Investment materials

1
Investment materials

• Dr. Waseem Bahjat Mushtaha
• Specialized in prosthodontics

2
Introduction

• When a restoration or appliance is being made by
  a (loss wax) process, the wax pattern is embedded
  in an investment material. The wax is then
  removed from this mould, and the space which it
  occupied is filled by the material of which the
  restoration or appliance is to be made

3
(No Transcript)
4
Requirements for investment materials

• All investment materials contain
• 1) A refractory substance a material that will
  not decompose or disintegrate on heating.
• 2) A binder a material which will set and bind
  together the particles the refractory substance.

5

• In addition the following properties are
  desirable
• a) The mould must expand to compensate for the
  shrinkage on cooling of the alloy.
• b) The powder should be of a fine particle size
  to ensure a smooth surface on the casting.
• c) The mixed unset material should have a smooth
  consistency.
• d) The material should have a suitable setting
  time.
• e) The set material should be permeable to allow
  air to escape as the molten alloy enters the
  mould.
• f) The strength of the material should be
  sufficient to withstand the force of the molten
  alloy entering the mould.

6
Types of investment material

• Three types are available. They all contain
  silica (SiO2) as the refractory constituent. The
  chief difference between them is the type of
  binder used, as follows
• a) gypsum-bonded investments are widely used for
  gold alloys, but are unsuitable for alloys which
  melt at temperatures approaching 1200C

7

• b) Phosphate- bonded materials are used for
  casting cobalt-chromium alloys, since they can
  withstand higher temperatures
• c) Silica-bonded investments are an alternative
  to the phosphate-bonded materials for high
  temperature casting

8
Gypsum-bonded investments
9
Gypsum-bonded investments constituents

• a. Silica is present in one of its allotropic
  forms (cristobalite or quartz) to
• 1) Act as refractory.
• 2) Provide mould expansion by thermal expansion
  and inversion

10

• B- Autoclaved calcium sulphate hemihydrate, for
  the following purposes
• 1) To react with water and on hydration to bind
  the silica together
• 2) To impart sufficient strength to the mould.
• 3) To contribute to the mould expansion by the
  setting expansion which occurs.
• C- A reducing agent such as powdered charcoal, to
  reduce any oxide formed in metal.
• D- Modifying chemicals such as boric acid or
  sodium chloride to inhibit shrinkage on heating.

11
Manipulation

• a) The mixing of an investment material is
  similar to that of dental stone. Use of the
  correct water/powder ratio is important to ensure
  that the correct strength, setting time and
  expansion are obtained.
• b) Before investing the wax pattern , it is
  washed with a non-foam detergent to remove any
  oil or grease, and to facilitate the wetting of
  the pattern by the investment mix.

12

• C) The casting ring is usually lines with a wet
  asbestos strip. This does two things
• 1- It facilitates mould expansion, since it can
  be compressed as expansion occurs, whereas a
  rigid ring on its own cannot do this.
• 2- It contributes the pattern may be carried out
• a) Under vacuum to prevent trapping air on the
  surface of the pattern
• b) painting investment material on to the pattern
  with a brush before carefully inserting it into
  the filled casting ring

13

• E- The mould is heated through 150-200C this
  dries out the excess water and burns off the wax.
  The mould is then slowly heated to above the
  temperature of inversion- usually to 700C it is
  held at this temperature for 30 minutes to ensure
  that the wax is completely burn out.

14
Dimensional changes of the mould

• A) Setting expansion is caused by the crystal
  growth of gypsum.
• b) Hygroscopic expansion. In one technique the
  investment is immersed in water after setting has
  begun. A greatly increased setting expansion
  occurs. So less thermal expansion is required.
  Increased hygroscopic expansion is obtained in
  the following cases
• 1) When a lower water/powder ratio is used
• 2) For an investment material of greater silica
  content.
• 3) If water of higher temperature is used.
• 4) For longer immersion in water.

