Types of porosity: gas and shrinkage
Gas porosity comes from air or gases trapped while the cavity fills, usually linked to turbulent flow or insufficient venting. Shrinkage porosity appears when the metal solidifies unevenly and the last areas to solidify do not receive enough liquid metal feed.
Telling the type of porosity apart is the first step to fixing it, because the causes and the solutions are different: gas porosity is tackled through filling and venting design, shrinkage porosity through feeding and wall thickness design.
Wall thickness and transitions
Abrupt thickness changes create late-solidifying areas surrounded by metal that has already solidified, which keeps them from receiving extra liquid metal as they shrink. Gradual thickness transitions reduce this risk significantly.
The thickest sections of a part should solidify before or at the same time as the thinner surrounding sections, or be designed with a dedicated feeding system that compensates for their shrinkage.
Filling and venting system
A poorly designed gating system creates turbulence that traps air in the stream of liquid metal before it can escape through the vents. Flow speed and path should be designed to fill the cavity progressively without excessive turbulence.
Vents should be placed at the last points to fill, with enough section to let air and gases out without freezing prematurely and blocking the outlet.
An oversized vent can create flash and a risk of metal spray. Vent design must balance gas evacuation with process safety.
Filling and solidification simulation
Software simulation predicts turbulence, air entrapment and late-solidifying areas before the tooling is built, avoiding costly trial-and-error iterations on the physical part.
Adjusting the design based on simulation, instead of discovering porosity in production or, worse, in service, significantly reduces the cost of correction.
Detection and acceptance criteria
Internal porosity is not always visible from the outside. Industrial radiography, leak testing or metallographic sectioning are common detection methods depending on how critical the part is.
Acceptance criteria should be set according to the part function: a critical structural part needs stricter criteria than a cosmetic or low-load part.
Checklist to prevent porosity
☐ Avoid abrupt section changes
☐ Design gradual transitions
☐ Favor directional solidification
☐ Identify mass concentrations
☐ Design a progressive flow path
☐ Avoid turbulent filling
☐ Place vents at the last points to fill
☐ Size vents correctly
☐ Simulate filling and solidification
☐ Adjust the design based on results
☐ Validate with a prototype part
☐ Document design changes
☐ Choose the inspection method by criticality
☐ Set acceptance criteria
☐ Record inspection results
☐ Update the design for recurring findings
Common questions.
Is porosity always a defect that scraps the part?
Not necessarily. It depends on the part function and the acceptance criteria defined. Minor porosity in a non-critical area may be acceptable, while the same porosity in a structural or pressure-tight area may be grounds for rejection.
Does filling simulation completely eliminate the risk of porosity?
It significantly reduces the risk by anticipating problem areas before the tooling is built, but it does not eliminate it entirely. The actual process, mold maintenance and molten metal control remain decisive.
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