die-casting mold flow analysis has been widely used in aluminum and magnesium alloy die-casting processes. In contrast, zinc alloy die-casting uses mold flow analysis to lag behind. Friends who have used mold flow software will find that the effect of zinc die-casting mold flow is not as obvious as that of aluminum die-casting, such as the distribution of pores, surface cold lines, and the accuracy of the filling process. In response to the above problems, this article introduces the characteristics of zinc alloy die-casting, including its material characteristics, hot chamber machine performance and process settings, product quality requirements and the characteristics of mold pouring design, so as to put forward the requirements for mold flow analysis.
Characteristics of zinc alloy materials and hot chamber die casting
The production cost of zinc alloy die casting is low. Compared with die-casting materials such as aluminum, magnesium, and copper, zinc alloys have a lower melting point. Take the most commonly used Zamak#3 as an example, the production temperature is around 440C, and the smelting fire consumption is low. https://www.cncmachiningptj.com/laser-cuttingMainly use hot chamber die casting machine for production; the advantages of hot chamber machine are short production cycle and relatively simple equipment (because the pressure chamber is immersed in the solution, there is no need for extra soup time and pressure chamber exhaust time, and no soup equipment. In addition. The solidification time of zinc alloy is very short, no third stage pressurization is needed, and the injection system is relatively simple.). Long mold life (up to a million mold times). The injection parameters of the hot chamber machine are few (although there is more nozzle temperature adjustment, but there is no three-stage boost, the first-stage stroke is very short, the second-speed is lower, and the demolding is relatively simple).
Zinc castings are easy to polish and surface electroplating treatment, there are many choices of colors and effects, and the overall cost is the lowest. Therefore, many hardware accessories with high surface quality requirements, such as sanitary ware, doors and windows, locks, ready-to-wear, leather goods, stationery, furniture, electrical appliances, decorations, etc., are widely used
However, zinc die casting also has many production disadvantages. Due to the high density of zinc alloy (Zamak#3 is 6.7g/cc) and the limitation of equipment, zinc alloy die castings are mostly small and medium-sized pieces. Most of the zinc die castings are produced on hot chamber die casting machines of 250T or below. In addition, zinc alloys have strict requirements on the content of heavy metals, otherwise intergranular corrosion will occur. Zamak#3 zinc ingot has the following specifications: iron<0.075%, lead<0.004%, cadmium<0.003%, tin<0.002%. Therefore, the recycling process of zinc alloy castings should be carefully controlled, and the proportion of recycled materials used should not be high to avoid heavy metal pollution. Therefore, too much casting residual material ratio and high scrap rate have a great influence on the cost structure.
In terms of production process, since it is mainly small and medium-sized parts and high surface finish, the filling time of zinc die-casting is very short, less than two hundredths of a second. In such a short filling time, it is very difficult to remove the entrainment in the cavity. Especially the pores close to the surface of the casting cause blistering during the surface treatment. For electroplated parts with high gloss requirements, any small defects are easily revealed, and the resulting waste loss is also particularly large.
Design of Casting Row for Zinc Die Casting Mould
For fear of affecting the surface quality, such as sand holes that often appear at the gate and slag ladle vent, mold designers tend to set the gate and vent thickness to be very thin, such as the gate as low as 0.3-0.35mm. thickness. This kind of consideration tends to make the gate area too small, causing the production operator to increase the punch speed to compensate for the lack of flow. This makes the gate speed too high, the mold is easy to flush, and the filling process is seriously entrained. Increase the loss of machines and molds. Rough runner design will also improve casting volume. Optimized pouring system can make high-quality castings, so it needs the assistance of software to improve the design.
Characteristics of Zinc Die Casting Mould Flow Analysis
CAD/CAE mold flow analysis is a commonly used auxiliary tool in the development stage of die-casting products. It helps designers to design products and molds and predict possible problems in different runner schemes. It has been widely used in the development of aluminum alloy automotive molds. Because zinc alloy molds are small, the gas distribution is difficult to predict, and the relative application area is low.
There are several differences between CAE simulation and aluminum alloy die-casting in zinc alloy die-casting: 1. The gate thickness is very thin, the resistance is large when filling the mold, and the splashing is serious. 2. The solidification range of zinc alloy is 381-387C. The thin gate solidifies quickly and cannot be fed during the pressure-holding stage, and it is easy to produce shrinkage holes. 3. The surface requirements of the casting are high, so the filling time needs to be very short. 4. The first-speed stroke is short and exhaust is difficult. 5. The filling curve is very different.
Process parameter recommendations
When evaluating the production of zinc castings, mold design engineers often use PQ diagrams to help select the appropriate die-casting model and process parameters, and calculate the gate area. To use PQ diagrams effectively, in fact, there must be an accurate empirical database as the basis for calculation. Using European die-casting experience and laboratory data, the PQ calculation module compiled by CASTLE only needs to input simple die-casting machine, casting information and requirements, and can suggest molds and process parameters, which has high reference value. The user does not need any basic knowledge.
Die casting simulation needs to set the parameters of one, two speed and switching position. The general simulation is to quickly switch to high speed at the gate position, and then quickly stop at the end of the filling; the injection curve of a typical zinc hot chamber is actually not like this. Because the injection system is different and the filling stroke is short, the punch has to switch to high speed before it reaches the nozzle, and the speed is close to the high point when it reaches the nozzle. When the punch reaches the gate, the speed reaches the peak. In the filling stage, the speed of the punch is already decreasing; when it reaches the slag bag or the exhaust port, the punch further slows down.
