1. Accurate- is free of distortion
2. Complete- has recorded all of the required anatomical parts
3. Detailed- complete reproduction of surface detail

A complete cast is not always an accurate cast

• Tray selection and modification
• flexing of the mandible
• removal without distortion
• proper disinfection procedures

• Proper storage of impression
• measured amounts W/P Ratio when mixing both alginate and the dental stone
• Proper mixing-Vacuum is preferred
• Proper pouring technique - 2 stage pour
• Proper setting-Removal-trimming

• Rigid- accuracy
• Retain impression material
• sterile-infection control
• proper size-assortment
• economical to use

1. Stock tray
• styrofoam-disposable
• plastic-disposable(what we're using)
• metal-perforated-reusable
• metal-solid-reusable
2. Individualized tray - modified stock tray
3. Custom tray - made for a specific patient (out of acrylic)

red rope wax

compound (requires heat to mold)

Irreversible Hydrocolloid

A hydrophilic elastic gel- a blend of dried sodium salt of Alginic Acid which is derived from a marine plant (12-18%) Calcium sulfate (12-15%) and water.

It contains sodium phosphate (2-3%) as a retarder and fillers such as diatomaceous earth and silicate powders (60-74%)

one scoop to one measure of water

Use 3 scoops for maxillary tray

2 for mandibular arch

Mixing= 45-60 seconds

Loading the tray = 30 seconds

Setting time = 2 min 15 seconds

Impression should be poured in 10-15 min

• Proper patient and clinician position
• Retract the lip with a mirror
• rotate tray into position - use incisors as a guide
• seat the tray - do not let the teeth touch the tray
• seat time= 15-20 sec

• Use Microstone- prepackaged 140gm
• measure 40 ml water
• water in first then the powder

Hand mix (with flat spatula)= 10-15 sec

Vacuum mix= 20 sec

Working time = 6-8 min

Separation time (final set) = 45 min

• Impression should be moist
• Add small increments from same area and vibrate until excess runs out the other side
• fill the impression and add retention "knobs"
• Set for 45 min

• Vacuum = 0 pressure when machine is off
• about 10 Hg when turned on
• Cover nozzle. Gauge should be in the green zone (25-30) in Hg

• Remove any stone that would lock-on and prevent easy removal of the cast
• Separate cast from the impression and rinse
• Trim to desired form for mounting on the Articulator or
• Add base to casts with second mix if a flat base is required

• Run vacuum mixer for 20-30 sec after mixing to purge water from system and protect the engine
• Do not get stone up into the vacuum hose- if you do, wash it out immediately
• be sure to was out vacuum mixing bowl and paddle

• Wet cast first
• do not trim away teeth or anatomic parts that you need
• wear safety glasses
• run water through cast trimmer until effluent water is totally clear

• Allow casts to dry after trimming
• remove all positive bubbles (blebs) carefully with a discoid-cleoid carver
• Do not hand articulate the casts to see how they fit together (until dry-24 hrs later)
• place in a safe place in drawer or locker