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Gypsum products and investment

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					                                ‫تسم اهلل الرحمه الرحيم‬
The doctor started the lecture by announcing that the exam will be in 14/8 and it's on-
line exam.

Gypsum products and investment..
What is gypsum material?
Its actually a mineral that is found naturally.
What is its application as a dental material?
It can be used either as:
    - impression material
    - constructing casts and dies (either a full mouth model or a model for one tooth
       which is die)
    - for making replicas.
What kind of properties we need die materials to have? (ideal properties)
    - dimensionally accurate
    - ease to use
    - sheep
    - good strength
    - abrasion resistant
    - accurate reproduction of fine detail
Not all of these properties you may found in die materials, but some of them are
necessary especially when you're making crown and bridges you need some properties
to be found in dental material.
How do these materials behave?
Chemically, they are exothermic reaction; which means it produces heat when it set.
When the material gets cool that means that the reaction is over, but that doesn’t mean
that the material has reached its full strength, but means the setting reaction is almost
over.
The mineral gypsum is a dihydrate (2 molecule of H2O) of calcium sulfate. When it is
heated up (to form it into a powder) it loses water and becomes hemihydrates of
calcium sulfate after it undergoes grounding.
There are different ways to produce a different type of gypsum, whether it was plaster
or stone or high strength/high-expansion stone…..etc
The different in the manufacture product is in the pressure that it used when the
material is made.
If the heating process occurs in atmospheric pressure at 115° C, the powder particle of
the gypsum will be irregular and porous and that’s what we call plaster or beta-
hemihydrate. (Type I or II gypsum)
     type I  impression plaster
     type II  model plaster


                                          -1-
If the heating process occurs under pressure, powder particles will be regular and less
porous which means stronger and we call it alfa-hemihydrate, or dental stone (type III
gypsum).
Increasing in pressure will make the particles more regular and result in denser stone
 die stone or high-strength stone (type IV gypsum)

When stone is mixed with silica it will be harder and so it can with stand high heat
and stress produced when molten metal is forced into molds to form indirect
restoration  dental investment.

As you can see in the pictures (slide 6) plaster of paris particle has an irregular shape
while dental/die stone have regular particle shape, which means more stronger.
In the case of dental stone the particles shape is rectangular.

Physical properties:
Notice that plaster has the highest W/P ratio , which means it has the highest amount
of water and this is because its irregularity and it has a lot of porous and that’s what
makes it weaker (compressive strength is low) .
Plaster is opposite of high strength/high expansion stone which has the lowest W/P
ratio and so the highest compressive stress (less porous).
 So we can conclude that strength and hardness are affected by:
    1- Porosity  increased porosity of the particles makes it necessary to use more
       water to convert hemihydrate particles back to dehydrate particles.
    2- Shape of particle  the larger more irregular shape of the particles prevent
       them from fitting together densely, and so the resulting product has more air
       space which will make the product weaker.
* Plaster cast is weaker than stone cast

Dimensional accuracy:
The higher the setting expansion, the lower the accuracy (the cast won't be identical to
the impression)
Plaster has the highest setting expansion and high strength stone has the lowest.
Setting expansion  when you mix the material with water, and after it set, crystals
will join each other, and the material will be expanded.
The control of setting expansion is critical for accurate models and cast.
So we can arrange gypsum according to the dimensional accuracy as the following:
Die stone > dental stone > plaster




                                           -2-
Solubility:
Set gypsum products are not highly soluble.
Solubility is directly related to the porosity of the material. The greater the porosity
the greater the solubility.

Therefore, plaster is much more soluble than stone.
Plaster > dental stone > die stone

Reproduction of details:
The greater the porosity the less surface detail is produced.
There must be compatibility between impression material and gypsum to improve
detail reproduction. So it is important to follow the manufacturer's directions in
selecting gypsum products that are compatible with impression materials.
The impression material of silicone and polyether form the best surface detail with
gypsum products.

