Casting

Question 1

Define the Casting process

Answer 1

materials are shaped by melting the material and pouring it into a mould where it cools into a new form

Question 2

What is a casting pattern?

Answer 2

a peice of cheap or sacrificial material with the geometry of the product which is used to create the cavity in the mould

Question 3

What are the two types of permentant mold casting?

Answer 3

Die casting
Injection molding

Question 4

What are the three types of temporary mould casting? for each is the pattern expendable or permanent

Answer 4

Sand casting –> permanent
Investiment casting, lost foam casting –> expendable

Question 5

What is a casting core?

Answer 5

A part used in a mould to create hollow or internal shapes

Question 6

Describe sand casting and its stages

Answer 6

1) reusable patterns are made from cheap materials
2) sand in a two part mould is packed around the pattern
3) the pattern is removed leaving a cavity
4) the cavity is filled with molten metal and is allowed to cool
5) the mold is broken open and the final part is finished into a usable product

Question 7

Name and describe the parts of a sand casting setup

Answer 7

Riser: A column of metal placed in the mould to feed the casting as it shrinks
and solidifies. Also known as a ‘Feed head’

Runner: The channel through which the molten metal is carried from sprue to
gate

Cope: Upper sand mould

Drag: Bottom sand mould

Gate: A channel through which the molten metal enters the casting cavity

Question 8

What are sprue pins?

Answer 8

wooden pins used to create the runner and riser in the sandcasting mould

Question 9

What is green sand ?

Answer 9

an oily sand containing high moisture and clay used to create mold in sand casting. also called petrobond

Question 10

What are the benifits of green sand?

Answer 10

1)fine round grain structure

2)good permeability to release gases and steam
3) strong and ridig enough to hold its shape during casting
4) good collapsibilty to shrink when cooling to prevent hot tearing and cracking

Question 11

Why do patterns need to be larger than the final part they will create?

Answer 11

metals shrink during the cooling processes making the final product smaller than the cavity it is in

Question 12

give an advantage and disadvantage of sand casting

Answer 12

ADV: large size, low unit cost, low setup costs ect
DIS: inprecise, rough finish ect

Question 13

Describe investment casting

Answer 13

1) a patter is made from wax or polystrene using injection moulding
2) The assembly is dipped in a liquid containing ceramic powder, with additional
powder sprinkled on, often repeated several times.

3)The mould is fired to harden and eliminate moisture, similar to the process of
pottery. melting the wax in the process

4)Molten metal is poured into the finished shell and allowed to cool.

5)Extraneous material is cut off, and usually, little additional finishing is needed.

Question 14

How does lost foam casting differ from investment casting?

Answer 14

Pattern is not melted away when the mold is created, instead the foam is vapourized when the molten metal is poured in

Question 15

Describe Die casting

Answer 15

1) a high precision mold is created from metal and reused for all processes

2) molten metal is poured in and allowed to cool creating the product

Question 16

What is the difference between hot and cold chamber die casting?

Answer 16

in hot chamber the metal is melted in a furnace connected to the die where a piston forces the correct amount of metal into the die

in cold chamber the molten metal is heated externally and poured into a piston which injects it into the die

Question 17

When is die casting used?

Answer 17

Non-ferrous metals (lower melting point than the die material)
high volume
high accuracy
low wall thickness

Question 18

describe a Misrun in casting

Answer 18

molten metal is poured too quickly into the mould or the mold does not allow the metal to flow everywhere leaving large holes where the metal has not reached

Question 19

what is porosity in metal casting?

Answer 19

When metal is poured too quickly gases are trapped creating defective air bubbles throughout the material

Question 20

what factors affect the casting process?

Answer 20

1) Flow of metal
2) solidfication of metal
3) cooling of the casting in the mould

Question 22

whats is equiaxed grain structure and where does it occur in a cast part?

Answer 22

crystal grains with roughly equal size in all directions caused by fast cooling of the metal. found in the core of the metal and the area in contact with the mould

Question 23

What is a columnar Grain structure and where is it found in a cast part?

Answer 23

crystal grains which are longer in one direction than the others caused by gradual cooling of metal. found between the core region and the area in contact with the mould

Question 24

How is soldification time cauculated

Answer 24

t = c(V/SA)^2
t -> time
c-> constant dependant on mould and metal
V-> volume
SA -> casting surface area

Question 25

How does cooling affect the casting structure?

Answer 25

as the solid metal cools in the mold it can undergo phase changes and contracts in the mold

Question 26

What is hot tear?

Answer 26

when a cast part splits during contraction as it cools in the mold

Question 27

How can cast parts be designed to prevent defects from cooling?

Answer 27

1) avoid sharp corners
2) use fillents to blend changes in cross sections
3) no rapid changes in cross sections
4) avoid regions of unnessasary thickness