Putting the metal into RPDs

Question 1

what is the lost wax technique

Answer 1

• Old technique, done for centuries
  ○ Precise
  ○ Useful
  ○ Accurate
• Whereby a wax shape made
• a mould is made to surround the shape
• The wax is removed from the mould by melting the wax
• The shape is filled with molten metal

Question 2

why is the lost wax technique of casting different for producing other denture castings compared to RPDs

Answer 2

This type of casting is different for producing other dental castings since this method involves a refractory model whereas other casting (crowns, bridges or implant bars) are waxed on a definitive cast and cast directly from the cast itself

• CoCr is designed on a model
• Then removed from the model
• Then made into cast
• Little difference between the techniques

Question 3

what is the refractory model made from

Answer 3

phosphate-bonded investment material

Question 4

what is the problem with using phosphate-bonded investment materials

Answer 4

phosphate-bonded investments are stronger than gypsum-bonded investment
BUT
on heating they are subject to surface wear and must be treated by hardening the surface of the model
○ Not as strong at the surface
○ Wear easily
○ Be careful - can chip and break easily

Question 5

what materials can be used to harden the refractory model

Answer 5

○ 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

Question 6

how is the model hardener procedure carried out

Answer 6

• 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 to the cast
○ Important to create a precision framework
○ Any dimensional changes means the overall result would prove to be inaccurate

• The model is allowed to cool (15-20 minutes)
○ If you place wax straight onto the hot model it will melt so it needs this time to cool

• The wax pattern can be placed directly onto the model surface

Question 7

what are wax patterns

Answer 7

• Profiled wax
• Self assemble CoCr model box
• Design something on your own - put wax into your model

Question 8

what are the stages in making the maxillary plate

Answer 8

• Stage 1
○ Fill the space made by the preparation wax for the internal finishing line
§ Looking to start where there are potential weak points with the CoCr framework - occlusal rests?
§ Mid palate - deep vault that the alloy could easily vault - needs extra wax

• 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

Question 9

how can the palate be adapted

Answer 9

• A wax sheet of 0.5mm is used and adapted in 2 stages, I and ii
○ Appropriate thickness for CoCr base
• The second sheet is placed overlapping the first sheet and using a fine blade the knife is drawn down the junction and the 2 pieces fitted together, iii
○ Deep vault in the palate - once you have wax in there it is likely to thin too much
○ Using 2 sheets stops this problem Overlaps first shape
• The 2 halves are adapted and lightly burnished to remove any visible junction, iv
○ Use instrument to burnish down junction

Good technique for deep palate

Question 10

how is a wax retention joined

Answer 10

attached close to the major connector and joined using joining wax
an external finishing line is added using round profile wax smoothed against the connector (represents where the acrylic will be finished when it comes to putting it into the CoCr)

Question 11

what is advisable for anterior teeth for retention

Answer 11

advisable to use posts as retention as space in this area can be limited for acrylic retention
can be on the fence with this idea
space can be a huge issue when setting teeth -having to over reduce the anterior can increase the translucency and then there is potential to see the metal posts through the teeth

Question 12

how are clasps included in this design

Answer 12

the appropriate wax pattern is selected and positioned on the prepared ledge

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

mesh is very thin - molten alloy will travel through and the clasp can be miscast so filling in the channels gives a better success rate

Question 13

what does a sprue do

Answer 13

A sprue conducts the molten metal into the mould

Question 14

where is a sprue attached

Answer 14

The sprue is attached to the thickest area and closest to the ingress of metal
○ The ingress of metal is where the metal is getting cast

sometimes parts are closer to the ingress of metal but they are thinner so would not support the sprue requirements

attached to a plastic cone which will be aligned in the casting machine
the plastic cone creates mould for the metal to pour into
the other wax attached to the cone is to stabilise it

the cone and sprue attachment must be higher than any part of the wax pattern

Question 15

how should the casting cool

Answer 15

• The casting should progressively cool from the exterior to the centre of the metal inflow
• This should prevent cooling shrinkage producing voids in the casting
Important to follow these processes

Question 16

why is the placement and number of sprues important

Answer 16

important to prevent air turbulence or air being trapped which would cause voids or porosity within the casting

