The carbon content of ductile iron castings in lost foam casting is generally 3.5% ~ 3.8%, which is easy to produce carbon defect inclusions. Considering the gas evolution and carbon content, the copolymer is selected for the white mold of lost foam casting nodular cast iron. The copolymer balances the gas evolution and solid carbon content of EPS and ep-mma (expandable polymethylmethacrylate), which reduces the possibility of carbon defects in lost foam casting nodular iron castings; Reduce the risk of porosity defects in castings. There are inclusions on the end face of the reducer shell made of lost foam casting nodular cast iron in mass production, and there are shrinkage holes at the geometric hot joints. The causes of the two defects are analyzed as follows.
1. Causes of inclusion
The cracking reaction of EPS and EPMMA at high temperature is as follows:
EPS:C8H8（s）→8C（s）+4H2（g）
EPMMA:C5O2H8（s）→3C（s）+2CO2（g）+4H2（g）
It can be seen from the above formula that the gas generation after pyrolysis of 1 mol EPS is 4 mol, and that of solid carbon is 8 mol, while that of 1 mol EPMMA is 6 mol and that of solid carbon is 3 mol. the two kinds of gaseous and solid substances have different emphases. The generation of gas and solid carbon provides a material basis for the porosity and inclusion defects of lost foam casting ductile iron castings. Copolymer (stmma) is a combination of these two structures, and its pyrolysis gas output and solid carbon are between the two.
In the process of thermal decomposition of the foamed pattern used for forming the nodular cast iron in lost foam casting, a large amount of pyrolysis products such as gaseous, liquid and solid products will be produced during the thermal decomposition. These products are affected by many factors in the transmission process, and the state, composition and discharge form of the pyrolysis products are varied. This is the most fundamental reason for the dispersion of carbon defects in the ductile iron produced by lost foam casting.
2. Causes of shrinkage cavity
From the carbon equivalent of lost foam casting nodular cast iron, since the mechanical properties of Y-type sample meet the requirements of QT450-10 and the spheroidization rate of grade 2 ~ 3 also meets the technical requirements of lost foam casting nodular cast iron reducer shell, the chemical composition of reducer shell will not be adjusted; At the same time, from the observation of the surface quality of the lost foam casting nodular cast iron reducer shell, there is neither cold insulation nor sand sticking problem. The influencing factors of spheroidizing treatment temperature and pouring temperature on the shrinkage cavity of the reducer shell are eliminated; The fundamental reason for the formation of shrinkage cavity in lost foam casting nodular iron castings is that when the alloy shrinks and solidifies in liquid state, a certain part of lost foam casting nodular iron castings (usually the hot spot of final solidification) can not be fed with liquid metal in time, where irregular holes with rough hole wall are produced. To solve the shrinkage cavity of reducer shell, we mainly start with the process measures of lost foam casting nodular cast iron.