15

• c) Thermal expansion. Investment materials
  containing cristobalite and quartz show rapid
  expansions between 200-300C and 500-600C
  respectively, due to displacive transformation of
  the silica. The amount of thermal expansion
  therefore depend on
• 1- Temperature.
• 2- Quantity of silica in the material
• 3- Allotropic form of silica used, for example
  the thermal expansion of cristobalite is greater
  than that of quartz at most temperatures.
• 4- water/powder ratio- thicker mixes have
  greater thermal expansion.
• d) Shrinkage on heating. This occurs due to the
  dehydration of the set gypsum in two stages
• 2CaSO4,2H2O (CaSO4)2,H2O 3H2O
• (CaSO4)2,H2O 2CaSO4 H2O
• The investment shrinkage is eliminated or reduced
  by the presence of small quantities of additives
  such as sodium chloride or boric acid.
• Other properties
• a) The total expansion of the mould is generally
  sufficient to compensate for the shrinkag on
  cooling of cold alloys ( about 1.5 by volume).

16

• b) The investments containing finer particles of
  silica and calcium sulphate hemihydrate give
  smoother surfaces on the finished casting.
• c) gypsum-bonded investments are easy to
  manipulate giving a smooth consistency mix.
• d) The setting time of theses materials can be
  easily controlled, as for stone and dental
  plaster
• e) The set investment is porous, as is set
  gypsum. This helps to prevent back-pressure
  porosity in castings.
• f) The strength of these materials, when set, if
  mixed at the correct water/powder ratio, is
  sufficient to withstand the forces of the molten
  alloy as it enters the mould. The autoclaved
  hemihydrate is used in preference to the calcined
  material for this reason

17
Limitations

• Above around 1200C, a reaction can occur between
  calcium sulphate and silica
• CaSO4 SiO2 CaSiO3 SO4
• The sulphur trioxide gas that is evolved
• 1) Causes porosity in the casting.
• 2) Contributes to the corrosion of the casting.
• For this reason gypsum-bonded investments are not
  used for the higher fusing dental alloys, such as
  cobalt-chromium. In this case phosphate or silica
  bonded materials are chosen.

18
Phosphate-bonded investments
19
Phosphate-bonded investments composition and
setting

• Magnesium oxide can react with a phospharic such
  as an ammonium phosphate in an aqueous system as
  follows
• MgO NH4H2PO4 MgNH4PO4 H2O
• The crystals of the magnesium ammomium phosphate
  bind together the particles of the silica
  refractory.

20
Manipulation

• These materials are mixed with water, similar to
  gypsum-bonded investments. However, the following
  difference in manipulation should be noted
• a) Because of the strength of the set material,
  metal casting rings are not needed. In their
  place, plastic rings can be used, they are
  removed after the material has set, but before
  the investment is heated.
• b) The investment is heated to 1000-1100C

21
Properties

• a) Expansion. The setting reaction is accompanied
  by an expansion, analogous to the crystal growth
  of gypsum. Also, thermal expansion occurs in
  heating.
• b) Porosity a set phosphate-bonded material
  shows a certain degree of porosity, again similar
  to the gypsum containing investments.
• c) strength the set materials increases in the
  strength during heating, possibly due to chemical
  interaction between silica and the binder, giving
  complex silicophoshates.

22
Silica-bonded investments

• Setting reaction
• a) Stage 1 hydrolysis. Ethyl silicate can be
  hydrolysed to silica acid, with liberation of
  ethyl alcohol
• Si(OC2)4 4H2O Si(OH)4 4C2H5OH
• In practical, a polymerised form of ethyl
  silicate is used, yielding a sol of polysilicate
  acid.

23

• b) Stage 2 gelation. The sol is mixes with
  cristobalite or quartz, then gel formation is
  made to occur under alkaline conditions by adding
  magnesium oxide. There is a slight shrinkage at
  this stage.
• c) Stage 3 drying. on heating, considerable
  shrinkage occurs and there is a loss of alcohol
  and water, leaving a mould made of silica
  particles tightly packed together.
• As alternative of the above, simultaneous
  hydrolysis and gel formation can occur, when an
  amine such as piperidine is incorporated.

24
Properties

• a) Dimensional changes. There is a large amount
  of the thermal expansion, due to the very large
  percentage of silica in the final material. This
  expansion is usually sufficient to compensate
  for
• 1- The setting shrinkage of the investment
  material.
• 2- The casting shrinkage of the alloy.
• b) porosity. The particles of the set material
  are packed so closely together that the porosity
  is negligible. Air spaces or vents must be left
  in the investment to permit escape of the air
  from the mould.

25
Venting