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Gypsum products and investments

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                    Introduction
   Gypsum is a mineral naturally found in nature.
   Dental applications:
       Impression materials
       Constructing casts and dies
       Making replicas




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    Ideal properties of die materials
   Dimensional accuracy.
   Ease of use.
   Accurate reproduction of fine detail.
   Inert.
   Good strength.
   Inexpensive.
   Abrasion resistant.
   Good color contrast
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         Properties and behaviour

   Chemical properties: (exothermic reaction)
Ca sulfate dihydrate heat         water loss ground Ca sulfate
  hemihydrate
 The production of various types of gypsum is basically the same:

    If the heating process occurs in atmospheric pressure at 115°C,

     the powder particles are porous and irregular (b hemihydrate,
     plaster).
    if heating process is under pressure, powder particles are

     regular and less porous (a hemihydrate, or dental stone).


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   A further increase in pressure and refining of the
    powder by grinding results in denser stone, high-
    strength or die stone

   When stone is mixed with silica: dental
    investment, a material that can withstand high heat
    and stress produced when molten metal is forced
    into molds to form indirect restorations


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        Gypsum products




Plaster of Paris               Dental stone




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                   Die stone
                 Physical properties

Type             W/P    Porosity   Compressive Abrasion     Setting
                                   strength    resistance   expansion

plaster          0.45   High       8.8 MPa      Low         High


Stone            0.30   moderate   20.6 MPa     Moderate    Moderate


High strength    0.23   Low        34.3 MPa     High        Low
stone

High strength    0.20   Low        48.0 MPa     High        High
High-expansion
stone
                                                                   7
        Physical properties (cont.)
   Strength and hardness: Affected by
      Porosity

      Shape of particles (large irregular particles don’t

       condense well leading to decreased density).
      Amount of water mixed with the powder. Higher

       porosity requires more water to be mixed to
       produce a cast which will be weaker.



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   Dimensional accuracy:
       The higher the setting expansion, the lower
        the accuracy.
       Setting expansion results from the growth of
        crystals as they join.
   Solubility:
       Set gypsum is not highly soluble
       The greater the porosity the greater the
        solubility.


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   Reproduction of detail:
       Greater porosity decreases surface detail
        production

       The impression material should be compatible with
        the gypsum to improve detail reproduction. The
        best compatibility is between silicon and
        polyethers and gypsum




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            Classification of gypsum
   Impression plaster (Type I)
    *seldom used*
   Model plaster (Type II): used
    for
      Diagnostic cast

      Articulation of stone cast

      Art portion of working cast

      Flasking procedure for
        acrylic dentures (cast end)
    The mix produces a weak cast
    compared to dental stone. It’s    Impression (negative replica), poured
    available is fast and regular     into gypsum to make a cast (positive
    sets.                             replica


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                  Plaster




Plaster

          Flask


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   Dental stone (Type III):
    used for making
      Full or partial denture
       models
     Orthodontic models

     Flasking procedure for
       acrylic dentures (teeth
       end)
    It requires less water,
    stronger than plaster
                                 Dental stone casts



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   Dental stone, high strength
    (Type IV):
      AKA die stone used in

       fabricating wax patterns of
       cast restorations (crown
       &bridge)
      Implants

      Precision attachments work

      Often colored pink or green

      Almost 2 times stronger than
                                      Die stone used in the fabrication
       type III stone                 of cast crown restoration


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   High strength, high expansion dental stone (Type V)
     Colored blue or green

     Most costly of all gypsum materials

     Lowest W:P ratio, higher compressive strength

     The need for higher expansion (0.1%) was to

      compensate for the solidification shrinkage of
      some alloys used as base metals used for dental
      casting


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                   Manipulation
   Selection: based on the desired properties and
    dental application. e.g.:
       For a diagnostic cast, plaster can be used.
       For a working cast, strength is required and
        accuracy, dental stone is the gypsum of choice
       Working models for cast restorations require die
        stone



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                Manipulation (cont.)
   Proportioning (W:P ratio)
       Golden rule: Manufacturer instructions should
        always be followed.
       Variations in W:P ratio affect the set materials’
        properties such as strength and accuracy.
            Too much water?
            Too little water?




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            Manipulation (cont.)
   Mixing:
     Manual: Rubber
      bowl and spatula.
      Powder is sifted into
      water to ensure good
      wetting and avoid
      clumps, and avoid air
      bubbles




                                   18
Mechanical (also used with vacuum)




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             Manipulation (cont.)
   Initial setting time-working time
       Working time start after mixing for 1 minute
       Initial setting time: time elapsed from the start of
        mix until loss of gloss (8-10 minutes)
       6-10 minutes of working time are available to pour
        the gypsum.




