TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 04/38 pp24-28 Volume 14, Number S1, June 2009

Perusing Piezoelectric Head Performance in a New 3-D Printing Design

RAHMATI Sadegh, SHIRAZI Farid†,**, BAGHAYERI Hesam†

Mechanical Engineering Group, Faculty of Engineering, Azad Islamic University, Majlesi Branch, Iran;

† Mechanical Engineering Department, Khaje Nasir Toosi University, Tehran, Iran

Abstract: Rapid prototyping (RP) is a computerized fabrication technology that additively builds highly com-

plex three-dimensional physical objects layer by layer using data generated by computer, for example CAD

or digital graphic. Three-dimensional printing (3DP) is one of such technologies that employ ink-jet printing

technology for processing powder materials. During fabrication, a printer head is used to print a liquid on to

thin layers of powder following the object's profile as generated by the system computer. This work looks at

redesigning 3DP machine, using piezoelectric demand-mode technology head in order to improve accuracy,

surface finishing and color quality of constructed models. The layers created with aforesaid system are be-

tween 25 to 150 μm (steps of 25 μm).

Key words: prototyping; three-dimensional printing (3DP); piezoelectric head

Introduction

Solid freeform fabrication (SFF) technologies are manufacturing/prototyping technologies that are char-acterized by layer-by-layer addition of material to fab-ricate components. These techniques are also known as layered manufacturing and rapid prototyping. The layer-by-layer building approach allows significantly more complex parts to be built in one fabrication step than was previously possible thus simplifying process planning.

SFF technology therefore can automate the process planning and fabrication of a part under computer con-trol so that the only input needed is a solid model of the part[1,2].

Over the last decade, many different technologies for SFF have evolved. Broadly, the SFF techniques available currently can be classified as stereo lithogra-phy, solid fusion and solidification, laminated object

manufacturing, and powder-based techniques. The ste-reo lithography technique selectively solidifies a liquid photopolymer while solid fusion and solidification fuse/melt the material and deposit it layer-by-layer. The laminated object manufacturing technology cuts out laminates from sheets of part material and glues or fuses them together. In most methods of SFF, special support structures are needed to support overhanging features of the part[1,3].

The two main powder-based techniques that have been commercialized are selective laser sintering and three-dimensional printing (3DP) printing. For powder-based methods, no support structures are typically re-quired to create complex shapes. Powder is selectively consolidated into a part and the remaining powder can be removed. In the SLS process, a thin layer of powder is deposited in a workspace container and the powder is then fused together using a laser beam that traces the shape of the desired cross-section. The process is re-peated by depositing layers of powder thus building the part layer-by-layer. In the 3DP process, a binder material selectively binds powder deposited in layers.

Received: 2008-11-09; revised: 2009-03-30

** To whom correspondence should be addressed. E-mail: s_farid_sh@yahoo.com; Tel: 98-912-1350938

RAHMATI Sadegh et al Perusing Piezoelectric Head Performance in a New … 25

Ink-jet printing head (IJH) technology is used to print the binder in the shape of the cross-section of the part on each layer of powder (Fig. 1)[4,5].

Fig. 1 3-D Printing process

Two kinds of drop-on-demand heads can be used in IJH systems, piezoelectric and thermal heads[6], and the thermal heads are used in current 3DP systems. Since thermal heads have some drawbacks, hence piezoelec-tric head has been employed for new generation of 3DP machines. In addition, piezoelectric technology can help to inject the live cells in to vital textures in order to create bones, members and dentures without any chemical or physical changes in cells.

1 Ink-jet Head Technologies 1.1 Thermal heads

In thermal systems there is a heating element as a thin-film resistor. When an electrical pulse is applied to the head, a high current passes through this resistor and the fluid in contact with it is vaporized, forming a vapor bubble over the resistor. This vapor bubble expands in fluid reservoir and is ejected as a droplet through the nozzle (Fig. 2)[6,7].

Fig. 2 Schematic of a thermal head

1.2 Piezoelectric heads

In this type of system a volumetric change in the fluid reservoir is induced by the application of a voltage pulse to a piezoelectric material element that is cou-pled, directly or indirectly, to the fluid. This volumetric

change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a drop that issues from a nozzle (Fig. 3)[6,8]. A result of simu-lated droplet ejection is shown in Fig. 4.

Fig. 3 Schematic of a piezoelectric head

Fig. 4 A result of simulated droplet ejection in piezo-electric heads[9]

When a voltage pulse is applied in the direction or-thogonal to the polarization direction of the piezoelec-tric element, it is deformed and the fluid in the channel reservoir is pressurized. When the pressure wave gen-erated in the channel is reflected between nozzle and common fluid chamber and resonated, the pressure ap-plied to the nozzle change in time, and as a result drop-let is ejected[9].

1.3 Comparison between thermal and piezoelectric technology

Thermal demand-mode ink-jet systems can achieve ex-tremely high fluid-dispensing performance at a very low cost. However, this performance/cost has been achieved by highly tailoring the fluid: thermal ink-jet systems are restricted to fluids that can be vaporized by the heater element (without igniting the fluid) and their performance/life can be degraded drastically if other fluids are used.

In practice, thermal ink-jet systems are limited to use for aqueous fluids while the work of piezoelectric demand-mode ink-jet technology does not depend on thermal process and because of this reason, does not

Tsinghua Science and Technology, June 2009, 14(S1): 24-28 26

create thermal stress on the fluids which is being jetted from the nozzles of head. Meantime, the diversity of fluids that can be jetted by the piezoelectric heads grows vastly[6]. In addition, some thermal ink-jet sys-tems in comparison with piezoelectric type produce more inconsistent droplets with satellite and misting, which causes dimensional error, rough surface finish-ing, and low color quality in constructed models[9].

