Before the proportion of molding sand is proposed, the basic range of molding sand properties should be determined according to the alloy type, molding process and method, sand casting characteristics and technical requirements. After determining the performance range and ratio, experiments should be carried out to determine the performance of molding sand, which can be put into production only after reaching the standard.
The raw materials of wet clay sand mainly include raw sand, bentonite, starch additives, etc. In order to obtain stable molding sand performance, the quality of raw materials must be guaranteed, that is, the factory should be equipped with special raw material procurement standards, personnel and equipment to ensure that qualified raw materials enter the factory.
The common testing properties of raw sand of sand castings include water content, mud content, particle size distribution, particle shape, etc; The common testing properties of bentonite include water content, colloid value, bentonite value, blue absorption, etc; Common quality analysis categories of starch additives in factories include water content, ash α Degree of hydration and swelling, etc.
The performance indexes of wet clay sand mainly include hot wet tensile strength, wet compressive strength, water content, compactness, air permeability, surface stability, crushing index, mud content, effective bentonite content, etc.
The hot wet tensile strength can be used as an index to evaluate the quality of bentonite and the anti sand inclusion ability of molding sand. The wet compression strength of high-pressure molding requires relatively high, but attention should be paid not to be too high, which will affect the collapsibility after pouring. Moisture content refers to the moisture content that can be removed after drying at 105-110 ℃. The moisture content of green molding clay sand has a direct impact on all aspects of the properties of molding sand. The water content is too low, the wet compressive strength is high, but the toughness is poor, it is not easy to start the mold and sand is easy to fall off; If the water content is too high, the molding sand has good toughness, but poor fluidity and low wet strength, which is easy to cause pores and sand inclusion, and the sand block is also hard after pouring. Compaction rate refers to the percentage of volume change of wet clay sand under the action of a certain compactness. In actual production, water content is usually controlled by controlling the compactness of molding sand. When the compaction rate of molding sand is too high, that is, it is too wet and sticky, the fluidity of molding sand is not good, the molding sand is not easy to compact, and the hardness of sand mold is low and uneven, resulting in large and large gaps on the surface of sand mold. Sand castings will have defects such as rough surface, mechanical sand sticking, porosity and sand expansion; When the compaction rate is too low, although the molding sand has good fluidity, easy compaction and good and uniform hardness, the molding sand is relatively dry and fragile, poor toughness, easy to be damaged during mold lifting, easy to be broken at the corner of the sand mold and easy to be washed.
Air permeability refers to the ability of molding sand to let gas escape through. If the air permeability of molding sand is too low, choking fire is easy to occur during pouring, resulting in pore defects; If it is too high, the pores between sand particles are large, and the liquid metal is easy to penetrate, resulting in problems such as sand sticking and rough surface. The permeability of wet molding clay sand depends on the type of alloy, pouring temperature, sand casting size, molding process and other factors. Surface stability and crushing index can be used to characterize the toughness of molding sand. In order to obtain high-quality sand castings in production, the molding sand and core are required to have the following properties:
1) Good formability, including good fluidity, plasticity, toughness and non mucous membrane;
2) Sufficient strength, including normal temperature wet strength, dry strength, hardness and high temperature strength;
3) Certain air permeability, small moisture absorption and low gas generation;
4) Higher fire resistance, better thermochemical stability, less expansion and contraction;
5) Good yield, collapsibility and durability.
The properties of molding sand are related to the type, composition and modeling method of materials. The properties, components and control objectives (reference values) of green molding clay sand used for various modeling methods of common sand castings are shown in the table.
| Properties of wet clay sand | Manual modeling | Shock compression modeling | High pressure molding |
| Wet compressive strength / kPa | 65-80 | 70-90 | 80-120 |
| Wet tensile strength / kPa | — | — | 11.0 |
| Wet splitting strength / kPa | — | — | 17.0 |
| Compaction rate /% | 50-60 | 45-55 | 34-40 |
| Moisture /% | 4.5-5.5 | 4.0-5.0 | 3.0-3.5 |
| Air permeability | 60-80 | 79-90 | 90-140 |
| Mud content of molding sand /% | 10-12 | 10-12 | 10-13 |
| Active bentonite content /% | 5-7 | 6-8 | 6-8 |