Runner In Casting

Runner In Casting Process

What is a runner in the casting process?

A runner is a horizontal channel in a mould that connects the sprue and gate together to supply molten metal to various parts of the mould through the in-gate during the solidification process of the casting.

The runner is an important element of a gating system that is filled with molten metal connected to single or multiple gates feeding the mould cavity with streamlined slag-free and dross-free molten metal.

The location of the runner is mostly in the drag section of the mould but can also be placed in the cope section depending upon the shape of the casting.

The runner is a horizontal channel filled with molten metal having a slag traping system used to avoid turbulence and improve the smooth flow of molten metal during the casting process resulting in the sound final casting.

The runner regulates the flow of molten metal in the channel connected to the ingate.

Shown below in diagram (a) runner in the sand casting process.

(a) Runner In Casting

A runner is designed in a trapezoidal shape in a cross-section for ferrous metal to get trapped for lightweight floating dross and slag.

The molten metal that is poured flows through the pouring basin, sprue, enters the runner through the sprue well.

When molten metal flows through the sprue towards the runner, the molten metal in the runner should always be full to trap the slag and dross in the slag trapping system.

Skim bob is an enlargement used in runners to trap the heavier and lighter impurities in the molten metal.

If the flow rate of the molten metal at the in gate is more than the runner, the runner will be partially full not letting slag and dross get trapped in the slag trapping system.

The runner channel should be fully filled during the casting process and should be placed on the drag side of the mould to trap impurities going to the mould cavity.

Insulation sleeves near the runner are placed in the form of padding to maintain the temperature.

Why are runners required in casting?

The runner is a common horizontal channel filled with molten metal that is connected to various in-gates that open in the mould cavity.

Runners are required in the casting process to supply slag-free molten metal to the mould cavity continuously through the ingates while the casting solidifies in the mould maintaining a laminar flow of molten metal in the passage.

Runner Extention

The molten metal that first enters the runner is full of slag, contamination, foreign particles, slag and sand. To ensure only clean molten metal passes through the gate runner extension is used in the gating system.

Dross accumulated at the top of the ladle makes its way to final casting if not trapped properly resulting in defects in casting such as slag inclusions, dirt and mould drop.

The runner extension has multiple functions and they are as follows:

Runner extension should trap the slag, dross and dirt in the mould cavity.
The other function of the runner extension is to absorb all kinetic energy built giving a smoother flow to the molten metal entering the mould cavity.
Runner extension is a slag-trapping system.
Runner extensions should reduce the velocity of the molten metal to avoid damage to the mould walls.
Should prevent backwash of unclean metal from entering the gate in the gating system.

Runner Design In Casting

The runner should be designed without any sharp corners and sudden changes to avoid sharp changes in the flow of molten metal to reduce erosion of the mould and efficient flow of molten metal.
The location of the runner is preferred in the cope section of the mould. The location of the runner also depends upon the metal used during the casting process. For aluminium, the metal runner is preferred to be in the drag section.
The runner is made larger in cross-section area as compared to the sprue to slow down the flow of molten metal by increasing the friction between molten metal and the runner reducing its velocity.
The area at the choke should be larger than the cross-section area of the gate to keep the channel full of molten metal. Ideally, the cross-section area at the choke should be 2 times larger than the area at the in-gate.
A runner should be designed in such a way that there is a smooth rate of flow of molten metal in the channel without any turbulence maintaining a sufficient quantity of molten metal in the system.
The cross-section of the runner should be a trapezoid shape to reduce the turbulence of molten metal in the channel.
A circular cross-section runner reduces frictional losses but is not easy to make.
The width of the runner should be twice that of the depth of the runner.
When multiple gates are used runner should be able to accommodate the flow of molten metal in the mould cavity through gates without any heat loss.
Runner length should be sufficient enough to trap gases, slag inclusions, eroded sand, dross and oxides if the runner length is larger than the required length it will cause surface turbulence and oxidation in the runner.
Cross section area at the exit of the sprue should be smaller than the cross-sectional area of the runner to reduce the velocity of the molten metal.
The feed rate, velocity and pressure of the molten metal at the entry of the gate by the runner should be controlled according to the size, complexity and shape of the casting.

Types Of Runner In Casting

How many types of runners are there in the casting process?

There are two types of runners in the casting process, vertical runners and horizontal runners. They have been explained below:

Vertical Runner

The runner in the casting process is normally horizontal but at times vertical runner is used to navigate the horizontal runner. This type of runner is called a vertical runner and is perpendicular to the parting line.

The vertical runner is supposed to reduce slag, turbulence, air pockets, oxides, debris and inclusions in molten metal entering into the mould cavity.

Horizontal Runner

A runner which is parallel to the parting line is called a horizontal runner. Function and working horizontal runner is the same as a vertical runner.

Factors For Selecting & Designing Runner In The Casting Process

Length of the runner. Runners with longer lengths will allow molten metal to trap dirt, impurities, inclusions, slag and foreign material.
Molten metal in the runner. The runner should be full of molten metal to supply gates with equal molten metal.
Number of runners. More than one parallel runner can be used in the casting process to feed large-size complex casting.
Type of pattern selection. Gate pattern in casting requires lengthy and larger runners for a uniform flow of molten metal to each gate as compared to other patterns in casting.
Cross section of the runner. To make sure that the flow of molten metal is uniform the runner is made with decreasing cross sections to accommodate for the equal flow of molten metal to the gate.
The shape of the runner. The runner is made trapezoidal and tapered towards the length to control the directional solidification of the molten metal during the solidification of casting.
Runner extension and type of slag trapping system used in the runner.
Velocity and momentum of molten metal in the runner. The velocity of the molten metal in the runner reduces the damage to the mould at the entry of the molten metal in the mould cavity.
Ease of removing, cutting and trimming of runner from the final casting.
Type of casting process used and runner selected for a particular casting process such as die casting and injection casting.
Location of the runner in the mould with respect to the parting line and mould cavity.
Heat loss of molten metal while molten metal passes through the horizontal and vertical runner channels.
Pressure and temperature of molten metal in the runner.