Investment preparation and burnout  The
use of gypsum?bonded investments poured around wax patterns to create casting
moulds is relatively recent compared to the long history of the lost?wax process,
and dates from the famous 17th century Florentine goldsmith, Benvenuto Cellini,
who devised a gypsum compound.
Modem investment powders are easy to use
and permit the rapid prep INVESTMENT WATER fN GRAMS IN GRAMS HEAVY
CASTING 1000 370 aration of good quality moulds. These are mainly
formulated from: Silica (Si02)  This
is combined with a mixture of finely?ground quartz and crystobalite. The latter
is a form of quartz with a high thermal expansion designed to match the shrinkage
of the molten metal during solidification
Calcium Sulphate Emi?Hydrate (caSO4?112H20)
Common
name gypsum. This acts as a "binder" and reacts with water forming gypsum
hydrate. Additives or Modifiers These control the setting time and
flow characteristics and to reduce the tendency to foam or "rise" during
vacuuming. The condition of the investment powder is very critical and can
have a strong influence on the quality of the castings produced. The following
points should, thus, be noted.  A
? Investment powder has a strong affinity for water and rapidly becomes damp in
humid atmospheres. Bags should therefore be kept sealed and dry at all times.
Dampness will increase the setting time of the material and considerably weaken
it. Surface roughness on the castings can also result ? also "flashing".
B
? Additives and modifiers con tained in modem powders may disappear after
a certain time, altering the characteristics of the material. Shelf life should
thus be observed. One result of this can be the appearance of greater fluidity
of the investment 46 "slurry". Attempts to correct this
by adding more powder to the mix will alter the water?powder ratio from that recommended
by the manufacturer and create problems. This is normally I Kg powder to 360?400
grammes or millilitres of water (see chart). Absolute adherence to these proportions
is important. C ? Powder should always be added to water, not vice?versa,
otherwise a "lumpy" mix will result with pockets of unmixed investment
producing weakness in the final mould. Mixing in a modern "under vacuum"
mixer takes care of this type of problem, guaranteeing a smooth and homogeneous
mix. Equipment of this type is shown in figure 44. D ? After mixing, the
slurry is dispensed into the flasks around the wax "trees" and re?vacuumed
with vacuum equipment as shown in figure 45. If large?scale operations are
envisaged, machines are available which carry out the whole mixing and dispensing
process under vacuum. Such a machine is depicted in figure 46. "Work
time" ? the time during which the slurry should be kept "on the move"
either by mixing, vacuuming or dispensing ? is normally about 9 minutes at 23’C.
Water and powder should always be at approximately this temperature. E ?
After investing, the flasks should be set aside and not disturbed for at least
one hour in the case of small or medium?sized flasks (120 mm diameter x 150 nun)
and two hours for larger sizes. They may then be placed in the furnace for dewaxing
and burnout. F ? If steam dewaxing is employed, the foregoing rest times
should be extended by 50%. If dewaxing dry, the flasks should be placed into a
furnace at 200’C, sprue entry downwards. Dewaxing time is about 4 hours
for small flasks and six hours for large. G ? Dewaxed flasks should then
be transferred into a burnout furnace already at 200’C. They are then fired
at a precisely?controlled timetemperature profile in extremely well?ventilated
conditions, these being important for the removal of gases such as S02. A clean
mould is vital for good quality castings; inadequately fired moulds will react
chemically with the molten metal, producing a variety of metallurgical problems.
To fiurther assist in this, the flasks should be stacked sprue entry upwards in
the furnace. H ? After firing, attention should be paid to the colour of
the investment. Correct firing conditions produce a "snow white" effect;
incorrect produces grey, which indicates tha curing has been complete and im purities
not dispelled. A mould it this condition will certainly produc defective castings.
Peak burnou temperature should be checked t( make sure that this is in line with
th( investment manufacturer’s recom mendations. It is possible, given
this latte circumstances, that the readin~ given by the furnace temperature controller
does not correspond to that actually achieved within the flasks, which, being
massive it thermal terms, require a fairly long period to either heat up or coo
down. Only experience can deter mine whether furnace heating o cooling rates match
those of the flasks, and these rates will vary according to whether the flasks
are large or small.  We
have carried out tests using a temperature probe (figure 47) inserted within flasks
of varying sizes within a burnout furnace. During the burnout cycle, this probe
and the furnace instrumentation were connected to a chart recorder and temperatures
of the firing chamber and flask interiors recorder on the paper
This showed
clearly that, only after about three hours did the two temperatures finally coincide
both on heating and cooling. This was a slightly surprising result and clearly
demonstrates that internal flask temperatures "lag" considerably those 48 of
the firing chamber. It is, thus, desirable to check the internal temperatures
of burned?out flasks using a portable temperature probe (figure 48) at the moment
of casting, as this may well vary from the temperature shown by the furnace controller. Curing
of flasks is a long and immensely important process and it is essential to employ
a programmable microprocessor type furnace time/temperature controller to give
the precise curing cycle, which involves: - Furnace switch?on time -
Rate of temperature climb to 7500C - Heat soak time at this maximum -
Controlled decrease to casting temperature I . The firing chamber of the
furnace must be large enough in relation to the charge of flasks to permit adequate
ventilation without obstruction; also to allow the insertion of loading tongs. L
. Mould temperature at time of casting depends upon the mass, cross section and
shape of the patterns and the type of alloy to be cast. For example, pieces
in thin section 18 carat alloy should be cast at 620’C while more massive
items need to be cast at between 570’C and 520’C. 
The
chart indicates flask temperatures at point of casting according to the alloy.
Temperatures given here refer to medium?sized, ring?type patterns (or bracelet
components, small brooches, etc.). Thinner pieces, such as chain links, will probably
require the flask temperature to be increased by about 30’C. These temperatures,
however, are merely a rough guide; only trials will reveal the optimum temperatures
for given components and alloys. With this in mind, a detailed chart listing all
the casting conditions employed will prove extremely useful ? particular
when it is likely that casting of the identical components is likely to have to
be repeated in the future. M ? Due to the permeability of investment, the
mould may be evacuated to 100?200 TORR in about 20 seconds whilst on the casting
arm, using the vacuuin system built?in to the casting arms of our centrifugal
machines (figure 49). 
This
vacuum is maintained during rotation and injection of the molten metal and eliminates
internal resistance to mould occupation due to air pressure ? thus facilitating
filling of very fine sections ? and also removes gases causes by the metal impacting
the gypsum within the investment ? a common cause of gas porosity. Casting carried
out without this feature can have rough surfaces, highly oxidised and frequently
brittle. 
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