3 ,',& ,
Chapter 4 The mould
4.3 Mould atmosphere
4.3.1 Outgassing pressure
The sudden heat from the liquid metal causes the volatile materials in the mould to evaporate fiercely.
In greensand moulds and many other binder systems the main component of this volatilization is water.
Even in so-called dry-binder systems there is usually
500 1000 1500 Figure 4.9 Products of decomposition of
expanded polystyrene (Goria et al. 1986).
Temperature (“C)
The mould 107
apparent volume of gases, and thereby invalidating the experimental results.) The really important quantity given by these curves is the rate of evolution of gas. The rates, of course, are equal to the slope of the curves in Figure 4.10, and are presented in Figure 4.11. Only a few results are presented for clarity. It is sufficient to note that the rates of outgassing are very different for different chemical systems.
enough water to constitute a major contribution to the total volume of liberated gas. On contact with the hot metal, much of the water is decomposed to hydrogen as is seen in the high hydrogen contents of analysed mould gases (Scott and Bates 1975).
In the case of the mould, the generation of copious volumes of gas is usually not a problem. The gas has plenty of opportunity to diffuse away through the bulk of the mould. The pressure build-up in a greensand cavity during mould filling is normally only of the order of 100mm water gauge (0.01 atm) according to measurements by Locke and Ashbrook (1972). This corresponds to merely 10 mm or so head pressure of liquid iron or steel.
However, even this rather modest pressure might be unusually high because their experimental arrangement corresponded to a closely fitting steel moulding box, and escape for mould gases only via the cope. Even so, in greensand systems where the percentage of fines and clay and other constituents is high, the permeability of the mould falls to levels at which the ability of the mould volatiles to escape becomes a source of concern.
The venting of the mould by needling with wires is a time-honoured method of reintroducing some permeability.
Chemically bonded moulds are usually of no concern from the point of view of generating a back pressure during the filling of the mould. This is because the sand is usually bought in as ready washed, cleaned and graded into closely similar sizes (a 'three pan sand'). In addition, only a few volume per cent of binder is used, leaving an open, highly permeable bonded mould. A single measurement by the author using a water manometer showed a pressure rise during the filling of a cylinder head mould of less than 1 mm water gauge. Even this negligible rise seemed to decay to nothing within a second or so.
In the case of cores, however, once the core is covered by liquid metal, the escape of the core gases is limited to the area of the core prints, if the metal is not to be damaged by the passage of bubbles through it. Furthermore, the rate of heating of the core is often greater than that of the mould because it is usually surrounded on several sides by hot metal, and the volume of the core is, of course, much less. All these factors contribute to the internal pressure within the core rising rapidly to high values.
Many authors have attempted to provide solutions to the pressure generated within cores. However, there has until recently been no agreed method for monitoring the rate or quantity of evolved gases that corresponds with any accuracy to the conditions of casting. A result of one method by Naro and Pelfrey (1983) is shown in Figure 4.10. (This method is an improvement on earlier methods in which the water and other volatiles would condense in the pipework of the measuring apparatus, reducing the
-
Self-set systems_ _ _ _
Vapour catalysed Heat cured'F
0 60 120 180 240
Time (s)
Figure 4.10 Gas evolution from various binder system\
using an improved test procedure thut includes the contribution from water and other volatiles (Naro and Pelfrey 1983).
Taking this recent method of estimating Q, the rate of volume of gas generated from a given weight of core in ml - I s-' (or preferably the identical- sized unit Lkg- s ') as being of tolerable relevance to the real situation in castings, we can construct a core outgassing model. We shall roughly follow the method originally pioneered by Worman and Nieman (1973).
We first need to define the concept of permeability. This is a measure of the ease with which a fluid (the mould gas in our case) can flow through a porous material. Permeability P, is defined as the rate of gas flow Q (as a volume per unit time) through a permeable material of area A and
5 -
~ Self-set _ _ _ _ Vapour catalysed
Heat cured
v)
Y 1
3 (!
m Silicate
ester
c
C ! ! core
!:I
.--
L c 02
0 a, iI! \oil !.m
[r c
\. \.
