Principle Of Gating

Gating system:

1)      The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity.

2)      The gating system is composed of

a)      Pouring cups and basins

b)      Sprue

c)      Runner

d)     Gates

e)      Risers

3)      Various components of gating systems shown in fig.

4)      Since the way which liquid metal enters the mold has a decided influence upon the quality and soundness of a casting, the different passages for the molten metal are carefully designed and produced.

5)      A gating system should avoid sudden or right angle changes in direction.

6)      Sudden change in direction causes mold erosion, turbulence and gas pick up.

7)      If possible the gating system should from a part of the pattern. It

a)      Avoids cutting a runner and gates

b)      Permits sand to be rammed hard.

c)      Helps prevent erosion and washing away of the sand as molten metal flows into the mold.

Requirements / purpose / function of the gating system:

A gating system should,

1)      Fill the mold cavity completely before freezing.

2)      Introduce the liquid metal into the mold cavity with low viscosity and little turbulence, so that mold erosion, metal oxidation and gas pick up is prevented.

3)      Help to promote temperature gradient favorable for proper directional solidification.

4)      Incorporate traps for separation of non-metallic inclusions which are either introduced with the molten metal or are disclose in the gating system.

5)      Regulate the rate at which liquid metal enters into the mold.

6)      Be practicable and economical to make and consume least metal.

For proper functioning of the gating system, the following factors need to be controlled:

a)      Types of pouring equipments, i.e. ladle, pouring basins etc.

b)        Temperature / fluidity of molten metal

c)      Rate of liquid metal pouring

d)     Type and size of sprue

e)      Type and size of runner

f)       Size, number and location of gates connecting runner and casting.

g)      Position of mold during pouring and solidification.

Pouring cups:

a)      Makes it easier for the ladle or crucible operator to direct the flow of metal from crucible to sprue.

b)      A pouring cup is a funnel shape cup which forms the top portion of the sprue.

Pouring basins:

a)      Makes it easier to ladle operator to direct the flow of metal from crucible to sprue.

b)      Helps maintaining the required rate of liquid metal flow.

c)      Reduces turbulence and overtaxing at the sprue entrance.

d)      Helps separating dross, slag etc. from metal before it (i.e. metal) enters the sprue hole.

Sprue:

1)      A sprue feeds metal to the runner which in turn reaches the casting through the gate.

2)      A sprue is tapered with its bigger end at the top to receive the liquid metal. The smaller end is connected to the runner.

Gates:

1)      A gate is a channel which connect a runner with the mold cavity and through which molten metal flows to fill the mold cavity.

2)      A gate should feed liquid metal to the casting at the rate consistent with the rate of solidification.

3)      The size of the gate depends upon the rate of solidification.

4)      A small gate is used for a casting which solidifies slowly and vice versa.

Types of gates:

a)      Top gate  / drop gate

b)      Bottom gate

c)      Parting line side gate

Multiple gate system:

a)      Simple system

 

b)      Simple inclined system

c)      Inclined step with common junction

d)      Reversed down runner

Turbulence:                moving unsteadily and violently

Erosion:                      gradually wear away

 Design of gating system:

Objectives achieved from good design:

a)      To fill the mold cavity rapidly without breaking the flow of liquid metal and without using very high pouring temperature.

b)      To avoid erosion of the core and mold cavity.

c)      To stop scum, slag, dross and eroded sand particles from entering the mold.

d)     To minimize turbulence and dross formation.

e)      To prevent aspiration of air or mold gases in the liquid metal stream.

f)       To obtain favorable temperature gradient to promote directional solidification.

Defects occurring due to improper design of gating system:

a)      Oxidation of metals

b)      Inclusion of slag, dross and other foreign materials.

c)      Cold shut

d)     Mold erosion

e)      Rough surface

f)       Shrinkage

g)      Porosity

h)      Entrapped gases

i)        Misrun

j)        Penetration of liquid metal into the mold walls.