| Constituents and properties of the ingredients | Properties | Uses | |
| Cobalt Chrome | Cobalt (Co) 55-65%:
Chromium (Cr) 28-30%: From a solid solution. A higher Cr content increases corrosion resistance but too much and the metal will ne brittle. Co increases the hardness Molybdenum (Mo) 4-5%: Refines the grain size and therefore gives hardening effect Titanium(Ti) 5%: Decreases the particle size and improves flow. It helps increase the melting range as well as increase the strength Carbon (C): increases strength and hardness although too much will result in a brittle alloy. Manganese (Mn) Iron (Fe) Silicon (Si) all less than 1%: increases the hardness |
corrosion resistant
hardness, high melting point and is hard-wearing even at high temperatures. |
Construction of partial denture framework |
P1- Compare the constituents, properties and uses of commonly used dental alloys. Bellow shows the different alloys used in dentistry, also stating the constituent’s properties and uses
| Bonding Alloys (PFM alloys (porcelain fused to metal) | Constituents | Properties | Uses |
| High Noble alloys | High Noble: These types of alloys contain a high Gold content with varying amounts of Platinum and Palladium plus others. The ingredients are
Au-86% Pd-2.6% Pt-8.5% In-1.4% Plus, Fe, Li, Ru, Ta |
These alloys are the most expensive, they are easy to manipulate in the laboratory. Very low corrosion properties in the mouth | Full- cast, ceramic bonding applications. |
| Noble Alloys | These types of alloys contain very little Gold, some have no gold content at all. A typical composition is:
Pd-60.55% Ag-28.1% Sn-2.5% In-6.6% Ga_2.1% Traces of Ru |
Corrosion in the mouth is low, strength and hardness are equal to or greater than high-noble alloys. These alloys are expensive | Full- cast, ceramic bonding applications. |
| Base Metal Alloys | These types of alloys generally contain Nickel-Chromium or Cobalt-Chromium. Furthermore, these alloys offer a cheaper alternative to high noble and Noble alloy system. The typical composition is:
Ni-54% Cr-22% Mo-9% Nb-4% Fe-4% Traces of Ta |
Extremely high strengths and hardness (most difficult to manipulate out of the 3 in the laboratory) high corrosion, low cost. Also, they have twice as high
an elastic modulus as do the high-noble and noble-metal alloys. |
Full-cast, ceramic bonding. Partial denture, wrought applications, dental implant application |
| Wrought Alloys: Stainless steel | Constituents | Properties | Uses |
| Ferritic | Chromuim (cr) 11.5-2.7%
Carbon(C) 0.2% Maximum |
Ferritic are not as strong or corrosion-resistant as austenitic steels, they are defined by their superior weldability and engineering properties | Little application in Dentistry |
| Martensitic | Chromium(cr) 11.5% to 17%
Nickel (Ni)- 0% to 2.5% Carbon (C) 0.15% to 0.25% |
Can be heat treated and has less corrosion resistance than other types of stainless steels. Poor weldability and is magnetic. Also, can be hardened by heat treatment resulting in high strength and hardness | Used for surgical and cutting instruments |
| Austenitic | Chromium(cr) 16-22%
Nickel (Ni)- 7%-22% Carbon (C) 0.25% |
It has greater ductility and ability to undergo more work without breakage. Also has substantial strength when working. Most corrosion resistant metal out of the 3 | Used for orthodontic wires, endodontic instruments, crowns in paediatric dentistry |
M1- Assess the ideal properties for alloys used in the construction of dental appliances.
The ideal properties of any dental application are that they should have good strength, hardness, ductility, corrosion/tarnish resistant, biocompatible, cost effectiveness, TEC and easy to use. Metal ceramic crowns made from high noble alloys tend to have a higher gold amount, making it soft which makes it a disadvantage as it becomes less durable, for example Gold-platinum alloys which has 85% gold composition (type 1). However, this can be changed by using silver as a hardener to increase strength. Furthermore, another advantage from using high noble alloys is that it has a very low corrosion resistance properties in the mouth due to the platinum. The disadvantage of using high noble alloys for PFM crowns is that it is very expensive due to the high amounts of gold input.
