MME 345

Lecture 18

The Design of Gating System5. Design of gating system elements 2

Ref:

[1] P. Beeley, Foundry Technology, Butterworth-Heinemann, 2001

[2] J. Campbell, Castings, Butterworth-Heinemann, 2001

Topics to discuss....

1. Design of runner

2. Design of gate

3. Design of inclusion control

1. Design of Runner

The runner is that part of the filling system that acts to distribute the melt

horizontally around the mould, reaching distant parts of the mould cavity quickly

to reduce heat loss problems.

The runner is usually necessarily horizontal

follows the normal mould joint in conventional horizontally parted moulds

For vertically jointed moulds, or investment moulds where there is little

geometrical constraint, the runner can be inclined uphill.

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Runner should be arranged under the casting, so that the runner is connected

to the mould cavity by vertical gates.

this will fill the runner completely prior to rising through the gates and into the mould cavity

In a two-part mould, the runner has to be moulded

in the drag, and the gates and casting in the cope

In a three-part mould, the joint between the

cope and the drag contains the mould cavity,

and the joint between the lower mould parts

(the base and the drag) contains the running channels

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• the gates will inevitably start to fill and allow metal into the mould cavity before the runner is full

• the first metal and its load of slag enters the gates immediately before filling the runner and thus prior to the chance of trapping slag against the upper surface of the runner.

• In short, the runner in the cope results in the violation of the fundamental 'no fall' criterion.

Moulding runner in cope

The runner in the cope is not recommended for any type of casting

- not even grey iron !!

This is particularly used in iron and steel foundries to minimize the danger of run-outs

and separation of metal and slag

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Optimum runner sizes

Runner / Sprue

Exit Area Ratio

Webster’s Findings

(1964)

1.0 High metal velocity

2.0Optimum metal velocity, runner fills rapidly and

excludes air bubbles reasonably efficiently

3.0 Starts to be difficult

4.0 Simply wasteful

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Recent findings

Recent video X-ray radiographic studies have made it clear that even the

expansion of the area of flow by a factor of 2 cannot prevent a serious amount

of surface turbulence.

The best that can be achieved without damage is by a 20 per cent increase in

area of the runner. Any greater expansion of the runner will cause the runner to

be incompletely filled and so permit conditions for damage.

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The expanding, rectangular runner

Figure 2.27 Plan views of a square section sprue connected to a shallow rectangular runner

showing attempts to expand the runner (a and b) that fail completely. Attempt (c) is better but

flow ricochets off the walls generates a central starved, low pressure region; (d) a slot sprue and

slot runner produce a uniform flow distribution in the runner shown in (e) (recommended) and (f)

(probably acceptable)

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Tapered runner

• When the runner has two or more gates, the momentum

of the flowing liquid causes the furthest gate, number 3

to be favoured.

• The rapid flow past the opening of gate 1 will create a

reduced pressure region in the adjacent gate at this

point, drawing liquid out of the casting! The flow may

be either in or out of gate 2, but at such a reduced

amount as to probably be negligible.

• For the present case, it would have been best to have

only gate 3.

• Where more than one gate is attached to the runner, the runner needs to be reduced in cross-section as each

gate is passed.

• A smooth, straight taper geometry does a reasonable

job of distributing the flow evenly.

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One of the most effective devices

to reduce the speed of flow in the

runner is the use of a filter.

The close spacing of the walls of its

capillaries ensures a high degree of

viscous drag.

Flow rate can often be reduced

by a factor of 4 or 5.

Use of filters to reduce flow velocity

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In general, it is important that the liquid metal flows through the gates at a

speed lower than the critical velocity so as to enter the mould cavity smoothly

If the rate of entry is too high, causing the metal to fountain or splash then the

battle for quality is probably lost. The turbulence inside the mould cavity is the

most serious turbulence of all

2. Design of Gates

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Do not place the gate at the base of the sprue so that the high velocity

of the falling stream is redirected straight into the mould.

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A separate runner and gate provides a number of right-angle changes of direction

of the stream before it enters the mould.

These provisions are all used to good effect in reorganizing the metal

from a chaotic mix of liquid and gases into a coherent moving mass of liquid.

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Total gate area

Gate should be provided with

sufficient area to reduce the velocity

of the melt to below the critical

velocity of about 0.5 m/s

If the area of the gate is too small

then the metal will be accelerated

through, jetting into the cavity a

though from a hosepipe

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A quick check

For an Al alloy (density 2500 kg/m3) to be cast at 1 kg/s, with the metal

velocity at the gate of approximately 0.5 m/s, we need approximately 800 mm2

of gate area

If we wished to fill the casting at twice this rate we would require 1600 mm2

If it is decided that the metal can be allowed to enter at twice the speed, it

would require only 400 mm2

Gate area = Pouring rate

Gate velocity x Metal density

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(a) The touch gate, (b) knife gate, (c) pencil gate, (d) normal and reverse horn gates

Types of gates

• Easy knockout

• Difficult to make

• Grey iron

• Heavy section

• Greensand

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0.8 – 1.2 mm overlap

Not suitable for any film forming

metal-mould combination• jetting off liquid • irregular filling

• suitable for greensand-grey iron

Junction effect

When gates are placed on casting, they create a junction.

