Metal Casting Flashcards
important advantages of casting processes
(1) complex part geometries are possible;
(2) some casting operations are net shape processes, meaning that no further manufacturing
operations are needed to accomplish the final part geometry;
(3) very large parts are possible;
(4) casting is applicable to any metal that can be melted;
(5) some casting processes are suited to mass production
limitations or disadvantages of casting
(1) limitations on mechanical strength properties;
(2) porosity;
(3) poor dimensional accuracy compared to other forming processes;
(4) safety hazards due to handling of hot metals;
(5) environmental problems
Name the two basic mold types that distinguish casting processes
expendable molds and permanent molds.
Which casting process is the most important commercially
Sand casting
What is the difference between a pattern and a mold
The pattern represents the final part to be manufactured and is used to produce the mold,
by forming the interior shape. The mold contains the hollow section (cavity) into which molten
metal is poured or injected to form the desired part
What is the most commonly cast metal in sand casting
Cast Iron
What material could the dies be made from in die casting
When casting low-melting point metals, the dies are typically made from steel (tool
steel) or cast iron. If cast iron is the molten metal being used, the dies must be made of a material
with a higher melting temperature such as ceramic
Which die-casting machines usually have a higher production rate, cold-chamber or hot-chamber,
and why?
Hot-chamber machines are faster because cold-chamber die casting machines require
molten metal to be ladled into the chamber from an external source. Ladling takes more time than
injecting the molten metal into the die as in the hot-chamber operation
What is flash in die casting?
Flash is a thin portion of metal at the exterior of a casting that results from molten metal
being squeezed into the spaces between the die halves of the mold at the parting line, or into the
clearances around the cores and ejector pins
How does solidification of alloys differ from solidification of pure metals? Include a basic, labelled
cooling curve for each in your answer
Pure metals solidify at a single temperature equal to the melting point. Most alloys
(exceptions are eutectic alloys) start to solidify at the liquidus and complete solidification occurs at
the solidus, where the liquidus is a higher temperature than the solidus
Describe the reasons for the flat thermal arrest region in the cooling of a pure metal
The change in state from liquid to solid releases energy as ‘latent heat’, as atoms lose
kinetic energy. Local solidification releases this energy as heat to the surrounding material which
remains at the freezing temperature until no more latent heat can be extracted (i.e. all material has
solidified). After this, the whole material continues to cool to ambient temperature
What are some of the operations required in sand casting after the casting is removed from the
mold
(1) trimming, in which the sprues, runners, risers, and flash are removed
(2) core removal
(3) surface cleaning (sand blasting, polishing etc)
(4) inspection
(5) repair if needed
(6) heat treatment
(7) machining
What are some of the general defects encountered in casting processes?
misruns, in which the casting solidifies before filling the mold cavity;
shrinkage cavity, which is a depression on the casting surface or an internal void in the casting
caused by solidification shrinkage;
sand blow, a large cavity caused by the release of mold gases
Write down the relationship known as Chvorinov’s rule in casting, and define the variables
TTS = Cm(V/A)
n
, where TTS = total solidification time, Cm =
mold constant, V = volume of casting, and A = surface area of casting. n = exponent with typical
value of 2.
What are the most common metals used in die casting
zinc, tin, lead, aluminum, brass, and magnesium
