Shell casting process points

Shell casting process points Shell casting uses a mixture of high-strength thermosetting materials such as silica sand or zirconium sand and resin to form a thin shell mold and stop pouring to obtain the casting, which is called shell casting.

The principle is as follows: Put the preheated (about 280-320℃) metal template on the tipping bucket filled with resin sand, and turn the tipping bucket to make the resin sand fall on the template. The resin melts when it encounters heat and adheres to the sand to form a thin shell. Then reset the tipping bucket to make the unsintered resin sand fall. Heat the template with the shell to soften the thin shell and then peel it.

Generally, the shell is formed in two times. The first layer is the surface layer, which can make the casting bright; the second layer is the reinforcement layer, which is to strengthen the strength of the shell. Heat and soften after each shell formation. The commonly used process has high requirements for mold quality. The shell thickness is very important to ensure the uniformity of the casting material, so the mold layout and mold manufacturing accuracy are very high. For the process without sand box and steel shot filling, because the shell mold is subject to the influence of high-temperature molten iron, the shell thickness should be thicker.

High requirements for coated sand quality

Shell casting has high requirements for shell sand quality. It requires shell sand to have high thermal strength. The shell is not easy to be damaged during transportation, so it requires high room temperature strength; at the same time, from the perspective of preventing pores, it requires low emission; there are also high requirements for particle size, which is more suitable between 15H~21H. Too fine has poor air permeability, and too coarse is easy to stick to sand; for long rod-shaped parts such as crankshafts and camshafts, in order to prevent bending deformation and ensure the dimensional accuracy of castings, the thermal shrinkage rate of shell sand is required to be low.

Strict requirements on molding process parameters Key points of shell casting process

Making process parameters are very important to ensure the quality of shell and casting, mainly including sand injection pressure and time, mold temperature, softening time, etc.

(1) Sand injection pressure and time: Appropriate sand injection pressure and time are important factors to ensure a clear, bright and strong shell. If the sand injection pressure is too low and the time is too short, it is easy to cause the phenomenon of not being blown tight. However, if the pressure is too high and the time is too long, the shell is prone to cracks, leakage and molten iron leakage will occur during pouring, and the casting will have the defect of insufficient pouring.

(2) Mold temperature: The higher temperature should ensure the hardening and softening reaction of the resin in the shell coating sand. If the temperature is too low, the core of the shell will not soften and become "porridge-like"; the higher temperature should ensure that the resin does not overburn and lose its adhesion. If the temperature is too high, the shell will be overburned and "carbon black"; too high or too low temperature will not make the shell obtain high strength, and even cause shelling. Under the condition of not causing "overburning", in order to improve labor productivity, a higher molding temperature is generally used.

(3) Softening time: The purpose of softening is to further react the plastic shell under the action of heat until it is completely solidified into a hard shell. The length of the softening time depends on factors such as mold temperature and shell thickness.