Guide
When do you need vacuum die casting instead of standard high pressure die casting?
Vacuum die casting is high pressure die casting with one added step: a pump pulls the air and gas out of the die cavity and the metal delivery path before and while the metal is injected. What changes is how much gas ends up sealed inside the part, and that decides whether the casting can later be solution heat treated, welded or trusted to hold pressure without a sealing step.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design engineers and sourcing managers deciding whether a die-cast part needs vacuum assist written into the RFQ, or whether conventional high pressure die casting (HPDC) plus the right inspection and sealing steps will do the job. It works from the downstream requirement backward: heat treatment, welding, leak tightness and machined sealing faces. It then covers drawing language that makes either route verifiable.
On this page
How does vacuum die casting differ from standard HPDC?
In standard HPDC the air in the cavity is pushed out ahead of the metal through vents and overflows, while in vacuum die casting it is actively removed by a pump before and during the shot. Both fill the die in a fraction of a second under high pressure; only the handling of the air differs.
In a cold-chamber machine, which is how aluminum is cast, molten metal is ladled into a sleeve and a hydraulic plunger drives it into the locked die at high pressure (NADCA, cold chamber process). The cavity, the runners and the sleeve all start the shot full of air, plus vapor from die lubricant. In a conventional die, that gas has to escape through thin vents and into overflows at the far end of the flow. The overflows are trimmed off afterward; NADCA lists gates, runners and overflows among the metal trimmed off as flash (NADCA, secondary processing).
Passive venting is a race: fast, turbulent metal wraps pockets of air into the flow before they reach a vent. A vacuum system connects the cavity to a pump through a valve or chill block, draws the air out as the plunger starts to move, and closes before metal reaches it. The ASM Handbook defines the process the same way: evacuating air and gases from the cavity and the metal delivery system before and during injection (ASM Handbook Vol. 15, 2008).
Vacuum assist is not the same thing as squeeze casting or semi-solid casting. Those change how the metal fills the die; NADCA groups all three together in its standard for high-integrity and structural die casting (NADCA, high integrity standards). If a drawing just says “high-integrity casting,” ask which of the three the supplier means.
Why does trapped gas matter in a die casting?
Trapped gas becomes gas porosity: small rounded voids that weaken the section, open up when the part is machined, and expand when the part is heated. Vacuum reduces this type of porosity; it does little for shrinkage porosity, which comes from the metal contracting as it solidifies.
Die castings have a built-in defense. The metal against the die steel freezes first and forms a dense, fine-grained skin, and that rapid freezing tends to push porosity toward the middle of the wall (NADCA, structural criteria). NADCA gives the skin a typical depth of 0.015 to 0.020 in (0.38 to 0.50 mm) on the same page. So an as-cast surface can look sound over a porous core, and every machined face cuts through that skin and exposes whatever is beneath it, which is why a sealing face or an O-ring groove is where porosity problems tend to surface.
Identify the void type before paying for vacuum. Gas voids tend to be round and smooth-walled; shrinkage voids tend to be irregular and sit in the heavy sections that freeze last. Vacuum attacks the first. The second is a design and gating problem, and a heavy boss full of shrinkage will not be fixed by a vacuum valve.
Can a vacuum die casting be heat treated or welded?
Often yes, with the right alloy, which is the main reason vacuum die casting exists. A conventional die casting is generally not solution heat treated or welded, because the gas inside it expands at high temperature and blisters the surface.
NADCA states the rule and the exception plainly. Aluminum die castings are not usually solution heat treated; low-temperature aging treatments such as T5 are used for stability and modest property gains, and heat-treatable specialty alloys such as AA365 exist for structural work. The same page says die castings are not generally gas or arc welded or brazed, but that high-integrity processes combined with specialty alloys have allowed welding in specific applications (NADCA, aluminum alloys). For magnesium, NADCA explains the mechanism directly: temper designations are not used for die-casting alloys because even small amounts of porosity can cause blistering at heat-treating temperatures (NADCA, magnesium alloys).
