Uniform freezing of melt in the mould
Melt must spread in all direction at uniform velocity in all direction so that it can freeze uniformly. Velocity can vary because of
- variation of cross sectional area of melt front on account of obstruction due to core (hole or cut out in the part) or
- variation in wall thickness
- variation in surface temperature.
See how freezing of melt takes place while melt spreads in the mould with velocity.
Flow depends on injection speed set on the machine and resistance flow offered in the mould whereas Freezing depends on cooling rate, a function of cooling circuit design and water flow rate and inlet temperature.
Remember melt freezes while it moves. Freezing is time dependent under the influence of mould cooling. Complete filling of mould depend on adequate (high enough) injection rate to counter the influence of freezing.