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Effect of nozzle/drop generator design & construction on

Printer reliability and print quality


These slides list and discuss the problems of printer reliability and performance stability that are related to drop generators/nozzles designs, ink types and formulations. The slides do not attempt to address:

• the design details of drop generators/nozzles

• the details of ink formulation

• the possible root causes of nozzle blockages (this subject is discussed in another pdf document on our download page)

Effect of nozzle/drop generator design & construction on

Printer reliability and print quality

Nozzle/drop generator design & construction Intended audience


These slides are aimed at: • those hard working , well travelled and resourceful service

engineers who are at the coal face keeping those CIJ printers running reliably and customers happy

• the tireless and meticulous wet test engineers and technicians

• the trusted CIJ operators

• those important , indispensable CIJ ink chemists

• and also those R&D engineers and physicists that are designing new drop generators or investigating the root cause of reported field printer reliability and robustness issues

Continuous Ink Jet ( CIJ) drop generator also referred to as nozzle

Definition

The drop generator also called “nozzle" is made up of five

main parts. These are:

1. The piezoelectric assembly also called drive rod or resonator

2. The ultrasonic interface and mounting between the resonator and the ink chamber

3. The ink chamber

4. The nozzle plate or nozzle orifice and its mounting

5. The ink



Definition


Ink chamber

Acoustic interfaces

Nozzle plate and its mounting

Ink

Resonator mounting and seal

Resonator or drive rod

1. The requirements for the efficient modulation, satellite free and stable break-up of an ink jet can be met in a large number of drop generator/nozzle designs and shapes that meet the basic constraints for the acoustic wave in the ink.

2. These acoustic constraints are determined by the resonator or drive rod interface at the entry to the ink chamber and at the interface between the nozzle plate or orifice and the ink chamber


Continuous Ink Jet ( CIJ) drop generator also referred to as nozzle Acoustic constraints


Definition of Modulation or Print Window

1. The modulation voltage or print window is defined as a range of modulation voltages within which:

• the jet break-up is stable and satellite free, • there is a pronounced upright pear-drop shape near the break-up point • The phasing is stable, and • the print quality is good.

i. Example of wide modulation voltage windows

• Vmin=25V, Vpeak=65; Vmax=145; (Vset = 54V) – print window= (145-25)= 120V

• Vmin=45V, Vpeak=85; Vmax=195; (Vset =75V) – print window= (195-45) =150V

ii. Example of narrow modulation windows

• Vmin=15V, Vpeak=22V; Vmax= 31V; (Vset=19 V) ; print window= (31-15)= 16V

iii. In a printer the modulation voltage set could, in these examples be: 54V, 75V and 19v

2. Therefore a nozzle/drop generator and ink combination that gives the widest print window is preferred. Such wide print window means that the whole printer has a robust operating range and is less sensitive to small variations in environmental and operating conditions as well as to ink types and ink formulations. A system with narrow print window can result in degraded print quality, difficulty in modulation voltage set up (even with auto modulation).


Example of Modulation/print window

Narrowing of print window at high temperatures


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Modulation Vmin (V)

Modulation Vset (V)

Modulation Vmax (V)

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Narrowing of print window at low temperatures


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very narrow print window at high temperatures

Components parts of the acoustic interfaces

The important parts that form these 2 basic acoustic interfaces in a practical nozzle or drop generator are:

• The mounting of the resonator to the ink chamber

• The mounting of the drop generator to the printhead chassis

• The seal between the resonator and the ink chamber (Oring groove design, material and spec)

• The Fixing of the resonator or the PZT to the ink chamber

• The nozzle orifice design, geometry, shape, fabrication and quality

• The nozzle orifice fixing or attachment to the nozzle plate or to the ink chamber

The above are different from one company drop generator design to another and, in most cases, these vary between nozzles/drop generator of the same design supplied by the same company. This is because of mechanical variations and tolerances in the component parts of the nozzles.


Variations of Acoustics Interfaces

from one nozzle design to another

1. The conditions of these 2 interfaces can vary because of different designs or during manufacture and/or assembly of a nozzle/drop generator as a result of:

• Resonator or drop generator mounting design and material. Due to the piezo used to modulate the ink jet being a lossy material and self- heats when first excited. This changes its piezo properties slightly , hence the modulation level to the jet. This effect is exacerbated if the mounting or manifold in which the resonator is held is made up of plastic instead of metal since, in this case, heat produced is conducted away less easily.

• Variation in ink type (variation from one ink to another)

• Variation in the orifice geometry shape and surface roughness from one nozzle to another (tolerances and mechanical variations)

• Variation in the orifice jewel mounting and fixing method from one nozzle to another of the same company and from one nozzle of one company to the nozzle of another company

2. Any one of more of the above variation can result in nozzles from different supplier or even from the same supplier exhibiting different modulation voltage windows. Therefore, it is possible that some nozzles will exhibit narrower modulation windows than other and be more sensitive to changes in environmental and operating conditions.


1. The conditions of these 2 interfaces can also , and often do, change (vary) during operation as a result of:

• Changes in the ink temperature (i.e changes in ink viscosity in the ink chamber)

• changes in the ink chemistry due to temperature or age

• Changes in the orifice jewel mounting and fixing conditions (due to seal aging )

• Changes in the orifice geometry shape and surface roughness (in the case of micro EDM metal orifice wear and tear)

• Changes in the resonator mounting stiffness. This is particularly so:

• when the resonator is mounted in a plastic ink chamber,

• when the PZT crystal is glued (aging or degradation of the glue line)

• when the seal (oring or plastic) between the resonator and the ink chamber ages or is affected by one of the ink constituents or by mechanical stresses and strain over time.

• When the Oring and plastic seals are not well specified (regarding size and/or material). In this case, the energy transferred towards the ink will slowly be lost and the modulation voltage required increases (modulation drift)…becomes inefficient. The opposite can also occur, whereby the modulation become very efficient and the print window becomes narrow or non-existent

2. Any one of more of the above changes will affect the modulation or print window. The modulation voltage could become unstable and start drifting, it could narrow and become more sensitive to changes. Worse still the jet could become impossible to modulate with any available voltages. In these severe cases, even printers with temperature control at the printhead and with dynamic closed loop auto modulation will start seeing their performances and print quality degrading. Unstable modulation voltage setting, phase drifts, charge and EHT faults start occurring quickly and increasing frequency.


Variations In Acoustics Interfaces Properties

During Operation & Resulting Consequences


Summary

Nozzle/drop generator design, manufacture and assembly have major influence on the robustness and width of the operating print window i.e the reliability of a printer system over the specified temperature variations, ink range and operating conditions. Therefore, they have critical influence on

o the printer reliability and uptime o The print quality

The design of the component part of the drop generator is critical, the materials specifications and selection are important and the quality of the manufacture and assembly is crucial.

Sellenis engineers have the expertise and the know how in this field. This ensures that Sellenis nozzles incorporate the best design, the best materials and the highest quality of manufacture and of assembly to ensure consistent print quality, printer robustness and uptime.


Documents

Sellenis - Effect of nozzle/drop generator design ......Sellenis engineers have the expertise and the know how in this field. This ensures that Sellenis nozzles incorporate the best