overview of dental casing materials types , uses, setting reactions, advantages and disadvantages etc

1. DENTAL CASTING
INVERSTMENT MATERIALS
Presented
at Dental Training
School
on 15th March 2019
by Philip Shaibila
Dip Dental Tech.

2. INTRODUCTION
• Investing is the process of covering or enveloping, wholly or in part, an object such as denture,
tooth, wax form, crown, etc. with a suitable investment material before processing, soldering or
casting.
• Dental casting investment is defined as a material consisting primarily of an allotrope of silica
and a bonding agent. The bonding substance may be gypsum (for use in lower casting
temperatures) or phosphates and silica (for use in higher casting temperatures).

3. IDEAL PROPERTIES
• Refractory substance: A material that will withstand high temperatures without
decomposing.
• The mold must expand to compensate for casting shrinkage on cooling the alloy.
• The powder should have fine particle size to give a smooth surface on the casting.
• Easily manipulated with enough working time.
• It should have smooth consistency when mixed.
• The set material should be porous enough to permit the air or other gases in the mold
cavity to escape easily during casting procedure.

4. CONT…..
• On being heated to higher temperature the
investment must not decompose to give of gases that
may corrode the surface of the alloy.
• It should have enough strength at room
temperature to permit ease in handling and enough
strength at higher temp, to withstand impact force of
the molten metal
• Casting temp should not be critical
• Material should be economical

5. TYPES OF INVESTMENTS
• Gypsum bonded investment
• Phosphate bonded investment
• Silica bonded investment

6. GYPSUM BONDED INVESTMENT
• Gypsum bonded investments are the oldest materials and are used for casting
conventional gold alloys.
• They are the mold materials used in the casting of dental gold alloys with liquidus
temperatures no more than 1080 ºc. Used for conventional casting of gold alloys
inlays, onlays, crowns and Fpd.

8. COMPOSITION
• Refractory; 55-75%
• Binder (25-45%)
• Modifier (4-7%)

9. MANIPULATION
• Mix the investment compound by hand for approximately 30 seconds with a
spatula, allow to stand under a vacuum for approx. 30 seconds. Then mix under
vacuum for 60 seconds with a mixing and evacuation unit.
• After mixing, place the mixing bowl on a vibrator (approx. 30 seconds) and
allow the investment compound to flow together. Then let the compound to flow into
the mould, shaking gently and ensuring that no bubbles form. After the mould has
been filled, switch off the vibrator.
• Allow the mould to set for 45-60 minutes, depending on the size If the mould is
preheated too soon the investment compound can crack

10. USES
• They are used for casting metal
inlays, onlays, crowns and bridges.
• They are used for casting gold
alloys

11. THREE TYPES OF GYPSUM BONDED
INVESTMENT MATERIALS
• Type 1: thermal expansion type; for casting
inlays and crowns.
• Type 2: hygroscopic expansion type; for casting
inlays and crowns
• Type 3: for casting complete and partial dentures

12. FACTORS AFFECTING THE SETTING EXPANSION
• Composition: more finer silica particles -more HSE. α hemihydrate greater HSE
than β hemihydrate.
• W: P ratio: less water, more powder in mix.
• Spatulation: more mixing time- more HSE.
• Time of immersion: immerse in water before initial set.
• Confinement: less opposing force from walls of casting ring (wet cellulose),
immersion in water bath at 37.7°C- expansion of wax pattern.
• Water: more immersion water.
• Shelf life: fresher investment.

13. ADVANTAGES
• Easier to manipulate.
• Adequate strength.
• Adequate porosity.
• Less expensive.
• Amount of dimensional change easier to control.

14. DISADVANTAGES
• Thermally unstable at high temp.
• Powder is hygroscopic.
• Improper wax burnout & high casting force may produce cracks in investment
leading to fins.

15. PHOSPHATE BONDED INVESTMENT
• These are used for casting cobalt chromium alloys. They can withstand high temp.
Silica bonded investment: These are alternatives to phosphate bonded investment
for high temp, casting. Principally used in casting of base metal alloy partial
denture.

