Basic Information about Metals and alloys-Dental Materials Lecture

             Metals are normally crystalline solids at room temperature,although there are some  exceptions   such as hydrogen(a gas) , gallium and mercury(liquids).

Metals are noted for the ease with which they give up their valance elecrtons; this accounts for certain characteristics such as good electrical and thermal conductivity and the tendency for metals to form cations(positive ions) in solution;metals are also strong, lustrous and ductile.All metals have crystal lattice, it can be:cubic, body-centered cubic and face- centered cubic.

Alloys:two or more metals that are mutually soluble in each other in the molten state.

Liquidus temperature :the temperature below wich a metal is completely solid(i.e..the temperature at which it begins to melt.)

Solidus temperature : the temperature above which a metal is completely melted .

Grain: a single crystal of metal as seen in the metal‘s microstructure.Grain growth-enlargement of grains by heating.

Pure metals such as gold, silver  are not used in prosthodontic(their relatively poor physical properties make them a poor  choise for intraoral use).

.Generally alloys are used in dentistry because  alloying strengthens a metal.

Grain refiner:a metal such as iridium that is added to a metal or alloy because of its high melting temperature.Since it remains solid while the rest of the metal is molten, its small particles act as seeds around which grains of the solidifyring metal form.This enhances the physical properties of the solid metal.

Sag deformation: of metal or alloy; for our discussion, this occurs at high temperatures such as during porcelain firing.

Yield strength- the stress at  which  dislocation motion, permanent deformation  begin.

During solidification(i.e. when a metal or alloy goes from liquid to solid),the following process occurs:

Random atoms to embroys(temporary nuclei) to nuclei to dendrites to grains.As the grains grow, they contact each other.The border or junction between them is called a grain boundary.

Generally, a metal to bee used under intraoral conditions should have a fine-grained structure because it tends to be more resistant to permanent deformation.

Smaller grain size  can be achieved by rapidly cooling the molten metal and by inclusion of a grain-refiner such as iridium(as noted previously).When iridium is added in a small concentration(0.005%=50ppm), the tensile strength and ductility  increas.The is little, if any, effect on hardness and yield strength.

 

Effects of some metals:

Gold: melting temp.-1064˚C, increases resistance to tarnish and corrosion,increases ductility and malleability;

Copper: principal hardener, is necessary for heat treatment(when added in a concentratio>12wt%)

Silver: melting temp.-960.5˚C; main purpose is to modify the red color produced by gold and copper,reduces melting temperature,increases ductility and malleability.

Platinum: melting temp- 1773˚C; raises melting temperature, increases tensile strength, decreases the coefficient of thermal expantion, reduces tarnish and corrosion.

Palladium:melting temp.-1555˚C;  raises melting temperature, increases hardness, acts to absorb hydrogen gas which can cause porosity in the castting, prevents tarnish and corrosion, has a very strong whitening effect on gold alloys even when used at a low concentration(i.e.5wt%)

Zinc: acts as a scavenger and prevents oxidation of the other metals during the manufacturing process, increases fluidity and decreases surface tension, which  improves castability.

Iridium: acts as a grain refiner, this increases tensile strength and percentage elongatio(i.e. ductility); it also increases, to a much lesser degree(if at all), hardness and yield strength.

Titanium: derives corrosion resistance by forming a surface layer of titanium oxide.

Chromium: it is included as a component of many alloys in order to improve corrosion resistance. Passivating effect-Chromium is electrochemically active it reacts  readily forming a layer of chromic oxide which protects the metal or alloy from further decomposition.Pasive elements are: Cr, Al and Ti.

CORROSION

Surface deterioration of a metal because of its interaction with the surrounding environmentis called corrosion.Resistance to corrosion is criticaly important for a dental alloys because corrosion can lead to roughening of the surface, weakening of the restoration, and liberation of elements from the metal or alloy.Liberation of elements can produce discoloratio of adjacent soft tissues and allergic reactions in susceptible patiens.Commonly, noble metals are added to an alloy to reduce its tendency to corrode.These include gold, platinum, palladium,ruthenium, rhodium, osmium and iridium.

There are many types of corosion: galvanic, pitting, crevice, errosion, intergranular, stress corrosion, biological.

