Putting Metal Into RPDs Flashcards
what is the lost wax process?
Duplicating casts
- Mould made in investment material
- Wax burnt out or melted
- Filled with melted alloy
Similar to how jewellery made
Precise, useful and accurate technique
why is it important to be skilled when making CoCr?
Different methods
Many stages
- Many variables
Timely
Trained individuals
Can waste time and money if done poorly
- Work is allocated to the whole lab – not so heavy on one team, spread workload
lost wax steps
whereby a wax shape made,
a mould is made to surround the shape,
the wax is removed from the mould by melting the wax and the shape is filled with molten metal (burn out wax)
what is the difference between lost wax technique for metal framework compared to other castings
This type of casting is different for producing other dental castings since this method involves a refractory model whereas other casings (crowns, bridges or implant bars) are waxed on a definitive cast and cast indirectly from the cast itself.
refractory model is made in….
in phosphate-bonded investment material.
It is vitally important that the investment is mixed according to the manufacturers instructions regarding powder/liquid ration and mixing time.
important stage to remember when made the phosphate bonded refractory model
Although phosphate-bonded investments are stronger than gypsum-bonded investments on heating they are subject to surface wear and must be treated by hardening the surface of the model.
Wear easily – chip and break
Use surface hardening to strengthen refractory cast
3 materials used to harden refractory material
Beeswax – the model is heated and dipped into hot wax
Resin – the model is heated and dipped into cold resin
- Most common
Aerosol – the Cold model is sprayed
how is the refractory model hardened?
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
- Want no dimensional change in the model
Good for precision framework made on top of so accurate
The model is allowed to cool (15-20 minutes)
what is placed on the cooled refractory model?
The wax pattern can be placed directly onto the model surface, once fully cooled
what are wax patterns
Profiled wax of different patterns
- Like self-assemble CoCr building box
Stippled wax for upper base plates
Different clasps
Lingual bars
Can use these components or make your own
2 stages in making maxillary plate in wax on the refractory model
Stage one - to fill the space made by the preparation wax for the internal finishing line.
- Fill the space made from the preparation of wax finishing lines
- Add a little bit extra wax in those areas
Occlusal plane – not as supported as base plate
Stage 2 – Although the plate will be 0.5mm a sheet of 0.2mm is adapted over the centre of the plate to provide strength and resistance to flexure
- Middle of palate has a deep vault – melt wax and add 2mm of wax
- Prevent alloy warping – need extra wax for support and strength
width of wax sheet used on palate
0.5mm
where is the second sheet of wax placed
The second wax sheet is placed overlapping the first sheet and using a fine blade the knife is drawn down the junction and the two pieces fitted together, iii.
- Separate the wax down the centre of deep vault to avoid thinning of wax
Ensures better guaranteed thickness
The two halves are adapted and lightly burnished to remove any visible junction, iv.
- Burnish down junction before - casting
what is a wax retention used on refractory model
A wax retention pattern is chosen and attached closed to the major connector and joined using joining wax
- 5 or 6 different types, can also design own
- Grid to hold acrylic
- Join at bottom
For anterior teeth or single teeth it is advisable to use posts as retention as space in this area is limited for acrylic retention
Advisable
Space can be hard when setting acrylic teeth
May alter shade if having to reduce it so then metal visible
how is wax retention finished
An external finishing line is added using round profile wax smoothed against the connector
- Round profile wax – smooth against the connector acrylic will be finished once dentures put on
Clasps placement on wax model
The appropriate wax pattern is selected and positioned on the prepared ledge
- Block out the model
- Make ledge in wax where occlusal rests and occlusal approaching casts have a ledge
Allow you to position exactly where you want them
Note: the mesh has been filled to the clasp attachment to join with the major connector, this ensures the clasp is fed from the thickest part of the casting and not from the mesh which is thinner than the clasp which might result in a miscast
- Cast is spread from thickest part (not the thinner mesh)
- Cast molten alloy would travel through mesh can cool too fast so clasp is incorrectly casted
important technique to use in clasp attachment
Note: the mesh has been filled to the clasp attachment to join with the major connector, this ensures the clasp is fed from the thickest part of the casting and not from the mesh which is thinner than the clasp which might result in a miscast
- Cast is spread from thickest part (not the thinner mesh)
- Cast molten alloy would travel through mesh can cool too fast so clasp is incorrectly casted
function of a sprue
conducts the molten metal into the mould
