The invention relates to a resistance welding device, in particular for joining elements made of thermoplastics in the process of resistance welding with high process dynamics.

Resistance welding (also known as contact welding or hot electrode welding) is a known and widely used method of joining elements made of thermoplastics. The method consists in pressing down the elements to be connected with a heated slat or a specially designed heating element, and then creating a weld as a result of plasticization of the material under the influence of heat, supplied from the outside or generated inside the material, most often in effect of electric current flow.

There are known machines for resistance welding. Traditional welding machines are equipped with an actuator comprising electrodes connected to a heating element which is directly pressed down against the material to be welded. Electricity from the transformer is a heat source. In order to cool down the heating element, a cooling system for example with cold air being passed through an air duct in the actuator, is used. Such a solution is known, for example, from publication US2004250939.

Similarly, US2017203499 discloses a device for thermal caulking of plastics, equipped with a heating rod with a metal welding tip, heating means for heating the same, a cooling conduit, cooling fluid supply means and a holder that provides a support structure for the heating rod with the tip and the cooling conduit. In addition, the device is provided with a controller for controlling both the heating means and the coolant supply means, which together constitute an external module connected by wires to the holder of the caulking device.

The above-mentioned solutions of classic resistance welding machines, however, are associated with certain disadvantages, resulting both from their construction and the efficiency parameters obtained, which are lower than in the case of, for example, ultrasonic welding machines. Due to the necessity of using large and expensive transformers, they must be remote from the actuators. Weight, dimensions and high-intensity currents generated by external transformers force the use of long, thick and expensive copper cables, which constitute additional and significant mechanical limitations, mainly due to the space they occupy. Additionally, the cables oxidize due to high temperature, which leads to their heating. This phenomenon, consequently, further aggravates the problem with copper conductors, as it can lead to burnout or generally to difficulties related to the repeated execution of the welding process and its efficiency.

Another problem with traditional resistance welders is heat leakage from the heating elements the thick copper wires. As you know, copper is a very good conductor - not only electrical, but also thermal. It is, therefore, equivalent to the significant heat removal by thick copper wires, at the expense of the heating element, which in turn significantly reduces the efficiency of the entire device with a simultaneous constant increase in electricity demand.

One of the methods of avoiding the above-mentioned disadvantages of the prior art is the use of heaters adapted to the nominal voltage in the electrical mains. In such a solution, energy can be supplied via thin cables directly from the mains, thus bypassing transformers. Unfortunately, by eliminating one problem, this solution creates a new one, namely related to the low dynamics of the process, resulting from the power limitations of the heaters, as well as the finally large volume and weight of unnecessarily heated material for the final welding. Moreover, in this solution, the amount of energy needed to heat the plastic is only a minimal part of the energy used for the heating and cooling process, while most of it is wasted unnecessarily, which significantly reduces the economic efficiency of the process.

In order to increase the efficiency of resistance welders and the process parameters achieved by them, solutions based on impulse power are used, enabling one-time (i.e. impulse) supply of high power and high frequency current, which allows for quick heating of the heating element. In addition, specially formed heating elements, known for example from metal welding processes, are implemented. Such a solution is disclosed, for example, in the document DE3605115, in which the front surface of the heating element has annular or circular embossing, allowing, on one hand, better management of heat distribution, and on the other hand, avoiding damage to the material to be welded.

In the light of the above-discussed solutions from the state of the art, as well as the related disadvantages affecting the usable and efficient welding parameters, the task of the present invention is to propose a new construction of a resistance welding device for plastics, ensuring high process dynamics, in particular short heating and cooling times of the heating element and at the same time low heat losses and compact design of the entire device. A resistance welder according to the invention, in particular for welding thermoplastics, comprises an elongated actuator with a distal end and a proximal end, a heating system with a transformer, a cooling system and an electronic power supply and control system. The actuator has a power inlet at its distal end and a heating element at its proximal end. The welding device is characterized by the fact that the heating system with the transformer, the cooling system and the electronic power supply and control system are placed inside the actuator and in that the transformer is connected to the heating element.

Preferably, the transformer has a core, a primary winding and a secondary winding, the transformer secondary winding having a first pole and a second pole, the first pole being integrated with the actuator body and the second pole positioned inside the transformer and extending along the axis of the welder actuator, wherein around the second pole of the secondary winding, a toroidal transformer core with the primary winding wound on it, is provided.

Preferably also, a transformer attachment is formed in the actuator of the welder at the distal end of the second pole of the secondary winding.

