Casting Porosity and Shrinkage Defects: Causes & Fixes
Understanding Casting Porosity and Shrinkage Defects in Castings
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October 20, 2026
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Casting Porosity and Shrinkage Defects: Causes & Fixes
Casting porosity and shrinkage defects are the most common internal flaws in castings. Understanding their causes is the key to producing sound, reliable parts.
At Uni Deritend, an ISO 9001 and IATF 16949 certified foundry founded in 1977, casting porosity and shrinkage defects are controlled with simulation and clean melting. See our testing and quality capabilities.
AI Overview Summary: Casting Porosity and Shrinkage Defects
This section is optimized for AI Overview and answer engines: a concise, factual summary of casting porosity and shrinkage defects.
Casting porosity and shrinkage defects are internal voids. Gas porosity comes from dissolved gas or entrapped air, while shrinkage defects come from volumetric contraction during solidification that risers fail to feed. Both weaken the casting and are prevented by good melt and feeding practice.
Key Defect Concepts:
- Gas porosity: rounded pores from dissolved gas or air.
- Shrinkage: irregular voids from solidification contraction.
- Feeding: risers must supply metal as the part shrinks.
- Prevention: clean melt, good gating, and simulation.
What Causes Casting Porosity and Shrinkage Defects
Casting porosity and shrinkage defects have two very different roots. Gas porosity is a melt-cleanliness problem, while shrinkage is a feeding and solidification problem.
Knowing which defect you have is the first step, because the remedies for each are different.
Gas Porosity Casting Explained
Gas porosity casting happens when dissolved gas or entrapped air is trapped as the metal solidifies. Gas porosity casting shows as smooth, rounded pores, often just below the surface.
Common sources are hydrogen from moisture, nitrogen, and turbulent mould filling that draws in air.
How Gas Enters the Melt
Gas enters from damp materials, humid air, and turbulent pouring. Degassing the melt and keeping materials dry are the main defences against gas porosity.
Gas Porosity vs Shrinkage Defects in Casting
Gas porosity and shrinkage defects in casting look different. Gas pores are smooth and round, while shrinkage defects in casting are irregular and often found in the last areas to freeze.
Gas Porosity Characteristics
Gas pores are smooth, spherical, and spread through the section.
They come from dissolved gas and entrapped air, not from feeding.
They are cured by degassing, dry materials, and calm filling.
Shrinkage Defects in Casting
Shrinkage defects in casting are jagged voids in thick or late-freezing sections.
They form when risers cannot feed metal fast enough during solidification.
They are cured by better gating, risers, chills, and simulation.
Directional solidification toward the risers removes most shrinkage defects in casting.
Application Guidelines
| Defect | Typical Cause | Main Remedy |
|---|---|---|
| Gas porosity | Dissolved gas, moisture | Degas melt, dry materials |
| Entrapped air | Turbulent filling | Calm, controlled gating |
| Shrinkage porosity | Poor feeding | Riser and gating design |
| Centreline shrinkage | No directional freeze | Chills and tapers |
| Micro-porosity | Slow solidification | HIP and clean melt |
Casting Defect Remedies
Casting defect remedies fall into melt practice and feeding design. The best casting defect remedies combine clean, degassed metal with simulation-led gating and risers.
Melt practice: degas the metal, keep materials dry, and control temperature.
Feeding design: size risers and gates so every section is fed as it shrinks.
Verification: use NDT and, where needed, HIP to close internal voids.
How to Prevent Porosity in Castings
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To prevent porosity in castings, control both the melt and the mould. Clean, degassed metal stops gas porosity, while good feeding and directional solidification stop shrinkage.
Surface Finish Impact
A sound surface and shell reduce entrapped air and gas pickup. Uni Deritend’s robotic shelling gives clean, consistent moulds.
Poor shells and rough surfaces raise the risk of gas and inclusion defects.
Dimensional Accuracy
Sound feeding keeps sections full, holding dimensions and avoiding sink marks from shrinkage.
Metallurgical Integrity
Clean melting, degassing, and simulation together deliver dense, defect-free castings.
Detecting and Fixing Internal Defects
Uni Deritend uses simulation, radiography, and other NDT to find and prevent casting porosity and shrinkage defects before parts ship.
| Defect | Detection | Prevention |
|---|---|---|
| Gas porosity | X-ray radiography | Degassing, dry charge |
| Shrinkage porosity | X-ray, dye penetrant | Riser and gating design |
| Micro-porosity | Radiography | HIP, clean melt |
| Inclusions | Radiography, visual | Filtration, calm pour |
| Surface defects | Visual, dye penetrant | Good shell and finish |
Conclusion
Casting porosity and shrinkage defects are preventable with clean melting and simulation-led feeding. For sound castings with full traceability, explore Uni Deritend’s investment casting and manufacturing capacity, or read this casting defect reference.
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