Printing on plain paper with a thermal inkjet printer - in reference to the HP DeskJet printer - technical

Hewlett-Packard Journal, Dec, 1988 by Steven J. Bares

Printing on Plain Paper with a Thermal Inkjet Printer

THE HP DESKJET PRINTER is the first low-cost inkjet printer to produce true letter-quality print on plain paper. Intended for office use, the DeskJet printer was designed specifically to meet customer demand for products that use available office papers. Approximately three million tons of paper is produced and distributed annually to offices in the U.S.A. alone. This paper is made on various types of machinery using many different processes and materials. A major challenge during the development phase was to construct a printhead that would produce reliable print quality over such a wide range of papers. To meet this challenge, the design team needed to identify and obtain samples of the types of papers available to the user and to understand the chemical and physical variability of these papers. This included obtaining information about the paper industry's raw materials, manufacturing processes, and distribution networks. This paper summarizes the results of these studies. Topics covered include the types of papers found in the office, the properties of these papers, and the effect of these properties on inkjet print quality.

What Is Plain Paper?

To design a printhead that produces consistent letter-quality print on plain papers, the development team needed to obtain paper samples for testing during the design phase. However, simply calling a paper company and requesting samples of plain paper will result in an incredulous response. The properties of all papers are radically different and depend entirely on the paper's intended use. Hence, there are no "plain" papers, only papers that are sold into a particular environment.

In U.S. offices, thousands of papers can be found, manufactured by over 75 paper companies. To conduct a complete study of all papers bought by thousands of distributors and sold to literally millions of offices nationwide would involve a vast expenditure of resources. Our first major dilemma in selecting papers for testing was to understand the processes by which business papers are brought from manufacturers into offices. This complex and dynamic distribution network is diagrammed in Fig. 1. We began by focusing our efforts on identifying high-market-share, manufacturer-labeled office papers--only one of the possible channels for paper distribution. It was felt that this approach would identify the office papers having the largest overall market share while minimizing the expenditure of resources.

Laboratory testing of over 150 of these selected papers revealed a wide range of properties for two general types of office papers, which we will call correspondence and copier papers. Correspondence papers are rough, have a large amount of starch on the surface (are hard sized), are typically acidic, and usually contain cotton fiber as well as traditional wood pulp. The relatively homogeneous nature of these correspondence papers simplified the task of designing a compatible printhead.

The specifications for copier papers, which represent over 88% of the paper tonnage shipped to U.S. offices, are well-documented and conform to an industry standard. Given our favorable experience with nonstandardized correspondence papers, we had high expectations of rapidly achieving excellent print quality on copier papers as well. However, when we tested copier papers, we found that their only homogeneous property pertinent to inkjet printing was their surface smoothness. Other potentially important factors affecting inkjet print quality (discussed later) showed significant variations.

The next step toward the goal of obtaining consistent print quality on office papers was to select a small subset for printhead optimization from the original 150 identified by market research. The criteria used to consolidate these papers were driven by both the need to cover the broadest market representation and the desire to include the widest ranges of print quality, fiber content, filler content, pH, type of internal sizing, smoothness, and many other variables. This work resulted in the selection of eight representative papers, which were purchased and used throughout the design phase. After selection of the eight-paper set and before the optimization phase, sufficient amounts of each paper sample were purchased so that paper consistency was maintained and only the ink and printhead design varied. The names of these eight papers and some of their properties considered important to the design of the DeskJet printhead are shown in Table I.

Similar research into European office papers was performed in parallel with the work reported above. This research focused on office papers in Germany, France, England, Switzerland, Holland, and Sweden. Because of the expense of market research, we limited our study to these countries and assumed that the information collected was representative of the office papers of all Western Europe. Our preliminary analysis of European office papers, obtained before the completion of the entire study, guided our choice of the eight-paper representative set for the U.S.A. Our analysis of European papers showed that they, like the office papers found in the U.S., displayed a wide range of properties, the major difference being a higher number of alkaline papers (pH above 7) in Europe. We determined that the range of properties covered by the U.S. eight-paper set was sufficient to represent its European counterparts. Hence, our optimization of the DeskJet printhead was performed on the eight-paper set and it was assumed that these papers typified those found in both U.S. and European offices. No detailed studies of office papers found in other countries were performed, although samples of papers from around the world were occasionally tested.