Biophysics Spring semester Dentristy...only followed by casting, hereby this method is not suitable for direct scanning or intraoral application. Optical scanners operate with the

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3D printing in dentristy

Biophysics – Spring semester

Dentristy

Digital technologies in dentristy

syllabus

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The steps of digital technology

Digital technologies can be classified as a rapid prototyping process, which is developing fast

therefore medical and indrustrial applications are expandly available. With other words digital

technologies are additive manufacturing procedure which print slight layers on each other in

order to create the object of interest. This technology is in contrast with the traditional

(subtractive) manufacturing where 3D objects are constructed by successively cutting down

from a solid block of material. Additive manufacturing process composed of 3 steps: (1)

making digital models by scanning with scanners. (2) the Computer Aided Design (CAD)

software slices the digital model into horizontal layers with identical thickness. The file

format is STL file (comes from the first additive tecnhology: StereoLithography word) in

which the surface of the object is dispieced into small polygons by using geometrical data of

the object of interest (figure 1). (3) Computer Aided Manufacturing (CAM) is the method to

create 3D object of interest by 3D printer which takes time from hours to days depending on

the size and the complexity of the object.

Figure 1. STL file

The development of digital technologies have been started from 1971 in the field of dentristy.

The first dental CAD/CAM (Computer Aided Design/Computer Aided Manufacturing)

system was introduced in 1983. During the next three decades the technology have been

developed significantly which can be divided into distinct groups (figure 2).

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Figure 2. Classification of CAD/CAM technology in dentristy.

Scanners

The first step for creating a digital 3D model is scanning the geometrical data of the object’s

surface which is processed by the computer. Nowadays several types of advanced scanners

are available for this reason. They can be divided into two groups based on the operation

scanning:

Mechanical scanners scan the surface of the object with a ruby sphere having small

diameter. Therefore the object has to be in solid state. In general, the object can be scanned

only followed by casting, hereby this method is not suitable for direct scanning or intraoral

application.

Optical scanners operate with the help of a light source (it can be a laser as well) to create

the geometrical data by the so-called triangulation method. In this case, the light source and

the camera are in a well-defined position and angle to each other. Structured light is projected

from the light source onto the object and the camera captures the shape of the target. Knowing

the angle surface the shape can be reconstructed by measuring the distorsions between taken

image and reflected image. The intraoral scanners work with this method.

With the help of intraoral scanners the situation in the oral cavity can be scanned to create a

digital 3D model. In strip light projection system the scanner projects gray code image to

measure the parameters. If the spatial object is illuminated by projecting structured light the

lines rather approach or retreat from each other as well as the thickness of the lines vary. After

multiple scanning (maximum 6 measurement) and with the help of an algorythm the surface

parameters of the object will be calculated by a computer. Intraoral scanners are able to scan

2-3 tooth at the same time resulting in the visualization of a whole dental arch. For fabricating

the perfect prosthesis the situation in the oral cavity must be scanned precisely. This is a

crucial point of making indirect prosthesis. All the work phases –from the preparation

untilinserting the prosthesis in the oral cavity – are performed by computer aided processes.

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The imaging system of intraoral scanners are similar but data input for 3D image generation

differs.

Photopolymer based systems

In order to print the object of interest abutment is necessary which is made of the same

material as the object. Stereolithography (SLA) technology based on photopolymerization

during which the printer press liquid state of matter material in one layer by using ultraviolet

irradiation resulting in solidification of the material. This cycle is repeated several times until

the 3D object will be created layer by layer. The advantage of this technology is the high

accuracy but disadvantage has to be taken into account as well: the physical properties of the

applied materials are not good as in other systems. Without UV-light application the material

can be degraded rapidly. The resolution in SLA can be found between 0,05-0,1 mm.

Powder based systems

In this system abutment is not needed. Further advantage is that different colours can be

applied during the creation of the object. Thus the colour of the binder matrix sticking

together powder particles and that will generate different colours in the object. Therefore

CAD design requires more time because basically the STL file does not consist colour data.

Selective Laser Sintering (SLS) based on heating particles above melting point by laser

thereby the components will combine to each other. In general alloys are used for this reason.