'..
I
60 120 180 240
Time (s)
Figure 4.11 Rates of gas evolution from various sand binders based on the slopes of the curves shown in Figure 2.21.
length L and driven by a pressure difference AP:
P, = Q L / A AP
The SI units of P, are quickly seen to be:
[ P , units] = [litrets] [ml/[m*][~al
= 1 s-1 m-1 Pa-'
Consider now our simple model of a core shown in Figure 4.12. The measured volume of gas evolved
Molten metal Mould
Area Ac
Figure 4.12 Core model, showing heated layer thickness d outgassing via its print. (In this particular case, metal flash along the sides of the print forces gas to exit only from the end area A , )
per second from a kilogram of core material is Q.
If we allow for the fact that this will have been measured at temperature T I , usually above 100°C (373 K ) to avoid condensation of moisture, and the temperature in the core at the point of generation is T2, then the volume of gas produced in the core is actually QT2/T1 where T is measured in K . For the casting of light alloys the temperature ratio T2 IT, (in K remember) is about 3 , whereas for steels it is nearly 6.
If we multiply this by the weight of sand heated by the liquid metal, then we obtain the total volume of gas evolved per second from the core. Thus if the heated layer is depth d, the core area A , and density p, then the volume of gas evolved per second is QdA,pT2/Tl. If the core is surrounded by hot metal, this volume of gas has to diffuse to the print and force its way through the length L of the print of area A p . We shall assume that the pressure drop experienced by the gas in diffusing through the bulk of the core is negligible in comparison with the difficulty of diffusing through the print.
Considering then the permeability definition only for the pressure drop along the print, we obtain the pressure in the core (above the ambient pressure at the outside tip of the print):
(4.2) P = QdA, p
.
LT2 IApTIP,This simple model emphasizes the direct role of permeability P, and of Q, the rate of gas evolution.
It is to be noted that the high casting temperature for steels is seen in Figure 4.1 to result in values for Q approximately twice those for aluminium alloys. Thus cores in steel castings will be twice as likely to create blows than cores in aluminium alloy castings. For this reason, an enclosed core that would give no problems in an aluminium alloy casting may cause blows when the same pattern is used to make an equivalent bronze or iron casting.
Our model also highlights the various geometrical factors of importance. In particular, the area ratio of the core and the print, Ac/Ap, is a powerful multiplier effect, and might multiply the pressure by anything between 10 and 100 times for different core shapes. Also emphasized is the length L of the core print. If the print is a poor fit then L may be unnecessarily lengthened by the flashing of the metal into the print so as to enclose the flow path in an even longer tunnel. If the liquid metal completely surrounds the end of the print too, then, of course, all venting of gases is prevented. Gases are then forced to escape through the molten metal, with consequential bubble damage to the casting.
The important practical conclusions for good core design to be drawn from the model are:
1. High permeability.
2. Core sand binder with low volatile content and/
or low rate of gas evolution.
The mould IO9 the metal, and the production of internal surfaces of the casting that are unacceptably rough.
These cores are bonded with a chemical binder that is cured by heat or chemical reaction to produce a rigid, easily handled shape. The numerous different systems in use all have different responses to the heat of the casting process, and produce gases of different kinds, in different amounts, at different times, and at different rates (Figures 4.10 and 4.1 1 ).
For instance, the silicate ester produces most of its gas early, whereas the core oil shows a rapid but limited early evolution, and then a considerable delay before a second, more severe outgassing. These results are not to be taken as absolute in any sense.
The manufacturers’ products are changing all the time for a variety of reasons: health and safety;
economics; commercial; changes in world markets and supplies of raw materials, etc. Thus binder formulations change and new systems are being developed all the time. At present the phenolic isocyanate-urethane systems are among the lowest overall producers of volatiles, which explains their current wide use as intricate cores, for instance in the case of water jackets for automobile cylinder heads and blocks.
