Abstract: A metal flow system, for high pressure die casting of alloys using a machine having a pressurized source of molten alloy and a mould defining at least one die cavity, defines a metal flow path by which alloy received from the pressurized source is able to flow into the die cavity. A first part of the length of the flow path includes a runner and a controlled expansion port (CEP) which increases in cross-sectional area, in the direction of alloy flow, from an inlet end of the CEP at an outlet end of the runner to an outlet end of the CEP. A CEP exit module (CEM) forms a second part of the length of the flow path from the outlet end of the CEP. The increase in cross-sectional area of the CEP is such that molten alloy, received at the CEP inlet end at a sufficient flow velocity, undergoes a reduction in flow velocity in its flow through the CEP whereby the alloy is caused to change from a molten state to a semi-solid state.