finishing/flasking Flashcards
why must a wax trial look presentable
- as its goingto be tried in the patient’s mouth
- it won’t have good retention however as it is wax
what type of technique is used for processing the denture
- a ‘lost-wax’ technique
what is used in the processing of a PMMA denture
- metal dental flask is used
what happens during finishing/flasking
- a two part closed mould is made, enclosing the wax denture using dental plaster
- the two part mould is opened and the wax is evacuated from the mould
- the space left is filled using PMMA
what happens during investing of the denture
- the waxed denture is sealed onto he duplicate cast and embedded in dental plaster into the deeper half of the denture flasks
- the wax palate is left exposed
- on setting the plaster mould is coated with a separating medium and mould completed
- following setting the dental flask is immersed in hot water for 10 mins to soften wax
- the flask is separated and the wax evacuated with boiling water
what is a separating medium often used
- sodium silicate
what type of technique is used during investing the denture
- hooded technique
why must you ensure you font have any air blows
- as acrylic will fill this
what is not good about the hooded technique
- not ideal as can’t see the buccal surface so don’t know if all the wax is gone or not
- need to make sure that when you add the acrylic its pushed as buccally as it can be
what is another method of investing the denture
- plaster over plaster dentate
how is the mould filled/packed
- the plaster surfaces and cast are coated with a ‘mould seal’ to prevent plaster adhering or penetrating the denture
- when the mixed PMMA has reach dough stage it is packed into the mould with special attention being paid to packing the mould around the teeth
- the mould is closed under pressure to expel excess material which on re-opening is removed
- trial packing
what is the procedure for packing
- on setting the mould is placed in hot water for 10 mins to melt the wax inside the flaks and to assist the opening of the mould
- on opening the flask the wax is evacuated with boiling water and the mould cleaned with a detergent to remove any wax residue
- the two halves of the mould are coated with a mould seal and PMMA is packed into the mould at appropriate dough stage
- flask opened and excess removed
- two halves removed and placed into a spring compress and processed
why is a trial of the packing stage conducted
- to ensure the mould is completely filled
what is used to mix PMMA
- powder = polymer
- liquid = monomer
- hydraulic press
what must you be careful of with the monomer
- it is very corrosive so needs to be in a controlled environment
what is the ratio for mixing of PMMA
- 10mls to 24 grams
what can happen is there is too much powder used in mixing of PMMA
- granular porosity
what is contraction porosity
- when there is not enough pressure applied or when there is not enough acrylic as it expands to fill the spaces and then contracts
what happens if the temperature is too high during PMMA mixing
- if the temperature is too high then the monomer boils too quickly and causes bubbles leading to gaseous porosity
- needs to be heated slowly
why must you bench cool when mixing PMMA
- if try to cool it by placing it in cold water then the internal stresses will cause it to fracture or crack
why is it unacceptable for patient to go away with porosity dentures
- unhygienic
Where is porosity most common
- thicker areas of the denture
how is the denture removed following processing
- can be tricky and messy
- the plaster is removed from the mould and the plaster mould removed from the denture and cast
- can saw away the mould from cast
what is the cold cure method
- use lab putty with an acirttvaot and mould over the wax trial on the denture
- then remove putty and boil off the wax
- stick the teeth to the putty
- now have a cavity for space to pour acrylic into
- place flask under pressure and cold water
what is good about cold cure method
- very fast way of creating a denture
- 5-10 min set
what are the negatives of cold cure method
- acrylic is often softer this way then heat cure
- not polymerised correctly
- will wear down quicker then heat cure denture
- acrylic doesn’t look as natural this way as just one single colour
how is occlusion check following processing
- the duplicate model with denture is secured to the original mounting with plaster bandage
- the occlusion is check and refined as required against the opposing dentition
- the incisal post at rest on the incisal table indicated occlusal vertical dimension
- need to use articulating paper to find high points
why might occlusion need altered
- processing can sometimes alter the occlusion and the pin may not be on the table
- it may have been raised
what is done after the occlusion is adjusted
- the duplicate model is removed from the denture by sawing the model from the denture
- the denture is finished and polished
what can be done following polishing
- denture can be fitted to the definitive cast
why must acrylic be finished
- it has lots of sharp corners and angles
- use hand tools
why don’t you do much finishing and polishing on the fitted surface
- this could affect the fit of the denture
- just need to remove any bits that may irritate the patient
what must you do if the clasp scrapes or damages the cast
- needs changes as this will damage patient’s tooth
- need to remove the sharp edges
how should clasp arm sit
- nowhere near the tissue or undercut as will have difficulty getting denture in and out if it sits below the undercut
- clasps should be closely adapted to the cast = hugging the tooth
what is the finishing process in brief
- the denture is fitted to definitive cast
- the anterior flange should have a thin edge to blend with the surrounding soft tissue
- the denture is finished above the surgery line
- the post dam and palatal periphery is thinned to blend with the plate
- the occlusal vertical dimension must be the same as the original restoration
- there should be no sharp edges
- the denture should be policed only on the non-impression surface
