BORDER SEAL
After finish the festooning of all polished surfaces and carving of the cast in the post dam area. Now the next step is to do border seal. In this step, we firstly seat the record base correctly on the cast, then drop melted wax on the interface between border of record base and the cast. Then smooth the wax. Care must be taken in that to ensure not to leave any space between base and cast. Like this mistake will failed the denture.The festooned wax denture base must be converted to resin to construct a denture. This is done by using the wax denture as a pattern, we make mold to which the denture base resin is inserted and cured.
Flasking is done in three steps: The first step: the cast is placed on the base of the flask. The second step: the upper half of the flask is placed and stone is poured to the occlusal surface of the teeth. The final stage: is accomplished when the top portion of the flask is pored and the lead is placed on the flask.
Flasking :is the process that carried out to make a gypsum mold of the try-in denture to replace the wax pattern by acrylic resin to produce finish denture by lost wax technique.
The flask consist of four pieces : The base( which contain the cast.) Round knockout plate. The top half of the flask. Lead.
Step of Flasking
The master cast, which is removed from the mounting plaster; is soaked in water at room temperature for 5 min. so that the flasking plaster will not come into contact with a dry surface and set prematurely.Select a flask that fit accurately with the size of the cast without rocking, then check if it there is enough space between the incisal and occlusal surfaces of teeth and the top of the flask, if there is no enough space trimming the base of the cast should be done. After that apply a separating medium to the cast and the inner surface of the flask.
Step of Flasking(2-layers)
Mix the plaster to smooth consistency, then invest the lower half of the flask and centre the cast in the lower half making sure that the base of the cast is parallel to the base of the flask.The cast is pushed until the bottom of the cast touches the base of the flask, the plaster must be brought up to the periphery of the wax but not carried over it, note that the posterior portion of the cast is left with the edge of the flask.
Step of Flasking
Any gross excess should be wiped away before the plaster sets and after setting the plaster must be smoothed so that no undercuts will cause breakage during subsequent separation of the other section. All plaster must be removed from the metal edges of the flask, after that a separating medium must be painted over the plaster before adding the upper half of the flask.Step of Flasking (2-layers)
Put the upper half of the flask and make sure that two sections meet accurately and there is intimate contact between the upper and lower edges of the flask.Then place a new mix of plaster over the teeth and wax surface until the flask is filled and this section of the flask is vibrated to remove any air bubbles from the plaster, this is important as the presence of air in plaster will possibly produce rough surface on the resin and flask cover is placed gently.
Step of Flasking (3- layers)
when the upper half of the flask is placed on the lower one correctly, the plaster is poured at the level of occlusal surfaces of posterior teeth and incisal edges of the anterior teeth to prevent movement during processing. Then allow the plaster to set and apply a mixture of plaster to fill the upper half of the flask, this procedure is called (capping) .The flask is then covered gently. The plaster is left to set completely for 45 minutes.
After Flasking of the denture, the wax must be eliminated to form a mold into which the resin may be packed. this done by: place the flask with the clamp into a boiling water 4-5 minutes , this will soften the waxed denture base which can be easily removed from the mold when the flask is opened After 5 minutes remove the flask from the boiling water and open the flask by inserting a wax knife between the upper and lower halves and gently separate them. Remove the waxed denture base, all the teeth should remain in the upper half of the flask and use a boiling water to remove the remaining wax.
Wax elimination
Wax elimination: it is a process of eliminating wax from a mold into which acrylic resin will be packed by placing the flask in hot water after setting of the flask plaster.
A detergent solution or wax solvent is used to scrub the mold , the mold should be thoroughly scrubbed with a brush which reaches to all areas of the mold. Flush the mold with a clean hot water to remove all traces of detergent solution. It is essential to remove all wax residue so that the acrylic resin will not adhere to a surface coated wax, then allow the flask to dry.
Wax elimination