1
Q
What is the purpose of the sprue?
A
- 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.
2
Q
What are the most common sprues made out of?
A
- Wax
- Plastic (Rigid)
- Hollow Plastic (Allows for better escape of the wax during burnout)
- Metal
3
Q
What are the different sizes of sprues and which ones are best?
A
- 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
- Attached at the bulkiest, non-critical part of the pattern
- Visualize “flow of molten metal” into the pattern
- Create path of least resistance and turbulence
4
Q
Why is a larger sprue sometimes disadvantageous?
A
- A larger sprue can be disadvantageous because of more finish work, more gold required, and you can cast gold a couple of times but after that, it isn’t good anymore.
5
Q
Is a sprue with a larger or smaller gauge going to be bigger?
A
- The larger the number gauge, the smaller the diameter
6
Q
At what angle should the sprue be placed on the wax?
A
- A 45 degree angle on the thickest portion of the pattern. The gold needs to change direction at almost a 90 degree angle when it enters.
7
Q
What are the keys to attaching the sprue to the wax pattern?
A
- 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
8
Q
What is the crucible former?
A
- Usually made of rubber for easy separa.tion from investment and casting ring
- Attaches to other end of the sprue
- Creates funnel shape through which the molten metal flows
9
Q
What is the casting ring?
A
- Surrounds wax pattern
- Confines the investment material to createthe mold
- Fits into rubber crucible former
- Ring liner allows for investment expansion
- (We did not use a ring liner because we didn’t use a metal casting ring, we used a plastic one.
10
Q
What are the nine steps of the investment procedure?
A
- Attach sprue to wax pattern
- Add small amount of wax and smooth at point of attachment
- Remove pattern from die, exercising EXTREME CAUTION NOT TO DISTORT!
- With forceps, insert other end of sprue into crucible former
- Try on castng ring to check sprue length (~ 6 mm from pattern to top of ring)
- Remove casting ring, then add small amount of wax and smooth at the point of attachment
- Place liner in casting ring and moisten
- Replace casting ring onto crucible former
- You are now ready to invest the pattern
11
Q
What two components is a dental casting mold made of? And what purpose do they each serve?
A
- Refractory Material
- A material that doesn’t burn up, heat resistant, but helps in the expansion. It is silica.
- Binder
- Provides Strength
12
Q
What are the four modifiers of investments (dental casting molds)?
A
- Accelerators
- Retarders
- Dimension Control
- Coloring Agents
13
Q
What are the three main types of investment?
A
- Gypsum Bonded (With less liquid, you get more expansion)
- Phosphate Bonded (With more liquid, you get more expansion)
- Ethyl-Silica Bonded (Know that this exists, but we won’t talk about it really)
14
Q
What are the main characteristics of gypsum bonded investments?
A
- Not chemically stable above 1200°F (650°C)
- 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
15
Q
What two things contract and what two things expand in regards to the investment process?
A
Contract
- Wax Pattern
- Cooling Alloy
Expand
- Stone
- Investment
- Setting
- Hygroscopic
- Thermal (Oven)
- Less H2O
- Casting Ring Liners
16
Q
A casting that is slightly undersized is advantageous for accurate seating. True or False?
A
False. Should be slightly oversized.
17
Q
If you have a clean tooth with the entaglio surface intact and everything but the crown still won’t seat properly and all the way down, what should you check first?
A
Always check the prep to make sure there is no leftover cement from the IRM, and then the next thing you check are the proximal contacts.
18
Q
What are the factors that increase expansion in gypsum bonded investments?
A
- ↓ Water/Powder Ratio
- Prolonged Spatulation
- Water Bath Set (100°F for 1 hour immediately aier investing) Hygroscopic Expansion
- Use a Dry Ring Liner
- Use Two Ring Liners
- Use Ringless Investment Technique
19
Q
What are the factors that reduce expansion with gypsum bonded investments?
A
- Add water to the mix
- Eliminate the casting ring liner
20
Q
What are the characteristics of phosphate bonded investments?
