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
