The Three Stages of Injection in High-Pressure Die Casting

The Three Stages of Injection in High-Pressure Die Casting

Blog Article

High-pressure die casting (HPDC) is a manufacturing process widely used to produce complex metal parts with high precision. At its core lies the injection phase, a critical sequence that directly impacts casting quality, defect formation, and production efficiency. This phase comprises three distinct stages—first, second, and third injection phases—each with specific mechanical requirements, velocity parameters, and material behaviors. Understanding these stages helps optimize process parameters, reduce defects like porosity, and ensure consistent part quality.

Filling Ratio and Its Impact

The foundation of successful injection lies in the filling ratio, expressed as the percentage of molten alloy poured into the shot sleeve relative to its total chamber volume. Calculated as (Shot Volume / Shot Sleeve Volume) × 100, this ratio determines how air escapes during cavity filling. A low filling ratio (typically below 50%) increases first-phase travel distance, raising risks of air entrapment and porosity defects. Conversely, higher ratios reduce air mixing but may cause overflow issues. The shot volume includes casting weight, overflow, runner, and biscuit volumes. When unknown, overflow and runner volumes are estimated as 40% of casting weight converted using liquid aluminum density (2.5 g/cm³).

Active sleeve length—calculated as (Plunger Stroke - Plunger Tip Protrusion) + Sprue Bush Length - Cone Height—directly affects shot sleeve capacity. For example, a machine with a 500mm plunger stroke and 50mm sprue bush requires precise active length measurements to avoid premature solidification. Advanced die casting mold design optimizes these parameters to balance filling ratios and phase transitions.

First Injection Phase: Slow Shot Stage

During the first phase, molten metal moves from the shot sleeve to the cavity gate at velocities between 0.05–0.7 m/s. This slow movement minimizes air turbulence while positioning material for rapid filling. The travel distance is calculated as Active Sleeve Length - Second Phase Length - Biscuit Thickness. If the second phase starts too early (before metal reaches the gate), air entrapment increases. If delayed, slow filling causes premature solidification and incomplete parts.

Critical Slow Shot Velocity Formula (NADCA):
Vcritical = Ccc × √(Filling Ratio / Plunger Diameter)
Where Ccc = 0.579 m0.5/s (metric) or 22.8 in0.5/s (imperial)

Second Injection Phase: High-Speed Filling

Once metal reaches the gate, the second phase begins with plunger speeds of 0.4–6 m/s. This rapid filling ensures the cavity and overflow channels fill completely within milliseconds. The travel length depends on post-gate volume and plunger area: Second Phase Travel = Volume After Gate / Plunger Area. Gate velocity (30–40 m/s) is maintained using the continuity equation: Plunger Velocity = (Gate Area × Gate Velocity) / Plunger Area. Insufficient velocity leads to cold shuts, while excessive speed traps air despite vents.

Third Injection Phase: Intensification and Solidification

The final phase applies high pressure (600–800 kg/cm²) to compensate for shrinkage as molten metal solidifies. Aluminum’s density increases from 2.375 g/cm³ (liquid) to 2.7 g/cm³ (solid), requiring additional material pushed through the biscuit. Intensification pressure is hydraulically amplified using the ratio (Cylinder Diameter / Plunger Diameter) × Hydraulic Pressure. For pressure-tight castings, 800 kg/cm² ensures minimal porosity. Proper timing is critical—gates must solidify after the cavity to maintain pressure effectiveness.

Biscuit thickness is designed to solidify last, allowing continuous material feed. Cooling rates depend on alloy properties and die temperature. Aluminum typically solidifies in 3–4 seconds,