1MMEMMEmaterials & metallurgical engineering

Prof. A. K. M. Bazlur RashidDepartment of MME, BUET, Dhaka

MME345: Lecture 12

The Gating Systems4. Design of gating system elements, Part 2Ref: J. Campbell, Castings Practice: The 10 Rules of Castings, Elsevier

12.1 Design of runner

12.2 Design of gate

12.3 Design of inclusion control

B. Rashid, DMME, BUET Lec #12: Design of gating system elements, Part 2 Page 2 of 26

2 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.

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 completelyprior 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

3 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

in iron and steel foundries to minimize the danger of run-outs and separation of metal and slag

The runner in the cope is not recommended for any type of casting - not even grey iron !!

Optimum Runner Sizes

Runner / Sprue Exit Area Ratio

Websters 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

4 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.

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)

5Tapered 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.

Fir 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.

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

6 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

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

7 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.

A separate runner and gate provides a number of right-angle changes of direction of the stream before it enters the mould.

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

8Some useful quick rules

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

Types of Gate

Easy knockoutDifficult to make

Grey ironHeavy sectionGreensand

9Junction 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.

Between these three junctions, T-junction poses the post serious problems.

L-junction is an intermediate case, while a casting extension poses no problem

10

Figure 2.33 Solidification sequence for T-shaped castings (A = arm, J = junction, L = leg) Figure 2.34 Array of different T-junctions

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?

11

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.

Dross Trap or Slag Trap

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

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!

12

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.

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

13

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.

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

1. the entrance at the base of the trap2. the exit to be sited at a substantially higher level3. both entrance and exit to have similar tangential direction, and4. an adequate height above the central axis to provide for accumulation of separated debris

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

14

The minimum particle diameter which can be centrifuged clear of the exit

b2 = 18a3n / pAVrDr

a = inlet and outlet thickness of trapA = height of trapr = radius of trapn = viscosity of liquidV = velocity of liquidDr = density difference between liquid and slag

Example:r = 100 mmA = 2a = 25 mmN (for molten steel) = 5.5x10 3 N s/m2

V (for 1-m high mould/sprue) = 4.5 m/sDr = 3500 kg/m3

b = 0.1 mm

V (for 100-mm high mould/sprue) = 1.4 m/sb = 0.2 mm

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 bad

15

Placement of filters

16

MME345: Lecture 13

The Gating Systems5. Calculation of gating system dimensions

Next Class