Pros Tho Don Tics Ll Laboratory 3(1)

8/2/2019 Pros Tho Don Tics Ll Laboratory 3(1)

1/22

1


2/22


3/22

3

Brief Introduction

1) Flasking

Flasks

What are the steps to convert the wax transitional RPD into the final acrylicdenture?

Lost wax technique is the basic process, but the specific techniques fordentures are (in order):

1. Flasking2. Dewaxing3. Packing4. Curing5. Deflasking

6. Laboratory remount7. Finishing and Polishing

Is defined as the process of investing the cast with the waxed denture in a flask to make a mold that is used to make the acrylicdenture .(see images 1,2)

There are different shapes for flasks (round, ovoid,squared). Each flask has two portions (a cover and a base)

and there is an opening between them (see image 2)

Closed flasks

1

2

Opened flasks


4/22

4

Notes

The upper flask differs from the lower flask :

In the UPPER , the cover and the base are parallel to each other, so whenyou place the cast inside the base its borders will almost leveled with the

land area of the cast, and when you pour plaster you can remove the excessquickly with no worries about the different heights. However; in theLOWER it's oblique so that the retromolarpad is higher than the anteriorpart of the ridge, and this oblique angle will ease the separation at thedewaxing stage,.(see image 3 , page 5)** It is written in the top of the cover of the flask (Upper or Lower) to helpyou distinguish between them.

1. In order to separate the gypsum from the flask, you should paintVaseline on the internal surface of the flask so that you can clean it uplater on, also put a thin layer of Vaseline on your cast base to keep itsafe so that you can re-mount it on the articulator after denture

processing to see if teeth occlusion has changed or not.2. If there is a space underneath the baseplate, when you pour stone it

might get below it and stick to the cast! So you have to seal and closethe space between the baseplate and the cast all the way around thedenture by using hot wax so that nothing can get under the denture.Usually do this step when the cast is still mount on the articulator so itdoesnt change the occlusal relationship.

3. This wax that you put around the denture will turn into acrylic, so youhave to be careful not to put wax on the teeth or their roots.


5/22

5

The flasking Procedure Place the cast in the base of your flask; pour plaster around the lower portion in order to prevent RPD

movement inside the flask.In complete dentures Plaster goes over everythingEXCEPT land area ,wax and teeth . However, inPartial Dentures it differs by that the plaster layerwill be higher in order to cover all undercuts.Complete dentures' casts don't have any undercuts,but in RPD's casts which have teeth on them thereare a lot of undercuts that have to be covered withplaster to ease the opening of the flask.. So wherever there is a natural toothit has to be blocked out using plaster.

You should cover the natural teeth and the retentive part of the claspotherwise the clasps will be free in dewaxing process ,so you have to catchthem from one side before they become free.But teeth are long so the plaster will be in more vertical shape , morevertical means more friction so you have to trim teeth in an angled way toease the separation afterwards. So trim 1 mm above clasps ,and 1 mmabove the lingual gingival margins. DO NOT damage the wax or artificialteeth during trimming.Trim the back portion of your cast specially the lower cast because it's veryhigh.

In summary; you have to cover Natural teeth on the cast in a sloped wayand cover the retentive part of the clasp, in the other hand don't cover thewax or the artificial teeth of the denture.

In the lower cast it's the same; the plaster goes up to the retromolarpad areabut it doesn't cover the lingual area.

This is the lower flask

3


6/22

6

** The mixing ratio for plaster: 100 g powder 50 ml of water,

You should make the posterior edge of the plaster tapered in order to make

it opens and closes more easily. And always remember if any plaster gets toteeth or wax you should remove it.

When the plaster layer sets you will be ready to pour the second part of the mold which is 50/50 plaster and stone, but before that you have toseparate between these two layer because gypsum will stick to gypsum, andif you try to open the flask the plaster will break during dewaxing which is

a big problem, so use a separating agent between them which isVaseline .

If there is an under cut in the plaster ,,when you pour the other layers it willget inside this undercut making the separation of the upper and the lowerparts harder which may break the cast .

You can add plaster to close these undercuts,but sometimes it will not connect perfectlywith the previous layer, so you have to fill itwith wax or a lot of Vaseline. During thedewaxing process it will melt away forming aspace between the two portions and make thedewaxing easier, but when finishing the denturecome out it will turn into acrylic that will needtrimming .