A
- 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°F
- 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
21
Q
When are ethyl-silica bonded investments used?
A
- Used for high-melting base metal alloys
- Partial denture frameworks
22
Q
What does quenching do to a casting?
A
- 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
23
Q
What are the main six types of casting alloys?
A
- Type I: Soft For simple inlays
- Type II: Medium Complex inlays, onlays, SUCrown
- Type III: Hard Crowns and Fixed dental prosth
- Type IV: Extra Hard Partial denture frameworks
- Metal-Ceramic Alloys
- Base Metal Alloys
24
Q
What is the fusing temperature of gold vs metal-ceramic alloys?
A
- Fusing temperature = Melting point of the alloy
- Au alloys: ± 1700°F (925°C)
- Metal-Ceramic Alloys: ± 2550°F (1400°C)
25
What is the gold effect? What makes it yellow?
* 9 karat jewelry gold with 37.5% Au and a considerable amount of Cu is quite yellow
* 20 karat dental gold with 85% Au and no Cu is not very yellow
* In general, more gold, less tarnish and more biocompatible (Au is less reactive than most metals)
* Avoid Ni
* Karat (24 Parts)
* 24 karat gold is pure gold
* 18 karat gold is 75% gold
* Fineness (1000 Parts)
* 1000 fine is pure gold
* 750 fine is 75% gold
26
How many ounces is 20 dwt (pennyweight)?
1 oz. 12 oz. to 1 lb.
27
What are the six different types of gold alloys and what are their percentages?
28
What are the six zones we deal with when finishing a casting?
* Zone 1: Intaglio Surface
* Zone 2: Sprue Area
* Zone 3: ProximalContacts
* Zone 4: Occlusal Surface
* Zone 5: Axial Walls
* Zone 6: External Margin
29
How much space should exist between the internal surface of the casting and the die?
* There should be a uniform space of 25 to 35 μm.
30
How tightly should the marginal area of the casting fit up with the margin on the die?
* The marginal area only ( ~ 1-2 mm) of the casting should create a tight seal at the die margin (10 μm or less gap)
31
What should we know about pins helping retain our build-ups?
* There is a question about you’ve placed a pin within the confines of the tooth far enough that it will help to retain your build-up, but it’s not so far in that by the time you’re done, it’s completely encased within the buildup material. **This is OK, if you expose the pin during your preparation, sometimes that happens.**
32
What do you do with stained sclerotic dentin?
* **Stained sclerotic dentin at the marginal areas (get rid of it all the time)**. Stained around the dentin near the pulp (sometimes you keep it around the pulp)
33
Is it a good idea to do a direct or indirect pulp cap underneath a tooth that is going to get a crown?
* No. High likelihood that it will need a root canal at some point during its lifetime.
34
The roughness of the intaglio surface of the crown, does that aid in retention? The roughness of the prepared tooth?
Yes. No.
35
What material leaves a sticky unpolymerized oxygen inhibited layer?
* Bis-GMA = Bisphenol A-glycidyl methacrylate
36
How do you get suck back?
* The gold in the sprue solidifies before the cold in the casting.
37
When you heat crystobalite or any of the quartz or silica, what happens to it?
* **It goes through a phase change.** Silicon dioxide is quartz, silica, crystobalite (a form of quartz in investments, and expands as the investment is heated, it is a phase change, from alpha to beta).
38
Which investments contain the most quartz? Gypsum bonded or phosphate bonded?
* Quartz causes expansion, the answer is phosphate bonded. More powder gives more expansion with gypsum bonded and more liquid gives more expansion with phosphate.
39
At what temperature does gypsum become unstable?
650 C
40
Can you put your investment in the oven, take it out, and do it a week later, take it out, another week and a half put it in, is there a problem with this?
* Yes, you can’t keep running it through these cycles.
41
What other metal in gold alloy tends to bring out the yellow color in gold?
Copper
42
You have an even greater exothermic reaction when the investment is heated with \_\_\_\_\_\_\_\_\_\_\_?
