Hot Runner System Introduction
In the hot runner system, the plastic in the runner between the major running channel of mould and the gate of the molded product is kept at a sufficient temperature during the course of injection processing; therefore, the plastic won’t solidify and no freezing material will be formed in the runner.
(1) Thermal Insulation Runner This is one of the simplest in hot runner system. The poor thermal conductivity of the molded materials in the hot runner as well as the large section of the runner causes the core part to fuse, and the core part passing through the runner keeps injecting new fused materials into the mould, which meanwhile also preserves the fusion state of the core part. For a mould with single cavity, a thermal insulative sprue bush can be used, as indicated in Fig.1-37. Should this principle be used for a multi-cavity mould, then a thermal insulative runner will be formed, as indicated in Fig.1-38. The large section of the runner therein should be as close to the cavity as possible. The air around the sprue bush can reinforce insulation. When using such insulative runner, there requires uninterrupted production and rapid production cycles, since when the production is interrupted, the runner will solidify, and under such occasion the runner must be removed so that new insulative runner can form. If the mould will be more advantageous. Inner surface of the main spure bush and the gate connecting with the cavity most be conical so that the solidified plastics within the gate can be drawn out with the product during each injection cycle. A small conical mark of the gate is thereby left on the molded product.
(2) Heated Hot Runner and Nozzle of Hot Runner Through heating the system, working state of the mould can be restored after the production interruption without filling the runner with fused materials first. Moreover, when setting the injection time for each cycle, consideration for removing frozen runner can be saved. Currently, almost all hot runner system provided by standard mould elements suppliers is well-qualified, and therefore, self-made hot runner system is seldom seen. In principle, the heated runner falls into inner heated hot runner system and outer heated hot runner system.
1) Inner Heated Hot Runner. As indicated in Fig.1-39, in an inner heated hot runner system, the heating element 2 is located in the center of runner groove, with the fused molding materials flowing there around. Since the materials feeding runner 3 lies in the manifold pipe plate 1 which will not be heated, a solidified fused material are provided with broad softening temperature range, or whether the decided runner requires far less outer thermal insulation , which shall contribute to more compact mould structure.
2) Outer Heated Hot Runner. In the outer heated hot runner system, the feeding runner of fused materials is heated exteriorly. The manifold pipe with a runner therein must be sufficiently insulated from the surrounding part of the mould so as to reduce thermal loss and temperature fluctuation. As indicated in Fig.1-40, manifold pipe 8 which contains a feeding runner is heated by a tubular heater22, while fused materials in the manifold pipe are transmitted by a hot nozzle 25 into the gate. A locating ring 13 and a locating pin 20 are set to ensure correct position of the manifold pipe and the clamping plate of the fixed half 4. Except for the aforementioned contact points, certain space is left between the manifold pipe and surrounding part of the mould. A metal reflection plate 9 controls thermal radiation loss of the hot manifold pipe within a minimum range. Usually, the thermal transmission from a hot mould to the cold mould plate of an injection machine is insulated by an insulative 5, the heating is controlled by a thermocouple 23. Fused materials into the manifold pipe. The end hermetic seal 12 at the end of the runner must be in a shape without a dead angle, since the materials shall change direction herein and flow into the hot nozzle. The nozzle 25 is mounted on mould plate 1. Due to the effect of thermal expansion and cold contraction, the manifold pipe shall slide on the nozzle which is harnessed with a mental O-ring 26 so that the contact point can be sealed. When there are no impurities in recycled materials if they are used for production. The filter core 15 which is located in the spure bush 14 shall prove to be very useful.
The design of the nozzle is determined by the requirement for dimension of the gate mark on a product. A gate can be of various forms, depending on the removal measures of the gate mark. Due to technical reasons, the type of gate, apart from meeting specific requirement for the dimension of gate, should ensure minimum pressure loss during the process of injection. Fig.1-41 illustrates a heating nozzle penetrating into the gate hole in the shape of a needlepoint, which forms very minute gate mark. Under such instance, however, the pressure drop and the shearing stress acted on the molding materials are used; the gate may even be blocked.
A hot runner nozzle with a needle valve (valve gate) is indicated in Fig, 1-42 in which the gate mark produced by a valve tends to be clear, round and smooth, and meanwhile low pressure loss is caused during the injection. Prior to injection, the needle valve is controlled by the cylinder, moves rightward and opens the gate hole so that fused materials can pass. Before the fused materials at the gate completely solidify, the needle valve is pushed to the left and closes, leaving a round and smooth gate hole mark. Same with the multi-point hot runner as illustrated in Fig.1043, that in Fig.1-42 is heated by a tubular heater and an inner-hole insert is used to help fused materials alter their flow direction.
Fig.1-43 illustrates a gate from wherein a hot nozzle is matched with four small mould cavities. Similar with the method of using a shearing gate, the multi-edge gate of a product can also equipped with one nozzle, as indicated in Fig.1-44.
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