MOULDING DEFECTS AND REMEDIES
BLOW HOLES OF ANGLES:Nature of Defect: Smooth wall cavities at the angles of the casting
Remedies: The cause is insufficient feeding and rate of solidification. Influencing by moulding sand is possible reduce the moisture content and the organic addition of the sand improve the venting of the mould.
METAL PENETRATION:
Nature of Defect: Firmly adhering agglomerates of sand/metal at hot spots and in softly rammed areas.
Remedies: Reduce compatibility of the sand. Improve ramming of the moulds. Increase amount of carbonaceous additive and coke in the moulding sand.
SWELLING:
Nature of Defect: Irregular bulges, in most cases widely spread, sometimes with rough surface and/or penetration.
Remedies: Reduce compactability of the moulding sand ram moulds better and more evenly. Make sand less sensitive to over-and underrating by increasing the amount of bentonite.
SCABBING:
Nature of Defect: Mushroom type defect with sharp edges parallel to the casting surface.
Remedies: Increase bentonite level. Use carbonaceous additive with a wide softening range. Increase the amount of carbonaceous additive in the moulding sand, Reduce amount of new sand.
SLAG INCLUSION
Nature of Defect: Non-metallic irregular shaped inclusions, when occurring, Tend to be on upper casting surface.
Remedies: The Defects are caused by slag being present with the molten metal in the pouring ladle. If the formation is due to eroded sand, reduce the amount of inert fines and use a more active instrous carbon producer increase milling time of the moulding sand.
SAND INCLUSIONS
Nature of Defect: Irregularly formed sand inclusions just underneath the casting surface, associated with products from the metal slag reaction in other areas.
Remedies: keep the moulding sand sufficiently plastic by good sand preparation and sand cooling Adjust to optimum compactability. Reduce the tendency of the mould to stick to the pattern by the use of parting agents.
SURFACE ROUGHNESS:
Nature of Defect: Rough casting surface finish due to coarse sand
Remedies: Use finer sand increase amount of lustrous carbon producing material in the moulding sand. Increase the amount of fines. E.g. coke in the sand.
PIN HOLES
Nature of Defect: Accumulation of small subsurface rounded holes defects appear after annealing and light machining.
Remedies: Normally the defect is of a metallurgical nature. The mould material may have a certain effect, In case of pin holes the moisture, the organic additions and the amount of nitrogen in the moulding sand should be reduced.
PENETRATION
Nature of Defect: Roughness caused by the molten metal penetrating the immediate sand grains of the mould surface, particularly in softly rammed areas.
Remedies: Use fines sand improve ramming of the moulds. Reduce compactability at the moulding sand. Increase amount at carbonaceous additive. Increase amount at coke in the moulding sand.
SURFACE BLOWHOLES:
Nature of defect: Cavities with smooth walls singularly or in groups.
Remedies: Low casting temperatures are often the cause. Benefits may be obtained by increasing mould permeability. Reduce finer or add coarser new sand. Reduce organic additions to the sands.
SCARRED SURFACE:
Nature of defect: Scarred casting surface around heat spot, particularly with S.G. cast iron.
Remedies: Reduce amount of impurities present in the sand that are destroying the oxide skin.
MICRO-CAVITIES:
Nature of defect: Porous structure found in the area at the casting which is last to solidify.
Remedies: Defect occurs with alloys having a wide solidification range. Little influencing by the moulding sand. Reduction of organic materials and an increase in the moisture level will improve thermal conductivity of the moulding sand.
