| Sprueing of wax patterns and
some notes on the metaHurgy of the process  Much
time and attention has been spent in considerations of how toenhance the quality
and reliability of the casting process, but very little regard has been paid to
the process of solidification of the alloys used which is absolutely crucial for
the final quality.
Melting is an extremely traumatic condition for any metallic
alloy and, to restore its original, pre-melt, condition, it is essential that,
during solidifaction, it recreates its original crystalline structure. A brief
explanation here may be of interest. A typical gold alloy is composed of
many billions of atoms of gold, silver, copper and other consitutants. When the
alloy is a solid, all of these atoms are bonded together in a fied relationship,
but not stationary. If imaginary lines are drawn joining the centres of
each atom, regular geometrical shapes are the result; this is known as the crystalline
structure. One can imagine this in casting ? good or bad ? depend exclusively
on the subsequent solidification, which can broadly be compared to the freezing
of water. Solidification, or freezing, starts from the walls of the container
and progresses towards the centre. At a certain point in this process it is thus
possible to identify a solid phase, a liquid phase and a viscous phase. Impurities
in particle form tend to be progressively pushed towards the section which solidifies
last. As the melt cools, solidification develops throughout the liquid mass
as groupings of crystals in three dimensions, propagating in fir?tree and
the waxes should be checked to ensure that they are free of these surface inclusions.
When using vacuum injection nhance the quality and reliability
the form of adjacent cubes with an f the casting process, but very atom at each
comer (figs. 20 & 21). ttle regard has been paid to the As previously
stated, the atoms are rocess of solidification of the not stationary but in a
permanent  Iloys
used which is absolutely state of balanced oscillation, the rucial for the final
quality. amplitude of which is elated to the elting is an extremely traumatic
kinetic energy of the component ondition for any metallic alloy and, atoms and
determines the hardness
o restore its original, pre?melt, con? of the alloy.
This oscillation exists ition, it is essential that, during even at O’C
and diminishes down to
olidification, it recreates its original absolute zero
at ?273’C. With a rise stalline structure. A brief expla? in temperature,
this oscillation
ation here may be of interest. increases in amplitude, the
crystal typical gold alloy is composed line structure dilates, and the metal
f many billions of atoms of gold, thus expands. When this amplitude silver, copper
and other constitu? exceeds a certain level, the atomic
nts. When the alloy
is a solid, all bonds are ruptured and the crystal if these atoms are bonded together
line structure breaks down (fig. 22);
n a fixed relationship, but not the
alloy then becomes a liquid. tationary. page
23  The
metallurgical properties of the forms known as "dendrites" (after the
acient Greek word for tree). Those that grow in a direction opposite to the diffusion
of heat are known as "columnar dendrites" and when growth is irregular
and disorderly are known as "equiaxial dendrites".
Figures 23
and 24 depict the external face of the alloy in solid black, the solidifying sections
in small squares and the still liquid section in white. As the dendrites
grow they broaden and form a myriad of microscopic islets which, upon solidification,
weld together forming a granular structure which may be clearly observed under
a microscope. Each granule represents a section of the alloy which has solidified
crystallising in a given direction. The left?hand section of figure 25 depicts
an equiaxial dendrite; the right?hand section depicts a columnar dendrite. Figure
26 shows a typical microscopic view of metallic granular configuration. If
cooling of the metal after casting does not occur in a well?defined direction,
irregular faults will inevitably occur in the component. We will now examine
this process on the microscopic (as opposed to atomic) level. The black
portion of figure 26 shows the growth of dendrites forming the grain structure.
Very small inclusions or impurities are about to be trapped and the empty spaces
will become typical voids or porosity.  The
crosshatched area depicts the
section which is still liquid. If contamination
of the melt has occurred during the melting/casting process by foreign bodies,
oxides or gaseous inclusions, these will remain trapped between the grains at
point of solidification and degrade the metallurgical characteristics of the component
that has been thus badly cast These contaminants will nersist through any
subsequent remelting of the material.  To
summarise, good solidification depends on these factors:
1) Heat diffusion 2)
Contamination during melting/ casting 3) Formation of the various phases 4)
The number of initial nuclei of crystallisation 5) The type of dendritic
growth (columnar and equiaxial).  From
the preceding, it is quite easy to understand the types of factors that may produce
porosity in castings. The causes, excepting contamination (which may be eliminated
by taking care not to introduce impurities) are exclusively to do with the manner
of cooling and solidification. Therefore, conditions must be established to ensure
that:
A ? The component is continuously as c anges in qui to so B
? Solidification takes place uniformly and gradually.  To
ensure that the first condition is met, it is always necessary to provide a "thermal
reservoir" capable of supplying, without interruption, molten metal during
all the progressive contractions of the solidifying component. One may think in
terms of a metal feed tank in the centre sprue (in the case of "tree"
sprueing) or within the base in the case of "button" sprucing (fig.
27).
