< CASTING FAULTS 2
HOT TEARING
Hot tearing is caused by the contraction of a cast around a solid core mass during cooling. The core is rather more rigid than the metal at this stage, so the cooling cast splits along any line of weakness before the core can give. Spheres, hemispheres and other volumetric forms are especially prone to this effect. The resulting tear in the cast is easily identified by it’s jagged crystalline face, similar in appearance to a SHRINKAGE FRACTURE (see above). Where possible, any vulnerable sand pattern design or wax pattern is ribbed and reinforced across it’s internal surfaces to help resist this effect. Alternatively, the core itself can be designed to collapse as the metal contracts, though the use of collapsible cores in art founding is unusual. More commonly, the body of the cast is kept well fed with metal from wide bodied runners and large volume cup reservoir, this help to minimise contraction. When a hot tear does occur, the edges of the fissure are normally easily pulled together and welded before making good the seam.
SCABBING
Scabbing, as the name suggests, indicates a flake-like disfigurement across the cast’s surface. Scabbing is usually seen on casts made from CERAMIC SHELL moulds, usually because a section of the investment’s FACING (or PRIMARY) COAT has lifted away from the wax pattern during the mould’s construction. INVESTMENT LIFT can be caused by the use of an inferior quality or degraded SLURRY, but also results from the accidental heating up of a partially invested wax – heating causes the underlying wax pattern to expand and disturb earlier applied facing coats. The lifting of a facing coat enables molten metal to run between layers of ceramic investment, resulting tin a ‘sandwich effect’ of refractory trapped under cast metal. This ultimately shows up as scabbing on the cast’s surface. Repair involves removing both metal scab and the trapped investment, before making good the affected area either with weld or by patching.
INCLUSIONS
Inclusions are non-metallic particles trapped in a cast metal surface. Typically, inclusions are found in ‘clusters’ towards the lower or uppermost parts of a cast, close to drain and riser outlets, as well as areas of undercut detail and edges – all of which can act as a trap for foreign debris. Inclusions arrive in the cast from a number of sources, commonly as stray refractory such as sand or investment particles trapped in the mould’s cavity, or else as dross and slag entering with the charge during the pour. Inclusions are usually localised and the offending material can often be removed by grinding or drilling the affected area out, then making the repair good with weld.
Artists and designers using foundries should be aware that minor casting faults, whilst undesirable, are to some extent an intermittent feature of the art founding process. In the event of being confronted with a cast with an obvious fault, it is nearly always worthwhile discussing the matter with the founder before rejecting the work out of hand. Many casting faults can look far worse on a newly broken out cast than they actually are, remember that in skilled hands even extensive repairs are often possible without detriment to the quality and overall standard of the finished artwork.
RECONSTRUCTING CAST SECTIONS 1 >
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