Acrylic RPD Processing:
- similar to complete denture
- same idea; converting base plate – wax, shellac, cold cured acrylic, light cured acrylic- into heat cured acrylic.
Properties of Heat Cured Acrylic to be used as the final material of RPD:
- strong "high toughness", but shellac is a very weak material
- more hygienic; can be polished to highly smooth surface.
There are different techniques but we use the gold standard technique which is called "lost wax/acrylic technique".
We replace the base plate with heat cured acrylic but we leave the acrylic teeth and the clasps.
The clasps and the remaining teeth in RPD make minor differences in the processing.
Steps of processing:
3- Acrylic Packing.
4- Heat Curing.
Sequence is very important.
In the previous labs we made 3 clasps, base plate and we did the setting of teeth.
In processing, the clasps and the teeth are kept in their places and just the red modeling wax will be replaced by heat cured acrylic.
Flask is made of 2 parts, upper and lower –written on them U & L-.
Every half has 4 components, 2 parts and 2 lids for coverage.
- Pre-vaselination the flask using Vaseline which is petroleum gel to make the de-flasking step easy.
- Fix the model inside the lower part of
the flask with gypsum type II but here
gypsum will reach the retentive arm of
the clasps and the area of the trimmed
remaining teeth –to prevent hard de-flasking and breakage of the cast or teeth as there are undercuts- not like complete denture where we cover until the border of the model.
- In the first fixation of the cast no need for more than plaster of Paris which provide easy de-flasking as it is weak material.
- if retentive arm of clasps not covered by gypsum, when gypsum added to the remaining parts of the clasp, 3 surfaces will be covered but the forth which touches the tooth may not so in de-waxing there is high chance for displacement of the clasp … the clasp is not in the undercut area any more so retention is lost.
- any tooth not important .. trim it so we will leave the abutment teeth where clasps are attached.
- The end of this stage:
Model with retentive part of the clasps covered and the remaining teeth trimmed … nothing visible except acrylic teeth and the wax base plate.
- Leave it to set –it needs 10-15 mins to set-.
- Put Vaseline for easy de-waxing all around the gypsum but not on teeth & wax, if Vaseline is put on teeth and wax, it will eliminate the anatomy.
- Put the other half of the flask and put mixed gypsum with the usage of vibrator.
Here we use mixed Gypsum because if we use gypsum type II with pressure teeth may be displaced inside the gypsum as it is weak. And if we use dental stone, it is hard to clean the denture –from fissures and embrasion- during finishing and polishing.
- So we use mixture of both plaster of Paris and dental stone so we will get strong material and easy to be removed from the anatomy.
- If you want to make it easy … pour until surface of teeth then pour layer of plaster of Paris to get easier de-flasking… close it by the cover so excess gypsum will go out… leave it to set for 45 mins.
- If you want to accelerate the reaction use hot water, salts increase it to 20% potassium sulfate … tera alba.
Tera alba: is the remnant material of gypsum in the rubber bowel –any thing we add to the mixture from previous mixture will accelerate the reaction-.
Linea alba: occlusal line formed due to biting.
Materia alba: 1st layer of the plaque.
The mixing ratio:
* Plaster of Paris 50-55 ml water : 100 g powder
* Dental stone 30 ml water : 100 g powder
* Modified dental stone 20 ml water: 100 g powder
- Hot water path… open the flask into two parts … leave it for 5 min for complete removal of wax …
- The end result is:
Part has the clasp retainer but retentive part inside gypsum
Second part has the acrylic teeth
- check for stability of the both parts as any mobility during packing and pressure will be a displacement in the denture.
- Also if no holes in the apical part of the teeth, do it in this stage to get retention mechanically but if the teeth are porcelain there is retentive metal component.
- use a separator called cold mold seal (sodium alginate) … make 2 coats but don’t cover the teeth or the clasps as no need for separator here.
- Heat cured acrylic; it is found in powder –MMA- and monomer.
Powder is polymer chain, the monomer will do polymerization reaction, powder-powder so co-polymerizations.
- No way to know the exact chemically, how much monomer needed from the monomer to do polymerization reaction for the polymers you have … always we will get excess monomer
Always try to add powder to liquid… so you will get less voids, and some manufacturer instructions say add powder to liquid until the surface of the liquid is almost powder.
Mixing done in glass cup as the reaction is an exothermic reaction so easy cleaning and not stick to rubber as if we use a rubber bowel.
