Definition: It is the negative replica of the structures of the oral cavity be it hard(tooth) or soft(gingiva, mucous membrane) structures, which are used to fabricate the cast or model of the patient, and for making crowns, dentures, and other prosthesis.


I) Based on mode of setting & elasticity

a) Rigid

i) Reversible (thermoplastic)

  • E.g. impression compound

ii) Irreversible (thermosest)

  • E.g. Zoe impression paste, Impression plaster.

b) Elastic

i) Hydrocolloid

a) reversible

  • E.g. Agar

b) Irreversible

  • E.g. Alginate

ii) Elastomeric impression materials

  • E.g. Polysulfides Polyether, Addition silicones, Condensation silicones

II) Based on type of impression & area of use

A) Dentulous

i) Primary

  • E.g. alginate

ii) Secondary:

  • E.g. Elastomers, Agar

B) Edentulous

i) Primary

  • E.g. Impression compound, Impression plaster, Alginate

ii) Secondary

  • e.g. ZOE impression paste, Elastomers for CD

III) Based on their use in dentistry

A) Edentulous- For complete denture prosthesis

  • e.g. Impression compound, zoe impression paste, alginate, Elastomers

B) Dentulous

i) FPD

ii) RPD

E.g. Agar, Alginate, Elastomers

IV) Based on the amount of pressure applied

i) Mucocompressive

  • e.g. Impression compound

ii) Mucostatic

  •  e.g. Impression plaster

V) Based on the manipulation

A) Hand mixing

i) Kneading

  • e.g. Impression compound – wet kneading, putty consistency Elastomers

ii) Circular motion (with glass slab & spatula)

  • eg. Zoe impression paste, polysulfide

iii) Vigorous mixing

  • e.g. Alginate (figure of 8 motions)

B) Mechanical mixing

VI) Based on the tray used for impression

A) Stock tray

i) Types of tray

a) rim locked perforated

  • e.g. Alginate Elastomers

b) water cooled

  • e.g. Agar

c) Plastic

  • e.g Orthodontic Tray

ii) Based on type of Perforation

a) Perforated

  • e.g Alginate, Elastomers

b) Non Perforated

  • e.g Impression Compound

B) Special Tray

i) base plate wax

  • e.g. Zoe impression paste, medium body

ii) resin