Analysis of Defects in Melting Precision Casting Shell - Cracks

Analysis of Defects in Melting Precision Casting Shell - Cracks

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

There are two types of shell cracks: first, cracks appear in the sprue cup, and in severe cases, cracks appear in the sprue cup; The second is the occurrence of cracks on the surface of the shell, with curved and varying depths of cracks on the surface of the shell.

1. Reasons for occurrence

① The temperature of the dewaxing solution is low, and the dewaxing time is too long. Due to the fact that the thermal expansion coefficient of the wax material is greater than that of the shell, slow dewaxing will result in the shell being subjected to various stresses during the dewaxing process; If the strength limit of the shell is exceeded at this point, cracks or even cracking will occur.

② During roasting, the shell enters the furnace at a high temperature, heats up too quickly, or is discharged from the furnace at high temperature for rapid cooling; Multiple calcinations of the shell or mold result in microcracks or even cracks, reducing its strength; The high temperature strength of the shell is low, which causes cracks to occur during calcination.

③ Mechanical damage to the sprue cup during cleaning.

2. Preventive measures

① The wax mold is immersed in the qualified coating, moving up and down and continuously rotating. After lifting, excess coating is dropped to evenly cover the surface of the module; No local accumulation or lack of coating (missed coating) is allowed; And timely and evenly sprinkle sand.

② Appropriately increase the temperature of the dewaxing solution and control it at 95-98 ℃; Shorten the dewaxing time, preferably 15-20 minutes, not exceeding 30 minutes; The key points of dewaxing water glass shell: high temperature and fast. If necessary, improve the dewaxing method.

③ Choose a reasonable roasting process, with a roasting temperature of T=850-900 ℃ and a time of 0.5-2 hours for ammonium chloride hardened shells; And strictly implement it; If necessary, use stage heating or cooling; The shell should not be baked more than twice.

A well roasted shell appears white, powdery white, or pink; The poorly baked shell appears dark or dark gray, indicating a significant amount of carbon residue in the shell.

④ Be careful when cleaning the sprue cup to avoid mechanical damage; If necessary, improve the structure of the sprue cup.

Precision machining