The basic raw materials of ordinary sand casting 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. 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 process 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 casting is the most popular and simplest type of casting that has been used for centuries. Sand casting is used to make 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.
Ⅰ. Selection of process parameters of sand casting
1. Machining allowance:
The so-called machining allowance is the surface that needs to be cut on the casting. A certain amount of machining allowance should be reserved in advance. Its size depends on the type of casting alloy, the molding method, the size of the casting, and the machining surface in the mold, the location and many other factors.
2. Draft angle:
In order to make the pattern easy to take out from the mold, the slope added to the vertical wall perpendicular to the parting surface is called the draft angle.
3. Casting rounded corners:
In order to prevent stress and cracks on the wall connections and corners of the casting, and prevent the sharp corners of the mold from damage and sand holes, when designing the casting, the connection and corners of the casting wall should be designed with rounded corners.
4. Core head:
In order to ensure the positioning, fixation and venting of the core in the mold, the shape and core must be designed with a core head.
5. Shrinkage allowance:
Due to the cooling shrinkage of the casting after pouring, this part of the shrinkage size should be added when making the pattern.
Ⅱ. Advantages of sand casting process
1. Clay is rich in resources and low in price. Most of the used clay wet sand can be recycled and reused after proper sand treatment;
2. The cycle of manufacturing the mold is short and the work efficiency is high;
3. The mixed molding sand can be used for a long time;
4. Sand casting application is widely used. Small pieces, large pieces, simple pieces, complex pieces, single pieces and large quantities can be used;
Ⅲ. Disadvantages and limitations of sand casting process
1. Because each sand mold can only be poured once, the mold is damaged after the casting is obtained and must be re-molded, so the production efficiency of sand casting is low;
2. The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor;
3. Castings are prone to defects such as sand washing, sand inclusion and pores.