Evaluation of Jetting Performance in the Inkjet Printing Process
Evaluation of Jetting Performance in the Inkjet Printing Process
When formulating raw materials for inkjet printing inks, it is essential to consider the compatibility of each ink component with all printing materials.
Printing materials vary widely and differ significantly in their properties. For example, metal components (such as orifice plates, sensors, and filters) are highly sensitive to ink vehicles with extreme pH values, while plastic components may dissolve or swell upon contact with organic solvents or monomers. Therefore, evaluating the effect of individual ink formulation components alone is insufficient. Ink compatibility testing should be carried out as a final validation step. This evaluation is typically performed by immersing individual printhead components in the ink and operating the ink supply system under accelerated conditions for extended periods. Once an ink meets the physicochemical requirements of a specific printhead, its jetting performance must then be evaluated.
Droplet Formation
Droplet formation is arguably the most critical performance factor, as it directly affects the entire printing process.
Well-established theories—such as the Navier–Stokes equations and Rayleigh instability theory—describe the influence of fluid properties on jet stability and filament breakup behavior. In recent years, extensive computational simulations have been applied to predict jetting behavior for specific nozzle geometries and to guide the selection of ink additives. Nevertheless, optimal ink formulations remain largely based on empirical methods.
Liquid is ejected from nozzles typically on the order of 10 μm in diameter with a defined momentum that depends on the kinetic energy of the droplet. The primary parameters governing jetting behavior are surface tension and the rheological properties of the ink. Depending on the printhead design, typical inkjet inks exhibit surface tensions in the range of 25–50 dyn/cm and viscosities between 1 and 25 cP.
By considering relationships among dimensionless parameters such as the Reynolds number, Weber number, and the combined Ohnesorge number—along with ink density and nozzle diameter—an operational “jetting window” can be defined.
Surface tension can be controlled through appropriate selection of ink vehicles, for example, by adding isopropyl alcohol to water or by incorporating surfactants, often at very low effective concentrations (commonly silicone- or fluorine-based surfactants). Ink viscosity can be adjusted through the formulation components, including soluble organic additives (such as binders) and the quality, type, and concentration of dispersants.
Ink Stability
Ink stability is one of the key factors ensuring consistent print quality. Over time, ink components may interact, leading to precipitation, phase separation, or gelation. These issues not only degrade jetting performance but may also damage printing equipment. Therefore, long-term stability must be considered during ink formulation.
Ink stability is typically evaluated through a series of accelerated aging tests, such as exposure to elevated temperature, high humidity, or intense light, to observe changes in physical and chemical properties. Additional methods include centrifugation to detect sediment formation and viscometry to monitor viscosity changes.
To enhance ink stability, various strategies can be employed. These include adding suitable stabilizers (e.g., antioxidants or biocides) to prevent oxidation or degradation, optimizing formulation ratios and concentrations to reduce precipitation and phase separation, and using advanced manufacturing and packaging technologies to maintain stability during storage and transportation.
Ink Latency
After ink reaches the nozzle, volatile components may evaporate, causing the ink composition near the nozzle to differ from that of the bulk ink. This results in changes to physicochemical properties—such as increased viscosity, reduced surface tension, or partial precipitation—which can lead to jetting difficulties after prolonged idle times, commonly referred to as the “first-drop problem.”
During this idle period, ink may remain inside a single nozzle without jetting; this behavior is known as latency. In drop-on-demand (DOD) printers, solvent- or water-based inks typically exhibit latency times ranging from seconds to minutes, whereas inks without volatile solvents (such as 100% UV-curable inks) may remain stable for days.
To achieve good latency performance, the following guidelines should be followed:
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For solvent-based inks, use solvents with low volatility and low evaporation rates (typically referenced relative to n-butyl acetate).
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For water-based inks, add humectants or co-solvents to inhibit water molecule association and slow evaporation.
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For both water- and solvent-based inks, select solvent systems and co-solvents (e.g., N-methyl-2-pyrrolidone for aqueous inks) that maximize solid solubility until complete dissolution.
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For pH-sensitive inks that require high solubility or high electrokinetic potential, use non-volatile pH regulators (such as high-boiling-point aniline derivatives).
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For colored inks, ensure sufficiently low viscosity even at high pigment loadings by selecting appropriate polymeric stabilizers.
Ink Supply
For ink to be properly ejected from the printhead, it must pass through tubing and multiple filters. Two parameters are critical to achieving optimal inkjet performance: the rheological properties of the ink and the size of particles encapsulating the colorants.
Most inks behave as Newtonian fluids with sufficiently low viscosity to flow through ink tubing with millimeter-scale diameters. Filters in the ink supply system and at the nozzle are designed to prevent large particles from reaching the nozzle. Pigment agglomeration often leads to increased viscosity, which can disrupt ink flow, clog filters, and reduce flow rates.
To ensure good ink flow and minimize clogging, pigment particle sizes should be significantly smaller than the nozzle diameter. An optimal guideline is approximately 1% of the nozzle diameter (e.g., for a typical 40 μm nozzle, particle sizes should be below 400 nm).
Ultimately, ink rheology must be controlled through careful selection of formulation components such as polymeric binders, dispersants, and the volume fraction of dispersed particles.
Ink Recoverability
Since all solvent- or water-based inks ultimately dry and fix on the substrate, some volatile components are inevitably present. As a result, inks may occasionally fail to jet after idle periods. Partial recovery of jetting performance at the nozzle typically occurs through purging into a waste or recirculation system.
Ink formulations should therefore be designed to re-dissolve into the bulk medium, allowing agglomerated particles to redisperse. This capability can be preliminarily assessed using dried ink samples by measuring how quickly dried ink regains its original properties upon contact with fresh ink. In practice, recoverability is evaluated by jetting ink until a stable and acceptable jetting performance is restored.
Ink Adhesion to Substrates
In addition to jetting performance and stability, ink adhesion to the substrate is a critical determinant of print quality. Different substrates exhibit distinct surface properties and chemical characteristics, necessitating tailored ink formulations for specific printing applications.
Good adhesion can be achieved through several approaches, including selecting ink formulations with functional groups capable of interacting with substrate surface groups to form chemical bonds; adding suitable additives (such as coupling agents or crosslinkers) to enhance interfacial bonding; and applying surface pretreatment techniques (such as plasma or laser treatment) to modify substrate surface properties.
Drying speed and curing mechanisms must also be considered, as they influence the degree of ink–substrate bonding. Consequently, ink formulation must balance printing speed, substrate type, and the required adhesion strength.
Environmental Sustainability of Inks
With increasing environmental awareness, the ecological impact and sustainability of inks have become major concerns. Traditional ink formulations may contain hazardous solvents and additives that pose risks to both the environment and human health. As a result, the development of environmentally friendly inks has become a key focus of modern ink research.
Eco-friendly inks typically feature low volatility, low toxicity, and biodegradability. These goals can be achieved by using natural or renewable raw materials, reducing harmful solvents and additives through formulation optimization, and adopting advanced manufacturing and recycling technologies to minimize wastewater and exhaust emissions.
Ink reuse and recycling should also be considered. Developing recyclable ink formulations and recovery technologies can reduce both ink consumption costs and environmental impact, contributing to sustainable development while enhancing corporate competitiveness and social responsibility.
Conclusion
In summary, the formulation of inkjet printing inks requires a comprehensive consideration of jetting performance, stability, substrate adhesion, and environmental sustainability. Through continuous optimization of formulations and manufacturing processes, it is possible to develop inks that deliver excellent performance while meeting environmental and market demands.