Guide
What causes casting shrinkage defects at bosses and junctions, and how does the drawing prevent them?
Casting shrinkage defects are voids that form inside a casting when a region of metal is still liquid after the metal around it has frozen, so the volume it loses on solidifying cannot be refilled with liquid. They collect at the last places to freeze: heavy bosses, thick pads, and the junctions where walls meet. They show up as dark, irregular indications on an X-ray, or as rough, jagged cavities that open up when a face, bore or thread is machined into the casting.
By Saola Manufacturing engineering team · Published · Updated
Shrinkage is often blamed on the foundry, and the pouring practice does matter. But the location of a shrinkage cavity is mostly decided by the part geometry, which is set on the drawing. A boss that is much thicker than the wall feeding it, or a junction where three or four walls cross, creates a hot spot that no reasonable feeder can reach. This guide is written for the design, quality or sourcing engineer who has a sand or gravity casting with shrinkage at X-ray or after machining. It covers how to recognize shrinkage and tell it from gas porosity, how to rank the causes, which fix fits which cause, and what to write on the drawing and RFQ so the next lot does not repeat it.
One point of vocabulary first. “Casting shrinkage” also names the patternmaker’s allowance, the amount a pattern is made oversize because solid metal contracts as it cools to room temperature. That contraction changes the size of the casting but does not create voids. The voids come from the earlier stage, the volume lost as liquid turns to solid, which has to be fed with liquid metal from a feeder (riser) or from a neighboring section that is still molten.
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What does a shrinkage defect look like, and how is it different from gas porosity?
Shrinkage cavities are irregular, branched or sponge-like and sit in the heavy sections that freeze last. Gas pores are smooth and rounded and tend to sit near cores, upper surfaces and poorly vented areas.
Foundries sort the two by shape, by where the voids sit and by how they are spread. Shrinkage forms between growing crystals, so its walls are jagged and it either fills one pocket or links up into a sponge; it follows the heat, collecting where the casting froze last. Gas pressure pushes outward, so gas pores come out round and tend to stand apart as single holes or clusters (NFFS, gas or shrinkage). The Non-Ferrous Founders’ Society note behind this also cautions that gas and shrinkage can coexist in one cavity, so treat shape as evidence to weigh rather than a verdict.
The ASTM reference radiographs give shrinkage its own category. In the steel casting set, Category C is shrinkage, split into linear, feathery and sponge types and a combination type, each shown at several severity levels (ASTM E446). Recording the type and level, rather than just “porosity,” gives the foundry and you a common record to compare lot against lot.
After machining, look for these signs:
- Location. The cavity sits under a boss, in the middle of a thick pad, or at a corner where walls join, not spread evenly over the part.
- Walls of the cavity. Under a loupe the inside is rough, often with a fern-like crystal pattern from the dendrites that grew into the last liquid.
- Depth. The cavity often grows as the cut moves toward the center of the heavy section, since the center is usually the last part to freeze.
- Repeatability. The same feature on many parts shows it, which points to geometry or feeding rather than a one-off melt problem.
What causes shrinkage at bosses and junctions?
The root cause is an isolated hot spot: a region that freezes after the sections around it, so its feed path is cut off before it has finished solidifying. Geometry creates most hot spots; feeding and pouring decide whether the foundry can still reach them.
The usual causes, geometry first:
- A heavy section fed through a thin one. A solid boss on a thin wall, or a thick flange beyond a thin web, freezes last. The thin section freezes first and closes the path from the feeder, so the boss shrinks with nothing to refill it.
- Wall junctions. Where walls meet in an L, T or X, the metal at the joint is thicker than either wall. The common foundry check is to inscribe the largest circle that fits in the section; where that circle is larger than in the walls around it, the joint is a hot spot. A cross (X) junction concentrates more mass than a T or L, so it is the hardest to feed.
- Oversized fillets and pads. Generous radii that are right for stress can add mass at an inside corner. A cast-on pad left solid for later drilling is a boss in disguise.
- Feeder too small, too far away or in the wrong place. Each feeder can only feed a limited distance along a section. University of Iowa researchers used casting simulation to develop feeding-distance rules for steel castings (Carlson et al., 2002), which shows why a feeder that is big enough can still be too far away.
- Alloy freezing behavior. Alloys that freeze over a narrow temperature range tend to form concentrated cavities and centerline shrinkage; alloys with a long freezing range tend to form dispersed micro-shrinkage that may only show as a leak after machining.
- Pouring temperature and mold conditions. Excess superheat, a mold that holds heat at an inside corner, or a core that is surrounded by metal on all sides can move the last-to-freeze point into the part.
The underlying physics is captured by Chvorinov’s rule: a section’s freezing time grows with its modulus, the ratio of its volume to its cooling surface. A boss and a wall can look alike in one drawing view yet have very different moduli, which is why a quick look at the drawing misses hot spots that a modulus check or a simulation finds.
How do you diagnose shrinkage on X-ray or after machining?
Confirm the defect type, map where it occurs across parts, and compare the location with the section geometry. If the cavity is irregular, sits at a heavy section or junction and repeats on the same feature, treat it as shrinkage and look first at the geometry and feeding.
