Joining method for metal-metal and metal-plastic bonds and parts produced therewith

Abstract

The present invention relates to a joining method for metal-metal and metal-plastic bonds and to parts produced therewith. For example, the parts may be a cooling element, through which a fluid can flow, for accumulators of electric or hybrid vehicles. The joining method comprises a preparation of the parts and subsequent integral bonding of the parts by means of a powder coating.

Description

[0001] Joining processes for metal-metal and metal-plastic connections and parts manufactured using these methods

[0002] The present invention relates to a joining method for metal-to-metal and metal-to-plastic connections and parts produced therewith. The joining method is particularly suitable for large-format parts that are assembled from two or more individual components and require high precision in their geometry. For example, the parts could be a cooling element for batteries of electric or hybrid vehicles, through which a fluid can flow. The joining method comprises preparing the parts and then bonding them together by means of a powder coating.

[0003] State of the art

[0004] Components in the automotive industry must meet various requirements, which, especially in the case of joined assemblies, can pose a challenge in combination. First and foremost, the connection between the individual parts in the joined assemblies must exhibit sufficient stability to withstand even high peak loads in an accident. Another essential requirement for metal parts is corrosion resistance. For this purpose, the metal parts are typically treated with a corrosion inhibitor after cleaning, unless the material itself provides sufficient resistance. Common methods include spray and immersion pretreatments with various phosphating agents or conversion coatings with crosslinking polysiloxanes. Subsequently, coatings such as cathodic dip coating (e-coating) or powder coatings are applied.Depending on the type of assembly and the joining process, the parts are usually joined beforehand, or less frequently afterwards, by material or form bonding. For metal parts, a material bond between the individual components is generally achieved by welding or brazing, followed by e-coating.

[0005] Residues from the flux used in soldering often cause problems with the cross-linking of various coatings and sometimes even prevent it altogether. A consistent, homogeneous surface is, however, a prerequisite for a reliable subsequent process. Completely removing flux residues, however, presents considerable difficulties. On an industrial scale, especially for large-area components that need to be manufactured in large series, chemical pretreatment is the only viable option. However, neither the usual alkaline nor acidic chemicals can remove these residues in a

[0006] November 13, 2025 1 / 17 TK0001P-WÖ chemical cleaning and pretreatment plant will be safely removed.

[0007] Another problem related to residues, particularly flux residues, concerns fluid-cooled cooling elements for the batteries of electric or hybrid vehicles. During operation, the batteries heat up considerably due to heat loss. At these elevated temperatures, chemical reactions could cause premature aging or damage to the batteries. To maintain the batteries at an optimal operating temperature that prevents damage, they are therefore equipped with a cooling system. Depending on the size of the batteries, this cooling is achieved using large-area cooling elements through which a fluid flows and which can be connected to the batteries. One common fluid used in this process is the ethylene glycol / water mixture, which is well-established in the automotive industry.

[0008] The individual components of these cooling elements usually consist of metal sheets laid side by side to form cooling channels through which the fluid can flow. To save weight, the metal sheets are often made of aluminum or aluminum alloys. As mentioned above, they are joined using a metallurgical bond, such as welding or brazing. Currently, the only method used industrially by Western vehicle manufacturers is brazing in the form of Controlled Atmosphere Brazing (CAB), although it is relatively expensive and requires an exceptionally large furnace due to the component sizes involved. The cooling channels are generally formed into the individual metal sheets of the cooling element using mechanical forming processes, such as pressing.

[0009] A disadvantage is that the production of such cooling elements is very complex. This is due both to the energy- and cost-intensive machines and ovens required for such large cooling elements for electric vehicles, and to the high quality and safety requirements regarding leak tightness and geometric distortion caused by high heat input of at least 400 °C during welding and brazing processes. Typical cooling elements for electric vehicles, for example, measure approximately 2 m x 1.3 m. Any distortion or leakage resulting from this size necessitates costly corrective processes and leads to scrap in series production, because to ensure adequate cooling, the cooling elements, including the cooling channels, must make perfect contact with the surface of the batteries across their entire area, and no cooling fluid may escape.Therefore, increased effort is required to maintain the tightest tolerances over such a large area under such temperature loads.

