Investment Materials

Question 1

How are metal/alloys inlays, onlays, crown and bridges made

Answer 1

the technique used involves casting the metal allow which is done under pressure by centrifugal force

Question 2

What does the process of casting require

Answer 2

a mould cavity of the required shape meaning the alloy must be surrounded by an investment material to maintain the molten alloy and ensure the alloy dimensions are sustained

Question 3

Describe the process of casting

Answer 3

• A wax pattern is made of a crown, inlay etc and this gives a positive replica
  
  • The investment material is poured around the wax pattern and is allowed to set forming a negative replica in what is called a mould
  • The wax is then eliminated by placing the mould in boiling water or by burning it in the oven
  • This leaves a cavity of the required shape surrounded by investment material and the alloy is poured into the mould cavity via sprue hollow tubes that allow the alloy to flow in
  • The sprues are prepared in the investment material

Question 4

What are the investment types

Answer 4

dental stone or plaster (gypsum)
• Gypsum bonded materials - gold casting alloys
• Phosphate bonded materials - base metals/cast ceramics
• Silica bonded materials - base metal alloys

Question 5

What can gypsum be used for

Answer 5

acrylic dentures

Question 6

What can gypsum bonded materials be used for

Answer 6

gold casting alloys

Question 7

What can phosphate bonded materials be used for

Answer 7

base metals/cast ceramics

Question 8

What can silica bonded materials be used for

Answer 8

base metal alloys

Question 9

What are the requirements / ideal properties of investment materials

Answer 9

expansion
porous
strong
smooth surface
chemically stable
easy removal from cast
handling
relatively inexpensive as it is destroyed

Question 10

Why is expansion important

Answer 10

compensate for cooling shrinkage of the alloy

Question 11

Why is porous important

Answer 11

if trapped, gasses will produce gaps in the alloy prosthesis which would result in a weak prosthesis therefore we want the investment material to be porous so that the gases can be released and stored within it

Question 12

Why is strong important

Answer 12

it needs to be strong at room temperature for ease of handling and it has to be strong at casting temp to withstand casting forces

Question 13

Why is smooth surface important

Answer 13

so alloy wont require much finishing

Question 14

Why is chemically stable important

Answer 14

porosity, surface detail

Question 15

Why is easy removal from the cast important

Answer 15

technician time

Question 16

Why is handling important

Answer 16

so it is not complicated

Question 17

What are the typical contractions from alloy melting pt. to room temp

Answer 17

• Gold alloys - 1.4%
• Ni/Cr alloys - 2.0%
Co/Cr alloys - 2.3%

Question 18

What are the 2 components of the investment material

Answer 18

binder

refractory

Question 19

What does the binder consist of

Answer 19

ypsum, phosphate, silica to form a coherent solid mass that provides substance and depends what type of investment material it is

Question 20

What does the refractory consist of

Answer 20

type of silica(quartz or cristobalite) and it can withstand high temperatures and also gives expansion which is crucial

Question 21

What does the graph show

Answer 21

how quartz and cristobalite expand as temperature increases
○ Quartz expands by 0.8% when going from 0 to 1000 degrees
Cristobalite expands by 1.3%

Question 22

What is the inversion expansion of silica

Answer 22

• In the previous graph we could see how quartz’s linear thermal expansion rose gradually until 570 degrees then it climbed more rapidly and here we can see why
  
  • Quartz below the temperature of 570 exists as alpha quartz and with this squashed crystalline lattice structure shown on the length
  • Beyond this temperature its structure changes to beta quartz where it explodes to its maximum volume

Question 23

What is the composition of the powder of gypsum bonded investment

Answer 23

§ Silica (60-65%)
§ Calcium sulphate hemihydrate (30-35%)
§ Reducing agent for oxides
§ Chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)

Question 24

What is the setting of gypsum bonded investment

Answer 24

○ As in gypsum products: hemihydrate –> dihydrate

Question 25

What are the dimensional changes in gypsum due to

Answer 25

silica's thermal and inversion expansion | gypsum - hygroscopic expansion and contraction above 320 degrees

Question 26

What is hygroscopic expansion

Answer 26

□ The mechanism for this is not fully understood □ The water molecules are attracted in gaps between crystals of calcium sulphate hemihydrate by capillary forces, forcing crystals apart □ Can be up to 5 times that of setting expansion

Question 27

What factors effect the hygroscopic expansion

Answer 27

® Lower powder/water ratio ® Increased silica content ® Higher water temperature ® Longer immersion time in water

Question 28

What factors effect the contraction above 320 degrees celsius

Answer 28

□ Water loss | □ Presence of sodium chloride and boric acid

Question 29

What are the properties of gypsum bonded investment

Answer 29

○ Expansion § Expands by 1.4% which is sufficient for gold casting which shrink by that same amount § Total expansion sufficient for gold alloys ○ Smooth surface (good) § Fine particles give good surface ○ Manipulation (good) § Easy § Setting time controlled ○ Porous § Good thing so can uptake gasses released when casting the alloy ○ Strength Adequate if correct powder/liquid ratio and correct manipulation

Question 30

What happens when gypsum bonded invesmtent material is casted at 700 degrees

Answer 30

there is an unwanted reaction at 700 degrees ○ This reaction is between CaSO4 and C (wax residue or graphite in investment) ○ It results in carbon monoxide release and calcium sulphide ○ The calcium sulphide reacts with calcium sulphate to produce sulphur dioxide gas

Question 31

What is heat soaking

Answer 31

○ It is crucial these gases escapes and to ensure this happens, the investment must undergo heat soaking where it is held at a high temperature to allow some time for these gases to naturally escape

Question 32

What is the composition of phsophate bonded investment in the powder

Answer 32

§ Silica § Magnesium oxide Ammonium phosphate

Question 33

What is the composition of phsophate bonded investment in the liquid

Answer 33

§ Water or colloidal silica □ The reason that colloidal silica is used to mix with phosphate bonded powder is because it increases strength and gives hygroscopic expansion and this expansion compensates for alloy shrinkage when it is cooled to room temperature

Question 34

What is the setting reaction of phosphate bonded investment

Answer 34

○ Ammonium phosphate + magnesium oxide + water --> magnesium ammonium phosphate

Question 35

What happens to phosphate bonded investment on heating

Answer 35

○ At 330 degrees, water and ammonia are liberated ○ Then complex reactions occur with silico-phosphates formred which generally increases strength ○ The chemistry isn't important, the outcome is

Question 36

What are the properties of phosphate bonded investment

Answer 36

``` ○ High 'green' strength meaning we don’t need metal casting ring for support ○ Easy to use ○ High strength ○ Porous ○ Chemically stable ```

Question 37

What is the setting reaction for silica investment

Answer 37

stage 1 - prepare stock solution stage 2 - add powder (quartz or cristobalite) - gelation stage 3 - drying (tightly packed silica particles)

Question 38

What are the silica investment dimensional changes

Answer 38

○ Contraction at early stages of heating § Water and alcohol loss form gel ○ Substantial thermal and inversion expansion Lots of silica present

Question 39

What are the other properties of silica

Answer 39

○ Strength - sufficient ○ NOT porous - needs vents § Gases normally released during casting process cannot be captured meaning alloy itself is porous and weak so to overcome this special equipment with vents is used ○ Complicated manipulation