In addition, metal ceramic crowns which are made from noble alloys can also be used. They have relatively high strength, hardness, and ductility. For example, using Gold-Copper-silver palladium alloys composition to make PFM alloys. They have less gold and substitutes it with 15% copper and 25% silver to increase strength and hardness. This is an advantage as it improves durability and lifespan. A disadvantage of this composition is that the corrosion resistance reduces due to the silver, copper and gold which all decreases the corrosion resistance, therefore it won’t be easy to use as it will be less biocompatible. Furthermore, another composition used in noble alloys is Palladium-copper-gallium. An advantage is adding gallium which is added to reduce the temperature at which the alloy is completely melted: this enhances bonding of porcelain to the metal and increases strength which is an advantage in the melting point area and strength.
Furthermore, metal ceramic crowns can be made from base metal alloys. The advantages of using base metal are that they have greater rigidity then other alloy types and have excellent sag resistance. However, the disadvantages of base metal alloys are that they are difficult to melt as their melting points are very high; for example, nickel-based alloys reach a melting range of 1275 degrees and Cobalt-based reaches 1400-1500 degrees. Adding to that, they have lower ductility and greater hardness meaning they are less burnish able and more difficult to finish and polish than the high noble and noble-metal alloys. Also, they are more prone to corrosion (due to the beryllium) under acidic conditions, difficult to solder, and can cause an allergic reaction in susceptible patients. Base metal alloys reduce biocompatibility because of beryllium as they can cause allergic reactions due to beryllium and nickel.
An example of a base metal alloys is nickel-chromium-beryllium which is used in making of metal-ceramic restorations. They contain 60% of nickel to increase the hardness which makes it an advantage when making the metal-ceramic restoration as it will not deform, and 20% of chromium with 1-2% of beryllium is used in the composition. This composition gives excellent sag resistance as well as high strength and rigidity. Small amounts of beryllium like 1-2% can reduce the liquidus temperature and therefore, make casting easier, which is an advantage. More advantages are that it gives the metal-ceramic restoration higher tensile strength, ductility, and possible hardness and yield strength. Disadvantages are that beryllium can increase corrosion making it less biocompatible in the mouth.
Another example is using Cobalt-chromium alloys to make metal-ceramic restorations. The ideal properties are that they should be easy to handle and be able to finish and polish. However, the low ductility and high hardness make them difficult to finish and polish. Another disadvantage is that it has high melting points, making it difficult to manipulate the metal ceramic restorations, so it needs special casting equipment. The advantages are that they are used as an alternative to the nickel-based alloys for patients who are allergic to nickel-based alloys and nickel.
Alloys for removable partial metallic dentures
The three types of alloys used are: Type 4 gold-based alloys, Chromium-based alloys, Cobalt-based alloys
The type 4 gold-based alloys are durable enough to function sufficiently when used to make frameworks. Furthermore, the Cr- and Co-based alloys are changed from their metal-ceramic counterparts, in that they contain a small quantity of carbon (usually from 0.1 to 0.5 wt%) for hardening which is essential in the ideal properties.
These alloys have advantages, for example, compared to the Type IV gold-based alloys in that they were stronger and 30% harder, less dense (therefore lighter and cheaper making it easier to use and cost effective), and had twice as high rigidity and elasticity. The better rigidity makes them a good choice for removable partial metallic denture frameworks because the frameworks could be cast in thinner sections and still retain the important property of rigidity. Also, because they stretched less, undercuts used for clasp retention could be less distinct and still function properly.