Work through this checklist with the part, the drawing and the casting supplier.
- Classify against the right reference set. Compare the film or digital image with the ASTM reference radiographs for the alloy and wall range, listed in the table below, and record the category, type and severity level, not just “porosity.”
- Map the location. Mark every indication on a copy of the drawing for several parts. Shrinkage clusters at the same boss or junction on every part; random gas pores move around.
- Section one part. Cut through the worst feature and polish it. A rough, branched cavity at the thermal center confirms shrinkage. CT gives the same answer without cutting if it is available.
- Check the section geometry. Inscribe circles in the drawing section at the defect and in the wall that feeds it. If the circle at the defect is larger than the feed path, the feature is isolated.
- Ask for the methoding. Request the gating and feeder layout and, where the foundry runs one, the solidification simulation. Carlson and Beckermann showed that a dimensionless form of the Niyama criterion, a thermal parameter taken from simulation, can predict where shrinkage porosity forms and how much (Carlson and Beckermann, 2009).
- Ask what changed. If earlier lots were sound, compare the foundry’s records for the bad lot: feeder and neck size and position, chill placement, pouring temperature, yield changes, and any simulation or thermocouple data. These are the items the NFFS note lists for a shrinkage review. A new pattern, a repaired pattern or a moved feeder is a common trigger.
- Check whether machining removed the skin over a hot spot. If the defect only appears after a face is cut deep into a boss, the machining stock or datum scheme may be the trigger rather than a change in casting quality.
| Casting type | Film reference set | How shrinkage is shown |
|---|---|---|
| Steel, up to 2 in (50.8 mm) wall | ASTM E446 | Category C: Ca linear, Cb feathery, Cc sponge, Cd combination; severity 1 to 5 |
| Steel, 2 to 4½ in (50.8 to 114 mm) wall | ASTM E186 | Category C: Ca linear, Cb feathery, Cc sponge; severity 1 to 5 |
| High-strength copper-base and nickel-copper, up to 6 in (152.4 mm) | ASTM E272 | Category C: Cb feathery (up to 2 in); Ca linear and Cc spongy (over 2 to 6 in); severity 1 to 5 |
| Aluminum and magnesium castings (other than die castings), up to 2 in (51 mm) | ASTM E155 (digital: E2422 aluminum, E2869 magnesium) | Reference plates in 1/4 in and 3/4 in sections, used for up to 1/2 in and over 1/2 to 2 in |
| Aluminum and magnesium die castings | ASTM E505 | Three discontinuity types graded at four severity levels, in 1/8 in and 5/8 in sections |
How do you fix or prevent shrinkage at a boss or junction?
Remove the hot spot in the design where you can, by coring bosses, evening out walls and breaking up junctions. Where the mass has to stay, give the foundry a feed path to it with padding, a feeder or a chill, and keep critical machined faces out of the last-to-freeze zone.
Use the table to match the fix to the cause found in the diagnosis. Design fixes belong to the drawing owner; methoding fixes belong to the foundry but often need drawing approval because they add metal or leave marks.
| Cause found | Drawing fix | Foundry fix (needs your approval) |
|---|---|---|
| Solid boss on a thin wall | Core the boss to a wall similar to the parent wall; cast the hole and machine it to size | Feeder on top of the boss or a chill against it |
| Cross (X) junction of walls | Offset the walls into two staggered T junctions | Chill at the junction |
| T or L junction hot spot | Use modest, uniform fillets; core out the inside corner where load allows | Chill or local feeder |
| Thick flange beyond a thin web | Taper the web so it thickens toward the flange, or lighten the flange with pockets | Padding (added taper metal) along the feed path |
| Solid pad left for drilling | Cast the hole, or reduce the pad to the machining allowance | Chill under the pad |
| Sealing face cuts into a hot spot | Move the face, reduce machining stock there, or change the datum so less is removed | Feeder or chill placed to make that face freeze early |
| Dispersed micro-shrinkage that leaks | Add a leak test and allow impregnation if function allows | Steeper thermal gradient with chills; review pouring temperature |
Design first. Coring a boss or staggering a junction removes the hot spot for every part, every lot and every foundry. Foundry fixes work, but they depend on a specific pattern and method, so they can be lost when the tool is repaired or the work moves.
Directional solidification. The goal of any foundry fix is to make the casting freeze in order, from the far end toward the feeder, so the last liquid is always in the feeder and not in the part. Padding, chills and feeder placement are all ways to set that order. A drawing that lets walls thicken gradually toward a feeder location makes this easy; abrupt changes from thin to thick make it hard.
Machining and porosity. Shrinkage near the surface is often sealed under the dense skin and harmless until a cut opens it. The machining allowance guide and the guide to datums on castings show how to keep cuts shallow over heavy sections.
What should the drawing and RFQ say so shrinkage does not recur?
Mark radiographic zones with the ASTM reference set, defect category and allowed severity for each zone, state which machined surfaces must be free of exposed cavities, and tell the foundry what it may change. A general note such as “castings shall be sound” gives no basis to accept or reject a lot.
