Processing of thermally conductive potting compounds by using a direct injection process
Thermally conductive materials are used for numerous applications in which efficient thermal management is required. They dissipate heat in a targeted manner and help to maintain the performance and reliability of systems – in electronic components and parts as well as in the rapidly growing field of electromobility applications. In view of the development of high-voltage batteries with high performance and other requirements in automotive engineering such as smart battery design, packaging optimisation and lightweight construction, conventional thermal gap fillers are reaching their limits. Instead of pressing thermally conductive materials under pressure, the focus is increasingly shifting to direct injection processes. They are attractive thanks to their sustainable advantages, both for thermal management and for the entire manufacturing process.
Battery technology in electric cars is making rapid progress. 800 V technology is increasingly becoming the new standard, and even 1000 V technology is no longer a rarity. OEMs and suppliers are continuously working on increasing the power density, storage capacities and thus the ranges that can be achieved with a single battery charge. However, the trend towards high-voltage batteries is placing even greater demands on an optimised thermal management and the thermal connection of the batteries to the cooling plates for active cooling.
In parallel with this step in performance, battery design is also evolving, towards smart designs that are even better adapted to the vehicle design. Thermally conductive gap fillers are rarely the first choice for more complex battery pack designs. Thermally conductive potting compounds, on the other hand, which are applied using a direct injection process, are able to reliably reach even small intermediate areas in the batteries and ensure complete heat dissipation.
Comparison of battery designs
These advantages of thermally conductive potting compounds exist regardless of the battery design, for prismatic cells as well as for pouch cells or round cells. This is because cell formats for automotive applications are constantly evolving as efficiency increases, and the number of designs is becoming increasingly diverse. Pouch cells, also known as “coffee bags”, are in widespread use today, with the increase in battery capacity compared to round cells being one of their main advantages. Prismatic cells, on the other hand, are constructed in a similar way to pouch cells, but they are also packed in a housing made of a solid material, predominantly metal. This facilitates the construction of battery packs, but places even greater demands on thermal management. Classic round cells are characterised by the fact that long strips are arranged in a cylindrical shape – this means advantages for automated large-scale production, but also makes cooling challenging.
Optimising of thermal management
Regardless of the battery structure and design, the direct injection process offers very good conditions for an optimised thermal management. Thanks to direct injection of the thermally conductive potting compounds, all intermediate areas of the battery can be reached reliably and without gaps, while also compensating of gap tolerances and manufacturing tolerances in just one working step. The high thermal conductivity of up to 3.5 W/mK, which at least equals and often exceeds the values of conventionally processed gap fillers, guarantees a reliable thermal battery connection.
Advantages for large-scale production
In addition to the material properties, there are also lasting advantages on the processing side. A crucial point here is that the direct injection process can take place on the production line itself with an excellent service lifetime. This accommodates new vehicle designs in which the battery cells are connected to the chassis on the production line. However, pressing of thermally conductive gap fillers under pressure runs contrary to this concept of the manufacturing process. For example, tilting can lead to an incomplete connection from the cell to the thermally conductive material and therefore to the cooling plate, resulting in poorer heat dissipation.
Low viscosity, excellent flow properties
In general, the direct injection method offers a number of processing advantages. While thixotropic gap fillers only start to flow under a shear force and have high viscosity leading to i.e. poor flow performance – potting compounds offer impressively low viscosity and, as a result, excellent flow properties. This ensures highly effective processability on a large series scale with consistently high process quality, including full-surface and uniform filling of all intermediate gaps. It also minimises existing process challenges by eliminating the risk of unwanted, detrimental tilting of the battery blocks with the direct injection process.
Resource-saving application
In addition, the issue of sustainability is now becoming increasingly important. Requirements such as energy efficiency throughout the entire process, conservation of resources and optimisation of the carbon footprint are top of the list. The trend towards lightweight construction is also opening up new fields of activity. The direct injection process also offers a number of advantages in this context, as the constant, reproducible degree of wetting with low-viscosity potting compounds results in optimised material usage. This means that the quantity of potting compound is always in line with demand, which conserves resources and has a positive effect on both cost efficiency and the ecological balance. Series applications show considerable potential for optimisation and material savings.
Sustainable and safe
An important prerequisite for direct injection to be directly integrated into automotive manufacturing processes is the absence of silicone, which is guaranteed with thermally conductive potting compounds from Kisling, part of the Würth Group. The company can also supply formulations in accordance with UL 94 V0 (fire classification test) and, as a contribution to sustainable production, CMR-free versions, i.e. free from carcinogens, mutagens and reprotoxic components, which guarantee a high level of safety for employees.
Conclusion
It is not only the considerable process and efficiency advantages that make direct injection processes increasingly interesting for the thermal management of vehicle batteries – from cars and commercial vehicles to high-performance e-bikes. In addition, the process is mature and is already being used in the first large-scale automotive series by leading OEMs. Kisling has many years of proven expertise in this field and supports new direct injection projects literally from the very first minute, from the design and selection of materials to the ongoing optimisation of formulations in order to optimally meet requirements for thermal conductivity and low viscosity for reliable and robust processing – while at the same time enabling exceptionally high thermal conductivity for optimised thermal management. Thanks to smart battery designs, the trend is clearly moving towards direct injection processes, which are increasingly replacing conventional methods. This already applies to passive battery cooling and is even more relevant where active cooling is needed.
Dr Michael Karcher Market Management & Application Technology at Kisling AG – Member of the Würth Group