Classification of gypsum:
The criteria for selecting a gypsum product are determined by the desired physical
properties and behavior.
1. Type I gypsum: impression plaster
Nowadays it's rarely used in dentistry.
2. Type II gypsum: model plaster:
It is used for:
    - Diagnostic cast  when you use the cast for treatment planning because it
        provides a 3-D record of the patient's hard and soft tissues.
    - articulation of stone cast  so it's used also in attaching stone cast to the
        articulator
    - art portion of working cast
This product is produced in white color to distinguish them from dental stone.

3. Type III  dental stone
It is ideal for making:
    - full or partial denture model
    - orthodontic models necessary for accurate reproduction.
    - cast requiring higher strength and abrasive resistance.
    - Flasking procedure for acrylic denture.
Stone is usually colored yellow or white.
It requires less water and so it is stronger than plaster.

4. Type IV die stones (dental stone, high strength)
It usually has a pink or a green color


                                           -3-
AKA die stone is used in making wax patterns of crown and bridges (cast restoration)
It is almost 2 times stronger than type III stone. A hard, abrasive resistant surface is
needed to resist the abrasion of sharp instrument used to curve on these stone dies.

5. Type V  high strength, high expansion dental stone
   - Colored blue or green
   - Is the most costly of all the gypsum product
   - Lowest W:P ratio (correct it in the slide), and so highest compressive strength.
   - Has the higher expansion to compensate for the solidification shrinkage of some
      alloys used as base metals for dental casting

How we select the type of stone we want to use?
The selection is based on the desired properties of the material.
For example, we use for a diagnostic cast a plaster since accuracy isn’t important, but
in the case of working cast, good strength and accuracy is required so we use dental
stone.
     Working models for cast restoration (contain metals) require die stone or high
       strength, high expansion stone.

How do we measure or proportion water to powder.
Always follow the manufacturer instructions.
Variations in W:P ratio affect in the strength and accuracy of the material
  - too much water  the mix will be runny, difficult to control it, takes longer to
      set, and it will be weaker
  - too little water  the mix wont flow easy, and the mixture may not reproduce
      the details and so your cast will be inaccurate.

Mixing:
There is two ways for mixing: manually or mechanically
Manually:
We use rubber bowl and spatula. The measured amount of water is placed into the
mixing bowl and the powder slowly sifted into the water. By sifting powder into water
an even wetting of the powder particles takes place and clumps are avoided.
Spatulation should continue for 1 minute at 2 revolutions per second until a
homogeneous mix is produced.
    Increase in spatulation time and rate will shorten the setting time and increase
      the rate of setting expansion.




                                          -4-
Mechanically:
It's used when the control of spatulaion is necessary.
Also used with vacuum to remove bubbles.

Initial setting time and working time:
Initial setting time  defined as the time elapsed from the start of mixing until loss of
gloss (8-10 minutes)
So first of all the material looks shiny and later on after water absorption it will looks
glossy (muddy).

Final setting time:
It is reached when the material can be safely handled. At this time the chemical
reaction is complete and the model is cool (exothermic reaction)

Setting expansion:
Result from crystal growth during setting. It can be decreased by adding borax,
potassium sulfate, sodium chloride.

Hygroscopic expansion:
If gypsum soaked during setting, water will fill the pores and so volume will increase.
Strength increases 2-3 times after 24 hours.

There are certain tests that are done to the material to identify the setting time, one of
these test is: initial Gillmore test for initial set  in this test, needles are used to
indent the material. When there is no indentation can be seen in the surface of the
material it will be the initial setting time.

Other test like vical penetrometer which is used for the next stage of setting after loss
of gloss, the plunger rod is released onto the mix.

