First, the hot runner mold overview
1. What is a hot runner?
Hot runner technology is an advanced technology applied to plastic injection molding casting flow channel system, and is a hot spot in the development of plastic injection molding process. The so-called hot runner molding means that the plastic sent from the nozzle of the injection machine to the gate is always kept in a molten state, and it is not required to be solidified as a waste material every time the mold is opened, and the molten material retained in the pouring system can be injected at the time of another injection. Cavity.
The ideal injection molding system should form components of consistent density that are unaffected by all runners, flash and gate nozzles. Relative to the cold runner, in order for the hot runner to do this, it is necessary to maintain the molten state of the material in the hot runner without being sent out with the formed part. The hot runner process is sometimes referred to as a hot manifold system, or as a no-pass molding.
Basically, the hot manifold can be considered as an extension of the barrel and the nozzle of the injection molding machine. The function of the hot runner system is to feed the material to each gate in the mold.
2. Classification of hot runner systems
In general, hot runner systems are classified into single-head hot runner systems, multi-head hot runner systems, and valve gate hot runner systems. The single-head hot runner system is mainly composed of a single nozzle, a nozzle head, a nozzle connecting plate, a temperature control system and the like. The single-head hot runner system plastic mold structure is relatively simple. The molten state plastic is injected into the nozzle connecting plate from the injection molding machine, 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 by adjusting the thickness dimension of the nozzle connecting plate, so that the fixed mold fixing plate presses the end surface of the nozzle connecting plate, controls the axial displacement of the nozzle, or directly uses the nozzle of the injection molding machine to abut the nozzle connecting plate. The end face can also achieve the same purpose. A lead slot is provided at a suitable position of the fixed mold fixing plate, and the power cord is taken out from the mold and connected to the terminal block mounted on the mold.
The plastic mold structure of the multi-head hot runner system is more complicated. The molten plastic is injected into the nozzle connecting plate by the injection molding machine, flows through the hot runner plate to the nozzle, reaches the nozzle head, and is injected into the cavity. The nozzle and the fixed plate of the hot runner system have radial dimension D matching requirements and axial dimensional limit requirements. The nozzle head and the fixed mold insert have a radial dimension d matching requirement to ensure that the molten plastic does not overflow to the non-cavity portion, and the hardness of the fixed mold insert is required to be hardened by about 50HRC. The distance L between the parting surface and the axial positioning surface of the hot nozzle must be strictly controlled. The size should be determined according to the actual distance L' of the nozzle at the normal temperature state plus the actual extension amount ΔL of the nozzle at the normal working temperature of the mold. In order to ensure that the nozzle and the hot runner plate are reliably attached without deforming the hot runner plate, an adjusting pad is arranged above the top of the nozzle, and the adjusting pad together with the axial positioning surface of the nozzle limits the axial movement of the nozzle. And the deformation that may occur in the hot runner plate is effectively controlled. In the normal temperature state, the gap between the adjusting pad and the hot runner plate and the fixed mold fixing plate is controlled to be 0.025 mm so that the mold is heated, and the adjusting pad is just pressed at the working temperature state. The locating seat of the hot runner system and the locating pin together control the position of the hot runner plate in the mold. The positioning seat and the fixed template have the radial dimension D2, and the depth h must be controlled accurately. The axial direction of the positioning seat acts to support the hot runner plate and directly withstand the injection pressure of the injection machine. The positioning pin has a matching requirement with the hot runner plate fixing plate. There must be sufficient clearance between the hot runner plate and the formwork to wrap the insulation. The hot runner plate and the fixed plate must have sufficient wiring slots for the power cord to be drawn from the mold to the terminal block mounted on the mold. There is a radial dimension D1 matching requirement between the nozzle connecting plate and the fixed mold fixing plate, so that the injection head of the injection molding machine cooperates well with the nozzle connecting plate on the mold. In the vicinity of the hot runner plate, the fixed plate, the hot runner plate fixing plate, and the fixed mold fixing plate are screwed together to enhance the rigidity of the hot runner plate.
The valve gate hot runner system has the most complex plastic mold structure. It has the same structure as the plastic mold of the ordinary multi-head hot runner system, and an additional valve needle transmission device controls the opening and closing movement of the valve needle. The transmission device is equivalent to a hydraulic cylinder, which is connected with the mold by the hydraulic device of the injection machine to form a hydraulic circuit, realizes the opening and closing movement of the valve needle, and controls the molten plastic injection into the cavity.