[0010] November 13, 2025 2 / 17 TK0001P-WG

[0011] If the metal parts are soldered or welded, the aforementioned problems with flux residue also occur. If the flux seeps out at the connection point of the cooling element, a surface problem arises on the outside of the cooling element. This leads to surface defects on the outside during subsequent coating processes, such as e-coating, but also during other coating processes. This can be a safety issue, because the cooling element is bonded to the underbody panel after e-coating using adhesive technology and is an essential component of the battery housings of electric vehicles.

[0012] The further problem mentioned arises from a chemical reaction between the cooling fluid, such as the ethylene glycol / water mixture, and the flux used during soldering inside the cooling channels. This results in a significant increase in viscosity, potentially leading to gelling of the cooling fluid. Consequently, the cooling function is impaired, which can result in a safety-critical overheating of the batteries.

[0013] A typical, state-of-the-art process for the entire joining and coating process, using the cooling element for batteries as an example, involves first degreasing and rinsing the pressed metal sheets of the two cooling element halves, and then joining them by soldering (CAB). The next step involves another cleaning and rinsing, followed by a cathodic dip coating (KTL) for corrosion protection. Finally, the cooling element is cleaned again, this time manually with a solvent. The finished cooling element can then be bonded to the vehicle's underbody panel.

[0014] Attempts to develop adhesive bonds for the cooling element components have not yet been successfully brought to market maturity, as stability problems and long-term tests have been observed, which – similar to the reactions with the flux – are due to an impairment of the cooling fluid through contact with the adhesive.

[0015] There is therefore a need for a joining process that does not have the aforementioned disadvantages. In particular, there is a need for a joining process that simplifies, reduces the cost of, and makes less prone to failure the production of a cooling element for batteries in electric or hybrid vehicles.

[0016] Description of the invention

[0017] The invention relates in a first aspect to a joining method for metal-metal and metal-plastic connections between at least one first part made of metal or plastic and one or more second parts made of metal or plastic, comprising at least the

[0018] November 13, 2025 3 / 17 TK0001P-WG

[0019] Steps: a) Chemical pretreatment of the first and second parts to be joined, b) Horizontal positioning of the first part, c) Powder coating of contact surfaces with the second part(s) with a plastic powder, d) Horizontal positioning of the second part(s) and placement onto the first part with plane-parallel alignment of the contact surfaces, the placement being carried out at such a low speed that the powder applied to the contact surfaces is not worn away by the air displaced between the parts, e) Joining the placed parts by heating the parts and melting the powder, f) Optional powder coating of the joined parts, g) Baking of the powder and the optional powder coating.

[0020] The inventive method enables the individual parts to be joined securely and precisely without distortion. Unlike soldering or welding, this method allows connections not only between two metal parts, but also between metal and plastic parts. This offers advantages in the example of the cooling element, as it enables a hybrid design characterized by high thermal conductivity to the accumulator on the metal side and low weight and cost on the plastic side. Furthermore, there is less thermal conductivity and radiation to the outside, allowing the generated heat to be efficiently dissipated away from the accumulator by the cooling fluid.

[0021] The joining method according to the invention is in principle also suitable for creating connections between two plastic parts and is therefore highly flexible in its application, although various methods are known in the prior art for purely plastic connections which can also be used.

[0022] Preferably, the first part is made of metal. This offers advantages when powder coating the contact surfaces, which can then be electrostatically coated more easily.

[0023] Furthermore, in the inventive method, no distortion occurs with large-format parts, since the parts are not exposed to such high temperatures as in soldering or welding. It is important, however, that the lower first part, onto which the powder is applied, is positioned horizontally in a perfectly flat position, and that the second parts are positioned accordingly.