·         Galvanic corossion: The potential difference that exist when two dissimilar metals are immersed  in a corosive or conductive solution is responsible for galvanic corrosion.The less-resistant(anodic) metal is corroded relative to the cathodic metal(it is called galvanic or electrolytic cell).The difference of electrical potential created by two or more dissimilar metals in the mouth or even by a single metal that conflicts with the body‘s own bioelectrical current is called oral galvanism.Other names for oral galvanysm are galvanic mouth currents, “mouth battery“, and metal tension fields.All regulating events in the human body are communicated by electrical charges.Therefore, any conflicting electrical charges that emanate from dissimilar metals in the oral cavity create an imbalance that can lead to pathogenicity.Variuos fluids in the mouth- saliva, bone fluid, and dentinal plasma- act as conductors for electrolytes(minerals that dissolve in a fluid medium into electrically charged ions).Whenever a non-precious metal post is placed in a root canal for reinforcement,or an amalgam buildup is installed ,or a gold crown constructed as a final  restoration, measurable electrical currents emanate from that tooth. This leads to a disruption of the body‘s own internal electrical currents, which in turn has a negative impact on the functioning of the immune system, rendering us more vulnereble to inflammation and infection elsewhere in the body.The infection and toxity that can indirectly result from this „battery effect “ in the mouth, silently permeating the body, and causing untold damage and ill health.Can be metalic pain or/and taste in the mouth too.These problems are exacerbated by using different metals in the mouth, as they cross-react with one another.For example, even more galvanic currents will be created if a gold crown is instaled, followed by a non- precious metal partial(a removable bridge) that contacts with gold crown.Should be used only one kind of alloy  in the mouth for avoiding „battery effect“.The gold being the most desirable choise as it does not  readely oxidise. 

·         Pitting corrosion: pitting  is a form of extremely localized attack that produces damage characterized by surface cavities.(i.e. produces holes- pits in the metal).This corrosion is caused by atacts of chlorides on a protective(passive) layer, which is enhanced by presence of oxidising chemicals.It is an especially insidious form of corrosion because in cause restorations to fail after only a small percentage of the designed for weight loss.Use materials with alloying elements designed to minimize pitting susceptibility.Other methods for preventing pitting corrosion include reducing exposure to aggressive ions be shielding the part, reducing the concentration of these ions in the environment surrounding the part,and minimizing the effects of and  exposure to corrosive factors of those design elements that must not be weakened by pitting.To reduce corrrosion by this mechanism, metals and alloys  used in the mouth should be polished to remove surface irregularities.

·         Crevice corrosion: an intense localized corrosion frequently occurs within crevise and other shieled areas on metal surface exposed to corrosive attack.This type of attack usually is associated with small volume of stagnant liquid at design details such as holes, gaskes surfaces, lap joints, and crevices under bolt and rivet heads.Crevice corrosion like pitting could be very damaging because the destructive action is extremely localized.Crevice corrosion or concentration cell occurs in poorly gasketed flanges and under bolt heads and attachments components immersed  in liquids.

·         Errosion- corrosion: deterioration at an acceleated rate that is caused by relative movement between a corrosive fluid and metal surface.Then the metal tries to grow a new layer via corrosion, which is removed again.

·         Intergranular corrosion: It is the result of  heating an alloy(e.g. stainless steel) can suffer local(normally the grain boundaries) depletion of an essential element for corrosion protectio(e.g. chromium).This is called sensitation.When such an alloy is exposed to oxiding media, the corrosion proceeds along the grain boundaries and the alloy disintegrates into grain.

·         Stress corrosion cracking(SCC) : Specific combinations of alloys and environment can lead SCC.When the metal is mechanically stressed while being exposed to this environment(the stress can be also result from the fabrication process).The metal then fails at a load far below its nominal  mechanical strength.

·         Biological corrosion: When the metalic surface is immersed in water a biofilm will being to form if there is any bacteria in the water.A biofilm is a microbial mass composed of aquatic bacteria, algae, or other micro-orgamizms.The biofilm begins  when organic material is absorbed onto the surface of the metal, bacteria grow, oxygen is exluded, which creates a place where the  passive film may break down.

PROBLEMS OF METALS BIOTOLERATION

Ideally, a dental material that is to be used in the oral cavity should be harmless to all oral tissues-gingiva, mucosa, pulp and bone.It should contain no toxic, leachable, or diffusible substance that san be absorbed into the circulatory system, causing systemic toxic responses, including teratogenic or carcinogenic effects.The material also should be  free of agents that could elicite sensitication or an allergic response in a sensitized patient.Allergic reactions can be local and /or systemic.

It is important to use material that will not trigger an immune response(i.e. biocompatible material).They are called biomaterials.

Titanium

is used in implantology as it is inert and has a very high level of compatibility with human tissues.Importan that the bone cells grow well around implant.Others biomaterials are used in imlpantology too:bioglass,titanium plasma spray, titanium nitrid,aluminium oxide, zirconium oxide,silicium carbide,dens hydroxyapatite, tricalcium fosfate,calcium fosfate ceramic.

Dental amalgam

can cause local soft tissues reaction.Large amalgam particles that are embedded accidentally in the gingiva during placement of a restoration may elicit chronic inflammation.Pigmentation of oral mucosa is called „amalgam tattoo“An increased content of mercury has been observed in gingival biopsies from areas in close contact with amalgam, and in oral lichen planus lessions.

