The plastic mold of single-head hot runner system has a simple structure. The molten plastic is injected into the nozzle connecting plate from the injection molding machine, reaches the nozzle head through the nozzle and is injected into the cavity. It is necessary to control the dimensions D, D and L, adjust the thickness of the nozzle connecting plate to make the fixed template press on the end face of the nozzle connecting plate to control the axial displacement of the nozzle, or directly press the nozzle of the injection molding machine on the end face of the nozzle connecting plate to achieve the same goal. A conductor groove is arranged at the proper position of the fixed template, so as to lead the power cord out of the mold and connect it with the wiring base installed on the mold.
The plastic mold structure of multi-head hot runner system is complex. The molten plastic is injected into the nozzle connecting plate by the injection molding machine, flows to the nozzle through the hot runner plate, reaches the nozzle head, and then is injected into the cavity. The nozzle and fixed template of hot runner system have the requirements of radial dimension D 1 and axial dimension limitation. The nozzle and the fixed die insert have the requirement of radial dimension d to ensure that the molten plastic will not overflow to the non-cavity part, and the hardness of the fixed die insert is required to be hardened to about 50HRC. The distance L between the parting surface and the axial positioning surface of the hot nozzle must be strictly controlled, and its size should be determined according to the actual distance L' of the nozzle at room temperature plus the actual extension δ L of the nozzle at the normal working temperature of the mold. In order to ensure that the nozzle is reliably attached to the hot runner plate without deforming the hot runner plate, an adjusting pad is arranged above the top of the nozzle, which together with the axial positioning surface of the nozzle limits the axial movement of the nozzle and effectively controls the possible deformation of the hot runner plate. At room temperature, the gap between the adjusting pad and the hot runner plate and the fixed template is controlled at 0.025mm, so that the adjusting pad is just pressed when the mold is heated. The locating seat and locating pin of the hot runner system jointly control the position of the hot runner plate in the mold. The radial dimension D2 of the positioning seat and the fixed formwork must match, and the depth h must be accurately controlled. The axial direction of the positioning seat supports the hot runner plate and directly bears the injection pressure of the injection machine. The locating pin has matching requirements with the hot runner plate fixing plate. There must be enough space between the hot runner plate and the template to wrap the thermal insulation material. The hot runner plate and the fixing plate must have enough wiring slots to allow the power cord to be led out of the mold and connected with the wiring base installed on the mold. There is a matching requirement of radial dimension D 1 between the nozzle connecting plate and the fixed template, which makes the injection head of the injection molding machine and the nozzle connecting plate on the mold cooperate well. Near the hot runner plate, the fixed template, the hot runner plate fixed plate and the fixed die fixed plate are connected by screws to enhance the rigidity of the hot runner plate.
The plastic mold structure of valve hot runner system is the most complicated. It has the same structure as the common multi-head hot runner system plastic mold, and a set of valve needle transmission device is added to control the opening and closing movement of the valve needle. The transmission device is equivalent to a hydraulic cylinder. The hydraulic device of the injection machine is connected with the mold to form a hydraulic circuit, which realizes the opening and closing movement of the valve needle and controls the injection of molten plastic into the cavity. First of all, according to the structure and use requirements of plastic parts, the position of feed inlet is determined. As long as the plastic structure allows, the nozzle and nozzle head in the fixed die insert do not interfere with the molding structure, and the inlet of the hot runner system can be placed at any position of the plastic part. The position of the feed inlet of traditional plastic injection molding is usually selected according to experience. For large plastic parts with complex shapes, the entrance position of injection molding can be simulated by computer aided analysis (CAE), and the cooling effect of each part of the mold can be analyzed to determine the ideal entrance position.
Secondly, the nozzle form of hot runner system is determined. The use characteristics of plastic parts and products are the key factors to choose the nozzle form, and the production batch of plastic parts and the manufacturing cost of molds are also important factors to choose the nozzle form.
Thirdly, according to the production batch of plastic parts and the tonnage of injection equipment, the number of cavities of each mold is determined.
Fourthly, the number of nozzles is determined by the position of the inlet and the number of cavities in each mold. If a product is molded, each mold selects a feed port, and as long as there is a nozzle, it is a single-head hot runner system; If a product is molded with multiple cavities in one mold, or a mold has more than two inlets, multiple nozzles are needed, that is, a multi-head hot runner system is selected, except for the mold structure with a runner.
Fifth, according to the weight of plastic parts and the number of nozzles, the radial size of nozzles is determined. The same type of nozzle has multiple size series to meet the molding requirements of plastic parts with different weight ranges.
Sixth, determine the mold structure size according to the plastic structure, then select the nozzle standard length series size according to the thickness size of the fixed mold insert and the fixed mold plate, and finally trim the thickness size of the fixed mold plate and other dimensions related to the hot runner system.
Seventh, determine the shape of the hot runner fixing plate according to the shape of the hot runner plate, arrange the lead groove of the power line on the plate, and design enough cooling water circuits near the hot runner plate, nozzle and nozzle head.
Eighth, complete the design drawing of plastic mold for hot runner system.
