conductor passages also in these embodiments the conductors in the conductor passages 4 can be cast simultaneously with the molded part and thus formed from the matrix of the mmc material or of a different metal when already bonded to the tube 3 before casting the molded part fig3 shows a casting mold 1 in which the cavity 2 is only partially filled with a preform 7 of a mmc material in all other aspects this casting mold corresponds to the casting mold described in connection with fig2 fig4 shows a casting mold 1 in which the cavity 2 is filled with preforms 8 9 of either different material or of same material but different density i e bulk density or apparent density different surface treatment or the like the preform 8 may be made from e g a pressed silicon carbide and the preform 9 may be made also of silicon carbide that is however only loosely introduced into the cavity 2 to effect a different density thus the molded part has different expansion coefficients and the production of such preforms is considerably simpler and cheaper alternatively the preform 8 may be made of silicon carbide and the preform 9 of aln or vice versa in any event also in this manner areas of the molded part can have different thermal expansion coefficients fig5 shows a sectional view of a casting mold 1.