SAND CASTING EQUIPMENT.

The sticky sand defects on the surface of sand castings can be divided into mechanical sticky sand and chemical sticky sand. Mechanical sand sticking is caused by molten metal drilling into the pores between the sand grains. Chemical sticking is the chemical reaction between SiO2 in the sand mold and metal oxides such as FeO to form ferrous silicate (ferroolivine) and adhere to the casting. The chemical composition of ductile iron castings is high in carbon, no ferrous silicate is produced, and the sticky sand produced is only mechanical sticky sand.

1. Nodular cast iron large pieces sticky sand
The production of large parts adopts dry clay, resin sand and water glass sand. High-quality coatings such as zircon powder, chromite powder, brown corundum powder, magnesia powder, graphite powder and other refractory aggregates are usually used for pouring larger castings. However, in order to save production costs, paint manufacturers may add some inexpensive materials such as quartz powder, bauxite powder, coke powder, etc. to the powder, which will reduce its anti-sticking effect.

2. Wet type medium and small ball iron parts stick to sand
Most of the casting sand contains coal powder, which produces a large amount of reducing gas during casting, which will not cause chemical sand sticking. The following discusses the influence of various factors on mechanical sand adhesion.

2.1 Sand tightness
The compactness of manual molding and seismic pressure molding is generally low. The sand particles on the surface of the sand mold are relatively loose, and the molten metal is more likely to penetrate the pores between the sand particles. The pits and corners of the sand mold cavity are also partially There may be looseness. Workers can use their fingers to plug tightly, and use the tip of a hammer to squeeze locally. Whether the high-productivity and high-density molding has local looseness depends on the fluidity of the molding sand. Therefore, many factories try to reduce the compaction rate of the molding sand to improve the fluidity of the molding sand. It is very effective to use microseism in the process of adding sand and compacting. In addition, it also depends on the setting of the compaction pressure of the molding machine.

2.2 Pouring temperature
The molten metal has high temperature and good fluidity, and it is easy to penetrate into the pores between sand grains to produce mechanical sand sticking and rough surface. However, in consideration of avoiding defects such as porosity and cold insulation in the casting, the pouring temperature cannot be reduced arbitrarily. Especially when producing complex thin-walled castings, higher pouring temperatures are required.

2.3 Granularity and permeability of molding sand
On the one hand, the thickness of the wet sand should be ensured smooth exhaust after pouring. On the other hand, the air permeability of the wet sand should not be too high to prevent the molten metal from easily penetrating into the pores of the sand. There are many vent holes in the sand molds for small parts produced by hand molding, and surface sand is often used. The sand grains can be finer, and the air permeability of the molding sand is 30-60. The grain size of the machine-molded green sand is roughly 70/140 mesh, and the air permeability is mostly in the range of 60-90. If the high-density sand is relatively dense, the sand must have higher air permeability. The particle size is mostly 50/140 or 140/50 mesh, and the air permeability is mostly concentrated in 100-140. Generally, the raw sand for the sand core of internal combustion engine castings has a particle size of 50/100 mesh, which is coarser than that of molding sand. In the long-term production, a large amount of core sand will be mixed into the molding sand to make the particle size of the molding sand coarse, resulting in the air permeability of some factories as high as 160 or more. Unless paint is sprayed on the surface of the sand mold, the surface of the casting will be rough or even localized mechanical sand. If the particle size of the molding sand becomes coarse, some foreign factories add new fine-grained sand of 100 mesh and 140 mesh when mixing the sand to correct it.

2.4 Sand coating
For larger wet ductile iron castings with a machine shape of more than 100 kilograms, the lower sand mold can be sprayed with alcohol-based paint, and the core can be set and molded after being ignited. Generally, the upper type does not need to spray paint because the metal hydraulic head is smaller than the lower type. Another advantage of spray coating is to improve the erosion resistance of the sand surface.

