Molds for casting plastics can have complicated developed geometry, which greatly complicates the fabrication of their working area. The optimum solution, in this case, is their manufacture from individual elements using welding technologies, which, in addition to the complex of physical and mechanical properties of the product, also provide strict requirements for the geometry of the product. Such requirements corresponds to plasma welding. However, its features, in particular, the high localization of thermal energy, which helps to minimize the level of residual stresses and deformations in the welding zone and the formation of a weld cross-section not typical for arc welding, are not taken into account in standard standards for the implementation of welded joints. The application of modern mathematical statistical modeling to the sample of standard geometry and the conditions of assembly with welding of details with standard seams allowed them to establish their basic installation parameter (assembly gap), suitable for plasma welding of different types of welds for the considered range of thickness of parts and to optimize technological parameters with a probability of 95% plasma welding.