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Aluminium die casting glossary

61 terms used every day in aluminium die casting, from alloy and melt to die, shot, heat treatment and inspection, defined in plain technical language.

A

Alloy: Aluminium mixed with silicon, copper, magnesium and other elements to give the castability and properties a part needs.
As cast (F, M): A casting that has not been heat treated. F is the Aluminum Association temper; M is the old British designation.

B

Biscuit: The slug of solid metal left in the shot sleeve in front of the plunger after each HPDC shot. It is trimmed off with the runner.
Blister: A raised bubble on the surface caused by gas trapped just below the skin expanding when the casting is heated.

C

Clamping force: The force that holds the two die halves closed against metal pressure, stated in tonnes or kN. It sets the size of part a machine can make.
Cold chamber machine: An HPDC machine where metal is ladled into a separate shot sleeve for each shot. Used for aluminium because molten aluminium attacks a submerged injection system.
Cold shut: A visible line where two metal fronts met but did not fuse, usually because the metal or die was too cold or the fill too slow.
Core: A die part that forms an internal feature or hole. A moving core is pulled out before ejection.
Cycle time: The time for one complete casting cycle: close, inject, solidify, open, eject and spray.

D

DCTG: Dimensional casting tolerance grade under ISO 8062-3. Lower numbers mean tighter tolerances.
Degassing: Removing dissolved hydrogen from molten aluminium, usually by bubbling nitrogen or argon through a rotating impeller.
Density index: A measure of gas porosity potential: the density difference between a sample solidified in air and one solidified under reduced pressure, as a percentage.
Die: The steel tool, in two halves, whose cavity gives the casting its shape. Also called a mould or tool.
Die life: The number of shots a die makes before it needs major repair or replacement, limited mainly by heat checking and erosion.
Die release agent: A water based spray applied to the die between shots to cool the surface and stop the casting sticking.
Draft angle: The taper on walls parallel to the die opening that lets the casting release cleanly.
Dross: Oxides and other non metallic material that forms on and in molten aluminium. It is skimmed off and controlled with flux.

E

Ejector pin: A pin that pushes the solid casting out of the moving die half. It may leave a small round mark.
Erosion: Wear of the die surface where fast moving metal strikes it, typically opposite the gate.

F

Fettling: Removing runners, overflows, flash and parting line material from a casting by hand tools, grinders or trim presses.
Fill time: The time to fill the die cavity during the fast shot, typically a few tens of milliseconds in HPDC.
First article inspection (FAI): A full dimensional and material check of the first parts from a new or changed tool before series approval.
Flash: Thin fins of metal that escape between die parts at the parting line, slides or ejector pins.
Flux: A salt mixture added to molten aluminium to separate oxides into the dross and clean the furnace walls.

G

Gate: The narrow opening through which metal enters the die cavity from the runner.
Gate velocity: The speed of metal through the gate during the fast shot, typically tens of metres per second in HPDC.
Grain refinement: Adding titanium and boron, usually as Al Ti B master alloy, to produce fine, uniform grains, mainly in gravity castings.
Gravity die casting: Pouring metal into a reusable steel die under gravity, often with a tilting machine. Also called permanent mould casting.

H

Heat checking: A network of fine cracks on the die surface caused by repeated heating and cooling. It leaves a raised pattern on castings.
High pressure die casting (HPDC): Injecting molten metal into a steel die at high speed and pressure, then intensifying the pressure as it solidifies.
Hot tear: A crack that forms while the casting is still partly liquid, where shrinkage is restrained by the die or a section change.

I

Intensification: The third phase of an HPDC shot: a rapid pressure increase after filling that feeds shrinkage and reduces porosity.

L

Leak test: A pressure or vacuum test that proves a casting will hold air or fluid, usually by pressure decay or helium.

M

Misrun: A casting that is incomplete because metal solidified before the cavity filled.
Modification: Adding strontium or sodium to change the silicon in Al Si alloys from coarse plates to a fine fibrous form, improving ductility.

O

Overflow: A small pocket at the end of the fill path that collects cold metal, oxides and air, so they stay out of the casting.

P

Parting line: The line on the casting where the two die halves meet.
Plunger tip: The end of the plunger that pushes metal through the shot sleeve. It wears and is replaced regularly.
Porosity: Voids inside a casting from trapped gas (rounded) or from shrinkage during solidification (irregular).
PPAP: Production Part Approval Process: the documents and samples a supplier submits to show a process can make good parts consistently.
PQ2 diagram: A chart that matches the machine's injection capability to the die's flow resistance, used to check that a die can be filled at the required gate velocity.
Projected area: The area of the casting, runners and overflows seen from the direction of die opening. Multiplied by metal pressure, it gives the opening force.

R

Reduced pressure test (RPT): Solidifying a melt sample under a partial vacuum to reveal hydrogen porosity, used to calculate the density index.
Runner: The channel that carries metal from the biscuit or sprue to the gates.

S

Shot profile: The recorded plunger speed, position and pressure through each phase of an HPDC shot.
Shot sleeve: The steel cylinder into which metal is ladled before the plunger injects it into the die.
Shrinkage: The volume reduction as metal solidifies. If it is not fed, it leaves shrinkage porosity, usually in thick sections.
Slide: A die part moved by a hydraulic cylinder or angle pin to form an undercut or side feature.
Sludge: Hard intermetallic particles of iron, manganese and chromium that settle in a holding furnace when the sludge factor is too high for the temperature.
Soldering: Molten aluminium bonding to the die steel, so the casting sticks or tears on ejection.
Solution treatment: Heating a casting close to its solidus to dissolve magnesium and copper into solid solution, followed by a quench. The first step of T6.
Spectrometer: An instrument that measures the chemical composition of an alloy from a sparked sample in seconds.
Squeeze pin: A local pin that pushes into a thick section during solidification to feed shrinkage.

T

T5: A casting cooled from the casting process and then artificially aged. Common for HPDC parts that cannot be solution treated.
T6: Solution treated, quenched and artificially aged for maximum strength. Called TF in BS 1490.
T7: Solution treated, quenched and over aged for better ductility and stability, used for structural castings.
Trimming: Removing the runner, biscuit, overflows and flash from an HPDC casting in a trim press.

V

Vacuum die casting: HPDC with air drawn out of the cavity before filling, giving lower gas porosity and allowing heat treatment and welding.
Vacuum impregnation: Sealing micro porosity by filling it with a resin under vacuum and pressure, so a porous casting becomes pressure tight.
Venting: Channels and chill vents that let air escape from the cavity as metal fills it.

W

Wall thickness: The section of the casting walls. HPDC suits thin, uniform walls; gravity casting suits thicker sections.

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