In the SLS system checking the object has to be taken into consideration, oddly density. Since

heavier materials fall into the bottom of the vat altering the homogeneity.

Extrusion based systems

The most common extrusion based technology is Fused Deposition Modelling (FDM) which

uses a heating chamber to liquefy polymer that is fed into the system as a filament. The

filament is pushed into the chamber by a tractor wheel arrangement and this pressing effect

that generates the extrusion pressure. Finally the molten material stratified on each other layer

by layer. In these system abutment is needed but the material of the abutment can be different

from the object’s. If the abutment if water-lyophilic it can be discarded from the surface of the

object later on. This process demands significant consideration in order to avoid damage of

the object. In general layer thickness is 0,254 mm in the FDM systems.

Photopolymerization processes

In photopolymerization processes liquid, radiation curable resins, or photopolymers as their

primary materials can be used. Most photopolymers react to radiation in the ultraviolet (UV)

range of wavelengths, but some visible light systems are used as well. During irradiation,

these materials undergo a chemical reaction to become solid by association of monomers.

Photopolymers are widely used in the field of dentristy, notably in the coating and printing

industry, such as sealing the top surfaces of teeth to fill in deep grooves and prevent cavities.

In these applications, coatings are cured by radiation that blankets the resin without the need

for patterning either the material or the radiation. Numerous wavelength range of the

electromagnetic radiation can be used in commercial areas, for instance: gamma or X-ray

radiation, UV and visible light. Lasers in the spectrum of UV or visible light are the most

widespread such as Helium-cadmium laser (325 nm) and Nd-YVO4 laser.

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Figure 5. Schematic illustration

of Two-photon system

Three main subtypes can be distinguished in SLA processes:

1, Vector Scan – most commonly used SLA process (figure 3)

2, Mask Projection – the whole layer is polymerized simultaneously (figure 4)

3, Two-photon – The resolution of the created object is high (figure 5)

Figure 3. Schematic illustration of Vector Scan

Figure 4. Schematic illustration of Mask Projection

In vector scan and two-photon system, scanning laser beam is necessary. In mask projection

direct laser beam cannot be applied for irradiating the large surface simultaneously. In order

to eliminate this problem, complex optical system has to be applied. In two-photon process,

photopolymerization occurs at the intersection of two scanning laser beam, although other

configurations use a single laser and different photoinitiator chemistries. Another distinction

is the necessary to recoat a new layer of resin, in the vector scan and mask projection

approaches, while in the two-photon approach, the part is fabricated below the resin surface,

making recoating unnecessary. Approaches that avoid recoating are faster and less

complicated.

In vector scan and mask projection laser beam is able to connect the subunits to each other on

the surface only, as well as to the polymerized layer, respectively. Therefore fabricating a new

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Figure 6. SLA monomer molecules

layer of polymer has to be applied. For this reason, lowering the platform is the easiest

solution.

Materials used in Stereolithography

The molecular structure of thermoplastic materials are linear or branch-like which facilitate

fast melting or hardening even several times. On the other hand in photopolymers cross-links

are generated which does not facilitate material alteration after polymerization. First SLA

technology used acrylate polymer for creating object. The advantage of acrylate is the good

reaction skill but inaccuracy can be occured originated from significant shrinkage. The

uppermost layer of acrylate can be polymerized in 46% followed by further photochemical

reaction when the new layer coats the former one. When light is trasmitting through the new

layer enable to initiate reaction over and over in the polymerized layer. Moreover, due to the

new layer the inhibition effect of oxygen does not show up. By the consecutive chemical

reactions the degree of shrinkage increases leading to volume change during polymerization

in the printed object.

Applying epoxy resin much accurate, stronger and harder material can be produced than with

acrylate. While the shrinkage of acrylate is between 5-20% until epoxies does alltogether 1-

2%, due to the chemical reaction during the opening of the ring-like structure of the epoxies.

Therefore the volume alteration of the material is negligible. Further advantages is that

oxygen does not influence binding therefore it contains less photoinitiator. The advantages of

epoxies are the slow reaction skill, high moisture sensitivity as well as the polymerized object

is fractile. Adding small amount of acrylate the drawback of epoxies can be minimized.

