Waxes And Casting Flashcards
Requirements for inlay wax
On softening, should be able to mould into homogenous mass without formation of lamination or flakes
Low thermal co efficient of expansion
Wax should burn out of mould without leaving residue
What type of wax do you use for direct technique inlay
Type 1 wax
Wax colour should contrast with hard and soft tissue
Fully mouldable at temperature above that of mouth, hard at mouth temperature so that can withdraw without distortion
What wax do you use for indirect technique inlay
Type 2 wax
Should not chip or flake on carving to a fine edge at room temperature
Low solidification temperature, melt at low temperature
Solidification temperature should not be too low so that will not flow during carving
What is the main component of inlay wax
Paraffin wax which has low melting temperature
Properties of wax
Low thermal conductivity, take time to heat uniformly and to cool
High clt hence shrinkage errors when direct wax pattern is made, especially cooling under pressure
Elastic modulus ????
Adequate flow at specific temperatures to record detail, low flow at other temps for dimensional stability
Describe the flows of type 1 and type 2 wax
Type 1: flow less than 1% at 37ºC
Type 2: flow less than 1% at 30ºC
Both minimal flow at 45ªC
How should you soften wax for direct inlay technique
Soften wax using annealer or flame (use hot air above flame)
What are wax sprues used for
Venting sprues to allow air to escape during casting, prevent formation of back pressure porosity
What should you fill core of metal sprie with and why
Fill with sticky wax to prevent distortion on spruing and to increase surface contact between external surface of sprue and wax pattern to strengthen attachment
What happens if you use a corrosive metal for a sprue
Contaminate alloy which will absorb contaminants, casting contaminated
What is the diameter of the sprue
1-2.6mm
What is a wax reservoir
Small amount of wax attached to sprue 1mm away from pattern , especially for small sprues
What is the function of wax reservoir
Prevent localised shrinkage porosity at junction of wax pattern and sprue
Molten metal at the reservoir last to solidify, immediately fill the voids in the mould at legit wax pattern due to shrinkage
What happens if sprue want large
Metal cool down quickly for gases from wax in the mould to be eliminated especially dense investments, result in back pressure porosity
What is back pressure porosity
Incomplete casting, casting with rounded porosity, full wall of mould space not in contact with metal
Where should be sprue be placed
Bulkiest portion of pattern away from margins
Eg marginal ridge for class 2
Cannot place at stamp cusp
Why do you add wax at the point of contact between sprue and wax pattern
Flare the area so that metal flows into mould more evenly through wider neck, reduce porosity at point of contact
Why is sprue placed at 45º
Minimise turbulence and time because hit both walls at same time
How long should sprue be
Not more than 6.5mm away from open end of ring. If investment is too thick, result in back pressure porosity. If investment too thin, molten metal shoot through and crack wall
When should you invest
Immediately after removal from mouth or die
Why should you keep repair and carving of inlay wax pattern to a minimum
Introduce stresses that will distort
Requirements of investment material
Harden in relatively short time
Produce smooth surface and fine details and margins on casting
Must not decompose and give off corrosive gases
Porous so that gas can escape
High temperature strength
Sufficient expansion
What happens if investment gives off corrosive gases
Cast absorb gases and become brittle and darkened in colour
How to remove gyosum bonded investment for gold alloy
Throw in cold water and investment break down
How to remove phosphate bonded investment
Does not break dow, remove using sand blaster
At what temperature should maximal thermal expansion be atained
Temperature not greater than 700ºC
When do you get significant amount of metal thermal shrinkage
Solid state to room temperature
What is the expansion gypsum bonded investments must reach
1.25 +- 0.1% which is the shrinkage of gold alloys
What is the binder in gypsum bonded investmeent
A-hemihydrate
25-45% of investment material, provides rigidity to investment material
What is the refractory in gypsum bonded investment
Silica
Regulate thermal expansion, provide high temperature strength
How does refractory cause thermal expansion
When silica heated, change in crystalline form from alpha to beta. Decrease in density and resultant increase in volume
Compare expansion of quartz and cristobalite
Quartz has lower expansion during conversion
Role of copper and carbon in gypsum bonded investment
Reducing atmosphere to reduce amount of oxides formed, improve casting quality and prevent darkening
What is the role of modifier in gypsum bonded investment
Eg boric acid, sodium chloride
Prevent most of the shrinkage when heated above 300ºC. If no modifier, reduce amount of shrinkage so that less absolute shrinkage
3 types of gypsum bonded investment
Type 1: casting of inlay/crown, thermal expansion
Type 2: casting of inlay/crown, hygroscopic expansion
Type 3: investment of denture metal bases with gold alloys, thermal expansion
Seraphina is an example of
Phosphate bonded investment
How does silica cause expansion of investment
Interfere with intermeshing and interlocking of crystals as they form
Range of hygroscopic expansion
1.2-2.2%
What must the thermal expansion of type 1 gypsum investment be
1.0 to 2.0%
What must the thermal expansion of type 2 gypsum investment be
0.0 to 0.6% because expansion should be mainly hygroscopic
What are the two components that form the binder in cera fina
Phosphate bonded investment
Ammonium diacid phosphate (soluble in water to form phosphate ion)
MgO2 (react with phosphate ion at room temperature)
What provides green strength to phosphate bonded investment
Green strength is rtp strength.
