Sanzhuji's analysis on green sand casting1.0

At present, green sand is the most widely used and most convenient molding method, accounting for about 60-70% of all sand molds used, but this method is not suitable for large or thick castings. Compared with the dry type, the surface drying type can save the oven, save fuel and electricity, and shorten the production cycle. Therefore, it has been promoted in the production of medium and large iron castings. Usually use coarser sand (to have high air permeability), add more clay and moisture, and sometimes add 1 to 2% of wood chips in the molding sand (to improve the ability to resist sand and scars), and the surface of the cavity must be coated coating.

1. Features of wet casting
The basic feature of the wet casting method is that the sand mold (core) does not need to be dried, and there is no hardening process. Its main advantages are large production flexibility, high productivity, short production cycle, easy to organize flow production, easy to realize the mechanization and automation of the production process; low material cost; saving drying equipment, fuel, electricity and workshop production area; extended The service life of the sand box, etc. However, the use of wet casting can also easily cause some casting defects, such as: sand inclusion, scarring, rat tail, sticky sand, pores, blisters, bulging sand, etc. With the development of casting science and technology, the understanding of the principle of interaction between metal and mold is more profound; the quality of molding sand is more effective; coupled with modern sand processing equipment, the quality of molding sand is guaranteed; advanced molding machinery makes molding sand The compactness is uniform, the mold is smoothly drawn, and the quality of the mold is high, which promotes the expansion of the application range of the wet casting method. For example, in industries such as automobiles, tractors, and diesel engines, thin-walled iron castings with a mass below 300-500kg. have now successfully adopted wet casting. Modern modeling methods include: ordinary machine modeling, micro-seismic compaction modeling, multi-contact high-pressure modeling, injection molding, impact modeling and static pressure modeling. The characteristics of various modeling methods and the dimensional accuracy and surface roughness values ??of the castings produced.

2. Green sand is composed of raw sand, clay, additives and water in a certain proportion. Clay is the main binder of green sand. The clay has cohesiveness and plasticity after being wetted by water, and hardens after drying, and has dry strength. The hardened clay can restore its cohesiveness and plasticity after adding water, so it has better reusability. The usual feeding sequence is to first mix the recycled sand and dry materials such as fresh sand, clay powder, coal powder, etc., and then add water to mix to the required moisture. The formula of molding sand should be based on pouring
The types of alloys, casting features and requirements, molding methods and processes, cleaning methods and other factors determine the range of performance that the molding sand should have, and then based on the types and specifications of various molding raw materials, sand treatment methods and equipment performance, sand-iron ratio and other factors Drafted.

3. Add to the green sand
Coal powder can prevent sand sticking defects on the surface of castings and improve the surface finish of castings. The content of pulverized coal in the molding sand used in wet iron castings is often in the range of 3-8% (mass fraction), depending on the size and thickness of the casting.

4. High-quality molding sand should have various properties necessary for casting high-quality castings. According to the type of casting alloy, the size, thickness, pouring temperature, metal hydraulic head, sand compaction method, compaction pressure, mold extraction method, pouring system shape, position and vent condition, as well as the air-dry condition of the sand mold surface, etc. Different requirements put forward different requirements on the performance of green sand. The most important thing is that the wet performance that directly affects the quality of the casting and the molding process includes moisture, air permeability, strength, compaction rate, deformation, crushing index, fluidity, mud content, effective clay content, particle composition, and burmese. , Sand temperature, gas generation, effective coal content, ignition loss, sand resistance, sand stick resistance, etc.

5. Moisture, optimum wetness and compaction rate are required to obtain the required wet strength and toughness. Clay sand must contain an appropriate amount of moisture. Too dry or too wet is not suitable for modeling, and it is difficult to cast qualified castings. Therefore, the degree of dryness and wetness of the molding sand must be kept within an appropriate range. There are several ways to judge the dryness and wetness of molding sand:

5.1 Moisture is also called water content or humidity. It represents the mass percentage of moisture in molding sand. This is the most commonly used traditional method for determining the degree of dryness and wetness of molding sand in general factories. The principle of measurement is to weigh a certain amount of molding sand and put it into a drying device at 105~110°C to dry it.
After the mass difference is calculated, the moisture of the molding sand is calculated.

5.2 Sand mixing or molding workers who feel that they are actually operating when they are kneaded by their hands often judge the degree of dryness and wetness of the molding sand according to whether the sand is easy to form agglomerates and whether the sand is touched when kneading the molding sand by hand, and also judge whether the sand is soft and deformed during the pinching action. Plasticity of molding sand; strong force when crushing sand mass according to fingers
To judge whether the strength of the molding sand is appropriate.

5.3 The compaction rate refers to the compaction of green sand with a pressure of 1 MPa. The optimal compaction rate of manual and machine modeling sand in dry and wet conditions is close to 50%; it is .35~45% for high-pressure modeling and air punching; extrusion It is 35-40% during modeling; no matter how little the effective swelling, coal and ash content in the molding sand is, as long as the compaction rate is controlled within the above range, the degree of dryness and wetness felt by the hand will be in the most suitable state. At this time, the moisture of the molding sand can be called the most suitable moisture.

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