Nowadays, most of the materials used in SLA are the appropriate proportion of compounds of

epoxies and composites. Ingredients of the applied materials are the following: photoinitiator,

reactive solvent, material influencing elasticity, stabilizer and liquid monomer. During

irradiation chemical reaction is initiated in the initiator which becomes reactive. Polymer

bundles are connected with strong covalent binding by association of monomers. There are

two main chemical reaction occur on polymerization: free-radical polymerization (acrylate)

and cationicof photopolymerization (epoxy- and vinylether).

In figure 6. at least one vinyle group (R) can be found in the structure of the molecules. In the

vinyl groups cross-links between polymer chains are created between C-C double bond. Free-

radical polymerization is characteristic for acrylates during which following the activation of

the initiator, monomers build up in polymers. Since acrylates are photosensitive on light

exposure the reaction is fast but it is not applied itself in these days because of the prevoiusly

mentioned drawbacks.

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Cationic polymerization based on epoxy compounds. Due to the ring-like structure of epoxy

by opening of the ring further reaction can happen accompanied by small volume change

since the number and the type of the chemical reaction does not change. Therefore the main

ingredient of the SLA materials is epoxy. The reaction is an exotherm process (approximetly

85 KJ/mol). Despite the significant heat production photocatalizator is needed for launching

the reaction.

On free-radical reaction two photon are able to generate one free-radical averagely. One free-

radical can launch more than 1000 monomer connection. Growing of the polymer chain can

be interrupted by two free-radical connection or if two free-radical extinguish each other

without any connection. In another case when the free-radical is surrounded by polymers,

though it is capable of reaction. These reactive agents can facilitate the reaction later on

during which oxygen or other diffusable material can be the initiator not the monomers or

polymers. This is going to lead for the senescence of the material.

During cationic reaction due to the energy of absorbed light cation is generated in the initiator

which connecting with a monomer thereby launching the polymerization process.

Photoinitiator is an agent which can convert the energy of light into chemical energy and

launch the chemical process. The following materials can be used as monomers:

polymetylacrylate ((aliphatic, cycloaliphatic or aromatic (met)acrylate)), and for cationic

reaction for instance epoxies (aliphatic, aromatic, cycloaliphatic or compound consisting of

heterocyclic structure), such as poly-glycidyl ester.

Dental application of SLA

SLA technology is used for manufacturing surgical templates and inserting implants.

Furthermore the transparency of the applied material, as well as anatomical models can be

generated by the application of colourful base materials in the past years. The advantage of

the technology is the accuracy and the good physical properties. Though these good physical

properties can be available by post-treatment.

Powder based systems

Selective Laser Sintering (SLS) device was the first which applied powder state of matter for

creating an object by the help of digital technology. First of all, plastic was used, but later on

alloys and ceramics became widespread in this technology.

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Figure 7. Schematic illustration of the SLS process

At the beginning of the process the powder leveling roller apply a thin powder layer

(averagely 0,1 mm) onto the build platform (figure 7). Further processes are performed in a

closed vat filled with N2 gas in order to minimize oxidation and degradation of the material.

Above the moving platform infrared heater supports pre-warming during which the material is

kept slightly under melting temperature. Due to this step high fraction of energy of the applied

laser is obtained for sintering process. When suitable amount of powder applied and the

temperature is also optimal CO2 laser beam is focused on the build platform. Due to the

energy of the laser beam the powder droplets will be associated. Unsintered powder around

the sintered object serve as abutment. When the device finished the actual layer the platform

is lowered with one layer thickness and a new layer of powder is applied on the surface of the

platform followed by the association of powder over and over by the laser. In the terminal

phase of the process the sintered object is kept in the closed vat supporting slow fall in

temperature and the avoidment of oxygen. Degradation, rupture or internal tension can occur

in the presence of fast decrease in temperature or oxygen.

Powder based systems can be divided into different process:

solid-state sintering

chemically induced sintering

liquid phase sintering

full melting

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Solid-state sintering

In this process the compounds are not molten during association, they kept in solid state.

