One of the sections of elementary science upon which the successful operation of a casting machine depends is heat, and it is one that is less understood than the mechanics which govern the machine operation. A normal mistake made is to confuse heat with temperature, and even to suppose they are Words describing the same thing. A gallon urn of boiling water and a pint kettle of boiling water tested with a thermometer will register the same temperature, but they do not contain the same quantity of heat. In fact, it is obvious that the water in the urn took more heat to raise it to boiling temperature than did that in the kettle. If we now add a piece of ice to each container the temperature fall in the kettle will be much greater than that in the urn.

To bring the analogy nearer home, we can. appreciate that a nugget of ‘Monotype’ metal added to a caster pot will therefore fall in temperature) than a at the same temperature. A ‘Monotype’ caster pot of molten metal contains a quantity of heat put into it by the heater elements sufficient (a) to ensure that the metal is fully fluid when pumped into the mould, (b) to supply heat to maintain the mould temperature and (c) to cater temporarily for heat losses due to the melting of new metal and to radiation and conduction.

The amount of heat vital to a casting machine operation is broadly independent of the specification of the type metal alloy used. For instance, the composition casting temperatures used for a 7/15 and a 10/16 metal are the same. The machine characteristics decide the range of casting temperatures of the metal at which various type sizes can be cast, and this temperature range determines the range of type alloys suitable for casting.

Units of Heat quantity

Initially when heat is applied to a substance,it absorbs that heat and as it does so, its temperature rises. The amount of the temperature rise for the same amount of heat absorbed is different for different substances. The quantity of heat is measured in either calories or British Thermal Units (btu) and for convenience we will use the latter units (one therm, the unit of gas supply, equals 100,000 btu) which is approximately equal to 29 units of electricity. One btu is defined as the quantity of eat required to raise the temperature of one pound of water by 1°F. Water absorbs a great deal more heat for the same temperature-rise than ‘Monotype’ metal and, fact, only 0.038 btu is necessary to raise one pound of 10/16 alloy by 1°F. The ratio of the heat-absorption of ‘Monotype’ 10/16 metal to that of water is therefore 0.038 :1; and the figure (0.038) is referred to as the specific heat of ‘Monotype’ 1/16 metal.

Type alloys owe a large part of their success to the fact that they melt easily. That is to say that not only do they require relatively small amounts of heat to raise their temperature to melting point and to cause them to melt, but also they have relatively low melting-points.

Calculation of heat

Let us now consider in closer detail the amount of heat in the 85 lb of molten metal in the caster pot at a temperature of, say, 700°F.

The amount of heat absorbed by ‘Monotype’ metal in raising its temperature to the casting point of 700°F is calculated in two parts (a) and (b) below. The first part is obtained from the product of the weight of metal in lb, the temperature-rise from, say, 60°F to 700°F, i.e. 640°F, and the specific heat factor of the alloy.

(a) 85 lb metal × 640°F × .038 = 2067.2 btu

Part-way through the temperature rise, the alloy reaches its melting-point temperature and a great deal of extra heat is absorbed, without the temperature rising, during that time that the melting takes place and before the temperature of the now molten alloy can be raised further. This extra heat is called the latent heat of the alloy and for 10/16 it is 26.2 btu per lb. (Remember that the heat thus taken, in to produce melting will be given cut again in solidification.)

(b) 85 lb × 26.2 btu per lb = 2227 btu

The 85 lb of molten metal in a caster pot at 700°F therefore has absorbed and contains altogether 4294 btu. This may appear to be a great quantity of heat but it is an enlightening illustration of the easy melting property of the alloy to point out that the amount of heat in an 85lb caster pot at 700°F would not raise the temperature of three gallons of water to boiling point.

We have not, therefore, a great quantity of heat stored up, so steps must be taken to see that it is used as efficiently as possible.

Apart from the heat required to keep the metal fully fluid, some of the heat is used: (a) to melt new metal added to replace type cast: (b) to-melt the tangs when they fall on to the molten metal surface: (c) to replace wastage, through radiation, convection and conduction to parts of the machine and the air surrounding the pot. This heat-wastage is accelerated when the machine is in a cold draught.

The heat used or wasted is replaced by the pot-heaters in response to the thermostat (rapidly if both are efficient).

One kW of electricity will produce 0.95 btu per second at 100% efficiency. Assuming a reasonable use-efficiency of 170%, a 3kW tubular heater will produce 1.9 btu per second. A 2lb nugget of cold metal requires 101 btu to raise it to 700°F: about the same amount of heat is required to counteract a temperature-fall from 720°F to 690°F in 85 lb of metal. The 50 seconds taken by a 3 kW heater to supply this amount of heat will be significantly longer if either element or thermostat is not in first-class condition.

