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Cnc Milling-Sand Cast

Modern castings are becoming more complex, thinner and more integrated, and higher requirements are placed on the manufacturing level of high-quality castings. These trends have increased the difficulty of casting manufacturing and prolonged the production cycle. The moldless CNC milling sand mold technology is a brand-new manufacturing method that combines modern advanced CNC technology and traditional casting technology. It has the advantages of fast manufacturing speed, low cost, high machining accuracy, and low pollution. It is especially suitable for The rapid manufacturing of various complex sand molds in single parts and small batches plays an important role in improving the manufacturing capabilities and levels of key parts in the aviation, aerospace, and automotive fields. Domestic and foreign researches have been conducted on the CNC milling technology of moldless sand molds, and the CNC milling sand molds have been successfully applied to the manufacture of complex parts. However, there are still some key technologies in the preparation of milling sand blanks and CNC milling processing technology. The problem needs to be solved urgently.
The physical manifestation is that there are easy chipping, jagged edges, cracks, and uneven surface roughness and unevenness of sand molds in the sand mold CNC milling processing.

In response to these problems, this article starts with the preparation of sand blanks suitable for CNC milling and the CNC milling processing technology. The gemstone sand with an angle coefficient ≤ 1.1 is used as the raw sand, and the impact of different particle size sand grain gradations on the performance of the sand blank is analyzed. , The effect of single factor change of resin content and curing agent content on the performance of furan resin self-hardening sand was carried out, and compared with the sand blank of coated sand; in addition, the milling process parameters (spindle speed, feed Speed, milling depth and milling width) affect the surface roughness of sand molds, and reveal the milling mechanism of sand molds. The study found that: using nearly spherical pearl sand as the raw sand, the surface of the sand grains is smooth, and there is no mutual interlocking between the sand grains. At the same time, the raw sand is selected with a particle size of 140/270 mesh, compared with 70/140, 100/ The 200 mesh particle size gradation is more suitable for CNC milling. The main reason is that the arrangement of sand particles is more compact, the fine sand is filled to the coarse particles more fully, and the small particles are closely and orderly distributed between the large particles, so that the sand particles are “bonded”. The number of “bridges” increases, the size of “bonded bridges” decreases, and the microscopic defects of the sand blanks decrease. The content of furan resin in the resin sand has a significant effect on the strength of the sand blank. With the increase of the binder content, the strength and gas evolution of the sand blank show an upward trend. When the addition amount reaches 3%, the tensile strength of the sand blank When it reaches 1.20MPa, the gas generation volume reaches 18ml/g; and the content of the curing agent is greater than 50%, the impact on the strength of the sand blank is not significant, but the gas generation volume is increased. The tensile strength of the coated sand blank with 2% resin content is more than 3MPa, and the gas emission is only 10ml/g. Combining the characteristics of CNC milling and casting, the coated sand blank has obvious advantages when used in CNC milling.

The study found that: using nearly spherical pearl sand as the raw sand, the surface of the sand grains is smooth, and there is no mutual interlocking between the sand grains. At the same time, the raw sand is selected with a particle size of 140/270 mesh, compared with 70/140, 100/ The 200 mesh particle size gradation is more suitable for CNC milling. The main reason is that the arrangement of sand particles is more compact, the fine sand is filled to the coarse particles more fully, and the small particles are closely and orderly distributed between the large particles, so that the sand particles are “bonded”. The number of “bridges” increases, the size of “bonded bridges” decreases, and the microscopic defects of the sand blanks decrease. The content of furan resin in the resin sand has a significant effect on the strength of the sand blank. With the increase of the binder content, the strength and gas evolution of the sand blank show an upward trend. When the addition amount reaches 3%, the tensile strength of the sand blank When it reaches 1.20MPa, the gas generation volume reaches 18ml/g; and the content of the curing agent is greater than 50%, the impact on the strength of the sand blank is not significant, but the gas generation volume is increased. The tensile strength of the coated sand blank with 2% resin content is more than 3MPa, and the gas emission is only 10ml/g. Combining the characteristics of CNC milling and casting, the coated sand blank has obvious advantages when used in CNC milling.

The results of the numerical control milling process experiment of the coated sand blanks show that the milling process parameters have a greater impact on the quality of the processed sand molds, and the surface roughness of the sand molds increases with the increase of the feed speed and the milling depth, and with the increase of the spindle speed On the contrary, it reduces, and the milling width has little effect. The order of influence degree is: milling depth>feed speed>spindle speed>milling width; among them, when the milling layer thickness is greater than 2mm, the surface quality of the sand mold decreases sharply. Therefore, reducing the milling depth and feed rate, and increasing the spindle speed can improve the surface quality of the sand mold. When the optimal milling process parameters are n = 2000r/min, v = 300mm/min, pa = 1mm, and ea = 2mm, the surface roughness of aluminum alloy castings cast by milling coated sand is 8.6μm, which is significantly better than 21.3μm of self-setting sand under the same conditions.

The SEM scanning electron microscope of the milled sand surface showed that the sand particles themselves were not damaged at all, and the fracture was obviously broken at the “bonded bridge” between the sand particles. Further observation of the sand debris and surface micro-morphology found that the number of pits on the surface of the sand with a larger milling layer was significantly greater than that of the sand with a shallower milling layer. As the milling depth increases, the sand produced by sand milling gradually transforms from quasi-continuous powder-based chips to fractured-block-based sand chips. As a result, the sand particles on the free surface of the milling sand mold will emerge with the massive sand chips, forming milling. Cracks and pits of different depths appear on the surface, which is the root cause of the rough surface of the sand mold. On the basis of the above research results, the preparation of typical complex blade sand molds was carried out, and the programming strategies, processing parameters and processing tools suitable for the rough machining, semi-finishing and finishing of the blade sand molds for CNC milling were formulated and optimized. The preparation of the complex sand mold was completed within a period of time, which reduced 51 hours compared to its laser sintering process. At the same time, the processing accuracy and surface roughness of the sand mold were improved, and the rapid milling of precision sand molds was realized.