Effective Binders in Investment Casting: Types, Uses, and Casting Procedures

Effective Binder In Investment Casting

A material that will set and bind together the particles of a refractory substance known as a “binder”.

In gypsum-bonded investment: Dental stone (alpha hemihydrate) acts as a binder for investment used in casting gold-containing alloys.

• Gypsum-bonded investment permits the pouring of the mix into the mold.
• Gypsum-bonded investment  binds and holds the silica particles together.
• Gypsum-bonded investment imparts strength to the mold.

In phosphate-bonded investment: The binder consists of magnesium oxides (basic) and a phosphate that is acid in nature. Originally phosphoric acid was used, but monoammonium phosphate has replaced it because it can be incorporated into the investment powder.

“Types Of Binders Used In Investment Casting Processes”

In silica-bonded investment: The silicate is a binder.

• It is derived from ethyl silicate or aqueous dispersion of colloidal silica or sodium silicate.
• Ethyl silicate has the disadvantage of containing inflammable components which are required for manufacture.
• Sodium silicate and colloidal silica are more commonly used as binders.

“Importance Of Binders For Mold Strength In Investment Casting”

Soldering investment:

Binder:

• Calcium sulfate
•  Hemihydrate.

Casting Procedures And Defects

Write in detail about investing and burnout procedure for gold fixed partial denture.
Answer:

Investing Procedure for Gold Fixed Partial Denture:

• We use gypsum-bonded investing material type III for gold fixed partial dentures.
• Apply a wetting agent on the wax pattern.
• Seat casting ring into crucible former taking care that it is located near center of the ring.
• Mix investment and vibrate.
• The same investment is applied to the wax pattern with a brush to reduce trapping air bubbles.
• The ring is reseated on the crucible former placed on the vibrator and gradually filed with the remaining investment mix. Allow it to set for 1 hour.

“Early Signs Of Problems With Investment Casting Binders”

Burnout Procedure for Gold Fixed Partial Denture:

• Burnout is started when the mold is set.
• Store in a humidor if burnout is to be delayed. Heating should be gradual.
• Rapid heating produces steam which causes the walls of the mould cavity to fake.
• The ring is placed in a burnout furnace and heated gradually to 400°C for 20 minutes maintained it for 30 minutes.
• In the next 30 minutes, raise the temperature to 700°C and again maintain for 30 minutes.
• Casting should be completed as soon as the ring is ready.