انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية طب الاسنان
القسم ترميم ومعالجة الاسنان
المرحلة 3
أستاذ المادة احمد غانم مهدي الهلال
4/27/2011 3:02:32 PM
Investing Procedure:
We need:
1. Casting ring.
2. Liner (asbestos liner).
3. Crucible former.
4. Wax pattern.
5. Investment material.
Line the inside of the ring with asbestos liner then we attach the ring to the crucible former so that the wax pattern will be in the center of the ring . The liner gives greater expansion, the amount of expansion depend on the condition of the liner , most liners give more expansion ( hygroscopic ) than that obtained from dry liner .
Method for mixing invest. Material :
Its either (1) manual or by (2) mechanical means .
Whether by manual or by mechanical means the following points should be considered :
1. The manufacturer instructions and water \ powder ratio .
2. The mix should be free from air bubbles .
3. Temp. of water should be about the room temp. to avoid distortion of w. p. , if not , it will cause distortion (expansion ) of w. p.
Mechanical is by vacuum mixer to ensure that the mix is completely free of air bubbles then we fill the casting ring (we apply the invest. to the w. p. by a brush then we fill the ring ) .
Vacuum tech. for investing : We again start to mix the powder & liquid in a special glass bowel then after mixing by hand we attach the glass bowel with the machine , after we finish mixing we invert the glass bowel to start filling the ring , then leave the ring for 1 hr , we wait until complete setting of investment material inside the ring , some times we leave it for 1 day & then separate the ring from the crucible former .
Now , the investing ring is ready for the burn out procedure .
Burn out procedure :
Heating of the invest. Ring in other mostatically controlled furnace (oven ) until the traces of wax are vaporized so that mold cavity is created into which the molten metal can be placed .
***The temp. reached by investment determine the thermal expansion .
Advantages of burn out procedure :
1. Elimination of the wax & plastic material from the mold cavity of the investing ring .
2. Elimination of the moisture or water from the investing ring.
3. To produce the necessary expansion in the investment to compensate for the solidification shrinkage of the alloy after casting procedure .
4. To rise the temp. of the mold to the proper point that permits throwing of the molten metal in to the mold cavity when the casting is made , other wise incomplete casting might result as a result of rapid solidification of the alloy as it inters the mold cavity.
Techniques of heating of investing ring :
1) High heat tech. : temp. about 1200 f.( 650 c ) if thermal exp. Is used for compensation expansion.
2) Low heat tech. : the temp . is 900 f. (480 c ) is used if hygroscopic exp. is needed for compensation exp .
Procedure of burn out :
1. After we separate the ring from the crucible former , we place the ring inside the oven with crator down at 600 f , we put it over 1\2 hr. by burn out in such position (inverted position of the ring ) , we allow the wax and water vapor to run out freely from the mold cavity
2. Depending upon the tech. used increase the heat to the final burn out temp. 900 f if low heat tech. and 1200 f if high heat tech. and we leave it for an hour or some times we can transfer the ring to high heat oven & hold it for one hour .
3. The ring should be crator up & this is about 10 min before casting procedure , this is done to allow the oxygen to contact the internal area of the mold cavity so we ensure complete removal or elimination of wax residue and residual carbon ( it is removed by oxidation in the form of carbon oxide or dioxide ).
General instructions during burn out procedure :
1. The position of the ring : it should be placed in the center of the oven so that the atmosphere surrounding the ring is the same that recorded on the pyrometric indicator .
2. The burning out time is the same whether we place the ring in to the room temp. or 600 f (temp. of oven ).
3. Initial burning out temp. (about 600 f ) cause too rapid heating with danger of cracking investment or cracking with the mold cavity so we should never place the ring in over heated oven .
4. Time required for proper burn out depend on :
a. size of the ring .
b. number of the rings .
c. type of the oven .
Casting procedure :
It is defined as introducing the molten alloy in to previously prepared mold cavity in the investing ring .
Casting equipment : Present day casting machine still use air pressure or centrifugal force as mean of filling of the mold with the molten alloy .
casting machine require :
1. Heat source to melt the alloy .
2. Casting force so that the casting will be completed .
The casting force must be high enough to overcome
1. The high surface tension of the molten metal as well as ,
2. The resistance of gas within the mold .
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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