To separate gypsum from gypsum use Vaseline To separate gypsum from new acrylic use tin foil substitute (sodiumalginate /cold mould seal)


7/22

7

6 If the surface is rough I can use sandpaper underwater to make everything smooth.

Always be sure that the teeth are centeredcorrectly, any irregularities may cause difficultiesin dewaxing , and this layer is strong enough tokeep the acrylic in place and weak enough to beseparated in deflasking process.

Next, after the second layer sets pour the third layer that is pure DentalStone on the top to prevent the movement of teeth .You just need to coverthe cusp tip with it. It also helps in deflasking procedure and to keep theshape of the denture.It will go around the teeth to keep them in place, paint alayer of the separating agent between the 2 nd and the 3 rd layers or not.( yourchoice ).

** The mixing ratio for Gypsum type 3(dental Stone) is: 100 g powder to30 ml of water.

You should mix it on the vacuum mixer to be more accurate and not tohave any bubbles between the flask and the teeth.**In private labs they just make two layers, they may use stone all the way,or 50/ 50 (plaster and stone) all the way, in order to save time but it maycause inaccuracy or difficulties in removing the cast.(( THIS IS WHATWE DID IN THE LAB )).


8/22

8

2 Dewaxin 9

With this we have finished our talk about Flasking , and now we will talk about Dewaxing which is the 2 nd step of this lab.

After flasking you have to do dewaxing ,,, where youshould put the flask in a dewaxer machine (water pathwith a timer and a thermosta t) .Put it for 10 minutes in a 95 C it will soften the waxinside. Once it is soften open the flask through theprovided opening (it will be so hot ) .Then put it again inthe dewaxer machine while it is opened for another 10minutes" total 20 minutes " ,then with a hot water jetwash away any remaining wax until you have a cleansurface .

After Dewaxing the baseplate is thrown away (it has atemporary job during fabrication ) .

At this point I am ready to do packing ,but before packing you shouldpaint Tin Foil substitute over everything in the mold except the teeth andthe wires,thus the acrylic will NOT stick to gypsum . either stone or plaster. If you painted on teeth, they will not connect to acrylic.

This layer of tin foil substitute (cold mold seal) should be very thin, and

you should put more than one thin layer. If it was so thick it will change thedepth of the sulcus. Then, you can use a brush to remove the separatoragent from teeth. Wait for it to get dry otherwise it will not work as aseparating agent.


9/22

9

3) Packing

Dont use Vaseline as separating agent between stone and acrylic; it willnot work well as a separating agent in packing.

A short summary of packing would best be like this,

Mix the acrylic according to certain ratios, wait till the dough stage, put the acrylic inside the flask, close it so that the excess will get out , use your hand to press it first then use Denture Hydrolic Press

So starting off with the first thing which is mixing of acryl:

Now what Im going to do is that Im going to mix the acrylic , u se porcaline container for mixing acrylic, you need 10 ml of the monomer fora denture then add powder in (1:3) ratio, why?

NOTE :* Tin Foil Substitute is made of Na alginate *

Why sodium alginate is used as a separating agent?

Because its reaction is similar to the reaction of alginate impression

material.

Na alginate + Ca sulfate Ca alginate + Na (in solution) + Sulfate (insolution)Ca alginate will make a network and precipitate.The cast is made from Ca sulfate; the same material that react with Naalginate


10/22

10

This is the packing machine ,the main concept of this deviceis Hydrolic pressure

10

11

Flask clam

The answer is when acryl reacts its reaction isexothermic, plus when methylmethacrylate (monomer)react with itself there will be change in size; contraction(reduction in size).

Some of the mistakes that might occur during mixing and pouring the acryl are:

Not enough monomer during mixing will lead to porosity or Localized Porosity.

Adding too much monomer will result in shrinkage. If the mix is not homogenous will also lead to

Localized Porosity.

There are different stages of the setting reaction of acrylic which are :

1. Powder and liquid stage : you can clearly see in thisstage that the powder and liquid are not the same(two different distinguishable materials). Here theshape of the mix is exactly like sand on beach.

