INVESTMEN

Question 1

An investment is

Answer 1

a ceramic material which is suitable for forming a mold into which molten metal or alloy is cast. The procedure for forming the mold is described as ‘investing’

Question 2

An investment is consisting of

Answer 2

a refractory and a binder

Refractory: material which resists heat, Silica (silicon dioxide) SiO2 (usually
quartz and cristobalite).

Binder: Material that holds the investment together

Question 3

refractory materials are

Answer 3

materials that can withstand high temperatures

Question 4

REQUIREMENTS OF AN
INVESTMENT MATERIAL

Answer 4

1.smooth consistency.
2- The powder particles size should be sufficiently fine.
3- The investment must expand to compensate for the casting
shrinkage
4- It should not crack or decompose on heating.
5- should be sufficiently permeable to allow air to escap
6- should have sufficient strength to withstand the force of the molten alloy

Question 5

casting investment materials can be classified according to the binder:

Answer 5

Gypsum-bonded investment, Used for casting of conventional gold alloys

Phosphate-bonded investment, •Used for casting of metal-ceramic alloys
•Used for casting of cobalt chromium alloys

Silicate-bonded investment , Used for casting of base-metal alloys partial
denture (e. g. cobalt chromium alloys)

Question 6

Gypsum-bonded investments
Composition:

Answer 6

1- Autoclaved calcium sulfate α-hemihydrate , to serve as:
A binder and provide sufficient rigidity to the mould.

This type of investment is used for casting of gold alloys with melting temperature
range below 1000˚C (1832˚F), because if gypsum-bonded investment is heat
above 700˚C (1292˚F) it will shrink and decompose and fracture.

2- Silica (SiO2) 60% , Silica is a refractory material which adequately
withstands the temperatures used during casting.

3- Modifiers: Such as boric acid, sodium chloride
4- Coloring agent.
• 5- Reducing agents: Such as carbon and powdered copper to provide a non-oxidizing atmosphere in the mould when casting of the gold.

Question 7

Silica t is available in three
allotropic forms

Answer 7

– quartz, cristobalite and tridymite–

Question 8

Silica (Quartz and cristobalite ) is
added in the investments to:

Answer 8

• a. Acts as refractory substance during heating of the investment.
• b. To regulate the thermal expansion

Question 9

Thermal expansion: This expansion occurs because of

Answer 9

displacive transformation of silica (inversion) from low (α) to high (ß) form.

Question 10

Hygroscopic setting expansion: If the setting process is

Answer 10

allowed to occur under water , greater in magnitude than the normal setting expansion