THERMAL TESTING OF A 3D PRINTED SUPER DENSE MESH HEATSINK AGAINST STATE-OF- THE-ART FINNED GEOMETRY Robert Smith, P.E., Chief Technologist AUGUST 29, 2015 QRP 6764 Airport Road West Jordan, UT 84081 www.qualifiedrapidproducts.com 8/29/2015 copyright 2015
Abstract Two 3D printed heat sinks were tested against each other. One had the geometry of a typical finned heatsink and the other had a 3D mesh geometry that can only be 3D printed. The mesh heatsink had 30% more surface area than the fined heatsink but the same overall volume. The results showed that the mesh heat sink s pressure drop was so much higher than that of the finned sink that it eliminated any advantage of the higher surface area. The pressure drop was on the order of 5X higher. This does not mean that 3D printed mesh heatsinks cannot perform better than the state-of-the-art finned heat sinks. It does, however, mean that the same free air ratio does not translate into comparable pressure drop and that the pressure drop must be comparable to that of finned sinks to compete. More testing with less dense mesh is recommended.
Introduction A test was conducted on the lattice mesh heat sink to compare it against a leading commercially available geometry from Alpha heat sinks (Z40-12.7B). The Alpha heat sink is generally build of 6063 Aluminum and is bonded together. The purpose of this test was just to compare the geometry differences so both the Alpha geometry and the mesh geometry were both printed and compared. The Alpha printed heat sink was also compared against the Alpha vendor datasheet to compare 3D printing vs. the 6063 bonded version. The objective was to compare pressure drop as well as thermal resistance over a function of airflow. Figure 1: 3D printed heat sinks printed for both Alpha geometry and mesh fin geometry Approach Concept Laser CL31 aluminum was used to print the geometry. Both parts met the geometric requirements of the Alpha vendor datasheet including the smooth bottom finish. The printed geometry tested consisted of a mesh fin geometry with 32 fins across the length of the sink in both directions. The pins were approximately as thick as the Alpha fins were but they tapered from the base to the tip. The total volume between the 32 fin sink and that Alpha sink was the same, but the surface area of the 32 fin heatsink was 31% higher than that of the Alpha heatsink. QRP followed the test procedure used by Alpha and published on http://www.micforg.co.jp/en/temeasuree.html.
A test plenum was printed on an FDM printer to channel the flow through the heatsink. A tiny type T thermocouple was placed in a tiny hole on the bottom of the heatsink. (Omega part number 5TC-TT-T-40-36). A foil heater was applied on top of the thermocouple on the bottom surface of the heat sink that was 0.5 inches square (Birk part number BD3546 53.0-L24-03) with a resistance of 53 ohms. Figure 2: Bottom surfaces polished with 400 grit sand paper
Figure 3: Test apparatus used
Figure 4: Alpha Z40 heat sink datasheet
Figure 5: Test set-up with no heat sink installed in test fixture. (Insullation not applied yet) (Other sample parts included in thie figure were not part of the scope of this test). One test was conducted with no heat sink installed and the bottom plugged up with a smooth surface mimicking the rest of the walls so that the pressure of the system could be subtracted from that of the heat sink. The first objective was to try and get results that approximated the Alpha datasheet for the Alpha shaped heatsink. The first attempt showed extremely high temperature values. For the second attempt a tiny hole was drilled in the bottom of the heat sink and the thermocouple was placed inside the hole with a small dab of silicon-like adhesive. The pressure curves were assigned a second order polynomial trendline to identify any potential bias in the pressure gage. This Y-intercept value was subtracted from the values measured in the test. Results Figure 6 shows the performance of the Alpha geometry compared against the vendor datasheet. The pressure drop ended up being lower. The test set-up was inspected to see if the gaps on the side of the heatsink were too large and they appeared to be very close to the spacing of the fins as specified in the original test procedure. Higher altitude of Utah testing (4600 ft) may also account for some of the lower pressure drop. The baseline printed heatsink was slightly bowed when post machining so there is a little more air gap at the top of the sink. This may explain some of the lower pressure drop and lower performance.
Static Pressure (mm w.g.) The thermal resistance was higher for the printed heat sink. Part of this may be due to the material thermal conductivity but maybe not all. The values are within the same order of magnitude of the vendor sheets and the two sinks were tested in the same way. Therefore, the comparison of the two geometries is deemed valid. Figure 7 shows the pressure drop across the 32 fin heatsink compared against the Alpha sink. It was much much higher. Figure 8 shows the thermal resistance comparison between the two heat sinks. The difference was not that different for the two sinks and the 32 fin sink was slightly worse. 5 4.5 4 3.5 3 2.5 2 1.5 1 Printed Alpha pressure Z40-12.7B spec mm H.G. Z40-12.7B spec C/W Printed Alpha Resistance 0.5 0 0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 AIRFLOW-(l/s) Figure 6: Performance of Alpha geometry against vendor datasheet
Thermal resistance ( C/W) Static Pressure (mm w.g.) 32 27 22 17 32 fin Z40-12.7B spec Alpha Geometry 12 7 2 0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000-3 AIRFLOW-(l/s) Figure 7: Pressure drop across 32 fin heatsink 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Z40-12.7B spec 32 fin heat sink Alpha geometry heat sink 0 0.5 1 1.5 2 2.5 3 3.5 AIRFLOW-(l/s) Figure 8: Thermal resistance comparison Conclusions The pressure drop across the 32 fin heat sink was considerably worse than that of the Alpha geometry so much that the added surface area could not compensate for it. Not only that, but the 32 fin sink performed less even when the pressure drop was driven to the same values. This does not make sense given that the surface area was so much greater unless all of the air tended to bypass the sink around
the sides and the top where there was a small gap in the test chamber. We suspect that this was the case, and this attests again to the extra high pressure drop which was just made the heatsink less useful in this cooling configuration. It is recommended that new geometries be tested with more open space between the fins to reduce pressure drop.