This paper presents the results of testing the mechanical properties of maraging steel 1.2709 that were obtained by the SLM method under uniaxial and triaxial states of stress. The triaxial state of stress was realised by making circumferential notches in the samples with different radii of rounding. The specimens were subjected to two types of heat treatment, which consisted of ageing at 490 °C and 540 °C for 8 h. The results of the tests that were conducted on the samples were considered as references and compared with the results of the strength tests that were conducted directly on the SLM-made core model. Differences were found between the results of these tests. Based on the experimental results, the relationship between the equivalent strain of the specimen in the bottom of notch εeqand triaxiality factor η was determined. Function εeq= f(η) was proposed as a criterion for the decrease in the plasticity of the material in the area of the pressure mould cooling channel. Using the FEM method, equivalent strain field εeqand triaxiality factor η were determined in the conformal channel cooled core model. Based on the proposed criterion of plasticity loss and the results of numerical calculations, it was shown that the values of equivalent strain εeqand triaxiality factor η in the core that was aged at 490 °C did not meet this criterion. On the other hand, the values of strain εeqand triaxiality factor η did not exceed the safety limit when ageing was carried out at 540 °C. The plasticity loss method presented in this paper assumes that the value of the triaxiality factor in the vicinity of the channel is influenced by the shape, cross-sectional dimensions and trajectory of the channel axis. Using the methodology proposed in this paper, it is possible to determine the value of allowable deformations in the cooling channel zone and to determine whether the heat treatment applied to the SLM steel does not cause an excessive reduction in the plastic properties.