Internally heated sprue bushing with thermally conductive sleeve

In an internally heated sprue bushing assembly and having a dual element heating element for independently heating one, two, or both of two heating zones of the sprue bushing, the improvement comprising:

• a bushing casing-core element including an generally cylindrical outer sidewall and an endwall integrally joined together and a generally cylindrical inner core axially extending therethrough and generally centrally disposed therein, said integral inner core being radially separated from said sidewall by an annular compartment axially extending through said bushing casing-core element, said inner core having a melt runner passage interposed between and in flow communication with a respective inlet and outlet of said sprue bushing assembly, said inner core further having an outer layer of thermally conductive material axially extending along the outer surface of said inner core between the inlet and outlet of said sprue bushing assembly, said inner core outer layer including means for monitoring the temperature of said inner core during operation of said sprue bushing assembly, and a dual heating element adapted to fit within said annular compartment in surrounding relation to said inner core outer layer, said heating element including first and second electrically non-conductive sleeves, each of said first and second sleeves being circumferentially wound with separate resistance wires which form heating coils on each of said first and second sleeves, said first and second sleeves including three aligned passages axially extending within said first and second sleeves, each of said passages being adapted to receive opposite ends of resistance wires and electrical conducting means in electrical engagement therein, a first passage of said first sleeve receiving one end of a first sleeve resistance wire and a first electrical conducting means therein in electrical engagement therewith, a third passage of said second sleeve receiving a second sleeve resistance wire and a third electrical conducting means therein in electrical engagement therewith, a second passage of said first and second sleeves receiving, respectively, the other end of said first sleeve resistance wire and the other end of said first sleeve resistance wire, the second opening of said first and second sleeve means further receiving a length of a second electrical conducting means therein in electrical engagement with said first and second sleeve resistance wires, thereby forming separate electrical circuits for each of said first and second sleeves.

An internally heated sprue bushing assembly having an inlet for receiving pressurized melt and an outlet for the discharge of said pressurized melt, said sprue bushing assembly comprising:

• a unitary bushing casing-core element having an integral outer sidewall, an endwall and an elongated inner core, the outer surface of said elongated inner core and the inner surface of said sidewall defining a generally annular compartment within said bushing casing;
• heating means in surrounding relation to said elongated inner core, said heating means axially extending a preselected length within and occupying a portion of said annular compartment;
• a compacted unified mass of particulate material occupying the remainder of said annular compartment;
• and
• a melt runner passage in said elongated inner core interposed between and in flow communication with said inlet and outlet of said sprue bushing assembly.