an ohmically or electrothermally controlled valve gate for injection molding apparatus comprises three concentrically located tubes in intimate contact with each other . the innermost tube is an electrode and also serves as the passage for flowing molding material . the outermost of the three tubes is also an electrode . the intermediate tube is a resistive heating element comprising a mixture of at least two highly compacted powders . one powder is an electrical conductor , while the other powder , which serves as a binder for the first powder , is an electrical insulator but a good thermal conductor . current flow between the electrodes occurs at points of contact between adjacent particles of the conductive powder . thus , the electrical resistance and therefore the generation of heat is controlled by the composition and quantity of binder in the heating element . the generated heat is conducted through the inner electrode tube to the flowing plastic to melt the plastic and &# 34 ; open &# 34 ; the gate . interruption or reduction of current flow reduces the heat and permits the plastic in the passage to freeze , thereby &# 34 ; closing &# 34 ; the gate .