[0024] November 13, 2025 4 / 17 TK0001 P-WG plane-parallel to this. In particular, deflection of the large parts must be prevented by a sufficiently large contact surface or a sufficient number of contact points. This plane-parallel alignment must not change when placing the second part, as otherwise the applied powder layer may become uneven or patchy. Likewise, the application speed must be calculated so that the air displaced when the parts are brought together does not lead to unwanted removal of the powder layer. Naturally, the larger the parts, the slower the application speed. In addition to the optimal stability of the joint, this is also relevant, for example, for the cooling channels formed in the cooling elements, whose geometry and thus the flow characteristics for the cooling fluid can be negatively altered by unwanted powder deposition.

[0025] Another major advantage of the inventive method lies in the curing process. The initial joining of the parts only needs to be carried out to a low degree of adhesion, sufficient for vertically suspending the joined parts for the optional powder coating. In the final curing process, both coatings—the powder coating of the contact surfaces and the powder coating—are cured simultaneously. This avoids double heating, saving both energy and time. Even if the optional powder coating is not applied, a significant amount of energy and time is still saved, since curing the powder coating of the contact surfaces can be carried out much faster and at lower temperatures than a CAB brazing process.If powder coating is also applied, the combined curing process can, in many cases, eliminate the otherwise additional baking step without requiring extra time and energy.

[0026] The chemical pretreatment of the parts in step a) is carried out in the same way for both metal and plastic parts. Both types of material must be cleaned before joining. Cleaning can be performed chemically in each case. However, the chemicals used and the process steps for cleaning metal and plastic parts differ. A common practice in the prior art is to clean / pretreat metal and plastic components on two separate, different cleaning and pretreatment systems.

[0027] With the inventive method, the inventors utilize a variably usable cleaning and pretreatment system on which, contrary to the prior art, both metal and plastic components can be chemically cleaned / pretreated. Advantageously, this system includes the use of differently fillable cleaning chambers, which carry out the corresponding pretreatment processes via a control program. The control program can, for example, adjust the

[0028] November 13, 2025 5 / 17 Nozzle pressure, exposure time, concentration of the medium and / or temperature of the medium.

[0029] Preferably, the chemical pretreatment in step a) comprises at least the following steps in the specified order: a1) Alkaline degreasing, a2) Rinsing with water, a3) Acid pickling, a4) Rinsing with water, a5) Application of a conversion coating with an aqueous, silane-based solution, a6) Rinsing with water.

[0030] The inventors have surprisingly discovered that this sequence, when implemented and ensured with optimal plant engineering conditions for the chemical pretreatment of the parts to be joined, results in particularly good joint stability. The individual rinsing steps a2), a4), and a6) can also comprise several separate rinsing cycles, with the required rinsing water being drawn from separate basins. Depending on the requirements, the various rinsing basins are filled with, for example, normal water or demineralized water, which is then used for the rinsing cycles. An intelligent, pump-controlled cascade system allows for a uniform and homogeneous treatment of the individual baths, regardless of the degree of soiling. The multiple rinsing cycles result in overall lower water consumption.The conductivity of the water can be controlled more precisely and reliably throughout the entire pretreatment process in order to achieve a higher quality of water across all rinsing zones.

[0031] In various embodiments, rinsing with water can be carried out by passing it close to the surface via nozzles at a pressure of 1.1 bar to 3.0 bar, preferably 1.2 bar to 2.75 bar, and more preferably 1.3 bar to 2.5 bar. "Close to the surface" in this context means a distance of less than 40 cm from the sprayed surface. A short distance between the nozzles and the component, especially in combination with a higher spray pressure than in conventional rinsing systems, enables a precise, focused mechanical impulse, which significantly increases the efficiency of surface cleaning.

[0032] November 13, 2025 6 / 17 TK0001 P-WQ

[0033] In particular, it is advantageous if rinsing with water in a cleaning and pretreatment system is carried out with 3 to 9 nozzle arrays / m in the transport direction, preferably 4 to 8 nozzle arrays / m, more preferably 5 to 7 nozzle arrays / m, and the nozzle arrays each have 17 to 26 nozzles / m in the vertical direction, preferably 18.5 to 24.5 nozzles / m, more preferably 20 to 23 nozzles / m. For example, 6 nozzle arrays per meter per zone, each with 21.5 nozzles per meter vertically, can be used. A nozzle array here refers to a vertical arrangement of several individual nozzles past which the part to be treated can be passed horizontally. This, in combination with the described pressure, yields the required amount of water per square meter in the inventor's system.

