Vacuum Pressure Impregnation (VPI) -Sealing Porosity in Die Castings

Vacuum pressure impregnation (VPI) is a post-casting process that seals the internal pore network in die castings, converting parts that fail pressure tests into reliably pressure-tight components. VPI is an automotive-qualified process used globally on millions of hydraulic and pneumatic die castings annually.

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How VPI Works

VPI uses pressure differential to force sealant into pore networks inaccessible from the casting surface:

1. Vacuum: Parts are loaded into a pressure vessel evacuated to <1 mbar. Air is drawn out of open pore networks.
2. Impregnant injection: With vacuum maintained, methacrylate resin floods the vessel, surrounding the parts. Resin fills evacuated pore networks.
3. Pressure: Vessel raised to 7-10 bar, forcing resin into the finest pore channels.
4. Wash: Parts are removed; surface resin is washed away, leaving resin only within pores.
5. Cure: Parts immersed in hot water (90-95°C) or IR oven. Heat polymerizes the resin into a solid, chemically resistant plug.

The cured resin bonds to pore walls and cannot be removed by operating pressure, service temperature, or chemical exposure within the sealant's specification range.

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What VPI Seals -and Cannot Seal

VPI seals:

• Interconnected micro-porosity networks (gas porosity, micro-shrinkage)
• Pore networks connecting two surfaces through walls up to 25 mm thick

VPI cannot seal:

• Through-cracks (structural defects -requires welding)
• Surface-open porosity exceeding ~0.5 mm diameter (sealant washes out)
• Macro-shrinkage visible to the naked eye

VPI is a standard post-process for castings with HPDC-typical micro-porosity -not a remedy for out-of-specification gross defects.

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Performance Data

VPI-treated A413 and A380 aluminum castings at KastMfg production programs:

Part	Alloy	Test	Pressure	Result
Hydraulic valve body	A413	Air decay 100 bar, 60s	Pass	<2 cm³/min
Pneumatic actuator body	A380	Air decay 16 bar	Pass	<0.5 cm³/min
Compressor housing	ADC12	Helium leak	Pass	<10^-6 mbar·L/s
Coolant manifold	A380	Nitrogen 5 bar	Pass	No decay

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VPI vs Vacuum-Assisted HPDC (VADC)

Factor	VPI (post-casting)	VADC (during casting)
Timing	After casting	During casting
Tooling investment	None	Vacuum system per machine
Per-part cost	$0.50-3.00	Added cycle time
Porosity	Sealed (functionally tight)	Physically reduced (Class 1-)
T6 heat treatment enabled?	No -porosity still present physically	Yes -porosity physically absent
Best for	High-volume moderate-pressure programs	T6-required or ultra-low leak rate

Choose VPI when: Standard HPDC castings need 50-100 bar leak performance; T6 heat treatment not required; large volume makes VADC tooling investment unattractive.

Choose VADC when: T6 is required; helium leak rates below 10^-7 mbar·L/s; structural integrity at porosity sites is critical.

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Sealant Compatibility

Modern anaerobic methacrylate VPI sealants are compatible with:

• Hydraulic oils (mineral, synthetic, biodegradable)
• ATF (automatic transmission fluid)
• Engine coolant (ethylene glycol/water)
• Refrigerants (R134a, R1234yf, CO2
• Most industrial fluids

Service temperature: -0°C to +175°C cured. No softening or outgassing within this range.

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VPI inquiry: yaoqingpu1983@gmail.com | +86 138 1403 4409 | No.6, Rungu Road, Nanjing, China