gas cavity in investment casting process

In investment casting process,we often meet defects like gas cavity,sand inclusion,cold insulation,ect.Now we would like to talk about the effects of gas cavity on investment castings,which is caused as the gas is not timely escaped before crusts of liquid metal.

*It would reduce effective bearing area  when gas cavity is produced in investment castings.Also stress concentration will be caused around the holes,which will reduce the impact resistance and fatigue resistance of castings.

*Gas cavity will also decrease the density of castings,causing castings which need to bear hydrostatic test scraped.

*It would also have bad infuence on corrosion resistance and heat resistance porosity of investment castings.

So how should we prevent producing gas cavities?We could reduce gas content of liquid metal,increase the permeability of sand and ass strain relif in the highest point of cavity.