Vacuum die casting has been used for this purpose for decades. A 1994 SAE technical paper describes a production vacuum process, in service from 1992, that made alloy 356.0 aluminum brackets able to take a full T6 heat treatment, with strength matching comparable permanent-mold and lost-foam castings, and it mentions the same technology being used for leak-tight compressor housings (SAE 941745, 1994).
Two points for the specification. First, vacuum alone does not make a part heat treatable; the alloy has to be a heat-treatable casting alloy, and the general-purpose aluminum-silicon-copper grades used for most housings (A380 and ADC12 among them) are not usually solution treated. The ADC12 vs A380 guide covers those grades. Second, ASTM B85 says it is written for general-purpose die castings and may not address the properties, integrity testing and verification needed for highly loaded or safety-critical parts (ASTM B85/B85M-25). A structural vacuum casting therefore needs its own agreed requirements, not just a B85 callout.
When is vacuum assist worth specifying instead of impregnation?
Specify vacuum when the part must be heat treated, welded, or strong in sections that will be machined; accept conventional HPDC plus impregnation when the only problem is leakage through fine porosity in a part that is otherwise structurally sound.
Impregnation forces a sealant into the surface pores of a finished casting. NADCA notes that its cost can be kept down by pressure testing every casting and impregnating only those that fail (NADCA, secondary processing). The US military standard on the subject, MIL-STD-276A, is scoped to the impregnation of structurally sound castings (MIL-STD-276A, 1992). That wording is the useful boundary: impregnation seals leak paths, but it does not restore the strength lost to voids, and it cannot survive a later process that heats the part enough to blister it.
| Requirement | Conventional HPDC | Conventional HPDC + impregnation | Vacuum-assisted HPDC |
|---|---|---|---|
| How cavity air leaves | Pushed out through vents and overflows | Same as conventional | Pumped out before and during the shot |
| Gas porosity | Present, mostly in the core of the wall | Unchanged; surface-connected pores are sealed | Reduced |
| Shrinkage porosity | Depends on design and gating | Unchanged | Not addressed by vacuum; still a design and gating issue |
| Pressure tightness | Test; leakers rejected or sealed | Leak paths sealed after testing | Improved at the source; testing still advisable |
| Solution heat treatment (T6) | Not usually done; blistering risk | Not suitable; blistering risk remains | Possible with a heat-treatable alloy |
| Welding | Not generally done | Not generally done | Possible in specific applications with suitable alloys |
| How to verify | Radiography per ASTM E505, sectioning, leak test | Leak test after impregnation | Radiography per ASTM E505, mechanical tests agreed with the buyer |
A cover that is machined on one gasket face, leak tested and powder coated is usually a conventional-HPDC part, with impregnation kept as a controlled rework step. A suspension bracket that is solution treated and aged, or a housing that is welded to a tube, is a vacuum candidate from the first design review, because no amount of sealing afterward fixes the blistering or weld porosity. The expensive case is a conventional casting that later picks up a heat treatment or weld in an engineering change without anyone revisiting the process.
Vacuum adds valves, seals, a pump system and die maintenance, so ask for it as a separate quote line and see what it adds for your part.
How should a drawing specify porosity for either process?
Specify the result, not the process alone: name the zones that matter, the acceptance level against a reference standard such as ASTM E505, and the leak test, then state vacuum assist as a requirement only if heat treatment or welding depends on it.
ASTM E505 is a set of reference radiographs showing categories of discontinuity in aluminum and magnesium die castings at four levels of increasing severity. It does not set a pass mark by itself; the standard is designed so that the buyer writes acceptance in terms of those images (ASTM E505-22). A workable drawing note therefore has three parts: the zone (for example, “within the sealing face and bolt bosses, Zone A”), the reference (“per ASTM E505”) and the accepted severity level for each category in that zone. Leave non-critical zones looser or unspecified.
Machined faces need their own line. Because porosity sits under the cast skin, say how much stock comes off each sealing face and what is acceptable once it is exposed, for example no visible voids in the seal land after machining. The tolerances guide covers how cast and machined dimensions sit together on one drawing. For a part that will be leak tested, give the test medium, pressure and pass criterion as numbers you own, and state whether impregnation is permitted as rework and to which standard.
