In the field of industrial manufacturing, sand casting is a widely used and long history of metal molding process.
1.Mold making
According to the requirements of the casting drawings, select the appropriate mold material (such as wood mold, metal mold or plastic mold), design and make molds. In the design of the mold, should be divided along the casting of the largest cross-section of the parting surface, in order to ensure that the modeling after the smooth demolding. At the same time, the structural characteristics of the casting and the production batch should be fully considered to optimize the mold structure and improve the durability and production accuracy of the mold. For example, for castings with complex shapes, it may be necessary to design more than one parting surface and core extraction mechanism to ensure the accurate positioning and molding of the sand core.
2. Sand mold preparation
Choose suitable silica sand, and according to the material and requirements of the casting, add the right amount of clay, water and additives, and mix it in a certain proportion. Strictly control the performance index of sand, such as particle size distribution, strength, fire resistance, etc., in order to meet the use of sand mold requirements. In the sand mold preparation process, also need to give full consideration to the use of recycled sand, through reasonable treatment and quality control, improve the recycling rate of sand, reduce production costs and environmental pollution.
3.Molding and core making
Molding
Molding methods can be divided into two categories: manual molding and machine molding. Manual molding has a high degree of flexibility, suitable for single piece, small batch production or complex shape and small batch of castings. Common manual molding methods include sand digging method, loose block method and so on. Machine molding uses high-pressure molding machines or 3D printed sand molds and other equipment, which can significantly improve the molding efficiency and quality, and is suitable for mass production or the production of complex castings. Compared to manual molding, the efficiency of machine molding can be increased by 3 - 5 times.
Core making
The sand core is an important part of the inner cavity of the casting. Depending on the shape and complexity of the cavity, the core box is designed and manufactured accordingly. In the core making process, the prepared core sand is filled into the core box, and the sand core is formed through the process of compacting and curing. For castings with complex cavities, such as turbine blade cooling channels, it is necessary to combine multiple sand cores together and embed air venting holes in the sand cores to prevent air hole defects from occurring during the pouring process. After modeling, the sand molds and cores are dried, usually at 200 - 300°C for 6 - 12 hours, to improve the strength of the molds and cores.
4. Gating system design
The design of gate system plays a vital role in the quality of castings. It consists of outer gate, gate, cross gate and inner gate, and each part should realize smooth transition to reduce the turbulence and oxidation of the melt. When designing the gate system, the location, size and number of gates need to be reasonably determined according to the shape, size, weight of the casting and the fluidity of the metal liquid. For example, for thick-walled castings, the gate should be opened as far as possible in the thick-walled part of the casting or at the hot joints, in order to facilitate the sequential solidification of the liquid metal and make-up shrinkage.
The design of the riser should be set in the hot joint area of the casting, such as thick-walled joints. Commonly used forms of risers include top riser, side riser, and so on. The top riser can effectively reduce the porosity of the casting, generally can be reduced by 30% - 50%. At the same time, in order to adjust the solidification sequence and prevent deformation of the casting, it is also necessary to reasonably set the vent and cold iron. Cold iron is usually placed in the thick-walled part of the casting or the area that may produce shrinkage, in order to accelerate the cooling rate of the part, so that it can realize simultaneous solidification with the thin-walled part or sequential solidification.
5.Melting and pouring
According to the material of the casting to choose the appropriate melting equipment, such as electric furnace or medium frequency furnace. In the melting process, the melting temperature and time should be strictly controlled to ensure that the composition and quality of the metal liquid meet the requirements. For example, the melting temperature of cast iron needs to reach 1400 - 1500℃, and the melting temperature of aluminum alloy needs to reach 680 - 750℃. At the same time, attention should be paid to the requirements of the melting environment to avoid contamination of the liquid metal.
Before pouring, the ladle should be preheated, generally to 600 - 800 ℃, in order to prevent the molten metal temperature from dropping too quickly. When pouring, the flow rate of liquid metal should be controlled, for gray cast iron, the pouring speed is generally controlled at 0.5 - 1.5 kg/s. The pouring operation should be smooth and even, to avoid liquid metal splashing, oxidation and air absorption. After the pouring is completed, let the casting model stand and cool down, for thick-walled parts, can be appropriate to extend the holding time, generally 2 - 4 hours, in order to ensure that the casting is completely solidified.
6. Sand fall and cleaning
When the casting is cooled to a certain temperature, it will be placed on the vibrating sand machine for sand treatment. Vibrating sand dropping machine by generating vibration, so that the sand and casting separation. In the process of sand dropping, process parameters such as vibration frequency and amplitude can be adjusted according to the strength of the sand mold and the structure of the casting, in order to improve the efficiency and quality of sand dropping. After sand dropping, the old sand can be recycled and reused after crushing, sieving and other treatments.
Subsequently, the casting is cleaned to remove the oxide skin and flying burrs on its surface. Shot blasting, sand blasting, grinding and other methods can be used to clean up to improve the surface roughness of the castings. For example, the surface roughness of the casting can be reduced from Ra 12μm to below Ra 6μm by cleaning.
7. Inspection and post-treatment
Inspection
Carry out comprehensive quality inspection on the cleaned castings, including appearance inspection, size measurement and internal quality inspection. Appearance inspection mainly checks whether there are defects on the surface of the castings, such as porosity, trachoma, cracks, etc.; dimensional measurement adopts measuring tools or measuring instruments to measure the dimensional accuracy of the castings to ensure that it meets the design requirements (the general dimensional tolerance level of CT8 - CT10 level); internal quality inspection can be used for non-destructive testing methods, such as X-ray inspection, to check whether there are defects in the casting such as shrinkage, shrinkage holes, cracks, etc., and carry out inspection according to the corresponding testing standards. The internal quality inspection can be carried out by non-destructive testing methods such as X-ray flaw inspection to check whether there are defects such as shrinkage, shrinkage holes, cracks, etc. in the castings and make judgment according to the corresponding testing standards.
Post-treatment
According to the use requirements of castings, qualified castings for the corresponding post-treatment process. Common post-treatment processes include heat treatment and surface treatment. The purpose of heat treatment is to eliminate the casting in the casting process of internal stress, improve the casting hardness, strength and toughness and other mechanical properties. Surface treatment can enhance the corrosion resistance, wear resistance and aesthetics of the castings, such as painting, plating, chemical oxidation and other treatments on the castings.