A relationship exists between the maximum weight of material that can cast per minute and the melting capacity both of the metal in the pot and of the heater element. The increase in capacity of the caster crucible from 65 lb to 85 lb resulted in an increase of over 1000 btu in the heat-content of the pot. An increase from 2.4 to 3 kW in the heater resulted in an increase in heating capacity of the heater. This in turn has enabled casting temperatures to be unaffected even when a heavy casting programme requires equally heavy replenishment with relatively cold metal.

Heat wastage

Apart from the waste of heat due to the operation of the machine in a draught, there are other causes of undesirable heat dissipation. The foremost of these is due to hand-feeding. As we have already said, a 2lb nugget of metal will need to absorb over 100 btu from the molten metal before it melts and the whole pot-temperature returns again to 700°F. The result is that the nugget melts slowly, leaving a scum of metal and dross on the surface.

The use of automatic feeding ensures not only preheating of the ingot, but allows only a few ounces of metal to melt at a time. The immediate heat-requirement is therefore only a tenth (or less) of that required for the rapid melting of the nugget in hand-feeding, and a steady casting temperature is readily maintained.

Further heat-wastage occurs when type is thrown on to the metal in the pot. Such material will float on the surface, reducing its temperature and melting so slowly that a scum of mixed metal and dross is the result. Test or reject lines should therefore be ‘boxed’ for remelting.

Mould temperature

The type metal pumped into the mould cavity transfers most of its heat to the mould and matrix. This causes the mould temperature to rise but, since the specific heat of steel is three to four times that of the-type alloy, the heating process is relatively slow and the maximum temperature is probably no more than 200°F throughout the mould. Until sufficient heat build-up in the mould has occurred, the type will be chilled.

As we have noted, the quantity of heat input via the type metal depends on the metal temperature and the type body-size. Since, however, by the nature of the machine, type is cast intermittently, the heat input also is intermittent and the quantity of heat transferred to the mould per minute will also depend on the casting rate. The relation might be stated as: heat input per minute varies as type size, casting temperature and casting speed.

The mould temperature must broadly, a constant one and therefore the heat must be, broadly correspondingly constant.

It will be obvious that the factors of metal temperature, casting speed and type size are not only important in themselves but are interlocked in their relation to heat input. If one of them is altered at least one of the others must also be changed, if the mould temperature is to be constant.

Although it is true that the rate of water-flow through the mould dissipates heat from it, if we remember water’s enormous capacity to absorb heat, we must concede that the fractional control of water-flow to effect small variations in mould temperature calls for extremely accurate adjustment. Only in the largest composition body-sizes can alterations in water-flow be an influence of significance in control of the mould temperature.

It will now be seen that in changing the product to be cast from one body size to another, at least one other change must be made. Either the casting speed or the casting temperature must be adjusted to balance the heat input in order to maintain a constant mould temperature. On this point we can note that a 6-pt piece of type contains 45% less volume of metal than its counterpart in 8pt, giving, when cooling and solidifying, 45% less heat input to the mould at the same metal temperature and casting speed. The extra heat required when changing from 8pt to 6pt must be provided by raising either, or both, the metal temperature and the casting speed. Too often, metal temperatures are raised to overcome chilled typecasting when it might be better to raise the casting speed. The fallacy of increasing pump-spring pressure to cure chilled casting should now be obvious.

The function of the water-cooling through the mould is to prevent it from overheating, which would occur as soon as the heat build-up from the type metal being cast is greater than that needed for maintaining the correct mould temperature. When the smallest type sizes are being cast, therefore, the minimum water-flow is necessary and in casting all sizes of type the relation of the water-flow to body size must be considered.

Other variables

Now let us examine machine operation affect the heat The presence of excess oil in the mould cavity and on the matrix face is a constant cause of chilling. The reason is that the oil is vaporised by heat but in the process heat is used up — heat that must be taken from the type metal as it is injected. The result is a lower heat input to the mould, loss of heat in the metal being cast, and chilled type.

Another frequent cause of heat-loss in the mould that is not so obvious arises from hollow or porous type. This occurs when, due to bad nozzle condition or seating or incorrect pump action or off-centre injection, the metal injection is imperfect. The type body then contains more than its normal content of air; therefore, less than its proper weight of metal; therefore there is a reduction in the amount of heat in the type available to maintain the mould temperature. This condition is made worse when the metal enters the mould as a spray and the fine metal particles lose heat rapidly. No alteration of metal temperature or casting speed will correct such faults, and increased pump-spring pressure must make them worse.

I hope that this explanation of the function of heat in the operation of composition casting will draw the attention of operatives to its importance as a controllable factor in their work. Only then will the enormous potential of ‘Monotype’ casters for high-quality production be fully realised and obtained.