Ready for use criteria:
    The material is considered ready to use when compressive strength reaches 80%
     of the strength after 1 hour.
    Most products are ready to use in 30 minutes.
    Ensure that no part of the tray is connected to the gypsum before separating cast
     from impression




                                           -5-
How to increase setting time?
1. Altering the W:P ratio 
If we increase the water, the model will be weaker, the setting will be delayed and the
model will be inaccurate.
If we decrease water, the setting time will be shorten, and the mix will be difficult to
manipulate coz there's no enough water. And of coarse the model will be inaccurate.
2. spatulation: if you increase the speed of spatulation while mixing the material, you
will accelerate setting and increase setting expansion.
3. Temperature: increasing the temperature at certain level will accelerate the setting
time, but if the temperature increased above the limits, there will be a retarding effect.
4. Accelerator and retarders:
Manufacturer uses certain chemicals that increase or decrease the gypsum solubility
which will alter the setting time such as potassium sulfate or slurry water
     Slurry water  it's when the water and crystals from ground set gypsum
       retrieved from the runoff water of model trimmers. It accelerates the reaction by
       establishing sites for crystallization.
In another hand, blood, saliva, and alginate are considered to be setting reaction
retarder.
All impressions must be rinsed free of any organic matter before the impression is
poured.

How do we pour the material?
The cast is composed of two parts, anatomical part  which replicate the hard and
soft tissues and The Base  which aid in handling the cast.
There are two methods for pouring:
Pour it in two stages
   1. Double-pour method: the anatomic portion of the arch is poured and left in the
        upright position. After 10 minutes a second mix is produced for the base. This
        mixture is then placed in a base former.
   2. single step method:
   Both anatomical and the base of the cast are prepared at the same time. It requires
   skill and accurate timing.
   If the mixture has reached its initial set when inverting, the union between art and
   anatomic portion will be incomplete.
   3. boxing method:
   a strip of wax is used to surround the impression, forming a wall into which the
   gypsum is poured.
   Storage and clean up:
   *Gypsum should be stored in airtight dry area. Prolonged exposure to moisture will
   decrease the solubility of water and so it will retard the setting rxn.
   * Equipments should be kept clean to avoid unwanted accelerationof setting
   * Routine disinfection of impressions should be done in the dental office.


                                           -6-
   * Casts should have set for 24 hours before being disinfected.
   * Casts should be sprayed rather than immersed.
   * Sodium hypochlorite, iodophors, chlorine dioxide are used as disinfectants

Trimming:
It's defined as the process of removing excess hardened gypsum from the cast for ease
in working with the cast.
Bases made from dental stone should be soaked in water for 5 to 10 minutes before
trimming. Anatomic portion should never be soaked.

Other types of die stone:
   1. metal plated die stone
   2. some time they add resin and certain chemical that make it harder
   3. epoxy die stone

Investment material (gypsum and silicone)
Used to form metal casting through the lost wax technique. So it withstand high
temperature without melting.
This materials are strong enough allow molten metal to be poured into them.

Chapter's answer:
  1. A
  2. C
  3. B
  4. C
  5. C
  6. A
  7. C
  8. D
  9.
  10.D




                                         -7-
Dental waxes:
Dental waxes uses in clinics and laboratories:
Uses of wax are written in the slide.

Composition:
   1. natural waxes produced from  beewax (animals) or paraffin wax (minerals)
   2. synthetic waxes
   3. additional component as gums, oil

properties:
   1. Melting range a range of temperature at which each component of the wax
       start to soften and then flow. Component with lowe melting point is soften
       firstly.
   2. Flow  it describes the movement of wax as it approaches the melting range.
       Viscosity of wax decreases as the temperature increase.
    When using a wax for bite registration, it is important that the wax doesn’t
       require       temperature much greater than mouth temperature to soften it.
   3. Dimensional change  waxes contract in cooling & expand in heating.
This is important, because since waxes easily affected by temperature your wax will
undergoes dimensional changes and inaccuracy if it stands for a long time. So, it is
always recommended to pour it within 30 minutes.