Write “vacuum die cast” on the drawing only when a T6 heat treatment or a weld makes the process itself the requirement. Otherwise a process callout without an acceptance level gives you no way to reject a bad lot: the supplier ran the vacuum, and the parts still leak.
What should you ask a supplier quoting vacuum die casting?
Ask what vacuum means on their machine, how they know it worked on each shot, and what evidence ships with the parts. The same questions apply to a caster in Vietnam, China, Mexico or Ohio; distance only makes the written answers more important.
Useful questions for the quote stage:
- Is the quoted process conventional venting, vacuum assist, or a high-vacuum structural process, and is it the same process that will run in production?
- Is the cavity vacuum level monitored and recorded for each shot, and are shots outside the limit scrapped automatically?
- Which radiographic acceptance level, per ASTM E505 or a digital equivalent, will be used at first article and how often afterward?
- If a heat treatment follows, who performs it, to what temper, and how are blisters inspected?
When die-cast parts move from China to Vietnam or elsewhere in Southeast Asia, process detail is easily lost in the transfer. An existing die may have been built with vacuum valves and chill vents that the new cell does not use, and a conventional shot through a vacuum-designed die can produce parts that look right and fail later at heat treatment. Write the process and acceptance criteria into the RFQ. The RFQ preparation guide has a checklist for the rest of the package, and the die casting page lists the alloys cast here, ADC12 aluminum and Zamak 3 zinc, so you can see before you send a drawing whether your alloy is one of them.
Frequently asked
Is vacuum die casting the same as vacuum impregnation?
No. Vacuum die casting removes air from the die before the metal goes in. Vacuum impregnation is a sealing step done after casting, which uses vacuum to pull sealant into pores. One prevents gas porosity; the other seals leak paths through it.
Does vacuum die casting eliminate porosity?
It reduces gas porosity but does not remove shrinkage porosity, which depends on wall design and how the last-freezing sections are fed. Specify an acceptance level rather than “porosity-free.”
Can I T6 heat treat an A380 or ADC12 die casting if it is vacuum cast?
NADCA describes aluminum die castings as not usually solution heat treated and points to heat-treatable specialty alloys for structural work. Treat T6 as an alloy decision first and a process decision second, and confirm it with the caster and alloy producer.
Will vacuum casting help a part that blisters in powder coating?
It can, if the blisters come from gas trapped near the surface expanding during the cure. Check first with sectioning or radiography, and see the finishing comparison for how coatings behave on cast parts.
What should I send to get a die-casting quote with porosity requirements?
The drawing with zoned porosity notes, the alloy by standard, any heat treatment, weld or leak test, and annual volume. Send it for a quote or email rfq@saolamfg.com.
Sources
- Vacuum High-Pressure Die Casting, W. A. Butler, ASM Handbook Vol. 15: Casting, ASM International, 2008 (abstract)
- Vacuum Die Casting Technology with True T6 Heat Treating Capability, C. A. Nelson, SAE Technical Paper 941745, SAE International, 1994 (abstract)
- Aluminum Alloys: Heat Treatment & Welding, NADCA Design, North American Die Casting Association, accessed September 2026
- Magnesium Alloys, NADCA Design, accessed September 2026
- Structural Criteria (skin effect), NADCA Design, accessed September 2026
- Secondary Processing Costs (flash trimming, impregnation), NADCA Design, accessed September 2026
- Cold Chamber Process, NADCA Design, accessed September 2026
- Standards for High Integrity and Structural Die Casting Process (table of contents), NADCA, accessed September 2026
- ASTM B85/B85M-25, Standard Specification for Aluminum-Alloy Die Castings (scope), ASTM International, 2025
- ASTM E505-22, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Die Castings (scope), ASTM International, 2022
- MIL-STD-276A, Impregnation of Porous Nonferrous Metal Castings, US Department of Defense, 1992