Keywords

1 Motivation

In the construction of electric devices like motors, there is a great potential to reduce the use of expensive and rare materials like neodymium, cobalt or copper. To achieve this goal the continuous power density needs to be increased. Many mechatronic power converters are limited in this regards by the cooling capacity. Here we see the possibility to increase the performance with improved internal cooling channels in the device’s encapsulation. Were standard methods for cavities like cores are not applicable anymore inserts with cooling channels, can offer more flexibility. Therefore, this paper will emphasize on the possibilities and challenges of additively manufactured metal inserts in injection over molded devices.

2 State of the Art

Some of the most advanced electric motors are designed with internal cooling channels [1]. So the thermal path can be greatly shortened. In addition, the devices can be embedded, potted or molded in high thermal conductive materials.

An example for an electric motor with internal cooling channels through the groove is the DEmiL project [2]. Here, the channels were created directly in the potting process. Core bars with a triangular cross-section formed the channels between the windings after demolding. In bench tests, a 2.76-fold higher power density was demonstrated in comparison with a similar, conventional jacket-cooled machine. However, the techniques used in this project cannot be used for devices whose structure requires 2 or 3 dimensional internal channels. Such shaped cores could not be demolded due to the resulting undercuts.

The use of additive manufacturing for an improved cooling is often seen in (experimental) high-power heat sinks and exchanger in power electronics [3][4]. These parts benefit from flow optimized and narrow sized channels enabled by the design freedom of additive manufacturing.

Mechanical metal inserts in injection molded components are well known. Threaded inserts are for example widely used to form and reinforce the area around fixation points. The mechanical optimization of the plastic/metal contact of additively manufactured parts was investigated by Saurav Verma [5].

3 Proposed Advantages Through the Enhancing of Injection Molded Encapsulations with Additive-Built Inserts

The benefits of mixing additive manufacturing with injection molding becomes relevant when the advantages of one specific production technique is used to cancel out the disadvantages of the other. Additionally, the insert can and should combine other functions, which are enabled by (AM-) implants as shown in Table 1.

Additive manufactured cooling channels are reinforced by the over molded plastic. So, manufacturing costs can be kept low, as wall thicknesses and thus laser time and material consumption can be minimized. This can further be exploited when the processing metal powder of the LPBF process is kept inside the channels during the over molding. So, the channels are stabilized against indentation by the injection pressure.

By using the injection molding process for the encapsulation, not only the inserts can be designed as small as possible, but complex external elements can also be integrated in the plastic [6]. The very good flow properties of the material allows filling of even the thinnest gaps, so that a very good thermal interconnection can be realized while mechanical vibrations are damped. Injection molding processes are widely established in the industry and enable low unit costs with increasing quantities, since the high mold costs can be amortized by the high part volume.

Figure 1 shows a conceptual bearing-seat-insert in a plastic housing as an example for a well-designed insert, that utilizes many of the design directives, implicated by Table 1.

Table 1. Suspended disadvantages of plastic injection molding (IM) and metal additive manufacturing (AM) by the combination of both.
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Fig. 1.
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Example for function combination in a cooled bearing seat insert

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4 Motivation in the HEaK Project

In the research project “High-efficiency electric motor with additively manufactured cooling system in plastic overmold (HEaK)” [7], the power density of the widely used hairpin-wound electric traction motors is to be increased by a factor of 2 by inserting cooling channels inside the motor. Due to the twisted and welded and thus fanning winding head, no straight cores can be inserted in the mold. Therefore, additively manufactured channels are inserted, to allow the cooling medium to be directed around the undercut winding head (Fig. 2).

