investing and casting gold alloys Flashcards
1
Q
Investing:
A
Investing: surrounding the wax pattern with a material that can accurately duplicate its shape and anatomic
features.
2
Q
Burnout:
A
Burnout: removal of the wax pattern so that a mold is created into which the molten alloy can be placed.
.
3
Q
Casting:
A
Casting: introducing the molten alloy into the previously prepared mold.
4
Q
noble metal function in alloys
A
tarnish resistance and ductility
5
Q
silver function in alloys
A
lightens color, adds ductility
6
Q
copper function in alloys
A
increase hardness and strength
7
Q
zinc function in alloys
A
reduces oxidation (scavenger)
8
Q
most widely used?
constituents of dental casting alloys
A
Most widely used alloy: 75% gold (18-karat; 18/24 pure)
Gold, platinum, palladium (noble)
Silver
Copper
Zinc
9
Q
Functions of the investment:
A
- A precisely detailed reproduction of anatomical form;
- Sufficient strength to withstand the heat of burnout and the actual casting of the molten metal;
- Compensation expansion equal to the alloy solidification shrinkage, about 2%.
10
Q
Types of compensation expansion (compensation for alloy shrinkage):
A
- Setting expansion of investment:
- Hygroscopic expansion
- Wax pattern expansion
- Thermal expansion of investment:
11
Q
due to? affected by?
Setting expansion of investment:
A
due to nucleation; affected by pattern, liner compressibility and water: powder ratio
12
Q
Hygroscopic expansion of investment:
A
when water added to setting investment immediately after ring is filled (submerged in water bath 37oC, I H)
13
Q
due to?
Wax pattern expansion:
A
wax expands while investment is still fluid, from heat of the chemical reaction or warm water bath (hygroscopic technique).
14
Q
due to? what is responsible?
Thermal expansion of investment:
A
occurs as mold is heated for wax elimination (BURNOUT). Silica (refractory) is responsible for thermal expansion because of solid state phase transformations.
15
Q
Types of Investments Materials:
A
- Gypsum bonded investments:
- Phosphate bonded investments:
- Silicate investments:
16
Q
Gypsum bonded investments:
contents?
a. Gypsum is unstable above? only use with?
A
gypsum (binder); crystobalite or quartz (refractory) - responsible for expansion.
a. Gypsum is unstable above 650oC (1200oF); used only with types I, II and Ill gold alloys.
17
Q
- Phosphate bonded investments: contents? Used for?
A
- Phosphate bonded investments: magnesium oxide or ammonium phosphate binder and high
concentration of silica refractory; Used for metal-ceramic allloys (above 650oC (1200oF).
18
Q
- Silicate investments: used with?
A
- Silicate investments: used w/ very high casting temperatures (base metals).
19
Q
Components of the Investment / Casting Process
A
- Casting ring and liner
- Crucible former
- Sprue former
20
Q
role, liner allows for?
- Casting ring and liner
A
- Casting ring and liner - ring holds investment in place; liner allows for expansion.
21
Q
Crucible former
A
Crucible former - constitutes base of casting ring during investment and helps position pattern
22
Q
Investing Procedures
A
- Wax pattern is cleaned (surface tension reduced for “wetting” of investment).
- Hand or vacuum investing: pattern is accurately enveloped in investment; ring is filled.
- Investment Technique:
a. Hygroscopic Technique: ring in 37oC water bath for 1H
b. Thermal Technique: investment bench sets for 1H.
23
Q
Objectives of Casting
how fast should alloy be heated/to what condition?
prevent what chem rxn? how?
cast should have what level detail? how?
A
1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with a well adjusted torch.
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold
24
Q
Spruing
A
- The process of attaching a sprue former/sprue pin to the wax pattern is
called as spruing
25
spruing purpose
To provide a channel through which molten alloy can reach
the mold in an invested ring after the wax has been eliminated.
26
* Basic requirements of the sprue:
1) Must allow the molten wax to?
2) Sprue must enable the molten metal to?
3) Metal must remain molten slightly longer?
1) Must allow the molten wax to escape from the mold.
2) Sprue must enable the molten metal to flow into the mold with as little turbulence as possible.
