6Desirable Properties of Impression Materials Biocompatible: non toxic- non irritant.Acceptable to the patient: pleasant odour, taste, esthetic colour.Ease of manipulation, reasonable cost.Reasonable setting time.Adequate shelf life.Adequate strength: not tear or break on removal from mouth.
7Compatible with model materials. Dimensionally stable: neither expands nor contracts.Adequate flow properties: to register fine details.Readily disinfected without loss of accuracy.
8Classification Non Elastic Elastic According to the ability of a set material to be withdrawn over undercuts.Non ElasticElastic
15TypesType I: low fusing compound softens at °CSupplied in the form of sheets, or cakes used as: impression for edentulous patients.Or in the form of sticks or cones or cylinders.Used as: impression for a single tooth preparation as in (crowns), known as Copper Band Impression.
18Manipulation Type II: High fusing compound softens at 70°°C. Used as tray material to hold a secondary impression materialManipulationHeating:In a water bath-soaking in water in cases of sheets and cakes.Over a flame in case of sticks.Avoid over heating.Kneading: with the fingers.Importance: assures uniform heatingIncreases the flow.Gives workability.
27ManipulationThe compound disc is completely immersed in a water bath at C for about 4-5 minutes to ensure complete softening. Gauze is placed at the bottom of the water bath to prevent adherence. However if it is left in too long some of the constituents may be leached out into the water bath, altering the properties of the material (it is often the plasticizer stearic acid that is leached out). If the compound is kneaded water will become incorporated and act as a plasticiser. Kneading of the compound will incorporate water into the mass and thus increase the flow in the material (water = plasticizer).The compound is loaded on to the tray and firm pressure is used to seat the tray home in the mouth. After the impression has been taken it is carefully examined, a common mistake is not softening the compound enough. In this is the case the impression can be reheated in a separate bowl of water (cross infection control) and the impression is repeated.
28Cooling:Assure uniform cooling by spraying with water at 16-18°C before removal from the mouth.
29Properties Flow type 1: 85% at 45°C less than 6% at 37°C. Type II: 70% at 45°C less than 2% at 37°C.Viscous material does not record fine details.
30Low thermal conductivity. High expansion and contraction coefficients.Linear contraction on cooling from mouth temperature to room temperature (0.3%-0.4%).
31Dimensional Changes: due to relief of internal stresses (relaxation) Warpage distortion of the impression.Model should be poured as soon as possible.No separating medium.Separation of model.
32PROPERTIES Poor surface detail High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth to room temperature)Distorts when removed over undercut areasPoor dimensional stabilityCan be modified by re-heatingNon toxic and non irritantGood shelf life
33ADVANTAGES & DisADVANTAGES 1. Non irritant and non toxic 2. Reusable (but with re-use the constituents are leached out) 3. Can be reheated and readapted 4. Can support other materials for wash impressionsDISADVANTAGES1. Poor dimensional stability 2. Poor surface detail 3. Expansion coefficient 4. Will distort if removed from undercuts