Moreover, preferably, the transformer is connected to the heating element via an electrode system.

Also preferably, the electrode system includes a central electrode extending along the axis of the actuator and an outer electrode that forms part of the actuator body.

Preferably, the heating element has a head portion and lateral electrical cables extending therefrom, through which the heating element is connected to the outer electrode.

Preferably also, the heating element has an electrical and thermal insulator that constitute its support.

In addition, according to the invention, the actuator, in the region of the distal end, has a welding extreme position adjustment system, which preferably consists of an external thread and a nut mounted thereon.

Preferably, the actuator has a housing with respect to which the actuator is slidable, the length of the housing being shorter than that of the welder's actuator.

Also preferably, the range of the actuator movement relative to the housing is limited by the position of the nut of the welding extreme position adjustment system. Preferably, the housing in the region of its distal end has a rotation locking system for the actuator, preferably consisting of a longitudinal groove in which a retainer integral with the actuator is seated.

Preferably, the cooling system is an air supply channel extending along the longitudinal axis of the actuator, the inlet of the air supply channel being located at the distal end of the actuator and the outlet of the cooling system channel being located at the proximal end of the actuator so that it directly cools the heating element.

According to the invention, the electronic power supply and control circuit is housed in the actuator.

Preferably, the power and control electronics include an electromagnetic interference filter, a rectifier, a control circuit, and a power switch.

In the welding machine according to the invention, the electronic power supply and control system may additionally be equipped with a position measuring system with a position sensor and/or a temperature measuring system with a temperature sensor.

The welding device construction used in accordance with the invention, wherein all its main components are placed within the actuator, including a heating system with a transformer, a cooling system as well as an electronic power supply and control system, based on impulse power supply, offers advantages in the form of a small space requirements, low transmission losses, increased dynamics of heating and cooling of the heating element and the associated shorter operating cycle, as well as reduced operating costs.

An unquestionable advantage of the invention is also the construction of the transformer and its connection to the heating element. The transformer has a ratio of N:l, where N is the number of turns of the primary winding, and a single turn of the secondary winding is integrated with the body of the welding device's actuator. This solution allows for further reduction of the size of the welding machine and losses arising in it, as well as the independence of its structure from external thick copper cables. The exposed secondary winding of the transformer, which is also part of its body, transmits hundreds of amps without any problems thanks to the large cross-section area. Moreover, said exposed secondary winding has a positive effect on the cooling of the transformer, which results from the easy heat dissipation outside the transformer to the environment. Moreover, the proposed structure of electric connections in the heating element turned out to be advantageous, because it makes low its electric resistance, as well as power losses in the lead zone, which result in a significant temperature drop in the non-welding area of the heating element connections and isolation of the hot working zone from well- conductive copper electrodes. For example, when the working surface of the heating element leads is enlarged three times, the following power distribution is obtained: up to 20% in the area of the first lead, over 60% in the heating zone and up to 20% in the area of the second lead

The object of the invention has been shown in the exemplary embodiments in the drawing, in which fig. 1 is a longitudinal section of the welding device according to the invention, fig. 2 is an enlarged cross-sectional view of the welding device in the area of the transformer, electrode system and heating element, fig. 3 shows the transformer in a perspective view, in partial section, Figs. 4a, b and c are respectively a perspective view of a heating element, a top view and a section along the line AA, Fig. 5 is an exploded view of the power distribution in the heating element, Fig. 6 is a diagram of the power supply and control electronics in the welding device according to the first embodiment, and fig. 7 is a diagram of a power and control electronics in the welding device according to a second embodiment.

A detailed description of the embodiments of the invention will be given below, in particular with regard to the mechanical construction of the resistance welding device and the electronic power and control components used therein. In this respect, the purpose of the aforementioned mechanical structure is primarily to ensure the pressing of the heating element against the surface to be treated, as well as to provide a housing for the electronics, without which the welding process would not be possible. Additionally, this mechanical structure provides guidance, i.e. maintaining a constant operating direction of the device in the welding process, while avoiding the vertical, horizontal and axis of rotation movement of its individual elements. The task of the electronic system is, in turn, to adjust the load to the characteristics of the power source, galvanic separation of the heating element circuit from the power circuit, as well as controlling the power and therefore - the temperature of the heating element.

