Classification of Hot Runner Systems
Generally speaking, hot runner system is divided into single-head hot runner system, multi-head hot runner system and valve gate hot runner system. Single-head hot runner system mainly consists of single nozzle, nozzle head, nozzle connection plate, temperature control system and so on.
Single-head hot runner system plastic mold structure is simple. The molten plastic is injected from the injection molding machine into the nozzle connection plate, and after reaching the nozzle head through the nozzle, it is injected into the cavity. It is necessary to control the dimensions d, D, L and the axial displacement of the nozzle by adjusting the thickness dimension of the nozzle connecting plate so that the fixed mold fixing plate presses the end face of the nozzle connecting plate, or directly use the injection molding machine nozzle to hold the end face of the nozzle connecting plate, which can also achieve the same purpose. A lead slot is set at a suitable position of the fixed mold fixing plate to allow the power cable to be led from inside the mold to connect with the terminal block installed on the mold.
Multi-head hot runner system plastic molds are more complex in structure. The molten plastic is injected from the injection molding machine into the nozzle connection plate, flows through the hot runner plate to the nozzle and then reaches the nozzle head and is injected into the cavity. The nozzle of the hot runner system has a radial size D1 fit requirement and an axial size limit requirement with the fixed mold plate. The nozzle head has a radial dimension d with the fixed mold insert to ensure that the molten plastic does not overflow into the non-cavity parts, and the hardness of the fixed mold insert is hardened by about 50HRC. The distance L between the parting surface and the axial positioning surface of the hot nozzle must be strictly controlled, and the size should be determined according to the actual distance L' of the nozzle at room temperature plus the actual extension ΔL of the nozzle at the normal working temperature of the mold. In order to ensure that the nozzle and the hot runner plate fit reliably and do not deform the hot runner plate, an adjustment pad is provided on the top of the nozzle, which together with the axial positioning surface of the nozzle itself limits the movement of the nozzle in the axial direction and effectively controls the possible deformation of the hot runner plate. At room temperature, the adjustment pad is controlled with 0.025mm gap between the hot runner plate and the fixed plate of the mold so that the adjustment pad is properly pressed at working temperature after the mold is heated. The hot runner system has a positioning seat and a positioning pin that together control the position of the hot runner plate in the mold. The positioning seat has a radial dimension D2 with the fixed template, and the depth h must be controlled accurately. The axial direction of the positioning seat plays the role of supporting the hot runner plate, and directly bears the injection pressure of the injection machine. The positioning pin and the hot runner plate fixed plate have the matching requirements. There must be enough clearance between the hot runner plate and the template to wrap the insulation material. The hot runner plate and fixing plate must be provided with sufficient wiring slots to allow the power cable to be led from inside the mold to connect with the terminal block mounted on the mold. There is a radial dimension D1 fit requirement between the nozzle connection plate and the fixed plate of the fixed mold so that the injection head of the injection machine fits well with the nozzle connection plate on the mold. Near the hot runner plate, the fixing plate, hot runner plate fixing plate and fixing mold fixing plate are connected with screws to enhance the rigidity of the hot runner plate.
Valve gate hot runner system plastic mold structure is the most complicated. It has the same structure as the common multi-head hot runner system plastic mold, but there is an additional set of valve pin transmission device to control the opening and closing movement of the valve pin. This transmission device is equivalent to a hydraulic cylinder, using the hydraulic device of the injection machine connected to the mold, forming a hydraulic circuit to achieve the opening and closing motion of the valve needle, controlling the molten plastic into the cavity.
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