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Mechanism

Gating system design and parts in casting

By Ricardo Lopez
After the mould has been prepared, the molten metal is to be poured or injected into the mould cavity. Introducing the molten metal into the mould cavity in a manner which does not erode the mould, fills the mould completely without entrapping gases and permits cooling and solidification to progress in such a way that the accompanying shrinkage may take place without resulting in voids, cracks or porosity within the casting.

Due to this, the molten metal is introduced into the mould cavity from the ladle, through a Gating System. the gating system for a casting is a series of channels which lead molten metal from the ladle into the mould cavity. The following is the process and design of Gating system.

Parts and Design of GATING SYSTEM

A Gate is usually made up of

  • Pouring basin
  • Sprue
  • Runner
  • Gate
  • skim bob
  • choke
  • cavity
  • Riser

Pouring Basin

The part of Gating system is made on or in the top of the mould. The main purpose of pouring basin is to direct the flow of metal from the ladle to the sprue, to help to maintain the required rate of liquid metal flow. Sometimes, the metal is directly poured into the top of the sprue, which is made with a funnel-shaped opening. However, better results are usually obtained with the use of a pouring basin. If the metal is poured directly down the sprue entrance lot of vortexing and turbulence, take place, resulting in unsound and defective casting. Pouring basin reduces turbulence and vortexing at the sprue entrance.It is placed nearly at the edge of moulding box to compensate metal shrinkage or contraction.

Sprue

The vertical passage that passes through the cope and connects the pouring basin with the runner or gate is called the sprue. As the metal flows down the sprue, its velocity increases. Hence the section of the sprue should decrease, otherwise, the sprue will not remain full of metal with the metal leaving the walls of the spure. This will results in aspiration of gases through the surrounding moulding sand. The sprues are generally tapered downwards to avoid aspiration of air and metal damage. The cross section of a sprue may be square, circular or rectangular. Sprues up to 20mm diameter are round in section where as large sprues are often rectangular.

Runner 

 In large castings the molten metal carried from sprue to gates around the cavity through a passageway called runner is generally preferred in drag, same times in cope depends on the shape of casting. The Runners are of large cross-section and often streamlined to slow down and smooth out the flow, and are designed to provide approximately uniform flow rates to the various parts of the mould cavity. It provides to avoid aspiration and turbulence. Runners are commonly made trapezoidal in cross-section.

Gate

A gate is a passage through which metal flows from the runner to mould cavity. The location & size of gates is so arranged that they can feed liquid metal to the casting at the rate consistency with the rate of solidification.The gates should break off easily from the casting after solidification. For this, at the junction to the cavity, the gates are much reduced in thickness. The best cross-section for gates is a trapezoidal one that smoothly passes into a rectangular section at the junction of the cavity.
According to position in the mould, cavity gating may be classified as

  1. TOP GATING
  2. PARTING LINE GATING
  3.  BOTTOM GATING

Skim bod

It is extension along the runner provides the elimination of ingredient particle in the circular shape portion. Thus it prevents these impurities from going into the mould cavity  And also gives a slow and progressive flow of molten metal into a mould cavity.

chock

It is a part of gating system which has smaller cross-section area. It is placed after the skimboard. Its function is to control the flow to lower the flow velocity in the runner and also to minimize sand erosion in the runner. It’s another end is connected to gates. Depending upon the chock location, gating system is classified as
  1. Pressurised
  2.   Unpressuriesed

Cavity

It is the hallow shape or space that is obtained by molding process. The molten metal enters in the mould cavity. The finished product is obtained by mould cavity shape.

Riser

Risers are a part of the feeding system.The risers is a passage of sand made in the mould during ramming. It provides the direction solidification of molten metal It escapes the gases in cavity during casting. It also indicates the filling of cavity. The rate at which the pouring metal is stop or not. The riser is placed top most portion of the mould cavity.
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