Many industrial processes utilize electric heaters to provide localized heating of tools, equipment, or the work product. For example, the injection molding process requires heaters to melt and maintain the fluidity of the thermoplastic injection material. In such a process, it is often necessary to provide supplemental heat in critical areas either at an injection nozzle or in runners that lead to mold cavities. One type of heater that is often used is industrial applications, including the plastic injection molding industry, is a cartridge heater. Such cartridges heaters are typically inserted into bores appropriately located on the processing equipment such as thermoplastic molding equipment. In plastic molding applications the cartridge heater temperatures can exceed 600.degree.-700.degree. F.
A typical prior art cartridge heater is shown in FIG. 1 and is generally designated by the numeral 10.
Cartridge heaters comprise a body portion 12 generally configured as an elongate cylinder with lead wires 13, 14 extending from one end 15 of the body portion 12. The body portion 12 is typically 1/8" in diameter or greater. Caps or plugs 16, 17 enclose the ends of the cartridge heater.
The body portion 12 of conventional cartridge heaters comprise a single heater core 18 with a single metallic sheath 20 encasing the core 18. The core 18 has a ceramic coil base 19 on which is wound a heater element 21. The heater element 21 is surrounded by dielectric material 22 separating the heater element 21 from the sheath 20. Heat generated by the heater element 21 is transferred through the dielectric material 22 to the sheath 20. Typically, the lead wires 14, 16 would each comprise a conductor wire 23 and a flexible nonmetallic sheath or sleeve 24 extending from the end 15 of the body portion 12 out of a plug 25 fixed in the metallic sheath 20. The nonmetallic sleeves 24 are often a glass silk or a silicone fiberglass material which provides a flexible insulating sleeve over the lead conductors 21. Such sleeves 24 generally have a temperature tolerance of under 500.degree. F and require a mechanical connection 26 to the body portion 12. With such sleeves 24, the temperature of the cartridge, especially at the lead wire connection end, must be limited to prevent degradation of the lead wire sleeve 24. This can result in a dead or unheated zone at the lead wire end 15 of the body portion. Moreover, the mechanical connections 26 often lack sufficient mechanical support and the leads often break or fail.
Furthermore, the flexible lead wire emerging from the rigid body portion 12 creates an area highly susceptible to bending and failure at the connection or transition 27 between the lead conductor wire and the body portion. This is a common location of lead failure.
The mechanical connection 26 between the lead wires 14, 16 and the body portion 12 also provide routes for entry of contaminants. Similarly, the engagement 27 between the caps or plugs and the metallic sheath 20 may provide a contamination route. Conventional cartridge heaters have significant problems with failure due to contamination. The contaminants may come from such sources as lubricants, cleaning solvents, plastic materials, or fumes. Wherever the cartridge has seams, seals, or connections on the body portion or the lead wires, the opportunity for contamination exists. Any contamination of the dielectric material or the heater element within the cartridge can cause instantaneous failure of the cartridge heater. This type of failure can be highly destructive due in part to the single sheath construction of conventional cartridge heaters. Said failures may weld the cartridge heater to the host equipment and damage the host equipment causing extended break down periods and difficulty in replacing the destroyed cartridge.
The above difficulties, especially the difficulties in attaching lead wires to the body portions, has limited the reduction in diameter of such cartridge heaters.
In an attempt to remedy mechanical support problems and contamination problems, additional metallic fittings or support members, not shown, have been utilized at the lead wire connections to the body portion for support and/or sealing of the leads. These fittings have not proven to be totally satisfactory in sealing out contaminants and in supporting the lead wires and are not practical in smaller diameter heaters. The fitting or support members also add significant cost to the price of the cartridge heater.
Particular applications of cartridge heaters frequently require nonuniform heat distribution along the length of the cartridge. For example, a specific application may need concentrated heat production at the ends of the cartridge as opposed to concentrated heat production at only the mid-portion of the cartridge. This is difficult or impossible to accomplish in conventional cartridge heaters which have the heater coil extending only longitudinally within the cartridge with caps or plugs between the heater element and the ends of the body portion.