Sika has developed a new adhesive solution to meet this challenge.
The lightweight concept of reducing automotive carbon dioxide emissions and the development of electric vehicles are two global trends in the current automotive industry. In these areas, the use of aluminum profiles and aluminum castings is increasing. Castings are often used as connection nodes, which presents new technical challenges for bonding these parts together with adhesives. Sika has developed a new adhesive solution to meet this challenge.
1.Technical challenges
Car manufacturers adopt different concepts to solve the problem of lightweight design in order to meet the requirements of further reducing carbon dioxide emissions of cars. The most important of course is the concept of mixed materials. This involves optimally combining different materials (such as steel, aluminum, and plastic) based on their characteristics and using them in the same body. The different thermal expansion coefficients of each material can cause problems in the cooling process and, in extreme cases, lead to bonding failure. To solve this problem, Sika recently developed Sika Power MBX technology. The use of different materials can cause problems with lightweight structures, and structural obstacles must also be overcome, such as the connection of cast aluminum nodes to aluminum profiles. In a skeletal structure (also known as a space frame structure), the sections are connected by nodes, as shown in Figure 1.
As with push-in connections, the bonding of such joints is often not easy to resolve. Due to the small gap size, most of the adhesive applied to the profile or casting before the bonding process is removed. Therefore, such joints are now basically mechanically connected, such as riveting.
In the battery structure of electric vehicles, the size of the battery limits the structural space and often requires a linear structure. The wider use of profiles in body construction increases the demand for suitable adhesive solutions. Sika developed the SikaPower Smart-Flow injection adhesive to bond such push-in connections.
Another field of application is the bonding of plastic reinforced parts (such as Sika glass fiber reinforced polyamide parts, trade name Sika Reinforcer) into the car body, as shown in Figure 2. There are several options for connecting these structural components to the vehicle body. Structural components can be hooked into the body. During thermal curing, the adhesive foam expands to the metal, forming a bond. Alternatively, a structural adhesive can also be used to directly bond such structural components into the cavity. In Sika, this process is called High Strength Bonding (HSB). From a mechanical point of view, this process has many advantages, as shown in Table 1.
Compared with Sika Reinforcer, bonding structural parts with adhesive (HSB) makes the structure stiffer and more resistant to impact. However, manufacturing challenges may arise. In particular, the reliability of the nesting process for individual parts is often not guaranteed. SikaPower Smart-Flow injection adhesive provides a solution for this. In this case, part or all of the adhesive can be injected directly into the already connected metal structure. This method is particularly suitable for strengthening A-pillars and B-pillars and for reinforcing battery frame structures in vehicles. For this application, it is possible to design a reinforcing part to allow the adhesive to flow from the injection point to the four side positions, thus ensuring the best combination of the part with the metal structure.
2.Smart injection adhesive
High temperatures and / or high pressures are required to inject standard body adhesives into the cavity. In addition, it is necessary to provide a direction in which the adhesive flows. Usually in the closed cavity, only the cavity needs to be filled. However, targeted injection becomes particularly difficult when the cavity is not closed. The components mentioned at the beginning are usually not form-fitting or sealed, which means that they are open on at least one side. When the adhesive is injected into a structure where one side is open or the side is not completely closed, the adhesive will diffuse not only in the desired direction, but also in other directions. The guiding effect of the channels in the module provides little help. In contrast, Sikapower SmartFlow Adhesive only spreads in designated channels, even if the channels have openings on the sides, as shown in Figures 3 and 4. This characteristic has proven to be particularly advantageous in the presence of widespread tolerances on the body. For simplicity, the injection cavity in this report is referred to as the channel, and the side without form-fitting is referred to as the shoulder, as shown in Figure 3. The height of the shoulder indicates the distance between the two connecting portions in the shoulder region.
The new bonding technology has been tested and validated on different laboratory models. In all the illustrations shown in this article, the channel depth is 3 mm and the shoulder height is 1 mm. Although the difference is small, the adhesive flows only in the channels, not the shoulders. Studies have shown that the ratio of channel depth to shoulder height is the deciding factor and must be taken into account when designing. For a given 3 mm channel depth, the larger the selected shoulder height, the worse the adhesive spreads in the channel, and the better the fluidity of the electrophoretic coating.
If electrocoating is not necessary, the shoulder height is preferably less than 1mm. If perfect electrocoating must be guaranteed, the shoulder height must be 1 to 2 mm. However, this is not a problem, as the shoulder-height up to 3mm provides good erosion resistance for pre-treatment and dip coating and good stability during thermal curing. In addition, a certain shoulder width is necessary to prevent the adhesive from overflowing into the shoulder from the channel. Shoulder width> 5mm proved to be ideal. Extensive injection tests have shown that both narrow and wide channels can be filled without problems, as shown in Figure 5. During the test, there were no problems filling channels up to 20 cm wide. With these specifications in mind, the channel can be integrated directly into the structural component or in the casting with the use of plug-in connectors.
For injection, the important factor is to choose the correct injection speed to prevent the adhesive from overflowing at the edges. Excessive injection speed can cause overflow; however, if the speed is too low, the adhesive may not spread through the channel as expected. Basically, a low and controlled adhesive shoulder flow is preferred. Temperature also has an impact on injection results, as the viscosity of the adhesive depends on the temperature. With a fully automatic application system, you can focus on the most important injection parameters, such as extrusion speed and temperature. This enables consistent, comparable results. The injection point should also be carefully selected. It should be noted that the volume of the cavity must be approximately the same in all flow directions in order to prevent overflow near the injection point.
In order to test the flow properties of the adhesive as realistically as possible, the developers set up several test bodies to test the adhesion of the reinforcement to the steel structure with the selected test piece. The reason for choosing this geometry is because the test channel is large. The relevant structural parts are produced using a 3-D printer as shown in Figure 6. Use appropriately shaped plexiglass housings instead of metal structures to best track flow behavior. Tests confirm that large channels can be filled without overflowing the edges and shoulders. Studies have also shown that typical components such as holes or recesses in such components do not cause problems with the spread of the adhesive. In this case, the adhesive flows around the obstacle and then flows together again without leaving any air pockets, as shown in Figure 6.
Adhesive, battery structure, bonding technology
If you need to fill a larger channel or shorten the filling time, you can also inject adhesive at multiple locations at the same time, as shown in Figure 7. This innovative adhesive is based on the well-known SikaPower adhesive. Therefore, the most important product performance of the new adhesive is also comparable to the known system shown in Table 2.
3.Summary
SikaPower SmartFlow offers a new bonding solution. This means that push-in connections, such as those in electric vehicles or lightweight designs, can be reliably glued. The same adhesive can also be used to glue structural parts over the entire surface, resulting in higher part stiffness and strength.