As shown in Fig. 1, the resistance welding device 1 has an actuator 2 of an elongated shape with a distal end 3 and a proximal end 4, the proximal end 4 (in a proximal direction) being located in the welding working area and the distal end 3 (in a distal direction) - in the area of connecting power cables to the actuator 2, the cables being connected to the electrical network (not shown in the drawing). The above-described definitions of the proximal and distal directions remain analogous for all of the elements considered below in the resistance welding device 1.

Inside the actuator 2 of the resistance welding device 1, along its longitudinal axis (this axis is not shown in the drawing, but its position remains obvious to those skilled in the art) there are successively arranged: an air supply channel 5, constituting a cooling system, with an inlet 51 of the air supply channel at the distal end 3 of the actuator 2, the electronic power supply and control system 6 in the form of an integrated circuit on the printed circuit board, the transformer 7, the electrode system 8 and the heating element 9 of the resistance welding device 1 being located at the proximal end 4 of the actuator 2.

In addition, the actuator 2 has a housing 10 (constituting a guiding element) mounted on actuator’s body, with respect to which the actuator 2 moves linearly during the welding process. According to the presented embodiment - the length of the housing 10 is shorter than the length of the entire actuator 2. Said housing 10 is supported at its proximal end by a compression spring 11, the position of which is limited by the spring support 12 on the body of the actuator 2. Moreover, the displacement of the actuator 2 relative to the housing 10 is carried out by sliding elements 13 embedded in the housing 10.

The travel range of the actuator 2 relative to the housing 10 is limited by the welding extreme position adjustment system formed in this embodiment by a nut 14 mounted on the male thread 15 formed within the distal end of the body of the actuator 2. Additionally, the position and movement of the actuator 2 relative to the housing 10 is fixed by the rotation locking system of this actuator 2, it is a retainer 16 embedded in the body of the actuator 2, which, according to the embodiment shown, is a pin. The pin 16 moves in the groove 17 in the housing 10. The pin 16, in addition to limiting the range of linear movement of the actuator 2 with respect to the housing 10, at the same time prevents the actuator 2 from rotating with respect to said housing 10.

The miniaturization of the resistance welding device 1 underlying the invention and the related placement of all the systems inside the actuator 2 forced the use of an electronic power supply and control system 6 in the form of an integrated system, entirely placed on the printed circuit board and located inside the actuator 2, behind the inlet of the power cables ( not shown) and downstream of the supplied air inlet 51 and in the region of the duct 5, so that the cooling function of this system is always ensured. A detailed description of the electronic supply and control system 6 will be presented further down in the description.

Downstream of the control electronics 6, within the actuator 2, there is a transformer 7 located. As shown in Figs. 1-2, and in particular in Fig. 3, the transformer 7 comprises a core 71, a primary winding 72 and a secondary winding 73. The secondary winding 73 of transformer 7 has a first pole 731 and a second pole 732. The first pole 731 of the secondary winding 73 is the housing of the transformer 7, which is also a part of the body of the actuator 2.

Such an integration of the secondary winding 73 of the transformer 7 with its mechanical housing enables a significant reduction in costs and miniaturization of the entire device, as well as shortening the length of the electrodes and obtaining proportional cross-sections adapted to the large currents flowing in the secondary winding 73.

The second pole 732 of the secondary winding 73 is located in the center of the transformer 7 and extends along the axis of the actuator 2 of the welding device 1. Around the second pole 732 of the secondary winding 73 there is a toroidal core 71 with the primary winding 72 wound on it. To increase the power of transformer 7, its core 71 consists of four toroidal cores 711 connected together.

The transformer 7 additionally has a fastening 74 by means of which the transformer 7 is mounted inside the body of the actuator 2. The fastening 74 is formed at the distal end of the second pole 732 of the secondary winding 73 and its transverse dimension in this embodiment is smaller than the transverse dimension of the transformer 7 housing formed by the first pole 731 of the secondary winding 73.

Such an integration of the secondary winding 73 of the transformer 7 with its mechanical housing enables a significant reduction in costs and miniaturization of the entire device, as well as shortening the length of the electrodes and obtaining proportional cross-sections adapted to the large currents flowing in the secondary winding 73.

From the secondary winding 73, via the central electrode 81 and the outer electrodes 82, respectively, the current is transmitted to the heating element 9, while an electrical insulator 19 is used at the contact point of transformer 7 and outer electrodes 82. Similar to the first pole 731 of the secondary winding 73 in the transformer 7, the outer electrodes 82 are part of the actuator 2 body.

