Porosity vs Shrinkage in Iron castings
Who this helps: Design Engineers / Buyers troubleshooting iron castings (gray/grey & ductile/SG, incl. ADI) where leak paths, blisters, or cavities threaten function and cost.
What you’ll get: a practical diagnostic map (porosity vs shrinkage), root-cause levers you can control (melting, molding, cores, feeding), copy-paste drawing notes, plus a prove-it plan (NDT, sectioning, leak tests).
Prepared by YB Metal Solution. Share your drawing and nonconformance photos via /rfq—YB Metal will return a part-specific defect containment + corrective plan with inspection steps and target checkpoints.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- Quick definitions & why it matters
- How to tell them apart (diagnostic cues)
- Metallurgical roots—porosity (gas/blows)
- Metallurgical roots—shrinkage (macro/micro)
- Process levers & proven fixes
- What to put on the drawing (copy–paste)
- Proof it’s gone—inspection & evidence pack
- Related internal resources
- What YB Metal delivers
- FAQs
Quick definitions & why it matters
Graphite form:
- Porosity (gas/blows): near-round or elongated voids from entrapped gases (moisture, binder decomposition, air). Often near surfaces, cores, pockets, or poor venting areas.
- Shrinkage (macro/micro): feeding failure during solidification; last-to-freeze zones form dendritic, irregular cavities (macro) or fine interdendritic voids (micro).
Why it matters: Porosity kills pressure-tightness and paint adhesion; shrinkage undermines fatigue strength and machinability. The fixes are different, so correct diagnosis saves weeks.
How to tell them apart (diagnostic cues)
| Feature | Porosity (Gas/Blows) | Shrinkage (Macro/Micro) |
|---|---|---|
| Shape/texture | Smooth-walled, round/elliptical; sometimes clustered “honeycomb” | Irregular/dendritic, spongy texture; often connected towards hot spot |
| Location | Near core prints, blind pockets, coating pools, thick coatings; under skin | In hot spots/last-freeze regions, junctions, heavy pads, riser necks |
| Depth | Often sub-surface or just under coating | Tends to sit behind hot sections; can be deep |
| Links to process | High moisture/LOI, under-baked cores, poor venting, turbulent pour | Undersized risers, wrong modulus, no chills/sleeves, high pour temp |
| Leak behavior | Random pin leaks; pressure rises then drifts | Larger, consistent leaks; bubbles localize at hot spot |
For a shrinkage-focused deep dive, use:
Metallurgical roots—porosity (gas/blows)
- Sand system: moisture drift, high LOI, residual organics; trapped steam in dense or un-vented sand.
- Cores: under-cure/bake, solvent-heavy washes, blocked core vents; wash not fully dried.
- Metal handling: turbulence → air entrainment; re-oxidation films; dirty ladles; wrong pouring height.
- Geometry: blind pockets, vertical “cups,” tight radii that trap gas.
Iron-specific notes
- Graphite (gray/ductile) provides some expansion during eutectic solidification, but gas porosity is unaffected by feeding—you must give gas a way out (vent, bake, dry, de-turbulate).
Related articles to tune the upstream steps:
Metallurgical roots—shrinkage (macro/micro)
Why it happens
- As iron solidifies, the last-to-freeze hot spots need liquid feed-metal. Without feeder modulus > hot-spot modulus, cavities form.
- Junctions (X, T, Y), heavy pads, and isolated bosses collect heat; without chills/sleeves or proper necking, they starve.
Iron-specific opportunity
- Graphite expansion during eutectic solidification helps compensate, but not at thick junctions/late freezes—hence the need for risers + chills + directional solidification.
Process levers & proven fixes
If it’s Porosity (gas/blows)
- Sand & cores
- Lock moisture/LOI windows; bake cores fully; ensure wash is dry before molding.
- Add/clear vents; avoid coating pooling at core prints & blind pockets.
- Metal & pouring
- Reduce turbulence: proper pouring height and gating; skim clean metal; keep slag off.
- Stable superheat—not excessive.
- Design tweaks
- Add weep/drain paths and fillets that don’t trap air.
- For thin-wall parts, consider shell molding to improve venting/skin:
- Verification
- Section suspect areas; do dye-penetrant on machined faces; pressure test for pin leaks.
If it’s Shrinkage (macro/micro)
- Feeding design
- Riser modulus > hot spot modulus; use exothermic sleeves; place necks to pull heat.
- Add chills to flip freezing order; cut heavy pads or core-out bosses.
- Pouring & solidification
- Avoid too-high pour temps (bigger hot spots); stabilize inoculation to control graphite nucleation.
- Process choice
- For heavy sections, resin sand can give better feeding paths; for thin walls, shell helps uniformity:
- Verification
- X-ray (macro), destructive sectioning of hot spots, and metallic density checks; repeat on pilot lots.
What to put on the drawing (copy–paste)
If pressure-tight (leak-critical):
Casting shall be free of through-porosity. Pressure test: [medium] at [pressure/time]; zero visible bubbles.
Surface integrity on sealed faces: no open porosity after machining.
Record DFT and adhesion for coating if used as sealant aid.
If shrinkage-sensitive zones exist (junctions, bosses):
No macroporosity/shrinkage acceptable in zones A/B/C (see view). Validate by [X-ray / sectioning] at PPAP. Supplier to propose riser/chill plan; freezing order must be directional from zone C to riser.
Tolerances & machining stock references:
Proof it’s gone—inspection & evidence pack
Containment (short term)
- 100% pressure test on leak-critical parts; mark/segregate.
- Targeted machining then dye-penetrant on suspect faces.
Validation (root cause closed)
- Shrinkage case: X-ray maps of hot spots; 2–3 cut-ups confirming soundness; riser/chill layout attached.
- Porosity case: moisture/LOI and core-bake logs, wash dryness checks, and a reduced-turbulence gating sketch.
- Program docs: include in PPAP/FAI: tensile (per grade), hardness, metallography, CMM for criticals.
- PPAP Levels & Docs
- Foundry Quality KPIs
Related internal resources
Process capability & finish:
Surface Finish by Process
Core/venting fundamentals:
Core Design: Prints, Venting & Coating Best Practices
What YB Metal delivers
YB Metal quotes with a machining plan attached:
- A diagnostic brief distinguishing porosity vs shrinkage on your part (photo-marked).
- A feeding & venting redesign (risers/chills/vents, gating sketch) tied to your hot spots.
- Process check windows: moisture/LOI/core-bake/wash dryness; pour temperature and transfer time.
- Pilot evidence pack: X-ray map or cut-ups, leak-test logs, metallography images, and CMM FAI on critical features.
Need a plan for your casting? Upload your drawing and NCR to /rfq—we’ll return recommendations and a quote.
FAQs
CTA — specify with proof, not guesses
Cut cycle time without burning inserts. Upload your drawing to /rfq—YB Metal will send cutting data, tool list, and a coolant/filtration plan tailored to your part.