Muffle setting Polymerization of bases and finishing

Muffle setting Polymerization of bases and finishing

Introduction

The quality of adaptation and integration of any partial removable prosthesis depends on the precision of the different recordings undertaken in the clinic and compliance with the technological chain in the laboratory, among them: the stage of polymerization of the resin bases.

Definition

Polymerization is a laboratory step consisting of replacing the wax of the teeth assembly with a material: acrylic resin, bio-compatible, resistant to oral pressure, physical and thermal shocks and allowing the fixing of hooks and prosthetic teeth.

The imperatives of mittening 

• To be carried out in a muffle of width and length in relation to the volume of the model and its mock-up.
• Ensures a plaster thickness at all points exceeding 1cm.
• Allow recovery of models if possible.
• Ensure the separation of the different parts of the muffle.
• Anticipate and compensate for all expansions of the acrylic resin during polymerization.
• Remove, eliminate any risk of excess thickness, distortion and porosities

Description of a mitten

It is a kind of elliptical bronze box which is made up of several parts:
  -A base part in which the model and its model will be placed,

   -A counterpart in which the artificial teeth will fit.

   -A cover, or not.

   -A guidance system ensures the adaptation of the different parts,

   -A fixing system (flange or keys) 

There are the flange block and the key block.

Practical implementation of a mitten setting

1. Wax rehabilitation: 

• Checking the thickness of each model : must be uniform at all points (1.5 mm in the center and 2.5 mm on the periphery).
• Secure the models : wax must be poured over the entire periphery of the support surface, thus creating a flawless seal.
• Wax finishing: with the aim of significantly reducing the time spent in the laboratory after polymerization. 

2. The mittening

• Isolate the base of the model with varnish.
• Vaseline all parts of the mitten that need to be clean and fit precisely.

• the model is tried in the part of the muffle in such a way that the walls of the latter are located at least 1 cm from the vestibular faces of the artificial teeth 

• The muffle part is filled with creamy plaster, and the model with the wax maquette are placed in it.
• Place the model so that the incisal block is placed vertically

•  The plaster is smoothed so as to fill all the peripheral undercuts. 

• Remove all traces of plaster from the edges of the muffle and the wax model.

• Once the plaster has crystallized, the exposed surface is isolated using a varnish
• Place the counterpart in good contact with the part
• and a second quantity of plaster of fluid consistency is brought to cover the artificial teeth, and the wax of the model.

    A : model. B : plaster of the lower part. C: plaster of the counterpart.        

Scalding

• After the plaster has completely set (1 to 2 hours), the muffle is placed in boiling water for 5 minutes. 

• the mitten is removed, the two parts will be separated. 
• The parts will be cleaned until all traces of wax have completely disappeared, by quickly passing the parts of the muffle through boiling water.

• Insulation is applied over the entire surface of the still-warm plaster.
• Allow the parts to cool (on a cooler) and begin preparing the resin mixture. 

Polymerization of resin

1. Mixing the resin 

-The ratio between powder (polymer) and liquid (monomer) has considerable importance on the dimensional stability of the future prosthesis,

– Two dosages for preparing the mixture:

                           .volumetric : 1 volume liquid / 3 volume powder,

                           . weight: 1 liquid / 2 powder,

-The principle is that the monomer must be in sufficient quantity to wet all the polymer grains, but it must never be in excess,

-the mixture must be pure, any introduction of foreign bodies risks inducing errors in the polymerization.

The acrylic resins used in PPA are thermopolymerizable resins. 

Whose polymerization reaction, which is a chemical reaction, will be induced (activated) by heat in a thermostatically controlled bath.

-The mixture goes through 4 phases: fluid, viscous, pasty, and rubbery, 

– fluid dissolution phase (or liquid): the monomer diffuses into the polymer; the mixture has a foamy appearance.

– viscous or sticky phase : the mass becomes sticky, threads form if it is stretched.

– plastic phase : the powder particles are all dissolved. The mass becomes plastic, does not stick to the pot or to the hands. This is the use phase, the consistency is ideal for stuffing .

– rubbery elastic phase (or evaporation): the monomer eventually disappears by penetrating into the polymer. The mixture becomes elastic and porous ; it can no longer be molded .

2. Resin preparation and stuffing

The preparation of the resin itself is done by mixing the liquid and powder of the resin in a glass container, following the manufacturer’s instructions. When the resin reaches the plastic phase, a quantity is applied to the ridges by digital pressure and another quantity to the teeth.

The stuffing should be done at the pasty stage (20 mins at 23 C*).

The muffle is closed and then placed under a press, where the pressure is gradually increased, until the excess resin is reduced to the maximum and the muffle is completely closed. 

Then the mitten is immersed in water at a moderate temperature. 

3. Cooking cycle:

The polymerization of the resin is done according to a cooking cycle composed of three stages:

   * temperature increase to 60 ° for 90 minutes 

   * cooking at a constant temperature of 100° for 30 minutes 

   * slow and linear cooling of 12H 

4. Demoulding: 

After gradual cooling of the muffle, we proceed to demoulding, 🡪 separation of the different parts of the muffle. 

The prosthesis is carefully separated from the working model using forceps, which will be broken in the majority of cases.

 Removal of plaster should be done with caution.

5. Finishing

After the flask and the polymerization, the finishing is an equally important step. The surface condition of the prostheses and their appearance must give an illusion of naturalness.

Excess resin that has melted on the periphery is removed, then the extrados is carefully polished. The intrados is examined and, unlike the extrados, it is not polished; only the prominent roughnesses are softened.

Roughing  : using grinding wheels mounted on a handpiece, we remove large burrs existing on the periphery.

Polishing: Using medium and small grit sandpaper, which can make the surface of the extrados smooth.

Polishing: then to give a certain shine to the extrados, using a large cotton brush and a paste.

Handling errors: 

These errors are the cause of either deformations of the prosthesis or porosities: 

– Wrong orientation of model and its mock-up in the muffle

– Presence of traces of humidity before stuffing or poor quality insulation,

– Failure to comply with the powder/liquid ratio,

-The presence of impurities,

– Too rapid or too sudden rise in temperature during polymerization as well as rapid cooling after polymerization ,

-Opening the muffle before cooling, 

Conclusion 

Errors 🡪 Deformation 🡪 Failure. 

Muffle setting Polymerization of bases and finishing

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The muffle The polymerization of the bases and finishinga polymerization of the bases and finishing