In order to truly simulate the filling process, CASTLE software, according to the characteristics of the machine, the acceleration and deceleration of the built-in punch, the user only needs to enter a simple one-two-speed data, and the injection curve can be executed close to the real injection curve.
Optimization of runners
As mentioned in the previous section, zinc hot chamber die-casting is different from aluminum cold chamber die-casting. It needs to switch to high speed very early. The high-speed flow of molten metal in the runner is easy to generate entrainment, and the runner must be designed according to the hydrodynamic characteristics as much as possible. CASTLE’s runner optimization module is a quick and practical auxiliary tool to help designers optimize the shape of runners; including detection of drift in the runner, excessive runner speed, or possible local turbulence, etc. You can also see the speed distribution of the gate, especially the design of multiple runners, which requires the overall runner balance. The software’s rapid flow channel optimization reduces the number of filling simulations.
Hybrid grid model
The meshing of the casting is an important step in preparing the die casting simulation. Zinc castings pay attention to beautiful appearance, and most of them are uneven in thickness. It is not easy to make grids to take into account the quantity and model accuracy at the same time. Because the zinc casting gate is very
Thin, not flat, and the gate area is the key position for filling-it is recommended to have at least 3 grids in the thinnest position, which is a great challenge to the mold flow software. In addition, zinc alloys cannot ignore the effect of fine pores on the surface quality, so fine and tiny features (such as fonts) should also be considered. Therefore, do not underestimate that the volume of zinc castings is smaller than that of aluminum castings, and it requires a large amount of mesh.
CASTLE uses an intelligent meshing method. It is based on the finite volume model commonly used in computational fluid dynamics as the main part of the grid. This model is conducive to calculating fluid motion and splashing, and is very suitable for simulating die-casting filling. The software’s unique automatic meshing can form a larger hexahedral mesh at the thick-walled position, and automatically transition to a fine hexahedral mesh at the thin-walled position. At the same time, in order to avoid the formation of stepped shapes on the boundary surface and cause calculation errors, the intelligent division method automatically forms a deformation volume at the position of the complex surface, so that the mesh division can use a relatively small amount of mesh to make an accurate model.
Filling calculation of liquid-gas two-phase
In the real world, we all understand that during high-speed filling, the molten metal is sprayed, and the front of the molten metal will not have a clear boundary, but droplets and air are mixed into a mist. During the filling process, the air is continuously heated, and the space is constantly replaced and compressed by the molten metal. The air is pushed by the squeeze and escapes from the exhaust port. The air pressure will affect the flow of molten metal during filling. If the remaining air is squeezed and cannot escape, it will compress and shrink into pores.
Traditional mold flow analysis software only calculates the flow of molten metal, and rarely considers the influence of changes in air properties in the cavity. In zinc alloy die-casting, we require a very short filling time, and the gate should not be too large, resulting in a high gate speed; if the exhaust area is not large, and the cavity air property changes are not considered, the filling flow state will be Inaccurate. CASTLE software calculates the zinc liquid and air flow at the same time, which can accurately calculate the entire filling process of the zinc liquid. This includes the actual exhaust area and location, as well as the atomization of the molten metal at the gate.
When calculating the flow, most casting simulation software does not consider the rheological properties of the zinc alloy during solidification; the viscosity change in this small temperature range is actually very important for the filling of thin-walled zinc alloy parts. Also important is the calculation of surface tension and surface wettability between zinc/mold and zinc/air. These attributes are all covered in CASTLE’s recharge calculation.
For zinc castings that require high surface finish, tiny pores close to the surface will blisters during electroplating. Using the stomatal ratio Display function, CASTLE software can Display the distribution positions of large pores and micro pores. The attached figure includes the 1% gas content distribution diagram, the combined distribution of residual pores and shrinkage cavities. If the location of the slag bag is right, most of the entrainment gas can be collected.
Recommended exhaust area
The exhaust design has a key influence on the gas content of the casting, which includes the choice of area and location. Some product customers consider the appearance of sand holes, and the slag bag opening may be smaller than the exhaust area, and the slag bag opening is the key exhaust area. In order to facilitate the evaluation of the efficiency of the mold exhaust design, in the exhaust calculation module of CASTLE, the user inputs specific process conditions to automatically calculate the air content of the casting. It also compares the effects of natural exhaust and vacuum die-casting, allowing users to judge whether the exhaust area is sufficient and whether they need to use a vacuum valve to meet the requirements. In the calculation of the attached drawing, the gate area of the casting is suitable, but the exhaust area is seriously insufficient, and the gas content of the casting will be very high. It is recommended that customers increase the exhaust area to improve the air content. At the same time, it can be seen that 22mm2 is the most ideal exhaust area, and a larger area cannot improve exhaust efficiency.
The software also has built-in exhaust effect calculations for different exhaust methods. The attached drawing clearly shows the difference in exhaust efficiency that can be reached by different methods such as vacuum valve, standard exhaust duct, and washboard under the same exhaust area. Help users in the initial stage of mold design, quickly evaluate and select the exhaust method or vacuum die casting.