MODELS & DIES

what properties/ characteristics would be required for a dental modelling material

• cheap

• easy to form (from an impression)

• safe during handling and if swallowed

• robust

• rigid, no flexion or deformation

• will not chemically react with water, wax, impressions etc.

• resistant to wear (apppliances will be made to fit the model and will need to be repeatedly tried on)

• resistant to heat (approx. 100°C)

what are the categories of modelling materials

• plastics/ polymers

• organic

• clay

• plaster (most common)

which criteria of ideal features of dental modelling materials does plaster of Paris/ Gypsum not meet

what material DOES react with plaster

agar

how can the reaction between agar and plaster be prevented

alum solution can be used as a barrier

what are the uses of plaster of Paris/ Gypsum

• models and dies

• impression material

• moulds for denture construction

• binder in refractory investments for casting

what is Plaster of Paris

• gypsum is a natural mineral that is mined

• quarries of Montmartre district of Paris produce burnt gypsum

• this dehydrated gypsum became known as Plaster of Paris

• upon addition of water, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or set

what pathways does gypsum from the ground go through to become dehydrated to form the end material

• open kettle at 110-120°C

• pressure vessel at approx. 125°C

• in the presence of deflocculants (CaCl₂ and MgCl)

outline open kettle at 110-120°C

• makes calcium sulphate beta-hemihydrate

  • beta hemi-hydrate produces large porous particles

» plaster of Paris

outline pressure vessel at approx. 125°C

• calcium sulphate alpha-hemihydrate in pressure vessel - this drives more water out of the gypsum

• produces small dense particles

» dental stone (buttery yellow colour)

outline in the presence of deflocculants (CaCl₂ and MgCl)

• calcium sulphate alpha-demihydrate

• add deflocculants which allows material to flow over each other so the material does not clump up

• produces very small dense particles

» improved stone (white colour)

what is improved stone used to make

crown and bridge

define model VS die

model: the whole mouth

die: a tooth (a section of the model), removable to be worked on

what are the types of Gypsum products

current ISO Standard for Dental Gypsum Products identifies 5 types of materials:

• Type 1 Dental plaster for impression

• Type 2 Dental plaster for model

• Type 3 Dental stone, die, model

• Type 4 Dental stone, die, high strength, low expansion

• Type 5 Dental stone, die, high strength, high expansion

key information about dental plaster

• cheapest

• referred to as the ‘beta’ form

• in the lab: white powder

• working time = 3-4 mins

• initial setting time = 5-10 mins

• slight expansion on setting of 0.2-0.3%

• exothermic setting reaction, cool once set

• softest but quickest setting

• rarely used on its own as a model (mostly mixed with dental stone)

• used as denture moulds

what kind of behaviour does plaster display

thixotropic behaviour

key information about dental stone

• referred to as the ‘alpha’ form

• often called Kaffir-D but use stone or KD

• in the lab: yellow powder

• working time = 3-4 mins

• initial setting time = 5-20 mins

• slight expansion on setting of 0.08-0.1%

• exothermic setting reaction, cool once set

• stronger than Plaster of Paris

• often used for ‘everyday’ dental models (mixed 50/50 with dental plaster to reduce cost and setting time)

what is the ratio of dental stone and water that is mixed

1:!

key information about improved stone

• a lot more expensive

• referred to as the ‘alpha-modified’ form

• ‘die-stone’

• in the lab: coloured powder e.g. blue

• finer, more regular particles

• working time = 5 mins

• initial setting time = 20+ mins

• even less expansion on setting of 0.05-0.07%

• it is harder than Plaster of Paris but takes longer to set

• significantly higher cost

• usually vacuum mixed

• used where accuracy and wear resistance is critical e.g. crown and bridge models

chemistry of plaster

reversible reaction

setting process for plaster

what are the stages of the setting reaction of plaster

how does plaster harden into a solid

spherulites of gypsum crystals expand, aggregate and impinge during the exothermic reaction when plaster is mixed with water

how can the setting time of plaster be increased

• if you need more time for plaster to set you mix it thinner

• this means the crystals have further to grow to impinge on each other so the plaster takes longer to set

what is a disadvantage of mixing the plaster thinner

• as the crystals get bigger they get more fragile

• therefore material that takes longer to set is also softer

expansion of plaster percentages

what can reduce the setting expansion of plaster

• accelerators and retarders

  • anti-expansion agents

what are bonus properties of plaster

• easy to trim or grind with a scalpel/ knife

• can be coloured (appealing shades)

• sticks to itself so models can be added to and modified

when making plaster moulds, how can you prevent plaster from bonding to itself

• use an alginate-based separating liquid

• paint it on the plaster and wait for it to dry

what is the water:powder ratio for plaster VS stone

• plaster: 50ml water per 100g powder

• stone: 20-35ml water per 100g powder

why does stone need less water than plaster

because the powder is ground finer than plaster

what does spatulation refer to and how does it affect setting time

spatulation: how long you are mixing for

• over spatulating decreases setting time (sets faster) because you start to break the crystals that have started to form

• this increases the nucleation sites so more crystals will form quicker

how does water temperature affect setting time

colder water = longer to set

give examples of retarders

• borax

• potassium citrate

• sodium chloride (large amounts)

give examples of accelerators

• potassium sulphate solution

• gypsum powder (adds new centres of nucleation)

• sodium chloride (small amounts, causes pink spots on material)

what is the typical composition of a dental plaster

powder:

• calcium sulphate beta-hemihydrate (CaSO₄)₂·H₂O

• borax (slows the setting rate)

• potassium sulphate (reduces expansion)

liquid:

• water

what other tests would have shown a greater difference between plaster and stone

• wear resistance test (scratching surface)

• surface hardness (indentation test)

• three-point bend

what are other model materials

• ‘Rhino Rock’ polymer

• 3D printed models

• digital models

outline Rhino Rock

• polymeric model material

• cures within 30 mins

• dimensionally stable

• zero expansion

• strong and impact resistant - will not chip

• white, opaque

• can be 3D scanned

what are disadvantages of Rhino Rock

• expensive

• dangerous to skin

outline digital models

• intra-oral scanning offers the opportunity for digital capture, design and manufacture

• no properties to work around

• only a file to store