3) Metal must remain molten slightly longer than the alloy that has filled the mold
27
wax sprue preffered why
referred because they melt at the same rate of the wax pattern
28
plastic sprue
they may be used when casting fixed partial dentures because of the added rigidity
29
metallic sprue forms
a- solid
b- hollowed
30
Lost Wax Casting Technique
1. Surrounded with a mold of heat-resistant refractory investment
material
2. Elimination of the wax from mold by heating (burnout)
3. Introduction of molten metal into the mold through a channel
(crucible and sprue) - casting
31
Investment Materials
* Requirements
1. Precisely reproduce?
2. Provide sufficient strength to withstand?
3. Expand sufficiently to compensate for? %?
1. Precisely reproduce the detailed form of the wax pattern
2. Provide sufficient strength to withstand the heat of burnout
and the actual casting of the molten metal
3. Expand sufficiently to compensate for the solidification
shrinkage of the alloy, 2%
32
# %?
Casting Investment Materials Composition
* Refractory (60-65%)
* Binder (30-35%)
* Modifiers (5%)
33
# affects? %? made of?
* Refractory component of investment materials
* Refractory (60-65%) – (affects expansion)
1. * Silicon dioxide ( quartz / tridymite / cristobalite)
34
# %? role? forms?
* Binder of investment material
* Binder (30-35%) - (holds set mass together) provides strength
1. * Gypsum (α-calcium sulfate hemihydrate)
1. * Phosphate (high-temp casting)
1. * Ethyl Silicate (high-temp casting base metals)
35
# %? role? forms?
* Modifiers of investment material
* Modifiers (5%) – numerous effects, mainly improves handling
1. * NaCl; Boric Acid; K2SO4; Graphite; Cu Powder; MnO2****
36
* Gypsum bonded investment material
1- binder?
2- Used to cast alloys that have melting temp. up-to?
3- amount of expansion?
4- used with which alloys?
1- binder is gypsum (calcium
sulphate hemihydrates)
2- Used to cast alloys that have
melting temp. up-to 1080°C
3- low amount of expansion
4- used with gold alloys type I,
type II, type III, and type IV gold
alloy
37
* Phosphate bonded investment material
1-binder?
2- Used to cast alloys with high
melting temp. up-to?
3- amount of expansion?
4-used with which alloys?
1-binder is magnesium oxide and ammonium phosphate binder
2- Used to cast alloys with high melting temp. up-to 1400°C
3- high amount of expansion
4-used with silver palladium, gold platinum, nickel chromium alloy
38
investment expansions
1. Setting Expansion
2. Hygroscopic Expansion and Wax Pattern Expansion
3. Thermal Expansion
39
1. Setting Expansion
* crystal growth?
* Enhanced by?
* Normal crystal growth as investment sets
* Enhanced by silica
40
2. Hygroscopic Expansion and Wax Pattern Expansion
* Exposure to?
* Wax pattern?
* Exposure to 100oF water bath during setting of investment
* Wax pattern softens / expands as heat generated during chemical reaction; ↑ water (?) “hygroscopic technique
41
3. Thermal Expansion
* expansion of?
* Heat during burnout causes expansion of the silica
42
Q :Why we need expansion?
A : To compensate shrinkage that had been developed during metal
solidification after casting
43
Spruing and Investing Process
* Surrounding wax pattern with? why?
* Surrounding wax pattern with a material that can accurately
duplicate its shape and anatomic features while leaving a
channel for entry of molten alloy.
44
Investment Casting Process
1. Attach sprue to wax pattern using pliable sticky wax.
2. Attach sprue to wax pattern using pliable sticky wax to create smooth, clean connection.
3. Mount sprued pattern to crucible former base, again making smooth connection point.
4. Metal rings and liner must be used with gypsum investments.
5. Place ring onto former base.
6. add measured water to bowl then invesment powder- hand mix then vacuum mix
7. invest with low vibration
45
HYGROSCOPIC TECHNIQUE
Immediately after investing, while investment is still fluid, immerse the ring under water in Hygrobath. (100°F for 30 – 40 minutes)**
46
burnout temps of high heat and hygroscopic techniques
* Hygroscopic Technique – burnout temperature is 950°F
* High Heat Technique: burnout temperature is 1,200oF
47
After quenching, gypsum investments:
easily break away
from castings for quick, clean recovery.
48
requirements of cast
fit, margins, surface?
* Excellent fit, sharp margins, smooth surface.
49
* Finish casting by:
removing sprue with separating disc,
adjust contacts and occlusion, and polish.
50
Casting Alloy Shrinkage Compensation
Molten alloys shrink upon solidification
* Gold alloys ≈ 1.5%
* Metal-ceramic ≈ 2%
* Base metal ≈ 2.5%
* Mold must be made correspondingly larger than original wax pattern
Mold expansion (investment) = alloy shrinkage
51
Alloys
* defined
* changes in behavior based on?
* Elements – described by?
* melting?
* A metal composed of two or more elements in which the two are
mutually soluble in the liquid state.
* “Alloy Systems” have changes in behavior with different % combinations of the respective elements.
* Provides unique properties –hardness, wear, strength, corrosion resistance and biocompatibility.
