, (1) lower half (which contains the cast), (2) upper half and (3) the cover or lid. The lower half may have a round plate, covering a round hole in the base of the lower half.
Molding or flasking Techniques:
1- Compression technique:
The stone cast with the record denture base, wax and teeth are secured to the lower half of the flask with gypsum investment material (plaster or stone). Then the upper half of the flask is put in place and gypsum is poured to the occlusal surfaces of the teeth. Finally the top portion of the flask is poured with investment and the lid is placed on the flask.
2- Injection molding technique:
It is a complicated procedure requiring special flask and equipment. In this technique the wax pattern is sprued and the material is injected into the mold. This process allows injection of further material during polymerization to compensate for the polymerization shrinkage.
The procedure of flasking a denture in compression technique is as follows:
Acrylic resin dough is made by mixing the powder (polymer) and liquid (monomer) to form 'dough' which is packed into a gypsum mold for curing. The ratio of powder to liquid is important since it controls the workability of the mix as well as the dimensional changes on setting.
The mixing should be done in a clean jar which should be covered to prevent evaporation of the monomer.
It should be done when the mixture reaches dough stage, as the dough is rolled into a rod-like form and placed in the upper half of the flask then a polyethelen (nylon sheet) is placed over the dough in the upper half and then the two halves of the flask are closed until they are almost in approximation, this is done to spread the dough evenly throughout the mold. Then the two halves of the flask are separated, the excess material at the borders of the denture is removed by a wax knife, and additional resin is added at any places that are deficient. At least two trial closures are done and before the final closure a thin layer separating medium is applied on the cast and the 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 