Die casting dies require high reliability and long service life. It is an effective casting production system that combines die casting machine and die casting process organically. Optimizing die-casting die design, improving process level and providing reliable guarantee for die-casting production are the direction pursued by large-scale die-casting die design.
Die casting die structure
Usually, the basic structure of die-casting mold includes: melting cup, molding insert, mold base, guide, core-pulling mechanism, ejection mechanism and heat balance system.
Design and development process of die casting mold
The mold design and development process, the work that designers need to do in the mold design stage and the overall idea of mold design, including some design and development processes related to standard certification, have a certain preventive effect on possible defects in the design stage.
Key points of die casting die design
Firstly, a reasonable casting shape is established by using rapid prototyping technology and three-dimensional software, and the parting surface, gating system position and mold heat balance system are initially determined.
According to the requirements, the two-dimensional casting drawing is converted into three-dimensional solid data, the reasonable shrinkage rate (generally 0.05% ~ 0.06%) is determined according to the complexity and wall thickness of the casting, and the position and shape of the parting surface are determined. According to the data of the die casting machine, the position and diameter of the injection punch and the number of die castings per die are selected to make the die castings arrange reasonably, and then the gating system and overflow system are modeled in three dimensions.
Secondly, the flow field and temperature field are simulated to further optimize the mold gating system and mold heat balance system.
After processing the data of casting, gating system and overflow system, the boundary condition data such as die casting process parameters and alloy physical parameters are input. The simulation software can simulate the filling process of alloy and the trend of liquid alloy in the mold cavity, and can also simulate solidification and temperature field, further optimize the gating system and determine the position of mold cooling point. The simulation results show the trend of liquid alloy and the distribution of temperature field in the whole filling process in the form of pictures and images, and the parts that may produce defects can be found through analysis. In the subsequent design, the filling effect is improved by changing the position and direction of the inner gate and increasing the slag collection ladle, so as to prevent and eliminate casting defects.
Thirdly, the overall structure of the die is designed according to the three-dimensional model.
While the simulation process is going on, we can design the overall layout of the mold, including the following aspects:
(1) Design the general layout of the die according to the data of the die casting machine.
Determining the injection position and punch diameter is the primary task of general layout design. The determination of injection position should ensure that the die casting is located in the center of the die casting machine plate, and the four pull rods of the die casting machine cannot interfere with the core-pulling mechanism. The injection position is related to whether the die casting can be ejected from the cavity smoothly. The diameter of the punch directly affects the injection ratio, thus affecting the clamping force required by the die-casting die. Therefore, determining these two parameters is the first step of our design.
(2) Design and form inserts and cores.
This paper mainly considers the strength and stiffness of forming inserts, the size of sealing surface, the splicing between inserts, the arrangement of push rod and cooling point, etc. The reasonable collocation of these elements is the basic requirement to ensure the service life of the die. For large molds, it is especially necessary to consider the combination of wearing parts and sealing surfaces, which is the key to prevent the early damage of molds and the loss of aluminum during die casting, and is also the need of large mold exhaust and mold processing technology. The mold forming part shown in Figure 4 adopts the splicing structure of 10 module.
(3) Design the mould base and core-pulling mechanism.
Small and medium-sized die casting dies can directly choose standard die carrier, and large dies must calculate the stiffness and strength of die carrier to prevent the elastic deformation of die carrier from affecting the dimensional accuracy of die casting parts. The key to the design of core-pulling mechanism is to grasp the fit clearance between moving components and the positioning between components. Considering the influence of thermal expansion on the sliding gap during the working process of the die frame, the fit gap of the large die should be between 0.2 ~ 0.3~0.5mm, and the butt gap of the molded part should be between 0.3 ~ 0.5mm, which should be selected according to the size of the die and the heating situation. The square key is used to locate the forming slider and the slider seat. Lubrication of core-pulling mechanism is also the focus of design, which directly affects the reliability of continuous work of die casting die. A good lubrication system is an important link to improve the productivity of die casting.
(4) Layout of heating and cooling channels and selection of thermal balance elements.
Because high-temperature liquid enters the mold cavity at high speed under high pressure, it brings a lot of heat to the mold insert. How to take away this heat is a problem that must be considered when designing dies, especially large die-casting dies. The thermal balance system directly affects the size and internal quality of die castings. Rapid installation and accurate flow control are the development trends of modern mold heat balance system. With the development of modern processing industry, the selection of thermal balance components tends to the design mode of direct selection, that is, the two-dimensional and three-dimensional data of components are directly provided by component manufacturing companies, and designers can choose according to their needs, which can not only ensure the quality of components, but also shorten the design cycle.
(5) Design the push-out mechanism.
The ejection mechanism can be divided into two forms: mechanical ejection and hydraulic ejection. Mechanical push-out uses the push-out mechanism of the equipment itself to realize the push-out action, and hydraulic push-out uses the hydraulic cylinder equipped with the mold itself to realize the push-out action. The key to design the ejection mechanism is to make the center of the ejection resultant force as concentric as possible, which requires the ejection mechanism to have good ejection guidance, rigidity and reliable working stability. For large molds, the weight of the ejection mechanism is relatively large, and the push rod between the components of the ejection mechanism and the mold base is easy to deviate due to the dead weight of the mold, resulting in ejection being stuck. At the same time, the expansion of the mold has a great influence on the ejection mechanism, so the positioning between the ejection component and the mold base and the fixed position of the ejection guide post are extremely important. The ejection guide posts of these molds are generally fixed on the ejection plate. The influence of thermal expansion on the push-out mechanism can be eliminated to the maximum extent by locating the template, shim and die frame with large diameter round pins or square keys. If necessary, the push-out components can be supported by rolling bearings and guide plates, and the lubrication between components should be paid attention to when designing the push-out mechanism. In mold designer, North America, a grease plate specially used for lubricating the push rod is usually added to the back of the movable die set to enhance the lubrication of the pushed parts. As shown in fig. 5, a lubricating oil plate is added to the bottom of the movable mold base, and the oil passage is communicated with the through hole of the push rod. When working, adding lubricating oil can lubricate the push-out mechanism and prevent it from getting stuck.
(6) Design of guiding and positioning mechanism.
In the whole die structure, the guiding and positioning mechanism is the biggest factor that affects the running stability of the die, and also directly affects the dimensional accuracy of die castings.
The guiding mechanism of the mold mainly includes: clamping guide, core-pulling guide and pushing guide. Under normal circumstances, the guide elements should adopt friction pairs made of special materials to reduce wear and resist wear. Good lubrication is also essential, and necessary lubricating oil paths should be set between each friction pair. In particular, the guide structure of super-large slider generally adopts the guide form of copper guide sleeve and hard guide post, and with good positioning form, the slider runs smoothly and accurately.
Mold positioning mechanism mainly includes: positioning between static and moving molds, positioning between push-out and reset, positioning between forming slider and slider seat, positioning between mold base push-out part and mold base, etc. The positioning between static and dynamic dies is a kind of movable positioning, which requires high accuracy. Small molds can be positioned directly through convex and concave surfaces between molding inserts. Special positioning mechanism must be adopted for large die casting die to eliminate the influence of thermal expansion on the positioning accuracy of the die. Other positioning structures are positioning between components, not fixed positioning. Generally, round pins and square keys are used for positioning. The positioning of the convex surface and concave surface between the forming inserts ensures the accurate positioning between the static and dynamic molds and prevents the wrong side of the mold.