LABORATORY WORK Flashcards
- A table is given
Anson: I think the question was to complete the table.
a. The aesthetic and framework try-ins were satisfactory. However, during denture delivery, you found an occlusal discrepancy of more than 3mm. What technical errors might have caused this, and what advice will you give to the laboratory to prevent this? (4)
Processing errors (e.g. packing of acrylic) + what measures can be do to minimise these errors
- Laboratory procedures for casting cobalt chromium, after master cast has been surveyed (5)
Unwanted undercut are blocked out of master cast
Duplication of master cast using agar x2: refractory cast on which waxing up is done, and a duplicated master cast for adjustment of the framework and the partial denture
Waxing up of framework on the refractory cast (investment cast) using pattern wax/resin and sprue former is attached, then invested
The mould is heated to 950oC for 2-3 hours to burn out the wax leaving a cavity in the mould, drive away water vapour and expand the mould
The mould is transferred to another unit where the metal (e.g. Co-Cr) is melted by oxygen-acetylene flame or electromagnetic induction and thrusted into the cavity by centrifugal force
The mould is cooled to room temperature for 30 minutes and investment removed. The framework is removed of its oxide layer
Wrought wire clasps may be soldered
Check occlusion using the duplicated master cast, and finish and polish using rubber points etc. and electrolytic brightening
- Outline lab stages for Co Cr framework fabrication after the stage of master cast surveying
- Block out unwanted undercut on the master cast after designing the outline
- Place spacer for acrylic retention mesh
- Duplication of master cast for refractory model and another duplicated model
- Impression material (agar)
- Refractory material (phosphate bonded investment)
- Strengthen refractory model by drying at 100 degree for 45 mins and soaking in beeswax at 140 degree for 15 s
- Outline design on refractory model and make wax pattern
- Sprueing attached to wax pattern
- Embedding the cast in casting ring
- Wax burnout at 900 - 1000 degree (also allow expansion to compensate metal cooling contraction)
- Co-Cr is melted by electrical induction and mould to the cast
- Cooling → remove investment → sandblast with Al2O3 → remove sprueing → electrolytic polish
- Check fitting and occlusal dimension on duplicated model (not master cast)
- Final polishing → delivery
Patient requests replacement of existing denture made 1 year ago. Given: I/O photo of upper cobalt chromium RPD.
a. Please describe the lab procedures for pouring an alginate impression (4)
- Use 2 stage pour technique
- Draw a line outside the impression 5 mm from the deepest part of sulcus extension
- Mix the artificial stone by ratio according to manufacturer’s instruction
- Pour in sufficient stone to just fill the impression and leave the surface rough under vibration and slowly flow to eliminate air entrapment
- Place impression on the tray stand
- When the stone become warm, soaked in slurry water (made by placing set stone debris in a container of water and leaving 48 hours)
- New mix of stone is then prepared as before and a mount built up on a tile
- Impression should be inverted and seated into soft mound of stone until the borders are covered up to the line drawn in the beginning
- Smoothen borders with plaster knife and remove gross excess to minimize trimming time
- Wait 1 hr for setting and remove impression from cast carefully without damaging cast
- Trimming
- Base of cast parallel to occlusal surfaces / vertical wall / 3 mm land area
Given: an upper mirror shot showing a denture made in China no clasps or rest seats, some crowns are present.
a. State the laboratory procedure of heat cured acrylic in making a denture base.
- Wax up the saddle area and set up the teeth on the duplicated master cast
- Sent to clinic for try in wax up
- Flasking and packing
- Embed the cast into plaster while keep intact for area involving saddle and tooth
- After embedding, put the flask in hot water to remove the wax
- Acrylic processing
- Mix the PMMA power and MMA liquid (change from sandy > stringy > dough)
- Pack the acrylic when it becomes dough into space prepared for acrylic base
- Put flask under hydraulic pressure and put in water bath (72 degree for 8 hrs > 100 degree for 1 to 2 hours) and then allow cooling
- Deflasking
- Check occlusion, remove plaster
- Trimming and polishing
- Metal burs, silicone polisher, pumice, Tripoli
Information given: A faulty RPD Design of upper arch of replacement of 21,22,23
Discrepancy: Major connector: lingual bar
Retention: only recriprocal arm present on 17 and 27. No clasp. Support: absence of rest seat to support saddle.
Indirect retention (cingulum rest on 23 and molars) not properly positioned.
a. What is the melting point of Co-Cr
1350 ~ 1370 (while casting temp is 1535)
Information given: A faulty RPD Design of upper arch of replacement of 21,22,23
Discrepancy: Major connector: lingual bar
Retention: only recriprocal arm present on 17 and 27. No clasp. Support: absence of rest seat to support saddle.
Indirect retention (cingulum rest on 23 and molars) not properly positioned.
b. What type of investment will be used for casting?
Phosphate-bonded investment
