impression

IMPRESSION MATERIALS USED IN ORTHODONTICS

Available in three viscosities

Light bodied

Medium bodied

Heavy bodied

COMPOSITION

Base:

Polyether polymer                   Cross linked to form rubber

Colloidal silica                        Acts as filler

Glycol ether (or) phthalate     Plasticizer

Accelerator Paste:

Alkyl aromatic sulfonate         Initiates cross linking

Colloidal silica                        Filler

Phthalate                                 Plasticizer

PROPERTIES

Pleasant odour and taste

Setting time is short 3.5minutes, mixing time is 30 seconds

Good dimensional stability

Curing shrinkage is 0.24%

Polyether absorbs water and can change in dimension. So should not be stored in contact with water or in humid climates

It is extremely stiff

Tear strength is 3000gm/cm2

Hydrophilic, has good compatibility with stone

Can be electroplated

Good shelf life i.e., more than 2 years.

IMPRESSION MAKING WITH ELASTOMERS

It involves five steps:::

  1. Preparing a tray
  2. Preparing the material
  3. Making impression
  4. Removing the impression
  5. Preparing stone casts and dies

            PREPARING A TRAY:

Impressions are usually made with custom trays. Perforated stock trays are used for making impression in putty.

To prepare a custom tray, an alginate impression is made and stone cast is constructed

Teeth are covered with modeling wax (1 or 2 sheets approximately 3-4mm) to act as spacer and stops are provided

Custom tray is prepared with tray material

Slightly roughened surface of tray inside improves retention.

PREPARING THE MATERIAL:

Materials are provided in two paste tubes

Same length of materials should be dispensed on to mixing pad

Catalyst paste is first collected on to spatula and then distributed over the base

Mixing should be thorough, uniform in colour with no streaks of base of catalyst

If one of the components is in liquid form such as catalyst for condensation silicone, a length of base is dispensed from tube onto the mixing pad and a drop of catalyst is added for each unit length of base.

Putty consistence are dispensed by volume using equal number of scoops and kneaded.

Automatic mixing and dispensing devices are available used for light, medium and heavy body consistencies.

It consists of a double barrel gun with mixing tip. The tip consists of spiral inside.   The mixing tips vary in diameter length and size of the tip opening for specific consistency.

 POLYETHER IMPRESSION MATERIALS

It has the advantages over hand mixing

Less air bubbles