When using resin cores or oil sand cores to produce general ductile iron cast iron that does not require high internal cavity, in order to prevent molten metal from drilling into the sand core, you can apply machine oil or other parts on the surface of the hardened sand core where the molten metal is easily penetrated Coating paste prepared by binder and graphite powder, quartz powder or other refractory powders can be cored after cooling and drying. When producing cast iron parts (such as internal combustion engine cylinder heads, body, hydraulic system valves, etc.) that require high internal cavity cleanliness and smoothness, the sand core must be coated as a whole and then dried on the surface. When producing small ductile iron parts by hand, a soft brush is often used to carefully apply graphite powder on the surface of the wet sand mold and sand core. Some are sprayed with a mixture of graphite and water, and poured after natural drying. Graphite powder can fill pores and is not wetted by molten iron, which makes it difficult for molten iron to penetrate between sand grains.

2.5 explosive sticky sand
Sometimes it can be seen that sand sticking defects appear on the upper surface of the casting position, which is different from the usual stick sand on the lower surface of the casting position. This is caused by an explosion when the molten iron level is adjacent to the upper surface of the cavity. At the beginning of pouring, the moisture of the sand mold evaporates and condenses on the upper surface of the cavity with a lower temperature. When the metal liquid level rises and contacts the upper surface of the cavity, the moisture suddenly evaporates and explodes. When the molding sand has high water content and pulverized coal content, the sand mold has poor ventilation conditions and the pouring speed is too fast, explosive sand sticking is more likely to occur.

2.6 Amount of pulverized coal in molding sand
Coal powder is the most important molding sand additive for wet cast iron parts to prevent sand sticking and improve surface finish. However, the commercial coal powder is mixed. The following will discuss the quality of pulverized coal, effective pulverized coal content, pulverized coal substitutes and new conditions in foreign applications.

2.7 Quality of coal powder
The better pulverized coal should meet the following performance indicators:

 2.71 ash content is not more than 10% (the lower the better, the foreign general commercial pulverized coal is not more than 7%). The purpose of measuring the ash content of coal powder is to check the amount of gangue remaining in the coal powder. The use of pulverized coal with excessive ash content in the foundry will not only prevent the castings from sticking to sand, but will also produce a large number of sand holes and pore defects. 

2.72 The volatile content is between 30% and 38% (preferably 32% to 37%). The purpose of the determination of the volatile content is to determine the type of bituminous coal, which is the raw material for pulverized coal. If the volatile content is less than 30%, it indicates that the coal is not pure. If it is higher than 38%, it may contain gas coal. 

2.73 Coke slag characteristics 5 to 6 grades (3 to 4 grades are still available). After measuring the volatile content, observing the residue in the crucible can get the characteristics of coke slag. According to the national standard coke slag characteristics are divided into 8 levels, which reflect the amount and amount of gas-solid-liquid three-phase colloids generated during the heating and retorting Duration. Coke slag characteristics 5 to 6 grade coal powder has the best anti-sticking performance. Such coal powder can block the pores between the surface sand grains during pouring, without causing the sand grains to form coarse particles that are bonded together. 

2.74 The amount of bright carbon generated is 10-18% (preferably above 12%). Bright carbon is a fine crystal precipitated by gas phase pyrolysis of the volatiles of coal powder in a reducing atmosphere at high temperature. It coats the surface of the cavity sand to prevent the sand from being wetted by the molten iron, thereby preventing sand sticking. defect.

2.8 Effective pulverized coal content
It is also necessary to know whether the effective pulverized coal that can work in the molding sand is enough, and how much pulverized coal remains in the old sand, in order to determine how much pulverized coal should be added during sand mixing. The method adopted abroad is to indirectly estimate the effective content of coal powder by measuring the volatile content, ignition loss, and total carbon content of molding sand and old sand. These tests are tedious and time-consuming, and no specific figures of effective coal powder can be obtained. In our country, the method of measuring gas generation is relatively time-saving, and the effective coal content can be measured and calculated in a few minutes. The principle is based on the amount of gas generated after the pulverized coal in an effective state is heated. Generally, the gas volume of high-density molding sand can be in the range of 16-22mL/g, and the gas volume of ordinary machine-shaped sand can be 20~26mL/g.

