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Getting the liquid metal out of the crucible ormelting furnace an d into the mould is a critical stepwhen making a casting: it is likely that m ost castingscrap arises during the few seconds of pouring of thecasting.

The series of funnels, pipes and chan nels to guidethe metal from the ladle into the mould constitutesour liquid metal plumbing, and is known as therunning system. Its design is crucial; so crucial, thatthis is without doubt the most important chapter inthe book.

Most castings are made by pouring the liquidmetal into the opening of the runnin g system und erthe action of gravity. This is a simple and quick wayto make a casting. Thus gravity sand casting andgravity die casting are im portan t casting processes atthe present time. Gravity castings have, however,gained a poor reputation fo r reliability and quality,simply because their running systems have ingeneral been b adly designed. Surface turbulence hasled to unreliability in leak-tightness an d mechanicalproperties.

Nevertheless, there are rules for the design of

gravity-running systems w hich, although admittedlyfar from perfect, are much better than nothing.They are mainly empirical, based on transparent-model work an d some confirmatory tests on realcastings. Although many uncertainties remain, theirintelligent use allows castings of the highest qualityto be made. They are therefore described in thissection, and constitute essential reading!

Historically, however, there has been a moveaway from gravity casting as a result of what havebeen believed to be insoluble barriers to the

attainment of high quality and reliability. Uphillfilling, against gravity, known as low-pressurecasting, has provided a solution to the elimination ofsurface turbulence, and thus provided the impetusfor the growth of low pressure die casting,low-pressure sand casting, and various forms of

uphill filling of investment castings. A form ofhigh-pressure die casting ha s also been developed totake advantage of the quality benefits associatedwith low-pressure filling followed by high-pressureconsolidation. These different techniques of gettingthe metal into the mould will all be discussed in thissection.

It is hoped to answer the questions 'Why is therunning system so complicated?' an d 'Why are thereso many different features?' It is a salutary fact thatthe apparent complexity has led to much confused

thinking.An invaluable general rule which I recommend toall those studying running and gating systems is 'If indoubt, visualise water'. Most of us have clear

perceptions about the mobility an d general flowbehaviour of water in the gentle pouring of a cup oftea, the sp lat as it is spilled on the floo r, the flow of ariver over a weir, or the spray from a hose. Ageneral feeling for this behaviour can sometimesallow us to cut throu gh the mystique, and sometimeseven the calculations! In addition, the application ofthis simple criterion can often result in the instant

dismissal of man y existing run ning systems intendedfor the production of a reliable quality of casting asbeing useless!

2.1 Surface-tension-controlledfilling

This section starts with the problem that the liquidmay not be able to enter the mould at all! This is tobe expected if the section thickness is small (ingeneral less than approximately 2 m m as we shallsee). It is an effect due to surface tension. If the

surface is subjected to being sharply curved against anon-wetted mould then it will be subject to arepulsive force which will resist the entry of themetal. Even if the metal enters, it will still be subjectto the continuing resistance of surface tension,which will tend to reverse the flow of metal, causing

Chapter 2

Fluid dynamics

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it to empty out o f the mould if there is any reductionin the filling pressure. These are important effects innarrow-section moulds (i.e. thin-section castings)and have to be taken into account.

We may usefully quantify our understanding ofthis problem with the well-known formula

P f - P6 = Y(l/ri + l/r2) (2.1)

where P 1 is the pressure inside the metal, an d P6 isthe ex ternal pressure of the local environm ent in themould. The two radii define the curvature of themeniscus in two planes at right angles. The equationapplies to the condition when the pressure differ-ence across the interface is exactly in balance withthe effective pressure due to surface tension. Todescribe the situation when the filling pressuresexceed the resistance of surface tension , for instancewhen filling a circular-section tube of radius r(where both radii are now identical), the relation

becomes:

P1 - P6 > 2Y/r (2.2)

For the case of filling a narrow plate of thickness 2r,the radius at right angles becomes infinite, so 1/rbecomes zero. The relation then becomes:

P1 - P6 > Y /r (2.3)

W e have so far assumed that the liquid metal doesnot wet the mould. If this happens then thecurvature terms 1/r become negative, so allowing

surface tension to assist the metal to enter themould. This is, of course, the familiar phenomenonof capillary attraction. The pores in blotting paperattract the ink into them, as does the wick of thecandle the molten wax. In general, the castingtechnologist attempts to avoid it in metal-mouldinteractions. However, it sometimes happens de-spite al l efforts to prevent it. These problems aredealt with in Section 3.4.

