined. The coil 25 ensures that the temperature in the molded parts to be connected is evenly distributed, and that cold spots are avoided where no connection can be established.

FR 2221259 also describes a method for inductively welding metal / thermoplastic cylindrical molded parts, wherein use is made of a magnetic coil movable around a mold. The molded parts to be connected are received in the mold and, if desired, are pressurized by an inflatable inner mold. FR 2240105 describes a similar method, wherein the outer template is, however, electrically conductive.

i 3

The object of the present invention is to provide a method for electromagnetic welding of molded parts from thermoplastic material with which also large and curved, thermoplastic molded parts can be continuously welded, wherein use is made of an electrically conductive component, such as carbon fibers already present in the material. .

To this end, the method according to the invention has the features as described in claim 1. In particular, a method for electromagnetically welding molded parts of thermoplastic material is provided, wherein the molded parts or the space between the molded parts contain an electrically conductive component, in which the assembly of moldings is pressed against a rigid mold in the intended shape of the assembly, and in which an inductor is guided over a path covering at least a portion of the mold surface, thereby inducing Foucault currents in the electrically conductive component and the thermoplastic moldings be heated at least locally at the height of the path through the mold material.

According to the invention, the thermoplastic molded parts are placed in a mold which presses the assembly of thermoplastic molded parts against the rigid mold at least during guiding of the inductor over the mold surface, optionally also during cooling. This limits the risk of delamination. Fiber-reinforced thermoplastic moldings are generally produced under elevated pressure. This also applies to pressed products made from carbon fiber reinforced laminate. If these molded parts are heated to the melting temperature of the thermoplastic and are not kept under pressure during that process, this can lead to air inclusions and delamination of the layers of fibers. The present method combines inductive welding with molding presses of thermoplastic materials.

A further advantage is that by using a normative rigid mold the thermoplastic molded parts are positioned and welded with high dimensional accuracy. The final geometry in that case is no longer determined by the pressure force of a pressure roller or skate, as is the case with the method described above.

! 4

It is advantageous to characterize the method according to the invention in that a mold is used which has a thickness less than the average mold thickness at least at the level of the inductor path. This allows the inductor to come closer to the assembly of thermoplastic molded parts, which advantageously influences the accuracy of the weld connection (for example, its thickness).

For connecting two thermoplastic molded parts, the most ideal location of the inductor is in principle a position between the two molded parts, namely where the thermoplastic must be melted. This is only possible by first keeping the two molded parts 10 at some distance from each other, bringing the inductor between the two molded parts, and then removing them in order to be able to press and consolidate the two molded parts on top of each other. Such a method is very similar to hot-plate welding, but is not the most effective method in this case. According to the invention it has been found that with electromagnetic welding it is more effective to place the inductor outside the two thermoplastic moldings to be connected. The electromagnetic field must then be strong enough to heat the nearest molded part through the thickness and to bring the concerned molded part above the melting temperature on the connection side. The material closest to the inductor will get the highest temperature in the open air. In the open air it could therefore happen that, in order to get the temperature on the connecting side high enough, the temperature of the surface on the inductor side becomes too high. The extent to which such an effect can occur depends on the properties of the thermoplastic material and on the thickness of the thermoplastic molding.

According to the invention, the thermoplastic moldings are placed in a rigid, preferably electrically non-conductive (or electrically insulating) mold. The inductor generates an electromagnetic field strong enough to heat the thermoplastic molded part closest to the inductor through the mold and thickness of the molded part itself. The mold is preferably also transparent to the electromagnetic field and is therefore virtually not heated by the field. However, the mold preferably has a certain thermal conductivity and heat capacity and can therefore remove heat from the surface of the thermoplastic moldings.

Because the mold removes this heat, the highest temperature no longer occurs at the surface of the thermoplastic moldings that is closest to the inductor 5, but much deeper in the material itself. By a certain choice of material for the mold material it can be achieved that the highest temperature will occur exactly between the two thermoplastic molded parts, in which the weld must be made.

5

It is advantageous if the method according to the invention is characterized in that the mold is actively cooled. Particularly suitable methods are characterized in that a mold is used from a non-metallic or a non-ferromagnetic material, from a ceramic material, or from composite.

10

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 piece, which preferably extends beyond the surface of the assembly. Such an extension piece can in principle be made from any electrically conductive material, but is preferably made from metal, from carbon, or comprises an adjustable resistor. The present preferred variant has the following advantage. The Foucault currents or eddy currents induced in the electrically conductive component in or between the thermoplastic molded parts are limited by the geometry of the molded parts. Edges, corners and holes in the 20 molded parts influence the distribution of Foucault currents and therefore also influence the developed heat. Due to the presence of this type of geometric disturbances in the field, parts can become hot, which do not need to become hot for the welding process. It is also possible that certain parts are difficult to heat because the Foucault currents are hindered. By shifting the boundaries of the area where Foucault currents can occur at certain locations of the thermoplastic molded parts, these problems can in some cases be solved. The boundaries of the area where Foucault currents can occur can be shifted by electrically connecting the electrically conductive component in the thermoplastic part to an electrically conductive extension, into which the Foucault currents can flow. With this solution, parts that were previously difficult to heat can still be made hot and high temperatures can be prevented in undesired places.

