Rapid manufacturing of parts for wind tunnel testing using polyjet technology.

Udroiu, Razvan ; Dogaru, Florin

1. INTRODUCTION

In the last years, there have been substantial changes in technology and its requirement. This changing environment created many new needs and opportunities, which are only possible with the advances in new materials and their associated manufacturing technology. Some of these new technologies are RapidX (Udroiu & Ivan, 2008). Under the umbrella of RapidX there are some specific terms uses in additive fabrication such as: Rapid Product Development (RPD), Rapid Technology, Rapid Nanotechnology, Rapid Prototyping (RP), Rapid Tooling (RT) and Rapid Manufacturing (RM). Additive fabrication refers to a group of technologies used for building physical models, prototypes, tooling components and finished parts, all from 3D CAD data or data from 3D scanning system.

The most popular RP technologies used worldwide are stereolithography (SL), selective laser sintering (SLS), 3D printing (3DP) and laminated object manufacturing (LOM). This paper is focused on 3DP technologies that represent 44.3% of all additive systems installed worldwide at the end of 2005 (Wohlers, 2006). 3D printing technologies can be divided in the following: inkjet printing, fused deposition modelling and polyjet (Park, 2008).

Rapid prototypes can be used for design testing. For example, an aerospace engineer might mount a model airfoil in a wind tunnel to measure lift and drag forces. This paper presents some research regarding the rapid manufacturing of products for wind tunnel testing.

2. MODELS FOR WIND TUNNEL TESTING

One of the most interesting and significant applications in the aerospace, automotive and wind energy sectors is with no doubt the wind tunnel model of products (aircraft, automotive or wind turbine) that can be tested before an important investment in manufacturing. The role of physical prototypes in the all industry sectors is essential.

A wind tunnel model needs to be manufacturing in a short time, with good surface quality and from materials with good mechanical characteristics.

Regarding of rapid prototyping applications for wind tunnel testing there are some researches projects that have used different RP technologies: stereolithography, selective laser sintering (SLS) etc.

The stereolithography process it was experimented (Landrum, 1997) to manufacturing a NACA 0012 airfoil section. It was resulted a surface finish with a noticeable distributed roughness as well as low chordwise ridges due to resin overcure in at the build layer interfaces. In the paper (Maheshwaraa et al., 2007) a UAV wing is fabricated using Duraform(r) Flex material in an SLS machine and postprocessed to remove unsintered powder. The UAV wing prototype is infiltrated using a mixture of ST-1040A and ST1040B polyurethane to make the UAV wing air tight.

In order to develop a European tiltrotor in cooperation, the companies AGUSTA and WESTLAND needed a prototype for wind tunnel testing. This prototype was been realized at 1:8 scale by the Rapid Prototyping Department of CRP Technology during the year 2007 (***, 2008), using SLS RP technology and WINDFORM GF materials (a composite polyamide based material, aluminium and glass filled). Thus it was possible to complete and test the model in the wind tunnel within a very short time, with excellent results and with really high-performing mechanical and aerodynamic properties.

This paper presents a new method for manufacturing aerodynamic models used in wind tunnel testing, using Polyjet technology by Objet Geometries.

3. POLYJET APPLICATION: RAPID

FABRICATION OF WIND TUNNEL MODELS

The various CAD packages use a number of different algorithms to represent solid objects. To establish consistency, the STL format has been adopted as the standard of the rapid prototyping industry.

First of all, it was designed the NACA airfoil section in SolidWorks software. A particularity of the 3D model (figure 1) is a series of small holes (0.8mm) on a high deep (127 mm). These holes are useful to measure the air pressure on different locations of the wing during the wind tunnel testing.

[FIGURE 1 OMITTED]

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The basic methodology for all current rapid prototyping techniques and of course for RP polyjet technology can be summarized in the following steps as follows:

* Designing of CAD part and then converting it to STL file format;

* Simulating and optimising (orientation of the model on the build try, material consumption and building time) the RP process. RP software processes the STL file by creating thin sliced layers of the model;

* Building of the model layer by layer on RP machine;

* Cleaninig and finishing the model.