Phosphate-bonded investments. Will cause wax pattern to expand as well.
43
With a gold crown on a molar, the functional cusp reduction is \_\_\_\_\_\_\_\_\_\_?
1.5 to 2 mm. Non-functional is 1-1.5 mm.
44
With PFM with porcelain covering the entire occlusal, the reduction is at least \_\_\_\_\_\_\_\_\_\_\_?
* 2 mm everywhere.
45
What is the axial reduction at the margin for PFM?
1.2 - 1.7 mm
46
Which tooth gets more wear and tear than any other tooth?
* **Lower First molars** get a lot of wear and tear, maybe more than any other tooth. They come in earlier when anterior guidance is yet to be established. So commonly, missing lower first molars is a common thing, and if you are, you can get the **second molar leaning forward and lingual, and the maxillary 1st can be hyper-erupted**. It **alters the curve of spee and wilson** and messes with occlusion. When mandible goes into left lateral excursion, you pick up some heavy balancing contacts back there. With that said, we see a lot of bridges that are put in to restore these first molars. What are the problems with this? **You have kind of duplicated the problems that were already existing.** In some cases, you can endodontically treat the hyper-erupted maxillary 1st molar, shorten it, ortho upright the mandibular second molar, and basically restore them. This can be more promising in the long run than a bridge that is thrown on there.
47
When should you not use a triple-tray?
* If you are doing more than 2 crowns, probably not wise to use a triple tray. Anything that gets more complicated than simple 1 maybe 2 crowns, working in the full-arch field on a semi-adjustable articulator. If any question on how you will proceed, use a **full-arch field.**
48
What are the two main disadvantages associated with composite as a core material?
* **Moisture absorption, and high thermal expansion.**
* Moisture absorption can be overcome pretty easily.
* There are enough advantages that people use it.
49
If you use amalgam for a build-up in a moist environment, what type of amalgam should you use?
Use a low-copper amalgam so you get less expansion with the moisture contamination.
50
Increased preparation height increase resistance and retention. True or False?
True. The only one that is sketchy is the surface area and widening the diameter, which decreases resistance I think. Know that caveat.
51
What are the characteristics of 4-META bonding materials?
* **C&B meta-bond is a bonding agent that will increase the bond strength.** If you are in a situation where you don’t have the crown height, grooves, boxes, situation you want, you might want to consider tin plating it (if it’s gold) and using a formetta cement.
52
Why is close marginal adaptation of the fixed prostheses important?
* **Dissolution of the luting agent over time (minimizing it)** is one of the most important things, as well as caries.
* If you have a tight margin and you cement something with zinc phosphate, it will be there forever. But if not as tight, it will be washed out in 10 years.
53
The three conflicting principles of tooth preparation are?
Mechanically, Biological, Esthetic. And know how they relate.
54
On a lingual axial chamfer, what is the depth on any PFM in the anterior?
0.5 to 1 mm
55
What is the thickness of the metal under a PFM, at minimum?
0.3 mm
56
If you get sparking with your electrosurgery?
Adjust the current down
57
If the cutting tip drags with electrosurgery, and doesn’t smoothly cut, what do you do?
Increase the current
58
What are some of the contraindications to anti-sialogogues?
Glaucoma, old people. A strong gag reflex is often accompanied by sialorrhea.
59
What is the liquid in IRM?
Eugenol, which soothes vital teeth.
60
What is the refractory material in investments?
* The refractory material in investments is the silica.
* Refractory is heat-resistant, it doesn’t burn. But helps in the expansion.
61
How many ADA type recognized gypsums are there?
5
62
What does quenching do to the casting?
It makes them less-ordered
63
What are two metals to be avoided?
Nickel and Beryllium
64
What are the noble metals?
**Gold, Palladium, Platinum**. Silver is not.
65
If there is any question about whether your pattern is going to draw, when do you take it off?
* Do it right after the coping stage, after you dip it.
66
What is the minimum distance allowed from the top of the casting ring to the top of the wax pattern?