Stages the heat cured acrylic go through:
1- Sandy stage: like beach sand but with water. Cover it with lid to reduce evaporation of monomer as the reaction is exothermic.
2- Fibrous stage: strings or sticky.
3- Doughy stage: the proper stage for packing
4- Rubbery stage: heat generation if felt by hands.
Avoid packing at both fibrous and rubbery stages; in fibrous stage the reaction is not completed but in the rubbery the material start r=to be rubbery with recall effect so distortion of the denture.
5- Hard set stage: after heat curing
We need to cure it in hot water to initiate the reaction as the initiator here is benzoyl peroxide, this material need 60 degree to destruct the bonds between molecules and start the reaction
But cold cure the initiator start the reaction when mixing occur, and the light cured the initiator needs light to start the reaction.
So we need minimum 60 degree to start the reaction and the reaction is exothermic so it will complete the reaction.
If upper RPD make the heat cured acrylic in the doughy stage as a ball and adapt it, then put the other half of the flask and using hydrolytic pressure excess will leak out … keep applying pressure until both halves of the flask meet each other.
Or during flasking escape channels are made and as the flask is opened posteriorly the excess will go out from the end in addition to the front and the sides until the two parts meet each other.
In hot water path.
Remember that here we want the temperature to exceed 60 degree to start the reaction but we don’t want the monomer to evaporate so we should not reach the boiling temperature of the monomer -100.8-.
If evaporation occur, remember that teeth, acrylic and gypsum are poor heat conductors so all vapor of monomer will stay inside the acrylic giving porosities.
Porosities will weaken the acrylic and if go to the surface there will be plaque accumulation, fungal infection, and bad odor. And if rough porosities soft tissue irritation and discomfort will occur.
We don’t want to leave excess monomer, if left in excess it is cyto-toxic material and it may react later on and distort the denture,
And we don’t want to reach the boiling temperature at first stage of curing preventing the evaporation of the monomer specially in thick acrylic RPD.
2 curing cycles :
1. Gold standard cycle "Slow / Long curing cycle
Room temperature at time zero
Switch on – Temperature , to make sure of complete degradation above 60 so 74 for 8 hours (above 60 good Rxn . this is Temperature in machine but inside more as Rxn is exothermic
After 8 hours , machine turn off
If rapid cooling >> Warpage distortion
So bench cooling or leave it in the machine to cool slowly
Modification to this cycle >> after it is completely set make the temperature 100 for 1 hour.
So to be sure that there is no excess monomer remains.
2. Fast / "short " cycle :
Same principle but the curing is for 2h at 74 degree then elevate it to 100 degree to get rid of the excess monomer.
So total it is 3 h.
But this method not good as 2 h might be not enough for the reaction to finished completely.
The worst thing you may do if you insert it in 100 degree water in the first stage as monomer will evaporate. Leading to porosities.
The Acrylic found in the lab:
Turn on the light cure machine until it reaches above 100 degree turn it off then put the flask in it; temperature is decreasing so no chance of evaporation, after 15-20 min turn it on again to remove the excess monomer then cooling.
In good separation the model will be without clasps as acrylic is stronger than gypsum so as par of the clasps embedded in it the retentive part will not stay in contact with the gypsum in this stage.
Hammering until denture comes out, the denture will have some sharp areas.
The Dr. showed us RPD with pink acrylic replacing one tooth.
The reason for this displacement during packing is that the tooth was not stable at de-waxing stage.
You don’t need to repeat the denture, just trim and fix the tooth with right relation then fix it using cold cured acrylic.
to remove sharp areas.
Start with acrylic bur with hand piece remove all excess material and sharp areas and irregularities but don’t touch teeth at all. And don’t touch the fitting surface unless there is sharp areas felt by hand.
If there is gypsum on tooth soak it with water then remove it with wax knife or Lacron carver. Or use round fine tip bur and don’t touch the clasps.
Then use sandpaper with mandrill.
to reduce discomfort, reduce plaque accumulation and infection and for esthetic if visible.
Use pumic –volcanic produst- with a wheel made of rugs and cloth using low speed hand piece ---- glossy.
Rouge (Iron oxide) which is white material.
Denture polishing paste ---- shine
Water and soap
Technician send the RPD to the Doctor soaked in water to prevent distortion by heat or monomer as it is a plastic RPD.
Then we insert it in patient mouth.
Key Words : Acrylic denture Processing,Dental Technician's duty in acrylic denture processing