METALLURGICAL TERMS

a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.

b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.

c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.

d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840^o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480^o Fahrenheit, and then immersing it in water.

Stages of manipulation

Definitions of intervals

• Mixing interval-length of time of the mixing stage.
• Working interval-length of time of the working stage
•  Setting interval-length of time of the setting stage

Definitions of times

• Mixing time-the elapsed time from the onset to the completion of mixing
• Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
• Initial setting time-time at which sufficient reaction has occurred  to cause the materials to be resistant to  further manipulation
• Final setting time-time at which the material practically is set as defined by its resistance to indentation

[All water-based materials lose their gloss at the time of setting]

WETTABILITY
To minimise the irregularities on the investment & the casting a wetting agent can be used .

FUNCTIONS OF A WETTING AGENT

1 . Reduce contact angle between liquid & wax surface .
2 .Remove any oily film left on wax pattern .

Mercury bioactivity

• Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
• Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
•  Half life for mercury elimination from body is 55 days .-
• mercury toxicity is <50 µm / m³ on average per 40-hour work week.
• Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
• Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam

Investment Materials

Investment is mold-making material

Applications

a. Mold-making materials for casting alloys
b. Mold-making materials for denture production

Classification

a. Gypsum-bonded investments (based on gypsum products for matrix)

b. Phosphate-bonded investments

c. Silicate-bonded investments

Components

a. Liquid-water or other reactant starts formation of matrix binder by reacting with reactant powder
b. Powder-reactant powder, filler, or modifiers

Manipulation

a. P/L mixed and placed in container around wax pattern
b. After setting, the investment is heated to eliminate the wax pattern in preparation for casting
 

Wax elimination (burnout):

Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.

• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
precious alloys.
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .

WAX BURNOUT AND HEATING THE RING

After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.
The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.

Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.

Hygroscopic Low-Heat Technique. 

After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.

So this technique obtains its compensation expansion from three sources:
(1)   The 37º C water bath expands the wax pattern
(2)   The warm water entering the investment mold from the top adds some hygroscopic expansion
(3)   The thermal expansion at 500' C provides the needed thermal expansion.

High-Heat Thermal Expansion Technique. 

After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent 
This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.  Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

Casting of glass or ceramic

A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .