polyethelen sheet is removed and then the two halves of the flask are closed under pressure by bench press of about 100 Kg/cm2. Then the flask is put in a spring clamp and the clamp is closed tightly.
It is polymerization of the hot cure acrylic to produce the final denture. The material is cured by heating in a water bath; pressure is applied during curing for the following reasons:
1- To decrease the effect of thermal expansion.
2- To decrease the polymerization shrinkage.
3- To increase the evaporation of monomer thus decreases porosity.
Types of curing cycles for heat cure acrylic:
1- Short curing cycle.
2- Long curing cycle.
1- Short curing cycle:
The denture is placed in water at room temperature and the curing temperature is programmed to 74 oC for one and half hour followed by 100oC for one hour.
2- Long curing cycle:
The curing temperature is programmed to 100 oC for 8 hours.
Polymerization can occur at any temperature but it is very slow at temperature below 70oC, the best curing cycle is the long curing cycle because most of the conversion of monomer to polymer occurs during the period at 70oC and during this time the dough itself may approach 100oC because the polymerization reaction exothermic. The monomer boils at 100.3oC so the dough must be kept below this temperature to avoid boiling of the monomer; on the other hand rapid curing cycle usually results in some gaseous porosity.
Deflasking is the process of removal of the processed denture from the flask and investment mold. Before deflasking of the processed denture begins the flask is left to cool to room temperature. If not, increased distortion of the acrylic may occur.
The denture should be remounted on the articulator as dictated by the indices with sticky wax. The incisal pin discrepancy should be noted. If the discrepancy is less than 2mm, it is acceptable. If the discrepancy is between 2-5 mm, occlusal correction can be accomplished.
If the discrepancy is more than 5mm, the entire treatment should by repeated.
7- Shaping and Polishing:
In this step any excess acrylic is removed from the processed denture by the use of stone wheel burs, stone burs, and steel burs. Care must be taken not to heat the denture during grinding, because this may cause distortion of the denture base. Finally, the denture should be smooth and clean, as no plaster and no deep scratches should remain after the preparation for polishing.
In polishing a rag wheel with pumice is used for smoothing the denture. Then a final high polish is given to the denture with a rag wheel and polishing material (Rouge).
Special-purpose Training Tests for study the topic level testing
1. Patient with complete loss of teeth on the upper jaw is made a complete removable denture. After plastic polymerization the flask is removed from the hot and is rapidly cooled under running cold water. What do you think will happen in a plastic prosthesis basis in this case.
A. -pressure porosity is formed
B. -gas porosity is formed
C. -granular porosity is formed,
D. -the color of plastic changes
E. +microcracks occur
2. The patient, 57, is made a removable plate prosthesis for the upper jaw. On clinical and laboratory stages of its manufacture dental wax is used. Which group is the material related?
D. + modeling
3. A patient, 73, addressed to the clinic with complaints of poor fixation of complete removable dentures, manufactured 10 years ago. On check-up: reducing the lower third of the face, a large uniform atrophy of the alveolar processes. Oclussion surface of artificial teeth is almost completely erased. After how many years of use removable dentures patient should change them.
A. -7-10 years
B. -7-8 years
C. -1-2 years
D. -9-10 years
E. +3-4 years
4.Patient,70, complaines of a lack of fixation of complete removable dentures on the upper jaw. Objectively: the edge of the prosthesis in the frenulum of the upper jaw is shortened, the locking valve is broken. What should be done to improve the fixation of the prosthesis.
A. +Denture reproduction
B. -Denture correction
C. -Expansion of denture basis edges