Framework construction Flashcards
What is the Lost-Wax technique
In this process, the wax, which is the model of the cast item, is completely burned-out, creating a void without leaving any residue, and then molten metal alloy (for instance cobalt chrome) fills this space during the casting process
What is a refractory cast
Model madein Phosphate-bonded investment material
Stronger tha gypsum so can withstand more heat
What materials can be used to harden the refractory model
Beeswax – the model is heated and dipped into hot wax
Resin – the model is heated and dipped into cold resin
Aerosol – the Cold model is sprayed
What is the process of hardening the model
The model is heated to 200°C for 45 minutes and then dipped for 10 seconds in the model hardener
The hardener is absorbed into the model – no dimensional change takes place
The model is allowed to cool (15-20 minutes)
The wax pattern can be placed directly onto the cold model surface
What is a sprue
A sprue conducts the molten metal into the mould
The sprue is attached to the thickest area and closest to the ingress of metal
What should prevent shrinkage and voids
The casting should progressively cool from the exterior to the centre of the metal inflow
How is the molten metal formed on the cast
First the wax pattern in formed on the cast in the desired shape of the metal framework
A sprue is then attached to the thickest area closesr to the ingress of metal and a plastic cone is attached
The cone and sprue attachment must be higher than any part of the wax pattern
Then the a mould is prepared around the refractory cast to hold the phosphate bonded investment cast material
The plastic cone is removed and one hour after setting the mould can be placed into the furnace (this melts and evapourates the wax)
The temperature of the furnace is increased from 00C to around 11000C in approximately 3-3.5 hours
Once at the correct temp the molten wax is poured in
Why does the mould need to be held at the precasting temp to heat soak
To ensure even the thinnest section of the mould is hot
Failure to ensure this will mean the metal freezes on contact and the mould is not filled.
Cobalt chromium alloy melts around 14000C
The mould must be hot to receive the molten metal or the mould will expand and crack or explode.
What may cause air bubbles in the metal framework
Air bubbles on the surface of the cast caused by possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast
How is the framework pollished
The casting is connected to the anode with a cathode consisting of stainless steel.
The anode reduces its size which produces a polished surface on areas on the impression surface that cannot be hand polished accurately.
The non-impression surface is polished using rubber cylinders and wheels.