Some geometries of junction create the danger of a hot spot. The result is

that a shrinkage defect forms in the pocket of liquid that remains trapped here

at a late stage of freezing.

The magnitude of the problem depends strongly on what kind of junction is

created.

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• Between these three junctions, T-junction poses

the most serious problems.

• L-junction is an intermediate case, while a casting

extension poses no problem

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Figure 2.33 Solidification sequence for T-shaped

castings (A = arm, J = junction, L = leg)Figure 2.34 Array of different T-junctions

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The term ‘inclusion’ is a shorthand generally used for ‘non-metallic inclusion’

Furthermore, one of the most common defects in many castings is the bubble,

entrained during pouring. This constitutes an ‘air inclusion’ or ‘gas inclusion’

Why did the bubble, instead of trapping in casting,

not simply rise to the surface, burst and disappear?

3. Inclusion Control

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The answer in practically all cases is that oxide films will also be present

Many bubbles, entangled in a jumble of films, never succeed to reach the

surface to escape

This close association of bubbles and films (since they are both formed by the

same turbulent entrainment process; they are both entrainment defects) is called

bubble damage

Whereas inclusions are generally assumed to be particles having a compact

shape, it is essential to keep in mind that the most damaging inclusions are the

films and are common in many of our common casting alloys.

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Dross trap or slag trap

Takes the first dirty and cold metal and keep it

away from the gate

• the best quality metal was concentrated in the

dross trap and all the dross was in the casting!

• this rather chunky form of trap sets up a

circulating eddy during filling

• dross arriving in the trap is therefore efficiently

floated out again, only to be swept through the

gates and into the casting a few moments later!

Metal flowing into the narrow section is trapped

• volume of melt that they retain is very limited

• can reflect a backward wave if the runner is

sufficiently deep

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A useful design of dross trap appears to be a volume at the end of the runner that is

provided with a narrow entrance to suppress any outflow. It is a kind of wedge trap

fitted with a more capacious end.

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Slag pockets

A rectangular trap is effective only when the liquid velocity through the runner is

within 0.4 m/s.

Use of traps of wedge-shaped design is completely ineffective because the

circulation pattern of flow would take out any material that happened to enter

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Swirl traps

Figure 2.52 Swril traps showing (a) incorrect

opposed inlet and exit ducts; (b) correct

tangential arrangements; (c) incorrect low exit;

(d) correct high exit.

The spinning of the liquid creates a

centrifugal action throwing the heavy

melt towards the outside where it

escapes through the exit, to continue

its journey into the casting.

Conversely, the lighter materials are

thrown towards the centre, where

they coagulate and float.

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Thus, a swirl trap, when correctly

designed, can be a useful device to divert

unwanted buoyant particles, both solid

and liquid, away from ferrous castings.

Regrettably, the swirl trap is expected to be completely useless for film-forming alloys

• films will be too sluggish to separate

• the swirl trap creating more films than it can remove

• in the case of alloys of aluminium and magnesium, their oxides are denser than

the metal, and so will be centrifuged outwards, into the casting!

Swirl traps are, therefore, of no use at all for light alloys

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The optimum design for the swirl trap must include the features:

1. the entrance at the base of the trap

2. the exit to be sited at a substantially higher level

3. both entrance and exit to have similar tangential direction, and

4. an adequate height above the central axis to provide for accumulation of separated debris

The minimum particle diameter which can be

centrifuged clear of the exit is given by the formula

b2 = 18a3n / pAVrDra = inlet and outlet thickness of trap

A = height of trap

r = radius of trap

n = viscosity of liquid

V = velocity of liquid

Dr = density difference between liquid and slag

Example:

r = 100 mm

A = 2a = 25 mm

n (for molten steel) = 5.5x10 – 3 N s/m2

V (for 1-m high mould/sprue) = 4.5 m/s

Dr = 3500 kg/m3

b = 0.1 mm

V (for 100-mm high mould/sprue) = 1.4 m/s

b = 0.2 mm

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Filters and strainers

Strainers

• 3 – 5 mm hole diameter

• Not much effective as filter

• Useful to laminise the flow

Steel wire mesh /

glass woven cloth

• 1 – 2 mm openings

• Very good in retaining oxide film

Ceramic block filters

• 2 – 0.05 mm pore sizes

• Very effective as filter in retaining

oxide films (in nonferrous alloys) and

liquid slag (in iron casting)

Can be clogged if the gating system is very bad28/31

Placement of filters

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Next ClassMME 345, Lecture 19

The Design of Gating System6. Calculation of gating system dimensions