[0034] In some design variations, rinsing is carried out with water at a rate of 70 l / m. 2 up to 190 l / m 2 , preferably 90 l / m 2 up to 170 l / m 2, preferably 110 l / m 2 up to 150 l / m 2 This refers to the surface of the parts to be rinsed. This ensures that sufficient cleaning performance can be achieved at all times.

[0035] In preferred embodiments, the powder coating in step c) is carried out by electrostatic powder coating or triboelectric powder coating. These methods have proven particularly suitable because they result in a uniform application that adheres so well that the second parts can be placed on top without a significant risk of being blown off.

[0036] The powder coating in step c) is preferably carried out using CNC-controlled application heads. A key challenge is the precise dosing and positioning of the powder application paths. This is particularly crucial when, as with cooling elements, areas of the parts outside the contact surfaces must be kept completely free of powder. The use of CNC (Computerized Numerical Control) controlled application heads allows not only highly accurate dosing and positioning, but also the consideration of different path widths. Depending on the part size and the extent and location of the surfaces to be coated, one or more application heads can be used simultaneously.

[0037] In various embodiments, the application head can be a nozzle or a metering head. Powder coating is typically applied using electrostatic spray guns. The powder is fed either by an injector or by pumps at the powder gun. In these cases, the application head is a nozzle that, according to the invention, is CNC-controlled and guided over the bonding surfaces. This is preferably done as close to the surface as possible to minimize contamination of the surfaces outside the bonding area.

[0038] November 13, 2025 7 / 17 TK0001P-WQ to prevent this as much as possible. Alternatively, for powder application to a spatially limited contour-like contact surface, a metered amount of powder can be deposited in the form of a bead. A metering head is used for this purpose. One or more suitably applied bead-shaped powder traces are then distributed on the joining surface when the second part is placed and formed into a uniform coating, so that a homogeneous joining surface coating is present at the latest when the powder coating is heated.

[0039] In advantageous embodiments, the application head additionally features a suction device and / or a pre- and / or post-heating device applied to the bonding surface or the powder. This allows excess powder to be extracted in a single operation. The heating device can be, for example, a laser, a microwave generator, an ultrasonic generator, or an infrared emitter. With its aid, the bonding surface can be heated before or after application, or the powder after application, to achieve targeted adhesion in the heated areas.This not only avoids or at least minimizes dusting when applying the second part, but also enables a particularly precise application by applying powder slightly beyond the joining surface, whereby it only adheres to the heated area of ​​the joining surface, while the loose powder from the surrounding area can be vacuumed away.

[0040] A corresponding powder coating process can therefore also involve the full-surface application of powder, followed by targeted heating of the bonding surfaces using a laser and subsequent extraction of the excess powder. It is advantageous if all these steps are integrated into a single multifunctional application head; however, they can also be performed separately using several specialized application heads.

[0041] Accordingly, in a highly preferred embodiment, the powder is deposited in the form of a bead onto the joining surfaces, excess powder is extracted, and optionally the joining surface or the deposited powder is heated before or after depositing.

[0042] It can be advantageous to add a volatile binder to the powder before application. This allows the powder to be mixed into a paste-like mass that can be easily applied with an extrusion metering head. Alternatively, the powder can be agglomerated into slightly larger particles that are less likely to be blown away from the coating area. Depending on the ambient conditions or the heat input devices used, the volatile binders can be organic.

[0043] November 13, 2025 8 / 17 Solvents, such as alcohols, or in the simplest case, water.

[0044] In various embodiments, the powder coating in step c) can cover at least 75%, preferably at least 85%, more preferably at least 95%, or at least 99% of the contact surfaces between the parts with the plastic. The powder application is thereby optimized to optimally fill the available joining surface in order to maximize adhesion. This applies particularly to the areas at the edge of the component and, in the case of joining parts that, like heat sinks with cooling channels, form cavities, to the areas between the cavities and near the boundary between the joining surface and the cavity. Since the geometry of the joining partners is predetermined, meaning that no increase in the contact areas between the joining partners is possible, optimization can only aim at utilizing the available surface as completely as possible.