In the second step, build orientation is important for several reasons. First, properties of rapid prototypes can vary from one coordinate direction to another. In addition, part orientation determines the amount of time required in building the model. Placing the shortest dimension in the z direction reduces the number of layers, thereby shortening build time. In this study case it was be done an optimisation of the part orientation on the build tray to minimize the building time and material consumption.

The RP software, in this case Objet Studio (figure 2) and Job Manager, slices the STL model into a number of layers and generates an auxiliary structure to support the model during the build.

Objet's PolyJet technology uses two different photopolymer materials for building models: one for the model, and another material for support (figure 3). Support structure is useful for delicate features such as overhangs, internal cavities, and thin-walled sections or complicated geometries, such as undercuts.

[FIGURE 3 OMITTED]

In this case study, we were used EDEN 350 rapid prototyping/ manufacturing machine (figure 4) which is available at Transilvania University of Brasov, Department of Manufacturing Engineering.

The FullCure materials used by Eden 350 3D printer offer excellent flexibility, impact strength and transparency, producing durable models suitable for snap fits and frequent handling. In figure 4 it is presented a sequence from the 3D printing process.

The great achievement of Objet's PolyJet technology is that it combines the advantages of two RP methods known so far (stereolithography and 3D printing), once with the elimination of their deficiencies. In summary the PolyJet process consists in the steps presented as follow.

[FIGURE 4 OMITTED]

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The PolyJet jetting head slides back and forth along the Xaxis, similar to a line printer, depositing a single super-thin layer (16 microns) of photopolymer onto the build tray. Two UV bulbs, located side by side the jetting bridge, emit UV light and immediately curing and hardening each layer. This step eliminates the additional post curing required by other technologies. When the build is finished, a WaterJet easily removes the support material, leaving a smooth surface.

Using polyjet technology it can be obtained parts with very small detail such as very thin walls (down to 0.4 mm) and small holes (up to 0.5 mm in diameter). Using other traditional or RP technology it is difficult to obtain these small details.

The rapid prototypes obtained by polyjet technology can be machining (milling, drilling, etc), gluing, painting and metal coating. In this case the two side holes were threaded (figure 5) to allow assembling the airfoil model within the wind tunnel.

Thus, polyjet technology can allow obtaining of aerodynamic models that can be tested with good results within wind tunnel.

4. CONCLUSION

This work demonstrates that the polyjet rapid prototyping technology can be effectively applied for fabricating test models that can be used in aerodynamic experimental investigations. The models are built with exceptionally high quality, accuracy and speed.

The next step will be focus on the fabrication of complex models such as aircraft models, component for automotive industry (spoiler), wind turbine blade models and so on.

5. REFERENCES

Landrum, B.; Beardt, R. M.; LaSarge P. A. & Sprecken N. (1997). Evaluation of stereolithography rapid prototyping for low speed airfoil design, AIAA-1997-719, Aerospace Sciences Meeting and Exhibit, 35th, American Institute of Aeronautics and Astronautics, Reno, NV, Jan. 6-9

Maheshwaraa, U.; Bourell, D. & Seepersad, C., C. (2007). Design and freeform fabrication of deployable structures with lattice skins, Rapid Prototyping Journal, vol. 13, No. 4, pp. 213-225, Emerald Group Publishing Limited, ISSN: 1355-2546

Park, R. (2008). Utilising PolyJet matrix technology and digital materials, TCT Magazine, Vol.16, No.3, Rapid News Publications plc, ISSN 1751-0333, UK

Udroiu, R. & Ivan N., V. (2008). Rapid-X Using 3D Printers, Supliment Of Academic Journal Of Manufacturing Engineering, No.2 /2008, pp.199-205, ISSN 1583-7904 Romania

Wohlers, T. (2006). Wohlers Report 2006: Executive summary, Rapid prototyping & manufacturing. State of the industry, TCT Magazine, Rapid News Publications plc, ISSN 17510333

***, (2008). 1:8 Scale Wind tunnel model of the external fairing of the European tilt rotor using SLS technology and Windform powders, Available from: www.crptechnology.com/sito/images/PDF/ cs/Case_Study_ Convertiplano_CRP_ENG.pdf, Accessed on: 2009-05-02