RPD LAB - PRIMARY IMPRESSIONS & POURING PRIMARY CASTS Flashcards
how many scoops of alginate and how much water for lower impressions
alginate - 3 level scoop
water - 3/4 of water
how many scoops of alginate and how much water for upper impressions
alginate - 4 level scoops
water - 1 cup
what does cold water do to alginate
slows down setting time
what does warm water do to alginate
decrease the setting time
how should you place the impression
place it front first and push back to prevent air blows
how much water and how much dental stone for each impression
25/30ml
100g dental stone
what much of a crust should we have
5mm
why do we saturate
it softens the particles of the dental stone which will cut down the spatulation time
what does an increased spatulation time do
increased spatulation time increases the set of the mix
how do we spatulate
place the spatula against the side of the bowl and rotate the bowl - we want a homogenous, thick creamy mix
what are the steps for the impression in the study cast preparation
- sift in the dental stone into the water and leave to saturate
- spatulate the mix
- fill impression from posterior aspect -
- invert impression
- agitate impression
- gradually refill impression
- clean off excess
- add additional material to create retention
- score lightly into surface for extra retention
why do we fill the impression from the posterior and vibrate rather than just filling from the front?
so we do not trap air into the occlusal surfaces or incisor edges
why do we invert the impression and vibrate the mix back out
to ensure the material is down into the incisal edges and occlusal surface of all the teeth
why do we agitate the impression
so any left over material will fill the occlusal surfaces
what should be the height of the retentions
1cm
why do we score into the surface and when
we score to add extra retention and we do it once the material has set so it looks matte
how much water and dental stone for impression base
30ml water
110g dental stone
50/50 dental stone and dental plaster
what are the steps for the impression base
- agitate mix into impressions
- use material to create base
- invert impression into base
- ensure the handle is horizontal
- remove excess and bring base material to meet the impression
- ensure we remove the material in the lingual aspect of the lower
- remove excess
- remove material overlapping impression
- trim cast
- rinse impressions
- smooth sharp edges
- smooth study cast with sand paper
how do we trim the cast?
we want the occlusal plane to be horizontal and we want to check that the back of the base is 90 degrees to midline
we want the occlusal plane to be 1cm deep in its deepest part
what are alginates based on
alginic acid
why do we use the snap removal technique for alginates
to obtain an elastic response
it ensures that the time for which the material is under compression is as short as possible
why do we want the time the material is under compression to be shorter
the longer the material is under compression the higher the amount of permanent deformation
why must the impression be rinsed after removal from the patient’s mouth
to remove any saliva as this will interfere with the setting of the gypsum model.
why should surface water be removed prior to pouring the model
residual water will dilute the model material and result in a soft surface which is easily damaged.
