DEVICE FOR WELDING THERMOPLASTIC MATERIALS

Description

[0001] The invention relates to a device for welding thermoplastic materials, in particular plastic films or fiber-reinforced tapes, with the features of the preamble of claim 1.

[0002] For spot welding plastic materials, especially thermoplastic films, heating cartridges with integrated temperature sensors are often used. The sensor position can be varied, but should be as close as possible to the tip of the welding punch, which is heated by the cartridge. The welding punch is usually designed to accommodate the heating cartridge. Thus, a simple component of a state-of-the-art device consists of a heating cartridge with an integrated temperature sensor and a holder that accommodates the heating cartridge and conducts the heat from the heating cartridge, via the welding punch, into the substrate to be welded. Of course, an external temperature sensor can also be attached; in this case, it must be connected to the holder of the heating cartridge. Such a component has a large mass and is therefore thermally inert and reacts slowly to control or regulation inputs.

[0003] Embodiments of the prior art are disclosed in JP S635928 A, EP 1422051 A1 or US 3178556 A.

[0004] The object of the invention is to provide a device for welding plastic materials which reacts faster to control or regulation interventions.

[0005] This problem is solved by a method having the features of claim 1. Advantageous embodiments of the invention are defined in the dependent claims.

[0006] According to the invention, the at least one heating device is itself designed as a welding punch, thereby reducing the mass compared to the prior art, i.e., a one-piece (in other words: monolithic, cast in one piece) design of welding punch and heating device is provided, so that welding can be carried out directly, i.e. without a previously common combination of welding mandrel and heating cartridge.

[0007] According to the invention, a control or regulating device is provided which is designed to control the at least one heating device for carrying out a pyrolytic cleaning process of the at least one welding stamp.

[0008] Preferably, the welding stamp is heated – preferably electrically – in such a way that at least one heating element (e.g., a resistance wire) – preferably electrically – is embedded directly in the material of the welding stamp (i.e., is completely in contact with the material of the welding stamp, in other words, without a cavity beyond the space occupied by the at least one heating element).

[0009] It is preferred that: the at least one welding stamp has a mass in the range of about 3 grams to about 30 grams, preferably in the range of about 5 grams to about 15 grams, and the at least one heating device (formed according to the invention by the welding stamp) can be operated with a power in the range of about 30 watts to about 600 watts, preferably in the range of about 100 watts to about 350 watts.

[0010] Due to the low mass of the welding stamp, the device can react very quickly to control or regulation interventions. The lower the mass of the welding stamp, the lower the required heating power of the heating device, and vice versa.

[0011] The welding die is designed to provide a welding surface for spot welding. Preferably, a flat surface with a size of up to 100 mm² is used as the welding surface. A surface smaller than 25 mm² is particularly preferred.

[0012] The welding stamp can be designed to be approximately cylindrical (essentially apart from the tip, which tapers to form the weld surface) and, viewed axially, has at least two zones, of which the zone forming the weld surface is naturally heated. For example, an unheated zone, a heated zone, and another unheated zone can be arranged in succession (viewed towards the tip). A temperature sensor can, for example, be located in the lower unheated zone.

[0013] To weld thermoplastic films, in the simplest case only the welding die needs to be brought into contact with the films. To weld two films together, the welding die must be heated to a temperature above the glass transition (for amorphous plastics) or to a temperature above the melting point (for semi-crystalline plastics).

[0014] During the welding process itself, at least one welding punch is pressed against the substrate to be welded by the pressing device. Whether the movement originates from the substrate (actually from a support on which the substrate is arranged) or from the welding punch, i.e., which component is moved by the pressing device, is irrelevant. Preferably, however, at least the welding punch is located on a movement mechanism, so that the welding movement originates from the welding punch and the plastic materials do not need to be guided to the welding punch.

[0015] In a preferred embodiment, a handling device is provided and the at least one welding punch is arranged on an arm of the handling device (e.g., a robot including a transfer head). This embodiment is particularly suitable for welding tapes.

[0016] The contact time is temperature-dependent; the higher the temperature, the shorter the contact time can be. The contact pressure is viscosity-dependent and therefore, as is well known, depends on the type of plastic used and the welding temperature.

[0017] Plastic films are defined as semi-finished products made of thermoplastic material, which have a length and a width, and whose thickness is at least five times smaller than their length or width. These semi-finished products may contain fillers such as talc, fibers, glass fibers, carbon fibers, natural fibers, flame retardants, UV stabilizers, and similar substances.

[0018] Preferably, the at least one welding stamp is made of a ceramic material (crystalline ceramic or amorphous ceramic, i.e., glass). For example, at least one resistance wire is embedded in the ceramic material, and preferably a temperature sensor is also integrated into the ceramic material. In a preferred embodiment, the temperature sensor is arranged in the immediate vicinity of the welding surface. In a highly preferred embodiment, the temperature sensor is arranged in the immediate vicinity of the welding surface of the welding stamp, in an area that is not directly heated.