Disadvantages of these alloys are; compared to the Type IV gold alloys they have higher casting temperature and higher hardness making them more difficult to fabricate, finish, and polish. Also, having low ductility. Low ductility means that clasps could only be permanently distorted (such as during chairside adjustment) a few times before they failed.
D1- Evaluate the effects of changing the constituents ration ono the properties of dental alloys.
Changing the ratio of different constituents in an alloy means that the alloys will change in properties and alter the suitability in different works and may affect dental appliances. For example, high noble alloy such as, gold-platinum alloys in making metal-ceramic restorations, gold 85% which, makes it a soft and easy to work with, class I. A disadvantage is cost effectiveness as 85% gold in the alloy is a lot, meaning that it will cost a lot for the dentist and might not use that composition; however, might be used in private work.
However, by decreasing the Gold to 60% it will make the density less strong, and the volume of gold particles will be lesser making the yellow colour less noticeable, ductility will affect the hardness of the gold and might be able to expand over time (known as thermal expansion cohesion) due to stress in a metal crown, corrosion resistance which will affect the biocompatibility of the dental appliance as it will corrode and react leading to allergic reactions.
In addition, tarnish will affect the colour of the gold making it lesser yellow hue and become dull therefore making it less aesthetically pleasing and needs to be polished occasionally. Furthermore, during this process of tarnishing leaching and allergic reactions in the mouth can leech out more. Lower melting point will help with manipulating the crown and is useful as it doesn’t need an expensive furnace to melt the gold as it melts at 1064 degrees. Increasing hardness of gold will help for studier fitting and longer lasting. Ultimately, workability will decrease, making it much harder to manipulate the crown.
Platinum which is combined with Gold has different effects in the properties. Platinum which has 12%, increasing this amount to around 35% it will increase strength meaning it will withstand great force or pressure, this is useful because when you bite it needs to withstand the biting pressure, so It doesn’t change in shape. Increase melting point (making it harder to melt and will increase the melting from 1140 to 1539 degrees) which is an advantage, because if the alloy requires to be soldered at some point, for example to join bridge components together. If the technician is concerned that a large casting may not be dimensionally accurate enough if you cast as one unit. Increasing hardness will make it more rigid so it won’t move and change shape over time, also excessive hardness can cause wear of the opposing dentition or restoration. Furthermore, it makes the colour of the gold less yellow which is an advantage as it might be aesthetically pleasing, increases corrosion resistance. By doing this, corrosion resistance will be high, therefore biocompatibility will increase in the mouth, so the patient will be unable to wear the crown or else there might be an allergic reaction. Furthermore, an advantage is that cost effectiveness will be low due to the composition meaning that the dentist will able to buy it.
Zinc at 1% is mostly used as a hardener, if you increase this amount to 5% the properties will be decreased, for example strength, density, melting point (decreases to 1462 degrees). Increases hardness will make it more rigid so it won’t move and change shape over time also excessive hardness can cause wear of the opposing dentition or restoration. Corrosion resistance and tarnish. Furthermore, by increasing Zinc it prevents oxidation of the other metals during the manufacturing process; increases fluidity and decreases surface tension, which improves castability
Furthermore, changing constituents of a noble alloy for example, Palladium-copper-gallium alloys. Palladium has 79%, by changing this to a lower value for insistence 35%. The effect will be that the properties will decrease for example strength, meaning it will be less robust and less able to withstand force of the crown. Melting point will still be high but will be reduced to around 1100 degrees this will affect the time taken in the furnace and can be melted in a basic furnace; hardness will decrease due to lesser particles of Palladium in the crown, the effect of this is that the rigidity will become weaker. The cost effectiveness of this product is quite high because it has expensive such as palladium and gallium. This composition is commonly used in private work and sometimes in NHS work.