2. Wet sandy stage

3. Stringy or the fibrous stage : If you hold the material at this stage andstick it between your fingers itll form fibers , just like holding a gumbetween your fingers

4. Doughy stage : This is the perfect timing and form for acryl to beplaced inside the packing machine.

5. Rubber stage : Using the acryl at this stage is bad , why ?As you know indental materials we have two different properties, we have Elastic andPlastic , the difference between these two properties is obvious that the

later will deform its shape when execrating pressure on it. If you shapedyour acryl at this stage, what will happen is that you will have somedimensional changes and this is not good , so no further manipulation canhappen at this stage

6. Finally is Harden stage


11/22

11

4 Curin

This is the flask clamp;itll be inserted like thisinto the curingmachine.

12

So we place the acryl in its doughy stage inside the flasks, but to make surethat both flasks are pressed as much as possible we are going to use theHydrolic press with the packing machine ( see image 10). A very importantnote is that the maximum pressure can be made on the cast (which is made

of gypsum type lll ) is 3000 Mp , above this pressure will causes fractures orcracks inside the cast . While the den ture is inside the packing machine we dont rush thingsbecause if we press too fast the excess will come out and no details wererecorded, we give the acryl enough time to cover all the possible details

After that Ill remove the excess in the flasks and then Ill put the upper andlower denture attached to the Flask clamp , the objective of the

clamp is not to excrete pressure and remove excess, its objective is simply to keep the flask close during the processing , there is a special screw andsprings in the bottom to keep the flasks close together. (see image 11)

After setting them in the clamp Ill do something called bench curing Illsimply leave them for half an hour to make sure that the monomer in theacrylic mix wet the teeth and make bond with them .

So packing is a simple process, mix the acryl place the acryl inside the flasks in its doughy form use packing machine to remove excess use flask clamps bench curing

We will take the flask clamps after leaving them on the

bench (bench curing), and put them inside the curingmachine (a hot water path that have a timer and athermosta t). The aim of this process is to convert the

heat cured acrylic from the dough stage to the final cured hard acrylic stage .(see image 12).


12/22

12

Long Acryl Curing Cycle

9 hrs

Water

Acrylic Long cycle

74

95

100

1

3

13

Before we discuss the actual procedure of curing, I want you to know aboutthe curing cycles of acryl. We have long and short curing cycles, each oneleads to the same thing which is converting the acryl to its harden form. Inthis lab the type of acryl we used was Heat Cured acryl , there are some

diffrances between long and short, lets talk more about them


13/22

13

Figure (13) is of great importance to help us understand the long curingcycle; you can see we plotted time vs. temp. Know that the acrylic in thisstage is present within the water inside the curing machine. So what happensto the water (rising temp. /lowering temp.) Will also happen to the acrylic.

You also have to know that the initiator for the acrylic (chemical reaction )starts at 74 degree , meaning that acrylic will start harden at 74 or slightlybelow .

With this reaction heat is generated so it is an exothermic reaction (but notethat inside the curing machine we have water and acrylic ,water also givesheat ).

Looking at the graph , Im heating up the water , rising the temperature untilreaching 74 degree the reaction of the acrylic will start (1 in the graph) , butthen I stopped Im not going to increase the temperature anymore (source of heat is fixed to 74) . What will happen now , is the acryl reaction will start Ihave a new source of heat this will lead to a peak as in (2 in the graph) ,acryl peak means that the acryl temperature went up until 95 and then wentdown back to 76 very fast. After the peak is done, Ill fix the temperature for 9hrs (overnight) and this is (3 in the graph).

The temperature of water boiling is 100C. The degree of acryl boiling is100.8C , you can see that the long cycle is a safe cycle , because thetemperature of water never reached 100C , it reached only 74C.


14/22

14

Short Acryl Curing Cycle

2 hr

1 hr

Water

Acrylic Short C cle

74

95

100

1

3

4

5

14

What if I cant wait 9hrs for the long curing cycle to complete? What if I

want to cure the denture in the shortest time possible? Ill probably use ShortAcryl Curing Cycle .

Graph 14 is of importance to understand the short curing cycle of acryl,.

We start the cycle by increasing the water temperature up to 74C as in (1 inthe graph) , the heat now is fixed to 74C .But the acryl has reacted and itll


15/22

15

Water

Acr lic

7

9

10

15

cause a peak as in (2 in the graph ) that was one hour of the 2hr for thisstage. After that will leave the fixed temperature for another 1 hr as in (3 inthe graph ) , after that we will increase the temperature of water to 95C as in(4 in the graph ) , after that the acryl will stay at 95C temperature for 1hr as

in (5 in the graph).

