Caring waxes and loss wax technique Flashcards
https://www.youtube.com/watch?v=2OTGeRCScYc
What are boxing waxes?
used to retain the gypsum when it is poured into the impression
What are beading waxes used for?
used to block out undercuts
Qhy are waxes ideal?
can be moulded easily to the shape required
can be easily removed after gypsum has set
relatively inexpensive
What temp can boxing and beading waxes retain their shape?
35 dc
What are carving (presentation) wax good for?
diagnostic wax up, show patient
laid down in pt model
more easily visualised by pt
What 2 material scanbe used for presentation wax?
inlay wax and carving wax
Qhat wax is mainly used as the prototype?
inlay
What technique is used for inlay wax?
lost wax technique
constituents oif inlay wax
paraffin (60%)
ceresin
carnaba
candeilla
beeswax
What are the 3 types of inlay waxes?
hard
medium
soft
What is a positive of inlay waxes?
high level of detail and dimensional accuracy required
What does stress releif lead to?
distortion of pattern and final resto will not fit
What temp should you eat wax to?
45-50 dc
20 mins
What colour are inlay waxes?
green, dark blue, purple
When wold you use pattern waxes?
to construct the wax ‘framework’ which then forms the metal skeleton/baseplate
What re the 2 types of inlay waxes?
prefabricated - in the shape of clasps etc
uniform thickness for the baseplate for a denture
What are the properties of preformed wax patterns?
slightly sticky - can be repositioned on model
need to be very ductile
must burn out with no residue below the casting temp of the alloy used
Why do you need to burn out wax?
no residue - no carbon incorporated in
What does a baseplate/modelling wax form?
base of a denture
pink in colour
What do you use the lost wax technique for?
fixed prosthodontic restorations:
inlays
onlays
crowns
bridges
removable prostheses
What temp is the wax pattern designed to burn out at?
below 600dc
What is the initial step in the lost wax technique?
wax pattern is made which corresponds to the shape of the object
What is the goal of the lost wax technique?
wax form to metal tooth to be placed in pt mouth or denture
with similar features to the wax
Describe what is going on here
wax pattern placed over DIE
sprue allows metal to flow in to the mould cavity - under high pressure
What is the die?
tooth prepped for crown
impression
a model of an individual prepared tooth - DIE
- this is a positive impression
take impression and take to lab
What is the crucible formed?
a structure which you place the sprue attached to the wax mould
What has been placed over the crucible and wax casting?
casting ring
open at top end
what is placed inside the casting ring?
casting ring liner
What gets brined out during burn out process?
casting ring liner
wax
Purpose of burnout phase?
not just a means to remove wax from the investment; it also cures the investment in preparation for the thermal shock of the metal entering the pattern cavity.
What is the investment material?
form of alloy
poured into casting ring - before burnout phase
What is the purpose of the resevoir?
compensate of the cooling and contraction of the metal alloy that’s started to cool
What is the metal casting ring lined with?
piece of lining material
aluminium silicate ceramic or cellulose lining paper liner
What is the purpose of the lining material in the casting ring?
help to counteract the effects of the expansion of the investment during the heating phases
Where should the wax pattern be situated in the casting ring?
hottest part of the ring (thermal centre)
longest possible flow time
What happens of the wax is to high up in the casting ring?
rupture the investment causing casting failure
due to air moving through microporosities in the investment
Do you wet the wax before placing in the casting ring?
sprayed with a wetting solution to reduce surface tension and prevent inaccuracies due to air entrapment
What do you pour the investment into the casing ring on?
vibrating plate
What are methods the alloy is heated?
naked flame e.g. oxyacetylene torch (if less than 1200 dc)
or by using electric induction casting machine
What alloys are quenched?
noble alloys
becomes granular and causes it to fracture
facilities burnishing and polishing
What happens if the golf is left to cool?
harder, stronger casting results but this makes the alloy more difficult to finish
When do you remove the casting form the alloy?
after cooling
How do you remove surface oxides form the crown after removing the investment?
sandblasting
What is the advantage of the lost wax process?
high level accuracy
dimensional change less than 0.05%
Why can the cast come out smaller than expected?
due to the mould not having been heated sufficiently
Why may the casting be distorted?
can be due to the wax pattern being damaged during the investing process or by incorrect sprue placement preventing the metal flowing to the extremities of the mould
How can you prevent distorting\/
thickening of the wax margins
How would you melt base metal alloys?
electric induction heating
melt at 1200-1500 dc
What happens if the alloy is cooled quickly?
coring
this is variable metals in the same alloy
electrolytic metals and prone to corrosion/weakness
How to overcome coring?
homogenisation treatment
reheat alloy and cool slowly
gaseous porosity?
dissolve oxygen when heated
metal voids
oxygen incorporated when alloy is heated/molten