Corrosion resistance will make it less biocompatible, the effect is that allergic reactions may happen to the patients. Furthermore, the colour change will turn less white making it less aesthetically pleasing but may be good for some patients. Copper has 7%, increasing the copper composition to 10% will make strength and hardness better. Also, when copper is increased the sag will be decreased so the deformation isn’t high, this occurs at high temperatures during porcelain firing. But significantly decrease the density and melting point which will be good as it can be melted faster (1190 degrees) and quicker. Increasing the composition of Gallium will increase corrosion resistance. Also, decreases the temperature at which the alloy completely melts; enhances bonding of porcelain to the metal and increases strength.
In addition, changing constituents of a based metal alloy, for example Nickel-chromium. The composition of the alloy has 60% nickel which Is used mostly for strength and corrosion resistance. However, by reducing that amount to 30% these qualities reduce, such as strength meaning it won’t be able to withstand pressure and make the yield strength much lower, density will be lower making the particles lesser, so the crown won’t have a good enough density efficiently. Cost effectiveness of Nickel-chromium is low because nickel Is cheap, meaning the dentist can use it.
Furthermore, melting point will be reduced, this will affect the melting time and will benefit the dental technician as they can use a resistance melting then using induction melting as it doesn’t need high temperature. Hardness will decrease due to lesser particles of Nickel in the crown, the effect of this is that the rigidity will become weaker. Also, the composition includes 20% chromium which is used for hardening, polishing and high corrosion resistance. However, changing it to 10% will reduce the hardening, polishing. Also, corrosion resistance will be reduced affecting the biocompatibility of the crown. Therefore, using this composition to make a crown will be difficult as it won’t be strong enough to support porcelain inside and be less durable.
P2- Explain the methods used to heat and melt dental alloys P3- Compare the temperature required to melt the commonly used dental alloys.
There are different melting methods, which are torch using flame, resistance, induction.
The different methods of Gas torching are, Gas air torch, Gas-oxygen torch and Oxy-acetylene torch.
Torch melting levels are:
1)Carbon-rich zone
2)Reduced Zone
3)Oxidizing Zone
Carbon-rich flame Oxygen-rich Flame Proper flame
Torch melting– Gas air torch- Gas-air torch is used to melt conventional noble metal alloys used for inlays, crowns and bridges) whose melting points are less than 1000 degrees. The method is that heat produced from the flame is then radiated throughout the dental alloy until it is molten and ready to cast, also torch melting can be used with centrifugal machines. Also, gas oxygen torch is used to melt metal ceramic alloys of higher temperature up to 1200 degrees (used for full cast, porcelain bonding application). Oxy acetylene torch: One volume of acetylene and two and half volume of oxygen are needed. This is more ideal for High noble alloys and noble alloys as their melting range is around 1000 to 1200. Torch melting are for lower temperature, mixture of natural/artificial gas, oxygen/tank gas-oxyacetylene. However, they are less fast than induction heating but faster than resistance heating
Resistance melting uses electricity passed through a wire which heats up to 4000 degrees. The type of melting is mostly used for based metal alloys (used for full cast, porcelain bonding, partial denture, dental implant applications). This alloy is wrapped around the crucible in which the alloy melts. It is used to melt ceramic alloys. Furthermore, this type of melting provides the best means of temperature control and can be used for all types of Crown and Bridges, it comes with choices of crucibles, carbon for high gold alloys and ceramic (and flux) for palladium alloys. Here, the alloys are automatically melted in graphite crucible. The crucible in the furnace is always against the casting ring. So, the metal button remains molten slightly longer and ensures complete solidification. However, they are, high risks of overheating of the alloys. Induction melting is more for higher temperature, and melts faster and can be easily over heated but it is expensive to buy, maintain and run.