Why did we increase the temperature up to 95 ( 5 in the graph ) ? Well wedid this because we are afraid of the free monomer (the excess) that mightcause irritation to the person , but when we increase the temperature wemake sure that there is no free monomer . Total duration of the short cycle is3hrs, comparing with the long cycle this is definitely shorter but the longcycle is safer.

The next talk is not about cycles, but rather a mistake that might be doneduring curing :


16/22

16

Graph 15 is of importance to help us understand the error GaseousProsoity, so lets see what it is.

The mistake here is that we increased the temperature of water up to 100C as

in (1 in the graph) , as you know by now acryl will have a peak as in (2 inthe graph) this peak reached a very dangerous degrees (for e.g. 110C ) , asyou know by now the boiling degree of acryl is 100.8 so the acryl in thiscase had boiled and it had porosities within it , itll continue with water as in(3 in the graph ) . So the danger is because the boiling temperature of waterand acrylic are nearly the same 100 and 100.8 respectively

This type of porosity happens w hen we dont use a curing machine , in somelabs they just bring a dash and they boil it using a cooking oven the only

way they know is when the water boils . And sometimes and might be toolate because the acryl has already been deformed.

Just know this small note, during the gaseous porosity, the temperature ishighest at the middle and not the corners .

If we to ask ourselves why Gaseous porosity happens? The answer could be one of these things or combination of them or all of them:

A sudden increase in temperature (too fast; didnt give enough timefor the temperature to escape) .

Rising the temperature beyond 100 degree . Having a thick denture: now some of you might ask where exactly the

thickest part of the denture is. Is it on the upper denture? Nopebecause the palate has even thickness. It is on the lower denture underthe teeth, I dont see it on the surface because heat can escape, sousually I see it buried inside the teeth in the lower denture.

If we ask ourselves how can we avoid having gaseous porosity mistake?Change the cycle (spending money on expensive curing machine that hasa fixed temperature rates and thermostat)

Note that we have other types of porosities, as Sub-surface porosity and Homogenous porosity (happens with bad mixing).


17/22

17

5) Deflasking

After we get the dentures out of the curing machine, we have to give themsometime (about 30min) before moving into the next step , which isdeflasking.

Up to this point doctor Esaam finished the lab and we started the practical part. The next talk is for those who are interested in summarizing the rest of

the steps .

After we let the dentures cool down from the curing machine , the first thingyou notice is that tin foil substitute will form a very thin dry layer, you cansee slight reddish shade (theyve added this reddish pigment so you can seewhere tin foil is on). Sometimes tin foil might unfortunately get into thesurface of the teeth, what you should do is wipe out the surface of thesecontaminated teeth using a piece of cotton dipped in acryl monomer , If Idont take out this excess monomer what are the results ? Teeth might fall

The curing cycles for Acryl

Short 1st stage : 74 C in water for (1, 5-2) hours THEN 2nd stage : 95 C for 1 hour

Long 74 C for 9 hours (overnight) It is safer .


18/22

18

off the denture, why? Read below about how a tooth attaches to the acrylicto know why:

1- One way is Chemical (between the artificial acrylic tooth and the

monomer of the denture base)2- The other way is mechanical, remember when you were making thedenture you did the ginval margins some of the facial gingival marginswent over the tooth , and some of the lingual gingival margins went alsoover the tooth .

, , .

selain itu terdapat juga cara mekanikal,sewaktu membuat gingival margin pada denture,sebahagian gingival margin melepasi cervical gigi,dnsebahagian lingual gingiva juga melepasi cervical gigi"

Reasons why we clean the tooth surfaces with acryl monomer are:

A) Itll remove the excess t in foil, which might lead to teeth falling off dueto insufficient retention

B) Itll also help to make the bond between the teeth and the acryl denturestronger

We have a layer called The Glazed Layer which is that shinny layer youcan spot on teeth , this layer will prevent bonding between the tooth andacryl


19/22

19

The Procedure

16

6) Laboratory Remount

Open the flasks (using the plaster knife to open the flak, sometimes wemight use a hammer) but keep in mind, using hammer is not the correct way

but most of the labs in the Arab world do it. What are the things that help meto easily separate the flask now? Can you remember? Well:

If you put good amount of Vaseline around the borders This layer (look the arrow on image 5) will also help me to open the

flask more easily.