Fig. 2.
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Left and Middle: Hairpin winded HEaK Stator with internal cooling channels; Right: channel-inserts

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The production process is as follows: The insert ring with all channels is manufactured from stainless steel as a tubular structure with closed ends using the LPBF (Laser Powder Bed Fusion) process. The surrounding powder can be directly reused according to known recycling processes, while a little powder initially remains in the tubes. After sawing from the building platform the ring can be inserted in the stator winding [8]. The stator is then overmolded with highly thermally conductive thermoset plastic. Thanks to the enclosed powder, the channels withstand the high injection pressures. Drilling and milling open the channel ends, and the powder can trickle out for further reuse (Fig. 3).

Fig. 3.
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Production steps (from left to right): Inserting the ring with all channels; overmolding with thermoset plastic; opening the ends of the channels

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4.1 Design of the Insert Structure

Strut Design. In order to be able to mount the insert structure on the stator, it should allow a flexibility, which enables the channels to be guided past the winding head undercut. The design freedom through the additive manufacturing is further used to implement this assembly function. For this the all-channels-connecting struts, are designed as a compliant mechanism, which allows a bistable, inward orientated rotation. This is achieved by disc spring-like ring struts. For an improved manufacturing process, the spring is designed in a double-strut design in the later versions (Fig. 4).

Fig. 4.
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Left: Two disc spring based strut designs (cutout) Blue: disc spring with gap around the channel; orange: double-strut design. Middle: spring characteristic: Right: Simulated Movement of one channel in the ring (displacement to scale)

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Design of the tubes. For the LPBF production-ready design of the tube structure, the aim is not to design any overhangs of more than 45° in order to avoid the need for support structures. In addition, a wall thickness of at least 0.35 mm is maintained. The cross-section of the tube element is implemented as a rounded square with a circular inner channel. This contour allows the minimum wall thickness in the direction of the winding head, so the channel to be run along it as narrowly as possible. At the same time, the thickened corners stiffen the tube for safer handling during installation. In the angled tube area, the inner contour changes into a “circle with roof”. This ensures that the inner walls do not have too much overhang (Fig. 5).

Fig. 5.
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Tube design of two cut out channels and final inserting ring

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Fig. 6.
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Position (green) of the inserted channels in the HEaK e-motor design

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5 Preliminary Investigations

5.1 Pressure Tests

To ensure that the additively manufactured tubular structures can withstand the static pressure during overmolding, a pressure test is performed. This test is also intended to ensure that the walls are printed tightly enough to prevent plastic from leaking in. For this purpose, individual channels printed on a test basis from titanium were placed in a pressure vessel filled with water and subjected to the pressure expected from injection molding (1 × 30 bar, 1 × 60 bar). After one minute, the water was drained, the test specimens were removed, externally dried and opened with a cutting grinder. It was shown that the printed tubes withstood the pressure without any leakage and that the enclosed metal powder showed no signs of moisture.

5.2 Powder Investigations

To determine, if the Ti6Al4V metal powder from inside the channels cold be recycled and reused, it was analyzed under the microscope and compared with new powder and powder from the printing bed. The left Fig. 6 shows new powder. In contrast, the middle figure shows the already sieved powder from the area next to the print object. In the right figure, the powder recovered from inside the cooling tube is shown. It can be seen in this optical analysis that the quality of the powder from the inside of the tube is not distinguishably worse than that of the powder next to the object. So it a recycling of this powder can be seen as a realistic option. From one HEaK-tube, 2 grammes of powder could be recovered (72 grammes per motor insert structure). So, by weight, roughly half of the inserting structure is enclosed powder (Fig. 7).

Fig. 7.
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Left: New powder; Middle: Powder from the printing bed; Right: Powder from the inside of the printed structure

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6 Conclusion

In this paper the integration of additive manufactured cooling channel inserts in over-molded high-performance devices was proposed as a concept and designed out on the example of the HEaK e-motor project. It is suggested to concentrate multiple functions in these inserts, to make up for their production costs. Furter cost saving could be achieved with thin-walled structures, filled with remaining powder which was removed after the molding process. First pressure tests of channel inserts and a powder investigation showed positive results.