Lecture on casting

Question 1

You want the wax pattern to be how long away from the case?

Answer 1

6mm minimum distance

Question 2

If you abrade your die you end up getting a

Answer 2

over finished crown margin

Question 3

Gold in reservoir

Answer 3

is last thing you want to cool (because as its cooling its shrinking, so yyou want molten metal sucked from crown to sprue not the other way around.

Question 4

purpose of spure

Answer 4

allows all remnant of wax to escape, and allows patten to flow into mold

Allows molten and volatile wax to escape from the mold
Allows molten metal to flow into the mold
The metal within the sprue must remain molten slightly longer than the metal that has filled the mold. This provides a reservoir to compensate for the shrinkage that occurs during the solidification of the casting alloy.

Question 5

Sprue materials

Answer 5

Wax
Plastic (Rigid)
Hollow Plastic (Allows for better escape of the wax during burnout)
Metal

Question 6

If you get bigger spure

Answer 6

you have to do more finish work, and more gold.

Question 7

Sprue gauges and large vs/small

Answer 7

10 gauge (2.5 mm diameter) for molars
12 gauge (2.0 mm diameter) for premolars and partial coverage castings
In general, larger is better as it provides a better reservoir to remain molten
Can have reservoirs built in

***Attach sprue at the bulkiest, non-critical part of the pattern

Visualize “flow of molten metal” into the pattern
Create path of least resistance and turbulence

Question 8

Sprue attachment to pattern

Answer 8

Should be carefully smoothed to minimize turbulence
Should not be restricted (“Necking”), or…
Increases chances of casting porosity and can reduce mold filling
Should not be excessively wide, or…
This portion will solidify last and create “Suck-Back” or “Shrink-Spot” Porosity

Question 9

crucible

Answer 9

Usually made of rubber for easy separation from investment and casting ring
Attaches to other end of the sprue
Creates funnel shape through which the molten metal flows

Question 10

Casting ring

Answer 10

Surrounds wax pattern
Confines the investment material to create the mold
Fits into rubber crucible former
Ring liner allows for investment expansion

Question 11

Investment procedure

Answer 11

1. Attach sprue to wax pattern
2. Add small amount of wax and smooth at point of attachment
3. Remove pattern from die, exercising EXTREME CAUTION NOT TO DISTORT!
4. With forceps, insert other end of sprue into crucible former
5. Try on casting ring to check sprue length (~ 6 mm from pattern to top of ring)
6. Remove casting ring, then add small amount of wax and smooth at the point of attachment
7. Place liner in casting ring and moisten
8. Replace casting ring onto crucible former
9. You are now ready to invest the pattern

Question 12

Because centrifuge spin put the wax pattern margins to the

Answer 12

right

Question 13

types of investments
- how many are there?
what are they?

Answer 13

Gypsum Bonded
Phosphate Bonded
Ethyl-Silica Bonded

Question 14

Gypsum bonded investments

Answer 14

Not chemically stable above 1200℉ (650℃)
Utilized for Types I, II, III and IV gold alloys
Cristobalite or quartz is the refractory material
Best surface reproduction, resulting in smoother castings
Softer investment, making it easier to retrieve casting
More porous, facilitating more complete mold filling
Thermal Expansion during burn out (SiO2 α to β phase change)
Probably easiest to work with

Question 15

Contraction vs. Expansion

Answer 15

Contracts
Wax Pattern
Cooling Alloy

Expands
	Stone
	Investment
		Setting
		Hygroscopic
		Thermal (Oven)
		Less H2O
	Casting Ring Liners

Question 16

Investment parts

Answer 16

**Binder
**Refractory Material (SiO2)
Silica
Quartz
Cristobalite (filler particle in investment)

Modifiers
	Accelerators
	Retarders
	Dimension Control
	Coloring Agents

Question 17

Gypsum bonded investments are

Answer 17

Not chemically stable above 1200℉ (650℃)

Utilized for Types I, II, III and IV gold alloys

Cristobalite or quartz is the refractory material

Best surface reproduction, resulting in smoother castings

Softer investment, making it easier to retrieve casting

More porous, facilitating more complete mold filling

Thermal Expansion during burn out (SiO2 α to β phase change)

Probably easiest to work with

Question 18

first place to check crowns when fitting is

Answer 18

proximal contacts
*assuming intaglio surface is free of interferences

this is the most common place that keeps crowns from seating.

Question 19

Factors that Increase Expansion in Gypsum Bonded Investments:

Answer 19

↓ Water/Powder Ratio
Prolonged Spatulation
Water Bath Set (100℉ for 1 hour immediately after investing) Hygroscopic Expansion
Use a Dry Ring Liner
Use Two Ring Liners
Use Ringless Investment Technique

we want the expansion to compensate for the metal and the wax conrtaction.

Question 20

Factors that Reduce Expansion with Gypsum Bonded Investments:

Answer 20

Add water to the mix
Eliminate the casting ring liner

NOTE: Exercise caution when deviating from manufactures recommendations!

Question 21

Phosphate bonded investments

Answer 21

Most are mixed with a suspension of colloidal silica in water

Adding H2O to colloidal suspension increases working time and decreases expansion

Some are mixed with water alone

Binder consists of magnesium oxide and ammonium phosphate

Stable at much higher burn out temperatures

Suitable for metal-ceramic alloys which typically fuse around 2550℉

More silica refractory material which creates more expansion in burn out oven

Compensates for additional shrinkage of metal as it cools from a higher casting temperature

Question 22

Ethyl-Silica Bonded Investments

Answer 22

Used for high-melting base metal alloys

Partial denture frameworks

Question 23

Quenching

Answer 23

Casting is placed in water when the button no longer has a red glow
Interferes with the crystalline lattice of the alloy
Results in a softer, more malleable alloy

Question 24

Casting allys

Answer 24

Type I: Soft Simple Inlays

Type II: Medium Complex Inlays, Onlays, Single Unit Crowns

Type III: Hard Crowns and Fixed Dental Prostheses

Type IV: Extra hard Partial Denture Frameworks

Metal-Ceramic Alloys:
Base Metal Alloys

Question 25

Gold is

Answer 25

inert, not tarnishable, biocompatible

so more gold the better

avoid nickel

Question 26

Gold yellow color in alloy is actually

Answer 26

not a indicator of how much gold is there.

Question 27

Metal and porcelain bond is better than

Answer 27

zirconia and metal bond