The filling theory of die-casting molds is the core of die-casting process design,
The key lies in the coupling of unsteady flow and solidification of molten metal at high speed, high pressure, and within an extremely short time. The mainstream classical theories include atomization jet filling, full-wall-thickness filling, and three-stage filling, while modern approaches primarily employ fluid simulation (FLOW-3D/ProCAST/Magma) + PQ² diagram + thermal boundary analysis as the dominant design methodology.
Three classic filling theories (basics)
1. Atomization spray filling theory (Frommer, 1932)
Core viewpoint: The molten metal enters the mold cavity as a high-speed jet, maintains a concentrated beam shape, and diffuses and generates vortices and entrainment after hitting the distal wall.
Flow pattern: The sprue speed is extremely high (50-80 m/s), and the jet penetrates the cavity without immediately filling the wall thickness.
Applicable conditions: thin-walled parts, small sprues, high filling speed (common for zinc/magnesium alloys).
Advantages: Fast process and good surface quality.
Disadvantages: Strong eddy current, excessive air entrainment, easy oxidation and slag inclusion, and multiple internal pores.
2. Full wall thickness filling theory (Brandt, 1937)
Core viewpoint: The molten metal advances as a whole, smoothly, and with full wall thickness, layer by layer like a "water wall", without obvious eddies.
Flow pattern: The sprue is wide and the velocity is low (10-30 m/s), and the molten metal moves forward simultaneously along the wall thickness direction.
Applicable conditions: thick walled parts, large sprues, low to medium speed (aluminum alloy die-casting parts).
Advantages: Smooth exhaust, few pores, high density.
Disadvantages: The process is long and prone to cold insulation, and thin walls are prone to under casting.
3. The three-stage filling theory (Barton, 1944-1952)
Core viewpoint: Integrating flow and solidification, divided into three stages, closest to reality.
Formation period of shell: metal injection → impact on the wall → expansion along the surface → rapid cooling to form a thin-walled solidified shell; Eddy currents are prone to occur at corners.
Internal filling period: Subsequently, the metal is filled inside the shell, the solidification layer thickens inward, and the liquid phase decreases.
Compaction and solidification period: After filling, increase pressure and compact, supplement shrinkage, and eliminate internal pores.
Applicable: The vast majority of actual die-casting (aluminum/zinc/magnesium).
Design guidance: Control the gate speed, temperature, and wall thickness to reduce the first stage vortex.
solidified surface layer
Key heat: latent heat (solidification heat release) ≈ 7 times sensible heat, determining the filling window
Solid phase line score:
Thin wall: 0.4-0.5 (good fluidity, good surface)
Thick walled: 0.75-0.85 (high density, few pores)
2. Non Newtonian fluid properties
High temperature molten metal is a viscoelastic non Newtonian fluid
As the temperature decreases, the viscosity increases sharply and the fluidity drops sharply
Surface tension and oxide film significantly affect flow state
3. Key parameter relationships (hydraulic foundation)
Charging speed:
V charge=ρ 2p ⋅ A internal A impulse
p: Injection specific pressure (MPa)
ρ: Metal density
A punch: punch area
A: Inner gate area
Filling time: t=Within A ⋅ v Charging V
V: Cavity volume
3、 Guidance of Filling Theory on Mold Design (Core Application)
1. Inner gate design (most critical)
Spray mode: small section, high speed, single-sided/point gate → suitable for complex thin-walled and appearance parts
Full wall thickness mode: wide cross-section, low speed, flat/side gate → suitable for thick walls, structural components, and airtight components
Three stage control: gate speed of 30-50 m/s (aluminum) to avoid strong eddy currents in the first stage
2. Exhaust and overflow design
Spray theory: It is necessary to have multiple exhaust slots and overflow bags to capture vortex entrainment and oxidation slag
Full wall thickness theory: exhaust is set along the end of the process, and overall propulsion is conducive to exhaust
3. Flow channel and mold temperature
Smooth transition, no abrupt changes, and no dead corners in the flow channel
Uniform mold temperature (temperature difference<50 ℃) to avoid premature solidification in local areas
4. Defect prediction (theory → defect)
Excessive spraying → pores, oxide inclusions, and streaks
Low speed → Undercasting, cold insulation, unclear contour
Process too long → End shrinkage and insufficient thin-walled pouring
Local eddy current → bubbles, pinholes
4、 Modern Fill Analysis Method (Mainstream in Engineering)
PQ ² diagram method
Establish a matching window with injection pressure P - gate flow rate Q ² to ensure the matching of the die-casting machine, mold, and alloy, avoiding overpressure/underspeed
Numerical simulation (CAE)
FLOW-3D、ProCAST、AnyCasting
Output: Flow field, temperature field, entrainment position, solid phase ratio, pressure distribution
Directly optimize the gate position, size, speed, and overflow layout
Experimental verification
High speed camera observation of filling morphology
Partial Shot Test to Observe the Flow Frontier
5、 Theoretical selection and design principles
Thin walled complex parts (<2mm): biased injection+three-stage, medium high speed, small gate, strong exhaust
Thick walled structural components (>3mm): biased towards full wall thickness, low-speed, wide gate, long process, sequential filling
Aluminum alloy: three-stage main, gate speed 30-50 m/s
Zinc/magnesium alloy: mainly sprayed, speed 50-80 m/s
