THERMAL SHOT-GATE FOR PLASTIC MOLDING APPARATUS

摘要

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 "open" the gate. Interruption or reduction of current flow reduces the heat and permits the plastic in the passage to freeze, thereby "closing" the gate.