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  • 7/30/2019 Lab Prostho 10 (1) (1)




  • 7/30/2019 Lab Prostho 10 (1) (1)



    Very important note: you should watch the video on the CD the CR gave

    us.. It helps a lot.. And sorry for any mistake.. It wasn't easy, the dr.

    explained a lot on his cast model.. I tried my best. ERRARE EST

    HUMANUM ! .

    There was a video about the procedures of fabrication of cobalt-

    chromium partial denture.So, we are going to do a quick review of the laboratory steps and then

    you'll go the central lab to see the Casting machine and casting furnace.

    What are the steps for the fabrication of Co-Cr PD?


    Primary cast (dental stone III)Primary impressionWe do initial surveying and design

    We fabricate custom tray

    Tooth \mouth preparationa-Secondary cast (Master )b-Verification surveying and final designc-Block out and reliefd-Duplicate master cast in Agar-Agar or

    Addition silicon- you get the REFRACTORY CAST made

    of Phosphate bounded investment

    material.e-Fabricate wax pattern

    f-Complete the investment process

    :sprout and investment


    h-De-investment and cut sproutsi-Finishing and polishing metal


    Secondary impression (Alginate)

    Mounting and teeth settingJaw Relationship record and Metal

    framework try in

    Wax try-in

    Acrylic processing

    Finishing and polishing


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    1- The initial survey and design are important to formulate thetreatment plan.


    Custom trays are not always necessary, they are necessary fordistal extensions and marginal edentulous areas ,short

    bounded saddle areas don't require custom tray.

    3- Mouth preparation in general involving extraction , tooth rest ,guide planes (all discussed earlier ). If we are to do border

    molding we do border molding

    4- The secondary impression is done using Alginate ,we need anelastomeric material we can't use a rigid material unless

    there's an edentulous area ,such as addition silicon

    ,dimensional silicon or Polysulphides5- Points from e-f are done on the refractory cast.6- We put a wax rim on the metal framework and we do Jaw

    relationship record in the same visit.

    7- The acrylic processing is done on the master cast, mountingand teeth setting is on the master cast , the finish and polish is

    on the master cast; so the master cast after we do duplication

    we keep it.

    Very quickly :

    Edentulous pt , we take primary impression ,using stock traysusing alginate for primary impression ,we can use utility wax to

    modify alginate trays( different from edentulous ones). We pour

    it we end up with primary cast made out of stone , we do initial

    surveying and design , we use it have an idea about the treatment

    procedure. In the lab, you do custom tray (two layers of spacersnear the teeth and one in the edentulous area )

    we make final impression and we end up with final cast (mastercast) type 3 dental stone or type 4 di-stone ;

    the final cast will have rest and guide planes on them ; this castwill have the right tissue contour to fit the PD ; the master cast I

    can't pour the metal ,because won't stand the temp. in

    Centigrade's it's very high necessary to do the casting ; So I do

    block out and relief ;

    shaped block out for the clasps

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    arbitrary block out to simplify the duplication relief to make room for the acrylic which decides where the

    internal finish line will be .

    In order to duplicate we need a special material , we usually use

    a silicon type material , additional silicon ,it's flow able and we

    use special molds to duplicate ; we end up with a mold which is

    a copy of the cast ;or we can use Agar Agar (thermoplastic

    setting ,while silicon chemical set);

    *Agar Agar is thermoplastic and hydrocolloid; water sensitive and can be

    re-used in the lab. While silicon can't be reused to do duplication butcan be report ;

    ** Distinction : silicon can be used to make only one copy of one cast

    but I can pour it many times. Agar Agar I can be use to make copy of

    many casts I can mount it and use it many times but I can only pour it


    Now I have the mold , copy of secondary cast, I'm going to pour it in amaterial stronger than master cast which can withstand high


