Risers are reservoirs built into casting molds to prevent cavities from shrinkage. They are located at the top of the casting and their volume is 25-55% of the casting. Riser design aims to overcome problems from uneven solidification like voids or stresses. The size and placement of risers depends on factors like the shape and freezing rate of the casting. Gating systems convey liquid material to molds, and their design controls shrinkage, flow speed, turbulence and traps dross. Gates are attached at thick parts to control shrinkage, and multiple gates may be needed for large castings.

1. Risers:
A riser, also known as a feeder is a reservoir built into a metal casting mold
to prevent cavities due to shrinkage. The risers are usually located at the
uppermost part of the section being fed. Depending on the metal being cast
their volume is kept between 25 – 55 % of the casting.It is important to note
that the risers are suitably located so that there is no necessity for excessive
metal removal to produce finished contour. Risers are connected to the
castings by the neck of metal called gate which enables the riser to be
removed easily from the casting after solidification.
Riser design :
If no riser is provided during casting, the solidification will start from walls
and liquid metal in the centre will be surrounded by a solidified shell and the
contracting liquid will produce voids towards centre of the casting. Further
cooling of the solid in centre sets up undesirable stresses in the casting.
Provision of risers overcomes these problems as these supply molten metal
for a solidifying casting. For this purpose, the risers must be large enough to
remain liquid after the casting has solidified and must contain sufficient
metal to provide for contraction losses. Further these should be so positioned
such that they continue to supply the metal throughout the solidification
period.
It can be shown that the heat loss or cooling rate from a casting or riser is
proportional to A/V i. e. Area/Volume.
If a casting due to its chunky shape has very slow freezing rate, then the
riser must be large so that it remains liquid after the casting has solidified,
In other words, Ac > Ar
Vc Vr
Ac and Ar = Areas of castings and risers
Vc and Vr = Volume of castings and risers

2. If casting is fast freezing one, then size of the riser need not be large.
Minimum size of the riser can be determined by a risering curve which is a
hyperbolic relation between the freezing ratio .
Ac/Vc = a
Ar/Vr + C
Vr - b
Vc
a = Freezing characteristic constant for metal( for steel
is 0.1)
b = contraction ratio from liquid to solid( for steel 0.3)
c = relative freezing rate of riser and casting.It is unity if the same mould
material is used for both casting and riser)
The proper placement of riser is equally important since it should be able to
feed the solidifying casting effectively. If the casting is of cubical or
spherical shape, ( i. e chunky shape having low value of Ac/Vc) then a
single riser is sufficient to feed casting on solidification.
However when value of Ac/Vc high as in case of plate and bar shaped
castings more than one riser may be required. If only single riser is used in
such cases,then the slushy state just prior to solidification may restricts metal
flow from a single riser and may cause centre line shrinkage
As a thumb rule it can be said that a single riser is adequate if the if feeding
length is less than 4.5 times the thickness of plate for 12-100 mm thick steel
plates. In the case of sq bars of size 50 –200 mm, a central riser can be used
for distances less than 6 times the sq root of bar size. Longer feeding
distances than above are possible by use of chills which will increase the
cooling rate and reduce the centre line feeding resistance. In case of alloys
having higher centre line feeding resistance than steel, chills have to be used
to ensure soundness of those parts of the casting enquiring the greatest
strength.

3. Gating System and design
The gating system serves many purposes, the most
important being conveying the liquid material to the
mold, but also controlling shrinkage, the speed of the
liquid, turbulence, and trapping dross. The gates are
usually attached to the thickest part of the casting to
assist in controlling shrinkage. In especially large
castings multiple gates or runners may be required to
introduce metal to more than one point in the mold
cavity. The speed of the material is important because
if the material is traveling too slowly it can cool
before completely filling, leading to misruns and cold
shuts. If the material is moving too fast then the
liquid material can erode the mold and contaminate
the final casting. The shape and length of the gating
system can also control how quickly the material
cools; short round or square channels minimize heat
loss.[8]
The gating system may be designed to minimize
turbulence, depending on the material being cast. For
example, steel, cast iron, and most copper alloys are
turbulent insensitive, but aluminium and magnesium
alloys are turbulent sensitive. The turbulent
insensitive materials usually have a short and open
gating system to fill the mold as quickly as possible.

4. However, for turbulent sensitive materials short
sprues are used to minimize the distance the material
must fall when entering the mold. Rectangular
pouring cups and tapered sprues are used to prevent
the formation of a vortex as the material flows into
the mold; these vortices tend to suck gas and oxides
into the mold. A large sprue well is used to dissipate
the kinetic energy of the liquid material as it falls
down the sprue, decreasing turbulence. The choke,
which is the smallest cross-sectional area in the
gating system used to control flow, can be placed
near the sprue well to slow down and smooth out the
flow. Note that on some molds the choke is still
placed on the gates to make separation of the part
easier, but induces extreme turbulence.[9]
The gates
are usually attached to the bottom of the casting to
minimize turbulence and splashing.[8]
The gating system may also be designed to trap
dross. One method is to take advantage of the fact
that some dross has a lower density than the base
material so it floats to the top of the gating system.
Therefore long flat runners with gates that exit from
the bottom of the runners can trap dross in the
runners; note that long flat runners will cool the
material more rapidly than round or square runners.
For materials where the dross is a similar density to
the base material, such as aluminium, runner

5. extensions and runner wells can be advantageous.
These take advantage of the fact that the dross is
usually located at the beginning of the pour, therefore
the runner is extended past the last gate(s) and the
contaminates are contained in the wells. Screens or
filters may also be used to trap contaminates.[9]
It is important to keep the size of the gating system
small, because it all must be cut from the casting and
remelted to be reused. The efficiency, or yield, of a
casting system can be calculated by dividing the
weight of the casting by the weight of the metal
poured. Therefore, the higher the number the more
efficient the gating system/risers.

6. extensions and runner wells can be advantageous.
These take advantage of the fact that the dross is
usually located at the beginning of the pour, therefore
the runner is extended past the last gate(s) and the
contaminates are contained in the wells. Screens or
filters may also be used to trap contaminates.[9]
It is important to keep the size of the gating system
small, because it all must be cut from the casting and
remelted to be reused. The efficiency, or yield, of a
casting system can be calculated by dividing the
weight of the casting by the weight of the metal
poured. Therefore, the higher the number the more
efficient the gating system/risers.