Resistance melting methods are:
1) Heating spiral
2) Power supply
3) Thermocouple
4) Crucible
5) Alloy
Induction melting works by an electrical current being passed through a copper coil inside which the crucible containing the alloy is placed. The melting range is around 2000 to 4000 degrees as induction is for those metals which are hard to melt for example titanium. The flow of the electricity creates an induction field. The eddy currents heat the metal eventually melting it. This method provides the fastest way of melting an alloy but can lead to problems with temperature control as it can easily overheat unlike torch melting which keeps a stable temperature. This method requires a pyrometer that is focused on the alloy to monitor of the temperature of the melt. It usually used mainly for based metal alloys for example cobalt-chromium-nickel alloys and titanium alloys due to the high melting point. The uses are for full-cast, metal-ceramic restorations, removable partial denture framework, dental implant applications, wrought applications
Induction melting methods are:
High-frequency coil- Cooling water(inlet)
- Colling water(outlet)
- High-frequency current)
- Crucible
- Alloy
High noble alloys have low melting points meaning that using torch melting will be more suitable for example Gold copper which has a melting range of 910-1065 by using Gas air torch which is suitable because the range of the torch is 1000 degrees. However, Gold platinum has 1045-1140 degrees which is unsuitable for gas air torch and needs gas oxygen torch instead as the melting point is 1200 unlike the gas air which has 1000 which can’t melt it. Noble alloys can also be melted using Torch melting but using gas oxygen, such as palladium copper which as a melting point of 1100-1190 degrees and silver palladium which has 1020-1100 degrees which can use torch melting gas oxygen combination.
Base metal alloys have high melting point which is usually above 1200, torch melting is incapable meaning that other machines such as resistant and induction melting methods should happen as their melting point is very high which is ideal for titanium-based alloys as they have a melting point of 1700 degrees which is perfect for resistant melting. However, resistance can’t be used as they take a longer time as that of induction making it insufficient to use. Nickel based alloys have a melting point of 1275 degrees which is lower but still can’t use torch melting method as it is 1200 degrees. Therefore, it needs to use resistance as it can control the temperatures and it can go to temperatures of 4000 degrees. Cobalt based alloys have a melting point of 1400-1500 degrees making it suitable for either resistance or induction as they go as high as 4000 degrees, however its unsuitable for torch melting as it is only for melting 1200 degrees.
M2- Discuss the various methods of changing the structure of dental alloys
Why do you need to change an alloy structure?
It is changed to improve its properties, for example using methods such as:
- Heat treatment
- Cold working
- Solid solution strengthening
- Precipitation hardening
Heat treatment includes rising the temperature of the metal alloy to an optimum point, keeping the temperature before cooling it. The temperature, holding time and cooling methods changes on the wanted impact. For instance: to soften a gold composite you will need to melt it at 705 degrees, leaving it for 15 minutes, then cool it quickly by using water. To make a gold combination harder you will need to warm is to 370 degrees C. Finally, leaving it 15 minutes and to be able to cool gradually to room temperature.
The changes of properties when using heat treatment are:
- modified structure of the alloy
- increases strength and hardness
- increases machinability
- improves properties of alloy
Cold working is the point at which a metallic alloy is cold worked the structure of the metal can be altered. This usually occurs because of separations at the grain margins and results in a firmer and sturdier metal alloy with better yield quality. When the metallic alloy has been ‘worked solidified’, more changes of the manipulation can occur leading to a fracture. Which is mostly common in wrought alloys when creating composites manufacturing of clasp arms for partial dentures or wire restorations for orthodontic appliances.
Solid solution is when atoms of base metal (solvent) and alloying elements (solvent) completely disperse together, the resulting phase is known as solid solution.
Interstitial solid solution-Solute atoms are much smaller than solvent atoms (different atomic level)
Substitutional solid solution- Solute atoms sizes are roughly like solvent atoms (different atomic level)
Solid solution strengthening means different metals in an alloy which have shared solubility with each other in their molten state. At the point when the metallic alloy is cooled from its molten state, the different metals still stay soluble in one another forming a solid arrangement. The diverse sizes of the particles in a similar lattice from mechanical resistance from separations bringing about a harder more stringer alloy. An example of a solid solution alloy is Au-Ag which has good strength.
