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A restricting approach of the rapid prototyping by material accretion.

1. INTRODUCTION

Since 1982, when Chuck Hull invented stereolithography (figure1), have been developed many methods of rapid prototyping by material accretion, which involve building a part by defining its geometry layer by layer, but between the main methods may be mentioned as follows: stereolithography, selective laser-sintering, laminated object manufacturing, fused deposition modeling (Ciucescu & Bontas, 1998).

The rapid prototyping by material accretion permits to eliminate the aberration which consists for an engineer to conceive an object in three dimensions, to represent it in two dimensions, and then, in workshop, to interpret this representation for making a prototype in three dimensions (Askeland, 1984).

These methods and other ones under development assures for the rapid prototyping by material accretion an increasing rate of its diffusion, the spread of these new methods into the broader market place, and of its infusion, the process of developing applications of these new methods to their maximum potential.

[FIGURE 1 OMITTED]

Despite these achievements, the rapid prototyping by material accretion methods present some technological limits, more or less important.

This paper presents the technological limits of rapid prototyping by material accretion in order to obtain microelectromechanical systems.

2. THE MAIN METHODS OF RAPID PROTOTYPING BY MATERIAL ACCRETION

All methods of rapid prototyping by material accretion involve building a part by defining its geometry layer by layer.

The stereolithography method was developed by 3D Systems Corporation (Valencia, California), which presented at AUTOFAC fair in 1987 the first machine: SLA 190. All SLA units direct the laser beam downward onto the free surface of the liquid photopolymer, which passes from liquid state to solid state. The focused spot of the laser, those the diameter is about 200 urn, may be oriented by scanning some mirrors. The laser beam traces, firstly, the boundaries of the slice cross-section being drawn; afterwards accomplishes the solidification of the appropriate cross-section, where the part is full. Non-uniform resin shrinkage produces distortion of the part (Ciucescu, 2007).

The selective laser-sintering method appeared in the year 1994 at a branch of Zeiss Group, named Electro-Optical Systems (EOS GmbH) company, placed in Planegg, near Munich. This method is based on a selective fusing or sintering of small particles by means of high power (50-watt) CO2 laser. A quantity of powdered material is moved upward by a piston within a feed cylinder, and is spread over a over the working area by means of a counter-rotating roller to obtain a thin layer. A laser beam, generating infrared radiation at 10,6 [micro]m, is then directed and focused with appropriate optics, onto the working surface. Some portion of the laser energy is absorbed and transformed into heat. In order to minimize the required laser output, the powder is maintained at an elevated temperature, just below the fusing point. Also, to avoid oxygen contamination of the bonding surfaces and the potential explosion or combustion hazard, the environment is made of nitrogen with maximum 2 % oxygen.

The laminated object manufacturing method appeared at Helisys company (Torrance, California) in the year 1991. The part is fabricated using laminated sheet material. Consecutive layers are joined using an adhesive that is both temperature and pressure sensitive. Each cross-section is cut using a 25-or-50 watt CO2 laser, emitting in infrared, at a wavelength of 10,6 [micro]m.

The fused deposition modeling (FDM) was developed by Stratatys company (Eden Prairie, Minnesota) in the early years of '90. The material is heated just beyond the melting point in a delivery head. The molten thermoplastic is then extruded through a nozzle in a form of a thin ribbon and deposited in zones, where the part is full.

Some usages of the rapid prototyping by material accretion are presented in figure 2.

[FIGURE 2 OMITTED]

3. THE MICROELECTROMECHANICAL SYSTEMS

The micro-electromechanical systems are considered for maximum feature size of 50 urn. Firstly, the structure of microelectromechanical systems was characterized by two aspects:

-geometric aspects by the fact that the structure was basically limited to planar geometry with a slight variation in the vertical direction;

-material aspect by the fact that the structure was limited to silicon-based materials.

The development of top technologies related to the computer manufacturing. demands a greater geometric and material flexibility.

Various and combined processes like bulk etching and wafer bonding permitted to increase partially the geometric flexibility.

By micro-machining methods and post-release assembly methods are able to make more complex 3D microelectromechanical systems.

Later, another methods like deep X-ray lithography, laser induced photo deposition and etching permitted to micro-scale three dimensional patterns.

The appearance of rapid prototyping by material incress methods gave a great hope to enable new structure and applications to micro-electromechanical systems.

4. THE TECHNOLOGICAL LIMITS OF RAPID PROTOTYPING BY MATERIAL ACCRETION

In the case of stereolithography, the most important thing is the slice thickness, which now arrived very small, at 50 [micro] m in order to reduce as much as possible stair-stepping errors in the Z axis of the object. Another important thing is the curl distortion of supports which hold the object in place during cladding. The release of Ciba-Geigy epoxy resins Cibatol SL 5170 and Cibatol SL 5180 appears like the best combination of mechanical properties. The advantages of epoxy objects made with these resins are:

-minimal mechanical anisotropy;

-high green strength;

-green dimensional stability;

-improved flatness;

-high curled strength;

-high curled dimensional stability.

The selective laser-sintering method produces a roughness surface due to:

-stair-stepping errors;

-horizontal aliasing errors;

-significant interstices between spherical particles which range in diameter from 50 to 125 [micro]m.

Also, it must to be taken in account that the volumetric shrinkage of crystalline materials is quite large leading to a significant distortion.

The laminated object manufacturing method has the negative aspect that the surrounding volume must be diced to enable to be broken away from the part upon completion. If for large solid parts this operation is not a problem, for finely detailed geometries it becomes a difficult one.

In the fused deposition modeling method, the spooled filament has a diameter of 1,78 mm, which means that the fine parts are obtained with great difficulty.

5. CONCLUSIONS

The micro-electromechanical systems are considered for maximum feature size of 50 [micro] m.

The development of top technologies related to the computer manufacturing. demands a greater geometric and material flexibility.

Even if the appearance of rapid prototyping by material accretion methods gave a great hope to enable new structure and applications to micro-electromechanical systems, up-to-now, the technological limits make them to be not considered for micro-electromechanical systems, exempting the streolithography.

As presented above, the slice thickness, which now arrived very small, at 50 [micro] m , reduces as much as possible stair-stepping errors in the Z axis of the object and permits manufacturing of micro-electromechanical systems with feature size ranged at highest limit of 50 [micro] m.

6. REFERENCES

Askeland, R.D., (1984), The Science and Engineering of Materials, PWS-KENT Publishing Company, ISBN 0534- 029757-4, Boston, Massachussetts

Ciucescu, D., (2007), The Science and Engineering of Materials, ISBN 973-30-1465-6, E.D.P., Bucharest

Ciucescu, D. & Bontas,, D., (1998), The Rapid Prototzping bi Material Accretion, ISBN 973-30-5467-4, E.D.P., Bucharest
Tab. 1. The number of rapid prototyping by material
accretion installations sold in the year 2004.

The methods of Vendor The number
rapid prototyping of rapid
by material prototyping
accretion by material
 increase
 installations

Stereolithography 3D SYSTEMS 469
 CMET/ Mitsubishi 43
 DEMEC/ Sony 28
 EOS GmbH 19
 Mitsui Engineering 1

Selective Laser DTM 43
Sintering EOS GmbH 4

Laminated Object Helisys 52
Manufacturing Sparx 15
 Kira 10

Fused Deposition Stratasys 44
Modeling Sanders 1
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Author:Ciucescu, Doru; Simionescu, Gheorghe
Publication:Annals of DAAAM & Proceedings
Date:Jan 1, 2008
Words:1256
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