537837

a small amount of water acccelerates the setting of zinc oxide eugenol impression paste

600343

Main causes of the porosity of alloy castings are:
1) Solidification defects
2) Trapped gases
3) Residual air

1. Solidification defects

Solidification defects cold lead to two different manifestations of porosity;
localised shrinkage porosity and Microporosity.


Localised shrinkage porosity is caused by insufficient feeding of the alloy
during solidification.

Microporosity is also caused by solidification shrinkage, but generally happens
in fine grain alloys when the solidification is too rapid for the microvoids to
segregate. This in turn is caused the mould or casting temperature being too
low.

2. Trapped gases

Many metals dissolve or occlude gases when they are molten. On solidification,
these gases are forced out of the casting causing what is usually called pinhole
porosity. These voids are rather small.


3. Incomplete casting
If the molten alloy is prevented from fully or partially filling the mould and
incomplete or even no casting at all can result.

704351

The working time for a polysulfide impression material can be safely and effectively increased by cooling the mixing slab to a temperature above the dew point

857445

342289

Heating a gypsum cast to a temperature above 90°C will remove water of
crystallization and strengthen it. This process is called "burning out" the
gypsum, and it is used to remove the remaining moisture and improve the cast's
stability and accuracy.
Gypsum casts are made by mixing plaster of Paris (calcium sulfate hemihydrate)
with water. When the material sets, it forms calcium sulfate dihydrate crystals
with water molecules trapped within the crystal lattice, known as water of
crystallization. Heating the cast above 90°C causes the water of crystallization
to be released, which results in the cast becoming stronger and more rigid. This
process does not cause the cast to expand but rather to shrink slightly as the
water is removed. Excess gauging water is typically removed during the initial
setting process.

109423

Strength and hardness of a gypsum model or cast are affected by water-powder ratio and porosity of cast

890189

Fusion Temperature of Impression Compound should occur above mouth temperature

482940

The Vickers hardness test is the most suitable for materials that exhibit
elastic recovery. The Vickers hardness test involves the use of a diamond
indenter in the form of a pyramid with a square base. The test applies a
controlled load to the specimen and measures the depth of the resulting
indentation. It is a micro-indentation hardness test, meaning it is used for
small or thin sections or for precise measurements. This test is particularly
useful for brittle materials, such as ceramics, because it creates a very small
indentation and applies a relatively low load compared to the other methods.

The other options are not as suitable for materials with elastic recovery for
the following reasons:

2. Brinell hardness test: This test uses a spherical indenter, usually a hard
steel or tungsten carbide ball, to press into the material with a significant
load. It is typically used for softer materials and is not as precise for small
indentations or for materials that are prone to plastic deformation, which may
affect the accuracy of the elastic recovery measurement.

3. Knoop hardness test: Similar to the Vickers test, the Knoop test uses a
diamond indenter but in the shape of a rhombic pyramid. Although it is also a
micro-indentation test, the Knoop indenter has a more pointed tip compared to
the Vickers pyramid. This can make it less ideal for certain materials that
exhibit elastic recovery since the sharp tip can cause more plastic deformation
and potentially damage the material.

While all three tests can be used to measure the hardness of materials, the
Vickers test is generally considered the best for materials that exhibit elastic
recovery due to its ability to produce precise and reliable results with minimal
plastic deformation.