Precipitation hardening is the point at which a metal is heated, and the particles end up soluble when quenched. Therefore, the particles become frozen in the metallic alloy. After warming these particles, diffusion happens rapidly resulting in the formation of smaller particles which are referred to as precipitates. Therefore, these precipitates will act as obstacles to dislocation, meaning there will be an increase in strength and hardness. It can be called as age hardening, and example is repeated firing of bonding alloys.
D2- Evaluate the methods of abrading and polishing the commonly used dental alloys
What is Abrasion?
Abrasion is the process of wear on the surface of one material by another material by scratching, gouging, chiselling, tumbling, or other mechanical means. The material that causes the wear is called an abrasive; the material being abraded is called substrate. Below are pictures of burrs we use for abrading a surface.
What is Polishing?
Polishing is the process of making rough surface smooth to the touch and lossy (mostly specular reflection of incident light. Polishing is usually preformed with very small-particle-size abrasives. Below is a picture of a dental technician polishing a denture using a polishing machine.
There are different types of abrasive equipment to use for example, bonded abrasives such as burr, disc, sandblasting, lathe, model trimmer and sand paper. Also, there is none bonded abrasives such as pastes and pumice, all of which can be different abrasive size and hardness. The reasons why we abrade polish substrates are that it will increase patients’ comfort, improve aesthetic properties, protect against tarnish and corrosion and help maintain oral health by reducing the risk of bacteria colonisation.
Factors which affect rate of abrasion and Polishing?
Large difference in hardness between the abrasive and substrate allows the most efficient grinding to take place. Brinell and Knoop hardness values are functions of a materials resistance to indentation, whereas Mohs value indicate one material’s resistance to scratching by another.
The particle size of an abrasive may be expressed in micrometres. By connection, particles are classified as fine (0to 10um), medium (10 to 100 um), and coarse (100 to 500 um), according to the average particle size of the sample. Larger abrasive particles will abrade a surface more rapidly than will smaller particles; however, they tend to leave coarser scratches in the abraded surface then do fine particles. Equivalent-sized scratches can be produced by different sizes of particles by varying the applied pressure.
The particle shape also influences rate of abrasion are sharp, irregularly shaped particles will abrade a surface more rapidly than will more rounded particles having duller cutting angles. However, the former will produce deeper scratches than the latter. The abrasion rate of the abrasive decreases during use; this is due partly to rounding of the particles and partly to contamination of the abrasive with some of the substrate material.
The greater the speed at which the abrasive travels across the surface of the substrate, the greater the rate of abrasion. The greater friction at higher speeds. However, tends to create higher temperatures at the surface of the substrate.
The greater the pressure applied, the more rapid will be abrasion for a given abrasive. Greater pressure produces deeper and wider scratches and creates higher temperatures.
Lubricants such as silicone grease, water spray, glycerol) are used during an abrasion for two purposes: to reduce heat build-up, and to wash away debris to prevent clogging or blinding of the abrasive instruments. Too much lubrications can reduce the abrasion rate because it may prevent some of the abrasive form meeting the substrate.
Methods for polishing and abrading restorative alloys are for example Gold alloys.
Gold alloys are finished by using coarse, medium, and fine abrasives in sequences. Coarse scratches are removed with fine pumice or coarse abrasive rubber wheels; the surface is finished with a rubber wheel impregnated with. Although relief polishing still takes place during the finishing of micro filled composite resins, the valleys are filler particles are so small that the surface appears glossy and feels smooth.
The new types of hybrid and small-particles copositive resins also tend to appear shiny and smooth by cannot be polished as easily as the micro filled composite resins. A normal way of polishing gold alloys are coarse grinding with a diamond stone or a green stone followed by a series of coarse to fine quartz or aluminium oxide abrasive disks or rubber wheels. Carbide burs with 12 or more blades have also been used to polish with 12 or more blades have also been used to polish composite resins. Finishing instruments for composite resins are available commercially. The effects of several polishing sequences on the surface roughness of composite resins.
