WO2013007988A1

WO2013007988A1 - Method for increasing the throw distance of thermal inkjet inks

Info

Publication number: WO2013007988A1
Application number: PCT/GB2012/051585
Authority: WO
Inventors: Julie CROSS; Stuart Cecil Andrew Molloy
Original assignee: Domino Printing Sciences Plc
Priority date: 2011-07-08
Filing date: 2012-07-06
Publication date: 2013-01-17
Also published as: GB201111719D0; GB2505855A; GB2492760A; GB201400533D0

Abstract

The invention describes a method of enhancing the throw distance of an inkjet ink, particularly a thermal inkjet ink, by controlling ink viscosity.

Description

METHOD FOR INCREASING THE THROW DISTANCE OF THERMAL INKJET INKS

Field of the Invention This invention relates to inkjet printing and, in particular, to a method of increasing the throw distance of thermal inkjet inks.

Background of the Invention In the field of industrial coding and marking various forms of indicia including codes, dating, and traceability information are applied directly onto products and/or product packaging. Industrial coding requires reliable non-contact coding technology that is able to mark a wide range of substrates, which can come in a variety of forms, e.g. curved, serrated, or lipped. The most versatile coding technologies are able to print with a gap between the coding equipment print head and substrate that is large enough to accommodate the range of forms that might be encountered in an industrial environment. This gap is termed throw distance.

Thermal inkjet (TIJ) printing is a desirable coding and marking technology as it offers significantly higher print resolutions than competing technologies in the field, such as continuous inkjet, but has had limited acceptance because of poor throw distance. Typically a continuous inkjet printer will operate at throw distances in the range 5- 12mm, whereas presently available TIJ printing systems are limited to throw distances of the order of 1mm.

Whilst the art published to date reveals significant concentration on improving the dynamics of ink drops ejected by inkjet systems (including thermal inkjet systems), little has been published directed specifically to the subject of increasing throw distance. Increasing throw distance in an inkjet printer is the stated objective of International (PCT) Patent Application W09842517. A drive arrangement for a piezoelectric inkjet print head is described which includes a double peak waveform. This is said to cause the droplet tail to break off early following ejection resulting in enhanced flight characteristics and increased throw distance.

Less relevant examples of the prior art describe both modifications to print heads and ink formulations to alter the dynamics of the droplets ejected by inkjet printing systems. British Patent application GB 2 325 438A describes analysing the performance of individually actuated printing elements to determine a printing characteristic of the print head, such as the ink jet velocity, or the amount of ink emitted. The method described uses modified control signals for the printing elements to adjust the printing characteristic of the print head. European Patent Application No. 1 544 262 describes a method to control drop velocity within 40% when going through firing frequency sweep, whilst European Patent Application No. 1 304 364 outlines the use of a specific solvent blend to reduce the amount of drop deceleration encountered with typical inks. In US Patent No. 6607268 metal salts are combined into the ink to increase drop velocity.

It is an object of this invention to provide a novel method of enhancing throw distance in an inkjet printer or to otherwise improve print quality, particularly in a thermal inkjet printer. Summary of the Invention

Accordingly, in one aspect, the invention provides a method of controlling the throw distance of thermal inkjet inks through control of the ink viscosity. Preferably the method includes increasing the ink viscosity to achieve substantially maximum throw distance.

Preferably said method comprises increasing said ink viscosity within the range from 1 - 10 mPa.s.

Preferably said method comprises selecting or adjusting said inks to have surface tensions in the range 18 - 60 mN/m . In a further aspect the invention provides a thermal inkjet ink the viscosity of which has been established to provide an enhanced throw distance. Preferably said ink has a viscosity in the range 1 - 10 mPa.s.

Preferably said ink has a viscosity of substantially 5 mPa.s.

Preferably said ink has a surface tension in the range 18 - 60 mN/m .

Many variations in the way the present invention can be performed will present themselves to those skilled in the art. The description which follows is intended as an illustration only of one means of performing the invention and the lack of description of variants or equivalents should not be regarded as limiting. Wherever possible, a description of a specific element should be deemed to include any and all equivalents thereof whether in existence now or in the future.

Brief Description of the Drawings

Figure shows the effect of ink viscosity on drop velocity for a range of throw distances;

Figure 2: shows the effect of ink viscosity on % grid non-uniformity for a range of throw distances for a high surface tension ink;

Figure 3: shows a similar plot to Figure 2 but for a low surface tension ink;

Figure 4: shows a comparison of calculated and observed ejection

velocities as a function of viscosity; and

Figure 5: shows a comparison of calculated and observed ink drop momentums as a function of viscosity. Detailed Description of Method

This invention is concerned with improving the throw distance of inkjet printing systems, particularly of thermal inkjet (ΤΠ) printing systems. Typical ΤΠ printing systems that are currently available have a throw distance of the order of 1mm. This significantly limits the application of these systems to industrial marking and coding.

In an effort to increase the throw distance of TIJ inkjet systems, without resorting to modifying readily available TIJ print heads and driver electronics, we have established, to our surprise, that altering the viscosity of the ink can be used to optimise the throw distance, whilst still achieving good image quality.

Typically TIJ inks have a viscosity of about 1 - 2 mPa.s at 25°C. Intuitively one would expect that increasing this viscosity would result in a loss of drop velocity for a given expulsion energy and, indeed, this is indicated in Figure 1 in which it can be seen that for viscosities between 2 and 3 mPa.s the drop velocity reduces. However as viscosity is increased still further, the rate of decline in drop velocity levels off for a stage. Turning to Figures 2 and 3, it will be seen that the % Grid non-uniformity is least (and thus print quality highest) when the inks used in the examples have viscosities lying in the range of 1 - 10 mPa.s and, more particularly, have viscosities of around 5mPa.s. It will be appreciated, however, that inks of different formulations might exhibit optimum throw distances at other viscosities.

