HPDC vs Gravity
Die Casting.

In high pressure die casting (HPDC), molten aluminium is injected into a hardened steel die at pressures of 50–150 MPa. Injection is fast — typically under one second — which enables thin walls, complex geometry, and short cycle times. The process is highly automated and suited to large production volumes.

In gravity die casting (GDC), molten metal is poured into a permanent steel die and fills under the force of gravity. The fill is slower and more controlled. Cycle times are longer, output per shift is lower, and wall thicknesses are generally greater — but the slower solidification and absence of injection turbulence produce castings with lower gas porosity and more uniform grain structure.

HPDC tooling is expensive. A production-quality HPDC die typically costs ₹8–25 lakh depending on component complexity, number of cavities, and required die life. This investment is justified at production volumes of 5,000 parts per year and above — at high volume, the per-part cost advantage of HPDC is significant.

GDC tooling costs considerably less — typically ₹2–6 lakh for a production die. The lower tooling investment makes GDC economically viable at volumes of 500–5,000 parts per year, and sometimes below that for high-value components. For prototype runs and design validation, GDC can also serve as a faster and cheaper qualification path before committing to HPDC tooling.

HPDC can produce wall thicknesses of 1.5–2.5 mm reliably, and even thinner walls in optimised designs. This is one of the key reasons HPDC is preferred for consumer and automotive components where weight reduction matters.

GDC minimum practical wall thickness is approximately 3 mm. Components with complex coring, thin ribs, or intricate internal channels are generally better suited to HPDC. For robust, solid sections — pump bodies, valve housings, structural brackets — GDC geometry limitations are rarely a constraint.

Gas porosity is an inherent consideration in any die casting process. In HPDC, the high-velocity injection entraps air and produces gas porosity throughout the casting — typically distributed but small. This porosity limits the heat treatability of HPDC alloys; most HPDC alloys cannot be solution heat treated without blistering.

GDC produces castings with significantly lower gas porosity because the gravity fill eliminates injection turbulence. Lower porosity means GDC castings can be heat treated — LM25 in T6 condition achieves tensile strengths of 230 MPa or more, versus 160–180 MPa for a typical HPDC casting in ADC12. For components that must hold pressure (pump casings, valve bodies), pass radiographic inspection, or carry structural load, GDC is often the technically correct choice.

HPDC alloys are selected for good flowability at high injection speeds. ADC12 (equivalent to A383) and A380 are the most common — both offer good castability and adequate strength for most applications. LM24 is also used where higher strength or elevated temperature resistance is required.

GDC alloys are typically from the Al-Si-Mg family. LM25 (equivalent to A356) is the dominant choice — it has excellent castability for gravity pour, responds well to T5 and T6 heat treatment, and delivers superior mechanical properties compared to HPDC alloys in their as-cast state. A356 is the ASTM equivalent of LM25 and is specified interchangeably on many European and North American drawings.

Choose HPDC when: annual volume exceeds 5,000 parts, wall thickness below 3 mm is required, cycle time and per-part cost are primary drivers, or the alloy and geometry are standard for HPDC.

Choose GDC when: volume is below 5,000 parts per year, the component must be heat treated for structural strength, pressure tightness or X-ray quality is required, the alloy specified is LM25 or A356, or the component carries structural load.

Hybrid programmes are possible — some OEMs run HPDC for high-volume standard variants and GDC for structural or low-volume derivatives within the same product family. SAPL operates both processes under one quality system, which simplifies supplier qualification for programmes requiring both.