Analysis of Defects in Melting Precision Casting Shell - Bulging

Analysis of Defects in Melting Precision Casting Shell - Bulging

The common defects of precision casting shell include cracks, deformation, bulging, ant holes, creep holes, and pores. Today, I will talk in detail about the analysis of the defects of bulging.

1、 Cause of occurrence

①. Various reasons for shell delamination:

② The natural air drying time before surface hardening is too long, which results in poor wetting and bonding with the second layer of paint.

③ After sanding the coating, there is floating sand on the surface; Or the particle size of the surface layer sand is too fine, there is too much dust in the sand, or the sand particles are damp and contain too much water; Causing poor adhesion between the second layer of paint and the surface layer of sand.

④ The short drying time after surface hardening and excessive residual hardening solution on the local surface of the mold shell are reasons for this.

2. The reason for the low high-temperature deformation resistance of the shell:

① The shell has low high-temperature strength and low resistance to high-temperature deformation. The bulging of the mold shell is mostly generated during the processes of dewaxing, roasting, or pouring.

② The high viscosity of coatings, excessive thickness of coatings, accumulation of coatings, low concentration of hardening solution, low temperature, short hardening time, insufficient hardening, and other reasons result in low strength and low resistance to high-temperature deformation of the mold shell.

3. The temperature of the dewaxing solution is low, and the dewaxing time is too long.

4. The baking temperature of small and medium-sized shells is too high and the baking time is too long; Or cooling too quickly; Or improper placement during roasting (stacking, squeezing) and other reasons.

5. Inappropriate coordination between roasting and pouring, such as pouring the shell in a cold state, causing the shell to rapidly heat up and have a high coefficient of thermal expansion, resulting in bulging of the shell.

2、 Preventive measures

1. Various measures to prevent shell delamination:

① It is advisable to maintain the temperature of the shell making site at 22-26 ℃; Properly reduce the viscosity of the coating and increase its fluidity; When necessary, add surfactants appropriately to the coating to improve its coating adhesion and coverage; Pay attention to the operation method, that is, immerse the mold shell into the coating, move it up and down and continuously rotate it. After lifting it up, drip off excess paint to evenly coat and cover the surface of the mold shell. Immediately sprinkle sand, and there should be no local accumulation of paint or sand, and it should be hardened.

② Before the hardening of the shell, it should be naturally air dried for a certain period of time, generally selecting 15-40 minutes, with the shell being "not wet or white"; After hardening, it needs to be air dried to allow the shell to continue to undergo penetration hardening, resulting in a more complete hardening.

③ Check the particle size and dust content of the molding sand (dust content ≤ 0.3%), and control the humidity of the molding sand during use to be ≤ 0.3%; And timely remove excess floating sand on the mold shell; The particle size of the surface layer sand should not be too fine, preferably 40/70 mesh.

2. Various measures to prevent shell deformation:

① Pay attention to the coating operation to ensure even coverage between coatings.

② Park the shell for 12-24 hours before dewaxing; The temperature of the dewaxing solution is controlled at 95-98 ℃, and the dewaxing time is 15-20 minutes, not exceeding 30 minutes.

③ The calcination temperature of ammonium chloride hardened shell is T=850-900 ℃, and the time is 0.5-2 hours; Control the cooling rate of the roasting furnace, etc.

In short, selecting a reasonable coating process, shell making process, roasting and pouring process, or adopting a suitable shell, can improve the shell's ability to resist high-temperature deformation.

Precision machining