Continuing now in our assumption that themetal-mould combination is non-wetting, we shallestimate what head of metal will be necessary to

force it into a wall section of thickness 2r for agravity casting made under normal atmosphericpressure. If the head of liquid is h, then thehydrostatic pressure at this depth is pgh, where p isthe density of the liquid, an d g the acceleration dueto gravity. The total pressure inside the metal istherefore the sum of the head pressure and theatmospheric pressure, Pa. The external pressure issimply the pressure in the mould due to theatmosphere Pa plus the pressure contributed bymould gases Pm. The equation now is

(Pa + pgh) - (Pa + Pm) > v/r (2.4)which gives imm ediately

P gh -Pm>V /r (2.5)

It follows that the back-pressure due to the

outgassing in the mould does lower the effectivehead which is driving the filling of the mould. It isgood practice, therefore, to vent narrow sections,reducing this resistance to practically zero.

It is also clear from the above result that,provided the mould is permeable and/or wellvented, atmospheric pressure plays no part in

helping or resisting the filling of thin sections in air,since it acts equally on both sides of the liquid front,cancelling any effect. Interestingly, the sameequation and reasoning applies to casting invacuum, which, of course, can be regarded ascasting under a reduced atmospheric pressure.Clearly, vacuum in itself is therefore not helpful inovercoming the resistance to filling provided bysurface tension (although, to be fair, it may help byreducing Pm by outgassing the mou ld to some extentprior to casting, and it will help where thepermeability of the mould is low, allowing an y

residual gases to be compressed ahead of theadvancing stream).

In the case of vacuum-assisted filling (not vacuumcasting) the atmospheric pressure is allowed to acton the liquid metal via the running system, but isremoved locally within the mould, by drawing avacuum either through the permeable mould, orthrough fine channels cu t through to the sectionrequired to be filled (as is com mon ly applied to thetrailing edge of an aerofoil blade section). In thisway Pm is guaranteed to be zero or negligible, and

Pa remains a powerful pressure to assist in theovercoming of surface tension as the equationindicates:

P a + P gh > Y/r (2.6)

It is useful to evaluate the terms of this equation togain a feel for the size of the effects involved.Taking, roughly, g as 10m/s

2, and for liquid

aluminium p as 2500 kg/m3

and y as l .ON/m (forsteels an d high-temperature alloys the correspond-ing values are approxim ately 7000 kg/m

3an d 2.ON/

m ), the resistance term y/r w orks out to be 2 k P a fo r

a 1-mm section (0.5mm radius) and 1OkPa for a0.1-mm radius trailing edge on the blades of aturbine wheel.

For a head of metal h 100 mm the head pressurepgh is 2.5 kP a, showing that the 1-mm section mightjust fill. However, the trailing edge has no chance;the head pressure is insufficient to overcome therepulsion of surface tension. However, if vacuumassistance were applied (not vacuum casting) thenthe additional 100 kPa of atmo spheric pressureshould ensure filling. In practice it should be notedthat the full value of atmospheric pressure is noteasily obtained in vacuum -assisted casting; in mostcases a value nearer half an atmosphere is moreusual. Even so, the effect is still important: oneatmosphere pressure corresponds to 4m head ofliquid aluminium , an d approximately 1.5m head of

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denser metals such as irons, steels and high-temperature alloys. In modest-sized castings ofoverall heights aroun d 100mm or so, these valuablefilling pressures are not easily obtainable by othermeans.