[0045] In particularly preferred embodiments, the plastic powder for powder coating in step c) comprises one or more of the polymers selected from the group comprising polyamides, epoxy resins, polyoxyalkylene polymers, polyisocyanates, hydroxyl-functional polyester resins, unsaturated polyester resins, hydroxyl-functional acrylic resins, glycidyl-functional acrylic resins, acid-functional acrylic resins, polyolefins, polyphenylene sulfide and polyvinyl chloride, as well as their copolymers and optionally reactive or non-reactive fillers.

[0046] As it turns out, the best adhesion results can be achieved with these polymers, which must be selected and optimized depending on the materials of the joining partners and the operating conditions of the component. The resulting bonds are both mechanically stable and unaffected by contact with cooling fluids in the case of cooling elements. They are neither attacked by the cooling fluid nor impair their adhesion, nor does the viscosity of a common cooling fluid such as an ethylene glycol / water mixture change. They can therefore simultaneously provide an excellent seal between the joining surface and a cavity. If necessary, the properties of the polymers can be further enhanced by the addition of fillers for the specific application.In addition to non-reactive fillers, reactive fillers are particularly suitable for this purpose. These fillers have reactive groups on their surface that allow them to react with the polymers. This ensures that they cannot be easily washed out of the bond by contact fluids such as cooling fluid.

[0047] In highly preferred embodiments, the plastic powder used for powder coating in step c) is a reactive system for a thermally curing powder coating system. This enables even further improved adhesion of the joint after the

[0048] November 13, 2025 9 / 17 The curing process is a pure joining process with polymers. In particular, these systems can also react better with and anchor themselves to the primer coating.

[0049] In further variations, the heating of the parts in step e) can be carried out completely in a heating oven or partially using an IR emitter. After precise positioning of the components, the powder is, in most cases, completely melted or at least partially melted in a heating oven, a process commonly known in industrial practice as "gelling." The latter is particularly useful if a powder coating process is to follow. Alternatively, spot heating can be achieved using infrared emitters to ensure a minimum adhesion for the subsequent powder coating process, sufficient to allow the component to be placed in the typical vertical hanging position without damage. This significantly reduces process time and costs.

[0050] The heating in step e) and / or the curing in step g) can be carried out at temperatures of 120 °C to 260 °C, preferably at 150 °C to 230 °C, more preferably at 170 °C to 210 °C. Preferably, the temperature in step e) is lower than in step g). On the one hand, the bonding with the powder can, in many cases, be achieved at a lower temperature than powder coating, thus saving time and energy. On the other hand, a major advantage of the process according to the invention is that the complete curing and cross-linking of the bonding layer can occur simultaneously with the curing of the powder coating. The externally applied powder coating is cured at the same time as the powder applied between the joining partners. Therefore, a lower temperature is sufficient in the first step, which does not yet have to lead to complete curing of the bond.

[0051] Preferably, the curing process in step g) is carried out in a convection oven. This offers the advantage that the heat distribution in the oven during the curing process is more constant across the entire surface of the parts, even when several units are stacked in a rack. Furthermore, in the case of cavities, this allows for better heating of the interior space due to increased air exchange.

[0052] In particularly preferred embodiments, the first part forms a cavity in certain areas with one or more second parts, preferably an outwardly open channel. Besides joining parts to form a compact component, the joining method according to the invention is particularly well suited to generating components with cavities, especially with channel structures that are open to the outside, as is the case with the cooling channels of cooling elements. Less demanding applications are also possible.

[0053] November 13, 2025 10 / 17 TK0001 P-WQ

[0054] Joining connections using the joining method according to the invention are conceivable, which result in a complete hollow body, wherein the joining surface therefore corresponds to a cross-sectional surface through the hollow body, but the real strength of the method lies in the formation of channels in the cavities. Here, the cavities are only formed in partial areas of the parts to be joined, so that sufficient joining surfaces are still available for a highly stable connection.

