Analysis of mechanical sand sticking defects of th

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With the vigorous development of China's automotive industry, the demand market for EPC Castings Used in automotive panels is also growing. However, the mechanical sand sticking defects of castings in the production process always affect their production quality and restrict their production efficiency. The casting process of EPC is generally to brush a certain thickness of fire-resistant paint on the surface of the processed plastic foam pattern, and then put it into the sand box. The self hardening resin sand is used to pound the shape outside, and it is poured under negative pressure to vaporize the pattern, and the liquid metal occupies the pattern position. After solidification and cooling, a new casting method is formed. Mechanical sand bonding, also known as "iron coated sand", refers to the infiltration of molten iron into the pores between sand particles, and the mechanical adhesion of sand particles to the casting surface after solidification. From the definition of technological process and mechanical sand sticking, it can be seen that coating preparation, brushing and molding sand pressing are the key processes to cause mechanical sand sticking of castings

1. The manifestation of mechanical sand sticking

EPC Castings generally have large tonnage and complex structure, and there are many transverse or longitudinal weight reduction holes. In the molding process, the sand mixing equipment is required to produce a large amount of sand. In this way, there are often individual weight reduction holes that do not pound the sand in place or miss the pound, leaving large pores between the coating and the molding sand (as shown in Figure 1), resulting in Mechanical sand sticking in this part. This kind of sand sticking form is exposed on the outer surface, mostly in a slope shape

in addition, the carbon equivalent, silicon, phosphorus and other contents of molten iron are relatively high, so it has good fluidity and strong permeability; The particle size of the original sand used for resin sand is generally 0 60 ~0. 355mm (30 ~ 50 mesh), the particle size is relatively coarse, and the gap between sand particles is large. Therefore, the coating on the surface of plastic foam pattern must have appropriate chemical composition and sufficient thickness to prevent high-temperature molten iron from penetrating into the gap of molding sand particles. When the coating thickness is insufficient, the gap between the bonded sand layer and the casting surface is very small or the coating gap can not be seen, which will also lead to the surface bonded sand. Its form is a uniform layer of "iron coated sand" bonded on the surface of the casting

2. Cause analysis of mechanical sand sticking

there are two reasons for the first type of defects: first, in order to ensure the uniformity of the wall thickness of the casting, the pattern designer designed a structure on the pattern that is not easy to pound sand or cannot pound sand. After chemical reaction, even a particularly narrow hole cavity appeared on the pattern. As shown in Figure 2a and figure 2B, the structures affect the compaction of molding sand at the dead corner of the lower right side and the upper right side respectively. In Figure 3, hole a is often located at the root of the lower side wall of the deep weight reduction hole, which seriously affects the compaction of molding sand at the upper arc of the hole. Although the modeler pays special attention to these parts during operation, the result is still sand sticking; Secondly, the barrel temperature: when the barrel temperature (plastic temperature) is high, the modeler is careless

there are also two reasons for the second kind of defects: the first is the preparation of coating components, the type, fire resistance and mutual ratio of coating aggregates, which have a great impact on the thickness requirements of coating layer and the effect of anti sticking sand. According to the actual use of coating in many EPC manufacturers, the aggregate in the coating is generally flake black graphite powder, high alumina, etc., and the thickness of the coating layer is generally 1 About 5 mm, which can achieve very satisfactory results. However, in the actual preparation process of the coating, due to the inconsistent technical level of the operators, under the same ratio and mixing process conditions, the viscosity and brushing performance of the prepared coating are quite different, which makes the high temperature resistance and anti permeability effect of the coating unstable, easy to cause sand sticking, and interfere with the effective judgment of the real cause of sand sticking of castings; The second is the thickness of the coating, which is too thick, and it costs labor and materials; If the coating thickness is too small, high-temperature molten iron will penetrate through the coating into the gap of molding sand particles, resulting in sand sticking

3. Preventive measures for mechanical sand sticking

through the above analysis, we mainly take the following preventive measures:

(1) strictly review the pattern structure. When reviewing the pattern structure, the casting engineer must carefully study the optical MOF film material to make a breakthrough, analyze whether the pattern structure is reasonable, and completely eliminate the unreasonable structure that affects the coating and prevents the compaction of molding sand, For the convenience of workers, we have carried out repeated cycle testing of human operations. For the type a structure in Figure 2a, holes can be opened in the upper part (as shown in Figure 4); For the structure of type 2B in Figure 2B, it should be changed to the structure of figure 5a, 5b or 5C; Figure 3 type of structure should be inverted 15mm on the edge of the hole × 15mm × Large inclined angle of 45 ° to facilitate the compaction of molding sand at dead corners

(2) strengthen the monitoring of the quality of molding sand, allocate full-time personnel to manage the process quality, and implement the whole process tracking, supervision and inspection of the quality of sand

(3) strictly control the quality of coating preparation and painting, especially the monitoring of coating layer thickness. Strict and flexible operations should be carried out according to materials, parts and time to ensure that the coating meets the process requirements

(4) strengthen the education of quality awareness. For castings with the above sand sticking defects, timely analyze and summarize the causes of sand sticking, and call relevant people to conduct on-site analysis against the defects

(5) adopt incentive mechanism to quantify the severity and quantity of defects, and give certain economic punishment to relevant persons

4. Conclusion

after a factory took the above improvement measures, the scrap rate of mechanical sand sticking decreased by 3. 5% in the second half of 2006 compared with the first half of 2006 51 percentage points, achieving satisfactory results and good economic benefits. (end)

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