GaN transistors are limited in their operational capabilities due to some limitations, of which the thermal management aspects. Until now, most of the existing heat-dissipator systems using additive manufacturing (AM) are based on a metallic finned heat sink, which is heavy and has a relatively high thermal resistance. Heat dissipation based on a phase change as operated in heat pipes is more efficient. Such heat sinks have been experimented with metals or ceramics and not by now with polymers. However, this may be of great interest. The use of polymer may enable reducing weight and cost of the thermal device. It may allow also improving the chemical compatibility of the heat pipe material and fluid, as this is often a severe issue. This work presents a characterization of a 3D-printed polymer-based heat pipe evaporator intended for GaN transistors. The electronic copper circuit on the polymer surface is created using plastronics technology. The metallized circuit presents an adequate electrical conductivity. The thermal characterization performed with the HFE 7000 fluid, shows that it is actually possible to cover the entire heated polymer wall with active nucleation sites once the full developed nucleate boiling regime is reached. The heat conduction through the insulating polymer wall appears as the limiting phenomenon for heat transfer.