According to the different types of casting alloys, it can be divided into casting sand for cast steel, cast iron, cast copper, aluminum alloy castings, and magnesium alloy castings. Different casting alloys have different pouring temperatures and casting properties, and the composition and performance requirements of the used molding sand are often different. The molding sand with clay as a binder is divided into clay wet mold (also known as wet mold sand), clay dry mold and surface dry mold according to its water content before pouring. The following mainly introduces the most widely used wet sand.
Characteristics of wet sand for cast iron castings
When using wet molds to cast iron castings, attention should be paid to preventing defects such as sand holes, pores, sand adhesion and sand inclusions from occurring in the castings. Attention should also be paid to the metallurgical quality and external quality of the castings. The performance requirements and formulation of the molding sand should be reasonably formulated according to the characteristics of the castings.
The water content of wet sand for cast iron parts is generally controlled at 4.5% ~5.5%, the air permeability is generally 50~100, and the wet compressive strength is generally 50~80kPa. Because the compaction rate is very sensitive to the change of moisture in the molding sand, factories often use the method of testing the compaction rate to monitor and control the appropriate moisture in the molding sand to meet the molding requirements. Generally, the compaction rate of wet molding sand is controlled at 50%~60% during manual molding, the compaction rate of molding sand for ordinary machine molding is controlled at 45%~50%, and the compaction rate of molding sand for high-pressure molding is controlled at 40%~45%.
The raw sand used for wet molds of cast iron parts generally uses round or polygonal natural silica sand with a particle size of 21 or 15. The particle size is best evenly distributed on four adjacent sieve numbers, and the retention amount on each sieve number is between 10%~35%. Such raw sand has a small and uniform expansion rate, which is conducive to preventing the occurrence of sand inclusion defects. When producing ductile iron castings, silica sand with a particle size of 30 or 21 is often used to improve the air permeability of the molding sand.
The wet mold sand of cast iron parts generally uses bentonite with good bonding properties. Some factories mix ordinary clay with bentonite. For small iron castings, calcium-based bentonite is generally used in wet sand, and sodium-based bentonite or activated calcium-based bentonite is suitable for medium-sized iron castings to improve the sand resistance of the sand. If the sand is all made of new raw materials, for small iron castings, the amount of ordinary clay added is 6%~8% (or 5%~6% bentonite), and for iron castings that are prone to sand inclusion, the amount of bentonite added is 6%~8%.
Characteristics of Wet sand for steel castings
Compared with iron castings, cast steel castings have the following characteristics: the pouring temperature is as high as 1580C, and the thermal effect of molten steel on the casting is intense and long; generally, bottom pouring is used, and the scouring force of molten metal on the casting is large; molten steel is easy to interact with the molding material, causing defects such as pores, sand adhesion and sand inclusion in the casting. Therefore, the requirements for wet sand for steel castings are higher than those for iron castings.
The moisture content of wet sand for steel castings should be strictly controlled, generally 4%~5%, the air permeability of the surface sand should be greater than 100, the air permeability of the back sand should be above 200, the wet compressive strength should be greater than 5SkPa, and the surface hardness of the mold (A-type table) should be 80~90. The compaction rate of the molding sand is controlled at 50%~55% during manual molding.
Steel castings generally use silica sand with a SiO2 content greater than 96%, a mud content less than 2%, and a refractoriness higher than 1580°. Natural silica sand can be used for small steel castings, and quartz sand or artificial silica sand is suitable for large and medium-sized steel castings. Sometimes, in order to solve the problem of high-temperature stability of molding sand, refractory materials such as magnesia sand and chromite sand are used to prepare surface sand, or quick-drying coatings such as chromite sand powder and zircon powder are applied to the surface of ordinary casting mold cavities to prevent defects such as sticking sand from the castings.
Bentonite or ordinary clay is often used as a binder in wet sand for steel castings, and the amount added is generally 9%~11%. At present, many factories use activated bentonite or natural sodium-based bentonite to improve the hot wet tensile strength of the sand and the ability to resist sand inclusion and scarring. In order to improve the toughness and surface air-dried strength of the sand, an appropriate amount of binders such as tung oil or dextrin are often added. In order to improve the surface strength of the mold, syrup, pulp waste liquid or water glass solution can be sprayed on the surface of the mold, but it should be left for a period of time or dried before closing the mold for pouring.
Post time: May-23-2025