3. Substitute for coal powder
Materials such as vegetable resin, oily bentonite and starch can be used to replace coal powder. Among them, the anti-sticky sand effect of starch material is basically equivalent to that of high-quality coal powder. But it is only suitable for the production of gray iron castings. If used for the production of ductile iron parts, it may produce subcutaneous pore defects, because it cannot produce sufficient reducing atmosphere. Some products are mainly coal powder, mixed with a small amount of other materials, and may also have anti-sticking effects. However, there are also some "pulverized coal substitutes" commodities whose specific formulas are unknown and lack quality evaluation and testing standards. Users should choose carefully and judge by pouring test. A small sand mixer can be used to add the same raw sand and bentonite, and different anti-adhesive sand materials. The mixed sand should try to maintain the same or close air permeability, the same molding hardness, and the same pouring temperature. Compare the surface finish of the casting, and then make a selection decision.

There are two main types of anti-sticking sand products produced abroad: 3.1 High-efficiency pulverized coal: 20-40% high softening point petroleum pitch is added to the pulverized coal to increase the bright carbon content to 12-20%, and the ability to resist sand is greatly improved. . Now our country also has several companies producing and supplying. 3.2 Mixed additives: it is a mixture of high-quality bentonite and high-quality coal powder. If you have special needs, you can also add starch, wood flour and other materials. The characteristics of products in one production line of a large foundry factory are similar, and the ratio of bentonite to pulverized coal does not need to be changed frequently. It is easy to control and manage with mixed addenda and simplify equipment. The formula is jointly formulated by the engineers of the supplier and the buyer according to the specific casting production conditions. Transported to the workshop by bulk tanker, pneumatically transported into the material tank. The user only needs to add one kind of additive when mixing sand.

4. Additional amount of pulverized coal

The supplementary amount of pulverized coal when mixing with single sand firstly depends on the quality of the pulverized coal itself. It is also affected by the sand/iron ratio, casting thickness, pouring temperature, cooling time, cleaning method, and specific requirements for the surface finish of the casting, etc. and other factors. xinyuanzhu group combined the actual casting production with a sand/iron ratio of 10:1, and ECOSIL coal powder consumption was 18kg/t Fe. That is, 10 tons of molding sand is used per ton of iron, and 18kg of ECOSIL coal powder is added, which is equivalent to 0.18% of the added amount of mixed sand coal powder.

If the sand/iron ratio in most factories in our country is about 6:1, the amount of ECOSIL pulverized coal mixed sand should be 0.30%. According to the foundry manual "molding materials" (page 103～104 of the second edition), the high-density molding line of my country Dongfeng Motor Corporation, FAW Casting Co., Ltd., China YTO Group Corporation, Shanghai Automotive Engine Co., Ltd., and Nanjing Texi Cast Iron Co., Ltd. 14 kinds of wet single sand formulas. When mixing sand, the maximum amount of pulverized coal is 3 to 4%, and the minimum is 0.3 to 0.5%.

In addition, there are 4 types of single sand for shock modeling from FAW, Teksid, and Shanghai Engine Works. The maximum amount of mixed sand coal powder is 3 to 5%, and the minimum is 1 to 1.25%. The supplementary amount of pulverized coal in most of the above-mentioned Chinese factories is much higher than that in the case of Southern Chemical Company. The reason is that the quality of pulverized coal used in these factories is too low, and high-efficiency pulverized coal is not used. Xinyuanzhu group combined with the experience of using high-efficiency coal powder, and concluded that the supplementary amount of coal powder for a single sand of general wet iron castings is between 0.15 and 0.3%, and that for individual thick and large parts is 0.5%. The sand-to-iron ratio of the air flushing line and the extrusion line of a foundry (in the same industry) in Fushun is 11:1 and 7.5:1 on average, and the high-efficiency pulverized coal added in the sand processing system shared by the two lines is only 0.08～0.12% Adding amount of pulverized coal sand mixed with ECOSIL in Germany. It can be seen that even though the price of high-efficiency pulverized coal is slightly higher (less than twice that of ordinary pulverized coal), the consumption is only a fraction of that of ordinary pulverized coal. The use of high-quality and high-efficiency pulverized coal not only greatly reduces production costs, but also saves storage and transportation costs. Moreover, the mud content and water content in the molding sand are greatly reduced, and the toughness, air permeability and mold release properties are improved. Not only the surface of the casting is smooth and clean, but also the defects such as pores and sand holes will be reduced.