Keywords: Gravity Casting Gravity Casting Machine Gravity Casting Gravity Casting Machine Gravity Casting Gravity Casting Process Gravity Casting Defects Gravity Machine Tilting Gravity Casting Machine Metal Mold Gravity Casting Machine Gravity Casting Plant Gravity Casting Equipment Gravity Casting Machinery Gravity Casting Equipment Sand Mold Gravity Casting Sand Mold Gravity Casting Process Sand Mold Gravity Casting Defects Metal Mold Gravity Casting Process Metal Mold Gravity Casting Defects; 1、 Overview& nbsp;& nbsp; Gravity casting refers to the process of pouring molten metal into a metal mold under the action of Earth's gravity to obtain castings. Generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, mud casting, etc; The narrow meaning of gravity casting specifically refers to metal mold casting& nbsp;& nbsp;& nbsp;& nbsp;& nbsp; The aluminum alloy metal mold gravity casting method is widely used due to its advantages of high productivity, clean working environment, smooth surface of castings, and dense internal structure. Especially for automotive engine components, many industrialized countries such as Japan, the United States, the United Kingdom, Germany, and Italy use metal mold gravity casting to produce aluminum cylinder blocks, cylinder heads, and pistons for automotive engines. In recent years, many manufacturers in China have also adopted advanced foreign metal mold gravity casting equipment or domestic gravity casting equipment to produce aluminum castings such as automotive engine cylinder blocks, intake pipes, and pistons. Metal mold gravity casting technology is also widely used in industries such as aviation, aerospace, power machinery, and instrumentation. Compared with other casting methods such as die casting, low-pressure casting, and sand casting, aluminum alloy metal mold gravity casting has the following advantages:;1) The overall quality of geometric dimensions and metallographic structure is good& nbsp;& nbsp; 2) Compared to low-pressure and high-pressure casting, gravity casting technology is relatively simple and flexible, with a high yield of castings and the ability to produce more complex castings& nbsp;& nbsp; 3) More conducive to mass production, achieving high automation and simplified maintenance; Under the same production scale, compared to high and low pressure casting, the one-time investment in casting equipment and metal molds and other tooling is lower.1. Metal mold: It is mostly made of cast iron, with gray cast iron being the most commonly used. When the performance requirements of castings are high, alloy cast iron, carbon steel, or low alloy steel can also be used. Metal molds can be divided into four types: whole body, horizontal mold, vertical mold, and comprehensive mold. Among them, vertical mold is easy to set up gating and placing metal cores, easy to exhaust and achieve mechanization, and has a wide range of applications. Due to the good thermal conductivity of the metal mold, the cross-sectional area of the runner is 20% to 25% larger than that of sand mold casting, and the length of the runner is also shorter. The metal mold has no breathability, so an air outlet should be set up on the upper part, an exhaust channel should be set up on the parting surface, and air outlets or ventilation plugs should be added to the parts that are difficult to exhaust& nbsp;& nbsp;& nbsp;& nbsp;& nbsp; 2. Process characteristics of metal mold casting: (1) The mold must be preheated: Due to the good thermal conductivity of the metal mold, castings are prone to casting defects such as cold shuts, inability to pour, and porosity, so preheating is necessary before pouring. The preheating temperature is determined based on the material of the casting, with gray iron castings generally ranging from 250 to 350 ℃ and aluminum castings generally ranging from 200 to 300 ℃. During the working process, attention should be paid to heat dissipation to avoid excessive mold temperature& nbsp; (2) The mold cavity must be coated with paint: Spraying paint on the surface of the mold cavity can protect it from direct erosion and thermal shock of metal liquid, and the cooling rate of various parts of the casting can be adjusted by using the thickness of the coating layer. The coating thickness is usually 0.3-0.4mm. The composition of the coating varies depending on the material of the casting, and the coating used for casting aluminum alloy parts is commonly prepared with zinc oxide powder, chalk powder, water glass, and water. (3) Timely mold opening and removal: The longer the casting stays in the metal mold, the greater the shrinkage. This not only increases the internal stress and crack tendency, but also makes it more difficult to remove the casting. Therefore, after pouring, while ensuring sufficient high-temperature strength of the casting, early mold opening and removal should be carried out. Generally, the opening time for small and medium-sized castings is 10 to 60 seconds after pouring.3. Advantages, disadvantages and applications of metal mold casting:; (1) Advantages:; 1) Metal mold casting with one mold and multiple castings saves molding materials and reduces environmental pollution& nbsp; 2) Simple process, easy to achieve mechanization and automation& nbsp; 3) The casting has high precision, low surface roughness value, and good mechanical properties. But its molding cost is high, the mold has no breathability and yielding properties, and the casting cools quickly& nbsp; (2) Disadvantages:; 