[0055] In another aspect, the invention relates to the use of a joining method according to the invention for the production of a cooling element for an accumulator of electric or hybrid vehicles.

[0056] In a further aspect, the invention relates to a cooling element for an accumulator of electric or hybrid vehicles obtainable by the joining method according to the invention, wherein the cooling element comprises at least two parts, at least one of which is made of metal and which are materially joined with a plastic, and wherein the contact surfaces between the parts are provided with the plastic to at least 75%, preferably at least 85%, more preferably at least 95% or at least 99%.

[0057] Example

[0058] The joining method according to the invention will be explained in more detail here using the assembly of a cooling element for batteries of electric or hybrid vehicles as an example.

[0059] The cooling element consists of two aluminum sheets: a trough sheet with a material thickness of 2 mm and dimensions of 1,809.00 mm x 1,280 mm x 30 mm, and a meandering sheet with a material thickness of 1 mm and dimensions of 1,748.50 mm x 1,170.00 mm x 5.2 mm. The required cooling channel was formed into the meandering sheet using a press. When the meandering sheet is placed on the flat trough sheet, the cooling channel is created by the indentations formed in the meandering sheet as the flat surfaces come into contact. The cooling channel runs in a meandering pattern across the entire surface.

[0060] The formed aluminum sheets were first subjected to a chemical spray pretreatment. Alternatively, the pretreatment could also be carried out in an immersion bath. For this, the first step involved alkaline degreasing with alkali salts and surfactants.

[0061] A first rinse with water was then performed. Tap water is sufficient for this purpose. However, if the system operator has a water treatment system with evaporator technology, the distillate from this can also be used.

[0062] November 13, 2025 11 / 17 TK0001P-WG

[0063] The third step involved acid pickling with inorganic acids.

[0064] This was followed by a second rinse with softened water or demineralized (DI) water.

[0065] In the fifth step, a conversion coating was applied using an aqueous, silane-based solution. This was again applied by spraying. An immersion method would also be possible.

[0066] The sixth step, the final step in the chemical pretreatment, involved a third rinsing of the treated sheets with deionized water.

[0067] The pre-treated sheets were then dried in a convection oven at 140 °C.

[0068] The cooling element's tray plate was then positioned horizontally on a suitable support, ensuring a form-fitting fit. The contact surfaces of the aligned tray plate with the meandering plate were subsequently powder-coated using an epoxy polyester polymer powder. The powder coating was applied electrostatically using flat and round nozzles. The coated area covered the entire contact surface of the two parts.

[0069] The next step involved horizontally positioning the meandering plate of the cooling element and placing it onto the tray plate. The speed was 5 m / min to prevent unwanted atomization of the powder.

[0070] The joined halves were then joined to the cooling element by heating the parts and melting the powder. Heating to 140 °C took place in a convection oven.

[0071] The assembled cooling element was then suspended vertically and subjected to an electrostatic powder coating.

[0072] Finally, the powder and powder coating were baked on for cross-linking and curing. This was done at 190 °C in a convection oven.

[0073] November 13, 2025 12 / 17

Claims

TK0001P-WG Claims 1. Joining method for metal-metal and metal-plastic connections between at least one first part made of metal or plastic and one or more second parts made of metal or plastic, comprising at least the steps: a) Chemical pretreatment of the first and second parts to be joined, b) Horizontal positioning of the first part, c) Powder coating of contact surfaces with the second part(s) with a plastic powder, d) Horizontal positioning of the second part(s) and placement on the first part with plane-parallel alignment of the contact surfaces, wherein the placement is carried out at such a low speed that the powder applied to the contact surfaces is not worn away by the air displaced between the parts, e) Joining the placed parts by heating the parts and melting the powder, f) Optional powder coating of the joined parts, g) Baking of the powder and the optional powder coating.

2. Joining method according to claim 1, wherein the chemical pretreatment in step a) comprises at least the following steps in the specified order: a1) Alkaline degreasing, a2) Rinsing with water, a3) Acid pickling, a4) Rinsing with water, a5) Application of a conversion coating with an aqueous, silane-based solution, a6) Rinsing with water.

