Porosity in Die Castings: How to Prevent It in Zinc Die Casting

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Porosity is the most discussed — and most misunderstood — defect in die casting. In zamak alloy castings it shows up as micro- or macro-cavities inside the part, sometimes breaking through to the surface, and it can compromise leak tightness, mechanical strength and plating quality. Understanding its two distinct physical origins — trapped gas and solidification shrinkage — is the first step toward controlling it. This guide walks through the root causes, prevention strategies in both design and process, advanced technologies such as vacuum die casting, quality control methods and acceptable porosity thresholds. The approach at Micrometal, based in Erbusco (Brescia, Italy) with 35 years of experience in zinc die casting, is straightforward: prevent it upstream, don’t try to fix it downstream.

What is porosity in die castings: definition and types

Porosity is the presence of voids — either gas voids or shrinkage voids — inside or on the surface of a solidified casting. Sizes range from micropores (fractions of a millimeter) to macro-cavities visible to the naked eye. In hot chamber die casting, the process typically used for zamak, porosity forms through two clearly distinct mechanisms, each with its own causes and remedies.

Gas porosity

This originates from air and gases trapped in the die cavity during filling. Molten metal, injected in a matter of milliseconds, doesn’t give the air enough time to fully escape through vents and parting lines. Bubbles remain trapped and appear as rounded, often smooth-walled pores.

Shrinkage porosity

This forms naturally as hot liquid metal transitions to a solid state: volumetric contraction during solidification creates irregularly shaped cavities, typically in the thicker sections that solidify last. As referenced technical literature confirms (zinc.org — Mechanical Properties), practically all zinc die castings contain some degree of porosity: the term “sound casting” is therefore not an absolute. The EN 12844 standard governs zamak die casting alloys and their quality requirements.

Main causes of porosity in zinc die casting

High injection pressure allows the die to fill rapidly, which is necessary so the entire cavity fills before any portion of the casting solidifies — avoiding discontinuities even in the thinnest sections. But that same speed is exactly what causes the defect: there’s very little time for air to escape (Wikipedia — Die casting).

Here is the logical chain connecting typical causes to the two types of porosity:

Hot spots are sections of the casting that cool more slowly than surrounding areas because of greater volume: they generate abnormal shrinkage, porosity and cracking, and can be prevented with proper die cooling design (Wikipedia — Casting defect). An unfavorable gate location can also prevent molten metal from reaching every region of the cavity along the shortest flow path, leading to defects (zinc.org — General Remarks). For a complete overview of related defects, see our zinc die casting glossary.

How casting design affects porosity

The battle against shrinkage porosity is won — or lost — at the design stage. Zamak design guidelines (zinc.org — Design Rules) offer precise, quantitative criteria.

The inscribed-circle rule

If you imagine inscribing a circle within different sections of a casting, shrinkage porosity can occur in the thicker areas when the diameters of two adjacent sections differ by more than 3:1. If the ratio exceeds 6:1, the effect becomes severe and strength in that area is significantly reduced. Design should therefore aim for gradual thickness transitions.

Uniform wall thickness and reduced thermal mass

Uniform wall thickness supports even solidification. Where strong sections are needed, it’s better to replace solid mass with ribs and fillets: this achieves the same stiffness while reducing thermal mass — and with it, the risk of hot spots and shrinkage cavities. Sharp corners should always be radiused.

At Micrometal, these checks are part of the DFM (Design For Manufacturing) analysis carried out during tooling engineering. Reviewing our zinc die casting design guidelines and our machinery fleet page helps set up a project correctly from the start, avoiding costly changes once the tool has already been built.

The role of the gating system, runners and vents in prevention

If part geometry drives shrinkage porosity, the gating system is largely what governs gas porosity. Gate, runner and vent design is where a die caster’s real know-how comes into play.

Vents placed along the parting lines let air escape during filling: they are the primary remedy against gas entrapment (Wikipedia — Die casting). Gate and runner sizing must ensure orderly, non-turbulent flow, which reduces air entrainment in the molten metal. A shorter flow path and a balanced cavity fill minimize colliding metal fronts that generate bubbles.

Critical process parameters: speed, pressure and temperature

With the same tool, injection parameters make the difference between a compliant casting and a porous one. Each lever acts predominantly on one of the two porosity types.

Parameter	Acts against	Technical effect
Injection speed	Gas porosity	Controls flow regime and air evacuation; poorly calibrated speed increases turbulence and entrapment
Intensification pressure	Shrinkage porosity	Compresses the metal during solidification, compensating for volumetric contraction and closing micro-voids
Die temperature	Shrinkage + hot spots	A die held at roughly 150-200°C manages solidification rate and reduces hot spots
Pour temperature	Fill quality	Zamak is injected at approximately 415-430°C: this ensures fluidity without overheating, which would otherwise promote gas pickup

Settings are not universal — they change with the alloy. The four zamak alloys Zamak 3, Zamak 5, Zamak 2 and ZP8 have different solidification behaviors (Zamak 3 is the most ductile, Zamak 2 the hardest and strongest, ZP8 the highest in aluminum content). With our fleet of 11 hot chamber machines — 7 Agrati, Italpresse and Frech presses plus 4 robotic cells, in the 20-90 ton range — we fine-tune parameters specifically for each component and alloy, following EN 12844 guidelines and NADCA process best practices.