Part of the reason for the historical success enjoyed by the phenolic urethane binders is their high strength, which means that the addition levels needed to achieve an easily handled core are low.
This is one of the important factors in explaining their position near the bottom of Figure 4.10; the volume of gas evolved is, of course, proportional to the amount of binder present. This self-evident fact is clearly substantiated in the work of Scott et al. (1978), shown in Figure 4.13. (If allowance is made for the fact that these workers used a core sample size of 150 ml, corresponding to a weight of approximately 225 g, then the rate of evolution 3. Large area prints.
4. Good fit of prints.
The provision of a vent such as a drilled hole along the length of the print will effectively reduce L to zero; the model predicts that the internal pressure in the core will then be eliminated (the only remaining pressure will, of course, be that to overcome the resistance to flow through the core itself). The value of vents in reducing blowing from cores has been emphasized by many workers. Caine and Toepke (1966), in particular, estimate that a vent will reduce the pressure inside a core by a large factor, perhaps 5 or 10. This is an important effect, easily outweighing all other methods of reducing outgassing pressure in cores.
Vents can be moulded into the core, formed from waxed string. The core is heated to melt out the wax, and the string can then be withdrawn prior to casting. This traditional practice was often questioned as possibly being counterproductive, because of the extra volatiles from the wax that, on melting, soaks into the core. Such fears are seen to be happily unfounded. The technique is completely satisfactory because the presence of the vent completely overrides the effect of the extra volatile content of the core.
A final prediction from the model is the effect of temperature. In theory a lowering of the casting temperature will lower the internal core pressure.
However, this is quickly seen to be a negligible effect within the normal practical limits of casting temperatures. For instance, a large change of 100 K in the casting temperature of an aluminium alloy will change the pressure by a factor of approximately 100/900. This is only 11 per cent. For irons and steels the effect is smaller still. It can therefore be abandoned as a useful control measure.
We shall now move on to some further general points.
Cores are almost never made from greensand because the volatile content (particularly water, of course) is too high and the permeability is too low.
In addition, the cores would be weak and unable to support themselves on small prints; they would simply sag. If greensand is used at all then it is usually dried in an oven, producing ‘dry sand’ cores (their name should be more accurately ‘dried sand’
cores). These are relatively free of volatiles, and are mechanically strong, but retain the poor permeability of the original greensand. They therefore usually require additional venting. This is usually time consuming and labour intensive.
Sand cores are therefore nowadays generally made from clean, washed and dried silica sand that is closely graded in size to maintain as high a level of permeability as is possible. The limit to the size of sand grains and the permeability is set by the requirements of the casting to avoid penetration by
0.15r
0 1 2 3 4
Loss of ignition (COI) (per cent) Figure 4.13 Increase in the peak rate qf outgussing ( I S
loss on ignition (LOI) increuses. Data recalculated~frotn Scott et al. (1978).
measurements converted to 1. kg-’ s-l agree closely with those presented in Figure 4.11. This is despite the significant differences in the techniques. The data in Figure 4.11 may therefore be of more universal application than is apparent at first sight.) 4.3.2 Mould gas explosions
The various reactions of the molten metal with the volatile constituents of the mould, particularly the water in many moulding materials, would lead to explosive reactions if it were not for the fact that the reactions are dampened by the presence of masses of sand. Thus although the reactions in the mould are fierce, and not to be underestimated, in general they are not of explosive violence because the 90 per cent or more of the materials involved are inert (simply sand and possibly clay) and have considerable thermal inertia. Outgassing reactions are therefore rather steady and sustained.
These considerations d o not apply to the mould cavity itself.
In the mould cavity the gases from the outgassing of the mould may contain a number of potentially flammable or explosive gases. These include a number of vapours such as hydrocarbons such as methane, other organics such as alcohols, and a number of reaction products such as hydrogen and carbon monoxide.
Because of the presence of these gases, explosions sometimes occur and sometimes not.
