Sand casting is a method of pouring molten metal into a sand cavity and cooling and solidifying it to obtain a casting. When the casting is removed from the sand mold, the sand mold is destroyed, so it is also called one-time casting, commonly known as sand. And the casting process is mainly:
1. Make Mold and core box
The shape and the size of the core box should be determined according to the casting. The appearance, casting, and parts are different. In terms of size, the casting is equal to the part size plus the machining allowance. The shape is equal to the size of the casting plus the shrinkage (the shrinkage when the liquid metal solidifies, the type of casting material is different, and the shrinkage rate is different. The free linear shrinkage of gray cast iron is about 1%, and the free linear shrinkage of cast iron and white iron More than 2%, cast aluminum is about 1.3%.); In shape, castings and patterns must have draft angles (to facilitate mold release). Rounded corners (to facilitate modeling, avoid sand collapse); when castings have holes At the time, the core head should be on the pattern to facilitate the positioning of the core.
2.Configure molding sand and core sand
Molding sand. Core sand is made by mixing sand, binder and water. If the binder is clay, it is called clay sand; if the binder is water glass, it is called water glass sand; if the binder is grease, it is called oil sand. In order to manufacture cavity of various shapes, the molding sand should have good plasticity. In order to prevent the cavity from being damaged by force during manufacturing, repairing, moving and pouring, the molding sand should have a certain strength. In order to exclude the gas in the sand mold and liquid metal, the sand should have good permeability. To prevent sticky sand from castings at high temperatures, the sand mold should be refractory. In order to allow the casting to shrink freely when condensing, the molding sand should be able to automatically collapse at this time, that is, have a certain concession.
3.Core making and boxing
Modeling. Core making and boxing include: placing the pattern, filling and tamping the sand mold, lifting the mold, creating a pouring riser and vent, and boxing. Among them, the most important is the mold. Some castings, if only analyzed from the casting performance of metals, the structure may be reasonable, but from the convenience and possibility of modeling core making, it may be unreasonable, or even undesirable at all. The shape of the casting should be simple, and use less curved surfaces, especially non-circular curved surfaces, because it is difficult to make curved surface patterns; use cores as little as possible to avoid living blocks. Structural design should be done reasonably so that wooden molds can avoid smooth blocks Reduce the number of parting surfaces (that is, the interface between the upper and lower sand molds) as much as possible. The fewer parting surfaces, the fewer sandboxes are used, the less possibility of wrong boxes, and the convenience of modeling. At the same time, in order to facilitate the molding, the structural slope should be designed on the wall perpendicular to the parting surface. In addition, in order to ensure the convenience and possibility of modeling, it should be careful not to use a closed cavity, and to ensure that the core is placed firmly in the sand mold. Convenient, the core should be vented smoothly. For large and complex castings, it is better to use combination castings.
4.Melting and pouring
Special melting furnaces can be used for melting metals, such as cupola for gray cast iron, crucible furnace for non-ferrous metals, and electric furnace for steel casting. The process of pouring liquid metal into a mold is called pouring. When pouring, attention should be paid to pouring temperature and pouring speed. Pouring temperature is too high, liquid gas content is large. Shrinkage is large, easy to produce pores. Shrinkage pores, sticky sand and other defects; Pouring temperature is too low, easy to produce insufficient pouring. Cold partition. Subcutaneous pores and so on. Therefore, the pouring speed should be appropriate, and the pouring should be continuous to keep the outer gate full. The casting temperature of cast steel is high (about 1500 degrees Celsius), the fluidity is poor, the length of the helix is 100 mm, the shrinkage is large, the total volume shrinkage rate is 12.4%, and it is easy to absorb and oxidize during the melting process. Therefore, the casting performance of cast steel is poor, and it is easy to produce sticky sand. Insufficient pouring. Cold partition. Shrinkage cavity. Crack. Porosity and other phenomena; low melting point of copper alloy, good fluidity, tin bronze casting temperature is 1040 degrees Celsius, spiral length is 420 mm . The casting temperature of silicon brass is 1100 degrees Celsius, and the spiral length is 1000 mm. Copper alloys are easy to oxidize when smelting. When smelting, it is necessary to prevent the alloy from oxidizing and burning. The casting temperature of aluminum alloys is lower, generally around 680 degrees Celsius, and the spiral length is 700-800 mm. Aluminum alloy is easy to inhale and oxidize at high temperature, which affects its mechanical properties. Therefore, it is necessary to isolate the contact between the alloy liquid and the furnace gas when smelting, and adopt some purification measures.
5. Cleaning and inspection of castings
After the casting has cooled to a certain temperature, it can be unpacked and cleaned by falling sand. Unpacking prematurely will cause internal stress, deformation and cracking of the casting. General castings are unpacked at around 450 degrees Celsius. But the sand should not be too late, so as not to affect productivity. The time of falling sand is related to the shape, size, wall thickness and casting alloy of castings. For castings with a simple shape and less than 10 kg, it can generally fall within 0.5 to 1 hour after casting. Castings after falling sand must be cleaned to eliminate casting risers, cores, sticky sand and flash. The pouring riser on the gray cast iron parts can be removed with a hammer; the pouring riser on the cast steel parts must be cut off with gas cutting; the pouring riser on the non-ferrous metal parts can be sawed off with a hacksaw. The core of the inner cavity of the casting can be cleaned by hand or by vibrating core machine or hydraulic sand cleaning device. Surface sticky sand, burrs and burrs can be removed by roller, sand blasting, grinding wheel and other methods.