Casting Processes and Casting Discontinuities

Learning outcomes

The learner should be able to compare expendable and permanent mould routes, explain gating and feeding, predict discontinuities from process conditions, connect surface finish and section geometry to method selection, and plan evidence-based casting examination.

9.1 Process families

Sand casting uses a shaped sand mould and can produce very large or complex components, with relatively rough surfaces and moderate dimensional control. Investment casting builds a ceramic shell around a disposable pattern and supports fine detail. Permanent-mould and die-casting routes reuse metallic tooling and can provide higher production rate and improved finish. Centrifugal casting uses rotation to distribute liquid metal and is common for tubular or ring-shaped products. Continuous casting produces long semi-finished sections, while lost-foam and other specialised routes solve particular geometry or production problems.

Every route must deliver clean liquid at the correct temperature, fill the cavity without damaging the mould or entraining excessive gas and oxide, allow progressive feeding during freezing, and control cooling until the product is strong enough to handle.

9.2 Gating, risers and cores

The gating system carries metal from the pouring source to the cavity. Poorly designed flow can cause turbulence, erosion, oxide folding or premature freezing. Risers provide liquid to compensate for solidification contraction and should remain liquid longer than the region they feed. Chills increase local heat extraction. Cores create internal passages but can move, break, generate gas or leave inclusions.

Section changes create hot spots. A thick junction connected to thin walls can freeze last and collect shrinkage. Sharp corners restrain contraction and increase hot-tear risk. Simulation supports design, but production controls and inspection still verify actual performance.

9.3 Discontinuity families

• Gas porosity and blowholes: rounded cavities from gas generation, entrainment or reduced solubility.
• Shrinkage: cavities or porous networks where liquid feeding was inadequate.
• Cold shut or misrun: incomplete joining of flow fronts or incomplete filling.
• Hot tear: crack-like separation during late solidification under restraint.
• Inclusion: trapped slag, oxide, sand, refractory or other non-metallic material.
• Core shift and mould mismatch: dimensional displacement that can alter wall thickness.
• Segregation: local chemistry variation caused by solute redistribution and flow.
• Surface laps, scabs and penetration: mould-metal interaction or folding at the surface.

Repair welding may add weld discontinuities and a local heat-affected zone. The repair map is part of product history and should be available to the examiner.

9.4 NDT strategy

VT begins with overall condition, dimensions, surface quality and repair evidence. RT can reveal many volumetric density changes, although complex thickness and access can limit it. UT supports internal examination and wall measurement, but coarse grains, irregular geometry and rough surfaces may reduce performance. PT or MT finds suitable surface-breaking conditions; MT requires ferromagnetism. Leak testing may address pressure-boundary tightness.

The calibration or reference response should represent material, thickness, surface and geometry as required. A smooth wrought-steel block cannot automatically demonstrate sensitivity in a coarse cast component.

Practical checkpoint

Given a casting drawing, mark hot spots, cores, gates, risers and machining allowances. Predict at least four discontinuity locations. Build an NDT matrix showing method, access, orientation, expected response and limitation. Compare the prediction with radiographic or ultrasonic training evidence.

Common errors and misconceptions

Several common errors recur in this topic: naming all rounded indications gas pores; assuming machining removes subsurface casting conditions; ignoring repair welds and their documentation; expecting one UT frequency to work through every cast structure; and confusing dimensional nonconformance with an internal discontinuity.

Chapter summary

Casting quality depends on clean filling, controlled solidification and adequate feeding. Process geometry predicts where gas, shrinkage, inclusions, tears and dimensional errors may occur. NDT must accommodate rough surfaces, coarse grains, changing thickness and repairs.