Gating System: Elements of gating system, Design Considerations of Gating System, Riser, Considerations in Riser Design, Classification of Riser, Function of a riser, Riser Design (Caine’s Method), NRL (Naval Research Laboratory Method) /Shape Factor Method and Design of Runner

Gating System

Grating system consists of all the elements which are connected to the flow of molten metal from the ladle to the mould cavity. Aim of the design of gating system should be the production of defect free casting.

Elements of gating system are

(i) pouring basin

(ii) sprue

(iii) sprue base well

(iv) runner

(v) runner extension

(vi) ingate

(vii) riser

Design Considerations

Cavity (mould) should be filled in smallest possible time.

Metal should flow smoothly into mould without turbulence.

Unwanted material as slag, dross etc. should not be allowed to enter.

Metal should enter cavity in such a way that contamination by atmospheric air is prevented.

Flow of metal should not erode gating system components or mould cavity.

Sufficient molten metal should reach mould cavity.

Gating system should be economical to prepare and easy to break.

Casting yield should be maximized.

Riser

During solidification most of the foundry metals and alloys shrink in volume.

Voids (blank spaces) are created due to shrinkage if extra material is not fed in.

So a reservoir of molten metal is to be provided which will supply molten metal to the casting when needed. This reservoir is called riser.

Considerations in Riser Design

(i) Metal in the riser should solidify last.

(ii) Riser volume should be sufficient to compensate the shrinkage in casting. So, the risers can be made sufficiently large. But it would increase process cost because it has to be cut off from the riser as scrap.

Classification of Riser

(i) Top risers – open to atmosphere (last effective and convenient to make)

(ii) Blind risers – completely in mould (more effective and more convenient to make)

(iii) Internal risers – enclosed on all sides by casting (most effective and must convenient to make)

Function of a riser 

– feeding the casting during solidification so that no shrinkage cavities are formed.

It depends on

– metal poured and

– complexity of casting

Normally due to shrinkage the volume of material is reduced when cooled from liquid to solid.

Grey cast iron some times shows negative shrinkage. It occurs due to higher carbon and silicon content. Graphitization occurs which increases volume. Risers may not be critical here

Classification of Riser

Riser Design (Caine’s Method)

Generally, risers are considered as cyclindrical shape

Freezing Ratio

where SA = Surface Area

V = Volume of casting

Empirical Freezing Ratio

a, b, c are constants (value of different materials)

For steel, a = 0.10, b = 0.3, c = 1.00

(i) Assuming certain value of riser diameter X can be found out.

(ii) Then by using graph, the value of X and Y can be plotted if the assumed diameter

meets the above soundness curve then it is accepted [Fig.].

Freezing Ratio vs Casting Volume
Freezing Ratio vs Casting Volume

(iii) For example, value of X is 1.4 and Y is 0.4 for a particular diameter of riser then the point will be in soundness zone. So the riser is of required size. If it is in unsound zone then the suitable size is to be selected by hit and trial method.

NRL (Naval Research Laboratory Method) /Shape Factor Method

Shape factor method is used for rough measurement of riser.

This method is a simplification of Caine’s method.

It uses shape factor which replaces the freezing ratio.

From the graph shown in Fig, the volume ratio can be obtained for sound casting considering shape factor.

Shape Factor vs Casting Volume
Shape Factor vs Casting Volume

From the given data volume of the casting can be calculated. After Knowing the volume ratio, the riser volume can be calculated.

Design of Runner

It is generally located in the parting plane which connects sprue with ingates.

It is normally trapezoidal in cross section

Normally for ferrous materials runners are cut in cope and ingates in the drag.

It is done to trap slag and dross which are lighter in the upper part of runners.

For effective trapping of slag the runners should flow full.

Runner full
Runner full


Runner partially full
Runner partially full

In order to get a fully filled casting the cross section area of runner must be larger than cross section area of gate.

For minimum or less metal velocity in runner, area should be high

For minimum or less metal velocity in runner, area should be high

For this reason, generally runner to gate area ratio of 1.15 : 1 to 1.5:1 is used as a compromise.

Fluid flow across varying cross section
Fluid flow across varying cross section

The design obeys equation of continuity for fluid flow i.e.,

A1V1 = A2V2 = A3V3 = constant.

The shape of the runner cross section gradully varies from round, trapezoidal, elliptical wide and flat to narrow and thick.

A thick runner will solidify slower than thinner one.

A flat runner looses heat faster before it enters gate.

Normally maintained width to depth ratio is 1.6 : 1 to 1.8 : 1.

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