How do you identify sand casting defects and write acceptance criteria for them?

What do sand casting defects look like, and which ones matter?

Most sand casting defects fall into six families: gas cavities, shrinkage, inclusions, incomplete fill (misruns and cold shuts), mold-surface faults (scabs, rat tails, penetration) and cracks or hot tears. The ones that matter are those on a sealing face, a thread, a bearing bore or a highly stressed section, and those that open up when the part is machined.

Gas porosity and blowholes. Smooth-walled, rounded cavities, single or in clusters, often just under the upper surface or next to a core. On a machined face they show as clean round holes. A technical note from the Non-Ferrous Founders’ Society describes gas pores as often smooth and rounded because the gas pressure pushes outward, and points to a core, an upper surface, a blind pocket or a poorly vented region as the places where mold gas or trapped air shows up (NFFS, gas or shrinkage).

Shrinkage. Irregular, angular or branching cavities, sometimes a single open void and sometimes a sponge-like network, in the heavy sections, bosses and junctions that freeze last. The same NFFS note says shrinkage follows the thermal map and collects in heavy sections, isolated bosses, abrupt section changes and junctions. Because it sits inside the heavy sections, shrinkage is a common way for a casting that looks sound on the outside to become a leaker once a bore is cut through it.

Sand and slag inclusions. Non-metallic particles trapped in or on the casting: loose mold sand, broken core pieces or dross carried in with the metal. They blunt cutting tools, leave pits in machined faces and can act as leak paths.

Misruns and cold shuts. A misrun is a section the metal never filled, usually a thin wall or the far end of a flow path. A cold shut is a seam where two streams met too cold to fuse. The terminology in ASTM A802 treats laps, folds and cold shuts as one family, a casting surface that is folded over, and the 1995 edition of that text ties them to low pouring temperature, poor flow caused by oxide films, or both (ASTM A802).

Scabs, rat tails and buckles. Mold-surface failures. When the face of the sand mold expands with the heat of the metal and breaks away, metal runs behind it and leaves a raised, rough crust (a scab) or a long, narrow linear step (a rat tail). They are mostly cosmetic on an unmachined surface and a dimensional problem on a machined one.

Metal penetration and burn-on. Metal or metal oxide fused into the sand grains, leaving a rough, sand-encrusted skin that resists cleaning. A paper from the University of Iowa casting group, written for steel castings, traces burn-on and penetration principally to localized over-heating of the sand mold or cores, and names sharp corners, the areas under risers, surfaces next to thick sections and thin cores as the usual locations (Brooks and Beckermann, 2006).

Hot tears and cracks. Ragged, branching fractures at junctions where the solidifying metal was restrained by the mold or a core. They are normally rejected on a load-bearing or pressure-containing part, and the reference radiograph sets for steel (ASTM E446) and tin bronze (ASTM E310) show hot tears as a single illustration rather than as graded severity levels.

How do you find sand casting defects at incoming inspection?

Match the method to the defect and to the zone: visual examination against comparator plates for the surface, radiography for internal cavities and inclusions, liquid penetrant for fine surface cracks and seams, and a pressure or leak test for anything that must hold a fluid.

Visual examination. ASTM A802 (steel castings) and ISO 11971:2020 (steel and iron castings) both grade the surface against physical reference comparators (SCRATA plates in A802; BNIF or SCRATA plates in ISO 11971), so the inspector holds a plate next to the casting instead of judging “acceptable” by eye. Both are written for ferrous castings, so for a brass, bronze or aluminum casting the drawing needs written surface limits or an approved limit sample unless buyer and foundry agree on a comparator set.

Radiography. Reference radiographs give each internal defect a category and a severity level, and the acceptance level is then a single line on the drawing. Choose the set by alloy: ASTM E446 for steel, ASTM E155 for aluminum and magnesium, ASTM E310 for tin bronze and ASTM E272 for high-strength copper-base and nickel-copper alloys. Each set also has a thickness range, and per ASTM E446 that set covers steel castings with sections up to 2 inches (50.8 mm) thick, so heavier sections need a different set. The radiographic technique itself is a separate document such as ASTM E1030, which states that it controls image quality and is not intended to control the acceptability of the product.

