Print heads nozzles

Stripe, inspired by print head nozzle featuring ink drop. 

Figure 7.4 is a photograph of a portion of the peptide controls slide printer. A stack of microscope slides, ready for printing, is at the far left. The slides are automatically ejected from the stack, one at a time. The slides are moved on a conveyer to the right, positioning them under the print head. The print head has eight nozzles, out of which microliter-sized droplets are ejected onto an underlying slide. The slide conveyer then places a new slide under the nozzles, and the process repeats. 

The Chemical Inkjet Printer (ChIP), an inkjet printer-like device which is equipped with a piezoelectric reagent-dispensing system, has been developed by Shimadzu Corporation. The ChIP can dispense picoliter volumes of matrix solution onto a tissue section, which has four printing heads and nozzles that have no contact with the tissue section surface. 

Piezo. This is the same basic print head technology as in continuous inkjet. The ink jet droplets are forced out through the nozzle after an electrical signal to the piezoelectric crystal causes a pressure wave to be set up in the ink (Figure 2.35b). The wave can be produced in the bend mode, as in Figure 2.35b or in a push or shear mode. An important variation on piezo technology is the Xaar... 

The advantages of an electrostatic inkjet are that it allows you to print a more concentrated fluid than the formulation that actually passes through the print head, and that the achievable resolution is not a function of the nozzle diameter so that potentially higher resolutions than piezo inkjet are possible. Additionally, very small drops can be formed while still using pigments, as the size of the drop... 

Conventional water-based and non-aqueous inkjet inks are mixtures of several components, including volatile solvents, dissolved materials, and dispersed solids (for pigment inks). When the ink reaches the nozzles prior to jetting, the volatile components may evaporate from the nozzle. Therefore, the liquid in the vicinity of the nozzle can have a composition which differs from that of the bulk ink which is further back in the print head supply channels. This disparity causes differences in the physicochemical properties of the ink (e.g., an increase in viscosity or decrease in surface tension)... 

In rare ink formulations, the ink may have a non-Newtonian rheology, i.e., high viscosity at low shear rate and low viscosity at high shear rate (shear thinning inks). In such a case, once the ink reaches the orifice it may be jetting well, since the shear rate in the nozzles is very high, but it may not flow properly in the ink supply tubing and the narrow channels of the print head, in which the shear rate is low. 

The ink supply system and most print heads have filters which are aimed at preventing arrival of large particles to the nozzles. Aggregation of pigment particles usually causes an increase in viscosity, which can interfere with the ink flow through the ink supply system. The aggregates can block the filters and thus may decrease the flow rate over time, eventually causing starvation of ink in the print head. 

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. 

Fig. 4. Cross-section view of the nozzle region and ink channels of an inkjet print head, showing the sharp water concentration gradient that evolves at an inactive, exposed nozzle.

Piezo drop-on-demand print head s principle of operation is relatively simple in comparison to CIJ. The jetted fluid is held in balance between a shght negative pressure in the ink system, and capillary tension in small nozzles. Each nozzle is attached to a small chamber from which ink is ejected on demand when a piezo crystal coupled to that chamber flexes on apphcation of voltage. While the principle of operation is simple, producing a viable print head is not. The print heads themselves are complex constructions and have specific requirements on the fluid to enable rehable operation. 

A resin or binder is required to impart adhesion, and physical and chemical resistance to the image so that it will not scratch or wipe off. The resin may also be used to build up viscosity for those print heads that require a higher jetting viscosity. In order for the print head to function reliably, the resin should be able to redissolve in the ink. If the resin is no longer soluble in the ink when dried, there is a large risk of plugged nozzles. This requirement must be balanced with the requirement to make the printed surface scratch and chemical resistant. 

Jetting reUabiUty also Umits the molecular weight of the binder used. In the vicinity of the nozzle, the more volatile components of ink evaporate. When the resin drops out of solution this wiU cause nozzle blockage. In addition, high molecular weight polymers may lead to viscosities outside the print head range. Polymers used normaUy have a molecular weight below 100 000 and often below 50 000. Common polymers used are vinyl chloride/vinyl acetate copolymers, acryUc resins and polyketone resins. 

State-of-the-art piezo-based drop-on-demand printers are produced by companies such as Xaar (UK), Konica Minolta (Japan), and Fujifilm Dimatix (US). These print heads have around 180 to 360 nozzles per inch and are designed to print at basic resolutions of up to 360 dpi with drop volumes down to about 40 pi (picoliter). This type of print resolution gives features equivalent to approximately 100 /xm track width on PET or PI substrates. However, Xaar s new generation "gray scale" heads allow variable drop volumes down to around 3 pi to give printed features of around 50 /xm or less. 

The print head of standard, color inkjet printers (Cannon, Hewlett-Packard) is composed of a nozzle, heater, manifold chamber, and restrictor. Once the resistive element is heated (300°C) the fluid... 

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