Inks exhibiting the benefits of the invention typically have surface tensions lying in the range 18 - 60 mN/m³.

If it is assumed that the droplet ejected from a thermal inkjet print head can be considered to be a free jet that is ejected in response to a pressure inside the ink chamber, and that the nozzle is efficient, then the pressure inside the head can be calculated as:

d_jo 2

Where P_i is the pressure on the ink in the chamber

P. is the free jet velocity

d_J0 is the free jet diameter

d_i(]is the nozzle diameter

/ is the nozzle length

P_i is the ink viscosity

< is the ink surface tension

p_i is the ink density

If it is further assumed that the pressure created for ejection is independent of viscosity then the expected jet velocity for a range of viscosities can be calculated as shown in Figure 4 below. Figure 4 shows a range of velocities observed experimentally together with a line calculated as above and indicates a reasonably good fit to the model considering the experimental error in measurement. Accordingly, it can be stated with confidence that the ejection velocity is behaving in a manner consistent with theory and that there is nothing in the ejection mechanism that is particular to this ink. If it is also assumed that the volume of the ejected droplet is independent of the jet viscosity then the graph in Figure 5 can be calculated. As drop momentum is shown to reduce as viscosity increases, it can be deduced that the superior drop placement evident from Figures 2 and 3 is not a result of ejection velocity or momentum. Examples

A set of aqueous dye based thermal inkjet inks suitable for printing with a Domino G-200 printer were prepared using the formulations shown in Table 1, where the level of Glycerol was modified to change the viscosity of the ink. Component Inkl Ink 2 Ink 3 Ink 4 Ink 5

Glycerol / % - 16 30 35 40

N-Methyl-2- 4 4 4 4 4 pyrrolidone / %

Isopropyl Alcohol / 4 4 4 4 4

%

Diethylene Glycol / 4 4 4 4 4

%

Black Dye / % 2.5 2.5 2.5 2.5 2.5

Water / % 85.5 69.5 55.5 50.5 45.5

TABLE 1 Table 2 shows the viscosity of each ink when measured at 25°C using a Brookfield DV- 11+ viscometer at 60rpm.

TABLE 2

To assess the effect of throw distance and ink viscosity on image quality, 2D data matrix codes were printed onto plain paper at a range of throw distances and the 2D data matrix codes were analysed using an LVS Integra 9505 bar code verifier. Specifically, the % grid non-uniformity measurement was used as this is a quantitative measure of drop placement. It records the largest distance between the actual centre of a drop and the ideal centre of the drop. A % grid non-uniformity of greater than 15% is observable as a loss of image quality with the naked eye. Surprisingly it was found that inks with a viscosity of around 5mPa.s gave a significantly better print quality when measured in this way than inks of other viscosities.

Each ink was then placed into an HP45a cartridge and a drop visualisation and measurement system produced by JetXpert was used to measure the drop velocity and volume. As a result of these measurements, the drop momentum was calculated for the different viscosity inks at increasing throw distances.

Figure 1 shows that the drop velocity decreases when the ink viscosity is increased. This result is to be expected as the ejection of the drop is a result of a pressure impulse and for a fixed pressure, the speed of the drop will depend on its density, which will be increased with viscosity. The three lines on the chart represent different throw distances and as expected the drops slow, as they are subjected to aerodynamic drag. Such a result might lead to the conclusion that a major factor in gaining good printing performance at distance is to increase exit jet velocity. If drop placement accuracy at throw distance were simply a function of exit jet velocity then low viscosity inks would be expected to exhibit the best performance at longer throw distances contrary to that which has been observed. Examination of Figure 1 also shows that there is a slight drop off in velocity with increased slope at 5mPa.s, this might indicate a momentum peak for a 5mPa.s ink.

Figure 2 shows that an optimum ink viscosity can be chosen to provide the best image quality, i.e. the lowest % grid non-uniformity figure at a variety of throw distances for a high surface tension ink. As the throw distance is increased, only the ink with optimised ink viscosity (in this example approximately 5 mPa.s) achieves good image quality (i.e. a % grid non-uniformity figure of less that 15%). Figure 3 shows a similar effect for a lower surface tension ink. In this example, the optimum ink viscosity is around 4.8 mPa.s. While a specific embodiment has been described in detail above, it will be apparent to those skilled in the art that variations may be effected without departing from the scope of this invention and that, with different ink formulations in alternative thermal inkjet printing heads, other optimum viscosities may be observed. The important factor is that the performance of an ink at longer throw distances can be optmised by varying the viscosity of the ink. This is a contrasting and unexpected route to previous attempts to achieve increased throw distance that have focus sed on changing head drive or formulation chemistry to achieve higher ejection velocity at the nozzle exit point.

Claims

A method of controlling the throw distance of thermal inkjet inks through control of the ink viscosity.

A method as claimed in claim 1 including increasing the ink viscosity to achieve substantially maximum throw distance.

A method as claimed in claim 1 or claim 3 comprising increasing said ink viscosity within the range from 1 - 10 mPa.s.

A method as claimed in claim 3 comprising establishing said viscosity substantially at 5 mPa.s.

A method as claimed in any one of the preceding claims comprising selecting or adjusting said inks to have surface tensions in the range 18 60 mN/m³.

A thermal inkjet ink the viscosity of which has been established to provide an enhanced throw distance.

A thermal inkjet ink as claimed in claim 6 having a viscosity in the range 1 - 10 mPa.s.

A thermal inkjet ink as claimed in claim 7 having a viscosity of substantially 5 mPa.s.

A thermal inkjet ink as claimed in any one of claims 6 to 8 having a surface tension in the range 18 - 60 mN/m .