Final Material Flashcards
What is the purpose of the sprue?
- 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.
What are the most common sprues made out of?
- Wax
- Plastic (Rigid)
- Hollow Plastic (Allows for better escape of the wax during burnout)
- Metal
What are the different sizes of sprues and which ones are best?
- 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
Why is a larger sprue sometimes disadvantageous?
- 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.
Is a sprue with a larger or smaller gauge going to be bigger?
- The larger the number gauge, the smaller the diameter
At what angle should the sprue be placed on the wax?
- 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.
What are the keys to attaching the sprue to the wax pattern?
- 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
What is the crucible former?
- 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
What is the casting ring?
- 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.
What are the nine steps of the investment procedure?
- 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
What two components is a dental casting mold made of? And what purpose do they each serve?
- Refractory Material
- A material that doesn’t burn up, heat resistant, but helps in the expansion. It is silica.
- Binder
- Provides Strength
What are the four modifiers of investments (dental casting molds)?
- Accelerators
- Retarders
- Dimension Control
- Coloring Agents
What are the three main types of investment?
- 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)
What are the main characteristics of gypsum bonded investments?
- 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
What two things contract and what two things expand in regards to the investment process?
Contract
- Wax Pattern
- Cooling Alloy
Expand
- Stone
- Investment
- Setting
- Hygroscopic
- Thermal (Oven)
- Less H2O
- Casting Ring Liners
A casting that is slightly undersized is advantageous for accurate seating. True or False?
False. Should be slightly oversized.
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?
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.
What are the factors that increase expansion in gypsum bonded investments?
- ↓ 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
What are the factors that reduce expansion with gypsum bonded investments?
- Add water to the mix
- Eliminate the casting ring liner
What are the characteristics of phosphate bonded investments?
- 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
When are ethyl-silica bonded investments used?
- Used for high-melting base metal alloys
- Partial denture frameworks
What does quenching do to a casting?
- 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
What are the main six types of casting alloys?
- 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
What is the fusing temperature of gold vs metal-ceramic alloys?
- Fusing temperature = Melting point of the alloy
- Au alloys: ± 1700°F (925°C)
- Metal-Ceramic Alloys: ± 2550°F (1400°C)
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
How many ounces is 20 dwt (pennyweight)?
1 oz. 12 oz. to 1 lb.
What are the six different types of gold alloys and what are their percentages?
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
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.
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)
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.
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)
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.
The roughness of the intaglio surface of the crown, does that aid in retention? The roughness of the prepared tooth?
Yes. No.
What material leaves a sticky unpolymerized oxygen inhibited layer?
- Bis-GMA = Bisphenol A-glycidyl methacrylate