Gibe me working and setting time of gypsum priduct according to Phillips dental materials second south asian edition
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Phillips dental materials second South Asian edition gypsum products working time setting time
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Gypsum products: working time and setting time
Working time
Working time is the period from the start of mixing the gypsum powder with water until the mix is no longer suitable for its intended use, such as pouring an impression.
- It includes mixing, vibrating, and pouring.
- A working time of about 3 minutes is generally considered adequate for routine manipulation.
- Clinically, loss of surface gloss indicates that the working time is nearly over or has ended.
Setting time
Setting time is the time taken by the gypsum mix to change from a fluid slurry into a hard set mass, due to conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate.
[
\text{CaSO}{4}\cdot\frac{1}{2}\text{H}{2}\text{O} + 1\frac{1}{2}\text{H}{2}\text{O}
\rightarrow
\text{CaSO}{4}\cdot2\text{H}_{2}\text{O} + \text{heat}
]
It has two practical stages:
| Stage | Definition | Clinical sign |
|---|---|---|
| Initial setting time | Time from start of mixing until the material becomes semi-hard and can no longer be manipulated properly | Loss of gloss; initial resistance to a Gillmore needle |
| Final setting time | Time from start of mixing until the material becomes hard/rigid enough to be separated from the impression | A fingernail or blunt instrument does not indent the surface; resistance to the heavier Gillmore needle |
For practical laboratory use, gypsum casts are commonly considered finally set at approximately 30 to 45 minutes, although complete drying and optimal strength require more time. The gypsum setting-time description similarly identifies loss of gloss as a practical working-time indicator and 30-45 minutes as a practical final-set criterion.
One-line exam answer
- Working time: From the beginning of mixing until the gypsum slurry is no longer workable, usually about 3 minutes.
- Initial setting time: From mixing until the slurry becomes semi-hard, clinically indicated by loss of gloss.
- Final setting time: From mixing until the gypsum becomes rigid and can be removed from the impression, usually judged at about 30-45 minutes.
Give for each type just give the setting and mixing time no exp needed
| Gypsum type | Mixing time | Setting time |
|---|---|---|
| Type I - Impression plaster | 1 minute (hand) | 3-5 minutes |
| Type II - Model plaster | 1 minute (hand) | 8-16 minutes |
| Type III - Dental stone | 1 minute (hand) | 8-16 minutes |
| Type IV - High-strength, low-expansion die stone | 1 minute (hand) | 8-16 minutes |
| Type V - High-strength, high-expansion die stone | 1 minute (hand) | 8-16 minutes |
Mechanical mixing: usually 20-30 seconds.
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