Sintering means the fusion of particles without any melting at increased temperature.

Therefore temperature is heated between the absolute melting temperature and melting

temperature. The surface energy of particles decrease because of diffusion. Moreover the

surface energy is proportional to the total particle surface area therefore during the fusion of

particles at elevated temperature the total surface area of particles decreases and thus surface

energy also decreases. In order to achieve low porosity, long sintering time and high

temperature are required (figure 8).

Figure 8. Porosity decline during Solid state sintering process by the association of the particles.

(a) Closely arranged particles prior to sintering. (b) Particles agglomerate at temperatures above the

absolute melting temperature, as they seek to minimize free energy by declining surface area.

(c) During sintering, neck size and pore size decreases (Additive Manufacturing Technologies)

For the fusion of smaller particles lower energy is necessary and the process is done faster at

lower temperature. The drawback is the long sintering time to create the object. The particles

around the sintered particles warm up when reaching the temperature for consolidation and

they distract heat from the diffusion process. If distraction is significant then more particles

are enable to fuse to each other. That is the reason why greater particle size require higher

energy for sintering.

Chemically induced sintering

During chemically induced sintering a thermally-induced chemical reaction takes place

between two different type of powder-powder or powder-gas resulting in consolidation of the

powder. This type of fusion mechanism is characteristic for ceramic materials. Practical

example of a reaction is the following: laser processing of ZrB2 in the presence of O2 thereby

form ZrO2 and bind together a composite of ZrB2 and ZrO2. The porosity of the final product

is high which can be reduced by post-process infiltration or longterm cauterization.

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Liquid phase sintering

Undoubtedly, liquid phase sintering is the most divese technology which refers to the fusion

of powder particles when the portion of particles are molten by heating above melting

temperature. While the other portion of particles remain in liquid state of matter. In this

process the liquid portion acts like a glue and binds solid particles together. Thereby heating

of materials –having high melting temperature - until sintering would require too much heat.

Hereby, binder and structural material can be distinguished from each other. The most

appropriate if the structural material is different in size as well from binder material. Due to

binder material having smaller diameter porosity and shrinkage is decreased. An example of

the process when steel powder combined to polymer binder material. On the other hand

binder material has very low melting temperature which can be disadvantage. It is able to melt

at low temperature but faster solidification can occur leading to inequal space-filling between

structural material causing high porosity. In order to decrease porosity, post-treatment has to

be applied. Similarly, problem can occur, if the density of binder material and structural

material are highly differ. Since the powder applied from a storing vat and the distribution of

the two material will be inhomogeneous influencing the quality of the final product. Powder

particles manufactured by this process consist of polymer and alloy with high melting point or

ceramic as structural material or metal binding material and metal with high melting point or

ceramic structural material. There are new develepmental results in which binding material

compose coating around structural material. Thereby the energy absorbing ability and the

fusion of structural material elements to each other will ameliorate.

Full melting

During full melting process mostly alloys or semi-crystal polymers are used. Heat energy is

provided by laser or electron beam in which the whole layer thickness will be molten by

elevating the temperature above melting temperature. Nylon polyamides are most frequently

used between polymers. In order to reach the highest solidity, the material has to be molten

fully, such as alloys (Ti, stainless steel, CoCr). Reaching fast molten condition and solidity

the physical properties of alloys are better than alloys processed by traditional casting. During

indirect process the powder of alloys act as structural material which can bind to binder

material resulting in high porosity. In order to decline porosity first of all the excess material

which is unused for evolving binding is evaporated. The empty space is filled by infiltration

or minimized by post heat treatment. Infiltration can be controlled easier and shrinkage of the

final product is significantly lower as against secondary heat treatment.

Figure 9. Infiltration of full melting process

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During the previously introduced processes the product of interest was directly manufactured

from the applied material. In case of alloys there are further possibilities by the application of

powder based systems. Polystyrene or wax based powder are applied for pattern which can be

cauterized undrossy. Further application of the pattern is similar to the traditional wax losing

procedure. On the other hand casting mould is created. In this case sand particles are anchored

to each other with material providing bond by heat treatment. Finally, molten alloy is poured

into the formerly manufactured casting mould. If not alloy, but ceramic must be processed

with this procedure certain steps accord with the procedure of alloys. In the beginning,

ceramic has high porosity which can be reduced by secondary heat treatment or infiltration.

