The controlled metal flow is a fundamental and distinguishing feature of low pressure casting molds. It is a critical principle that sets this casting method apart from traditional high-pressure casting techniques. 

Pressure Differential:

The core concept of controlled metal flow in low pressure casting molds lies in the application of a relatively low pressure differential to move the molten metal from a crucible or holding furnace into the mold cavity. This pressure differential is typically in the range of 1 to 1.5 bars, significantly lower than the high pressures used in methods like high-pressure die casting. The use of low pressure allows for a more gradual and precisely controlled filling of the mold.

The low pressure differential is typically achieved by maintaining a sealed system, where the molten metal in the crucible is exposed to a controlled atmosphere, often consisting of air or inert gases. By regulating the pressure applied to the surface of the molten metal, the flow rate can be carefully adjusted to achieve the desired filling characteristics. This controlled approach minimizes turbulence, which is a common source of defects in castings, such as gas porosity and shrinkage voids.

Gravity Assistance:

In low pressure casting, gravity plays a significant role in facilitating the controlled metal flow. The molten metal in the crucible is typically positioned at a higher elevation than the mold cavity. As a result, gravity assists in guiding the metal into the mold. This gravitational force, in conjunction with the low pressure differential, ensures a smooth, controlled, and uniform flow.

The use of gravity minimizes the need for excessive mechanical force or hydraulic pressure, which can cause turbulence and defects. This gentle, gravity-assisted approach allows for the efficient filling of the mold without subjecting the molten metal to abrupt changes in velocity or direction, reducing the likelihood of issues such as air entrapment and misruns.

Mold Filling Control:

Another critical aspect of controlled metal flow is the precise control over the metal filling process. This control is achieved through the design of the gating system, which comprises components like the pouring basin, runner, and sprue. The pouring basin collects the molten metal from the crucible, while the runner and sprue guide it into the mold cavity.

The design and dimensions of these gating system components are carefully engineered to ensure a gradual and uniform flow of metal. Their size, shape, and positioning are optimized to prevent turbulence and air entrapment. In essence, the gating system acts as a hydraulic buffer, allowing the metal to flow steadily and evenly into the mold.

Moreover, the gating system design considers factors such as the metal's temperature, viscosity, and flow rate. These parameters are tailored to the specific alloy being cast, ensuring that the metal behaves predictably and consistently during the filling process.

Minimizing Oxidation:

In addition to controlling the flow of molten metal, low pressure casting molds incorporate features to minimize oxidation. Exposure to oxygen can lead to the formation of oxides on the surface of the metal, which can affect the casting's quality. To mitigate this, the controlled atmosphere around the molten metal is often inert, such as nitrogen or argon, which prevents oxidation and helps maintain the metal's purity.