* Elements – described by mechanical and physical properties (e.g., one Tm).
* Alloys have a melting range.
52
Corrosion defined
corrosion can lead to: (margin, porosity, strength, release of?)
* Progressive destruction of a metal by chemical or electrochemical reaction with environment.
* Leads to the following:
* Increased porosity
* Reduced marginal integrity
* Loss of strength
* Release of metallic products into the oral environment
53
tarnish and gold alloys
tarnish resistant, withstands oral environment
54
most widely used gold cast alloy
karat/fineness
75% Au
* Karat: parts gold / 24 (18 karat = 75% Au alloy)
* Fineness: #parts gold / 1000 parts alloy (750 fine)
55
benefits of gold alloy
* Tarnish / corrosion resistance and ductility****
56
Metallic Elements in Dentistry
base metals and noble metals
57
noble metals used
* Gold (Au)
* Platinum (Pt)
* Palladium (Pd)
* Iridium (Ir), Ruthenium (Ru),
and Rhodium (Rh)
58
Classifications of Dental Casting Alloys
Classifies casting alloys as Type I – IV.
Specifies relative hardness (yield strength and elongation) of each:
59
type 1 casting alloy
Type I – Soft (Low stress - Class I inlays)
60
type 2 casting alloys
Type II – Medium (Low stress - Class II inlays)
61
type 3 casting alloys
Type III – Hard (Onlays, crowns and short span FDPs)
62
type 4 casting alloys
Type IV – Extra Hard (Long span FDPs
and RPDs)
63
Ø% elongation measurement of:
ductility, how much margins can be closed by burnishing
64
Noble metal: chemical behavior
* Corrosion and oxidation resistance (chemically inert)
* Gold and Platinum Group* (Palladium - major)
65
* Base metal
* Base metal: undergoes oxidation readily
* Chromium, Nickel, Beryllium
66
# high noble, noble, predom base, Ti/Ti alloys?
Composition Classification of alloys
* High Noble Alloys:Noble metal content ≥ 60% (gold + platinum group*); 40% gold.
* Noble Alloys: Noble metal content ≥ 25% (gold + platinum group*)
* Predominantly Base Alloys: Noble metal content < 25% (gold + platinum group*)
* Titanium and Titanium Alloys
– Titanium ≥ 85%
67
Titanium and Titanium Alloys
* biocompatibility?
* Chemically active?
* Oxide layer?
* Bonding with?
* Bonds how?
* Difficulties in?
* Excellent biocompatibility
* Chemically active (non-noble)
* Oxide layer formed when exposed to air
* Bonding with porcelain
* Bonds with resin cements and polycarboxylic acid cements
* Difficulties in casting, welding
68
Base Metal Alloys
Ni-Cr / Ni-Cr-Be / Co-Cr
* Physical properties:
stiffness?
hardness?
melting temp?
density?
finish and solder?
oxide layer?
–Greater stiffness (Modulus of elasticity)*
–Greater hardness / strength (500 – 600MPa)
–Higher melting temp* – greater sag resistance
–Lower density – lighter restoration / difficult to cast
–More difficult to finish and solder
–↑ oxide layer formation – problems w/ porcelain bonding
69
Base Metal Alloys: Ni-Cr-Be
Biocompatibility Concerns
* Patient allergic response (Ni, Co)
* Carcinogenic (Be, Cd)
* Technician handling biohazard
* Dissolution / Leaching
70
Casting Errors: “Finning”
Too rapid burnout – cracking in investment
71
Casting Errors: Incomplete Margin
Alloy not hot enough / Insufficient casting
force Incomplete burnout
72
Casting Errors: Investment Cracked
Pattern touched ring liner
73
wrong end of casting ring
74
wax pattern dislodge in investment
75
distortion of wax pattern can be due to?
improper investment
76
suck back porositites due to?
sprue directed at right angle to wall of mold=hot spot created keeping alloy molten long afterf cast solidifies
77
shrink spot porosities
too thin or long a sprue former used, alloys solid in sprue before in casting
78
bubble at cast surfaces causes
no debubblizer
poor inital coating of investment
air entrapment
79
short margins with rounded buttons cause
alloys not hot enough/lack of pressure used
80
short margins with sharp buttons cause
pattern too far from end/ring
81
short rounded margin and shiny casting cause
incomplete burnout
82
holes in casting cause
thin wax pattern
83
excessively black/rough/ incomplete casting cause
burnout temp too high; too long in oven
84
no casting recoved causes?
pattern vibrated loose from sprue/ gold against wrong end of investment ring
85
purpose of the resivoir for casting
allows larger mass of alloy so pattern solidifies first and at resivoir last, thus shrinkage occurs here not at pattern