Magnesium ammonium phosphate
How does phosphate bonded investment get high temperature strength
Remaining phosphate react with silica at high temperature to form silico phosphate
How does viscosity change as spatulation continues
Water is formed, lower viscosity of mix
What is carbon added for in phosphate bonded investment
Produce clean casting and facilitate separation of casting from investment
What type of alloys should use carbon free phosphate bonded investment
Base metal and silver palladium alloys (fusion temperature above 1504ºC)
Only use for gold alloys
What happens if you use carbon in investment for base metal alloy
Embrittle alloy due to formation of carbide
Why is there thermal shrinkage between 200-400ºC when phosphate investment mixed with water
Decomposition of binder magnesium ammonium phosphate, with evolution of ammonia
What happens when you mix phosphate investment with silica sol instead of water
Silica sol is special liquid
Higher setting and thermal expansion than when mixed with water
No thermal shrinkage from decomposition of binder
Mixed investment can expand hygroscopically (negligible when only water used)
Increase strength of investment
Should you hand invest or vacuum invest phosphate investment
Hand invest
What is green shrinkage of silica bonded investment
Drying of gel below 168ºC, loss of water and alcohol, volumetric contraction leading to reduced size of mould
Why does silica bonded investment have significant thermal expansion
Both binder and refractory are forms of silica that can invert during heating
What happens when you heat silica investment over 700ºC
Polysilicic acid gel change to silica with resultant shrinkage
What is the refractory and binder for titanium investment
Magnesium oxide as both refractory and binder
Or
Zirconia particles held together by zirconium acetate
Why is it hard to get consistent casting for titanium
Formation of titanium oxide
Water immersion vs control water added technique
Soak ring under water and cause maximum expansion vs calculate amount of water for desired expansion
How does type of silica affect thermal expansion
Cristobalite expands to greater extent than quartz
Need significantly more quartz to counterblaance contraction of gypsum during heating and casting shrinkage (>75%)
How does water powder ratio affect thermal expansion
Greater water powder ratio, less thermal expansion
What do chemical modfiers do
Increase expansion without excessive amount of silica by eliminating contraction caused by gypsum below 700ºC
What happens when investment cools after first heating
Contracts to less than original dimension because of shrinkage of gypsum when first heated
On second heating reach same maximum but internal cracks may develop
How does water powder ratio affect strength of investment
Greater water powder ratio, lower strength
How do chemical modifidiers affect strength
Increase room temperature strength because more gypsum binder can be added without marked reduction in thermal expansion
How does particle size affect porosity of investment
More unfirom particles, greater porosity
How do particles affect hygroscopic expansion
Finer particles, more hygroscopic expansion because more particles per unit volume
How long should you heat investment for
60 to 90min
What temperature should you heat type 2 investment to
Keep thermal expansion to minimum 482ºC
What happens if you heat gypsum investment to too high temp
Give off sulfur gas which contaminates the gold colour, cannot be removed by polishing and embrittles casting
Casting rough due to disintegration of investment
Early wear of rings and heating element
Rate of heating for investments
Type 1 slowly heat from rtp to required temp
Type 2 can heat up slowly or place in pre heated oven
What happens if heating is too rapid (of investment)
Flaking/fracture of investment due to steam pressure
Cracking due to uneven heating esp cristobalite
Alter size of casting esp quartz