[0019] The use of a ceramic welding stamp offers several advantages: The end face of the ceramic piece can be easily machined to produce a flat welding surface; pyrolysis can be carried out particularly easily.

[0020] The device can include a holding device for compressing the plastic materials to be welded. This allows the plastic materials to be pressed against each other, particularly plastic films, before, during, and / or after welding. Furthermore, compressing the plastic materials after welding prevents them from adhering to the welding die, thus reducing the risk of a poor weld.

[0021] According to the invention, the device includes a control or regulating device, preferably for controlling or regulating: which includes at least one heating device and / or the pressing device and / or the holding device

[0022] The control device ensures that at least one welding stamp is operated at a suitable welding temperature and that this welding stamp is pressed against the plastic material with a defined pressure for a specific time. After the welding time has elapsed, the welding stamp can be lifted and the plastic materials are welded.

[0023] Preferably, a temperature sensor is provided (particularly preferably arranged on the at least one heating device and / or on the at least one welding punch), the signals of which can be supplied to the control or regulating device of the apparatus. If the temperature sensor is arranged on the welding punch, it is preferably located in the immediate vicinity of the welding surface.

[0024] The control or regulating device can be designed to monitor a welding process using the signals from the temperature sensor.

[0025] A temperature profile detected by a temperature sensor can be used as a quality parameter for a welding process. In spot welding thermoplastic materials, a temperature is selected at the welding punch, and the punch is then regulated to this temperature by supplying energy to the heating device (preferably a PID controller is used for this purpose). Since energy is transferred to the plastic during the welding process, the temperature at the welding punch changes. This is detected by the control device, which then attempts to restore the target temperature. Therefore, the temperature loss during a welding process can be considered a quality parameter.

[0026] When the same plastic material is repeatedly welded, the temperature loss at each weld point should be very similar. Therefore, the temperature loss per weld point can be considered as a quality parameter. Preferably, the temperature before the welding rod makes contact with the plastic material to be welded is used as the starting temperature. During the welding process, and preferably also for a certain period afterward, the temperature profile is monitored, and the minimum value is determined. The time period used to determine the minimum value can be slightly longer than the actual welding time.

[0027] The closer the temperature sensor is to the welding surface, the more sensitive the temperature reading will be. If the temperature sensor is located directly at the welding surface, there is no delay in the temperature signal, and only the actual welding process can be considered when determining the minimum value.

[0028] Due to structural limitations, the temperature sensor cannot always be integrated directly into the welding surface, and therefore it may only be located in the immediate vicinity. For this reason, the minimum value can also be determined by considering the time after the welding process has already taken place and the welding torch is no longer in contact with the substrate being welded.

[0029] The minimum contact time of the welding stamp with the substrate to be welded can therefore be considered. In a preferred embodiment, a contactless time after lifting the welding stamp is also taken into account. Preferably, this contactless time is 15 seconds, but particularly preferably it is less than 6 seconds.

[0030] The minimum value is then subtracted from the starting temperature, and this value can be used as a quality parameter. If this value changes, the weld quality has also changed.

[0031] In addition to temperature, the energy required for a welding process can also be considered. Here too, the observation period depends on the position of the temperature sensor and can therefore be longer than the time required for the actual welding process.

[0032] Quality assessment is particularly important for recurring welding processes. The tape laying process is one such method. In tape laying, a customized semi-finished product is built up from tapes.

[0033] In modern tape laying processes, tapes are laid down every second and then need to be welded. When spot welding thermoplastic films, the welding die becomes slightly dirty with each weld. This contamination builds up continuously. Once a certain level of contamination is reached, the weld quality deteriorates, and it can happen that the weld points can no longer be bonded securely.

[0034] Introducing a quality indicator can make this problem more easily identifiable. Preferably, the control device detects a deterioration in weld quality based on a changing quality indicator and outputs at least one signal. This signal could indicate that the welding stamp needs to be cleaned manually. Preferably, the welding process is interrupted in the event of a deviation, allowing the welding stamp to be cleaned. Particularly preferably, the welding stamp is automatically thermally cleaned in the event of a deviation. The welding process can then be repeated, thus ensuring the weld joint's durability.

[0035] Preferably, the welding stamp should be thermally cleaned. If the specified value deviates, the welding stamp temperature is temporarily increased to allow the welding stamp surface to be thermally cleaned (pyrolysis). A thermal cleaning cycle should be achievable in under 5 minutes. Preferably, however, the thermal cleaning requires less than 2 minutes. A cleaning cycle is defined as the time required to heat the welding stamp to a temperature at which the contamination can decompose and then cool back down to the actual operating temperature.

[0036] The control or regulating device is therefore particularly preferably designed to automatically or after receiving a command from an operator control the at least one heating device for carrying out a pyrolytic cleaning process of the at least one welding stamp.

