Design of exhaust system for sand core in sand mold casting

The reasonable design of the exhaust system of sand mold casting sand core should comprehensively consider the exhaust hole and exhaust channel of the sand mold casting sand core itself; The sand core of sand mold casting is matched with the mold for exhaust; The sand core of sand mold casting has good air permeability, low gas generation and good exhaust performance. The exhaust direction of sand mold casting sand core is generally upward and along the parting surface, as well as side and downward bottom.

1. Manufacturing method of exhaust hole and exhaust channel of sand core in sand mold casting

(1) Sand cores are cast by sand mold for simple parts. The types and exhaust methods of sand cores for this kind of sand mold casting.

(2) Sand cores are cast by sand mold for complex parts. For complex thin-walled parts such as cylinder, hydraulic pressure, automobile volute and intermediate shell, in particular, the exhaust conditions of sand mold casting sand core shall be improved. The coated sand small sand mold casting sand core can be used. The core head of sand mold casting sand core can be hollowed out with manual tools or the exhaust hole of required size can be drilled with electric tools. As shown in the figure, the middle case oil duct core is marked with red drilling line Φ 2.5 mm × 40 mm。

(3) Vent when hot core box is used for core making. In the design of the hot core box, the vent needle is designed, and the vent hole is formed during core shooting. For the larger coated sand core, the vent hole shall be made when the sand is not fully solidified during sand shooting, or the sand in the sand mold casting sand core shall be emptied by machine turnover to form the vent hole.

(4) Large and medium-sized complex sand mold casting sand core exhaust. For large and medium-sized sand cores with complex shape, vertical and horizontal exhaust ducts shall be set up, and the exhaust duct must be connected to the end face of the core head. Hollow nylon rope is often wound around the core head or core bone of the inner cavity. The exhaust duct between sand cores of sand mold casting is connected through the main exhaust duct and led out of the cavity, or discharged through the exhaust steel pipe drilled around the body in the subsequent process.

(5) Exhaust of large and medium-sized complex sand castings. For large and medium size sand mold castings with complicated inner cavity, in order to ensure the unsmoothed exhaust of sand core sand cores, the exhaust medium is embedded in the core making process (as shown in the illustration, foam plastics are used as exhaust medium). When pouring, the exhaust medium buried in the sand core of sand mold casting will be vaporized rapidly when heated, forming a complete exhaust channel, so as to ensure the smooth exhaust of sand core of sand mold casting.

(6) The 3D printing technology is used to form the flow channel core, water channel core exhaust hole and exhaust channel of complex sand mold casting that can not be made of resin sand and hot core and cold core, so as to ensure the smooth exhaust of sand mold casting sand core and achieve good technical results.

(7) For sand castings with complex inner cavity, how to correctly open the exhaust hole of sand core is often the key to obtain qualified sand castings. However, it is a very difficult operation procedure to set up exhaust holes in complex sand mold casting sand core.

ZHY casting senior engineer team believes that whether the vent hole of sand mold casting sand core is opened or not and its size depends on the type of core making material and the size of sand mold casting sand core and core head. The basic conditions for eliminating the invading pores caused by volatiles in the sand core of sand mold casting are as follows:

Calculation formula for exhaust hole area of sand core in sand mold casting:

Where: γ 1 is the specific gravity of liquid metal, g / cm3; γ 2 is the specific gravity of sand core for sand mold casting, g / cm3; ρ Is the air permeability of sand mold casting sand core, CM4 / (g · s), h is the distance from metal liquid level to the upper top surface of sand core, cm; G is the gassing property of sand core mixture, cm3 / g; C is the gas decomposition percentage of sand core; AV is the total cross-sectional area of sand core vent, cm2; ACM is the contact area between sand core and liquid metal, cm2; AP is the sectional area of the core, cm2, K is the conversion coefficient, 2.166 cm2 / s.

Therefore, the theoretical calculation method of cross-sectional area of exhaust hole of sand core in sand mold casting is more reliable and practical. If the specific gravity of the liquid metal is large, the pouring effective head is high, and the permeability of the sand core is high, that is, the cross-sectional area of the exhaust hole of the sand core can be greatly reduced. Under certain conditions, even complex sand cores can save the complicated process of opening exhaust holes. However, the specific needs to be verified by practice, which shall be subject to the fact that the sand castings do not produce porosity defects.

2. The sand core of sand mold casting is well matched with the sand mold

(1) The structure shall be designed reasonably and properly, the sand core of sand mold casting shall be bonded firmly, and the size of the core head of sand mold casting shall be large enough to prevent high-temperature metal liquid from surrounding the core head and blocking the exhaust duct, which will affect the exhaust capacity of sand mold casting sand core.

(2) The head and the core seat shall be matched and sealed in place. When closing the box, pay attention to seal the gap between the core head to prevent liquid metal from entering the air outlet, blocking the exhaust duct, and producing air holes, choking fire and other defects in sand castings.

(3) The exhaust duct between sand cores and between sand cores and sand molds of sand mold casting shall be enclosed with box sealing strip or asbestos rope to prevent liquid metal from entering the air duct. The upper mold exhaust duct can be drilled manually at the corresponding position of the lower mold exhaust duct and the sand core exhaust hole of sand mold casting.

3. Sand mold casting sand core has good air permeability, low gas generation and good exhaust performance

The coated sand with low gas output and high strength shall be selected. The coated sand core shall be as hollow as possible to improve the air permeability, sintering and drying degree and high temperature resistance of sand mold casting sand core, which can prevent pore defects.

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