The key points for this module are:
The steps involved in shell molding are:
A match-plate or cope-and-drag metal pattern is headed and placed over a box containing sand mixed with a thermosetting resin.
The box is repositioned so that loose, uncured particles drop away.
The sand shall is heated in an oven for several minutes to complete curing.
The shell mold is stripped from the pattern.
Two halves of the shell mold are assembled, supported by the sand or metal shot in a box, and pouring is accomplished.
Sprue is removed from the finished casting.
The characteristics of shell molding are as follows;
Shell is light and thin (5mm - 10mm)
Thermal conductivity is different from sand
The advantages in shell molding are:
Complex shapes can be produced with less labor
Better dimensional accuracy
It has a better surface finish
The disadvantages of shell molding are as follows:
It has lower permeability than sand molds
The decomposition of shell-sand binder produces a high volume of gas.
Investment casting comes from the word "invest" - "to cover completely," which refers to the coating of refractory material around the wax pattern. In investment casting, a pattern made of wax is coated with a refractory material to make a mold, after which wax is melted away prior to pouring molten metal.
The steps involved in investment casting are as follows:
Wax or plastic is injected into a die to make a pattern
Patterns are gated to a central sprue
A metal flask is placed around the pattern cluster
Flask is filled with investment mold slurry
After mold material has set and dried, patterns are melted out of the mold
Hot molds are filled with metal by gravity, pressure, or vacuum
The mold material is broken away from casting
Castings are removed from sprue and gate stubs ground off
Permanent mold castings are commonly made of steel or cast iron. The cavity with the gating system included is matched into the two halves to provide accurate dimensions and a good surface finish.
Die casting is a permanent mold casting process in which molten metal is injected into a mold cavity under high pressure (7 to 350 Mpa).
There are two main types of die casting.
Hot-chamber die casting: Metal is melted in a container, and a piston injects liquid metal under high pressure into the die
Cold-chamber die casting: Molten metal is poured into an unheated chamber from an external melting container and a piston injects liquid metal under high pressure into the die cavity
The following are some of the castings defects
Misrun: Caused when a casting solidifies before completely filling the mold cavity
Cold shut: Occurs when two portions of the metal flow together but there is a lack of fusion between them due to premature freezing
Sand blow: This is caused by the release of mold gases during the pouring
Shrinkage cavity: This is an internal void caused by solidification shrinkage that restricts the amount of molten metal
Hot tears: This happens when casting is restricted from contraction by an unyielding mold
Sand wash: This is the result of erosion of the sand mold during pouring.
Joining is the process in which two (or more) parts are coalesced at their contacting surfaces by application of heat and/or pressure.
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