Casting Flashcards
DMM1: manufacturing Deck 1
Define the Casting process
materials are shaped by melting the material and pouring it into a mould where it cools into a new form
What is a casting pattern?
a peice of cheap or sacrificial material with the geometry of the product which is used to create the cavity in the mould
What are the two types of permentant mold casting?
Die casting
Injection molding
What are the three types of temporary mould casting? for each is the pattern expendable or permanent
Sand casting –> permanent
Investiment casting, lost foam casting –> expendable
What is a casting core?
A part used in a mould to create hollow or internal shapes
Describe sand casting and its stages
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
Name and describe the parts of a sand casting setup
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
What are sprue pins?
wooden pins used to create the runner and riser in the sandcasting mould
What is green sand ?
an oily sand containing high moisture and clay used to create mold in sand casting. also called petrobond
What are the benifits of green sand?
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
Why do patterns need to be larger than the final part they will create?
metals shrink during the cooling processes making the final product smaller than the cavity it is in
give an advantage and disadvantage of sand casting
ADV: large size, low unit cost, low setup costs ect
DIS: inprecise, rough finish ect
Describe investment casting
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.
How does lost foam casting differ from investment casting?
Pattern is not melted away when the mold is created, instead the foam is vapourized when the molten metal is poured in
Describe Die casting
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
What is the difference between hot and cold chamber die casting?
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
When is die casting used?
Non-ferrous metals (lower melting point than the die material)
high volume
high accuracy
low wall thickness
describe a Misrun in casting
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
what is porosity in metal casting?
When metal is poured too quickly gases are trapped creating defective air bubbles throughout the material
what factors affect the casting process?
1) Flow of metal
2) solidfication of metal
3) cooling of the casting in the mould
whats is equiaxed grain structure and where does it occur in a cast part?
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
What is a columnar Grain structure and where is it found in a cast part?
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
How is soldification time cauculated
t = c(V/SA)^2
t -> time
c-> constant dependant on mould and metal
V-> volume
SA -> casting surface area
How does cooling affect the casting structure?
as the solid metal cools in the mold it can undergo phase changes and contracts in the mold
What is hot tear?
when a cast part splits during contraction as it cools in the mold
How can cast parts be designed to prevent defects from cooling?
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