There are different methods of abrading and polishing a restoration to the ideal appearance for example:
Carbide Tungsten which is usually for finishing burrs which have different dimensions such as lengths and widths, which is done by using flutes on a lathe that will tell you the level of harshness you want for the cutting movement, which is used for cutting preparations in dentistry. Another example is Diamond which is a very tough material and can be enough to abrade any surface it meets. They are usually found in model trimmer disks or dental burs in different lengths, sizes and abrasiveness. Furthermore, very small particle diamonds will come in a paste formula for polishing compounds. Which is mostly used in sectioning crown and bridge arrangements and to polish and finish compound dental appliances. Silicon carbide is an artificial material that has very strong and hard abrasive material which makes it very efficient in abrading. Examples are merged rotating devices and silicon carbide layered disks, which are mostly used as a final action.
Furthermore, Aluminium oxide is artificial abrasive. The powder is mostly used in sandblasting procedures and for getting ready the cementation and abrasion from air pressure. It is used in layered rotational appliances as an abrasive. The coarse material comes in different sizes and grits and has mostly changed emery. It is mostly used to finish metal alloys and ceramic dental materials and even the surface of enamel, so it is more aesthetically pleasing. Electro brightening is a procedure that gets ride of 00.1mm-0.12mm of the chrome base. Electroplating polishing will get ride of any elevated points or tips. Also, electro polishing helps to level out surfaces which where roughened to hard from the sand plaster. The main point of electro polishing is to roughen the surface; therefore, it will give the metal a nice shiny, lustrous area.
Shown above are the different types of coarse materials used when abrading a surface. They are different types of abrasives which can be classified as “bonded” and “non-bonded”. Examples of bonded are burr or a disk; examples of non-bonded are polishing past and pumice. In addition, there are mechanical and the use of electrolysis to achieving a excellent finish in your dental appliance. However, at the end of the day the product must be fit for purpose. Also, you should use correct equipment and substances when making your dental appliance; and follow the correct methods.
For example, if the dental technician uses an abrasive and dry adhesive on a surface, this will make complications such as damage and breakage; which is due to deep scrapes and friction. However, for example high diluted adhesive which is very fine is used, very little material can be taken off or removed. The correct burr grit and thinning of runny abrasive must be used to acquire the ultimate result in polishing and finishing of the surface from a dental appliance.
P7- Describe the constituents, properties and uses of the commonly used dental refractory materials
Dental Investment-
Investment materials are a ceramic material that are used for making a mould into which the alloy is poured. Recording all the wax design in a strong material which can keep up accurate detail through the burnout process for casting in metal compounds. Investment materials are accessible as two component systems. Powder and liquids. All investment materials contain silica as an refractory constituent (SiO2) the main difference are the types of binders used.
The ideal properties of investment materials are that:
- It should be reproducing the shape, size and details recorded in the wax pattern
- Easily manipulated and must readily wet the surface of the wax pattern
- Adequate setting time to allow the investing procedures
- It should be able to withstand the high temperatures during the burnout of the wax and casting of the molten metal.
The composition of investment materials
There are 3 types of investment materials:
Gypsum bonded which is commonly used in the casting of gold alloy approx., 700c but not stable for melting alloys above 1200c
Phosphate bonded is commonly used for high fused noble or base metal alloys which needs a higher melting temperature which is 4000 degrees, For example metal ceramic and CoCr (such as metal ceramic or CoCr).
Ethyl Silica Bonded is primarily used for casting base metal alloys with high temperatures. The examples of the uses of Ethyl Silica Bonded are partial dentures.
