Gating & Optimal Filling Time

The Optimal Filling Time is nothing but the time required to cast or mold a product .The efficiency of the casting is normally affected by two scenarios.

Slow Filling : If the velocity of the fill is very slow, it leads to cold shuts and misruns. The slow filling is avoided because of the early freezing in thin sections before the complete filling.

Fast Filling : The fast filling is governed by the onset of surface instability. The optimal filing time lies in-between this fast & slow filling. It also depends on the following factors like:

• Cast metal
• Weight
• Pouring temperature
• Minimum section thickness.

With the help of various experimental methods, researchers have developed different empirical equations, for determining the Optimal Filling Time for major metals.

For example if the time "f" is expressed as a function of casting, weight "W" in kg, section thickness t"" in mm and fluidity length Lf in mm.

A universal equation for filling time can be written as:
F = K0 (Kf Lf / 1000) (Ks + Kt t / 20) (Kw W)

Where,
K0 overall coefficient,
K0 overall coefficient,
Ks coefficients of size
Ks coefficients of size
Ks coefficients of size

Let us take an example of the metal, Grey Iron. For this particular metal the following values can be used:

K0 = 1.0,
Kf = 1.0,
Ks = 1.1 (for castings of size 100-1000mm),
Kt = 1.4 (for wall thickness up to 10 mm),
Kw = 1
P = 0.4.

A proficient casting engineer can set the values of the coefficients for each metal-process combination with his experience. These form an important part of the knowledge base to become an expertise in specific castings.

The Optimal Filling Time also depends on the following two factors, which are explained briefly below:

The Optimal Filling Time depends on the speed of flow of the molten metal. This mostly varies within the gating channels and the mold openings. The metal will be hot and fast at these locations and thus it leads to huge damage if the flow is not maintained properly. The speed of the molten metal depends on two factors:

The metallostatic head and the ratio of cross-sections of sprue exit, runner(s) and ingates(s), which are referred as the gating ratio. Thus the universally accepted fact is that the speed of molten metal should be lower than 1 m/s for ferrous metals and 0.5 m/s for aluminum alloys.

Let us see some simple examples about the gating ratio. Let As, Ar, Ag be the cross-sectional areas of sprue exit, runner and ingate respectively. A system, where the ingate area is more than the sprue exit area, is considered so that the metal slows down which thereby reduces the instability. The gating ratios include: 1:2:1.5 for ferrous and 1:4:4 for non-Ferrous metals. To reduce the speed of the molten metal, higher values of ingate areas such as (1:4:8) can be used. Thus optimal filling time depends on the above-mentioned scenarios.