Advanced technologies: vacuum die casting and CAE simulation

When the standard process isn’t enough, there are technologies that address the root causes more directly.

In vacuum-assisted high pressure die casting (VHPDC), a vacuum pump removes air and gas from the die cavity and the metal delivery system before and during injection. The result: significantly reduced porosity, the ability to heat-treat and weld the part (both impossible on standard castings, where heat causes trapped gas in the pores to expand, creating micro-cracks and surface blistering), and improved surface finish (Wikipedia — Die casting).

Aspect	Standard HPDC	VHPDC (vacuum-assisted)
Gas porosity	Present, controllable with venting	Significantly reduced
Heat treatment / welding	Not recommended	Possible
Typical applications	Non-structural components	Critical structural components
Cost / complexity	Standard	Higher

Vacuum assistance isn’t necessary for every zamak alloy, however: for Zamak 3/5/2/8 in non-structural applications, a well-designed standard HPDC process is generally sufficient. It’s an additional technology to be evaluated case by case, not a blanket standard.

Upstream, CAE/CAD simulation of fill and solidification makes it possible to predict where shrinkage cavities will form and to correct geometry and gating design before the steel is ever cut — avoiding higher scrap rates and higher unit costs (zinc.org). It’s the tool that turns porosity prevention from empirical craft into engineering discipline.

How porosity is measured and controlled: inspection methods

Prevention alone isn’t enough — verification is essential. Internal porosity, by definition invisible from the outside, requires dedicated inspection methods.

Radiographic inspection (X-ray)

The reference method for detecting internal cavities. The ISO 9915 standard specifies the rules for implementing radiographic inspection on alloy castings, including acceptance criteria for discontinuities. Reference radiographs ASTM E505 (zinc alloy die castings), ASTM E192 (microporosity) and ASTM E155 form the visual standard for classifying defect severity.

Leak testing

For castings intended to contain fluids or gases, leak testing (pressure decay or similar methods) verifies the absence of through-leak paths generated by interconnected porosity.

When porosity is acceptable: thresholds and impregnation as a remedy

Since some degree of porosity is inherent to HPDC, the right question isn’t “is there porosity?” but “does the porosity present compromise the part’s function?”

Technical sources (zinc.org — Mechanical Properties) indicate that when the porosity level falls between roughly 1% and 5% and individual pores are very small, the effect on strength is not significant. However, distribution matters as much as the nominal percentage. A casting with porosity concentrated in a structurally critical area is more problematic than one with porosity spread evenly throughout. Designers and die casters therefore need to ensure integrity precisely in functional areas.

Vacuum impregnation

For castings that must be leak-tight, vacuum impregnation is a downstream remedy: a sealant penetrates and closes porosity and leak paths. It is governed by military standards such as MIL-STD-276A and MIL-I-17563C. It’s important to understand its proper role: it is not a prevention technique, but a complementary post-casting solution addressing leak tightness alone.

Surface-breaking porosity is also the enemy of plating quality: surface pores trap plating solutions and cause defects in electroplated finishes, including the demanding Cu-Ni cycle (copper undercoat plus nickel). This is exactly why Micrometal’s approach is to prevent porosity upstream — through tooling design, gating system engineering and optimized process parameters — rather than relying on costly downstream fixes.

FAQ: frequently asked questions about porosity in zinc die castings

Can porosity be completely eliminated in an HPDC casting?

No. Technical sources (zinc.org, Wikipedia — Die casting) confirm that some residual porosity is inherent to the standard high pressure die casting process, even under highly refined conditions. The realistic goal is to minimize it and keep it away from critical areas.

Does porosity always affect mechanical strength?

Not always. With porosity between 1% and 5% and very small pores, the effect on strength is negligible. It becomes critical when concentrated in structurally stressed areas: distribution matters as much as the percentage.

Why is porosity a problem for plating?

Surface-breaking pores trap plating solutions and gases, causing blistering, staining and delamination in the finish. A surface free of exposed porosity is essential for cycles like Cu-Ni nickel plating.

When is vacuum die casting worth using?

When you need structural components, parts that must be heat-treated or welded, or particularly strict porosity requirements. For most non-structural applications in Zamak 3/5/2/8, a well-designed standard HPDC process is sufficient.

Do you have a component with leak-tightness, structural integrity or critical finish requirements? Request a technical consultation with Micrometal: our technical team evaluates geometry, alloy and process to minimize porosity starting from the design stage. Phone +39 030 7760830.

Technical review: Marco Sega

Content produced with the assistance of artificial intelligence systems and subject to technical oversight by our editorial team. Editorial responsibility: Micrometal S.r.l.