Basic Principles of Die Casting

1. Consider the injection pressure: multi‑stage injection with different functions

There are four stages for injection pressure: fast pressure, filling, pressurization, and pressure holding, each with distinct functions. In the initial phase, the piston pushes molten metal into the cavity at a relatively low speed until the cavity is fully filled.

Pressure holding stage：The injection punch transmits pressure to the cavity via the solidified residual material and the metal at the junction. This enables the solidifying metal to crystallize under pressure, producing castings with dense internal structure. The duration of pressure holding is mainly determined by die‑casting alloy properties, casting wall thickness, gating system and other working conditions.

Cooling stage：The cooling stage covers the time from the end of pressure holding until the casting is ejected in the die‑casting cycle.

Filling time：Adjust die‑casting filling time according to product geometry and wall‑thickness variation.

2. Critical die‑casting process parameters affecting casting quality

Multiple parameters exert great influence on final casting quality:

• Residence time of molten alloy inside the shot sleeve
• Punch speed or punch acceleration at the start of the first‑shot phase
• Transition point from first‑shot to second‑shot phase (displacement stroke for each injection stage)
• Punch acceleration time and filling time during the mold‑filling (second‑shot) phase
• Boost pressure and boost time for molten metal inside cavity after filling completion
• Solidification time of castings within the mold cavity

Derived core die‑casting process specifications include: slow injection speed, high‑speed (mold‑filling) speed, speed‑switch position, boost pressure, and cooling time.

Slow injection speed：Slow injection speed should be properly controlled to avoid turbulence of molten alloy inside the shot sleeve and excessive temperature drop of molten metal. The advanced 9‑stage low‑speed control is widely adopted nowadays. It simulates uniformly‑accelerated motion and minimizes gas entrapment inside the shot chamber.

High‑speed speed (secondary speed)：Also known as secondary speed, it refers to punch speed during mold filling. Proper control ensures favorable atomized metal flow at the ingate during cavity filling. Reference values shall be selected based on casting structure and wall thickness, then converted into punch travel speed according to ingate dimension and shot‑chamber size.

Speed‑switch position：Also called second‑speed switch position, it defines where the punch begins to accelerate. Acceleration can be triggered when the shot sleeve is fully filled, when molten alloy reaches the ingate, or after partial cavity filling. This position must be fine‑tuned according to product features and actual production defects.

Boost parameters (four controllable factors)：Four parameters should be managed for boost function:

• Start position of pressurization
• Pressure building time
• Pressure holding time
• Magnitude of boost pressure