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   Final setting time: is reached when the materials can be
    safely handled, the gypsum is cool (exothermic reaction
    is over).
   Setting expansion: Results from crystal growth during
    setting. Can be decreased by the addition of potassium
    sulfate, sodium chloride, borax.
   Hygroscopic expansion. If gypsum soaked during
    setting, water fills pores and increases volume
   It’s recommended to separate the cast from impression
    after 1 hour.
   Strength increases 2-3 times after 24 hours



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     Tests for initial setting, and final
                setting times
   Loss of gloss test for initial setting time: loss of gloss
    occurs as water is taken up by gypsum to form the
    dihydrate. The materials does not have measurable
    compressive strength.


   Initial Gillmore test for initial set: needles are used to
    indent the material until no indentation can be seen =
    initial setting time.


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                                           A
   Vicat penetrometer (A) for setting
    time: used for the next stage of
    setting (refer to figure) after loss
    of gloss, the plunger rod is
    released onto the mix. Time
    elapsed until the rod no longer
    penetrates is the setting time.

                                           B
   Gillmore test (B) for final setting    B
    time: a heavier Gillmore needle
    to determine final setting time.




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          Ready for use criteria
   The ability to judge readiness of gypsum to be
    handled improves with experience.

   Technically, the material is considered ready
    when compressive strength reaches 80% of the
    strength attained after 1 hour.

   Most products are ready to use in 30 minutes.
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   Clinical tip: before separating cast from
    impression, ensure that no part of the tray is
    connected to the gypsum

   Clinical tip: if alginate impression dried before
    cast separation, soak in water for 15 minutes.



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     How to control setting time
1. Changing water : powder ratio


 Increasing water          Decreasing water
 Retarded setting          Shorter setting time
 Weaker model or cast      Mix difficult to manipulate
 Inaccurate model          Bubbles inclusion in mix
         --------          Inaccurate model


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2. Spatulation: rapid and prolonged spatulation
   accelerates setting and also increases setting
   expansion.

3. Temperature: increasing water temperature to
   a certain level will accelerate setting.



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4. Accelerators and retarders:
     Manufacturers add accelerators or retarders to
      gypsum. These chemicals increase or decrease
      gypsum solubility respectively which will alter
      setting time accordingly.
     Clinicians can add accelerators such as
      potassium sulfate or set gypsum (slurry water),
      they act as sites for crystallization.
     Setting reaction retarders: blood, saliva, alginate.
      If left on impression, can affect surface details of
      impression. Impression surface need to be
      properly rinsed before being poured.
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       Pouring of the cast: the cast is composed of two
        parts which are prepared separately
         The anatomical part (hard and soft tissue), impression
          poured using a vibrator
         Art portion or base, which is important to aid in handling
          and articulating the casts. Can be poured in different
          ways:




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1. Double-pour method
        1               2




                3




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                    2. Single step
   Both anatomical and art portions of the cast are
    prepared at the same time. This method
    requires skill and accurate timing. Difficulty
    encountered:
       If mix is too runny?
       If mix started to initially set?




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              3. Boxing method
   A strip of wax is fitted around the impression then
    gypsum is poured. The wax border should extend at
    least 0.5 inch above the highest point of the
    impression.




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           Storage and clean up
   Gypsum should be stored in airtight dry area.
    Prolonged exposure to moisture can retard
    setting due to decreased solubility of powder.

   Relevant equipment should be kept clean to
    avoid unwanted acceleration of setting by set
    gypsum.


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                Infection control
   Casts should have set for 24 hours before
    being disinfected if necessary.
   Spray rather than immerse
   Disinfectants commonly used:
       Sodium hypochlorite
       iodophors
       Chlorine dioxide


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                   Trimming
   Plaster bases are recommended since trimming
    them is easier than dental stone.

   If base is made from stone, it should be soaked
    in water for 5-10 minutes to soften it before
    trimming. Important considerations when
    trimming?


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        Trimming considerations
   Proportion of base to
    anatomical part
   Parallelism
   Use of wax bite registration
   Outer border of cast
   Shaping of anterior part of
    upper and lower arches



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         Other types of Die stone
   Metal plated die stone: silver or copper plated
    to increase abrasion resistance

   Epoxy die stone: resin and a hardener are used
    to increase abrasion resistance.

   Resin reinforced die stone: resin is
    incorporated into the gypsum material to
    increase abrasion resistance.                     38
Investment materials


    Used to form metal
     casting through the lost
     wax technique




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                   References
   Dental materials, clinical applications for
    dental assistants and dental hygienists.
    Chapter 12

   Phillips’ science of dental materials.
    Chapter10



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