Differences in bonding technology explained

Every manufacturer of bonded lamination stacks has developed their own technologies and processes for bonding electrical sheet coated with bonding varnish (Backlack). At its core, bonding varnish (Backlack) is an epoxy resin that is mixed with hardener before coating so that it chemically hardens under pressure, temperature, and time. This ensures high-strength bonding of the electrical sheet over the entire surface. The manufacturers of the bonding varnish (Backlack) specify a processing window that indicates the duration of the application as a function of the temperature. The higher the temperature, the shorter the curing time of the bonding varnish (Backlack). Sufficient pressure is required to ensure that the bonding varnish (Backlack) layers flow together evenly.

Batch oven

The most well-known and widespread technology is bonding in the oven. Electrical sheets are stacked and placed under (bonding) pressure in bonding tools with springs. The bonding tools with the electrical sheets are then bonded in the oven at 200 °C for 2 hours and cooled down to room temperature for approx. 30 minutes. The springs are now released and the bonded stator and rotor stacks are removed. This process is reproducible and established. However, the process is not suitable for series manufacturing, as the cycle times are around 3 hours and even longer for large tools.

Further development: the continuous furnace

The continuous furnace is the further development of the aforementioned batch furnace. The tools are identical. The continuous furnace consists of a furnace section and a cooling section. The tools are moved through the furnace on conveyor elements. High air velocities and special nozzles speed up heating, which reduces cycle times. However, due to the faster heat input, the electrical sheets heat up faster on the outside than on the inside, which leads to an asymmetrical temperature distribution in the stator and rotor stacks. As a result, the bond is not always identical, and even warping can occur in the component. This can have a very negative effect on flatness, for example. In addition, the effect of the temperature difference outside to inside is further intensified by the geometry (e.g. a rotor assembly with magnetic pockets, where the magnetic pockets represent heat barriers). Although the process enables shorter cycle times (approx. 1.5 hours), good quality can only be achieved by optimizing the process and the furnace to the existing geometry.

Bonding in the tool

Another technology is bonding directly in the punching die. A heating unit is installed directly under the punching die. After punching, the laminations are guided through this heating unit, heat up, and are subjected to counterpressure. After a certain number of laminations, either a release layer is bonded on or the pile is ejected to create individual, separate stacks.

This leads to several conflicting objectives:

• High punching speed (high output) reduces the time of the laminations in the heating unit (less firm bonding)
• The punching die should be kept as homogeneously as possible at room temperature so that the cutting gap and the positions between punch and die are correct, and the heating unit must be very warm for a quick and effective bonding
• The heating unit can usually only fix and heat the laminations from the outside. The same problems arise as with a continuous furnace

These conflicts are thus usually resolved in such a way that a compromise has to be found between these issues. This results in bonded stator and rotor stacks that exhibit low strength, higher stacking errors, and large shape tolerances (perpendicularity and parallelism). But there are also disadvantages that only become apparent at second glance: a lower stacking factor, setting behavior during installation and over the temperature cycles, which in the case of segmented stators can mean, for example, a reduction in the inner diameter over time!

Bonding as a separate process

The SWD technologies BPS® (for segments) and EPS® (for large stators and rotors) are characterized by the fact that punching and bonding are carried out separately in their own tools. This means that each process can be operated at its optimum and no compromises are necessary.

The linking takes place via automation, which enables us to achieve maximum punching speeds and high precision as well as the shortest bonding times thanks to higher temperatures and more bonding pressure. Our bonding processes are now so sophisticated that we achieve bonding times of 3 to 4 minutes for small components such as segments, and dynamic bonding processes (dynamic control of temperature and bonding pressure) achieve maximum bonding, maximum stacking factor, best perpendicularities, and flatness. Our technology also enables extremely precise alignment of the laminations and thus the smallest tolerances. In addition, the segments do not exhibit any setting behavior, as the bonding varnish (Backlack) cures completely and the stacking error is minimal.

We also optimize the bonding recipes on an ongoing basis for large stators and rotors. The core of our technology is the uniform and wide-area heating of the laminations, regardless of the geometry. Coupled with the dynamic bonding process mentioned above, we can positively influence stack tolerances and properties in many dimensions and thus produce the best possible bonded stack. The dynamic bonding process also makes it possible to adapt to different material manufacturers and bonding varnish (Backlack) coatings in the shortest possible time.