Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
24 Cards in this Set
- Front
- Back
|
Review
|
the adoptaion of the casting for making onlays inlays , crown and bridges
|
|
|
investing material
|
it's ceramic material that is suitable for forming a mold which metal or alloy is cast
|
|
|
inveting
|
the operation of forming a mold
|
|
|
Lost wax technique
|
1- fabrication a pateern was and attach it to a sprue
2-investing the wax pattern by surrond it with investment 3-burning out the wax 4-melting the alloy and and forcing it inti the mold |
|
|
properties requried
I- FOR manipulation |
1- the investing material shouldn't be expensive
2- free from air bubbles 3-easy to mis and easy to obtain 4-flow readily to produce fine details 5-should have low contact angle 6-no adverce chemical reaction with wax 7-have adquate strength 8-produce smooth surface |
|
|
II-FOR COMPENSATION
|
1-should have enough expansion during hardening and heating to compensate he shrinkage of the wax pattern and the metal takes place
|
|
|
the amount of contraction :-
|
- 2% for gold alloy by indirect wax pattern
- 2.5 %those by direct technique - contraction is higher for higher mellting alloy |
|
|
III- FOR CASTING
|
1- The investment should gain strength upon heating
---> at room T :- to resist fructure suring removal of sprtue ---> at high T :- to support impact of molten alloy 2- the nvestment shouldn;t genet\rate toxic gas 3-should havs sufficient porosity to allow escape of gas |
|
|
IV- FOR FINISHING
|
1- investment should be weak after casting
2- don't react with the cast |
|
|
>CLASIIFICATION OF DENTAL INVESTMENT MATERIALS :-
I- ACCORDING TO TYPE OF BINDER:- |
1- gypsum- bonded investment
2-phosfate-bonded investment 3- silicate-bonded investment |
|
|
II- ACCCORDING TO ADA :-
|
TYPE 1 :- inlay,thermal
TYPE 2 :- inlay,hygroscopic TYPE 3 :- partial denture,thermal |
|
|
I- GYPSUM BONDED INVESTMENT :-
INDICATION:- |
- For alloys that don't required heating the investment above 700 (gold alloy)
- heating above 700 will lead to gypsum to decompose |
|
|
composition :-
|
1- SILICA :- 60%---65%
in one or in misture (quartz0 used to impact refractoriness and thermal expansion 2- gypsum:- 30-- 35 % in the form of CaSo4.1/2h2o BENEFITS :- -USED AS A BINDER MATERIAL -IMPACT THE STRENGTH TO THE MOLD -CONTRIBUTE TO MOLD EXPANSION 3-ADDITIVES:- -GRAPHITE:- reduce atmo during heating - BORIC ACID OR SODIUM CHLORIDE :- inhibit shrinkage of the binder upon heating |
|
|
investing equipment :-
|
1- casting ring lined by liner (ceramic paper<asbestos substitute<)
2- 3mm of clearance at the end of the ring 3- the liner dipped in water to be saturated |
|
|
function of the liner
|
1- facilitate setting expansion
2- contribute to hygroscopic expansion |
|
|
INVESTMENT EXPANSION
|
the induce expansion of the mold cavity to compensate the contraction caused by shrinkage of the wax and the cast alloy during hardening and heating
-achieved by :- -setting expansion -hygroscopic expansion -thermal expansion |
|
|
I-SETTING EXPANSION
|
INVOLVES THE
CaSo4.1/2H2O=====> CaSo4.2H2o - AND DUE TO FORMING OF CRYSTALS (DEPEND ON BENDING MATERIAL) |
|
|
II-HYGROSCOPIC EXPANSION (LOW HEAT TECHNIQUE)
|
achieved by allowing the gypsum bonded material to set while contact with excess water BY:-
- immersion technique -water added technique * the water added technique is more accurate than immersion technique |
|
|
III- THERMAL EXPANSION (HIGH HEAT TECHNIQUE)
|
compesation of wax and alloy contraction is achieved through thermal expansion (silica is the responsible)
|
|
|
allotropic of silica
|
-quartz
-tridymite -cristobalite (tridymite,cristobalite ) find in 2 polymorphic forms :- A) alpha form ===> more stable in room T B) beta form ====> mora stable at high T the invesrsion from alpha to beta accompanied by EXPANSION -tridymite :- has 3 stable polymorphic forms |
|
|
COOLING THE INVESTMENT
|
the refractory and the binder contract on cooling
-TO ROOM T:- an oveaall contraction occurs -ON REHEATING:- -the investmment doesn't expand to the previous level -heating and cooling may cause CRACKS in the investment |
|
|
INVESTMENT FOR CASTING HIGH MELTING ALLOYS
|
most palladium,base metal alloys,partial dentureand porcelin metal to metal alloy have HIGH MELTING T greater than 700 and reach 850--1100
so we use phosphate and silicate compounds this type has less than 20%BINDER and the remainder is QUARTZ |
|
|
I-PHOSPHATE BONDED INVESTMENT
|
DIFFER FROM GYPSUM IN :-
1- the S.reaction isn't simple hydration 2-the phosphate can react with SiO2 to give superior strength |
|
|
-COMPOSITION
|
-THE POWDER:-
1-SILICA :-act as refractory (80%) 2- ammonium dihydration phosphate:- source for phosphate ions 3- MAGNESIUM OXIDE:- REACT WITH PHOSPHATE IONS TO GIVE (magnesium ammonium phosphate (BINDER) MgO + Nh4H2PO4 ====> NH4MgPO4 + H2O - THE LIQUID :- colloidal suspension of SiO2 |
