Casting 2 Flashcards
(30 cards)
What kind of sand is used in casting?
Silica sand SiO2
What are the two types of sands and which is more preferable to foundaries?
Natural bonded (bank sand)
Synthetic sand (lake sand)
Foundries prefer synthetic sand because its composition can be controlled more accurately
Review the outline of production steps in a typical sand casting operation
Name the 3 basic types of sand molds
- Green-sand (water, sand & clay)
- Cold-box (organic & inorganic binders)
- No-bake molds (resin)
Titanium and zirconium (reactive metals) react aggressively with silica - so don’t use them with sand casting.
She’ll molding
Popular because of good dimensional tolerance, high accuracy and low cost
Outline of the process of shell casting
A pattern made of ferrous metal is heated to a range of 175-370°C and coated with a parting agent (like silicone). This pattern is then clamped to a box or chamber.
The box contains fine sand and is mixed with 2.4-4% of a thermosetting resin binder which coats the sand particles. The box is then either rotated upside down or the sand is blown onto the pattern to make a coating.
The shell hardens around the casting and the pattern is removed using built in ejection pins. Two half shells are made in a similar manner and are later pinned or clamped together to make a complete mold.
The thickness of the shell can be determined by controlling time that the pattern is in contact with the mold for intended usage (thicker molds for higher strength and ductility)
Design considerations for shell casting
The shell-sand binder produces a high volume of gas. Thus the molds must be vented well otherwise trapped air and gas can cause defects in the shell molding of ferrous castings.
Plaster mold casting (add relevant notes for this section)
Name the process used to increase permeability in plaster-mold casting. Briefly describe it.
The Antioch process. The molds are dehydrated in an autoclave for 6-12 hours, then rehydrated in for 14 hours
Name another way of increasing permeability in plaster-mold casting
Using foamed plaster containing trapped air bubbles
Why shouldn’t you use wood patterns for plaster based molds?
Because they are repeatedly in contact with water based plaster slurry and thus warp and degrade quickly
What kinds of patterns are best used with plaster based molds?
Patterns made of aluminium alloy, thermosetting plastics, brass and zinc alloys
What upper limit of temperature can plaster molds withstand?
About 1200°C
What materials are mostly used in plaster-mold casting?
Aluminum, magnesium, zinc and copper based alloys
What are the usual parts made in ceramic castings
Impellers, cutters, dies for metal working and molds for making plastic or rubber components.
The main takeaway is ceramic molds are used to make parts that endure a lot of strain
Why is ceramic casting good for very high temperature alloys?
The slurry is made of fine grained ZrSi04, mixed with bonding agents.
The high-temperature resistance of these molding materials allow these molds to be used for ferrous and other high-temperature alloys
What kind of components are usually made using expendable molds?
Cylinder heads, engine blocks, crankshafts, brake components, manifolds and machine bases
Add notes on evaporative casting
What types of parts are usually made in this investment casting?
Components for office equipment.
In mechanical aspects, things like gears, cams, valves and ratchets
Types of parts made in permanent mold casting
Automobile pistons, cylinder heads, connecting rods, gear blanks and kitchenware.
Parts can be made typically weigh less than 25kg
Vacuum casting uses two variations
Counter-gravity low-pressure vacuum casting (CLV) and counter-gravity low-pressure air casting (CLA)
CLV Parts usually involve reactive metal like aluminium, titanium, zirconium and hafnium
Slush casting (describe it)
Thin walled hollow castings can be made by permanent mold casting using the principle of slush casting.
Molten metal is poured into a permanent mold, and once a desired thickness is reached by leaving the metal to cool for a certain amount of time, the mold is inverted and the remaining molten metal is poured out thus leaving a hollowed out structure with walls with the desired thickness.
Used in toys with low-melting point metals like zinc, tin and lead alloys
