Drop ejection

Fig. 11-19. The drop ejection process in an inkjet printer (a) bubble nucleation (b) bubble growth and drop ejection (c) refill. [From J. H. Bohoiquez, B. P. Canfield, K. J. Courian, F. Drogo, C. A. E. Hall, C. L. Holstun, A. R. Scandalis, and M. E. Shepard, Hewlett-Packard J. 45(1), 9-17 (Feb. 1994). Copyright 1994, Hewlett-Packard Company. Reproduced with permission.]...

Droplets from the jet caused by liquid rushing to fill the cavity left by the bubble (see Fig. 14-89). These droplets range up to 1000 Im, their size depending on bubble size. This is important only at modest loadings. Once foam forms over the surface, drop ejection by this mode decreases sharply. 

Creating acceptable prints requires two steps (1) getting ink to fire reliably from a print head onto paper, and then (2) getting ink to behave in a desirable manner on paper. In the first part of this chapter, the inkjet printing process is discussed how is ink ejection impacted by ink properties What are typical impediments to reliable drop ejection ... 

Conversely, in a piezoelectric inkjet (PIJ) head, the deflection of a membrane drives ink through each nozzle — schematics of the various configurations used in PIJ heads can be found elsewhere. The timescale for PIJ drop ejection is similar to that in a TIJ head (Fig. 1), thus, both are capable of firing 10000 to 30000 drops from a nozzle each second. Typical nozzle diameters d = 10—50 fxm), ink viscosities ( 7 = 1—5 centipoise), ink surface tensions (a = 20—50dyne/cm), and ink densities (p = 0.9—l.lg/ml) are fairly similar for the TIJ and PIJ printers for office and home use. The resulting key fluidic parameters for such print heads are summarized in Table 2. 

Clearly, fluid inertia is important in drop ejection, since Re >> 1 however, flow is still considered to be in the laminar regime, as the... 

Table 2. Typical fluidic parameters for ink drop ejection from TIJ and PIJ print heads.

Despite the numerous obstacles mentioned earlier, inkjet manufacturers are able to produce print heads that rehably put drops on paper. The next challenge to achieving good print quality is to get ink to behave properly once it is on paper. In many ways, the ink characteristics needed for effective drop ejection (low viscosity, good stability/ solubility of components, poor wetting on print head exterior) are completely opposite those needed for good performance on paper. In this chapter, some of the basics of ink/media interactions are explored. 

Figure 4. Jet drop ejection height as a function of bubble diameter, water...

Noncontact Acoustic Drop Ejection of 2.5-500 nL compound DMSO solutions from source to destination plates for cherry-picking or dose-response series... 

The process of drop ejection is not as simple as taking a fluid chamber with a small hole and pressurizing it enough for fluid to start emerging from the ejection nozzle hole [10]. To accomplish monodisperse droplets ejected out of a nozzle, one needs the ability to produce highspeed fluid jets of approximately the diameter of the desired droplets. Additionally, the behavior of the jets has to be controlled precisely enough to cause them to consistently disperse into uniformly sized droplets. 

Also, the combined surface areas of the polymer and pigment particles adversely modify the viscosity of the inlqet such that drop ejection rates are limited to lower rates. Moreover, the pigment and the pol5mer often dissociate in the fibers of plain paper, reducing both the otherwise durability and optical density of the print. 

FIGURE 19.68 Drop ejection sequence for thermal ink jet. Nominal time frames and values for various parameters are given to indicate the scale of the three processes of nucleation, bubble growth, and jet formation followed by drop ejection and refill. 

FIGURE 19.69 Squeeze tube piezoelectric inkjet. Early implementation of piezoelectric transducers for drop ejection. (Source Advanced Technology Resources Corporation.)... 

FIGURE 19.70 Drop ejection sequence for piezoelectric printhead. Schematic of drop ejection via deflection of piezocrystal bonded to an ink capillary. In practice the piezodrivers were located well upstream of the... 

Current inkjet printers have hundreds of nozzles, and future inkjet printers are projected to have several thousands of nozzles in page-wide arrays to obtain very high print speeds, which will require very high levels of power. Here we consider nonthermal means of drop ejections to decrease the required power. 

Figure 6.12 Drop ejection from a MEMSJet drop ejector (Xerox MEMSJet).

Typically a drop can perform between 30 and 60 rotations with a steady rotation frequency of around 1.9 Hz, which is independent of the drop volume and time. During this stable rotation, the drop ejects smaller droplets from its tips. Each daughter droplet moves approximately 4 mm, flattens slightly, and then decays into several smaller droplets, that Anally disappear. 

---