[0037] When thermal cleaning is performed on a welding stamp, it must be heated to a temperature suitable for thermally decomposing the plastic. Since this temperature is usually far above the actual operating point, the welding stamp must be cooled down after heating and, if necessary, holding it at the operating temperature. To ensure cooling, no energy is supplied to the welding stamp after the thermal cleaning process is complete. Additionally, it is conceivable that the cooling process could be assisted by a cooling device for at least one welding stamp, for example, by blowing compressed air onto the welding stamp to accelerate the convection process.

[0038] Exemplary embodiments of the invention are discussed with reference to the figures. They show: Fig. 1 a schematic representation of a device according to the invention; Fig. 2a-c a device according to the invention in various stages of a welding process; Fig. 3 a representation of time-dependent quantities during a welding process

[0039] Fig. 1 shows a device 1 for welding thermoplastic materials (in Fig. 2 e.g., of two or more plastic films 10) with a welding die 2, which has a welding surface 3 for spot welding, and an interface 4 (shown exaggeratedly large, is preferably designed as a plug connection for power supply lines or is omitted entirely) for heating the welding surface 3. Via a pressure device, shown only schematically because it corresponds to the prior art, the welding die 2 can be moved (in the x-direction) and thus pressed against plastic materials arranged on a carrier 9 for welding.

[0040] Interface 4 can be powered by a power supply 8. The welding process can be controlled or regulated via a control device 6. For this purpose, signal connections are provided in a known manner (shown with dashed lines). The temperature of the welding stamp 2 (more precisely: the welding surface 3) can be measured via a temperature sensor 7 and reported to the control device 6.

[0041] In the Fig. 2a-c The diagram schematically depicts a welding process. The welding punch 2 is approximately cylindrical (essentially apart from the tip, which tapers to form the weld surface 3) and, viewed axially, has three zones: an unheated zone, a heated zone, and another unheated zone, arranged sequentially (in the direction of the tip). The temperature sensor 7 is located in the lower, unheated zone.

[0042] Two plastic films 10, which are arranged on the carrier 9 ( Fig. 2a ) and are held down by a holding device 5 (shown only schematically, as it is in accordance with the prior art; it can be controlled or regulated in a known manner by the control or regulating device 6), are welded together at one or more points using the device 1. The welding position in which the welding surface 3 is in position x 2 is shown in Fig. 2b depicted. In Fig. 2c The welding process is complete and the welded surface is in position x 1 .

[0043] The temperature profile T(t) can be described - see below. Fig. 3 - conclusions can be drawn about the quality of the welding process. Curve A shows a welding process of good quality, curve B a welding process of poor quality. Reference symbol list:

[0044] 1 Device 2 Welding die 3 Welding surface 4 Interface 5 Holding device 6 Control or regulating device 7 Temperature sensor 8 Power supply 9 Carrier 10 Plastic film T(t) Temperature profile of the weld area as a function of time tx 1 Position of the weld area where no welding process takes place x 2 Position of the weld area where a welding process can take place

Claims

1. A device (1) for welding thermoplastic materials, in particular plastic films (10) or tapes with fiber reinforcement, comprising at least one welding stamp (2) having a welding surface (3) for spot welding, at least one heating device for heating the welding surface (3) of the at least one welding stamp (2) and preferably a pressing device for pressing the at least one welding stamp (2) onto the plastic materials to be welded, wherein the at least one heating device itself is designed as a welding stamp (2), characterized in that an open-loop or closed-loop control device (6) is provided, which is designed to actuate the at least one heating device in order to carry out a pyrolytic cleaning process of the at least one welding stamp (2).

2. A device according to claim 1, wherein the at least one welding stamp (2) consists of a ceramic material.

3. A device according to claim 1 or 2, wherein: - the at least one welding stamp (2) has a mass in a range from about 3 grams to about 30 grams, preferably in a range from about 5 grams to about 15 grams, and - the at least one heating device for heating the at least one welding stamp (2) can be operated with a power in a range from about 30 watts to about 600 watts, preferably in a range from about 100 watts to about 350 watts.

4. A device according to at least one of the preceding claims, wherein the at least one welding stamp (2) is moveable for pressing by the pressing device.

5. A device according to at least one of the preceding claims, wherein the device (1) comprises a holding-down device (5) for holding down the plastic material to be welded.

6. A device according to at least one of the preceding claims, wherein the open-loop or closed-loop control device (6) is provided for open-loop or closed-loop controlling - the at least one heating device and / or - the pressing device and / or - the holding-down device (5).

7. A device according to at least one of the preceding claims, wherein a temperature sensor (7) is provided, the signals of which can be supplied to the open-loop or closed loop control device (6).

8. A device according to the preceding claim, wherein the temperature sensor (7) is arranged on the at least one welding stamp (2).

9. A device according to one of the two preceding claims, wherein the open-loop or closed-loop control device (6) is designed to monitor a welding process using the signals of the temperature sensor (7).

10. A device according to at least one of the preceding claims, wherein a cooling device for cooling the at least on welding stamp (2) is provided.

11. A device according to one of the preceding claims, wherein a handling device is provided and the at least one heating device and / or the at least one welding stamp (2) are or is arranged on one arm of the handling device.