Filling of die cavity:
The filling process occurs in three successive phases. The first phase is generally a slower movement of plunger to fill the sprue bush and runner. The second phase is the actual filling period of the cavity under hydrodynamic farce.
The third phase also called the final squeeze acts the hydrostatic pressure is applied to the casting before solidification.
The most important of all these three phases the second phase of cavity filling many theoretical studies and tests have been conducted to study this cavity filling. The important among these theories are frommer’s theory and brand’s theory.
Frommer’s theory: the first theoretical explanation about the cavity filling in pressure. Die casting was given by frommer in 1932. He conducted his experiment on a simple die cavity of rectangular shape. The cavity was made partly across on one of the smaller sides. He observed the following things.
The molten metal enters the die cavity through the gate as a stream with shape and cross section corresponding to the gate. It projects through the cavity until it bits the opposite side of the cavity violent turbulence. A pool is formed on the side of the metal flows out of the pool in a reduced speed along the side walls perpendicular to the pool surface. Swiths and eddies are created in the remainder of the metal that stays in the pool by the impact of incoming material. The metals which flow out of the pool are known as “fore runners”. Due to the reduction in flow energy of incoming metal remains in the pool. Only a part of the metal that flows out of the pool is chilled and retained on the side wallet. Therefore according to frommer’s observation cavity starts filing from the side opposite to the gate and the last area to the filled are near the gate side. The speed of the incoming stream and hence the speed at which the pool is increasing is faster than the speed of the fore runner so that with in a short distance the fore runners fuse with the incoming material.
Brandt’s theory: a different theory about the cavity filling with molten metal was explained by w.brandt in 1037. Brandt for his experiment also used a cavity of rectangular shape but made gating on the full width of one of the small side. A number of electrical metal contacts were built in the cavity at which the contacts closed indicated the flow of metal.
But in actual practice one can find that the castings which are not filly filled due to less metal injection are filled on the far end from the gate. However one can presume that castings with in gating injection with high velocity follows frommer’s theory where as castings with thick gating and slow injection speed follows Brandt’s theory.
