Casting 2

Question 1

What kind of sand is used in casting?

Answer 1

Silica sand SiO2

Question 2

What are the two types of sands and which is more preferable to foundaries?

Answer 2

Natural bonded (bank sand)

Synthetic sand (lake sand)

Foundries prefer synthetic sand because its composition can be controlled more accurately

Question 3

Review the outline of production steps in a typical sand casting operation

Question 4

Name the 3 basic types of sand molds

Answer 4

1. Green-sand (water, sand & clay)
2. Cold-box (organic & inorganic binders)
3. No-bake molds (resin)

Question 5

Titanium and zirconium (reactive metals) react aggressively with silica - so don’t use them with sand casting.

Question 6

She’ll molding

Answer 6

Popular because of good dimensional tolerance, high accuracy and low cost

Question 7

Outline of the process of shell casting

Answer 7

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)

Question 8

Design considerations for shell casting

Answer 8

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.

Question 9

Plaster mold casting (add relevant notes for this section)

Question 10

Name the process used to increase permeability in plaster-mold casting. Briefly describe it.

Answer 10

The Antioch process. The molds are dehydrated in an autoclave for 6-12 hours, then rehydrated in for 14 hours

Question 11

Name another way of increasing permeability in plaster-mold casting

Answer 11

Using foamed plaster containing trapped air bubbles

Question 12

Why shouldn’t you use wood patterns for plaster based molds?

Answer 12

Because they are repeatedly in contact with water based plaster slurry and thus warp and degrade quickly

Question 13

What kinds of patterns are best used with plaster based molds?

Answer 13

Patterns made of aluminium alloy, thermosetting plastics, brass and zinc alloys

Question 14

What upper limit of temperature can plaster molds withstand?

Answer 14

About 1200°C

Question 15

What materials are mostly used in plaster-mold casting?

Answer 15

Aluminum, magnesium, zinc and copper based alloys

Question 16

What are the usual parts made in ceramic castings

Answer 16

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

Question 17

Why is ceramic casting good for very high temperature alloys?

Answer 17

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

Question 18

What kind of components are usually made using expendable molds?

Answer 18

Cylinder heads, engine blocks, crankshafts, brake components, manifolds and machine bases

Question 19

Add notes on evaporative casting

Question 20

What types of parts are usually made in this investment casting?

Answer 20

Components for office equipment.
In mechanical aspects, things like gears, cams, valves and ratchets

Question 21

Types of parts made in permanent mold casting

Answer 21

Automobile pistons, cylinder heads, connecting rods, gear blanks and kitchenware.

Parts can be made typically weigh less than 25kg

Question 22

Vacuum casting uses two variations

Answer 22

Counter-gravity low-pressure vacuum casting (CLV) and counter-gravity low-pressure air casting (CLA)

Question 23

CLV Parts usually involve reactive metal like aluminium, titanium, zirconium and hafnium

Question 24

Slush casting (describe it)

Answer 24

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

Question 25

Die casting

Answer 25

• Cost of machinery is high but labour associated with it is very low because processes are automated or semi-automated

Typical parts made are housings for transmissions, business machine and appliance components, etc

Question 26

Insert molding

Answer 26

Similar to putting a wooden stick into a popsicle before it freezes. Components like pins, shafts and threaded fasteners can be die cast integrally

Question 27

Directionally solidified blades

Answer 27

Make more notes about this stuff. Used when the grain structure solidified is polycrystalline. Generally made of nickel-based superalloys

Question 28

What are ribs used for?

Answer 28

To increase the rigidity and strength of a casting without increasing the wall thickness which is often undesirable. They also aid in the flow of molten metal

Question 29

What are gates?

Answer 29

They are connections between the runners and the part to be cast.

Question 30

Why are multiple gates often preferable?

Answer 30

Multiple gates are crucial for large parts.

Gates allow for a lower pouring temperature as well as reducing the temperature gradient in the casting.

Gates should feed into thick sections of the castings