The devices which are at low temperature used for creating direct polymer and indirect metal

as well as ceramic product. These are the so called Selective Laser Sintering (SLS) devices.

These are using CO2 laser and N2 gas (averagely beside 0,1-3,0 % O2 concentration) which are

not competent for processing metals directly. If alloys should be processed one of the most

essential from several modification is changing the energy source. Nd:YAG laser is applied

instead of CO2 laser which energy can be absorbed easier by metal components. These are

the Selective Laser Melting (SLS) processes.

Materials of full melting

In general, all type of material which are able to become molten and recurrently consolidate

can be used in powder based systems. Thermoplastic materials are the best choice because of

the low melting point and low heatconductance ability. Most commonly applied materials are

polyamide based materials and polyamide enhanced with glass. Added glass enchances the

solidity and rigidity of the material but cut down the flexibility. Polystyrene based materials

are primarily used for manufacturing patterns. Amorphous materials such as final products

made from polycarbonate and polystyrene possess high porosity. On the other hand crystalline

materials such as polyamides, its density is significantly higher and the smoothness of the

surface and the mechanical properties are more advantageous. However, the processing is

more difficult, because the temperature has to be highly controlled and the shrinkage of the

final product is considerable. From among polymers, thermoplastic elastomers have specific

flexibility. Moreover application of high temperature does not lead to neither degradation nor

damaging of material structure during fall in temperature. It is worthy to mention, that

biocompatible material, for instance calcium-hydroxyapatite can be processed by this

technology and since the material similar to bone tissue it can be utilized in numerous field of

medicine. Nowadays, metals can be processed indirectly in a wide range with powder based

manufacturing system. RapidSteel (1990) was the first material in this case which contained

thermoplastic binder material, carbon steel and copper as infiltration material. The material of

interest is pre-warmed at 350-450°C, afterwards sintered at 1000°C followed by infiltration

with copper at 1120°C. The final result is a product consisting of 60% of steel and 40% of

copper. The more developed Rapidsteel 2.0 avarage particle size is 33 µm which contains

bronze as infiltration material. This one results in a more stable material structurally, due to

the lower infiltration temperature of the bronze.

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Dental application of full melting process

Creating the structure of fixed prosthodontics is the main application of SLS technology.

Mainly cobalt-chrome (Co-Cr) alloys can be used among alloys in the procedure. The device

is able to manufacture several structure of crown and bridge work simultaneously, thus it is a

cost-effectice technology. Furtermore the structure of removable partial denture (RPD) can be

created as well with high accuracy which does not show defaults of the traditional casting

technology.

Extrusion based systems

During this technology the material is pressed from the vat onto the building platform. If the

applied pressure is constant then the outflow is balanced through the nozzle tip. Moreover the

diameter of the pressed material is also constant. In order to support the constant diameter of

the outflowing material the nozzle tip has to be moved in constans velocity above the

platform. When passing through the nozzle tip the material is semi-solid and applying onto

the platform it has to be became solid without any flowing and form alteration. During

consolidation it will bind to the former applied layer as well building up the 3D product layer

by layer.

There are two way for manufacturing by extrusion based technology. In the first case, the

state of matter of the material is regulated by altering the temperature. The applied material is

heated until melting point in the vat applying in liquid state of matter onto the platform after

that and bind to the former layer before the solidification. In an alternative way, when the

liquid state of matter consolidates through chemical way. This chemical reaction can be done

by binder material, solvent, air or as a result of drying procedure. The latter methode is

applied when biocompatibility is a crucial point because of the connection with live tissues.

