There are three chemicals used in preparation of sand mould for casting
bentokol, bentonite, seacoal
0.5-.85, 1.1%, 1% were respectively used. This use brought many blow holes in the casting. To avoid it I changed the values to
1.1%, 0.5%, 0.2 respectively. Now I need to know which chemical has the effect for blow holes and what percentage should be selected?
You forgot to mention one important ingredient, water. Water probably has more effect on blowhole defects than any of the products you mention. You appear to be using very low quantities of bonding agents compared to the volatile materials. You may want to increase the bentonite contents to 6% based on sand weight and then adjust volatile content based on the metal being cast. A new sand with 6% bentonite may take around 2 -2.5% water.
The defects that you experienced may not likely be due to the addition of the additives that you mentioned above. Meanwhile, your binder (bentonite) is inadequate for proper binding, it should be increased to between 5 - 7% of the mass of the prepared sand. Addition of water helps to form gel with the binder in the sand there by enhancing the bindability of the sand, the water added should also be controlled to between 2 - 3 % depending on the time to cast the mould prepared with the sand. Avoid too fine sand to improve the permeability of the mould, you can increase your coal dust up to 3% as it will help to combat the reactive effects of molten metal on your mould surface, avoid excess water (moisture) in the sand, avoid casting relatively wet mould without drying and lastly control your melt against impurity pick up and you will overcome the defects.
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