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Research on Paper-Ink-Process Interactions in Electrophotographic and Ink Jet Printing*

Oittinen, P., AL-Rubaiey, H., Sipi, K. and Vikman, K. Helsinki University of Technology, Laboratory of Media Technology

E-mail: Pirkko.Oittinen@hut.fi, Hussain@cc.hut.fi, Katri.Vikman@hut.fi, Katja.Sipi@hut.fi

* Poster paper presented at the first DPP Conference, May 13-16, 2001, Antwerp, Belgium.

ABSTRACT

The research is motivated by needs of quality improvement in electrophotography and ink jet. From a materials perspective, understanding of the interactive phenomena between paper, inks and the process is a key asset in the development. The overall goal of the research is to gain understanding of the interactive phenomena by experimentation, characterization of print structure, measurement of print properties and result modeling. This paper focuses on the methodology in interaction and print structure research. Several approaches can be taken: 1- experimentation by variation of process parameters, 2- monitoring process phenomena in real time and 3- monitoring the development of print structure or print properties in the process. TONER TRANSFER

Methods

A commercial colour laser printer has been equipped with adjustment of transfer voltage and measurement of transfer current. Figure 1. illustrates the configuration. It allows recording of the flow of the current during the transfer stage. Measured current responds to any change in the input variables, such as the bulk and electrical properties of the paper, the q/m of the toner, transfer voltage and RH%.

Figure 1. Measurement of transfer current. Sampling at 50 Hz.

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Results

The method provides information of the transferred image related to any change in any individual or group of input variables. Some of those evaluations are:

• Use of transfer current to indicate transfer efficiency. • The relations of transferred toner amount and transfer voltage; to support deductions of

transfer phenomena and support quantitative modeling. • Study of the influences of different paper properties, or any input variables on the transfer

efficiency and print quality. Figure 2 shows the change in transfer current due to toner transfer at different levels of moisture content.

Figure 2. Influence of moisture content of paper on toner transfer current.

FUSING

Methods

The challenges of toner fusing as a research topic, arise from needs for energetically more efficient fusing methods and needs for better fusing quality. Different fusing installations have been constructed for these purposes. Table 1 gives a summary of the facilities currently available.

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Table 1. Facilities for fusing experimentation and their variables.

Results

Completed research has focused on fusing with a combination of nip and IR fusing. Figure 3 illustrates toner adhesion in single and double fusing experiments as function of print density. In general, the second step causes changes in most quality factors as well as their small-scale variation, but the first step is decisive.

Current research deals with print structure and fusing quality achieved in different fusing conditions. Also the feasibility of long nip fusing is studied.

Figure 3. Example of fusing experiments with different combinations of nip and IR fusing.

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AGING

Methods

In most of the current ink jet applications, the prints are expected to retain their optical properties for many years, even when exposed to various environmental stresses. In our test set-up, aging phenomena can be studied by monitoring changes in print structure caused by light, water and mechanical stress, see the following table. Temporal changes in print properties are measured as colour and density differences, whereas more detailed information of changes in print structure is gained from FTIR- and Raman spectroscopic measurements.

Table 2. Methods in spectroscopic print structure analysis.

Results

The objective of the present study is to characterize the effects of paper-ink interactions on ink jet print quality. An example of light fastness test results is shown in Figure 4. Colour co-ordinates and Raman spectra of light exposed yellow prints were measured, and intensity ratios for the most intense bands of yellow ink were calculated relative to a band at 1082 cm–1 originating from the CaCO3 filler. In Figure 4, the intensity ratios are presented as a function of b*, or print yellowness.

Figure 4. Raman intensity ratios of yellow print (reference band: CaCO3 at 1084 cm-1.

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PRINT STRUCTURE

Methods

Vibrational spectroscopic methods, or IR and Raman spectroscopy, give complementary information to optical print quality measurements. These methods enable an analytical study of the actual print structure. Methods used include ATR Rapid-scan PAS, DTGS and Microscopic Raman in x,y-direction and Step-scan PAS and confocal Raman in z-direction.

Confocal Raman allows sensitive depth profiling of ink jet colourants, toner pigments and print substrates. The principle of the technique is illustrated in Figure 5.

Figure 5. As laser light impinges on a sample, a difference in the refractive indices causes light reflection and

refraction at the optical boundary (a). Using a travelling objective the beam can be focused at different depths. In the basic mode, sample aberration, however, decreases depth resolution and causes variation in the sampling depth (b). Also light scattering diminishes the penetration depth and causes losses in spatial resolution (c). An immersion objective and suitable immersion oil (d) radically improve the depth profiling when light scattering samples, such as prints, are measured. [Vyörykkä, J. et al., Confocal Raman Spectroscopy for the Depth Profiling of Paper Coating Colours.. HUT, Dept. of Forest Products Technology, Lab. of Forest Products Chemistry]

Results

Changes in print structure caused by aging can be investigated in detail by spectroscopic methods. Figure 6 gives an example of a Raman spectroscopic depth profile of a yellow print, which has undergone a water immersion test WF-test).

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Figure 6. Effect of water immersion on yellow print.

CONCLUSIONS

Paper-ink-process interactions were discussed from the viewpoint of research approaches and methods. Especially the question of extending the traditional paper-ink approach to include the process toner transfer, fusing or aging was addressed.

Available set-ups support adjustment of process parameters, process monitoring and print structure analysis in a manner that is different in the three areas of the study. Also the needs are different. Despite some shortcomings the methods meet current needs.