Liquid penetrant. Penetrant per ASTM E165/E165M shows surface-breaking cracks, hot tears and cold-shut seams that are too fine to see. It does nothing for subsurface porosity, and it is best done after machining on the faces that matter.

Leak or pressure test. For a valve body, pump housing or manifold the only test that proves the casting is the one that pressurizes it. Shrinkage and porosity that are invisible on the outside are often cut open by a bore; the test belongs after machining.

Sectioning. When a defect type is in dispute, cutting one part through the suspect zone shows whether a cavity is a smooth gas pore or a dendritic shrinkage void, which tells the foundry which process change to make.

A practical receiving checklist for a new sand cast part:

• Check the first lot against the casting drawing for misruns and missing features before any machining.
• Inspect critical zones visually, using comparator plates for steel and iron or the approved limit sample for other alloys.
• Radiograph the zones the drawing names, at the agreed sampling plan, and file the films or images with the lot.
• Penetrant-test sealing faces and fillets after machining when cracks or seams are a risk.
• Leak test every pressure-containing part after machining, and record the lot and cavity of each leaker.
• Section one failed part per defect type before agreeing a corrective action.

How should acceptance criteria for sand casting defects be written on the drawing and PO?

Name the standard, the defect category and the severity level, and apply them by zone, so that a critical sealing area carries a tight level and a non-functional surface carries a looser one. A note that says only “castings shall be free of defects” cannot be inspected and will be argued.

Reference radiograph standards are written to be applied by agreement. The scope of ASTM E310 says its basis of application requires a prior purchaser-supplier agreement on the examination attributes and classification criteria, and E155 is used for other thicknesses only where purchaser and manufacturer have agreed. That agreement belongs on the drawing or the PO, not in an email thread. Useful elements:

• Zones. Mark critical zones on the drawing (for example Zone A: sealing faces and bores; Zone B: remaining machined surfaces; Zone C: as-cast surfaces) and give each its own level.
• Internal soundness. “Zone A: radiographic quality per ASTM E446, Category A and B severity level 2 max, Category C level 1 max, Categories D and E not permitted” (pick the levels with the foundry; these are illustrations of the format, not recommendations).
• Surface. For steel, “as-cast surfaces per ASTM A802” with the acceptance level stated for each comparator category that applies; for other alloys, “as-cast surfaces to approved limit sample, revision”.
• Sampling. How many castings per lot are radiographed or penetrant-tested, and what happens to the lot when one fails.
• Pressure test. Medium, pressure, hold time and leak criterion, after machining, 100 percent or sampled. The leak testing guide covers method choice.
• Repair. Whether weld repair, plugging or impregnation is allowed, in which zones, and whether each repair needs approval and a record.
• Records. Radiographs, penetrant reports and leak results identified by lot and, where possible, by pattern cavity.

Material specifications can also carry repair rules. The published abstract of ASTM B584, the copper alloy sand casting specification, states that the castings shall not be repaired, plugged, welded or burned in, so check that the alloy specification on the drawing and the repair note agree rather than contradict each other; a repair the drawing allows may still breach the material specification it cites.

Who does what on a Saola order for sand castings?

On a Saola order the pour is placed with a partner foundry we manage, and the castings are machined at our lead CNC plant. The quote names the production site and the country of origin for each step.

The sand casting program is run through partner foundries, which are not named, and it currently centers on brass and bronze bodies. Castings go through incoming inspection before they are bored, faced and threaded on CNC turning and CNC milling equipment at our lead CNC plant, T.K Viet Nam Precision Company Limited. That plant’s inspection equipment is a 2D vision measuring machine, height gauges and a hardness tester; radiography and penetrant testing are not on its list, so when a drawing calls for them, the quote states where that testing is done.

To get a quote that prices the acceptance level you need, send the casting drawing, the machined part drawing, the alloy specification, the zone map with radiographic and surface levels, the leak test requirement and your repair rules. The quote comes with a manufacturability note that flags heavy sections and junctions likely to need feeding. ISO 9001 certification is in progress, not held. Send the drawings through the quote form.