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Selection Of Materials To Make The Dental Braces Engineering Essay

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Dental braces (orthodontic braces) are devices used to straighten teeth through the use of force. Dental braces are often used in conjunction with other orthodontic appliances to widen the palate or jaws or otherwise shape the teeth and jaws. Nowadays, it is very popular.

Dental braces have 3 parts that are bracket, wire and rubber band. Dental braces usually are made from stainless steel and sometime will combine with nickel titanium. Braces also may make of ceramic or plastic due to the request from customer because of cosmetic reasons. There are other types of dental braces, such as gold dental braces, lingual dental braces and Invisalign dental braces.

Material Used

For the traditional dental braces which are most widely used, they are made from high grade stainless steel. Stainless steels are alloy steel. "Stainless steel" is a name which is given to the corrosion and heat resistant steels which contain minimum 10.5% chromium, carbon and iron. Stainless steel is different from carbon steel.

The primary component of stainless steel is iron, so it can resist oxidation. Because of the presence of the chromium, chromium oxide will be produced when react with oxygen. It adheres to the surface of the stainless steel in the form of a tough coating, and is capable of mending the damage. However, this self-healing process of chromium oxide will work only if there is sufficient amount of oxygen. Sometimes, molybdenum, nickel, copper, vanadium and titanium also will be added to stainless steels to increase the mechanical and manufacture properties of stainless steels.

The available grades of stainless steel can be classified into five basic families, which are ferrite, martensite, austenite, duplex and precipitation hardenable.The arrangement of atoms in the ferrite crystal (BCC) is different from austenite crystal (FCC). When nickel is added, the austenite structure is stabilized.

This picture shows optical micrograph of single stainless steel wire which surrounded with the eutectic silicon needle shaped particles (near to the interface) and the globular structure (far from the interface).

Commonly, stainless steel is available in the "Hot Rolled and Annealed" condition and has yield strength of about 42 ksi and can increase to 140 ksi by cold working. For different grade of stainless steels, there is different chemical composition of them.

Grade of stainless steel

C

Mn

Si

P

S

Cr

Mo

Ni

N

Fe

304

0.08 max

2.0 max

0.75 max

0.045 max

0.03 max

18.0-20.0

-

8.0-10.5

0.10 max

balance

316

0.08 max

2.0 max

0.75 max

0.045 max

0.03 max

16.0-18.0

2.0-3.0

10.0-14.0

0.10 max

balance

Physical Properties

Properties

Units

Value

Grade of stainless steel

304

316

Density

kg/m3

8000

8000

Melting point

°C

1454

1398

Co-Efficient of Expansion

(20-100 °C)

ìm/m.°C

18.2

17.5

Thermal Conductivity

W/m.K

16.2(100°C), 21.5(500°C)

16.3(100°C), 21.5(500°C)

Specific Heat (0-100°C)

J/kg.K

500

500

Stainless steel is less dense than brass, gold and other dense metal. Thermal conductivity of stainless steels is low (28 times lower than copper), so the wearers will not feel warm. Stainless steels also have low coefficient of thermal expansion so there is little or no change in size when the temperature changes. This coefficient of expansion varies between steel grades and also increases slightly with temperature.

Mechanical Properties

Properties

Units

Value

Grade of stainless steel

304

316

Tensile Strength

(min)

MPa

515

515

Yield Strength

0.2% Proof (min)

MPa

205

205

Elongation (min)

% in 50mm

40

40

Hardness (max)

Brinell (HB)

201

217

Modulus of elasticity

GPa

193

193

- Typical elongations of annealed materials

- Typical Tensile Properties of Annealed Materials

Stainless steels have higher tensile strength, proof stress and are more ductile compared to mild steel, aluminium and brass. For annealed austenitic stainless steels, the yield strength is a very low proportion of the tensile strength (40-45%) and will greatly increased to a higher proportion of tensile above cold work. Their high tensile strength (515 MPa) enables them can withstand high stress.

