This invention relates to a heat-generating device for molds of injection molding machines and, more specifically, to a tip heater for use on a runnerless injection molding heating probe.
Conventional heat-generating devices commonly called torpedoes or probes for runnerless injection molding are disclosed in U.S. Pat. Nos. 4,516,927 and 4,643,664 to Yoshida. These patents disclose pointed heat-generating devices in which a heat-generating wire disposed within a bore in a non-processed cylindrical material is joined to the non-processed cylindrical material by fusion to form an alloy. The alloy portion is then machined to form a point. The alloy point is commonly called a "tip heater". Tip heaters of the type disclosed by Yoshida are relatively easy to construct; however, tip heaters constructed by fusing the heating wire to a non-processed cylindrical material and then machining the alloy to a point have several disadvantages. Alloy materials which form the tip of conventional tip heaters have unpredictable hardness due to the fusion process by which the alloy is formed. When molding abrasive resins, it is common for alloy point tip heaters to fail due to wear at the alloy point. A need therefore exists for a heat generating device which has a tip heater having a known hardness to match the abrasive environment in which the device is to be used.
Another disadvantage of alloy pointed tip heaters is the tendency for the alloy material to exhibit microscopic porosity to molten polymer resins. Alloy points often have microscopic pores through which molten resin can enter into the body of the heating probe resulting in fouling of the probe and associated equipment over time. A need therefore exists for a probe having a tip heater which is not susceptible to resin porosity.
Another disadvantage of alloy pointed tip heaters is that alloy pointed tip heaters are difficult to repair once the alloy portion becomes damaged due to wear from molding abrasive resins, accidents, or abuse of the molding equipment. The repair difficulty is due to the uncertainty of determining exactly where the wire is fused to the unprocessed material. Since the exact location of the alloy-to-wire junction is unknown, it is difficult to know how much of the tip must be removed in order to effect repairs. A need, therefore, exists for a tip heater for use during runnerless injection molding which provides a pointed portion in which the wire to point junction is known thereby allowing for easy tip heater repair.
Accurate placement of wire to point junction also greatly improves consistency of thermal performance guaranteeing identical behavior between multiple probes in a mold.