THE TEN RULES

8/7/2019 THE TEN RULES

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THE TEN RULES:

(1) "NO POURING!" The best quality and most reproducible castings are those produced bythose few foundries designed to avoid the pouring of metal. Hard to believe? Not when

you think about what happens to metal when it is poured, and how that affects it after

solidification.

(2) "Do it s-l-o-w . . ." When the melt is never poured (SeeRule No. 1), and never exceeds aspeed of 0.5 m/s (about 20 inches/second), the casting can stay free from oxide cracks.

(3) "DON'T STOP" While the melt continues to rise smoothly in the mould, the liquid front

stays "alive", with the surface oxide continuously breaking and sliding off the advancing

meniscus to form the skin of the casting. The thin oxide on the advancing liquid front isnot therefore a problem; this steady advance will ensure a good filling condition and a

casting free from oxide cracks.

(4) "BUBBLES ARE BAD"Bubbles are the most common source of porosity in castings --

but their effect is usually mistaken for shrinkage, which it closely resembles, because the

bubbles and their trails are often irregularly shaped.

(5) "BIG BUBBLES ARE WORSE"The outgassing of cores can lead to huge defects, filling

whole areas of the tops of castings. However, even a small blow from a core can leave abubble trail that can create a leak defect. To avoid blows from cores, the core must be

vented to the atmosphere.

(6) "AVOID SHRINKAGE"Follow the three steps provided to assure that feeder design is as

reliable as possible.

(7) "ROCK OR ROLL"The great majority of castings in the market place have a freezingtime of several minutes. This is similar to the time taken for the convection of hot and

cold liquid metal in the solidifying casting to build up, and for the resulting convection

currents to start re-melting their way through the casting as it attempts to solidify. Foursteps can be taken to alleviate the problems this causes.

(8) "SEVERE SEGREGATION"All freezing will cause some segregation of alloying elements,

and some alloys segregate seriously, to the point at which parts of the casting will be well

outside chemical specification. In principle the problem can be predicted by computerpackages, and therefore allowed for in the design.

(9) "NO QUENCHING"Water quenched castings are effectively pre-loaded to approximately

50 % of their failure stress before being put into service. Polymer quenchants or air

quenching are much more reasonable alternatives.

(10) "THE TOOL RULE"The best castings startby thinking about the work that happens

aftercasting -- and that means providing pick-up points needed in machining and other

finishing operations. Only then will a trauma-free, integrated, supply of castings be

achieved, all accurately within dimensional specification.
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Rule No. 1:"NO POURING!"The best quality and most reproducible castings are those produced by those few

foundries designed to avoid the pouring of metal. Hard to believe? Not when you think

about what happens to metal when it is poured, and how that affects it after solidification.

Death by Oxides -- Nearly all metals, particularly aluminum alloys, contain oxides thatact as cracks. This can be death to the mechanical properties, particularly elongation and

fatigue. The biggest source of these troublesome oxides comes when the metal is exposed

to the oxygen in open air when the metal is in its molten state -- particularly whereagitation is involved.

view images of aluminum damaged by oxide trails.

Step One: Cleanup -- Liquid aluminum alloys need treatment to reduce hydrogen and

oxides. Some foundries de-gas with tablets, or with a simple open-ended lance, but this is

not enough. Current best practice is rotary degassing and should be specified when theorder is placed.

The most important outcome of this treatment is the removal of oxides. If oxides are

successfully reduced to a low level, hydrogen porosity will not be a problem no matterwhat the hydrogen level is!

Step Two: No Pouring! -- The subsequent handling of the melt requires great care, so as

to avoid the unnecessary re-introduction of oxides. Thus the melt should not be poured at

all if possible, since pouring folds in oxides.

If transfer to another crucible or furnace is necessary, any pouring height should bereduced to a minimum -- definitely less than four inches (100 mm), and preferably lessthan two inches (50 mm)!