The reasons have never been properly investigated.
This is an unsatisfactory situation because the explosion of a mould during casting can be a nasty event. The author has witnessed this in furan-bonded boxless moulds when casting an aluminium alloy casting weighing over 50 kg: there was a muffled explosion, and large parts of the sand mould together with liquid metal flew apart in all directions. After several repeat performances the casters developed ways of pouring this component at the end of long- handled ladles, so as to keep as far away as possible.
The cause always remained a mystery. Everyone was relieved when the job came to an end.
Explosions in and around moulds containing iron or steel castings are relatively common. One of the most common is from under the mould, between the mould and its base plate, after the casting has solidified, so that there is less danger either to personnel or casting.
With subsequent experience, and in the absence of any other suggestions, the following is suggested as a possible cause of the problem in the case of the light alloy casting.
Explosions can, of course, only happen when the flammable components of the gas mix with an oxidizing component such as oxygen from the air.
The mixing has to be efficient, which suggests that turbulence is important. Also, the mix often has to
be within close compositional limits, otherwise either no reaction occurs, or only slow burning takes place. The limits for the carbon monoxide, oxygen and inert (carbon dioxide and nitrogen) gas mixtures are shown in Figure 4.14.
100% CO2 + N2
4%
Figure 4.14 Shaded region defines the explosive regime for carbon monoxide, oxygen and a mixture of carbon
dioxide and nitrogen (Ellison and Wechselblatt 1966).
In the author’s experience, the mixing with air, which is essential for explosions, only occurs in moulds in certain conditions. These are moulds that are (i) open to air because of open feeder heads, or (ii) poured with oversize spmes that allow the ingress of air, or (iii) the use of double pouring, using two sprues, where the start of pour is not easy to synchronize, with the result that air is taken down one sprue at the time that metal enters down the other. Thus eliminating open feeders by the use either of blind feeders or chills promises to be a useful step. The provision of a properly calculated single sprue should also help.
‘What happens to the air already in the mould?’
is the next question. In a single-sprue system filling quiescently from the bottom upwards, the outgassing of the mould and cores will provide a spreading blanket of gas over the liquid. There will be almost no air in this cover, so that no burning or explosion can occur. The air will be displaced ahead, and will diffuse out of the upper parts of the mould.
Where the flammable gas blanket meets the air it is expected to be cool, well away from the liquid metal. Thus any slight mixture that will occur at the interface between these layers of gases is not likely to ignite to cause an explosion.
In the case of the casting poured from two sprues, the second stream of metal might arrive to spark the spreading front of gases from the first stream.
The mould I I I
necessary to prevent penetration of the metal into the mould.
However, in the case of grey iron cast in a mould rich in hydrocarbons (i.e. greensand with heavy additions of coal dust, or certain resin-bonded sands) metal penetration is prevented when the hydrocarbons in the atmosphere of the mould decompose on the surface of the hot liquid metal to deposit a film of solid carbon on the liquid.
Thus the reason for the robust non-wetting behaviour is that a solid carbon film on the liquid contacts a solid carbon layer at the mould surface. This twin aspect of non-wettability is considered further below.
For the casting of iron, powdered coal additions, or coal substitutes, are usually added to greensands, to improve surface finish in this way, providing a carbon layer to both the sand grains and the liquid surface. The reactions in the pyrolysis of coal were originally described by Kolorz and Lohborg ( 1963):
In the case of an open feeder, the cold downdraught of fresh air into the mould is likely to penetrate and mix with the flammable blanket, and be sufficiently close to the molten metal to be ignited.
A poorly designed turbulent filling system will undo all the good described above. The splashing of hot droplets and jets of metal through the vapour blanket, mixing it with the air, will give ideal conditions to spark an explosion.
That the event occurs from time to time in a random manner is to be expected. It is partly as a result of the randomness introduced by the turbulent mixing, and partly the sensitivity of the composition of the mixture, since Figure 4.14 confirms that only a limited compositional range is explosive.