With the rapid development of the automobile industry, the production and use of large-scale stamping parts has increased sharply, and the demand for medium and large-sized molds has also increased significantly. In recent years, a large number of researches and practices have shown that the yield strength of nodular cast iron is higher than that of ordinary steel. The matrix structure is the same as that of steel. The spherically distributed graphite is not easy to generate stress concentration. In addition, ductile iron can be immersed in lubricating oil, and the graphite in the structure has self-lubricating. The effect can effectively reduce the friction in stretching. In theory, the heat treatment process that can be applied to steel is basically applicable to ductile iron. Therefore, the microstructure of the ductile iron can be changed by appropriate heat treatment to improve its performance. The ductile iron material is the preferred material to replace the alloy mold steel, and the high-strength gray cast iron material can also replace part of the carbon steel.

The FM method is a solid self-hardening sand casting method, which is different from the dry sand binderless vacuum molding method. It has many years of production history and mature experience in China. In the production of cast iron parts, furan resin is usually used to fill the mold to cover the vaporized foam mold. After the mold is hardened, the molten iron is poured into the cavity, and the foam mold is burned out in the sand mold to form a casting. This method eliminates the process of demoulding, tumbling, lower core and boxing in general sand casting, which improves the overall dimensional accuracy of the casting, reduces the labor intensity and shortens the production cycle, and is suitable for casting large and medium single piece cast iron molds. method.

How to choose the right coating in the real casting process is a crucial step. In the past, in the process of coating configuration, the different materials and particle size distribution of the aggregates were adjusted according to the requirements of the castings to obtain the technical indexes of different refractoriness, coating strength, sintering performance and gas permeability of the coating. Considering the different mold casting size and wall thickness, the different characteristics of the paste-like solidification of ductile iron and the layered solidification of gray iron, the most suitable coatings often need to be customized. This method is difficult to achieve under the current commercial conditions of coatings. To this end, I have tested the use of two or more different types of coatings, in the use of the site according to the needs of the composite configuration of the process to meet the needs of the field. This not only takes into account the commodification, standardization, but also meets the individual needs.

1, paint selection and performance testing

According to the general requirements of cast iron lost molds, we chose two coatings as the basic coating. One is a graphite alumina-based paint specially used for cast iron parts (hereinafter referred to as E5), and the other is a high-refractory, high-breathing paint (hereinafter referred to as E2) with a brown corundum base as the main aggregate, as a follow-up test. And tested the base paint.

1.1, refractoriness
Refractoriness, also known as refractory degree, refers to the temperature at which a material reaches a certain degree of softening under high temperature. It is an important technical index for evaluating refractory materials. The degree of refractoriness is not a physical constant of a substance, but a technical index whose height is determined by the chemical composition of the material, the degree of dispersion, the proportion of the liquid phase in it, and the viscosity of the liquid phase.

The meaning of refractoriness is different from the melting point. The melting point is the temperature at which the liquid phase and the solid phase of the crystal are in equilibrium. The degree of refractoriness is the temperature at which a multiphase body reaches a certain degree of softening. For most common refractory materials, they are heterogeneous heterogeneous materials, without a certain melting point, and the liquid phase to complete melting is a gradual process. In a fairly wide range of high temperatures, the solid and liquid phases coexist. Therefore, the characteristics of softening and melting of such materials at high temperatures can only be measured in terms of refractoriness.

The test standard for refractoriness is in accordance with GB/T 7322-2007 refractory test method for refractory materials. The test principle is to plant the test cone of refractory material on the frustum together with the standard temperature measuring cone of known refractoriness under the specified conditions. The degree of refractoriness of the test cone is expressed by heating and comparing the bending of the test cone with the standard temperature measuring cone.

The test parameters are: rated temperature 1800 °C, heating rate 0-20 °C / min, cone table rotation speed 3r / min, control mode is microcomputer automatic measurement and control.

It has been found through experiments that the refractoriness of the coating E5 is 1550 ° C, and the refractoriness of the E 2 is 1680 ° C. The refractoriness of the coatings of the two materials is greater than the general pouring temperature of the cast iron parts. Considering that the actual wall thickness and the thermal joint of the casting have different requirements on the refractoriness of the coating, the two coatings can be compounded according to the actual use requirements of the casting, and the ideal state has been achieved.

1.2, gas permeability
There are many factors that affect the permeability of the coating. In the case of selecting the same bonding system and the same coating thickness, the change in the type and composition of the refractory aggregate in the coating has an important influence on the gas permeability. It was verified by making three kinds of representative different aggregates into test samples and measuring the gas permeability of the samples. The test method adopts the coating of the current common metal mesh sheet, and after drying, the gas permeability data is measured in the sand permeability tester.

The larger the particle size of the powder, the higher the gas permeability. This is because as the particle size of the powder becomes larger, the gap between the powders becomes larger, and the gas permeability of the coating naturally increases. And among the three aggregates of brown fused alumina, zirconium powder and bauxite, brown corundum has the best gas permeability. When the average particle size is 0.038mm, the permeability of brown corundum is better than that of the other two aggregates of 0.075mm. When the particle size of the three aggregates reaches 0.075mm, the permeability of brown corundum reaches the highest. It is twice as much as the other two aggregates. It can be seen from the results that the aggregate structure of the brown corundum can improve the gas permeability of the coating.

