| 摘要 |
This invention relates to an improved electrically heated sprue bushing for injection molding. The sprue bushing has a central runner passage for conveying melt from the molding machine or manifold to the cavity. The runner passage extends through an elongated inner core portion having an undulating outer surface in the form of a helical ridge with a rounded cross section. A single element electrical heating element with a helical configuration is located over the core portion and an outer shell portion is cast over them to form the sprue bushing as an integral unit. The inner core portion is formed of a high strength material such as a beryllium nickel alloy and the outer shell portion is formed of a highly conductive material such as a beryllium copper alloy. The coils of the heating element are separated from each other and threaded in the opposite direction to the helical ridge on the outer surface of the core portion. This allows the highly conductive material to surround the heating element with no insulative air gaps. Thus heat is rapidly conducted away from the heating element preventing burn out, and the undulating surface provides a maximum of surface contact for heat transfer to the core portion. The integral structure provides increased strength to withstand the repeated impact loading which allows the wall thickness of the inner portion to be reduced.
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