GATING SYSTEM

Gating systemGating system refers to all channels through which molten metal flows to enter the mould cavity. The gating system should be such that erosion of mould or core does not occur. It can be done by reducing the velocity of the molten metal within the cavity and avoiding direct impingement on mould walls or cores.The object of any gating system is to allow the mold to be filled as rapidly as possible with a minimum amount of turbulence and to provide sufficient hot metal to feed the casting during solidification to prevent shrinkage defects.The soundness of casting depends to a large extent on the way in which the metal enters the mould cavity and the rate of solidification.

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FUNCTIONS OF THE GATING SYSTEM

Gating system serves the following purposes, requirements and functions. To fill the mould cavity completely To help in proper directional solidification Provide means for separation of non-metal like inclusions Regulate the rate of liquid metal flow in to the mould cavity. Practical, economical and simple in design Minimum amount of metal in sprue, run ner. gates and risers should be

consumed because all these arc removed from the cast ing after solidification and the metal is rendered waste.

In addition to the above design considerations, proper functioning of me gating system also de pends upon the following factors:

(i) Temperature of casting and fluidity of molten metal (ii) Rate of molten metal pouring (iii) Shape and size of the sprue (iv) Shape and size of the runner

Types of gates.

The three main types of gates are:

(1) parting, (2) top. and l3) bottom parting gate : - between cope and drag is the easiest and fastest for

the molder to make. Its chief disadvantage is that molten metal drops into die cavity and may cause erosion or washing of the mold. In the case of nonferrous metals, this drop aggravates the dross and entraps air in the metal, which makes for inferior casting.

Top gates are used for gray iron castings of simple designs, but not for nonferrous alloys since they have a tendency to form excessive dross when agitated. A disadvantage of top gat ing is that it is conducive to a favorable tem perature gradient; a big disadvantage is that of mold erosion.

A bottom gate offers smooth flow with a minimum of mold and core erosion. Its main dis advantage is that it creates an unfavorable temperature gradient. The metal is introduced into the bottom of the mold cavity and rises qui etly and evenly. It cools as it rises, and the result is a condition of cold metal and cold mold near the riser and hot metal near the gate. The riser should contain the hottest metal in the hottest part of the mold so it can feed metal into the mold until the casting has solidified.

Elements of gating system

Pouring basinThe molten metal is not directly poured into the mould cavity because it may cause mould erosion. Molten metal is poured into a pouring cup or basin, which acts as a reservoir at the top of the sprue from which it flows smoothly into the sprue.When the total amount of metal poured exceeds 90 cubic inches, it is advantage to use a pouring basin. The best pouring basins are rectangular with a flat as opposed to rounded bottom. The basin should be large enough to "hit" when pouring the molten metal and should be deep enough to prevent a vortex from forming and drawing air into the sprue. Two and a half inches minimum depth works well for most smaller castings. The basin may be formed from green sand, however they are often made of baked sand. Pouring cups are less desirable than basins but also used.

Pouring cup or basin rules

The following rules are used in basin design:-Pouring cups, whether external or cut in the moulding sand, is used to help separate the dross and slag from the molten metal.The metal must be poured into the basin at a point that is remote from the sprue hole, to prevent vortexing and the turbulence develop.A pouring basin can be used to ease the filling of the mould and also to minimize the possibility of the air and oxide entrance into the mould cavity.Skimmer cores filter or delay screen can be used in the pouring basin, to help and provide cleaner metal into the mould.

Sprue rules

The sprue is the part of gating system into which the molten metal is poured. it is a vertical channel that connects the pouring basin with runners and gates. The following in a list of typical rules used in sprue design:The sprue should be sized to limit the flow rate of the molten. If the sprue is large, the flow rate of metal will be high.Height of sprue is determined by casting and top riser height.Sprue should be tapered by approximately 5% minimum to avoidaspiration of the air and free fall of metal.Some air is almost always carried down with the first metal that enters the mould. This should be washed out in a well bellow the sprue.The diameter of the well should be 2.5 time the width of the runner. The well should be extended down bellow the runner to depth equal to the depth of the runner .The well should have straight sides with no sharp corners and the bottom should be flat as possible.

Sprue base This is a reservoir for the metal at the bottom of the sprue to reduce the momentum of the falling molten metal. The molten metal, as it moves down the sprue, gains in velocity, some of which is lost in the sprue base well, and the mould erosion is reduced. This molten metal changes direction in the sprue base and flows into the runner in a more uniform way.

Runner rules

Runners are the passages that carry the molten metal from the sprue well to the gates through which the molten metal enters the mould cavity.

The rules that are commonly used, for runner design is as follows: The increase of cross-section area of runners from the choke area

decreases the velocity of the molten metal in the runner. There by reducing turbulence and helping the dross to rise or sink in the runner.

Rectangular cross-section area is preferred in sand casing. Runner’s extension (blind end) is used in most casting to trap any

dross that may occur in the molten metal stream. The runner’s area is three to ten time the cross-section area of

sprue exit.

Skim bobIt is an enlargement along the runner whose function is to trap heavier

and lighter impurities such as dross or eroded sand. So it prevents these impurities from going into mould cavity.

Choke: it is that part of the gating system which has the smallest cross sectional area. It perform the following functions: -

To control the rate of metal flow to help lower the velocity in the runner.

To hold back slag and foreign material and float these in the cope side of the runner.

To minimize sand erosion in the runner.

Gates rules. Gates are the passage between the runner and the part. The

following rules on gate design in common practice. Gate into thick regions Use standard sizes, shapes for the gates. Rectangular gates are

mostly used. The numbers of gates depend on the design of the casting and

risers.

Riser rules

Similar to the procedure used in gate design; the following rules are extracted from foundry man practices:-

Risers are attached to heavy sections of casting . Side risers are used for thin wall casting. Top risers are allocated on bosses, away from the gates. Risers are sized by the volume fed. In the case of multiple risers,

each riser is considered to be feeding the apart of the casting , e.g. the total volume of the casting is divided by the number of the risers suggested.

The connections between the risers and casting must be solidified just after the casting solidification and just before the riser does.

Types of Gating Systems  Pressurized gating system Un-pressurized gating system

Pressurized Gating System The total cross sectional area decreases towards the mold cavity Back pressure is maintained by the restrictions in the metal flow Flow of liquid (volume) is almost equal from all gates Back pressure helps in reducing the aspiration as the sprue always

runs full Because of the restrictions the metal flows at high velocity leading to

more turbulence and chances of mold erosion

Un-Pressurized Gating System The total cross sectional area increases towards the mold cavity Restriction only at the bottom of sprue Flow of liquid (volume) is different from all gates aspiration in the gating system as the system never runs full Less turbulence

FACTORS CONTROLING GATING DESIGN

The following factors must be considered while designing gating system. Sharp corners and abrupt changes in at any section or portion in

gating system should be avoided for suppressing turbulence and gas entrapment. Suitable relationship must exist between different cross-sectional areas of gating systems.

The most important characteristics of gating system besides sprue are the shape, location and dimensions of runners and type of flow. It is also important to determine the position at which the molten metal enters the mould cavity.

Gating ratio should reveal that the total cross-section of sprue, runner and gate decreases towards the mold cavity which provides a choke effect.

Bending of runner if any should be kept away from mold cavity. Developing the various cross sections of gating system to nullify the

effect of turbulence or momentum of molten metal. Streamlining or removing sharp corners at any junctions by providing

generous radius, tapering the sprue, providing radius at sprue entrance and exit and providing a basin instead of pouring cup etc.