Manipulation of Gypsum Product

MANIPULATION OF GYPSUM PRODUCTS

Gypsum is manipulated in three simple steps

1. Proportioning

2. Mixing

3. Caring of the cast

Rules of mixing gypsum

Rule no. 1:

Work Preparation

    Cleanliness of the tools is the first rule for a good laboratory. Blow as well as stirring spatula must be free from plaster residue and rinsed thoroughly with clear water after cleaning with plaster solvents. Any kind of residue on the mould may cause incorrect results. The mixing ratio stipulated by the manufacturer must be observed as precisely as possible. Weighing and measuring of plaster and water ensure reproducibility.
Scoop with hard plaster (negative example!!!)

Rule no.2

 Storage

The best possible storing for unopened dental plasters is in humidity-proof containers in dry rooms at 23°C and 50%air humidity. When stored in unsuitable rooms, storage suitability decreases considerably, since temperatures that are too high or too low, or air humidity that is too high will damage the plaster irreparably. When a package has been opened, it should be closed whenever material is removed in order to minimize absorption of air humidity. Under no circumstances should water penetrate into the plaster container, since the resulting reaction may waste the whole batch.

Rule no. 3

      Water

The highest precision in terms of results is achieved with distilled water. Yet, also mixing with normal tap water may bring about the desired results. However, it should be ensured that the water has room temperature.

Rule no. 4

       Dosing

Plaster powder and water must be measured or weighed precisely for each mixing procedure. It should be kept in mind that measuring water is only precise at room temperature (1ml=1g). Cold water has a smaller volume, which may cause incorrect dosing exceeding the desired mixing ratio. Likewise, if the water is too warm, its larger volume may cause incorrect dosing below the desired mixing ratio.

Rule no.5

      Soaking/Pre-mixing

The plaster powder is swiftly and loosely poured into the water. If this happens too fast, too much air is bound, which causes so-called plaster bulbs. After a soaking time of 20 – 30 seconds the plaster mush is stirred and premixed with easy stirring movements.

(Never pour water into gypsum/plaster.)

Rule no. 6

      Stirring

It is possible to either manually stir or use machines.
Mixing by hand must take 60 seconds (for impression plasters(type I) and articulation plasters(type II) 30 seconds). This is calculated on a basis of 2 – 3 rotations per second.
Using machines under vacuum, the plaster-water mixture is briefly pre-spattled and then inserted into the machine. Stirring should take  30 seconds.
It is important to note that adding plaster powder or water later may distort the results dramatically. In case the plaster mush turns out too thick or too thin, it should be disposed of and a new mixture be made. This saves future trouble with models that are too soft or imprecise.

Mix slowly and steadily from the bottom, using a side to side motion.

Stirring in a circular motion will introduce undesirable air bubbles into the mix.

The consistency of final mix should be that of heavy dairy cream.

Rule no. 7

      Casting (the act of forming an object in a mold or pouring the cast)

The models should be cast immediately after the mixing is completed. The incipient setting process may, at a progressed stage, not be able to render important details of the mould precisely. Using a vibrator is recommended in order to prevent formation of air bubbles and to introduce the plaster mush into each little crevice of the impression. However, extending processing time by shaking should be avoided, as this damages the crystal structure of the plaster and reduces its hardness.

Pouring of the cast

Pouring of the cast is done in two pour technique

1.  A thin layer of  gypsum is first  allowed to flow over the critical areas of the impression under a vibrator in order to ensure that  the  areas  are well registered and to reduce the air bubbles

2.  A thicker mix is poured over to make sure that  adequate bulk is obtained.

Rule no. 8

      Modelling

After vibrating and layering the plaster mush the model may be re-finished(eg. For articulating plaster). It should be observed that processing should take up to half the setting time. It can be observed at the beginning of the modeling stage that the mush only has a matt shine and assumes a doughy consistency. There should be no further processing once the mush loses its remaining shine.
If plaster should stick to plaster, the hardened plaster must be dampened, so that it does not absorb any water from the plaster mush layered on. This would prevent a good formation of di-hydrate and thus a tight connection.

Rule no. 9

       De-Moulding

After 30 – 45 minutes the impression has set enough so that it can be de-moulded. This is evident from a decreased setting heat. After that, the impression should rest for another 120 minutes in order to achieve acceptable hardness and stable expansion values. A resting time of 24 hours is ideal. After this period the reaction is almost completed; however, it will continue – only minimally – for 7 days

Rule no. 10

       Trimming

Finishing the model, i.e. sanding, sawing, and steaming are processes carried out with the help of water. This new contact of the model with water may increase expansion slightly. Also, heating (scalding) results in thermal expansion, which may result in inaccuracies. This type of expansion is, for the most part, reversible. It is important that the model should be briefly watered before scalding.