Initial And Final Setting Times

Initial And Final Setting Times

Question 1. Define the initial and final setting time of gypsum products. State their importance.
Or
Enumerate the factor that controls the setting time of gypsum hemihydrates.
Or 
Answer:

Setting time: The time elapsing from the beginning of mixing till the material hardens is called as “setting time”.

Initial Setting Time: The time elapsing from the mixing till the material becomes rigid (but not hard) it can be carved but not moulded, this is known as “initial setting time”. It is of 5 to 7 minutes.

Final Setting Time:  The time at which the material can be separated from the impression without distortion or fracture. It is 30 to 45 minutes.

Factors Controlling Setting Time of Gypsum Hemihydrates:

Factors controlling setting time are:

• Manufacturing process: Finer is the particle size of hemihydrate, the faster it sets.
• Mixing and speculation: As faster and longer the plaster is mixed, faster it will set because nuclei of crystallization are broken and well distributed within the mass.
• Water/Powder ratio: As more water is used for mixing,
  few are the nuclei per unit volume and so the setting time increases.
• Temperature: As room temperature is increased to the
  body temperature, the rate of reaction increases slightly, and setting time decreases.
• Accelerator: As chemical added decreases the setting time is known as accelerator. Sodium chloride and potassium sulphate act as accelerators in hemihydrates.
• Retarders: They form a layer on hemihydrates to reduce solubility. Acetates, borates, citrates tartrates, and salt like ferric sulphate, chromic sulphate, and aluminum sulphate are the retarders that act by nuclei poisoning and reducing the rate of solution of hemihydrates.

Question 2. Describe setting time in detail in relation to gypsum products used in dentistry. What are the different methods of changing the setting time?
Answer:

Setting time: The time elapsing from the beginning of mixing till the material hardens is called as “setting time”.

Setting time is divided into two parts, i.e.

• Initial Setting Time: The time elapsing from the mixing till the material becomes rigid (but not hard) it can be carved but not moulded, this is known as “initial setting time”. It is of 5 to 7 minutes.
• Final Setting Time: The time at which the material can be separated from the impression without distortion or fracture. It is 30 to 45 minutes. For methods of changing setting time refer to the heading factors controlling the setting time.