Simulation of High Pressure Die Casting (HPDC)
via
STAR-Cast
STAR Global Conf. 2012, 19-21 March, Noordwijk
Romuald Laqua, Access e.V., Aachen
High Pressure Die Casting: Machines and Products
Common Materials:
● Aluminum alloys
● Magnesium alloys
● Zinc alloys
● Copper alloys
HPDC process cycle, horizontal cold chamber machine
Parts considered for simulation
HPDC process cycle: 1. closing moving die parts
HPDC process cycle: 2. shot sleeve filled with melt, starting plunger movement
HPDC process cycle: 3. completed shot
HPDC process cycle: 4. ejecting and removing solidified casting
HPDC process cycle: 5. spraying of lubricant, casting cycle finished
Challenges in HPDC Simulation:
Moving plunger in filling chamber – moving mesh model necessary
Thin walled, complicated and large castings – challenging enmeshment and high cell count
Multi physics: melt, solid, gas – VoF model with HRIC scheme, combined with surface
tension model and correct wetting angle
Short pouring times, leading to high fluid velocities – small time steps (~0.1ms) mandatory
Extreme pressure ranges from 10 Pa initial cavity pressure up to 1000 bar in melt during
solidification – Compressibility model for melt and gas
Why simulate?
Goals & Objectives of HPDC Simulation:
Reduce iterations in tooling development: Cost for one mould insert 50-100k€
Reduce process development time: faster achievement of a stable process window
Better process understanding: helpful when negotiate with customers about necessary part design
changes
Typical defects in high pressure die casting:
Misruns: Melt solidifies before filling is completed
Cold shuts: Imperfect fusing of molten metal coming together from opposite directions in a mold
Porosity: small holes caused by insufficient feeding or dissolved gas
Air and oxides inclusions
Cold flakes: floating crystals, solidified at shot sleeve walls and transported into cast part
High Pressure Die Casting – Overview
Features
● Filling Simulation
● Gas is Compressible
● Liquid is Compressible
● Moving Mesh
● Phase Change
● Conjugate Heat Transfer
Simulation
● Shot chamber is half filled with liquid,
plunger follows shot control curve, pushing
the fluid into the cavity
High Pressure Die Casting: shot curve vPlunger=f(t)
Constant velocity until
mold is filled, followed
by pressure control, up
to 1000 bar
High Pressure Die Casting – Geometry and Mesh
Die parts
Shot sleeve
Cast
Chilled vents
(allow air to escape and
force melt to freeze)
Meshing
Structured (extruded) mesh
in shot sleeve
Polyhedral mesh in cast part and die
Cell count: 1.6 million cells in fluid domains
3.6 million cells overall
Meshing
● Two layers of prism cells on each side of casting-die
interfaces to resolve high temperature gradients
● Water and oil channels are not meshed
Process parameter setup, additional settings related to HPDC process
Shot sleeve components must be identified:
Empty filling chamber Shot sleeve walls Prefilled with melt
Process parameter setup
Shot curve definition:
Enter values or
read from file
Process parameter setup
Pressure curve definition:
Enter values or
read from file
Die cycle warm up simulation
Pure thermal simulation over
at least 5 casting cycles,
including all phases: shot,
solidification, die opening,
ejection and spraying
Final temperature
distribution in die is used as
initial state for main
simulation run with coupled
filling and solidification
Initial temperatures in die and shot sleeve
Cooling cycles in oil and
water channels are
modelled by applying mean
fluid temperatures on
channel wall boundaries
(channels are not part of
computational domain)
High Pressure Die Casting – Results
Pressure on
melt surface
Velocity on
melt surface
Time = 1.96 seconds Time = 2.05 seconds Time = 2.06 seconds
Temperature distribution on melt surface during mold filling
Front view Rear view
Air entrappments in casting after completed shot (2.52 s)
Initial melt temperature = 640°C Initial melt temperature = 680°C
Animated mold filling process with piston movement
Initial melt temperature = 680°C
Tliquidus = 613°C
Tsolidus = 555°C
Future steps of development of STAR-Cast
HPDC process is challenging to simulate seriously, but casting industry
seeks for a more detailed simulation tool with more physics inside
Migration to STAR-CCM+ will simplify the setup process, enhance
postprocessing capabilities and (probably) improve numerical stability