16. TYPES
• Type 1: For casting of inlays crowns and other restorations especially for alloys like
gold, platinum, palladium cobalt chromium and nickel chromium
• Type 2: For casting of removable partial dentures.

17. COMPOSITION
• Refractory filler (80%)
• Binder
• Other
• Colloidal silica

18. MANIPULATION
• 1. Open the pre-weighed bag
• 2. Measure the liquid
• 3. Put the liquid in the bowl
• 4. Add powder
• 5. Mix: hand mix according to direction and vacuum mixer direction
• a) Hand mix: The liquid is added to a clean, dry mixing bowl, and the powder is
gradually added to the liquid, using the care and caution to minimize air entrapment.
Mixing is formed gently until all the powder has been wet.
• b) Vacuum mixing: this is a type of mechanical mixing done under vacuum created by a
vacuum mix machine.
• 6. Pour the mixer into the ring: metal casting ring.

19. USES
• Construction of high melting temperature dental alloys.
• Soldering and porcelain veneering
• Casting of base metal alloy of partial dentures

20. SETTING REACTION
• Ammonium diacid phosphate reacts with magnesium oxide - green strength, or
room temperature stregth.
• Ammonium diacid phosphate is used in a greater amount.
• additional amount can react with silica at an elevated temperature – P2O5 and SiO2
forms silicophosphate which increases the strength of investment
• NH4H2PO4 + MgO + 5H2O - NH4MgPO4 6H2O

21. ADVANTAGES
• They have high fired strength.
• They have high setting and thermal Expansion.
• They can withstand temp more than 900°C.

22. DISADVANTAGES
• When used with higher melting alloys i.e. those with casting temp. ≥ 1375ºC,
these investments results in mold breakdown & rougher surface on castings.
• The high strength of these investments can make removal of the casting from
the investment difficult.

23. ETHYL SILICATE BONDED INVESTMENTS
• This type of investments loosing popularity because of the more complicated and time
consuming procedures involved, but it is still used in the construction of the high
fusing base metal partial dentures alloys. In this case, the binder is a silica gel that
inverts to silica (cristobalite) on heating. Several methods may be used to produce
silica or silicic acid gel binder.
• It is supplied as a powder and liquid or two liquids;
• Powder consists of refractory particles of silicas and glasses along with the calcined
magnesium oxide and some other refractory oxides in minor amounts.
• Liquid contains stabilized alcohol solution of silica gel.

24. COMPOSITION
• Refractory material – Silica
• Binder –Silica gel or ethyl silicat
• Modifiers;
• Magnesium oxide (strengthen the gel)
• Ammonium chloride – accelerator

25. MANIPULATION
• The powder is added to hydrolyzed ethyl silicate liquid, mixed quickly and vibrated
into a mold, which has an extra collar to increase the height. The mold is placed on a
vibrator that has a tamping action. This allows the heavier particles to settle while
the excess liquid and some of the finer particles rise to the top .The top of the mold is
prone to cracking due to greater drying shrinkage from evaporation of the ethyl
alcohol. In about 30 minutes the accelerator in the powder hardens the settled part,
and the excess is poured off (to avoid crack formation). The liquid powder ratio in
settled part is greatly reduced and the setting shrinkage is reduced to 0.1%.

26. USES
• Used for higher temp castings (1090°C1180°C) and compactible with higher fusing
alloys.

27. SETTING REACTION
• The silica is first formed by the hydrolysis of ethyl silicate in the presence of
hydrochloric acid, ethyl alcohol & water. The reaction can be expressed as:
• Si(OC2H5) + 4H2O - Si (OH)4 + 4C2H5OH
The second stage of reaction – gelation.
• Sol mixed with quartz or cristobalite + small amount of MgO to render the mixture
alkaline.
• Coherent gel of polysilicic acid formed – accompanied by slight ‘setting shrinkage’.

28. CONT……..
Third stage – drying
• Soft gel dried to a temp below 168 °C. During drying Gel loses alcohol & water to
form hard concentrated gel of silica particles tightly packed together. Volumetric
contraction accompanies drying “green shrinkage”.

29. T H A N K Y O U
WHENLIFEGETSSO HARD,USEWHATYOUHAVETO
SUCCESED.