- Zones. Divide the casting into critical zones, such as pressure walls, sealing faces and threaded bosses, and non-critical zones. Give each zone its own acceptance level.
- Reference set and level. Name the ASTM reference radiographs for the alloy and wall range, the shrinkage type, and the maximum severity level allowed in each zone. ASTM states the reference radiographs are for comparison only and that their use needs a prior purchaser–supplier agreement on acceptance levels (ASTM E272), so the drawing has to set the level.
- Sampling. State how many castings are radiographed on first article and per lot, and what triggers full inspection.
- Machined-surface criteria. For sealing faces and threads, state that no cavity may be visible after machining, or give a maximum size and spacing, and say which faces this applies to.
- Function test. If the part holds pressure, add the leak test method and limit, and say whether impregnation is allowed. The leak testing guide covers how to set that limit.
- Permitted changes. Say whether the foundry may add padding, chill marks or feeder contact pads, and where. If you want cored bosses or cast holes, show them on the casting drawing, not only on the machined-part drawing.
- Repair. State whether weld repair of castings is allowed, in which zones and with what re-inspection.
On the RFQ, send the X-ray images or photos of the cavities and the location map. A supplier who sees the actual defect can suggest a coring or wall change before the pattern is made, which costs far less than changing a finished pattern.
Can a casting with shrinkage be repaired or accepted?
Sometimes, if the defect is outside critical zones or within the severity level the drawing allows. Leaking micro-shrinkage can often be sealed by impregnation, but cavities in load-bearing or sealing areas need a design or method change, not a repair.
Impregnation fills connected porosity with a sealant under vacuum and pressure; it is covered for military parts by MIL-STD-276, which the vacuum impregnation guide explains. It seals leak paths but adds no strength, so it does not rescue a boss that cracks under thread torque. Weld repair can fill open cavities in many alloys, but it has to be allowed on the drawing, done to a qualified procedure and followed by re-inspection. For a nonconforming lot, record the zone and severity of each defect against the drawing, segregate the parts, and decide disposition with the engineer who owns the function, not only the inspector.
Who does what on a Saola order for a shrinkage fix?
Sand castings in brass and bronze are poured by a partner foundry we manage, and die castings by partner plants we manage; bores, faces and threads are then machined at our lead CNC plant, T.K Viet Nam Precision Company Limited in Vietnam. The quote names the production site and country of origin for each step.
Castings come to the lead CNC plant for machining, where cavities opened by a cut on a boss or sealing face are found and reported back with photos and locations, so the casting partner and you can decide whether the fix belongs on the drawing or in the method. See sand casting and die casting for the partner-plant programs, and CNC milling and CNC turning for the machining. If your part needs radiography, CT or a gravity (permanent-mold) process, say so on the RFQ and we will confirm before quoting whether it can be arranged and where. ISO 9001 certification is in progress, not held. Send the drawing, the X-ray images or cavity photos and the location map through the quote form.
Frequently asked
Is shrinkage allowance the same as shrinkage porosity?
No. Shrinkage allowance is the oversize built into the pattern so the casting ends at the right size after the solid metal cools. Shrinkage porosity is a void left when liquid metal could not feed a section as it solidified. A correct allowance does nothing to prevent porosity.
Why does a casting pass X-ray and still show cavities after machining?
Small or dispersed shrinkage may sit below the severity level that was accepted, or in a zone that was not radiographed, and only becomes visible when a cut exposes it. Check whether the machined face was inside a radiographed zone and at what level.
Will a bigger feeder always cure shrinkage at a boss?
No. If a thin section between the feeder and the boss freezes first, a larger feeder cannot push metal through it. The boss needs its own feed path, a chill, or a cored design that removes the mass.
Does casting simulation replace X-ray?
No. Simulation predicts where shrinkage is likely and is best used to change the design or method before the pattern is built. Radiography or sectioning confirms what the actual castings contain.
Can shrinkage be found without X-ray?
Partly. Sectioning sample castings, CT, leak testing and checking machined faces all reveal shrinkage, but only radiography or CT shows internal cavities on parts that are shipped.
Sources
- Gas or Shrinkage? Clues That Point You in the Right Direction, Non-Ferrous Founders’ Society, 2026
- ASTM E446-20, Standard Reference Radiographs for Steel Castings (thin-wall set, scope), ASTM International, 2020
- ASTM E186-20, Standard Reference Radiographs for Heavy-Walled Steel Castings (scope), ASTM International, 2020
- ASTM E272-21, Standard Reference Radiographs for High-Strength Copper-Base and Nickel-Copper Alloy Castings (scope), ASTM International, 2021
- ASTM E155-20, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings (scope), ASTM International, 2020
- Development of new feeding-distance rules using casting simulation: Part I. Methodology, Metallurgical and Materials Transactions B, vol. 33, K. D. Carlson, S. Ou, R. A. Hardin and C. Beckermann, University of Iowa, 2002
- Prediction of Shrinkage Pore Volume Fraction Using a Dimensionless Niyama Criterion, Metallurgical and Materials Transactions A, vol. 40, K. D. Carlson and C. Beckermann, University of Iowa, 2009
- ASTM E505-22, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Die Castings (scope), ASTM International, 2022