Not all sand can be used for sand casting production. It must be based on the properties of the casting metal liquid, the size of the casting, and the casting wall thickness, casting mold, and adhesive characteristics of the workpiece, and should be reasonably selected based on the quality requirements of the casting and the economic efficiency of production. For example, Sand used for copper casting usually differs from that used for steel casting.

When selecting, important indicators such as the purity, mud content, particle composition, particle shape, and sintering point of the raw sand are mainly considered.

1. Mud content of sand for sand casting

The mass fraction of the part with particle diameter less than 0.020mm (20μm) in the original sand is collectively called the mud content of the original sand. The mud content of the raw sand has a great influence on the strength, air permeability and refractoriness of the molding sand. A considerable part of the mud contained in the original sand is often not clay minerals, and its bonding properties are much worse than ordinary clay, which reduces the performance of the molding sand. The higher the mud content of the raw sand, the higher the cost of the sand core and mold shell. Therefore, for oil sand, resin sand and raw sand used for investment casting shells, the mud content is best controlled below 0.3%.

2. Particle composition of sand used in sand casting

The particle composition of sand used in sand casting includes particle size, distribution, particle shape and surface morphology. The particle composition of raw sand has a great impact on the strength and air permeability of the molding sand, as well as the dimensional accuracy and surface quality of the casting mold. It is one of the important performance indicators for judging the quality of the sand used for manufacturing.

The particle composition of the sand used for sand casting is determined after sieving with a casting standard sieve. China’s GB/T9442-2010 stipulates that the standard screen consists of 12 screen numbers and a total of 12 layers of chassis, as shown in the table below. The sieve number corresponds to the American ASTM standard and represents the number of sieve holes per unit inch length, referred to as mesh.

GB/T9442-2010 standard mesh number and mesh size for casting as specified

Mesh No./No.	6/1	12/2	20/3	30/4	40/5	50/6
Mesh size/mm	3.35	1.70	0.850	0.600	0.425	0:300
Mesh No./No.	70/7	100/8	140/9	200/10	270/11	chassis/12
Mesh size/mm	0.212	0.150	0.106	0.075	0.053

3. Particle shape of sand used in sand casting

The different contour shapes of various sand grains can be clearly seen using an optical binocular stereomicroscope. The grain shapes of raw sand used for sand casting are roughly divided into three categories, namely round, polygonal and angular, which are represented by the symbols “O”, “口” and “△” respectively, as shown in the figure below.

a). round sand   b). polygonal sand   c). angular sand

If one shape of raw sand is mixed with particles of other shapes, as long as it does not exceed 1/3, it will still be represented by the grain shape symbol of the main particle; otherwise, it will be represented by two symbols, and the larger number of grain shape symbols will be used. Ranked first, for example, □-△, □-O, etc.

The particle shape of sand used in sand casting is related to its mineral composition and formation process. Natural silica sand, such as river sand, lake sand, etc., are generally round or quasi-circular due to mutual friction during hydraulic transportation; mountain sand and silica sandstone are mostly polygonal; while broken artificial silica sand is sharp-angled.

 

In order to produce sand castings, not only advanced and reasonable casting processes and equipments need to be adopted, but also the structure of the parts themselves must meet the requirements of sand casting production, while ensuring the quality of the castings and simplifying the casting process.

The performance of the part structure that adapts to the requirements of sand casting process and ensures the quality of the castings is called the castability of the part structure.

The main steps for designing a qualified cast part include: a. Functional design; b. Modifying and simplifying the design based on casting experience; c. Metallurgical design (selection and applicability of casting materials); d. Considering cost-effectiveness.

When analyzing the castability of the part structure, it is necessary to first consider ensuring the quality and usage requirements of the castings, and prevent the tendency to pursue simplification of the casting process unilaterally.

The casting process plan should be formulated according to the actual production conditions and the production batch of the castings. It involves conducting a process analysis of the parts that are planned to be produced by the casting method.

After comparing various possible schemes, it is formulated based on real and possible conditions. The reasonableness of the structure of the casting is closely related to the type and performance of the casting alloy, the quality requirements and the quantity of the casting, the casting process method, and production conditions.

(1) Minimum wall thickness of sand casting

Under the premise of ensuring performance, in order to avoid defects such as incomplete pouring and cold shut, the casting should not be too thin. Each type of casting alloy has its appropriate wall thickness range. If chosen properly, it can not only ensure the mechanical performance requirements of the casting but also facilitate casting production, while also saving metal materials and reducing the weight of the casting.

The appropriate wall thickness should enable the casting to meet the requirements of mechanical properties without causing casting defects. Under certain casting conditions, the minimum thickness that a casting alloy can fill the mold is called the minimum wall thickness of the casting alloy.

The minimum allowable wall thickness that a casting alloy can fill the mold is related not only to the type of casting alloy but also to the casting method, the size and shape of the casting, pouring temperature, and the thermophysical properties of the mold. Under the conditions of ordinary sand mold casting, the minimum allowable wall thickness of the casting is shown in the following table:

Alloy Type	Outline Dimensions Of Sand Castings
	200	400	400 -800	800~12.50	1250-2000	2000
Carbon Steel	8	9	11	14	16~18	20
Low-Alloy Steel	8-9	9=10	12	16	20	25
High Manganese Steel	8-9	10	12	16	20	25
Stainless Steel, Heat-Resistant Steel	8～10	10-12	12-16	16～20	20-25	—
Gray Iron	3-4	4-5	5-6	6-8	8～10	10-12
Inoculated Chain Iron (Above Ht300)	5-6	6-8	8-10	10~12	12-16	16~20
Nodular Cast Iron	3~4	4-8	8~10	10~12	12～14	14 -16
High Phosphorus Cast Iron	2	2	—	—	—	—
Alloy Type	Outline dimensions of sand castings
	<50	50-100	100-200	200-400	400-600	600-800
Malleable Iron	2.5-3.5	3~4	3.5 ～4.5	4.0～4.5	5~7	6~8
Aluminum Alloy	3	3	4-5	5~6	6~8	8～10
Brass	6	6	7	7	8	8
Magnesium Bronze	3	5	6	7	8	8
Tin-Free Bronze	6	6	7	8	8	10
Magnesium Alloy	4	4	5	6	8	10
Zinc Alloy	3	4	—	—	—	一

(2) Strive for uniform wall thickness, reduce thick sections, and prevent hot spots

In the design of sand casting structures, it is often encountered that two or three walls are connected. Properly designing the wall connection is of great significance to preventing casting defects such as shrinkage cavities, shrinkage porosity, cracks, deformation, sand sticking, and improving casting quality. Heat nodes should be minimized and dispersed, and wall intersections should be avoided.

Castings with uneven thickness will form larger internal stresses during cooling, which can easily cause defects such as shrinkage cavities, shrinkage porosity, and cracks. Therefore, unnecessary thick sections should be eliminated and the arrangement of ribs should minimize intersections to prevent the formation of hot spots, as shown in the figure below.

a) un-reasonable                                  b) reasonable

(3) The inner wall of a sand casting should be thinner than the outer wall

 

During casting, the inner wall of the casting has poorer heat dissipation conditions, slower solidification speed, and lower mechanical properties compared to the outer wall.

At the same time, thermal stress is easily generated at the junction of the inner and outer walls during the casting process, leading to cracks in the casting.

For alloy castings, shrinkage cavities and shrinkage porosity are also prone to occur. Therefore, the inner wall of the casting should be thinner than the outer wall to ensure uniform cooling of the inner and outer walls, reduce internal stress, and prevent cracks. The improvement of the structure of a sand casting is shown in the figure left.