For accurate replica reproducing intra and extraoral tissues, impression materials are able to transform into a rubbery or rigid solid in the mouth within
To produce accurate replicas of intra and extra oral tissues, the impression materials should be:

Sufficiently fluid to adapt to the oral tissues. 
Viscous enough to be contained in a tray. 
Able to transform (set) into a rubbery or rigid solid in the mouth in a reasonable time (less than 7 min). 
Resistant to distortion or tearing when removed from the mouth. 
Dimensionally stable long enough to allow one or more casts to be poured.  
Biocompatible.
Cost-effective in terms of time as well as the expense of the associated processing equipment.

Phillips 12 ed page no 152
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