6

The invention also relates to a device for electromagnetic welding of molded parts from thermoplastic material, wherein the molded parts or the space between the molded parts comprise an electrically conductive component, which device comprises a rigid mold in the intended shape of the assembly of molded parts to be joined, and pressure means for pressing the assembly against the mold, which device also comprises an inductor and guide means for guiding the inductor along a path covering at least a portion of the mold surface, provided that the inductor Foucault currents in the electrically conductive component can induce and the thermoplastic molded parts are heated at least locally at the level of the path through the mold material.

The invention will now be further elucidated with reference to the non-limitative exemplary embodiment described in the figures. Herein: figure 1 shows a schematic side view of a device according to the invention; and figure 2 shows a schematic perspective view of a device according to the invention.

The mold 4 shown in Figures 1 and 2 is relatively thin at the location of the weld connection in order to get the inductor 3 close enough to the thermoplastic molded parts 1 and 2. The thickness of the mold 4 is partly determined by the rigidity of the mold material, the width of the weld and the pressure exerted on the welding surface. The material of the mold 4 is not electrically conductive and has a thermal conductivity and heat capacity adapted to the materials 1 and 2 to be welded. In general, the thermal conductivity and heat capacity are high to conduct sufficient heat away from the surface and thus prevent melting or softening of the surface. This material can be, for example, ceramic or a composite. The end result can then be a weld without marking on the surface which is closest to the inductor 3.

The shape of the inductor 3 is adapted to the connection to be welded and is moved along the weld by means of, for example, a linear guide or a robot. Applying pressure to the thermoplastic molded parts 1 and 2 can be done in various standard ways, such as with a bellows or a pneumatic cylinder. It is important that all metal parts are far enough outside the influence of the high-frequency magnetic field.

7

At edges, corners and holes in the electrically conductive component in or between the thermoplastic molded parts 1 and 2, the Foucault currents are locally hindered. Since this can result in a certain spot in the welding area not becoming hot enough and other unwanted spots too hot, an extension piece 5 is provided. The extension piece 5 shown in Figure 2 ensures that the

Foucaul currents are not deflected through the edge of thermoplastic part 2 but can flow through into the extension piece 5. The extension piece itself must therefore be electrically conductive and electrically connected to the electrically conductive component in or between the thermoplastic molded parts 1 and 2. The extension piece 5 can be removed after the welding process. The extension piece 5 can be a composite, metal or other electrically conductive material and is designed as laminate, foil, mesh, cable or wire. The extension may be connected to an adjustable resistor and be grounded to control the influence of the extension.

15

Claims (14)

1. Method for electromagnetic welding of molded parts from thermoplastic material, wherein the molded parts or the space between the molded parts comprise an electrically conductive component, wherein the assembly of molded parts is pressed against a rigid mold in the intended shape of the assembly, and wherein an inductor is guided over a path covering at least a portion of the mold surface, whereby Foucault currents are induced in the electrically conductive component and the thermoplastic molded parts are heated at least locally at the level of the path through the mold material. Method according to claim 1, characterized in that the assembly is pressed against the rigid mold at least during the guiding of the inductor over the mold surface. 15 3. Method as claimed in claim 1 or 2, characterized in that a mold is used which has a thickness less than the average mold thickness at least at the level of the inductor path. Method according to one of claims 1 to 3, characterized in that a mold is used made of substantially non-electrically conductive (electrically insulating) material, which is also sufficiently thermally conductive to extract heat from the contact surface of the adjacent thermoplastic molded part. Method according to one of the preceding claims, characterized in that the mold is actively cooled. 6. A method according to any one of the preceding claims, characterized in that the thermal conductivity of the mold material is used for dissipating heat. Method according to one of the preceding claims, characterized in that mold parts are used from a non-metallic or non-ferromagnetic material, from a ceramic material, or from composite. A method according to any one of the preceding claims, characterized in that the inductor is guided over the mold surface by means of a robot arm. A method according to any one of the preceding claims, characterized in that the inductor is guided over the mold surface by means of a linear guide. 10. Method as claimed in any of the foregoing claims, characterized in that the electrically conductive component consists of carbon fibers, or of metal mesh or of metal foil. A method according to any one of the preceding claims, characterized in that the electrically conductive component is electrically connected to an electrically conductive extension piece. 15 A method according to claim 11, characterized in that the electrically conductive extension extends beyond the surface of the assembly. 13. Method as claimed in claim 11 or 12, characterized in that the electrically conductive extension piece is made from metal, from carbon, or is a controllable resistor. 14. Device for the electromagnetic welding of molded parts from thermoplastic material, wherein the molded parts or the space between the molded parts comprise an electrically conductive component, which device comprises a rigid mold in the intended form of the assembly of molded parts to be connected, and pressing means for pressing the assembly against the mold, which device also comprises an inductor and guide means for guiding the inductor over a path covering at least a portion of the mold surface, provided that the inductor 30 can induce Foucault currents in the electrically conductive component and the thermoplastic molded parts are heated at least locally at the height of the path through the mold material.