3. Joining method according to claim 2, wherein rinsing with water November 13, 2025 13 / 17 TK0001 P-WQ is produced by passing near the surface of nozzles at a pressure of 1.1 bar to 3.0 bar, preferably from 1.2 bar to 2.75 bar, more preferably from 1.3 bar to 2.5 bar; and / or with water with a water quantity of 70 l / m 2 up to 190 l / m 2 , preferably 90 l / m 2 up to 170 l / m 2 , preferably 110 l / m 2 up to 150 l / m 2with reference to the surface of the parts to be rinsed; and / or in a cleaning and pretreatment system with 3 to 9 nozzle arrays / m in the transport direction, preferably 4 to 8 nozzle arrays / m, more preferably 5 to 7 nozzle arrays / m, and the nozzle arrays each have 17 to 26 nozzles / m in the vertical direction, preferably 18.5 to 24.5 nozzles / m, more preferably 20 to 23 nozzles / m.

4. Joining method according to one of claims 2 or 3, wherein the chemical pretreatment in step a) is carried out using differently fillable cleaning chambers which carry out the corresponding pretreatment processes via a control program, wherein the control program preferably effects an adjustment of the nozzle pressure, the reaction time, the concentration of the medium and / or the temperature of the medium.

5. Joining method according to one of the preceding claims, wherein the powder coating in step c) is carried out by means of electrostatic powder coating or triboelectric powder coating; and / or by means of CNC controlled application heads; and / or provides the contact surfaces between the parts with the plastic to at least 75%, preferably at least 85%, more preferably at least 95% or at least 99%.

6. Joining method according to claim 5, wherein the application head is a nozzle or a metering head; and / or additionally has a suction device and / or an upstream and / or downstream heat input device into the joining surface or the powder.

7. Joining method according to claim 5 or 6, wherein the powder is deposited in the form of a bead on the joining surfaces and excess powder is vacuumed up, and optionally the joining surface November 13, 2025 14 / 17 TK0001P-WG or the deposited powder is heated before or after depositing.

8. Joining method according to any one of claims 5 to 7, wherein a volatile binder is added to the powder prior to application.

9. Joining method according to any of the preceding claims, wherein the plastic powder for powder coating in step c) comprises one or more of the polymers selected from the group consisting of polyamides, epoxy resins, polyoxyalkylene polymers, polyisocyanates, hydroxyl-functional polyester resins, unsaturated polyester resins, hydroxyl-functional acrylic resins, glycidyl-functional acrylic resins, acid-functional acrylic resins, polyolefins, polyphenylene sulfide and polyvinyl chloride, as well as their copolymers, and optionally reactive or non-reactive fillers; and / or is a reactive system for a thermally curing powder coating system.

10. Joining method according to one of the preceding claims, wherein the heating of the parts in step e) is carried out over the entire surface in a heating oven or partially over the surface with an IR emitter.

11. Joining method according to one of the preceding claims, wherein the heating in step e) and / or the baking in step g) is carried out at temperatures of 120 °C to 260 °C, preferably at 150 °C to 230 °C, more preferably at 170 °C to 210 °C, wherein preferably the temperature in step e) is lower than in step g).

12. Joining method according to one of the preceding claims, wherein the baking in step g) takes place in a convection oven.

13. Joining method according to one of the preceding claims, wherein the first part forms a cavity with one or more second parts in partial areas, preferably a channel open to the outside.

14. Use of a joining method according to one of the preceding claims for the manufacture of a cooling element for an accumulator of electric or hybrid vehicles.

15. Cooling element for an accumulator of electric or hybrid vehicles obtainable by a joining method according to any one of claims 1 to 13, wherein the cooling element comprises at least two parts, at least one of which is made of metal and which are materially joined with a plastic, and wherein the contact surfaces between the parts cover at least 75%, preferably at least 85%, more preferably at least 95% of the surface area. November 13, 2025 15 / 17 or are at least 99% covered with the plastic. November 13, 2025 16 / 17