For those castings which have sections of only 1 or2mm or less, the surface tension wields a strong

control over the tightly radiused front. Filling is onlypossible by the operation of outside agencies, suchas the cen trifugal action of the jew eller's centrifug e,or the application of vacuu m assistance. Filling canoccur upwards or downwards without problems,being always under the control of the surfacetension, which effectively keeps the surface com-pact; no surface turbulence is possible to allow thesurface to break up into drops, or to allow theformation of freely falling streams and jets, whichwould normally lead to splashing. The integrity ofthe front is under the control of surface tension at all

times. This special feature of the filling of verythin-walled castings means that they do not requireformal running systems. In fact, such thin-walledinvestment castings are made successfully by simplyattaching w ax patterns in any orientation directly toa sprue (Figure 2.1). No well, runner or gate isnecessary.

If the section is halved, the required head forpenetration is, of course, doubled. Similarly, if themould shape is not a flat section w hich imposes onlyone cu rvature on the meniscus, but is a circular holeof diameter lmm, which, of course, curves thesurface additionally at right angles to the firstcurvature, then, as for Equation 2.2, the head is

doubled again.

2.2 Surface finish

When the pressure in the liquid metal becomessufficiently high, su rface tension is no longer able toresist the penetration of the metal into the spacesbetween the sand grains of the mould. The size ofthe holes between the sand grains can be roughlyestimated assuming that the radius of the inter-granular spaces is only approximately 15.4 per centof the radius of the sand grains (Ho ar, 1953).

The penetration of the mould in this way producesa 'furry' casting w hich may be quite unsaleable. Thepenetration may be only one grain deep, givingeffectively an excessively rough surface. Howeverpenetrations of 20-50mm are not uncommon inlarge castings. In a classical series of experiments,Hoar and Atterton (1950 to 1956) demonstrate thatonce the critical pressure difference to force themetal into the sand is exceeded, then penetrationoccurs rapidly, within a second or so. The depth ofpenetration is controlled by the freezing of the

leading edge of the advancing metal when it reachesthe freezing isotherm in the sand. Clearly, thisdistance is greater for larger castings. The mix ofsolid m etal an d sand is difficult to remove from suchcastings.

Attempts are made to resist such mould pen-etration by reducing the size of the pores by:

1. The use of finer sand for the mould or core. Forcores this approach is limited by the req uirem entto maintain the permeability of the core materialso that core gases can escape during casting. It is,

however, widely used for vacuum (V ) processmoulding, where the vacuum w hich is applied tomaintain the rigidity of the mould assists indrawing the liquid metal into the pores betweenthe sand grains. Thus whereas normal sandcastings have an average grain size in the range250-500 iim, sands for the V process are approxi-mately 50-150 ^m . This results in a serious dustnuisance, which is a great pity since the vacuummoulding process is otherwise excellent in itsenvironmental benefits. Newer plants are im-proving their designs to tackle this problem.

2. The application of a mould wash - a ceramicslurry applied as a paint to fill the spaces betw eenthe sand grains. The pores in the dried coatingare one or two orders of magnitude smaller thanthe pores between the grains, thus, in line withEquation 2.2, enabling the mould surface to

( S t e e l c o n t a i n e r )

( P e r m e a b l ep l a s t e r m o u l d )

V acu u m

Figure 2.1 A plaster mould encased in a steel bo x usingvacuum-assisted filling. No formal running system isrequired fo r such sma ll thin section ca stings.

To gain an idea of the head of metal required toforce the liquid metal into small sections, fromEquation 2.6 we have:

pg/i = v/ r

h = y/rpg (2.6a)

Using the values for aluminium and steel givenabove, we can now quickly show that to penetrate a1-mm section we require heads of approximately 80an d 60mm respectively fo r these tw o metals.

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withstand 10-100 times greater pressures beforemetal penetration.

Although the metal will often still succeed inpenetrating the sand coating through cracks, thepenetrated sand only adheres to the casting at theisolated points of failure of the coat, and so isrelatively easy to remove. This action is generally

used to counter metal penetration at the base of tallmoulds, which can sometimes be more than a metrehigh.