6 mm
67
What are the three basic requirements of the sprue design?
1. The sprue must allow the molten wax to escape from the mold.
2. The sprue must enable the molten metal to flow into the mold with as little turbulence as possible.
3. The metal within it must remain molten slightly longer than the alloy that has filled the mold. This provides a reservoir to compensate for the shrinkage that occurs during solidification of the casting alloy.
68
Why are wax sprues preferred over plastic or metal?
* Wax sprues are preferred for most castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax.
69
What are the size sprues recommended for molar and premolar castings?
* A **2.5-mm (10-gauge) sprue** is recommended for **molar** and metal-ceramic patterns. A smaller **2.0- mm (12-gauge)** sprue is adequate for **premolar** castings and most partial-coverage restorations.
70
What is the point of a casting liner around the casting ring?
* Normally a liner is placed inside the ring to allow for more expansion, because the liner is somewhat compressible. Use of two liners allows for additional compression and enables increased setting expansion of the investment material.
71
What two things can increase hygroscopic expansion?
1. **Wetting the liner increases the hygroscopic expansion** of the mold and should be carefully controlled. An absorbent dry liner removes water from the investment and makes a thicker mix, which leads to increase in the total expansion. To prevent expansion restriction, care must be taken not to squeeze the liner against the ring.
2. Increased expansion can be obtained by placing the mold in a water bath. This is because of **hygroscopic expansion.**
72
What are the gypsum-bonded and phosphate-bonded investments used for respectively?
* The gypsum- bonded investments are used for castings made from American Dental Association (ADA) type II, type III, and type IV gold alloys.
* The phosphate-bonded materials are recommended for metal-ceramic frameworks.
73
What is the binder and the two refractory materials of gypsum-bonded investments, and which two are responsible for the thermal expansion of the mold during wax elimination?
1. Gypsum is used as a binder.
2. Cristobalite and
3. Quartz as the refractory material, to form the mold.
4. The cristobalite and quartz are responsible for the thermal expansion of the mold during wax elimination.
74
What is setting expansion?
* As the gypsum investment sets after mixing, it expands and slightly enlarges the mold. The pattern, metal casting ring, and compressibility of the ring liner all influence this expansion.
* **The water/powder ratio can be altered to reduce or increase the amount of setting expansion.**
* **The use of less water increases the setting expansion and results in a slightly larger casting**.
* **Use of an additional ring liner increases the setting expansion, as does a slight increase in mixing time**. If a smaller casting is desired, more water can be used or the liner can be eliminated, both of which curtail the amount of expansion. When attempting to alter setting expansion, the clinician should not deviate more than minimally from the manufacturer’s recommendations, to ensure that there are no changes in the essential properties of the investment.
75
What is hygroscopic expansion?
* **Hygroscopic expansion occurs when water is added to the setting gypsum investment immediately after the ring has been filled.** Usually this is accomplished by submerging the ring in a water bath at 37°C (100°F) for up to 1 hour immediately after investing.
* **A significant amount of additional setting expansion results, enabling the use of a slightly lower wax elimination temperature**.
* **A wet ring liner also contributes hygroscopic expansion to the portion of the mold with which it is in contact.**
76
What is thermal expansion?
* **As the mold is heated to eliminate the wax, thermal expansion occurs.**
* **The silica refractory material is principally responsible for this because of solid-state phase transformations.**
* **Cristobalite changes from the a to the b (high-temperature) form between 200°C (392°F) and 270°C (518°F); quartz transforms at 575°C (1067°F).**
* **These transitions involve a change in crystal form, an accompanying change in bond angles and axis dimension, and a decreased density, producing a volume increase in the refractory components.**
77
What are the principle differences between gypsum-bonded and phosphate-bonded investments? And what are the characteristics of phosphate-bonded?
* The principal difference between gypsum-bonded and phosphate-bonded investments is the **composition of the binder and the relatively high concentration of silica refractory material in the latter**.
* The binder consists of **magnesium oxide and an ammonium phosphate** compound.
