Investment Materials Flashcards
How are metal/alloys inlays, onlays, crown and bridges made
the technique used involves casting the metal allow which is done under pressure by centrifugal force
What does the process of casting require
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
Describe the process of casting
- 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
What are the investment types
dental stone or plaster (gypsum)
• Gypsum bonded materials - gold casting alloys
• Phosphate bonded materials - base metals/cast ceramics
• Silica bonded materials - base metal alloys
What can gypsum be used for
acrylic dentures
What can gypsum bonded materials be used for
gold casting alloys
What can phosphate bonded materials be used for
base metals/cast ceramics
What can silica bonded materials be used for
base metal alloys
What are the requirements / ideal properties of investment materials
expansion porous strong smooth surface chemically stable easy removal from cast handling relatively inexpensive as it is destroyed
Why is expansion important
compensate for cooling shrinkage of the alloy
Why is porous important
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
Why is strong important
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
Why is smooth surface important
so alloy wont require much finishing
Why is chemically stable important
porosity, surface detail
Why is easy removal from the cast important
technician time
Why is handling important
so it is not complicated
What are the typical contractions from alloy melting pt. to room temp
• Gold alloys - 1.4%
• Ni/Cr alloys - 2.0%
Co/Cr alloys - 2.3%
What are the 2 components of the investment material
binder
refractory
What does the binder consist of
ypsum, phosphate, silica to form a coherent solid mass that provides substance and depends what type of investment material it is
What does the refractory consist of
type of silica(quartz or cristobalite) and it can withstand high temperatures and also gives expansion which is crucial
What does the graph show
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%
What is the inversion expansion of silica
- 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
What is the composition of the powder of gypsum bonded investment
§ Silica (60-65%)
§ Calcium sulphate hemihydrate (30-35%)
§ Reducing agent for oxides
§ Chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)
What is the setting of gypsum bonded investment
○ As in gypsum products: hemihydrate –> dihydrate
What are the dimensional changes in gypsum due to
silica’s thermal and inversion expansion
gypsum - hygroscopic expansion and contraction above 320 degrees
What is hygroscopic expansion
□ 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
What factors effect the hygroscopic expansion
® Lower powder/water ratio
® Increased silica content
® Higher water temperature
® Longer immersion time in water
What factors effect the contraction above 320 degrees celsius
□ Water loss
□ Presence of sodium chloride and boric acid
What are the properties of gypsum bonded investment
○ 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
What happens when gypsum bonded invesmtent material is casted at 700 degrees
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
What is heat soaking
○ 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
What is the composition of phsophate bonded investment in the powder
§ Silica
§ Magnesium oxide
Ammonium phosphate
What is the composition of phsophate bonded investment in the liquid
§ 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
What is the setting reaction of phosphate bonded investment
○ Ammonium phosphate + magnesium oxide + water –> magnesium ammonium phosphate
What happens to phosphate bonded investment on heating
○ 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
What are the properties of phosphate bonded investment
○ High 'green' strength meaning we don’t need metal casting ring for support ○ Easy to use ○ High strength ○ Porous ○ Chemically stable
What is the setting reaction for silica investment
stage 1 - prepare stock solution
stage 2 - add powder (quartz or cristobalite) - gelation
stage 3 - drying (tightly packed silica particles)
What are the silica investment dimensional changes
○ Contraction at early stages of heating
§ Water and alcohol loss form gel
○ Substantial thermal and inversion expansion
Lots of silica present
What are the other properties of silica
○ 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