Austenitic stainless steels have high elongations (60-70%). For grade 304 and 316 stainless steels, their elongation is about 40%. Stainless steels are ductile. This property enables stainless steels can be made into braces without fracture. Stainless steel has over 200 HB, which are harder than soft materials like copper and aluminium. But, it is softer than brass.

Stainless steel also has a modulus of elasticity in between of carbon steel and monel metal (approx. 193 GPa). Stainless steel is also more resistance to fatigue compared to aluminium. Wrought stainless steels have quite good creep strength at 700-750°C. It also is tough materials so that it will not break easily when sudden strong force applied.

Technological Properties

Characteristic

Stainless steel

Grade

304

316

Bending

E

E

Hot Forming

G

G

Cold Forming

G

G

Shearing

F

F

Machining

F

F

Code: E = Excellent G = Good F = Fair P = Poor

Stainless steel is also has good machinability and formability. These properties are very important to make braces because the wires are very thin. Besides, the arrangement of teeth of wearers is not same; the braces should be made into different shape.

Chemical Properties

Stainless steel is highly resistance to corrosion and does not get oxidized easily because of the chromium, nickel and molybdenum content. It also has good scaling resistance. Stainless steel is capable of retaining its strength and resistivity towards oxidation and corrosion even it at a very high temperature. At cryogenic temperature, it can remain its toughness. This is an important property to make dental braces.

Other Properties

Stainless steel is a quite cheap material. Besides, stainless steel also has a good appearance. So the wearer will not feel that the braces are very ugly. It also will prevent the wearers from hurting.

Reason of Stainless Steel used to make Dental Braces

Stainless steel has medium density (8000 kg/m3). Stainless steel is less dense than brass, copper and other dense metal. As the teeth are attached to our face bone, weight/density of the braces is very important because if the dental braces are too heavy, the wearer will feel pain. If the braces are made from stainless steel, the wearer will not feel strong pressure on their teeth, but will feel more comfortable, natural than copper braces and they can eat their food happily.

Stainless steel also has low thermal conductivity (28 times lower than copper), so the quantity of heat transmitted will few. The wearers will not feel warm as their body temperature increase. Stainless steels also have low coefficient of thermal expansion so there is little or no change in size when the temperature changes. Dental braces should design in this way. Because, if the size of braces change, the teeth is hard to be straighten as a constant force is needed to force the teeth to move.

Moreover, stainless steel has high tensile strength (about 515 MPa), proof stress and good ductility so it is suitable to make braces. As braces wire is so thin, its tensile strength must be high enough and its ductility must good. If not, the wire will break within the drawing process. Stainless steel dental braces are also easier to shapen.

Stainless steel is also tough and has medium hardness (>200HB).During operation, stainless steel braces will not break or deform when large force is exerted suddenly because of its strength, ductility and toughness.

Stainless steel also is resistance to fatigue and creep. Stainless steel braces wire is also elastic so that it can contract after it has been stretched and the teeth will be forced to move.

Stainless steel is also highly resistance to corrosion and oxidation. These properties are important as the dental braces wear in the mouth. Mouth contains some acidic (food) and alkaline (toothpaste) substances. These substances will cause corrosion. So, with this highly resistance stainless steel dental braces, corrosion will be prevented and so it can function well. Stainless steel is also not poisonous so that it does not harm the wearers.

Other topic: Ongoing Research on Dental Braces

Nowadays, ceramic and plastic also used to make dental braces (clear braces). This brace's colour is more close to natural colour of teeth and making the brackets less noticeable than stainless brackets and it does not stain. Its function is same as stainless steel braces but it often cause less gum soreness. So, ceramic braces' wearers will suffer less pain. Ceramic braces are also safer because ceramic braces not usually break or come off wearer's teeth. Plastic braces brackets are also lighter.

Gold also will be added to stainless steel to make braces because of some patients will allergic to nickel (a basic and important component of stainless steel). Many wearers also more prefer the look of gold.


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