1) It is not suitable to cast complex, thin-walled or large castings; 2) When used for alloys with high melting points such as cast steel and cast iron, the mold life is short; Grey iron castings are prone to white structure during casting.3) There are drawbacks such as low metal utilization rate, difficulty in pouring thin-walled complex castings, and relatively low tissue density of castings compared to pressure casting& nbsp;& nbsp; (3) Application: Metal mold casting is mainly used for batch and mass production of non ferrous alloys such as aluminum alloys, copper alloys, and small and medium-sized castings, such as pistons, cylinder bodies, hydraulic pump shells, bearing shells, and sleeves& nbsp;& nbsp; Comparison between Gravity Casting and Sand Casting  Gravity castingsand casting Floor area (with equal production)Approximately 100 square metersApproximately 500 square metersLoss (excluding the loss of the pouring port returning to the furnace) & nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp;& nbsp; 2%~3%6%~8%Pouring port ratio (the ratio of finished castings to pouring ports)4: 1 to 5:1, i.e. the pouring port is small2: 1 to 1:1, i.e. the pouring port is largeproduct qualityThe casting has uniform wall thickness, high tissue density, smooth inner and outer surfaces, consistent external dimensions, and fast precision machining speed.Castings are prone to slag inclusion, sand holes, looseness, rough internal and external surfaces, inconsistent external dimensions, slow precision machining speed, and inaccuracy. 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Yield92%~98%, with a high pass rate (referring to the placement of sand cores), and a higher yield without sand coresHigh scrap rateLabor efficiency(In the case of equal production)Only 7 people are needed (one 90KW power frequency continuous furnace, two sand core machines, two gravity casting machines, one sand mixing machine, employees: two sand core machines, three gravity casting machines, and two edge grinding machines), and each shift (8 hours) can cast 900-1200 moldsMore than 14 peoplePollution statusGravity casting requires low pouring temperature, low pollution, and low noiseCoal and oil furnaces produce smoke during the smelting process, causing significant pollution and noiseOperation situation Easy to operate, can learn to operate in a short period of time (2 hours), work is easy without deep technical skills, and human energy consumption is small.Proficient technical skills are required to operate, resulting in high human energy consumption and high labor intensity.cost accountingFewer operators, low consumption, low scrap rate, increased production, high efficiency, and stable qualityMany operators have high consumption, high scrap rate, low output, unstable quality, and high costs. The above comparative data is for reference only.2、 Aluminum Alloy Metal Mold Gravity Casting Technology;  (1) Aluminum alloy metal mold gravity casting process design: The key to metal mold gravity casting process design is the determination of casting pouring position, the design of pouring system, and the control and adjustment of mold working temperature& nbsp;& nbsp;& nbsp;& nbsp; l) Casting pouring position. It is directly related to the number of metal mold cores and parting surfaces, the position of liquid metal introduction, the smoothness of exhaust, and the complexity of metal mold structure, thus determining the difficulty of metal mold processing and operation, as well as the distribution of casting cooling temperature, thus affecting the production efficiency, dimensional accuracy, and other internal and external quality of castings. Therefore, the pouring position of the casting is an important aspect to consider first in the design of the casting process& nbsp;& nbsp;& nbsp;& nbsp; 2) Pouring system. The design of the casting pouring system determines the internal and external quality of the casting. The pouring system should have functions of skimming, exhaust, and feeding, while ensuring a reasonable solidification and cooling temperature field for the casting. A correct and reasonable pouring and pouring system can not only be estimated based on experience, but also be intuitively predicted by computer numerical simulation during the solidification process of castings, displaying the dangerous parts of castings that may produce shrinkage porosity (holes), guiding process design. By adjusting the pouring and pouring system structure and size, metal structure, controlling cooling speed or adjusting coating thickness, the temperature field can be adjusted to eliminate casting defects, For example, the blank of the aluminum cylinder head of an automobile engine cast by bottom pouring method, although the process measures of setting a large riser in the upper part that almost exceeds the weight of the casting and forced water cooling at the bottom are adopted, it is difficult to adjust the reasonable temperature field of sequential solidification, and it is difficult to eliminate shrinkage defects caused by overheating around the inner gate at the bottom. A certain factory introduced the aluminum alloy metal mold casting machine from Sifa Company in France, which uses this pouring system and the production process is unstable. 