High Pressure Die Casting:
A Complete Guide for Design Engineers

High pressure die casting (HPDC) is a manufacturing process in which molten aluminium alloy is injected into a hardened steel die under high pressure — typically between 70 and 700 MPa. The metal solidifies rapidly under pressure, producing a dense, dimensionally accurate casting with excellent surface finish.

The process is suited to high production volumes. Once a die is qualified, cycle times typically range from 30 to 120 seconds depending on component size and wall thickness. SAPL operates five HPDC machines ranging from 180 to 450 tonnes of clamping force, enabling components from small precision housings to larger structural brackets.

The process begins with die preparation. The tool — a two-piece hardened steel die — is cleaned, lubricated, and clamped shut under the machine's locking force. Molten aluminium alloy is ladled or transferred into the shot sleeve. A plunger then drives the metal through a runner system into the die cavity at high velocity.

Once the cavity is filled, intensification pressure is applied to compensate for solidification shrinkage. The casting solidifies within seconds. The die opens, ejector pins push the casting out, and the cycle repeats. Overflow wells at the periphery of the cavity capture the first, gas-rich metal to enter — these are trimmed in a subsequent press operation.

For components requiring tight-tolerance bores, profiles, or threaded features, the casting then moves to CNC or VMC machining. Powder coating or anodising may follow depending on the application.

The three alloys most commonly used in HPDC are ADC12, A380, and LM24, each with a distinct property profile.

ADC12 (equivalent to A383) is the most widely used HPDC alloy in India and globally. It offers excellent fluidity, good corrosion resistance, and is well-suited to complex thin-wall geometries. Tensile strength of approximately 310 MPa. SAPL uses ADC12 for ceiling fan covers, mixer grinder housings, and most consumer appliance components.

A380 provides slightly higher tensile strength (~320 MPa) with good thermal conductivity and machinability. It is the preferred alloy for automotive and industrial components where mechanical performance is critical. SAPL uses A380 for automotive pump housings, solenoid housings, and tractor covers.

LM24 is a eutectic alloy with outstanding fluidity — ideal for thin-wall, complex components where complete die fill is the primary challenge. Commonly used in electrical enclosures and intricate housings.

HPDC produces castings with linear dimensional tolerances typically in the range of ±0.1 to ±0.3 mm depending on feature location, wall thickness, and die parting line proximity. Features that lie entirely within one half of the die (no parting line crossing) achieve the tightest tolerances.

As-cast surface finish on HPDC components is typically Ra 1.6 to Ra 3.2 µm — significantly smoother than sand casting or gravity die casting. Features requiring Ra ≤ 0.8 µm, such as sealing faces or bearing bores, are achieved through CNC finish machining after casting.

SAPL achieves CNC machining tolerances to ±0.02 mm using our VMC machining centre with 4th-axis capability.

The geometry of a component determines whether it is suitable for HPDC and how cost-effective production will be. Several DFM principles apply universally.

Wall thickness should be uniform where possible, ideally between 1.5 and 4 mm for HPDC. Abrupt changes in wall section create shrinkage porosity and hot tears. Where thick sections are unavoidable, coring — the use of internal die inserts — can reduce mass and improve solidification uniformity.

Draft angles allow the casting to be ejected cleanly from the die. External surfaces typically require 1–2° of draft; internal surfaces (cores) require 2–3°. Zero-draft features require slides in the die, which increase tooling cost and cycle time.

Undercuts — features that would prevent the die from opening in the pull direction — require side actions (slides) in the die. Slides add cost and reduce die life. Where a design allows, undercuts should be relocated to a machined feature rather than a cast feature.

Fillets and radii at all internal corners improve metal flow, reduce stress concentration, and extend die life. Minimum internal fillet radii of 0.5 mm are standard; 1.5 mm or greater is preferred for structural features.

HPDC is the right process when volume justifies the tooling investment and when component geometry permits ejection from a two-piece die. For components produced in quantities below approximately 5,000 per year, gravity die casting or sand casting may be more economical due to lower tooling cost.

Gravity die casting using LM25 alloy produces components with superior mechanical properties — higher tensile strength, better elongation, and lower porosity — because metal fills the die under gravity rather than high-velocity injection. For applications requiring pressure tightness (hydraulic bodies, pump housings for higher pressures) or heat treatment, GDC or low-pressure die casting is preferred.

For components with extreme geometric complexity or very low volume, investment casting or CNC machining from billet may be appropriate. SAPL's engineering team conducts DFM analysis on every enquiry to recommend the most suitable process before tooling is committed.