The structure of the corundum powder is an irregular polygonal crystal, and the powder has a large hardness, and this property easily forms voids between the powders. Moreover, the brown fused alumina has small thermal expansion and uniformity, so that the volume under high temperature is stable and cracked, and the size of the gas permeable pores is not changed, so that the gas permeability of the coating is higher than that of other powders.

Through the gas permeability test of aggregates, we should choose brown fused alumina as the main aggregate to make coating E2 to meet the high gas permeability requirements of customers. However, the one-sided improvement of gas permeability is at the expense of reducing the surface quality of the coating. In order to ensure the gas permeability and also the surface quality of the coating, we have chosen E5 coating made of bauxite as the main aggregate. The two coatings are used in combination according to actual needs.

2, the use of paint

For the production of cast iron parts, graphite is an ideal choice for refractory aggregates. Graphite is easy to produce a reducing atmosphere at the mold interface, inert to iron oxide, and produces a joint between the mold and the casting interface at high temperatures. A bright carbon film that is not wetted by metals and metal oxides, allowing the sand layer to be easily peeled off. Finally, the surface of the casting is smooth, and the graphite has good thermal stability and a small expansion coefficient. The coating can withstand extreme temperature changes at high temperatures without cracking.

According to the experience of our comprehensive majority of customers. Unless the special complicated parts are used, the castings can be directly selected from the E5 type below 3 tons, which can basically meet the requirements of refractoriness and gas permeability; the casting ratio of the castings in the range of 3 to 10 tons of E5 and E2 is 6: 5, that is, 6kgE5 and 5kgE2 are used in combination; if the casting is more than 10 tons, the mixing ratio of paint E5 and E2 is generally required to be 3:10. Of course, it can also be adjusted according to the actual process of the customer when using it on site.

2.1 brushing
In some smaller companies, the brushing process is generally used in view of the site and turnover. Brushing method, as its name suggests, is mainly done by hand through the brush. Relatively speaking, the technical requirements of the coating method for the coating are not as strict as the spraying method and the dip coating method, but since it is mainly done manually, for the operation The technical and experience requirements are relatively high. The biggest disadvantage of the brushing method is the high labor intensity and low production efficiency. Almost all work is done manually.

When using the brushing process, the general control of the composite paint is between 90 and 100, and the paint is applied with 2 to 3 layers. It is important to note that since it is hand-painted, it is inevitable that there are more or less in some places, so it is necessary to prevent a large amount of paint from accumulating in corners and deep pits. Figure 4 shows the mold casting used for manual brushing.

For most large-scale companies, the flow coating process is generally recommended. At this time, the Baume degree of the composite coating is generally controlled between 65 and 70, and the coating is applied to 2 to 3 layers. Since the paint at this time is relatively thin, after the flow coating is completed, the excess paint will naturally flow away when the model is turned over, and there is no accumulation in some places as in the case of brushing. Figure 5 is a view of the case where the mold casting is flow coated.

3, field application practice

3.1 automotive stamping die castings
A large-scale foundry enterprise in Anhui uses furan resin sand as a mold, and uses a lost foam casting process to mass produce automobile stamping die cast iron parts. The coating process is to apply a composite coating with a weight ratio of 6:5 (E5:E2) on the pattern. The coating has a Baume degree of 68, the coating flow is applied twice, the total thickness of the coating is about 0.8 mm, and the pattern after drying is shown in Fig. 6. The casting material is HT300, the casting temperature is 1390 °C, the casting weight is 7.5 tons, the casting result is shown in Figure 7. After the casting is cleaned, the surface is smooth and the size is accurate.

3.2 Large machine tool castings
A foundry enterprise in Jiangsu Province also uses a furan resin sand lost foam process to produce large machine tool castings. The coatings are brushed, the Baume degree is controlled at around 90, and the weight ratio of the coating is 3:10 (E5:E2). The coating is twice, the coating thickness is about 1mm, the pattern after brushing the composite coating is shown in Figure 8; the casting material is HT250, the casting weight is 13.2 tons, and the casting temperature is 1450 °C.

4 Conclusion

For the use of the lost foam process for large mold cast iron parts, the choice and rational use of the paint is particularly critical. Through the research in this paper and the experience of similar factories, we can draw the following conclusions:

1) By selecting the composite use of two different properties of the lost foam coating, the field can be flexibly adjusted according to the process requirements, and the most suitable use effect has been achieved;
2) The refractoriness and permeability of the coating are important indicators for measuring the performance of the lost foam coating;
3) The use of graphite as a refractory aggregate for cast iron parts is an ideal choice, considering its refractoriness can be added as part of the refractory material;
4) The structure of brown corundum powder is irregular polygonal crystal, and the hardness of the powder is also large. This property is easy to form voids between the powders, so that the permeability of the coating is high;
5) The coating process can significantly improve the working efficiency by using the flow coating process, and the uniformity of the coating can be better ensured.