100% of the cylinder head needs to be infiltrated, and even for cylinder heads with severe shrinkage and porosity, infiltration cannot meet the pressure resistance requirements; Injecting aluminum liquid directly from the riser, the aluminum liquid is purified by a ceramic filter before entering the mold cavity, ensuring a reasonable cooling gradient of the casting, which is a bottom-up sequential solidification method, eliminating shrinkage defects and significantly improving the yield of the cylinder head. Foseco, a British company, has conducted detailed research and comparative experiments on two pouring methods, and referred to the latter as the DYPUR method. This method simplifies and compacts the mold, saves aluminum liquid, and has a high yield of castings. Even with the use of this method, the small amount of inclusion defects caused by the high drop of aluminum liquid have little impact on the mechanical properties and airtightness of the casting. Of course, in addition to considering the solidification temperature field of the casting, the location, structure, and size of the pouring system also need to consider the complexity of the mold, whether the metal liquid filling is smooth, and whether it has functions such as skimming and exhaust& nbsp; 3) Metal mold working temperature. Similarly, the working temperature of the metal mold and the temperature difference between various parts play an important role in the cooling temperature field of the casting. The forced water cooling and air cooling of the partially overheated area of the metal mold is to ensure that the area maintains normal working temperature, improve production efficiency, and eliminate overheating to ensure a normal cooling temperature field. The advanced and effective method for controlling the working temperature of metal molds is to control the outlet temperature of cooling water, which is regulated by the circulating speed of cooling water. The metal molds designed and manufactured by Italian Fata company and French Sifa company both have advanced water and air cooling devices. In addition, metal inserts with high thermal conductivity or large heat storage capacity can be embedded in locally thick and large hot spots, or the thickness and type of coating can be adjusted to ensure the formation of a reasonable cooling temperature gradient in the casting and eliminate local shrinkage porosity (porosity) defects& nbsp;& nbsp;& nbsp; (2) Aluminum alloy metal mold design and materials;  1) Metal mold design and manufacturing. Good metal mold design and manufacturing technology are key to meeting process design requirements and adapting to large-scale, high-quality casting production. The metal mold design mainly includes the metal mold structure, exhaust system, locking mechanism, cooling system, connecting mechanism, and casting ejection mechanism. Reasonable casting process and metal mold design can only be reflected through advanced metal mold processing and manufacturing technology& nbsp;  2) Metal type materials. Materials suitable for manufacturing metal molds should have sufficient high-temperature strength, certain thermal stability, thermal fatigue strength, and sufficient strength and toughness. Cast iron, cast steel, and copper alloy are generally used as aluminum alloy metal mold materials in China, with an average lifespan of around 10000 to 50000 times; Developed countries generally use the American Iron and Steel Institute (AISI) classification H-13, which is equivalent to domestic 4CrMoVSi steel. This type of steel has high thermal strength and hardness, as well as high wear resistance and toughness. When used as a mold, its casting size is stable and the mold life is long, generally greater than 100000 times. Of course, the lifespan of a mold is not only related to the material, but also to factors such as mold structure, casting alloy material, operation, and management& nbsp; 3、 Aluminum alloy metal mold casting equipment and automation& nbsp;& nbsp; (1) Metal mold gravity casting equipment; There are various structural forms of aluminum alloy metal mold gravity casting machines, which can be divided into fixed and tilting types according to their structural forms. The tilting metal mold gravity casting machine adopts a tilting mechanism with adjustable tilting speed, which completes the pouring process in variable speed rotation, greatly facilitating the filling and exhaust of metal liquid in the mold, greatly improving the quality of castings and reducing the scrap rate of castings; It can be divided into specialized metal mold casting machines and general metal mold casting machines according to their purpose; According to power, it can be divided into manual, pneumatic, electric, and hydraulic metal mold casting machines. Some powerful domestic casting equipment manufacturers can design and manufacture specialized metal mold gravity casting machines according to product and user requirements, such as aluminum cylinder heads, aluminum cylinder bodies, aluminum pistons, aluminum intake manifolds, and other metal mold casting machines, and have been put into long-term use in well-known domestic automobile parts manufacturers& nbsp;& nbsp; (2) The automated production line for metal mold casting includes: molten metal melting and conveying equipment, pouring machines, metal mold casting machines, core lowering and part taking robots, metal mold coating machines, casting transmission and riser cutting machines, heat treatment equipment, and furnace front rapid detection instruments. Automated production lines are suitable for specialized production and can efficiently and cleanly produce large quantities of high-quality castings.