DEVELOPMENT OF MOULD SEALING PASTE FOR MAGNESIUM ALLOY CASTINGS

摘要

Mould sealing agent/paste is used to seal the two sand mould haves while assembling moulds with core. Sealing paste is applied on peripheral edges of lower mould (drag) and upper mould (cope) is assembled with bottom mould. In between the parting lines of top and bottom mould there is chance,of minor gap due to imperfect mould/assembly/warpage. To avoid chances of molten alloy leakage from this gap, mould sealing paste is applied in sand casting method. Different types of sealing paste are used for non-ferrous alloys. Commonly used sealing material is bentonite-water mix. This is sticky in nature and provides fair sealing. When magnesium alloys are cast through sand casting method, performance of bentonite-water mix is not satisfactory. Sand moulds shall be thoroughly preheated to remove traces of moisture (since magnesium alloy is very reactive with moisture) thus moulds shall be warm during pouring of molten magnesium alloy. Due to warm mould, bentonite water mix starts drying and while assembling the mould there are chances of improper cope and drag closing resulting into gap between the two. Since Magnesium alloy reacts vigorously with moisture, it may start burning when it comes in contact with sealing material (bentonite/water) resulting into severe fire hazards. There are non reactive ceramic pastes with adhesive properties. Application of these pastes causes difficulty in separation of mould halves and may cause damage to the non ferrous castings (comparatively lower strength than ferrous castings) while knock out. To avoid the event of molten alloy leakage and fire hazard, alternate mould sealing paste was required. Vegetable oil (coconut oil with commercial purity) is mixed with bentonite powder (200 mesh size) to prepare the paste. This paste can be applied on upper portion of lower mould half at peripheral edges and top mould can be assembled on it. It provides perfect sealing and in extreme case of coming in contact with molten Magnesium alloy, doesnt initiate the burining of alloy and subsequent fire.