* **In contrast to gypsum-bonded products, this material is stable at burnout temperatures above 650°C (1200°F), which allows for additional thermal expansion**.
* Most phosphate-bonded investments are mixed with a specially prepared suspension of colloidal silica in water. (Some, however, can be mixed with water alone.)
* Compared to gypsum-bonded investments, **phos- phate-bonded investments offer greater flexibility in controlling the amount of expansion**. **The liquid/powder ratio needs only slight modification to effect a significant change in setting expansion.** Increasing the proportion of special liquid (colloidal silica) also increases expansion.
* **Phosphate-bonded investments have a relatively short working time in comparison with gypsum materials.** Their **exothermic setting reaction** accelerates as the temperature of the mix rises during manipulation.
78
What looks more yellow, 9-carat jewelry alloy with only 37.5% gold, or a metal-ceramic dental alloy with 85% gold but no copper.
The 9-carat jewelry alloy with only 37.5% gold. Color is not a good guide to gold content.
79
What must the manufacturer list in order to pass ADA requirements for an alloy suitable for dental restorations?
* To be accepted by the ADA as an alloy suitable for dental restorations, the manufacturer must list **the percentage composition by weight of the three main ingredients and any noble metal percentage.**
80
What are the ideal properties of an investment material?
1. Controllable expansion to compensate precisely for shrinkage of the cast alloy during cooling.
2. The ability to produce smooth castings with accu rate surface reproduction and without nodules.
3. Chemical stability at high casting temperatures.
4. Adequate strength to resist casting forces.
5. Sufficient porosity to allow for gas escape.
6. Easy recovery of the casting.
81
What are gypsum-bonded investments not suitable for?
* Not suitable for casting **metal-ceramic alloys** because the gypsum is unstable at the high temperatures required and sulfide contamination of the alloy can occur.
82
What are the seven factors that increase expansion of gypsum-bonded investments?
1. Use of a full-width ring liner.
2. Prolonged spatulation.
3. Storage at 100% humidity.
4. Lower water/powder ratio.
5. Use of a dry liner.
6. Use of two ring liners.
7. Hygroscopic technique with the pattern in the upper part of the ring.
83
Gypsum-bonded investments are more stable at higher temperatures in comparison with Phosphate-bonded investments. True or False?
* False. Phosphate-bonded investment materials offer certain advantages over gypsum-bonded invest- ments. **They are more stable at high temperatures** and thus are the **material of choice for casting metal-ceramic alloys**. They expand rapidly at the temperatures used for casting alloys, and their expansion can be conveniently and precisely controlled.
84
What are the three factors that result in increased expansion for phosphate-bonded investments?
1. Heat from the setting reaction softens the wax and allows freer setting expansion.
2. The increased strength of the material at high temperatures restricts shrinkage of the alloy as it cools.
3. The powder mixed with colloidal silica reduces the surface roughness of the castings and also increases expansion. Thus, expansion can be conveniently controlled by slightly diluting the colloidal silica with distilled water.
85
Castings made with phosphate-bonded investments are rougher than those made with gypsum-bonded investments and are more difficult to remove from the investment. True or False?
True
86
Once the investment is heated during the wax-elimination procedure, heating must be continued, and casting must be completed. Cooling and reheating of the investment can cause casting inaccuracy because the refractory mold and binder do not revert to their original forms (hysteresis). True or False?
True
87
About how many grams of noble metal alloy is needed for premolars, molars, or pontics?
* With a high- density noble metal alloy, 6 g (4 dwt\*) is typically adequate for premolar and anterior castings, 9 g (6 dwt) is adequate for molar castings, and 12 g (8 dwt) is adequate for pontics.
88
When removing a nodule on the finished casting, you should remove slightly less than the defect to ensure complete seating of the restoration. True or False?
False. Should be slightly more.
89
What two areas of the margins are the two most problematic for plaque control, and can only be finished and contoured in intraorally?
* Interproximally and Sub-gingivally
Fixed Prosthodontics
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