For: design engineers, manufacturing engineers, procurement, quality/inspection.
Porosity is voiding in a casting that can reduce strength or create leak paths. The key prevention step is confirming the mechanism: gas porosity is managed through gas and melt handling controls (often including degassing), while shrinkage porosity is managed through feeding and solidification control. To avoid repeat issues, specify where porosity matters, how it will be verified, and what evidence is required, rather than using blanket “no porosity” wording.
Answer: Porosity is voiding within a casting, and it matters most when integrity, leak-tightness, fatigue resistance, or reliable machining performance is required.
Why: Voids reduce effective material cross-section and can create leak paths or stress concentrations, especially at sealing faces, pressure boundaries, or fatigue-loaded zones.
How to decide:
When this advice may not apply: Some surface marks are not porosity and require confirmation before treatment.
Answer: Gas porosity is caused by gas entering or remaining dissolved in molten metal, then forming voids during solidification or being trapped during filling.
Why: Gas can originate from moisture sources, melt handling conditions, or turbulent entrainment during filling. It forms fine voiding when it cannot remain in solution as the metal cools.
How to decide:
When this advice may not apply: Gas and shrinkage mechanisms can be present together in the same casting.
Answer: Shrinkage porosity is caused by insufficient feeding as the casting contracts during solidification, creating voids in last-to-solidify regions.
Why: If a hot spot cannot be fed with molten metal as it solidifies, contraction leaves cavities or spongy voiding. Heavy sections and abrupt thickness transitions are the most common locations.
How to decide:
When this advice may not apply: Fine dispersed voiding can be gas-driven rather than shrinkage-driven, and visual appearance alone does not always distinguish them.
Answer: Gas porosity is usually fine and dispersed across broader areas, while shrinkage porosity is usually localised at hot spots or heavy sections where feeding is difficult.
Why: Gas mechanisms produce small voids more widely distributed through the casting, while shrinkage mechanisms concentrate voiding where solidification ends last and feeding becomes inadequate.
How to decide:
When this advice may not apply: Visual appearance alone can misclassify mixed mechanisms. Internal inspection is more reliable for confirmation.
How do I tell gas porosity from shrinkage porosity?
The short answer is this. Gas porosity is usually fine and dispersed – small voids spread broadly across the casting. Shrinkage porosity is usually localised – concentrated at hot spots or heavy sections where feeding becomes difficult.
The reason they look different comes down to mechanism. Gas mechanisms produce small voids more widely distributed through the casting. Shrinkage mechanisms concentrate voiding where solidification ends last, and where feeding has become inadequate.
So, what should you look for in practice?
Fine pinholes, or dispersed voids spread across the casting, suggest a gas-related mechanism first. Larger cavities concentrated at heavy sections suggest shrinkage first. If porosity only appears after machining – if it wasn’t visible as-cast – that points to sub-surface voiding, and internal inspection is the more reliable way to understand what you’re dealing with.
One important caution. Visual appearance alone can misclassify mixed mechanisms. In more complex geometries, both mechanisms can be present in the same casting. Where internal integrity needs to be evidenced, internal inspection methods are more reliable than visual assessment alone.
If you’re trying to distinguish the two – or confirm which mechanism is driving a repeat issue – our team is used to working through exactly these questions.
Sand casting is usually the better choice for complex or changeable designs at low to medium volumes, with lower upfront tooling commitment. Gravity die casting is usually the better choice for stable designs with repeat volumes, where consistent outputs and as-cast finish matter.
Answer: Porosity risk is reduced by confirming the mechanism first, then applying targeted controls for gas management or feeding and solidification control as appropriate.
Why: Porosity is a category, not a single cause. Prevention fails when gas controls are applied to a shrinkage problem, or when feeding changes are applied to a gas problem. Mechanism identification is the prerequisite.
How to decide:
When this advice may not apply: If the mechanism is unconfirmed, prevention actions may be partial or inconsistent in their effect.
Answer: Porosity confirmation requires an inspection method matched to the risk, with internal methods needed where internal integrity must be evidenced.
Why: Visual inspection detects surface-breaking or near-surface porosity but cannot confirm internal integrity. Where pressure boundaries, sealing faces, or structural integrity are at stake, internal inspection methods are needed and must be specified with acceptance criteria.
How to decide:
When this advice may not apply: Applicable standards can dictate inspection methods, acceptance criteria, and reporting requirements regardless of application detail.
Answer: Common mistakes include blanket “no porosity” wording, not defining critical zones, and specifying inspection without acceptance criteria or evidence outputs.
Why: Ambiguous requirements drive conservative assumptions or mismatched inspection. The result is inflated cost and, in some cases, functional risks that are still not controlled because the requirement was not linked to the function it was meant to protect.
How to decide:
When this advice may not apply: Some customer or sector specifications impose blanket requirements regardless of function. In those cases, the specification governs.
Answer: You need geometry, integrity requirements, critical zone identification, process route assumptions, and the inspection evidence required to assess porosity risk appropriately.
Why: Porosity acceptability depends on where it occurs and what the part must prove in service. Incomplete inputs lead to over-control on non-functional areas or under-control on critical ones.
How to decide:
When this advice may not apply: External standards or customer requirements may define acceptance and evidence outputs regardless of the application detail provided.
| Mechanism | Typical signs | Primary prevention | Confirmation method |
| Gas porosity | Fine pinholes, dispersed small voids across the casting | Moisture control, melt handling, reduced turbulence, degassing | Internal inspection where required |
| Shrinkage porosity | Larger voids localised at hot spots or heavy sections | Feeding strategy, solidification control, hot spot reduction | Internal inspection where required |
| Mixed mechanisms | Dispersed and localised voiding in the same casting | Confirm dominant mechanism, then apply targeted controls | Inspection that distinguishes internal discontinuity types |
| Post-process sealing (impregnation) | Microporosity causing leak paths on pressure or fluid-carrying parts | Impregnation seals voids but does not address the casting mechanism | Leak testing before and after to confirm sealing performance |
If you have a project, talk to our experienced sales team
Contact usAccess our library of aluminium sand casting photographs, video animations and articles.