The application of a core w ash is only marg inallysuccessful, however, in resisting metal penetrationwith the use of sand cores in low-pressure diecasting. This seems to be the result of the ratherpoor pressure control on most low-pressuremachines, and the additive effect of the momentumof the metal, giving a pressure peak at the instantwhen the liquid hits the top of the mould (see

Section 3.1.2). Sand cores are only rarely used,therefore, in low-pressure die casting. For similarreasons, sand cores have proved impractical forhigh-pressure die casting. In this case othe r solu tionssuch as water-soluble salt, glass, or ceramic cores,have been tried with mixed success. In general, onlywithdrawable steel cores continue to be usedsuccessfully.

Filling pressures are high in pressure die casting.This is widely alleged to be for the purpose ofincreasing surface finish and definition, i.e. theability of the metal to fill small radii so as to

reproduce fine detail. Pressure die casting machinescommonly operate at metal pressures of lOOOatm(100MPa). Equation 2.3 indicates that such press-ures will force the liquid into radii of only 10~

8m ,

approaching atomic dimensions! This is, of course, avast overkill. The student should therefore be on hisguard against such loose thinking. The highpressures are actually needed mainly to reduce thebubble defects produced by the turbulently en -trained mould gases - the bubbles are simplysquashed to acceptable dimensions. Considerationof Equation 2.3 reveals that a mere lOa tm ( I M P a )would reproduce a radius of 0.001 mm , which wouldbe more than good enough for most purposes.

Filling pressures are enhanced for the genuinepurpose of reproducing detail in the centrifugalcasting of jewellery. A casting travelling at 10m/s onan arm of radius 1 m will experience an accelerationof 100 m/s

2. This is close to 1Og. In the absence of

mould gases, and replacing the acceleration g due togravity with the total acceleration (g + Wg) = Ug asthe arm goes from the vertically up through thevertically down part of its stroke, Equation 2.3

predicts that an improvement in the fineness ofdetail by a factor of 11 should be achievable.In industrial uses of centrifugal casting much

higher accelerations are normally used, typically50-10Og. The high pressures in the liquid are,however, not normally need fo r filling, since the

moulds are usually of simple shapes. The techniqueis valuable for totally unrelated reasons: (1) forpipes an d cylinders and the like, because thecentrifugal action avoids the requirement for acentral cylindrical core to make a hollow shape; an d(2) for enhancing the pressure in the casting duringfreezing to reduce porosity. In this case, however, it

has to be pointed ou t that the production of shapedcastings by this route involves pouring the liquidmetal down a central down-runner, an d acceleratingit out along radial runners, to arrive in the mould atsuch a high speed that considerable damage is doneto both mould an d metal. The high centrifugalpressures are then needed to help to repair some ofthis damage in the casting. Shaped castings wouldprobably be cheaper and better if not centrifuged atall, but simply produced with a properly designedgravity-running system.

2.3 Running systems

2.3.1 Gravity pouring of open top moulds

Most castings require a mould to be formed in twoparts: the bottom part (the drag) forms the base ofthe casting, and the top half (the cope) forms the topof the casting.

How ever, some castings require no shaping of thetop surface. In this case only a drag is required . Theabsence of a cope means that the mould cavity isopen, so that metal can be poured directly in. The

foundryman can therefore direct the flow of metalaround the mould using his skill during pouring(Figure 2.2).

This is a successful and economical technique forthe production of aluminium or bronze wall plaquesand plates in cast iron, which do not require awell-formed back surface.

Other viscous an d poorly fluid materials are castin this way, such as hydraulic cement an dconcretes. Molten ceramics such as liquid base t aretreated similarly, as are organic resins an d resin/aggregate mixtures which constitute resin concretes.

The general principle of eliminating the runningsystem by simply pouring into the top of the mould(down an open feede r, for instance) may be the bestsolution in certain cases. Such an option should bechosen with care, however. For instance, it is likelyonly to be successful in those instances where themetal-mould system does not form deleteriousfilms, as in the grey iron-green sand system, orwhen high quality is unnecessary, which is rare!

2.3.2 Gravity pouring of closed moulds

Closed moulds represent the greatest challenge tothe casting engineer. There are numerous ways toget the metal into the mould, some disastrously bad,some tolerable, some good. To appreciate the goodwe shall have to devote some space to the bad. If