Why are risers not as useful in die casting as they are in sand casting?

There are a number of reasons that risers are not as useful in die casting as they are in sand casting.
Recall that in sand casting, a riser is sized and located so that it provides molten metal to the die cavity
to compensate for metal shrinkage. In sand casting, the cooling rate is relatively low, so that the cooling
rate can be effectively manipulated by placement and size of a riser. In die casting, it is essential that the
cooling rate be high, or else the economic justification for tooling and equipment cannot be made. Using
risers would of course slow the cooling time, and therefore they are economically undesirable. Further,
the metals that are used in die casting will therefore be ones that develop internal shrinkage porosity,
but do not separate from the mold wall, so that risers are not as necessary. Remember that in sand
casting, risers are located and sized to act as pools of molten metal to account for shrinkage of
the metal. Since the cooling rate of the sand cast product is slower, placing risers appropriately
can control the cooling rate and shrinkage rate. Another reason is that, in order to make die
casting economically feasible from tooling and material standpoint, its cooling rates must be
relatively fast. Using risers is this case would slow the cooling time and affect production rate.

Describe the drawbacks to having a riser that is (a) too large and (b) too small.

Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification.
Excessively designed risers waste more material in castings, while the shrink cannot be compensated if
the riser is too small

Large risers add to the cooling time of the part, slowing production. Also, large risers, when they
are removed, leave behind larger areas on the cast part that must be removed or dressed up
with some areas to solidify too soon and leave voids behind.
o Risers that are too small may not hold enough reserve metal to account for part shrinkage
during cooling, or they may freeze too quickly, leaving the mold incompletely filled, small risers
can require the casting metal to be heated to a higher temperature. Which increases the
chances of forming shrinkage pores, which leads to a lower quality part.

Why does die casting produce the smallest cast parts?

Die casting involves high pressures. So, it is possible to make cast pieces with lower wall
thickness than those cast by other methods. High pressure in die casting allows the liquid metal
velocity in the runner to be high compared to other processes, ensuring that the small parts cool
before the runner do. Applying vacuum to the die adds an extra level of assurance that this will
be the case.

Why is the investment-casting process capable of producing fine surface detail on casting?

The surface detail of the casting depends on the quality of the pattern surface. In investment casting,
for example, the pattern is made of wax or a thermoplastic poured or injected into a metal die with
good surface finish. Consequently, surface detail of the casting is very good and can be controlled.
Furthermore, the coating on the pattern (which then becomes the mold) consists of very fine silica,
thus contributing to the fine surface detail of the cast product
They are made from wax patterns. The pattern itself has excellent properties; since it is usually
made by die casting, rapid prototyping or machining. Since the wax mold is then coated with a
layer of ceramic slurry, the first layer can contain ceramic particles that are extremely fine in size,
which will result in a smooth surface on the finished casting.

Would you recommend preheating molds used in permanent-mold casting? Would you remove the
casting soon after it has solidified? Explain your reason.
By preheating a mold to be used for a permanent mold casting, there is less likelihood that the metal
will chill when toughes the mold surface, which could lead to low fluidity. Pre heating the molds for
permanent mold casting also lowers the fatigue and shock imparted on the mold by repeated casting
with molten metal. As for removing the casting quickly, the part must be given adequate time to cool in
the mold so that there is no possibility of it being distorted or acquiring other defects during the
shakeout process. Small parts require less time than large casting.
Preheating the molds in permanent-mold casting is advisable in order to reduce the chilling
effect of the metal mold, which could lead to low metal nuidity. Also, the molds are heated
to reduce thermal damage (fatigue, shock) which may result from repeated contact with the
molten metal. Considering casting removal, the casting should be allowed to cool in the mold
until there is no danger of distortion or developing defects during shakeout. While this may
be a very short period of time for small castings, large castings may require an hour or more.
Preheating the molds in permanent-mold casting is advisable in order to reduce the chilling effect of the
metal mold, which could lead to low metal fluidity. Also, the molds are heated to reduce thermal
damage (fatigue, shock) which may result from repeated contact with the molten metal. Considering
casting removal, the casting should be allowed to cool in the mold until there is no danger of distortion
or developing defects during shakeout. While this maybe a very short period of time for small castings,
large castings may require an hour or more.

Explain why squeeze casting produces parts with better mechanical properties, dimensional accuracy,
surface finish than do expendable-mold process.

The squeeze-casting process involves a combination of casting and forging. The pressure applied to the
molten metal by the punch or the upper die keeps the entrapped gases in solution, and thus porosity
generally is not found in these products. Also, the rapid heat transfer results in a ne microstructure with
good mechanical properties. Due to the applied pressure and the type of die material used, good
dimensional accuracy and surface nish are typically obtained for squeeze-cast parts

The squeeze-casting process consists of a combination of casting and forging. The pressure applied to
the molten metal by the punch, or upper die, keeps the entrapped gases in solution, and thus porosity is
generally not found in these product. Also, the rapid heat transfer results in a fine microstructure with
goo d mechanical properties. Due to the applied pressure and the type of die used, i.e. , metal, good
dimensional accuracy and surface finish are typically found in squeeze-cast parts.

Because squeeze casting combines forging and casting, trapped gases remain in solution when the
upper die or punch is applied due to the pressure exerted on the gas, and as result porosity is not
present in squeeze cast products. Heat transfer occurs very quickly in squeeze cast parts. The rapid
transfer of heat allows the metal's cooling with fine grain microstructure, ensuring excellent and
homogeneous physical characteristics. The applied pressure from squeeze casting, combine with
materials typically used for the process, produces parts with great accuracy dimensionally. Pressure
imparted by squeeze casting, along with metals typically used, produce parts with a fine surface finish.

Porosity that has developed in the boss of a casting is illustrated in Fig. P11.60. Show that the porosity
can be eliminated simply by repositioning the parting line of this casting.
Note in the figure below that the boss is at some distance from the blind riser; consequently,the boss
can develop porosity as shown because of a lack of supply of molten metal from theriser. The sketch b
elow shows a repositioned parting line that would eliminate porosity inthe boss. Note that the boss can
now be s upplied with molten metal as it begins to solidifyand shrink

If you need only a few castings of the same design, which three processes would be the most
expensive per piece cast?

Die casting, shell-mold casting, and centrifugal casting would be the three most expensive processes per
piece because these processes involve high equipment costs and a high degree of automation. Both of
these factors require large production runs to justify their high cost. The high tooling cost can be
mitigated somewhat by rapid tooling technologies, as discussed in Section 20.5 on p. 542. As an
interesting comparison, refer to the answer to Problem 12.3 for a discussion regarding the most cost-
effective means of producing only a few cast parts.