Analysis of Defects in Melting Precision Casting Shell - Deformation

Analysis of Defects in Melting Precision Casting Shell - Deformation

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

Shell deformation

Feature: The size of the cavity does not meet the requirements of the drawing

1、 Cause of occurrence

① The shell has low high-temperature strength and low resistance to high-temperature deformation. The majority of shell deformation occurs during the processes of dewaxing, roasting, or pouring. During pouring, high temperatures near the gate may cause deformation of the mold shell.

② The viscosity of the coating is high, the coating is too thick, and the coating accumulates; Due to low concentration of hardening solution, low temperature, short hardening time, insufficient hardening, etc., the strength of the mold shell is low.

③ 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 cause the shell to be subjected to various stresses during the dewaxing process, resulting in deformation of the shell during the dewaxing process.

④ The baking temperature of small and medium-sized shells is too high and the baking time is too long; Or cooling too quickly; Improper placement during calcination (stacking, squeezing) may cause local deformation due to imbalanced force on the support.

⑤ The roasting furnace cannot meet the process requirements.

⑥ Improper coordination between roasting and pouring results in higher strength of the shell in the hot state after roasting; When the temperature drops, the intensity decreases accordingly. If the shell is poured in a cold state, the shell will rapidly heat up and have a high coefficient of thermal expansion, leading to deformation of the shell.

2、 Preventive measures

Master, apply, and control the "ratio viscosity temperature" relationship curve of coatings; Pay attention to the coating operation to ensure even coverage of the coating; Control the concentration, temperature, and hardening time of the hardening solution to fully harden the mold shell.

① Park the shell for 12-24 hours before dewaxing; Choose a reasonable dewaxing process, control the temperature of the dewaxing solution at 95-98 ℃, and the dewaxing time is 15-20 minutes, not exceeding 30 minutes; And the placement of the shell during the dewaxing process.

② 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; Pay attention to the placement of the shells during the roasting process to avoid squeezing each other; Control the cooling rate of the roasting furnace, etc. A well roasted shell appears white, powdery white, or pink.

③ Strengthen the regular inspection and daily maintenance of the roasting furnace to meet the requirements of the roasting process.

Precision machining