After getting the flask out of the different layers of gypsum . There is still that gypsum surrounding thecast and the denture (look image arrow on image 16) ,now hitting with a hammer around this area issomething unacceptable and risky. What we can do is

place it in a special device Ultrasonic Water Bath this de vice will send ultrasonic vibrations that wecant hear, leaving it for half hour one hour the

gypsum will dissolve and any excess particles on the denture will fall off . Inthe Arab world, we use something less expensive way; Lemon acid (citric acid).

While removing the plaster off the flasks do we care about having the castsafe? The answer is yes, we need the cast to check the occlusion later.During this entire lab process the teeth will definitely change their position alittle bit, so we still need the cast to mount them on the articulator andcorrect that error later, this step is called Laboratory Remount as youknow from the theory part. We also have clinical remount which is differentfrom the laboratory remount, in clinical the correction procedure is done atclinic.


20/22

20

7) Finishing and Polishing 17

The arrow indicates a fittingsurface for the upper denturewhich is the palate area , thisis one of the examples about

the fitting surfaces

So , now that we know the aim of the laboratory remount it is very importantto know that while doing this step we are going to leave the denture insidethe casts and we are going to attach them to the articulator and check occlusion . Why dont we just remove the cast and put the denture on the

articulator without the cast? Well, when the denture is attached to the cast itis always stressed, if I remove the cast there might be relaxation and this willlead to errors during laboratory remount. So always remember laboratoryremount:

Aim is to: correct the slight mispositoned teeth during the entireprocess.

Is done: while the cast and denture are both attached to the articulator.

Alright, the denture now is clean and ready for the next process which is Finishing and polishing . Before we

start off with this process keep in mind that we dontdo anything to the fitting surface (look image 17)during this process , we leave it as it is , it is smoothenough and has good details . The only case we try topolish it is if we scratch it by mistake.

Acrylic bur Fissure bur : for clearing out the frenum area Sandpaper mandro : a long bur with a slight in

the middle to allow the insertion of the sandpaper Wheel brushes The doctor didnt talk about Acrylic bur , he immediately started with

the sandpaper mandro .


21/22

21

1. We firstly remove the large excess we use the sandpaper to removethe excess acrylic around the borders but we d ont use it in the

postdam area .2. After that we start polishing the surface , well start with sandpaper on

slow speed (10k- 15k rpm) , If I did good polishing for the wax I dontneed to use this very much expect on the borders areas. (The moretime you spend waxing the less time you spend finishing).

3. After that well use a special rubber polishing burs that has within therubber small particles that will give a shine surface.

4. After that Ill use a large trimming wheel ( look image 18 next page )these wheels are made out of cotton we dont usually use the normal

plastic brussures , during using these trimming wheels we dont use

them arbitrarily, but we do the following :a. The first stage we use these wheels wet with pumice material, it

comes as powder so we add water to it, and it forms like dough.We put it on the wheel and then the wheel goes around thedenture and itll give a very shiny nice appearance. (pre -finalshine )

b. The final shine comes from :i. Using the wheel dry , at low speeds

ii. We use rouge or what is known as , togive the final shine (look image) . We have two types of rouge in dentistry :

1. The red iron oxide ( picture ) is used to polishmetal ( crown , bridges and partial dentures madeout of metal ..etc ) , we dont use this polishacrylic. This is called Iron oxide

2. Triplen (not sure about the name) it is the whitematerial we use to polish the acrylic denture. Thisis called Aluminum oxide


22/22

22

18

When we have the final shine we leave it inwater, we never let the denture dry out why?

Because the denture has very small pores in it,acrylic has these microscopic pores in it.Imagine a sponge filled with water but then thewater evaporates what will happen to thesponge? The same thing with acrylic itll shrink.(Think of the final denture as a fish, you always have to put it inside water. If it leaves water itll die) .

Just to recap with you on the finishing and polishing steps , we start off with

the acrylic bur then we used the sandpaper-mandro bur then rubber polishingbur then we used the wet with pumice wheel bur then we used the wheel burdry.

~The end

~Done by : Enas Salameh and Osama Yousef .

Date post:	06-Apr-2018
Category:	Documents
Upload:	manar-malkawi
View:	222 times
Download:	0 times

Pros Tho Don Tics Ll Laboratory 3(1)

Documents