Electrically operated heated element with a flow channel for molten plastic
BACKGROUND OF THE INVENTION 
     The present invention relates to an electrically operated heating element with a flow channel for molten plastic and a central hot rod that is connected to an outer jacket at the downstream end of the heating element, said end being provided with outflow apertures. 
     An electrically operated heating element of this type is known from U.S. Pat. No. 4,285,660. The hot rod is connected at its point to the outer jacket by means of webs that demarcate one or more exit apertures for the molten plastic coming from the annular flow channel. There is a section between the point and the shaft of the hot rod that has an extensive area wetted by the molten plastic emerging in the vicinity of the exit apertures. The maximum cross-section of the section is equal to or greater than that of the shaft of the rod. 
     The flow channel for the molten plastic in this heating element is demarcated by the central hot rod and by the outer jacket of the heating element. The cross-section of the heating element is composed of the cross-section of the hot rod, of the annular flow channel, and of the outer jacket. 
     An electrically operated heating element that is immersed in the hot runner that conveys the molten plastic in a hot-runner tool is also known from U.S. Pat. No. 3,520,026. That heating element has a central metal core that is connected at the end remote from the electric connection with a metal outer jacket. A layer of insulation is positioned outside of the point of connection and between the metal core and the outer jacket. 
     The metal core is made out of copper or a similar material and has a low effective electric resistance, whereas the outer jacket is made out of a metal with a high effective electric resistance. Thus there is a high partial voltage at the outer jacket. Since the metal core has a large cross-section, the total cross-section of the heating element is correspondingly large. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a heating element of the aforesaid type that has a small cross-section and wherein the flow resistances in the flow channel are low and the flow of the molten plastic is extensively laminar. 
     This object is attained in accordance with the invention in that the hot rod is a metal tube that demarcates a flow channel and has outflow apertures at its downstream end in the vicnity of a point, in that the outer jacket is made out of a material with low specific and effective electric resistances, and in that there is a layer of insulation between the outer jacket and the tube. 
     In one practical embodiment of the invention the layer of insulation between the outer jacket and the metal tube is between 0.1 and 0.5 mm thick. The insulation can be ceramic. 
     The tube that demarcates the flow channel and is made out of a material with a high specific electric resistance preferably has the same cross-section over its total length, tapering into a point only in the vicinity of its downstream end, in which there are outlfow apertures. 
     Since the metal tube or heat conductor is embedded in a layer of insulation and in a metal outer jacket, it loses no heat. The tubular heat conductor can accordingly be operated at very low electric power. Thus, the molten plastic is not subjected to any thermal overload at the point where the metal tube is welded to the metal outer jacket. The tubular heat conductor is supplied with electric energy at the circumference as dictated by the conditions necessitated by the particular type of molten plastic in the vicinity of the point and hence of the outflow apertures. 
     A preferred embodiment of the invention will now be described with reference to the attached drawings, wherein 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical section through an electrically operated heating element; 
     FIGS. 2 through 4 illustrate stages in establishing a welded connection between the metal tube or heat conductor and the outer jacket of the heating element, and 
     FIG. 5 is an enlarged vertical section through the end of the element of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the heat conductor of the electrically operated heating element is a metal tube 1 made out of a material with a high specific electric resistance, a chrome-nickel steel for example. Metal tube 1 demarcates a flow channel 2 for the molten plastic introduced through an inflow aperture 3. 
     The flow channel 2 has the same cross-section over its total length except that it tapers at the downstream end into a point. In the vicinity of the point are flowthrough apertures 4, whence the molten plastic arrives in a chamber 6 through outflow apertures 5 in the heating element, whence it flows in turn through a nozzle aperture 7. 
     Between metal tube 1 and outer jacket 8, which is welded to the tube in the vicinity of the point, is a layer 9 of insulation. Layer 9 extends beyond outer jacket 8 and also constitutes an intermediate layer between metal tube 1 and an upstream metal section 10 of outer jacket 8 that is welded at its outer end 11 to metal tube 1. Current is supplied to metal tube 1 through a plug 13, a connector disk 12, and jacket section 10. Connector disk 12 is mounted between insulating disks 14 and 15 in the top 16 of a housing. Top 16 is screwed to the bottom 17 of the housing, which accommodates a nozzle aperture 7. Insulating disk 15 rests on a flange 18 in outer jacket 8. Outer jacket 8 is made out of a material with low specific and effective electric resistances and can be made for example of structural or of another type of steel. Layer 9 of insulation is between 0.1 and 0.5 mm thick and can be made out of ceramic. 
     The metal tube 1 in the illustrated embodiment is shaped into a point at its downstream end and has flowthrough apertures 4. As shown in FIGS. 2-4, outer jacket 8 has two webs 19 that are bent into a point and demarcate lateral outflow apertures 5. As will be evident from FIGS. 2 through 4, metal tube 1 is inserted into outer jacket 8 in such a way that webs 19 are displaced 90° with respect to flowthrough apertures 4. Once metal tube 1 has been completely inserted into outer jacket 8, webs 19 are attached to the point of the tube with welded seams 20 as is shown in FIG. 5. 
     As shown in FIG. 5 no insulation is present between apertures 4 and 5. 
     The outer jacket 8 of the heating element rests against the wall of a bore in the bottom 17 of the housing. The bore narrows at the free end of bottom 17 into a chamber 6 that is provided with nozzle aperture 7. 
     It will be appreciated that the instant specification and claims are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.