< CASTING FAULTS 1
POROSITY, BLOW HOLES & PIN HOLES
Porosity is typified as series of small holes in the cast which often impart a coarse ‘sand’ like quality to the metal surface. In more serious examples, the holes are pronounced and deep, creating rough irregular intrusions into the cast. There are a number of potential causes for this type of fault; the most usual being too heavy or too uneven a wall thickness (often this is also accompanied by localised shrinkage). Likewise, trapped gases in solidifying metal, a poorly vented mould or core mass, and excessive turbulence set up in the metal charge during casting may also contribute to a high incidence of porosity faults. Small pin holes and areas of minimal porosity are easily MATTED over using hand tools, though larger repairs may require patching, or at least filling with weld and reworking.
PITTING
Pitting or CRATERING (typified as smooth, rounded depressions), is usually associated with the presence of water in the mould during casting. Moisture can originate from a damp (insufficiently fired) mould, or else from a metal charge that has picked up excessive hydrogen and oxygen gases (which combine on cooling). Steam is created in the mould under either of the above conditions, and the resulting vapour reacts with the charge to create a distinctively pitted ‘lunar’ like surface on cast copper alloys.
MIS-RUNS, NO-RUNS & COLD SHOT
Mis-runs are areas of unfilled mould, resulting in a partial loss of the reproduction. Unlike the holes in a cast’s fabric which are caused by excessive shrinkage (which are typically rough edged), mis-runs are distinguished by usually having smooth, rounded-off edges. Mis-runs are often the result of an entering charge cooling off too rapidly and prematurely freezing in the refractory mould’s cavity before filling completes (this is also known as COLD SHOT). Inadequate feeding (through placing too few runners on the wax pattern), or having too narrow an air gap in the mould cavity (from too thin a wax pattern or oversized sand core), can also cause a mis-run. Likewise, a poorly secured core can float out of position within a refractory mould, thus closing the air gap and preventing the passage of the charge. Too few (or poorly positioned), RISER VENTS can create a back pressure effect in the mould by trapping displaced air, this prevents metal from flowing into affected areas. Small mis-runs can usually be repaired with weld, though patches are required if larger areas are affected.
SHRINKAGE
Shrinkage often arises where part of a design with a heavy wall thickness adjoins a thinner walled section. If these differences in wall thickness are not carefully controlled through feeding (see WAXWORKING and SAND CASTING), there can be a tendency for the two areas of varying thickness to shrink and ‘pull away’ from each other as the cast cools and contracts. The effects of shrinkage are often made significantly worse if a thick walled design is filled with overheated metal.
In severe cases, adjoining features can either detach, or the lighter section be completely drawn into the heavier portion, resulting in a section of the work being lost. Where shrinkage results in a fracture, the open surface of the split usually has a coarse, crystalline like surface. Shrinkage can also be accompanied by surface porosity in affected areas.
CASTING FAULTS 3 >
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