2 Control of the Ejection and Impact Phenomena

As the ink-jet printed models structures strongly de-pend on the velocity, the initial size and the path of the droplet just before spreading, it is essential to control these different characteristics as a function of the driv-ing parameters of the printing head[10]. To obtain this data, the mathematical equation was used based on two different voltages (5 and 12 V). This reveals that the increase of the amplitude (up to 12 V) leads to the formation of a satellite droplet, which catches up with the main one later. Moreover, this shows that the final volume increases with the amplitude of the pulse (Fig. 5).

Fig. 5 Resonance frequency vs. droplet volume

The equation is given as[10] 2 /(2 )dV r V f (1)

where Vd is the volume of droplet, r is the radius of the nozzle, V is the velocity of droplet, and f is the reso-nance frequency. As can be inferred from Eq. (1), when Vd is increased, the necessary velocity of droplet in-creases rapidly. Also, the frequency of head movement to print the layers decreases contemporary. Consider-ing these conditions, accurate dimensional layers of model are possible to be made. The only downside to these attitudes is the rate of building layers because the

frequency of working head has direct effect on the ve-locity of printing layers.

3 Nozzles

Another important parameter to build accurate layers is the inner nozzle diameter. When a nozzle diameter is decreased, the droplet volume decreases, however, the viscous resistance in the nozzle is greatly increased, and the energy loss grows rapidly. Figure 6 shows the relationship between the nozzle diameter and the drop-let velocity.

Fig. 6 Nozzle diameter vs. droplet velocity at differ-ent viscosity

In a situation where the binder viscosity is increas-ing, if nozzle throat area gets smaller, velocity drop is significant. In other words, increasing binder viscosity has predominant effect on velocity drop compared with increasing velocity by changing nozzle cross sectional area. The relation between inner nozzle diameters, droplet size, and droplet volume is shown in Table 1.

Table 1 Relation between inner nozzle diameter, drop-let size, and droplet volume[11]

Inner nozzle diameter (μm)

Droplet size (μm)

Droplet volume (pL)

30 35 20 50 55 90 70 70 180

4 Binder Properties

To adjust the fluid properties of the organic suspen-sions to be compatible with the type of printing head, viscosity and surface tension must be 5-20 mPa s and 35-40 mJ/N, respectively. This will provide the ratio of 1/2/e eR w to be in the adequate range (1-10) for

RAHMATI Sadegh et al Perusing Piezoelectric Head Performance in a New … 27

ejection of a consistent droplet. In fact successful drop ejection occurs when the ra-

tio 1/2/e eR w has a value ranged between 1 and 10 with

/ /Re We r , where Re is the Reynolds number (vr / ); We, the Weber number (V2r / ); , , and are the ink density, viscosity, and surface tension, respec-tively; r, the radius of the nozzle; and v the fluid veloc-ity[12]. When this ratio is too small, viscous forces are dominant which implies large pressure for ejection; in-versely, if this ratio is too large, a continuous column is ejected that can lead to the formation of satellite drops behind the main drop.

As demonstrated previously, piezoelectric head tech-nology is capable of jetting the binder from the nozzle continuously and more efficiently. Moreover, this tech-nology assures that the binder drops after leaving the nozzle, would rest accurately at the interested position. Therefore, in general, piezoelectric technology is the most adapted of the ink-jet printing technologies to flu-id jetting or micro dispensing and in particular to rapid prototyping applications[6]. Hence, a piezoelectric head with these specifications has characteristics as given in Table 2.

Table 2 Piezoelectric head characteristics

Print method: Drop-on-demand ink-jet Nozzle configuration: Monochrome: 48 nozzle (120dpi)

Color (48 nozzle×5 ) Cyan, Magenta, yellow, light cyan, light magnet

Print direction: Bi-direction with logic seeking Print speed: 238 CPS Print head life: 3000 million dots/nozzle Feed speed: 110 mm/s Max resolution: (720×2880) dpi

Figure 7 shows the nozzle configuration viewed from the back of the print head. The required energy to eject the binder droplet includes the energy to form the

Fig. 7 Nozzle configuration of piezoelectric head

droplet surface and the kinetic energy of the droplet. In addition, a considerable amount of energy is consumed for the flow of the binder in the nozzle. Fur-ther, even after droplet ejection, more energy is con-sumed until the residual oscillation of the binder is terminated.

5 Conclusions

The advantages and disadvantages of piezoelectric and thermal heads were investigated. Based on the results, parameters such as accuracy, life time and diversity of materials, and piezoelectric heads were recognized as the most adapted to rapid prototyping applications. Based on the new design, piezoelectric demand-mode technology was employed to jet the binder from noz-zles. The printed layer samples with piezoelectric head are shown in Fig. 8.

Fig. 8 A single layer printed by new 3-D printer

Parameters such as dimensional accuracy, surface finishing, and color quality of fabricated models of the new 3DP system demonstrate a significant improve-ment over the common 3DP models. Moreover, the ca-pability of layer dispending mechanism is improved by up to 3 times (minimum layer thickness is 25 m), and the surface finishing of fabricated models is also im-proved.

The fabricated models are colorful, with excellent accuracy and improved surface quality, compared with the fabricated models using current commercial 3-D printers. As a matter of fact, thin layer thickness has significant effect on surface texture quality of the model. Applying piezoelectric technology enables the binder to penetrate the required depth, resulting in layer thickness as thin as 25 m and improving surface texture quality. This work is currently in progress and initial results have been promising.

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