Bottom Line: Ultimately, the best quality and most reproducible castings are those

produced by those few foundries designed to avoid the pouring of metal, and in which theoxides suspended in the melt are allowed to sink or float, and the melt transferred into the

mould cavity without any turbulence whatever.
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Rule No. 2:"Do it s-l-o-w . . ."When the melt is never poured (See Rule No. 1), and never exceeds a speed of 0.5 m/s

(about 20 inches/second), the casting can stay free from oxide cracks.

Gravity, Acceleration, Speed -- Thus the optimum filling systems to avoid forming

oxides in the casting are (1) counter-gravity, and (2) properly controlled tilt-pouring (inwhich the mould starts from horizontal or above the horizontal, so that the melt is caused

to transfer horizontally into the mould cavity without flowing downhill).

However, most foundries use gravity pouring systems. Even if the melt falls only 12 mm(about 1/2 inch!), it has already reached a speed of 0.5 m/s. Thus all pouring introduces

problems to achieve a casting free from oxide cracks!

Design Features that ameliorate Pouring Problems -- To reduce these problems when

pouring, many precautions need to be built into the filling system design. The design isoften difficult, but the alternative is nearly always castings that are impaired, if not

actually ruined, by oxides, as recent research has demonstrated. (Unfortunately, mostfoundries remain unaware of these recent developments!)

Of the many possible design improvements, key beneficial features include:

(1) an off-set weir basin;

(2) a correctly tapered sprue;

(3) a slim runner, correctly profiled to slow flow, and distribute melt into gates; and

(4) gates entering the mould cavity only at the lowest points.

In particular, the gates are the key control point for making sure melt velocity is held to

below 0.5 m/s. Velocities higher than 0.5 m/s into the mould cavity will cause the melt tojet into the mould, creating oxide and bubble damage in the casting. Properties, leak

tightness, and corrosion resistance will all be randomly degraded in proportion to anyexcess velocity.

view diagram of oxide formation on the

surface of turbulent, high-speed melt.
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Sources of Trouble -- Buyers should note that particularly damaging features include:conical pouring basins, oversized sprues and runners, and wells at the base of sprues.

These all contribute to the entrainment of bubbles and oxides, and thus lead to the

random degradation of properties. These features must be avoided to successfully procure

reliable castings.

Bottom Line: Rule No. 2 is closely related to Rule No. 1 -- it is absolutely essential tofill below a top limit of 0.5 m/s, and acceleration due to gravity is far too great to control

below this limit. The first step is to avoid pouring, and the next step is to find other

control points to limit filling speed.

NOTES

off-set weir basin -- The entry point to the whole filling system. The pouring basin is

off-set from the sprue (in contrast to a conical basin which is set in-line with the sprue),

and a small vertical step (the weir) is an essential feature that controls the filling pattern.The design of the basin is critical to the success of most castings. Conical basins should

be avoided if possible.sprue -- The vertical channel from the filling basin that conveys the metal to the lowestpoint of the filling system. Best sprue designs taper, narrowing towards the base. The

degree of taper has to be calculated precisely. Too much taper is as bad as too little.

runner -- The horizontal channel from the sprue exit, distributing the metal to the gates.

gate -- The channels leading off from the runner into the mould cavity.


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Rule No. 3:

"DON'T STOP"While the melt continues to rise smoothly in the mould, the liquid front stays "alive",

with the surface oxide continuously breaking and sliding off the advancing meniscus toform the skin of the casting. The thin oxide on the advancing liquid front is not therefore

a problem; this steady advance will ensure a good filling condition and a casting freefrom oxide cracks.

When filling the mould, the melt front must never be allowed to come to a stop. If this

happens, the stationary front becomes covered with a thick oxide film, so that restarting

its advance may not be possible. The melt may break through and roll over the oxidelayer, trapping it in the casting as an "oxide lap". If the arrest of the front is prolonged,

the front may freeze, creating a "cold lap". Laps of any sort can act as cracks.

Interrupted pours are therefore a NO NO!