3. Combined hot runner and its advantages
Structure of hot runner system
Although there are many hot runner manufacturers and a variety of hot runner products in the world, a typical hot runner system consists of the following: Hot runner manifold (MANIFOLD), hot nozzle (NOZZLE), temperature controller ( Temperature control box), auxiliary accessories. Hot nozzles generally include two types: open hot nozzles and needle valve hot nozzles. Since the form of the hot nozzle directly determines the selection of the hot runner system and the manufacture of the mold, the hot runner system is often divided into an open hot runner system and a needle valve hot runner system. The manifold is used in a multi-cavity or multi-point feed, single point feed but level offset. The material is usually P20 or H13. The manifold is generally divided into two categories, standard and non-standard. The structure is mainly determined by the distribution of the cavity on the mold, the nozzle arrangement and the gate position. The temperature control box includes the main unit, cables, connectors, and wiring male and female sockets. Hot runner accessories typically include: heaters and thermocouples, runner seals, connectors, and junction boxes.
In a combined hot runner system, the nozzle and manifold form a simple unit. The melt flows directly from the manifold into the nozzle so that no deviations and flow* angles are produced. The threaded nozzle is embedded in the manifold, eliminating leakage between the nozzle and the manifold. Traditional bushing system designs create thermal expansion, and such combined systems are particularly effective at eliminating such leaks.
The combined hot runner system is located at the center of the mold and has little coupling to the mold. The material of manufacture does not require high thermal conductivity, nor does it require clamping and pre-tensioning of the mold. This minimal connection provides a high precision and stable temperature profile, so energy consumption is much lower than with conventional hot runner systems.
The combined hot runner system is capable of directly pre-assembling hydraulic lines that are independent of the mold. Valve ports that are directly driven by hydraulic equipment can also be mounted directly on the system, thus omitting the control valves on conventional machines, making injection molding more flexible. In addition, electrical and hydraulic lines can also be configured according to customer requirements. Since the system is subjected to electrical, temperature, hydraulic or pneumatic testing prior to delivery, the customer will be given a pre-installed system description that can be easily installed in the mold and put into production immediately.
When the mold or system requires routine maintenance, the combined hot runner system can also be removed from the mold in a simple step so that it can be repaired and tested independently of the mold.
Typically, combined hot runner systems reduce maintenance costs. The complete hot runner system is based on a combined pre-design and installation where the heater or thermocouple wires are removed during assembly. The wires connecting the heater to the distribution box are placed in a specially designed conduit which is advantageous for the removal of the mold or hot runner system. At the same time, the integrated hot runner system enables day-to-day maintenance without disassembly, saving time and reducing the chance of assembly errors.
Compared with ordinary flow channel molds, hot runner molds have the advantages of high injection efficiency, good quality of molded parts and material saving. With the development of polymer industry, hot runner technology is constantly developing and perfecting, and its application range is also coming. The more extensive.
The hot runner is heated to ensure that the plastics of the runners and gates remain molten. Since the heating rod and the heating ring are arranged near or in the center of the flow path, the entire flow path from the nozzle exit of the injection molding machine to the gate is in a high temperature state, so that the plastic in the flow path is kept molten, and it is generally unnecessary to open the flow path to take out the condensation after the shutdown. Material, just turn on the flow channel to the required temperature.
Hot runner injection molding was introduced in the 1950s. After a long period of promotion, its market share has increased year by year. In the mid-1980s, hot runner molds in the United States accounted for 15% to 17% of the total number of injection molds. Europe It is 12% to 15%, and Japan is about 10%. However, in the 1990s, hot runner molds in plastic injection molds produced in the United States accounted for more than 40%, and in injection molds for large products accounted for more than 90%.
Advantages of hot runner systems
Saving raw materials and reducing product cost are the most prominent features of hot runner molds. In the ordinary casting system, a large number of handles are generated, and in the production of small products, the weight of the pouring system aggregate may exceed the weight of the product. Since the plastic is always in a molten state in the hot runner mold, the product does not need to be trimmed, and basically no waste processing, so that a large amount of raw materials can be saved. Since no waste recycling, selection, crushing, dyeing and other processes are required, labor saving, time saving, energy saving and consumption reduction are required.
Since the injection material is no longer incorporated into the repeatedly processed gate material, the product quality can be significantly improved, and at the same time, since the pouring system plastic remains molten and the pressure loss during flow is small, it is easy to realize multi-gate and multi-cavity molds. And low pressure injection of large products. The hot gate facilitates the pressure transmission, and can overcome the defects such as depression, shrinkage, deformation and the like formed by the plastic parts due to insufficient feeding.
Wide range of applicable resins and easy setting of molding conditions. Due to the improvement and development of the hot runner temperature control system technology, the hot runner can be used not only for polyethylene and polypropylene with a wide melting temperature, but also for heat sensitive plastics with a narrow temperature range, such as polyvinyl chloride and polyoxymethylene. POM) and so on. For flow-prone polyamides (PA), hot runner molding can also be achieved by using a valve-type hot nozzle.
In addition, the simplification of operation and shortening of the molding cycle are also an important feature of the hot runner mold. Compared with the ordinary flow passage, the opening and closing stroke is shortened, not only the demolding and molding cycle of the workpiece are shortened, but also the automatic production is facilitated. According to statistics, compared with ordinary flow channels, the molding cycle after switching to hot runners can generally be shortened by 30%.
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