Gold

foil is a stable and relatively insoluble restorative material.There are patiens sensitized to gold may react to gold restorations.These reactions iclude burning sensation of oral mucous membrane in contact with the gold alloy, lichenoid lessions, and general systemic reactions.The insertion of gold may result in pulpal reactions, but these are generally thought to be caused by the conductivity, cavity preparation, dehydration of the cavity and micro leakage.

Allergic reactions have been described for many of metals, including palladium,nickel,chromium cobalt and berylium.

Approximately 10% of women and 1% of man are sensitive to nickel. Allergy to gold-based restorations is reported more commonly than allergic reactions to nickel-containing dental alloys.

Palladium

based alloys have been reported as causative agents in cases of stomatitis, oral lichenoid reactions, and disseminated urticaria. Palladium allergy seems to occur in patients who are sensitive also to nickel.

Chromium

cobalt  alloys have an excellent history of biocompatibility, although there are some reports of tissue sensitivity in a very limited population.

Beryllium and cadmium

(toxic metals) can be harmful too. There is no known skin hazard connected with touching or handling solid forms of beryllium oxide, metallic beryllium or beryllium containing alloys. The hazard  associated with chronic beryllium disease, so called “berylliosis” is  due exclusively with inhalation. It is very important for dental technicians.

DENTAL ALLOYS CLASSIFICATION:

Before discussing alloys characteristics, it is worth reviewing the terms high-noble, noble, base-metals ;

Classification according to the cost -precious, semiprecious and non-precious metals. In dental advertisements precious usually refers to silver, a metal that is precious but not  noble; silver is usually a major ingredient in most alloys advertised as precious. Nonpresious alloys are made from ignoble(base) metals(Co,Cr,Be,Fe)

Classification according to alloys composition:

High -noble metals must have a noble metal content of at least 60%, of which at least 40% is gold. They usually contain a small amount of tin, indium, and/or iron which provides for oxide layer formation. This increases the alloy’s bond strength to porcelain. The disadvantage of these alloys include their lower rigidity and poor sag resistance during porcelain firing. There are three subclasses of high noble alloys(Au-Pt; Au-Pd; Au-Cu-Ag-Pd)

Noble metals are defined on the basis of their chemical properties; that is they resist oxidation and are not attacked by acids .There are seven noble metals but widely are used gold, palladium, platinum(other-Os, Ru, Ir, Si, Rh). The alloys must contain at least 25% noble metal. They may be yellow or  white in color. There are three subclasses of noble alloys(Au-Cu-Ag-Pd; Pd-Cu-Ga; Pa-Ag)

Base-alloys contain less than 25% noble metal. They are used for full-cast and porcelain fused to metals restorations, for removable partial denture frameworks. As a group they are much harder, stronger, and have twice as high an elastic modulus as do the high-noble and noble-metal alloys. This latter property is advantageous because casting can be made thinner and still retain rigidity. For metal- ceramic purposes, this means that porcelain need not be over-bulked for esthetics, and long- span fixed dentures will be more resistant to flexure, which can lead to porcelain fracture. For removable partial dentures, greater rigidity is advantageous because major connectors can be made thinner(and therefore less objectionable to the patient) and still be rigid. The base-metals alloys also have excellent sag resistance. But they have disadvantages too. They are more difficult to cast than non base-metal alloys because of their very high liquidus  temperatures They  also exhibit  a greater casting shrinkage (about 2.3%)that must be compensated for because of their lower ductility and greater hardness, they are less burnish able and more difficult to finish and polish than the high-noble and noble-metal alloys. They are more prone to corrosion under acid conditions, difficult to solder, and can cause an allergic reaction in susceptible patients. Finally, beryllium and nickel can cause allergic reactions and may have carcinogenic effect .There are three subclasses of base-metal alloys(Ni-Cr-Be; Ni-Cr; Co-Cr)

Dental alloys classification according to the main element:

·         Gold

·         Palladium

·         Silver

·         Nickel

·         Cobalt

·         Titanium

Dental alloys classification according to their use:

1.      Gold- based alloys for full- cast restorations

2.      alloys for porcelain- fused –to- metal restorations

3.      Alloys for removable partial dentures

Gold- based alloys classification  according to physical properties:

·         type I –soft(for casting subjects to non bearing stress or very slight stress  bearig- two surface inlays)

·         type II- medium( for inlays, MOD, crowns)

·         type III- hard( for inlays,  crowns, short-span fixed partial dentures)

·         type IV extra-hard( for crowns, long-span  fixed partial dentures, removable partial dentures)

The most commonly used type III which composition is:gold-75%,silver-10%, copper-10%, palladium-3%, zinc-2%.