Immense importance attaches to the dimensional relationships of the centre
sprue or base and the individual sprues or runners connecting them to the patterns. The
fact that must be borne in mind is that the metal within the feeder system remains
liquid longer than that in the patterns. It is interesting to consider here the
case of dental lost?wax castings where porosity is absolutely inadmissable due
to possibilites of ingress of bacteria and other micro?organisms. Figure
28 shows the size of the feeder system provided for casting relatively small toothshaped
patterns. It is necessary to remember that the vast majority of alloys melt and
sol? idify over a range of temperatures. Diagrams in figure 29 show the cooling
curves of some gold/silver (Au?Ag) alloys indicating the respective "heatsoaking
interval". Point "S" indicates the start of sol idification while
point "F" marks completion of solidification. Fine gold and fine silver
solidify at a precise temperature, whereas their various alloys solidify
within a ?ange of temperatures known as the ’heatsoaking interval"
or "melting .?ange". The contraction from liquid o solid is represented
in figure 30 xhich also depicts the progressive eduction of the mass during tem)erature
variation. 1 . By contraction at liquidus (from )oint "D" to point
"C" we mean the eduction of the metal mass when )assing from casting
temperature to ?netting temperature. Casting tem)efuture is always higher thari
melting emperature otherwise the molten rietal will not have sufficient fluidity
o fully occupy the mould. rhere can be a difference between hese temperatures
of 50’C ? 150’C lependent upon the type of alloy ind the type of item
to be cast. 2 . Heatsoaking contraction (point ’C" to "B")
takes place during the nterval from casting temperature to netting temperature.
This phase indergoes the greatest mass reducion and thus requires the maximum
ittention. 3. Contraction in the solid state point "B" to point
"A") takes place vhen the solidified mass cools town to ambient temperature
from olidification temperature "B". rhese contraction processes
may ause voids in the cast patterns inown as "shrinkage porosity" vhen
these are relatively massive nd "microporosity" when small ind dotlike.
Diffuse microporosity mmediately sub?surface are prouced by inclusions of air
or gas enerated by investment and trapped hiring solidification. More attention
o this phenomenon will be given iarther on. The contraction process as such a
strong bearing on the ultinate quality of the castings that we fraw attention
to the following dia,rarns: Figure 31 shows the manner in which shrinkage
porosity forms after mass contraction if the solidifying casting is starved of
farther molten metal. Figure 32 shows the solidification process within
a correctly fed mould. The sprueing system allows adequate feeding during solidification
and no porosity results in the cast item.  If
the sprue size is not adequate to permit continuous feeding until solidification
is complete, the results may be as depicted in figure 33.
Another factor
of equal importance to the size of the sprue is its location on the cast
pattern. As a general rule, the sprue should be attached to the most massive part
of the pat tem and provision be made for a reservoir of molten metal to
feed thin, remote, sections (fig. 34). If these thin, remote, sections are
adequately fed with molten metal, porosity will not be experienced (fig. 35). The
foregoing section concerning sprueing principles and techniques is of importance
with regard to metal feeding. We shall now examine how to achieve uniform cooling
of the casting, from the exterior to the interior. A ? The central stem
of the "Tree" at the centre of the flask is the principal thermal reservoir
and its size must, thus, be proportional to the  weight
and size of the patterns. Some practitioners in this field assert that a tapered
form is the most appropriate in order to avoid excessive recycling of precious
metal with consequent "losses". Also, hot metal inevitably reached the
top of the "tree" first and, thus, the lower part, where the metal arrives
at a slightly lower temperature, requires the larger cross?section.
36 ?
Incorrect sprucing
3 7 & 38 ? Correct sprueing
39 ? Incorrect sprueing
40 ? Correct sprucing
41 ? Sprue systems joined to central "tree
(left?hand: correct, right?hand: incorrect) 
B
? It is important that all of the pattems in a given flask are of a similar size,
mass and cross?section. 
C
? Patterns should be attached to the central stem in a clean, fidy and regular
fashion. D ? The overall diameter of the assembled "tree"
must be in relation to the internal diameter of the flask. Ideally, there should
be a clearance of 10? 15 mm between the outermost patterns and the inside of the
flask. 
E
? Flask temperature at the moment of casting must be determined by pattern profile. 
F
? It is desirable that cooling of the molten metal should take place from one
side only of the flask, developing in one direction. Our vacuum system built in
to the casting arm is of value here, creating suction at the opposite end of the
flask to that at which the metal is injected, not only satisfying the foregoing
condition, but also removing harmful gases generated by the metal impacting the
investment. This point will be dealt with in more detail later. Figure 43
depicts the result of progressive cooling without the harniful phenomena described
earlier. It must be home in mind that all of the phases described in this sectionoccur
in a very brief interval of time and are influenced by metal temperature, alloy
composition, flask temperature, profile of the casting patterns and ? above all
? the method of casting employed. | 