However, stops can also occur as a result of part of the melt arriving at an overflow.

( view diagram of overflow.) Such "waterfall" conditions are to be avoided at all costs,and this is a further reason for providing ingates at every low point in the casting. Stops

can even occur when, after filling thin walls, the advancing front arrives at a large area

expanse such as the top of a box type casting (e.g. an automotive oil pan). The irregularfilling of such flat horizontal sections of castings can lead to severe lap defects, often

invisible and undetectable (except to destructive mechanical testing such as bend testing

of the casting). Thus horizontal surfaces of castings should be avoided by design ifpossible, or by tilting the mould if possible, or finally simply filling these regions at a

speed sufficient to reduce the problem to an acceptable level.
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Rule No. 4:

"Bubbles are Bad . . ."In addition to making holes in the casting, bubbles leave trails of oxides, making double

trouble.

Bubbles are the most common source of porosity in castings -- but their effect is usually

mistaken for shrinkage, which it closely resembles, because the bubbles and their trails

are often irregularly shaped. ( view diagram of bubble damage.)

They are manufactured with awe-inspiring efficiency in badly designed filling systems.Thus all those damaging features of filling systems listed forRule 2 apply here also.

Good filling systems (counter gravity, tilt pouring especially if started from above the

horizontal, and well designed gravity systems) all reduce bubble damage.
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Rule No. 5:

". . . and Big Bubbles are

Worse!"The outgassing of cores can lead to huge defects, filling whole areas of the tops ofcastings. However, even a small blow from a core can leave a bubble trail that can create

a leak defect. To avoid blows from cores, the core must be vented to the atmosphere.

( view diagram of a core blow.)

An ultimate test is for the foundry to video record the filling of the mould with the top of

the mould open, so the metal can be clearly seen covering the cores in turn. Any blowingoff cores is then immediately clear.
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Rule No. 6:

"Avoid Shrinkage"Feeders can't feed uphill. (Well, actually, they can, but not reliably.) (See also Rule 7).

Feeding downhill, aided by gravity, is safe. Thus feeders (risers) preferably have theirtops well above the top of the casting.

Ensure that feeder design is as reliable as possible. Therefore:

(1) Check that the design follows all the Feeding Rules (see book"CASTINGS"by J

Campbell, Published by Butterworths 1991), and

(2) Use a reliable computer modelling package, and

(3) Check test castings by X-ray radiography (or somewhat less reliably, by cut sections).
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Rule No. 7:

"Rock or Roll"The great majority of castings in the market place have a freezing time of several

minutes. This is similar to the time taken for the convection of hot and cold liquid metalin the solidifying casting to build up, and for the resulting convection currents to start re-

melting their way through the casting as it attempts to solidify. This little-known problemcan lead to unsuitable temperature gradients (for instance from bottom gating in an effort

to promote good filling) and so can undermine the effectiveness of feeders, and lead to

segregation and a kind of shrinkage damage that is difficult to eliminate.

The problem is a source of concern, because it is little understood, and little researched.Most computer packages cannot simulate it, and thus predict the wrong shrinkage pattern

in the casting.

Problems of convection are eliminated by "rock or roll," i.e.

(1) Careful horizontal transfer by tilt casting operations (requires a start tilt condition

above the horizontal, and a slow tilt speed);

(2) Counter-gravity filling followed by immediate roll-over of the mould through 180

degrees.

(counter-gravity filling as applied in the Cosworth Process.)
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Usually, however, neither of these solutions is possible because most foundries work only

with static moulds. In such cases the problem is reduced, but not necessarily eliminated,by

(3) Feeding by oversized feeders placed on the top of the casting and feeding under

gravity.

(4) Avoiding convection loops, especially in the rigging of investment castings (where

the problem is especially common).

As an aside, the problems of convection are automatically avoided in very thin or very

thick section castings. Thin section castings freeze quickly before convection can build

up to become important. Thick section castings take so long to freeze that there is plentyof time prior to freezing for hot metal to convect upwards into the feeders and for the

cold metal to sink to the bottom. Thus the melt can redistribute, favourably arranging the

temperature gradients before freezing starts.