As the gold-based  full casting alloys go from type I to  type IV, their composition changes: gold content, temperature, and ductility decrease, white copper content, proportional limit, hardness, tensile strength, and yield strength increase.

Alloys for porcelain-fused- to-metal(PFM) restoration:

·         High gold alloys(74-88%-Au and total noble metals content is about 97%), small amount of indium, tin, iron, gallium are  added for strength and to promote a good porcelain bond to metal oxide. Corrosion resistance is excellent because of  high nobility.

·         Gold- palladium alloys(no silver, 50%- gold, 40%- palladium)Disadvantage- the silver free alloys tend to have lower expansion values than the silver-containing group. So, gold palladium group  thermal expansion is incompatible with some of the higher- expansion porcelains. Corrosion resistance is excellent.

·         Gold-palladium- silver alloys(10-15%Ag,20-30% palladium)Disadvantage-have tendency to change the porcelain color(yellow-green) because of their silver content

·         Palladium- copper alloys(70%-80%-palladium, contain little or no gold,15%-copper,9%- gallium ).They are extremely rigid and are used for full-cast and metal ceramic restorations.

·         Palladium-silver alloys (50-50%Pd)Good corrosion resistance, solder well and have the lowest sag tendency of the precious metal alloys. Porcelain bond strength is also excellent.

·         Nickel-chromium alloys (70%Ni,16%Cr,2%Al,0.5%Be).  Al  and Ni form an intermetallic compound, that contributes to strength and hardness, and beryllium lowers the melting range, enhances fluidity, and improves grain structure.

·         Nickel- chromium-beryllium alloys

·         Cobalt- chromium alloys (60%Co,25-30%Cr,which imparts corrosion resistance.)They may also contain minor amount of molybdenum, aluminum, iron, gallium, copper, silicon, carbon, platinum. Manganese and silicon enhance fluidity of the molten alloys.

·         Titanium and titanium alloys

These alloys differed  from  the gold-based alloys used for full-cast restorations in the following ways:

1.      they had higher melting temperature(due to greater palladium content) to accommodate the temperatures required for porcelain application;

2.      they contained little, if any, silver which could cause the porcelain to turn green;

3.      they contain little, if any, copper because it could cause the alloy to sag during porcelain firing;

Alloys for removable partial dentures

·         Type IV gold- based alloys

·         Chromium-based alloys

·         Cobalt-based alloys

The type IV gold-based alloys were strong enough to function adequately when used to make frameworks. The Cr- and Co- based alloys differed from metal-ceramic counterparts in that they contained a small amount of carbon(0.1-0.5%) for hardening.

Amalgam - Dental Materials Lecture Note

Mercury+silver alloy= Dental amalgam

Amalgam restoration limited to the replacement of the tooth tissue in posterior teeth and are recognized by silver gray metallic appearance.

Mercury –dense liquid metal that is high toxic.

Silver alloys for dental amalgams:

Low cooper alloys : silver-65-72%,tin 26-29%,cooper 2-4%, Zinc up to 2%

High cooper alloys : silver-40-60,tin 27-30%,cooper 13-30%

Amalgamation –reaction that occurs b/n mercury and amalgam alloy.

2 types of amalgam alloy compositions are:

1. Silver-Tin alloys w/significant amount of Cooper.

2. Silver-Tin alloys without significant amount of Cooper.

The rardering of amalgam is the result of 2 phenomena:

1. Solutoin

2. Crystalization.

Properties:

1. Dimension change when amalgam hardness , it can be expansion or contraction

2. Strength.

If an amalgam restoration is subjected to the forces of mastication too soon after insertion, it may be seriously damaged by being sufficiency overloaded to cause fraction of the amalgam mass.

Creep of  amalgam is a dimension change that occurs under load as a result of the viscoelastic properties of amalgam.

Tarnish   and corrosion –discoloration and or failing by mechanical properties.

• Proportion of mercury and alloy is 1:1. The mercury must wet the alloy particles before the 2 components can react.
• Moisture contamination of the amalgam must be prevented before and during condensation of the mass into the cavity.

Key Words : fillings dental fillings amalgam fillings mercury free fillings tooth fillings temporary fillings composite fillings teeth fillings tooth colored fillings

Acrylic Denture Processing-Laboatory Proceedure : Prosthetic Dentistry Lecture Note

 

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:

1-    Flasking.

2-    De-waxing.

3-    Acrylic Packing.

4-    Heat Curing.

5-    De-flasking.

6-    Finishing.

7-    Polishing.

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.

1-    Flasking:

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.

So:

-         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: 1^st 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

2-    De-Waxing:

- 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.

3-    packing:

 - 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

*Note:

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.

4-    Curing:

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.

*Remember:

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.

5-    de-flasking

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.

6-    finishing:

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.

7-    polishing:

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