    I'll use investments materials ( because the Co-Cr-Mo (Cobalt-

    Chromium-Molybdenum) alloy has very high melting temp , so I use

    phosphate bounded investment . I mix it up , I have special liquid with

    high silica content ; I pour it up then sets and dries ;when I remove it

    ,sometimes the surface is not very hard,

    there's a thin layer of something that shines on the surface; what I do to

    remove it? I have a hot bath of bees wax, when it's hot I soak cast in it

    and I take it out again, the surface will absorb some of the bees wax , it

    will make the surface harder ;technically can't scratch the cast even by

    mistake during wax patter , so I do hardening of my refractory cast with

    bees wax.
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    on the master cast , what am I going to do? I want to make the metal

    frame work , so what I do is the wax pattern ,there are special wax

    patterns fabricated by companies so we have : nice even thickness steeled sheet wax ,for the palate mesh work -latex work I-bar ginigivally approaching clasp -circumferential clasps inlay wax to make the rest and meet everything together ;

    ** In wax pattern we make the external finish line ; the internal finish

    line formed by the relief.

    Everything which is wax will turn into metal .

    The problem is , this is not like acrylic , I can't just make another mold

    and open it again , so what I do is similar to acrylic fabrication ,I need

    to make lost wax process but the technique which I use is the sprout

    technique ; instead of making two-part mold I make one part mold

    ;what I do is I place a sprout ( a conical sprout) ;the objective of

    sprout when it's completely enclosed in investment is to allow thewax to escape ,to evaporate and burn out and for the metal to go

    inside to fill the mold .

    So how do I do the mold? I put it in a mold former, I seal it ,I mix my

    investment in a similar technique which I used with stone (in a

    vacuum mixer )we pour it and we end up with something like a block

    ,but when I remove the funnel ,there'll be a space , then I put it in the

    furnace at high temperature approximately 1000 C . for 2 things:

    setting expansions, from chemical reaction of setting andthermal expansion , because when I increase the temp it will

    became larger

    And when my mold is hot the material (metal) will flow mucheasier ,if my mold is cold and I pour metal inside it , it will cool

    down before it goes to the details ;so I warm it to allow

    expansion to compensate for the contraction of metal when it

    cools * I heat it up to allow metal to flow inside it .

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    I need to machines to do casting, I need a machine to heat up my

    mold and another to melt the metal and pour the metal inside the

    mold .** one is called the "CASTIING MACHINE " and the other is "CASTING

    FURNICE" .

    So I put the mold in the oven ;I leave it for several hours ,there's a

    special cycle to allow the wax to evaporate without destroying the

    mold. When it reaches the right temperature ,I take the mold out the

    oven and I put inside the casting machine ,in the casting machine

    either using a flame or using an electric coil ;the metal inside thiscontainer is heated and becomes liquid metal ; I need to get this

    liquid metal inside the mold (I don't hold it by hand ,there's a special

    machine) there are two basics techniques :

    1- centrifugal casting force: it start rotating very fast so that the metal

    inside will flow outside the opening of the machine and it'll go inside

    the mold, inside the mold because it's rotating so fast, the metal

    goes in the details of the mold

    2- vacuum casting ,(the air force will press on the metal pushing it

    inside; the air is hot air ); of course the machine is different from


    The temperature source can be either flame or induction casting(electric


    // The casting process is on the CD the Cr gave .

    After casting we'll move the mold from the casting machine and let it

    cool down, usually changes in color, it becomes white; inside it will be all

    metal ,so I'll tap it and inside I'll find something like to indicate where

    the sprout former were; I'll get the sprout and the metal casting .I use a

    special machine called "SAND BLASTER" ,is a very simple design, it has a

    tube with an opening at the top , with air pressure and Aluminum Oxide

    Abrasive container ..

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    the air will bring the Aluminum Oxide inside the tube and out of the tip

    it'll start shooting the aluminum oxide sand and we use it to abrade; the

    investment will become clean; if I use a bur maybe I'll hit the clasp or

    destroy the machine ,so the way I