Investment material
Refractory material: Exposure to high temperature without degradation (silica and quartz cristobalite tridymite)
Binders: which can bind the refractory material to form a coherent solid mass (Gypsum, silica phosphate, alpha hemihydrates)
Modifies: Added to the refractory and binder material to enhance their physical properties. (reducing, accelerating agents, colouring material, non-oxidizing agents, retarders agents)
Gypsum bonded investment:
These are mostly used for casting gold alloy with melting points of around 700-1000 degrees.
There are different types pf gypsum bonded investment for example:
Type 1: Thermal expansion, inlays or crowns.
Type 2: Hygroscopic expansion, inlay and crowns.
Type 3: Partial dentures with gold alloys
The constituents are
Refractory: crystalline polymorphs of silica (quartz or cristobalite) 55-75%. Binder: Calcium sulphate hemihydrate (plaster or stone) 25-45%. In setting, hemihydrate binder combines with water to form dihydrate(gypsum). Modifiers: Accelerators, retarders, reducing agents or additives that reduces the thermal contraction of the binder, colouring agents.
Properties of Gypsum bonded investment are that they have:
- Good strength
- Good porosity
- Controlled larger setting and high thermal expansion
- Simple methods of manipulation and casting process
- Inexpensive
- Cannot be used for titanium alloys because gypsum fused investment has low melting points
- Also, too high casting force and careless wax burnout method
Uses of gypsum bonded investment is mostly for Partial dentures, inlays and crowns.
Phosphate bonded investment
They are usually used for casting cobalt chromium metal alloys. They can withstand high temperature and meet required requirements, such as expansion. Examples can be magnesium phosphate mixed with ammonium phosphate.
The constituents are
Refractory: Colloidal silica liquid, increases expansion and improves casting surface smoothness. Binder: Magnesium-oxide (basic) and phosphate (acid, mono-ammonium). Modifiers: Carbon; to produce clean casting and facilitate the divesting. Don’t use with palladium-containing alloys because carbon reacts with Pa.
The properties in Phosphate bonded investment are:
- Easy to handle without breaking before burnout
- High strength to withstand centrifuged forces
- High temperatures to withstand burnout.
- Can have mould breakdown and rough surface on casting of high melting alloys above 1375 degrees
- High strength can make removal of cast difficult from investment.
Phosphate bonded investment is mostly used for base metal and gold casting alloys used to make copings and framework for metal-ceramic prosthesis. Also used for casting ceramics and for refractory die for ceramic build-up.
Silica bonded investment
These are used as a substitute to phosphate bonded investment for high temperature, casting. Principally used in casting of base metal alloy partial denture.
The constituents are:
The powder contains refractory particles of silica and glass along with calcined MgO and other oxides in small quantities. 2 liquids are ethyl silicate and acidified solution of denatured ethyl alcohol. Binder: Silica gel that reverts to silica (cristobalite) on heating. Refractory: Quartz or cristobalite and small quantities of magnesium oxide is added to the powder to reduce the ph of silica gel during manipulation.
The properties of Silica bonded investment are:
- Able to cast high alloys with satisfactory results
- Low cast strength makes removal of casting easier that phosphate bonded
- Can be complicated and time consuming to use
- The low strength and high thermal expansion require precise burnout process and firing schedule to prevent cracking
The uses are for casting high fusing metal and for partial denture alloys.
References:
https://slideplayer.com/slide/9695638/
https://www.slideshare.net/indiandentalacademy/abrasive-polishing-agents
https://pocketdentistry.com/21-alloys-used-in-dentistry/
https://www.slideshare.net/priyankachowdhary7/casting-procedure-casting-defects
https://www.slideshare.net/ShilpaDineshan/wrought-metal-alloys
https://www.metalsupermarkets.com/types-of-stainless-steel/
https://www.slideshare.net/kadarsh/stainless-steel-68819670
Dental Alloys Used in: Prosthodontics
https://moodle.lambeth.ac.uk/pluginfile.php/72548/mod_resource/content/2/Heating%20Alloys.pdf
Dental Materials and Their Selection Second Edition William J. O’Brien PhD
