Sand casting core type
Process flow of sand casting
Sand casting refers to a casting method in which castings are produced in sand molds. Steel, iron and most non-ferrous alloy castings can be obtained by sand casting methods.
Because the molding materials used in sand casting are cheap and easy to obtain, the molds are easy to manufacture. It can adapt to single piece production, batch production and mass production of castings. For a long time, it has been the basic process in casting production.
Overview of sand casting
The basic raw materials for making sand molds are foundry sand and sand binder. The most commonly used foundry sand is siliceous sand.
When the high-temperature performance of silica sand cannot meet the requirements of use, special sand such as zircon sand, chromite sand, and corundum sand are used.
In order to make the finished sand mold and core have a certain strength. It will not be deformed or damaged during handling, molding and pouring of liquid metal. Generally, a sand binder is added in the casting to bond the loose sand particles to form sand.
The most widely used molding sand binder is clay, and various drying oils or semi-drying oils. Water-soluble silicates or phosphates and various synthetic resins can also be used as molding sand binders.
The outer sand molds used in sand casting are divided into three types: clay green sand, clay dry sand and chemical hardened sand according to the binder used in the sand and the way it builds its strength.
Sand core type
In order to ensure the quality of castings, the cores used in sand casting are generally dry cores. According to the different binders used in the core. The core is divided into clay sand core, oil sand core and resin sand core.
Clay sand core
Simple core made of clay sand.
Oil sand core
The core sand made of dry oil or semi-dry oil as the binder is widely used. The oil has low viscosity, the mixed core sand has good fluidity, and it is easy to compact when making the core.
But the strength of the newly made core is very low. It is generally accepted by a profiling core plate, and then baked in an oven at 200-300°C for several hours, and the oil is oxidized by air to harden it.
The disadvantages of this core-making method are: the core is easily deformed during demolding, handling and baking, resulting in reduced dimensional accuracy of the casting; long baking time and high energy consumption.
Resin sand core
Various cores made of resin sand. After the core is hardened in the core box, it can be taken out. Which can ensure the tolerance of the shape and size of the core.
According to different hardening methods, the manufacture of resin sand cores is generally divided into three methods: hot core box core, shell core and cold core box core.
Hot Box Method for Core Making
Appeared in the late 1950s. Usually furan resin is used as the core sand binder, and a latent hardener (such as ammonium chloride) is also added.
When making the core, keep the core box at 200-300°C. After the core sand is injected into the core box, the ammonium chloride reacts with the free formaldehyde in the resin to generate acid at a higher temperature, so that the core will harden quickly.
It takes about 10-100 seconds to establish the release strength. The core is made by the hot box method. The dimensional accuracy of the core is relatively high .But the process equipment is complicated and expensive, energy consumption is high, irritating gas is discharged, and the working conditions of the workers are also very poor.
The shell core is made by the thermal method of covered sand, which has high strength and good quality;
Cold core box method
Appeared in the late 1960s. Use urethane resin as the core sand binder. When the core is made by this method. The core box is not heated, and amine steam is blown into it for a few seconds to harden the core.
This method is superior to the hot box method in terms of energy, environment, and production efficiency. In the mid-1970s, the furan resin cold box method, which was hardened by blowing sulfur dioxide, appeared again.
The hardening mechanism is completely different from the urethane cold core box method, but the process characteristics. Such as fast hardening and high core strength, are roughly the same as the urethane cold core box method.
Process flow of Sand casting
Sand casting is the most popular and simplest type of casting that has been used for centuries.
Sand casting is used to manufacture large parts such as gray cast iron, ductile iron, stainless steel and other types of steel. The main steps include painting, mold, core making, modeling, melting and pouring, cleaning, etc.
Drawing
The traditional method is to obtain casting drawings and then send them to the foundry. This process can be completed in the quotation.
Nowadays, more and more customers and foundries use computer-aided design instead.
Sand casting mould
In sand casting, the mold is made of wood or other metal materials. In this process, we ask our engineers to make the mold size slightly larger than the finished product, and the difference is called the shrinkage allowance.
The purpose is that the molten metal acts on the mold to ensure that the molten metal solidifies and shrinks, thereby preventing voids during the casting process.
Core making:
The core is made only by placing resin sand particles in a mold to form a casting on the inner surface. Therefore, the void between the core and the mold eventually becomes cast.
Forming
A pair of molds needs to be prepared during the smelting and molding process. Molding usually involves the support frame of the mold, pulling out the mold to separate it during the casting process, melting the previously placed core in the mold and closing the mold opening.
Clean
The purpose of cleaning is to remove sand, sand, and excess metal in castings. Welding and sand removal can improve the surface appearance of castings.
The burnt sand and scale are all demolished to improve the surface appearance of the casting. Excess metal and other risers are removed. One step closer to welding, polishing and other steps. Finally, check its defects and comprehensive quality.
Selection of pouring position
(1) The important processing surface of the casting should be facing down or on the side.
(2) The broad plane of the casting should face downwards.
(3) The thin-walled part with a larger area should be placed in the lower part of the mold or in a vertical, inclined position.
(4) For castings that are prone to shrinkage, the thicker section should be placed on the upper or side.
(5) The number of cores should be reduced as much as possible, and it is easy to place, fix and vent.
Selection of mold parting surface
1) It is easy to draw the mold and simplify the modeling process.
(1) The selection of parting surface should avoid loose blocks and cores as much as possible to simplify the process of mold making, modeling and molding.
(2) In order to facilitate the extraction of the mold, the parting surface should be selected at the source of the largest section of the casting.
(3) The parting surface should be as straight as possible and the number should be small.
(4) Try to make the casting have only one parting surface, so as to adopt two-box modeling with simple modeling process.
2) Try to put the important processing surface or most of the processing surface and processing datum surface of the casting in the same sand box
3) Try to make the cavity and main core located in the lower box
