The casting mold shell has influence on casting quality such as precision, thus the process of mold shell producing should be valued.
1. Shell overview
The common process of mold shell as follows: Mold degreasing - coating - sanding - drying, hardening - sealing layer - dewaxing - roasting. First, make 1~2 layer shell, then 3~6 layer back shell, and finally 1 layer closed layer (only paint without sand). The shell made in this way has two different structures. Generally, Generally, the overall shell is better for small and medium-sized parts.
In order to ensure casting quality, the mold shell has the following performance requirements: strength, deformation resistance, air permeability, line change, thermal conductivity, thermal vibration stability, thermochemical stability, etc.
The performance of the type shell may vary greatly due to its different materials and manufacturing techniques. The most common classification method is to divide the shell into water-glass shell, silica sol shell and ethyl silicate shell. The latter two types of shells are of high quality, which can be used in high precision casting production. Compared with the two, silica sol shell manufacturing is beneficial to environmental protection, which is being used more and more widely. Water glass shell is mainly used in low precision casting production.
2. Refractory for shell making
Shell refractory accounts for more than 90% of the weight of the shell, and its performance has influence on casting quality. Shell making refractory materials should have the following properties: have enough refractoriness, small and uniform thermal expansion coefficient, good thermal chemical stability, can guarantee shell sufficient strength, particle size appropriate, and non-toxic, etc.
2.1 Silica sand
Silica sand is acidic at high temperature, melting temperature is 1713 ℃, the melting point decreases while containing impurities. The linear expansion coefficient is large and uneven when the phase change is heated. So silica sand is not a high quality shell refractory. However it is still widely used in China for its low price. Silica sand (powder) is suitable for low precision carbon steel, low alloy steel, cast iron, copper alloy and aluminum alloy. While pouring it into alloys with high nickel content and active elements such as aluminum and titanium, the surface of the casting is easy to produce mucous sand, flax point, oxidized slag and other defects, can not be used.
2.2 Fused silica
Fused silica (quartz glass) is made by natural high purity silica melting by the electric furnace at the temperature of 1760 ℃ and then rapid cooling. Fused silica melt temperature is about 1713 ℃, low thermal conductivity, thermal expansion coefficient is almost the smallest of all refractories（0.51~0.63）*10-6/℃, so it has high thermal vibration stability. During roasting and pouring, the fused quartz shell is seldom broken by temperature change, It is an ideal refractory for molten casting shell making, but the price is high.
2.3 Fused corundum
Fused corundum is also called the white corundum, its melting point 2050 ℃, high density, compact structure, good thermal conductivity, small thermal expansion coefficient. It belongs to amphoteric oxide, which is usually weak alkaline or neutral at high temperature and has good chemical stability, it is a high quality investment casting shell refractory.
2.4 Zircon sand
Zirconium sand, also known as zirconium silicate, has high thermal conductivity, low thermal expansion coefficient, and good thermal chemical stability at high purity, which is a kind of high quality shell refractory, widely used in non-high temperature alloy and high alloy with high precision castings.