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The compound disc is completely immersed in a water bath at 55-60C for about 4-5 minutes to ensure complete softening. 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. The compound is loaded on to the tray and firm pressure is used to seat the tray 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. Impression Compound
Non-Elastic Impression Materi als DR.HINA ADNAN
Any significant deformation produces a permanent deformation. They are used where there are no undercuts and are mainly used for edentulous patient cases. The main non-elastic impression materials are: Impression compound Impression Plaster Zinc Oxide Eugenol Impression Waxes.
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. The compound is loaded on to the tray and firm pressure is used to seat the tray 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. Impression Compound.
Filler (e.g. talc or soapstone) 3. Lubricants (stearic acid or stearin) It is a thermoplastic compound i.e. it softens when heated and hardens when cooled. This process does not involve a chemical reaction. Impression compound is available in 2 forms: Properties of Impression Compound.
Softening temperature is 70 0 C 2. Sheet form - used to take impressions of the mouth. Softening temp is C. PROPERTIES 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 areas.
Poor dimensional stability Can be modified by re-heating Non toxic and non irritant Good shelf life.
Reusable (but with re-use the constituents are leached out) 3. Can be reheated and readapted 4. Can support other materials for wash impressions 5. Mucocompressive.
Expansion coefficient 4. Will distort if removed from undercuts.
The mix should be a smooth paste, free of air bubbles, which may appear on the surface of the impression leading to inaccuracy. The tray should be loaded and puddled into position, and held in place until set. As the impression is removed from the mouth it is not uncommon for pieces of plaster around the periphery of the impression to fracture off. These pieces should be retrieved and glued back onto the impression before it is cast..
varnish or soap solution) before being cast in stone or plaster Exothermic setting reaction On storage dimensionally stable but a small amount of shrinkage may occur Non toxic but may be unpleasant due to dryness and heat evolved during setting.
ADVANTAGES Good surface detail Excellent dimensional stability Rate of the setting reaction can be controlled by the clinician DISADVANTAGES Cannot be used for mucocdisplacive impressions Cannot be added to Properties affected by operator handling technique Taste and roughness may cause the patient to vomit
The material is mixed in a 1:1 paste ratio and used in thin sections only (2-3mm) as a wash impression. Vaseline is used as a separating agent on those areas requiring protection (soft tissues, teeth etc). Firm pressure is used during impression taking..
Constituents of a typical zinc oxide eugenol paste are: BASE PASTE Zinc oxide Inert oils (plasticiser) Hydrogenated resins (increases setting time and improves cohesion) REACTOR PASTE Eugenol Zinc acetate (accelerator) Fillers (talc or kaolin)
PROPERTIES Non toxic Adherence to tissues Mucostatic or mucocodisplacive (depending on brand used) Good surface detail in thin section Good dimensional stability (little or no dimensional change on setting, 0.1% dimensional change during setting) Can be added to with fresh zinc oxide eugenol Stable on storage and good shelf life
Can be added to 4. Mucostatic or mucocodisplacive DISADVANTAGES 1. Cannot be used in very deep undercuts 2. Only sets quickly in thin section 3. Eugenol allergy in some patients.
They do not set by chemical reaction. Normally used to correct small imperfection (e.g. airblows) in other impressions, especial zinc oxide impressions. They consist of a combination of a low melting paraffin wax and beeswax in a ratio of approximately 3:1 to ensure the wax flows at room temperature. A cast should be poured up immediately after taking the impression to avoid distortion which readily occurs in wax. These materials are not commonly used..
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