The material become liquid state of matter in the vat which can be filled directly with the

material of interest, but it is better to feed continously from an associated vat. The material of

interest can be liquid or solid state of matter (powder, pellet or filament). The appropriate

regulation of the temperature is crucial to avoid degradation of the polymer materials. The

diameter of the nozzle is important since it determines the size and the amount of the passing

material, thus defining the resolution of the device. The resolution has to be taken into

consideration during editing STL data in CAD model. The outflowing amount of material

depends on the geometry of the nozzle and the viscosity as well. Ideally, during extrusion

process the material does not change its shape and volume. However gravitation and surface

tension can influence the shape of the material. Fall in the temperature and losing humidity

rather influence the volume. For instance, gel state of matter which shrinking and become

porous during losing humidity constantly. If the material is molten fall in the temperature lead

to shrinkage similarly. Cooling does not have linear tendency therefore shape alteration has to

be taken into account beside shrinkage.

The best known type of extrusion based technology is Fused Deposition Modelling (FDM)

during which the filament material is forwarded by a pinch roller feed system into the pre

heating liquifier chamber. This roller provides the pressure of the material through the nozzle.

Among all the procedures the application of the materials are the widest in the FDM

technology, however manufacturing is time-consuming.

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Figure 10. Schematic illustration of extrusion based system

Materials of extrusion based system

Most frequently used material is ABS (akrylonitrile-butadiene-styrene) and the developed

variants (for instance ABSplus, ABSi, ABS/PC). ABSi is a translucent material which is

similar to the other ABS materials. Polycarbonate (PC) materials are used when ABS

materials are not completely suitable for the requirements. In general the applied materials are

amorf materials in contrast with materials consisting of high crystal phase. Amorf materials

do not have exact melting point and increasing the temperature and viscosity decline

significantly. Regulated viscosity can be an advantage when the material is leaving the nozzle

tip, its shape will be rigid and fast consolidation can be detected. Furthermore the fusioning

ability is better to the previously layer compering to the SLS process. The smallest layer

thickness is 0,078 mm with this technology. The properties of the final product is anisotropic

which means that the physical properties of the material are not the same in the three direction

of the space. Moreover extrusion based systems are suitable for manufacturing biocompatible

materials. These biocompatible and/or biodegrading materials utilized as scaffold which

contains micropores providing adhesion for live cells. On the other hand macropores can be

found as well, making space for cell divison, later on for biological tissue. Such a scaffold

material can be Poly-capro-laktone (PCL) which is biocompatible and degrading in live

tissues. Therefore this material can be used as absorbing suture. One of the possible way for

manufacturing biocompatible scaffold matierals is hydrogel application which is nontoxic.

These polymers can dispergate but cannot dissolve in water. Followed by the extrusion, water

is evaporated and the porosity will provide the space for growing of the tissues. Primarily, it

can be used for generating soft tissues. When the aim is to make hard tissue melting

technology is preffered for making scaffold. The porosity (60% in this case) is a significant

disadvantage but in this technology it is a benefit for growing the biological tissues.

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Dental application of extrusion based system

Surgical templates are most commonly made by extrusion based process. The ABS material

applied which is transparent and it can be sterilized. Wax based materials can be used for

making dental replacements. Drawback of the thermoplastic materials which used that the

surface of the final product is not perfectly smooth and porosity can occur very commonly as

well.

Printing processes

In the early 3D printing all type of technology used wax based material for manufacturing

product. Derived from the properties of wax, it was suitable for prototyping or making

pattern. In the 1990s, printers appeared on the market which were similar to SLS devices. In

these technologies binding material is applied on powder layer fusioning the appropriate

particles. By the help of this process various polymer, alloys or ceramic can be processed. In

another process which is similar to SLA, acrylate photopolymer is used by using UV light for

polymerization. Nowadays these three type of market leading printing processes are

widespread. Wax based system contains two jet, one of them spray the wax material and the

other spray the abutment material on the building platform. The thickness of one layer can be

0.01 mm. Photopolymer based system consist of more than 1500 jet outflowing monomers at

the same time which are solidified by UV light. The available layer thickness is 0.02 mm.

The abutment material is gel state of matter which can be eliminated easily by high pressure

water. In these days, there are diveses which are able to use numerous polymers at the same

time during the process.

