Under this category are those defects which are caused because of the characteristics of the moulding materials. The defects that can be put in this category are cuts and washes, metal penetration, fusion, run out, rat tails and buckles, swell, and drop. These defects occur essentially because the moulding materials are not of requisite properties or due to improper ramming.
Cuts and Washes
These appear as rough spots and areas of excess metal, and are caused by the erosion of moulding sand by the flowing molten metal. This may be caused by the moulding sand not having enough strength or the molten metal flowing at high velocity. The former can be remedied by the proper choice of moulding sand and using appropriate moulding method. The latter can be taken care of by altering the gating design to reduce the turbulence in the metal, by increasing the size of gates or by using multiple in-gates.
Fusion
This is caused by the fusion of sand grains with the molten metal, giving a brittle, glassy apperance on the casting surface. The main reason for this defect is that the clay in the molding sand is of lower refractoriness or that the pouring temperature is too high. The choice of an an appropriate type and amount of bentonite would cure this defect.
Runout
A runout is caused when the molten metal leaks out of the mould. This may be caused either due to faulty mould making or because of the faulty moulding flask.
Rat Tails and Buckles
Rat tail is caused by the compression failure of the skin of the mould cavity because of the excessive heat in the molten metal. Under the influence of the heat, the sand expands, thereby moving the mould wall backwards and in the process when the wall gives away, the casting surface may have this marked as a small line, as shown in Fig. With a number of such failures, the casting surface may have a number of criss-crossing small lines. Buckles are the rat tails which are severe.
The main cause for these defects are that the moulding sand has got poor expansion properties and hot strength or the heat in the pouring metal is too high. Also, the facing sand applied does not have enough carbonaceous material to provide the necessary cushioning effect. Proper choice of facing sand ingredients and the pouring temperature are the measures to reduce the incidence of these defects
Metal Penetration
When the molten metal enters the gaps between the sand grains, the result would be a rough casting surface. The main reason for this is that either the grain size of the sand is too coarse, or no mould wash has been applied to the mould cavity. This can also be caused by higher pouring temperatures. Choosing appropriate grain size, together with a proper mould wash should be able to eliminate this defect.
Swell
Under the influence of the metallostatic forces, the mould wall may move back causing a swell in the dimensions of the casting. Aa result of the swell, the feeding requirements of castings increase which should be taken care of by the proper choice of risering. The main cause of this is the faulty mould-making procedure adopted. A proper ramming of the mould should correct this defect.
Drop
The dropping of loose moulding sand or lumps normally from the cope surface into the mould cavity is responsible for this defect. This is essentially due to improper ramming of the cope flask.
Contact Person: Mr. Fred Zhang
Tel: 0086 574 28861663
Fax: 86-574-28861665
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