Chapter 17- Impressions Flashcards
Why are impressions made?
In order to make a cast
What are casts used for?
To make preparations
An impression for cast should meet what 3 requirements?
- Exact duplicate of the prepared tooth along with unprepared tooth structure beyond the prep
- Other teeth and tissue adjacent to the cast should be accurately produced to permit proper articulation of the cast
- Free of bubbles
In terms of wet ability, impression materials can be described as?
Hydrophilic or hydrophobic, hydrophobic cannot become wet
Greater contact angle increases the probability of?
Air entrapment during pouring
What can repel materials with a large contact area?
Hemorrhage or fluid in the gingival sulcus
Which materials are the least viscous?
Light bodied polysulfide and condensation silicone
Which materials are the most viscous?
Heavy bodied polysulfide
Does viscosity increase or decrease following mixing?
Increase
How does shear rate affect viscosity?
Lower pressure means less viscosity.
What conditions would this be seen in?
A syringe
What is the term for a material that becomes more fluid as flow rate increases?
Thixotropic
Shear rate for mixing material is how many times greater than mixing?
100x
What are the cheapest impression materials?
Hydrocolloid
Most expensive?
polyvinyl siloxane
