Additive manufacturing of mold inserts for injection molds is gaining more and more importance. Employing conformal cooling channels, both the warpage of the molded parts and the cooling time could be mitigated. In an industrial prestudy, a mold for manufacturing two poly-butylene-terephtalate half-shells of a sensor housing was successfully manufactured by laser powder bed fusion (L-PBF) using a maraging steel 1.2709 steel (X3NiCoMoTi 18-9-5). However, due to a new area of application, the parts needed to be made from polyetherimide (PEI), which requires mold temperatures of 140°C to 180°C and a corrosion-resistant mold steel. Using the software Sigmasoft v5.1, a simulation model of the parts and the mold was built. First, the serial process was cloned. Second, a conformal cooling system, consisting of several parallel low-diameter channels, was designed and implemented. Third, the coolants oil and water and their temperatures were varied to test their cooling performance. In no simulated process condition, oil cooling provided the necessary cavity surface temperature homogeneity and molded part planarity of less than 0.05mm. However, water of 150°C (overcritical above 0.476 MPa) in a volume rate of 8 l/min offered heat transfer coefficients above 25000 W/m²K in the channels, a maximum cavity surface temperature inhomogeneity of 4°C, and a maximum asperity of 0.059mm. In a next step, the mold inserts were manufactured by L-PBF using a corrosion resistant X20Cr13 steel powder. Finally, the mold inserts were successfully tested for PEI, where the planarity was even better than predicted.