Question 17

explain investing

Answer 17

• A mould is prepared around the refractory cast using the same investment as the refractory
○ Phosphate bonded material used to close that pattern

• A mould (muffle) former consists of a base and ring shape to hold the investment
○ Stick cast to the base
○ Ring shape goes around it

• Phosphate bonded investments do not require a supporting metal ring
○ Other casting methods for chrome shell crowns or other things need the support

• One hour after setting the mould can be placed into a furnace
○ These muffle formers are removed and the plastic is removed
○ The investment materials need to cool down - exothermic

Question 18

what should be noted for investing

Answer 18

The height of the sprue former is higher than any part of the casting and the casting increases in thickness from pattern to sprue to button (cone)
§ Make sure the sprue is higher than the rest of the wax

Question 19

how should the furnace be used

Answer 19

• The temperature of the furnace is increased from 0°C to around 1100°C in approximately 3-3.5 hours
○ Need to be present for the length of this time
○ Be available to cast at the right time

• The mould is held at the pre-casting temperature to ‘heat-soak’ for 30 minutes to ensure even the thinnest section of the mould is hot
○ To receive the alloy

• Failure to ensure this will mean the metal freezes on contact and the mould is not filled
• Cobalt chromium alloy melts around 1400°C
• The mould must be hot to receive the molten metal or the mould will expand and crack or explode- The investment material

Question 20

what happens in pressure casting

Answer 20

• High frequency induction casting machines use pressure and high temperatures to cast precious, semi-precious and base metal into specific shapes, such as a frame for a crown or bridge, or for a cobalt chrome partial denture
• These machines are designed to make casting precise parts for dental restorations as simple as repeatable as possible, and some automate much of the process, including melting the metal

• Other casting machines are less complex and require the metal to be melted before it is added to the chamber
Whether automatic or manually operated these machines then us centrifugal force to cast the liquid metal

Question 21

what should be done once the casting is completed

Answer 21

• When the casting is completed the mould is allowed to cool (manufacturers guidelines differ for different investments)
○ Wait a long time for it to cool

• The cooled mould is removed from the metal by tapping the residue of the casting button
○ Where the ingress was - metal button
○ Investment material break away

• Once the mould is removed from the casting is cleaned using a sandblasting cabinet with aluminium oxide (300 microns particle size)
30-40 minutes

• Remove Investment material
  ○ Keep tapping on material
  ○ Can see nothing with the investment material on
• Vibrations can break off elements of the framework
  ○ Use sandblaster
• Sprue is cut off

Question 22

what should you look for when inspecting

Answer 22

• 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
• Make sure you don’t have these little oxides on the fitting surface
  ○ Common
  ○ Should be removed by the technician

If sandblast doesn’t remove these then fine blast should remove it

Question 23

how is the fit of the base refined

Answer 23

• Any abrasion of the cast must be attended to by the technician
• Although the hardest gypsum plaster is used for metal base dentures this can be abraded
○ tooth enamel is much harder and will not ‘give’
○ Gypsum will break easily but still should scrape down enamel
• Might have to reduce a little to fit the patient
○ Check model, try and identify these areas
• Don’t bend clasps - makes the whole appliance worthless
○ Removes retention
• Reduce a little to try and get it try and fit
○ Either get pros lab to come and help at chairside or can bring it down to the lab

So much work involved to get tp txhis stage so don’t want to wreck it at this point

Question 24

what does space on the occlusal rest mean

Answer 24

something has warped or moved
cause a problem
difficult to fix but can be done

Question 25

what is the preparation for electro-polishing

Answer 25

• Strip surface layer away from the alloy
• Makes it shiny
• Can reduce thickness a little
  ○ Wax on occlusal rests / clasps when doing this as don’t want it getting any thinner and this stops the thinning
• Do this then fit it to the model

Some prefer to wait for this layer to be removed before fitting it to the patient

Question 26

do you need to polish retention?

Answer 26

this is debatable
Put acrylic over the top of it so not needed but some people like to present something that looks nice but can argue that polishing might make the metal shine through or produce retention
Doesn’t really matter