The advantages of 3D printing are the following: low cost, expandable, opportunity of

application of different materials and various colours. On the other hand there are drawback

as well, such as for direct printing only photopolymers and wax based materials can be

applied. In case of indirect printing using different materials like photopolymers and wax the

accuracy of the product will diminish compering to SLA or FDM processes. Printing is a very

complex process in which the first phase is transforming the applied material into liquid state

of matter. For instance, these can be the following: particles suspended in liquid, material

dissolved in liquid, solid material melting or monomer or prepolymer mixing with initiator.

On the other hand, there are difficulties regarding the liquid drops, such as flying orbital of

the drops, impact on the building platform as well as interaction with the substrate. Since as a

consequence of the impact on the platform liquid drops disintegrate into small drops

decreasing the accuracy of printing. In the point of view of the interaction solidification is

crucial. Basically, liquid leave the nozzle tip in two way: continous stream or drop-on-

demand way.

3D printing

In contrast with traditional printing technology, during 3D printing binder droplets are

mediated on powder based surface connecting powder particles to each other. In this case only

the binder droplets must be injected through the inkjet print head. The binder droplets

(avarage diameter is 80 µm) compose spherical agglomerate with the powder making a tight

connection to the previously layer as well. After each prepared layer the platform is decreased

with one layer thickness and the surface is coated with a new powder layer by the powder

nozzle tip, likewise the earlier demonstrated powder based systems.

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Figure 11. Schematic illustration of 3D printing

In 3D printing there are similar advantages, such as different material for abutment is not

necessary because the unused powder serve as abutment. After the 3D product the unused

powder is eliminated by high pressure air. Post-treatment is also available, when the

eliminated powder is substituted by a material increasing the hardening of the object. The

available scale of materials are extremly wide. Nowadays alloys can be used in printing as

well and the final hardness of the product is created by post-treatment. In this procedure,

binder droplets are cauterized by low temperature followed by sintering of the alloy at higher

temperature. Finally, infiltration is the final phase at lower temperature by copper or bronze.

Ceramics can be processed with similar procedure like alloys, but in this process there are

other complications yet. 3D printing is widespread in the field of dentristy, such as operative

dentristy, fixed prosthodontics, removable partial denture (RPD), implants, orthodontics and

oral surgery.

3D printing based on Computer Tomography (CT)

CT images can be used for creating 3D models in medical diagnostics possessing several

advantages: hardly approachable areas and the parts of malignances can be visualized in 3D.

On the other hand physicians can try the surgical intervention in advance thereby decreasing

the chance of complications during the operation. DICOM file of CT can be easily converted

into STL file, thus 3D model can be created with the help of CAD software. The resolution of

STL files are aligned to the size of CT voxels. Maximal resolution of a 3D model can be

achieved if each CT voxel is converted to STL file.

Figure 12. Low resolution = small STL file size (left), high resolution = big STL file size (right)

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CBCT (Cone Beam Computed Tomography)

Cone Beam CT is a maxillo-facial diagnostic process during which CT sensor come around

the patient’s head recording the image. Based on the CT images axial layers are made by

software (primer reconstruction) which summary contains the whole spatial imaging of the

patient.

Advantages of CBCT in contrast with CT

During evaluation of traditional CT panorama images magnification and deformation of the

images must be considered. If the ratio of the magnification is known it does not cause any

problem. The main problem comes from that the magnification is unbalanced at distinct parts

of the CT image resulting in deformation. By CBCT certain structures can be visualized

separately from several direction as well. Scanning time is between 20-40 sec by CBCT

whereas it takes some minutes by traditional CT. Furthermore the time of reconstruction is

shorter. The most important difference that CBCT works with a cone-shaped beam. Due to

the fast scanning time CBCT is able to work with lower radiation dosage, than traditional CT.

Source:

I. Gibson l D. W. Rosen l B. Stucker: Additive Manufacturing Technologies (Rapid

Prototyping to Direct Digital Manufacturing) ISBN: 978-1-4419-1119-3 e-ISBN: 978-1-

4419-1120-9. DOI 10.1007/978-1-4419-1120-9 Springer New York Heidelberg

Dordrecht London

Documents

Biophysics Spring semester Dentristy...only followed by casting, hereby this method is not suitable for direct scanning or intraoral application. Optical scanners operate with the