Metal framework construction

Question 1

what is the lost wax technique?

Answer 1

• used for casting precise metal framework for RPDs
• the wax is completely burned out, creating a void without any wax residue
• molten metal alloy fills the space

Question 2

what are the disadvantages of the lost wax casting technique?

Answer 2

• cobalt chrome material shrinks on cooling so must bear in mind
• molten metal may not enter the mould cleanly so the framework may be incomplete
• risk of porosity if the molten metal is overheated which can cause excessive corrosion, leading to structural weakness

Question 3

what can be used to harden the surface of the refractory model?

Answer 3

• beeswax - model is heated and dipped into hot wax
• resin - model is heated and dipped in to cold resin
• aerosol - cold model is sprayed

Question 3

what is the refractory cast/model? what is it made of?

Answer 3

• a model made from heat resistant materials
• made from phosphate-bonded investment material
• stronger than gypsum-bonded investments on heating and must be surface hardened

Question 4

what is the process of surface hardening? what is a potential disadvantage of this?

Answer 4

• heat model to 200 degrees for 45 minutes
• dip into model hardened for 10 seconds
• the hardened is absorbed into the model so no dimensional change
• cool for 15-20 minutes
• but hardener can leave a glaze over the model and can reduce surface detail

Question 5

how is the design of the denture made?

Answer 5

wax patterns with wax designs of clasps, major connectors and molten wax then the lost wax technique is carried out

Question 6

how is the maxillary plate design made from wax?

Answer 6

1. fill the space made by the preparation wax for the internal finishing line (A,B,C)
2. the plate will be 0.5mm, but a sheet of 0.2mm is adapted over the centre of the plate to provide strength and resistance to flexture (D)
3. a wax sheet of 0.5mm is applied and adapted in two stages (i, ii)
4. the second sheet is placed overlapping the first sheet and a blade is drawn down the middle and the pieces are fitted together (iii)
5. the two halves are adapted and lightly burnished to remove any visible junction

Question 7

how is retention design made from wax?

Answer 7

1. the wax pattern is chosen and attached close to the major connector and joined using joining wax (A)
2. an external finishing line is added using round profile wax smoothed against the connector (B,C)
3. for anterior saddles or single teeth, posts are used for retention as space is limited for acrylic retention

Question 8

how are clasps made from wax?

Answer 8

1. the appropriate wax pattern is selected and positioned on the prepared ledge
2. the mesh has been filled to the clasp attachment to join with the major connector, ensuring that the clasp is fed from the thickest part of the casting and not the mesh
3. then the wax pattern is sealed to the cast after all thicknesses are checked and all components are connected appropriately

Question 9

what are sprues in casting?

Answer 9

• a sprue conducts the molten metal into the mould
• they are attached to the thickest area (external finishing line) and closest to the ingress of metal
• it is attached to a plastic cone which is where the metal ingress is
• the cone and sprue attachment must be higher than any part of the wax pattern

Question 10

what is an investing mould?

Answer 10

• a mould (muffle) is prepared around the refractory cast using the same investment as the refractory cast
• consists of a base and ring shape to hold the investment
• one hour after setting, the mould can be placed into a furnace

Question 11

describe the heating process of casting

Answer 11

• temperature of the furnace is increased from 0 degrees to around 1100 degrees in approx 3-3.5 hours
• the mould is held at the pre-casting temperature (1100) to ‘heat-soak’ for 30 mins to ensure the thinnest section is hot
• if not, the metal freezes on contact and the mould is not filled
• cobalt chromium alloy melts around 1400 degrees
• the mould must be hot to recieve the molten metal or it can expand, crack and explode

Question 12

how do high-frequency induction casting machines work?

Answer 12

• use high pressure and temperatures to cast metals into specific shapes
• designed to make casting precise parts for dental restorations as simple and repeatable as possible
• then uses centrifugal force to cast the liquid metal

Question 13

describe the cooling process after casting is completed

Answer 13

• the mould is allowed to cool
• the cooled mould is removed from the metal by** tapping the residue** of the casting button (cone)
• once the mould is removed, the casting cleaned using a sandblasting cabinet with aluminium oxide

Question 14

what are air bubbles on the cast caused by? how can they be removed?

Answer 14

• caused by possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast
• removed with a sandblaster or smooth stone

Question 15

what is shown here?

Answer 15

abasion of the cast - must be attended to by the technician (tooth enamel is much harder and will not ‘give’)

Question 16

how can you remove rough areas on castings?

Answer 16

different burs can be used for trimming

Question 17

what should you do after trimming?

Answer 17

transfer the framework to the master cast to check the fit of the metal casting

Question 18

what is electro-polishing? how can you prepare for it?

Answer 18

• electrochemical process that uses electrical current to reduce the thickness of the cobalt chrome material
• in thinner areas (e.g. rests, clasps), wax can be added for protection to prevent these areas from weakening
• casting connected to anode, and cathode of stainless steel
• the anode reduces its size which produces a polished surface on the impression surface

Question 19

how is the non-impression surface polished?

Answer 19

with rubber cylinders and wheels