Control units for battery and cell management: Why heat staking is the optimal joining technique for different materials.
In the production of e-car batteries, there are numerous requirements for the components, which are also placed on the assembly technologies. Most importantly, these are low weight, vibration stability, process-safety and fast mass production. When designing and dimensioning a product, the method of joining the components must therefore be considered in the early design phase.From the various joining methods that are technically possible, a well-known tier 1 supplier to the automotive industry has chosen heat staking technology. They purchase the suitable heat staking machine for his process from bdtronic, for various reasons.
The right joining method for thermoplastics
Heat staking uses heat to melt and reshape plastic pins, forming a head and creating a durable joint between different materials. The pins are incorporated in the injection mold and can have different diameters – depending on the requirement for the holding force of the riveted joint. This is because they can hold very high tensile forces. For example, a 3mm diameter rivet head made of a glass fiber reinforced polyamide (nylon) can hold up to 530 N pull-off force. Basically, the achieved pull force depends on the material, riveting method and process time. Several pins are often used per component, which can be riveted simultaneously. A major advantage of heat staking is that existing material and the existing process step injection molding are used, and no additional parts need to be procured, stored, and fed to the production line. This makes the heat staking process cost-effective.
Compared to the screw connection, heat staking is ahead in terms of weight, because the plastic is lighter than the metal of the screws. Heat staking joins the joining partners permanently and the parts can only be separated by destroying the assembly.
Compared to ultrasonic riveting, hot riveting does not expose the components to vibrations. This has advantages when it comes to electronic components on printed circuit boards, for example, which can be destroyed or pre-damaged as a result. Furthermore, ultrasonic riveting often results in the formation of dust and flakes, which can contaminate the electronics and impair or disrupt their function.
BHS HOT AIR for clean results
In the example of the battery control units, the company explicitly opted for the BHS HOT AIR method from bdtronic. This method delivers clean results, because, compared to other hot riveting methods, the tool is independent of volume. This means that no flashes can form on the rivet head, even if the height of the pins varies due to product tolerances. The heads are not only more presentable, but there is also no contamination due to flaking of the burrs.
Furthermore, the BHS HOT AIR process makes it possible to close the gap that exists between the pin and the borehole of the joining partner. This results in added stability and a fixed position. When designing rivet pins, it is important to ensure that there is sufficient clearance around the rivet pin to the next heat-sensitive part. bdtronic provides design recommendations for this in the form of guidelines which are available for download on the website.
Materials suitable for heat staking
In heat staking, different materials can be firmly joined together, e.g. thermoplastics with metal or printed circuit boards, but also different plastics. In principle, it is suitable for all amorphous or semi-crystalline thermoplastics.
In the automotive industry, particularly high-performance thermoplastics are used as standard, such as PA, PBT, POM, or PP, but also high-temperature-resistant plastics such as PPA or PPS. The BHS HOT AIR® process is frequently used for these semi-crystalline thermoplastics with a high glass fiber content in particular because it is very robust and consistently delivers the highest strengths even with material-related fluctuations, such as moisture absorption, or during the injection molding process. The small temperature window of 10 - 20 Kelvin, in which the material must be formed, is very challenging when processing these materials. For all bdtronic heat staking processes, however, it is easy due to the exact temperature measurement, control, and closed-loop regulation.
In the example of the automotive supplier, semi-crystalline polymers are used to rivet a printed circuit board and, at the same time, the cell connectors and busbars to a frame.
Optimal manufacturing processes for high-quality components
The process settings for heat staking depend primarily on material properties, but also on the component design itself. Ideally, the manufacturing process is developed in parallel with the component, the production parameters are defined, the process is tested, and initial prototypes are produced.
The design guideline from bdtronic mentioned above already provides support during the design of the injection molded part and the design of the pin. It contains information on tool sizes and recommends an optimal rivet pin geometry. As soon as the first sample parts are available, the specialists at bdtronic's technology center provide support with a process simulation under near-production conditions. They use standard production machines as laboratory equipment to find the optimum between cycle time and strength requirements for the individual product. The defined parameter settings are in turn incorporated into the machine concept.
In the example of battery control units, pins with different diameters were defined which rivet different materials on several levels. A pull-off strength of over 100 N was required for each rivet pin - in pull-off tests at the technology center, the pins produced using the BHS HOT AIR® process also withstood forces of up to 540 N. The test results were recorded and verified in stress tests carried out by the customer.
Optimal cycle times and process control for mass production
Specifically in the mass production of parts, the processing and cycle times are hard cash. On the other hand, time has a direct influence on the quality of the riveted joint. In order to compensate for the supposed disadvantage in processing time, the process steps of heating and forming have been divided into two stages in the BHS HOT AIR process. This allows shorter cycle times for this fully automatic machine than with other heat staking methods.
In the application example, this resulted in a cycle time of 15 seconds including the run-in and run-out time of the workpiece carriers. With an availability of >99.5%, the machine now runs in 3-shift operation.
To ensure the quality of the rivets, the fully and semi-automatic systems from bdtronic have numerous sensors that continuously measure and regulate the process temperature. In the application example, additional optical controls were integrated into the process flow. With this system, the customer has full process control and a transparent process for riveting the control units for battery and cell management. Other benefits are low overall costs due to the low energy and air consumption compared to other heat staking methods, and the wear-free nature of the riveting tools.