A heating element 9 is located at the proximal end 4 of the actuator 2. As shown in particular in Fig. 4 a-c, the heating element 9 has a face 91 in the form of an embossed plate and electrical side leads 92 extending therefrom, by which the heating element 9 is connected with external electrodes 82. The above-described structure of the heating element 9, especially in the area of the leads 92, has a positive effect on the formation of the hot zone and cold zones, and with their appropriately selected width - it allows for precise control of the temperature distribution within the heating element 9 and ensures effective temperature growth, which also increases the efficiency of the welding process. The distribution of the hot and cold zones formed within the heating element 9 according to the present embodiment is schematically shown in Fig. 5.

The heating element 9, as indicated in the above description, is located at the proximal end 4 of the actuator 2, at the outlet 52 of the air supply duct 5, whereby cooling is directly directed to the inner wall of its head 91. Additionally, the lead surfaces 92 have an electric and thermal insulator 19 mounted thereon, as shown in particularly in Fig. 2.

Figures 6 and 7 show exemplary embodiments of a power and control electronic system 6 used in a resistance welding device 1 according to the invention. As shown in Fig. 6, the resistance welding device 1 is equipped at the input with an electromagnetic interference filter 20, which acts as a barrier to high frequency electromagnetic interferences. The necessity to use an electromagnetic interference filter 20 results from the fact that the resistance welding device 1 is powered by alternating current, which is associated with the problem of the formation of disturbances caused by an emission of electromagnetic radiation. Additionally, the use of the said electromagnetic interference filter 20 results from the necessity of meeting the stringent requirements of CE directives in this respect.

The next element of the electronic power supply and control system 6 is the rectifier 21 which converts at the input an alternating current supply, with a frequency in the range of 50-60 Hz, into a direct current. The constant voltage from the rectifier 21 supplies the control circuit 22 and the power switch 23. The control circuit 22 controls the operation of the power switch 23 so that the input of transformer 7 is again supplied with alternating voltage, but of high frequency (several dozen kHz). As a result, the transformer 7 can be significantly smaller while maintaining its high power. The control system 22 also controls the power supplied to the transformer 7 in proportion to the control signal S. The temperature reached by the heating element 9 is therefore proportional to the power supplied to the transformer 7, as a result of which the control signal S controls the temperature of the heating element 9. The transformer 7 lowers the voltage several dozen times and in the same proportion increases the current, so that the low resistance of the heating element 9 is adapted to the high supply voltage.

Fig. 7 shows another, advantageous embodiment of the electronic power supply and control system 6, in which - in order to ensure full control of the welding process - the temperature measurement system 24 with the temperature sensor 241 and the position control system 25 of the heating element with the position sensor 251 were additionally added. In the case of such a configuration of the electronic supply and control system 6, a feedback is provided that gives a complete picture of the process course as a function of time and power given by the control signal S.

The welding process performed by the resistance welding device 1 according to the invention is described below. The welding process begins with a placing the heating element 9 on the surface of the processed material, so that the actuator 2, together with the heating element 9, moves in the housing 10, which maintains it and sets the direction of movement by the planned depth of melting/deformation of the material. Then, the control signal S increases the power, so that the heating element 9 heats up to the operating temperature in a very short time (i.e. with a dynamics of 200°C/sec). At this point, the thermoplastic material against which the heating element 9 is pressed starts to melt and/or to deform under the influence of the temperature and the force generated by the pressure spring 11. The heating element 9 together with the actuator 2 gradually change their position until the rest position is reached, being determined by the position of the nut 14 in the control system of the extreme welding position. At this point, the welding process (i.e. the so-called main welding stage) is completed and the cooling process begins by supplying cooling air to the air supply channel 5. Due to supplying air directly to the inner surface of the heating element 9, the cooling process is short (the temperature drops at a rate of about 200°C/sec). A very important parameter influencing the dynamics of the heating and cooling process is the small mass of the heating element 9 and the resulting low heat capacity. After completion of the cooling process, the supply of cooling air is turned off and the heating element 9 is moved away from the treated (i.e. welded) surface.

The present invention finds its application in the resistance welding process of all kinds of thermoplastic plastic materials. Due to the fact that each type of thermoplastic material requires different processing parameters, in particular in terms of welding temperature, temperature of setting aside, welding time, cooling time and total process time - those parameters are individually selected for a specific application.

Of course, the present invention is not limited to the embodiments described above, but various modifications and developments are possible within the scope of the appended claims without departing from the spirit of the invention.