College of Engineering

Mechanical Eng. Dept.

Gating Design

Ch6

Gating Design

The gating system is that part of the mold cavity through which the metal is poured to fill the casting impression; its design is the principal means by which the foundry man can control the pattern of metal flow within the mold. The importance of gating technique arises from its fourfold purpose

1. The rate and direction of metal flow must be such as to ensure complete

filling of the mould before freezing.

2. Flow should be smooth and uniform, with minimum turbulence.

Entrapment of air, metal oxidation and mould erosion are thus avoided.

3. The technique should promote the ideal temperature distribution within

the completely filled mold cavity, so that the pattern of subsequent

cooling is favorable to feeding.

4. The system can incorporate traps or filters for the separation of

nonmetallic inclusions, whether dislodged in the gating system or introduced

Essential features of gating systems

In the simplest of all gating systems the metal is poured down an open feeder head situated at the top or the side of the casting. This technique is often followed for small castings, in which the provision of an additional system of mold passages would greatly lower the yield of useful metal; at the same time the temperature distribution is favorable and the distance through which the liquid metal must fall is short.

The essential features of a typical system

Principal features of a gating system. (a) Bush or basin, (b) sprue or down runner, (c) runner, (d) gates

Design of gating systems

Most modern studies of gating systems have been based upon

consideration of two laws of fluid dynamics. The first of these, the

Equation of Continuity, states that the volume rate of flow is

constant throughout a system and is expressed by

Q = A1 V1 = A2V2 ………………………… 1

where Q = volume rate of flow,

A = cross-sectional area of flow passage,

V = linear velocity of flow.

The linear velocity of flow in a system is related to other factors in Bernoulli's Theorem, which states that the total energy of unit weight of fluid is constant throughout a system:

Poring Time, t For cast Iron

Poring Time, t For steel

Pouring Time For Cast iron

Recommended pouring times

Friction

• for thin “plates”, fr =0.2 • for heavy “cubes”, fr =0.8

Some potential energy lost to friction (heat) as liquid moves against mould wall and liquid moves against liquid.

1 10 100 1000 10000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Frictional Loss, f r

Casting Weight, kg

Gray and Ductile Iron

Carbon Steel

Alloyed Steel

The Role of “Choke

Definition: Choke is that cross sectional area in a gating system which determines mold filling time.

Choke located at junction of runner and gate in a simple GATE-RUNNER (pressurized) system.

Choke located at junction of sprue and runner in a simple SPRUE-RUNNER (non-pressurized) system.

Selection of Gating System Type:

1. Use SPRUE-RUNNER system for large number of small castings in one mould where it is impractical to choke the castings individually – where choke dimensions are very small – very demanding on molding technique and pouring temperature.

2. Use GATE-RUNNER system on most other occasions.

Features of GATE-RUNNER and SPRUE-RUNNER SYSTEMS can be combined to form a HYBRID system. This is normally used where a complicated network of runners is required to deliver iron to casting cavities.