What is the difference between gravity die casting and high pressure die casting?

How does each process get metal into the mold?

In gravity die casting the metal is poured into a preheated metal mold and fills it under gravity, sometimes with the mold tilted to calm the flow. In high pressure die casting a plunger forces the metal into a clamped steel die at high speed and pressure, and holds it there while it freezes.

The US Department of Energy’s profile of the metalcasting industry describes both routes. For permanent molds, metal is introduced into the preheated mold either under gravity or with a low-pressure assist, and variants such as bottom filling and tilt pouring exist specifically to avoid the turbulence that causes porosity. For die casting, the same report gives injection pressures for aluminum and magnesium alloys ranging from 3,000 psi to more than 10,000 psi (DOE metalcasting profile, 1999).

Aluminum is die cast on cold-chamber machines: the melt is ladled into a sleeve and a hydraulic plunger seals the port and pushes the metal into the locked die (NADCA, cold chamber process). Zinc alloys such as Zamak 3 run on hot-chamber machines, where the injection mechanism sits in the molten metal.

The fill method also decides what can form the inside of the part. A permanent mold can take a sand core for passages that metal cores cannot be pulled out of; the DOE report calls that variant semi-permanent molding. A sand core would not survive a high-pressure shot, so every internal shape in an HPDC part has to be formed by steel cores and slides that retract before the part is ejected.

Why are A356 and A380 used in different processes?

The Aluminum Association registers A356.0 for sand and permanent mold castings and A380.0 for die castings, and the chemistries show why: A380.0 carries the iron that keeps aluminum from soldering to a steel die, while A356.0 keeps iron and copper low and adds magnesium so the casting can be heat treated.

The rows below are copied from the Aluminum Association’s registration record; the product code shows which process each composition was registered for.

Iron is the clearest split. NADCA explains that iron-free liquid aluminum attacks die steel and sticks to it, and that iron in the range of 0.60% to 1.20% in die casting alloys prevents this (NADCA, aluminum alloys). A356.0 is capped far below that range, which suits a slow gravity fill in a coated mold but not a high-velocity shot against bare die steel. The same NADCA page notes that copper raises strength and hardness but lowers corrosion resistance, which is the trade A380.0 makes.

Magnesium is the other split. The controlled magnesium in A356.0 is what lets the alloy respond to a full solution-and-age heat treatment, and it is the reason drawings so often read “A356-T6”. ADC12, the Japanese die casting grade common in Asian quotes, sits in the high-iron, copper-bearing die casting family with A380; the ADC12 versus A380 guide covers how those two relate. Asian foundries often quote gravity castings in JIS AC-series grades rather than AA numbers, so ask which standard the quoted grade comes from and compare it with the alloy on your drawing.

Which process leaves less porosity where you machine?

Neither process is porosity-free, but the porosity sits in different places. An HPDC part has a dense skin with gas porosity pushed toward the center of thick sections, while a gravity casting fills more calmly and depends on its gating and feeding to control shrinkage porosity.

NADCA describes the skin effect in die castings: metal at the die face freezes fastest into a dense, fine-grained layer that is typically 0.015 to 0.020 in (0.38 to 0.50 mm) deep, and that rapid freezing drives porosity toward the middle of the section (NADCA, structural criteria). A heavy machining cut removes that skin and opens the center. For HPDC parts, keep machining stock light on sealing and bearing faces, keep walls near-uniform so there are few thick sections, and put leak or X-ray requirements on the drawing rather than assuming them.

For permanent mold castings, ASTM B108 makes porosity control part of the foundry’s job: its abstract calls for radiographic or other approved examination until gating, pouring and other practices are proven to meet the purchaser’s quality standard. Both processes still need the machined faces identified on the drawing, and the CNC milling page shows the kind of post-cast machining such faces receive.

When should you move a part from gravity casting to HPDC, or back?

Move toward HPDC when volumes are high, walls can be thin and uniform, and the part works with as-cast or T5 properties in a high-iron alloy such as A380 or ADC12. Stay with gravity casting when the part needs T6 strength and ductility, sand-cored internal passages, or is too large or too thick for a die casting machine to fill soundly.

A conversion from gravity to HPDC is really a redesign. Expect to change the alloy, reduce and even out wall thickness, add draft suited to a steel die, move internal passages to shapes that pulled cores can form, and repeat the strength and leak testing.

Sourcing from Vietnam or elsewhere in Asia adds a vocabulary step. Write the process and the governing specification on the drawing, for example “A356.0-T6 permanent mold casting per ASTM B108” or “A380.0 die casting per ASTM B85”, and say whether the supplier may propose a different process. Finish belongs in the same note: NADCA gives 380 a middling 3 of 5 for anodized appearance, which the finishing comparison guide discusses. The RFQ preparation guide lists what else the package should carry.

For context on this site: the die casting Saola Manufacturing sources from partner foundries is high pressure die casting in ADC12 aluminum and Zamak 3 zinc, and the quote names the production site and origin. Gravity or permanent mold casting is not a process Saola offers. If you are weighing a move of a gravity-cast part to HPDC, send the drawing with the current alloy and temper, and mark which properties are fixed, so the fit with ADC12 is discussed before any tooling.