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Rule No. 8:

"Severe Segregation"All freezing will cause some segregation of alloying elements. Most casting alloys

segregate their elements to only a small extent, so that the problem is not noticeable (forinstance as in most Al-Si-Mg alloys).

However, some alloys segregate seriously, to the point at which parts of the casting will

be well outside chemical specification. For instance Al-4.5Cu alloys might havespecification limits of 4.0 and 5.0 per cent copper. Although the average percentage of

the copper in the casting will be normally nicely inside this limit, parts of the casting can

easily rise to 5.5 per cent or more in a chilled area, perhaps causing that locality to be toohard, strong or brittle. Conversely, other hotter parts will decrease to 3.5 % Cu or less,

causing those regions to be too soft and weak. Such problems occur naturally at abrupt

changes in section (which is a concern since such locations are usually also regions of

stress concentration), and at other parts where freezing patterns are altered (such as underfeeders or under chills).

Most foundries are unaware of the problem. A designer and his buyer need to proceedtherefore with caution. In principle the problem can be predicted by computer packages,

and therefore allowed for in the design (more information in "CASTINGS").
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Rule No. 9:

"No Quenching!"Never quench light alloy castings into water (either hot or cold, since both are extremely

bad). The US National Specifications that specify water quenching as part of the heattreatment of aluminium alloy castings are therefore not (definitely not) recommended.

Such procedures can effectively reduce the total strength of castings by half, andconstitute a major reason for casting failures in service.

Polymer quenchants or air quenching are much more reasonable alternatives. The slightloss of strength (about 5 to 10 %) as indicated by test bar results is more than

compensated by the avoidance of the 50 % or more loss that the casting as a whole

suffers when quenched into water. This is because water quenched castings are

effectively pre-loaded to approximately 50 % of their failure stress before being put intoservice.

Water quenching to give a planned residual stress can be beneficial. However, this is not

easy to arrange. It is normally only possible in round components such as discs withradial spokes like wheels and housings for turbine engines etc.


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"The Tool Rule"Datums and pick-up points need to be agreed between the pattern or tool maker, the

casting engineer, and the machinist to avoid unnecessary scrap after the casting has been

made, and the inevitable delays in supply.

This agreement has to be put in place at the time of the placing of the order for the parts.Thus it is essential that the buyer spends time with the people who machine his parts in

order to understand their needs in this area.

Once the pick-up points have been agreed, the patternmaker or tool maker can check thetooling, the foundry can check the casting, and the machinist can set up the part for

machining, all parties working from the same working datum points. Only then will a

trauma-free, integrated, supply of castings be achieved, all accurately within dimensionalspecification.

Strongly recommended are the robust, reliable and low cost "passive" location systems

(1)

A rectilinear job can use a classical "6 point" system. The incorporation of lugs that

simultaneously provide clamping points is helpful. This is an absolutely accurate system.(5 points is not enough, since the part is not uniquely located; 7 points is too many, and

will have some points in conflict).

(2) The traditional "cone, groove and flat" system is another absolute location techniqueuseful for some geometries of casting.


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(3) A part with some cylindrical form that needs to be held in a 3-jaw chuck needs the 3

locations at 120 degrees, plus end stop, plus a "clock" stop (to define its angle ofrotation). This is usually less easy to work as an absolute system, but has sufficient

accuracy for most applications.

Active systems are those that require several measurements to be taken from a casting

when it is presented to the machine tool, and an average is calculated (usually by built-incomputer). This approach may be necessary for flexible, open box type products (such as

an oil pan with flat, unstrengthened walls). However, this route is, of course, moreexpensive and the machine tools to deal with such castings not so widely available. Also,of course, the method does not have the benefit of providing the integration of the supply

chain that is so valuable for trouble-free supply.

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THE TEN RULES