Dental Impression is a negative replica of the oral cavity, many impression materials are present which are capable of getting the perfect impression if the proper guidelines are followed
It is a rigid, reversible impression material which sets by physical change.
According to ADA Specification Number 3:
- Low fusing compound – Green stick compound.
- Medium fusing compound – Impression compound.
- High fusing compound – tray compound.
- sticks, cakes, & cones.
- olors – green, brown, red.
- For making primary impressions.
- For individual tooth impression
- For peripheral tracing or border moulding.
- To check undercuts in inlay preparation.
Requirements of an ideal impression compound:
- It will harden after heat treatment when brought back to mouth temperature and be stable after hardening and removal from mouth.
- When softened it should have uniform consistency to help reproduce fine details of oral tissue.
- Should become hard uniformly throughout without discrepancies and distortion.
- It should not be harmful to Oral tissues when it is elastic in plastic state.
- Should not be harmful and not have toxic or Irritating materials to Oral tissue.
- Material should have cohesive property and not adhesive property.
- When exposed to flaming after removal from mouth should exhibit a smooth glossy surface.
- After hardening flaking and chipping should not be seen during trimming and reshaping using lacron carver.
- Rosin 30
- Copal resin 30
- Carbuna wax 10
- Stearic acid 5
- Talc 25
- Coloring agentapp.
- During softening of the material , the outside will soften first & inside last. So to ensure uniform softening the material should be kept immersed for a long time in water bath.
- The layer adjacent to the mouth tissue swill remain soft. Therefore Its imp to cool the compound thoroughly before removing the impression.
i) Thermal conductivity:
- has low thermal conductivity.
ii) Coefficient of linear expansion. (CTE)
- high CTE , 0.3% acceptable.
- high fusing compounds – < 2% at 37 .C & < 70-85 % at 45.C
- low fusing compounds – < 3% at 37 C& < 80-85% at 45 C
iv) Glass transition temp:
- It is the temp. at which the material loses its hardness or brittleness on heating or forms a rigid mass.
- App. 39.CMaterial removed from the mouth after glass transition temp. is reached.
v) Fusion Temp.
- It’s the temp. at which the impression compound becomes plastic.
- Its 43.5.C
vi) Dimensional stability.
- Poor dimensional stability.
- To prevent distortion pour the cast within 1 hr.
vii) surface detail reproduction is less because of high viscosity & low flow.
- Sticks:Smallamount softened over flame
- Cakes: Softened in a thermostatically controlled waterbath.
- Prolonged immersion in a water bath is not indicated overheating
- In water makes the compound sticky & difficult to handle.
- Avoid incorporating water while kneading.
- Cheap reusable
- Does not produce irritation to the patient impression
- Can be remodified & resoftned again till an accurate impression is obtained
- Difficult to record details accurately.
- Soft tissues are compressed due to pressure applied while taking the impression.
- Difficult to remove undercuts.
- Does not have an pleasant taste.
- Can be uncomfortable in patients because of the rigidity.