Classification of waxes:
Pattern wax:
is not very common in using, it include:
    A. inlay wax.  type I (placed directly in the prepared tooth in direct waxing
       technique.
    Type II (Indirect), melted on die outside mouth in indirect technique. Most
    common in use.
    B. casting wax
    C. baseplate wax.

Processing wax:
Used to aid in dental procedures, it includes:
   A. boxing wax  naturally it is sticky, it needs to be dry before it attaches to tray,
   B. utility wax
   C. sticky wax  comes in yellow/orange sticks. It is hard and brittle.
   It is important in holding component of metal together.

    There is various form of wax: sheets, ropes and sticks.



                                           -8-
‫:‪Impression waxes‬‬
‫:‪Used to obtain impressions of the oral structures, it includes‬‬
  ‫‪A. corrective impression wax‬‬
  ‫‪B. bite registration wax‬‬

‫:‪Manipulation‬‬
‫:‪Waxes are softened by‬‬
    ‫‪1. warm hands‬‬
    ‫‪2. dry heat‬‬
    ‫‪3. flame‬‬
‫.‪It should be added in layers into an object, and to be invested within 3 min. of carving‬‬
‫.‪It should be stored at below room temperature‬‬




                     ‫‪Forgive me for any mistake‬‬
‫‪And forgive me coz there are certain statements I couldn’t hear , but I‬‬
         ‫‪tried to write every thing I heard as much as I can‬‬

‫‪Done by: Banan al hawamdeh‬‬
‫‪Special thanx for Saba al syouri , Hiba abu alhomus , Halah Abu nijem , Aya‬‬

     ‫عه أثٍ عمُس األوصبزٌ زضٍ اهلل‬
      ‫عىه لبل : لبل زسىل اهلل صهً اهلل‬               ‫لبل صهً اهلل عهُه وسهم عه زة انعصح: لبل اهلل عص وجم‬
     ‫عهُه وسهم (( مه صهً عهٍ صالح‬                   ‫اوً واالوس وانجه نفً وجب عظُم،اخهك وَعجد غُسي،ازشق‬
     ‫واحدح صهً اهلل عهُه عشس صهىاد‬                                                            ‫وَشكس سىاي‬
    ‫و حطذ عىه عشس خطُئبد ، وزفعذ‬             ‫خُسي انً انعجبد وبشل وشسهم انً صبعد،ارحجت انُهم ثبنىعم واوب‬
     ‫نه عشس دزجبد ، و كزجذ نه عشس‬                  ‫انغىً عىهم، وَزجغضىن انً ثبنمعبصً وهم احىج شئ انٍ‬
                               ‫حسىبد‬            ‫اهم ذكسي اهم مجبنسزً فمه ازاد ان َجبنسىً فهُركسوً--اهم‬
                                                                                         ‫طبعزً اهم محجزً‬
                                            ‫واهم معصُزً الالىطهم مه زحمزً فبن ربثىا انٍ فبوب حجُجهم وان‬
     ‫مه قال تسم اهلل الرحمه الرحيم‬                                                 ‫نم َزىثىا انً فبوب طجُجهم‬
     ‫وال حول وآل قوج إال تاهلل العلي‬            ‫ان الجم انً واحد مىهم رهمُزه مه ثعُد، وان اعسض عىً واحد‬
                                                     ‫مىهم عبصُب وبدَزه مه لسَت لبئال:انً اَه ررهت انك زة‬
           ‫العظيم سثعا' تعد صآلتي‬                                                                 ‫غُسي؟؟؟‬
          ‫الصثح والمغرب كتة مه‬                   ‫انحسىخ عىدي ثعشس امثبنهب واشَد، وانسُئخ عىدي ثمثههب ولد‬
      ‫السعداء ولو كان مه األشقياء.‬                                                                     ‫اعفى‬
                                                                  ‫وعصرً وجالنً الن اسزغفسووً غفسرهب نهم‬




                                           ‫-9-‬

				
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