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It's heating the packed mould in a water path where both the temperature and time of heating should be controlled to avoid :
which results in high residual monomer content
which leads to gaseous porosity
The flask should be cooled slowly because of stresses resulted of the difference in contraction between the flask and the acrylic, slow cooling permits relief of these stresses by plastic deformation.
- Heat at 72 C for 8 hours or longer ( long cycle )
- Heat at 72 C for 1.5 hours then raise temp. to 100 C for one hour
Deflasking should be done with care to avoid breaking the denture.
The mixture should be packed in the dough stage for its proper consistency, In packing it's important that the mould is full and there's is sufficient pressure on it.
If packed in the sandy or sticky stage the material will be of too low viscosity to be packed well and will result in porosity in the final denture.
If packed in the stiff stage the material will be too viscous to flow in the flask.
The powder and liquid should be proportioned in the ratio of 3:1 by volume or 2.5:1 by weight.
...The reaction should be in a sealed jar to avoid loss of monomer by evaporation.
Metallic ,
Non Metallic ( Acrylic Resins )
-Advantages : Esthetically superior to metal bases and easy to repair
-Disadvantages : Lower strength and wear resistance to metals , Porous ,
Low thermal conductivity
Thermal expansion : change from room temperature to water temperature.
Contraction : polymerization shrinkage of resin.
Thermal contraction : change from temperature of water path to room temperature.
External porosity : shrinkage porosity due to under-curing
Internal porosity : gaseous porosity due to raising the temperature suddenly in the curing stage.
Cracks on the surface of the resin due to tensile stresses.