Ninth, the mature hot runner system must consider the degree of cooperation between the hot runner system and the plastic mold, that is, the design of the hot half mold. Hot half mold refers to the precision hot runner system processed by professional hot runner manufacturers for customers, which has the characteristics of simple maintenance, high matching accuracy and fast processing speed. Reduce the injection pressure and clamping force. An important step of hot runner is the design concept of hot runner. A detailed design concept, including manifold and pressure plate, will become an important part of mold audit.
Manifolds are used to ensure that melt channels can be arranged in the most efficient way. Ideally, the melt channels are designed in a symmetrical way, and the flow lengths and turns of all downward channels are the same. In the case of a multi-cavity mold or an asymmetric mold, the melt channel may include an artificial length and a turning point so that the system can be properly balanced. This concept helps designers and hot runner designers to ensure the best manifold design.
In order to control the seam line on parts requiring three injection ports, it is necessary to solve the problem of plastic flow balance. Through detailed manifold design, the flow balance and manifold layout can be evaluated to ensure that the downstream runner can meet the customer's mold base requirements. The end result is that a direct injection port and two hot and cold injection ports are combined in a cavity mold (Figure 3).
In addition, pressure plate technology should be adopted to ensure that the closing height and key functions required by customers can be designed. Since the hot runner nozzle is included in the nozzle, mold designer should also confirm whether the proximity and cooling of the injection port can meet the requirements of the hot runner manufacturer.
The main factors to evaluate the hot runner include: flow balance and manifold heat distribution; Channel scale; Strength of manifold material in high pressure application: size of injection port; Cooling and the proximity of the injection port; Resin component capable of resisting abrasion and corrosion.
Hot runner is a complex and superior mold part. In the mold production project, CAE computer-aided engineering analysis, resin testing and design concept can be completed by hot runner suppliers. At the beginning of the project, if the hot runner supplier participates in the work, the designer can further optimize the final product. At present, the main development trends of hot runner molds at home and abroad can be summarized as follows.
Miniaturization of components to realize multi-cavity and multi-gate filling of small products. By reducing the nozzle space, more cavities can be set on the mold, and the output of products and the utilization rate of injection molding machine can be improved. In 1990s, the nozzle developed by Master Company can be as small as15.875 mm; ; Multi-gate nozzle developed by Haschi Company, each nozzle has four gates, and the gate distance can be as close as 9.067mm；; The combined compound nozzle developed by Osco company, each nozzle has 12 gate probes, which can be used to mold 48 cavities. In view of the space limitation of small parts, MoldMaters Company developed a nozzle for small parts in 200 1 year, which includes an integrated heater, a needle tip and a melt channel. The volume diameter is less than 9mm, the gate distance is only 10mm, and the product with a molding weight of 1~30g can be molded. The miniature hot runner system developed by Shelley Hot Runner Company, with gate spacing close to 8.00 mm, is especially suitable for smaller electronic products.
At present, users' requirements for mold design and manufacturing cycle are getting shorter and shorter. Standardization of hot runner components is not only conducive to reducing the repetition of design work and die cost, but also very convenient for the replacement and maintenance of wearing parts. It is reported that Polyshot has developed a quick-change hot runner mold system, which is especially suitable for small injection machines with injection pressure of 70kN. Husky, Presto and Moldmasters have standard nozzles, valve stems and diverter plates, which are convenient for quick replacement and delivery of molds. It only takes four weeks to deliver molds abroad, and it takes two weeks to make molds at home at the earliest.
The overall reliability of hot runner die design is improved. At present, major mold companies at home and abroad attach great importance to the design of hot runner plate and the research and development of pressure distribution, temperature distribution and seal at the joint of hot nozzle. The development and utilization of laminated hot runner injection mold is also a hot spot. Laminated mold can effectively increase the number of cavities, but the requirement for clamping force of injection machine only needs to be increased by 10%~ 15%. Laminated hot runner molds have been industrialized in some developed countries abroad. Laminated hot runner mold has been widely used in domestic injection molding industry, such as disposable tableware, bottle caps, anti-theft buttons and handles of bottle caps and other small batch products. For example, the domestic Sherry Hot Runner Company has accumulated rich experience in the design, manufacture and use of laminated hot runners.
The purpose of improving the material of hot runner element is to improve the wear resistance of nozzle and hot runner, which is used to shape sensitive materials. If the nozzle is made of ductile alloy materials such as molybdenum and titanium, the hot runner element can be manufactured by injection molding and sintering of metal powder.
Develop an accurate temperature control system. It is an effective measure to develop a more accurate temperature control device to control the temperature of molten resin in the hot runner plate and gate during the molding of hot runner mold to prevent resin from overheating degradation and product performance degradation.
Use hot runner for * * * injection. Through the effective combination design of branch pipe and hot nozzle element, * * * injection molding can be combined with hot runner technology to form 3-layer, 5-layer or even more composite plastic products. For example, Kortec developed a melt conveying system and an injection nozzle. Incoe's multi-outlet and multi-cavity injection manifold production line can be used for multi-material and multi-component injection.