Substrates - The Media Enhancing The Message
As a designer you can appreciate the vital role played by the substrate or stock in maximizing the overall impact of the piece. Color, whiteness, thickness, texture and durability - these are all important aspects that contribute to the design and functionality of the final piece. When compared to xerographic printers and in some cases even conventional presses, your substrate options are greatly expanded with the HP Indigo digital press. Xerographic papers are often characterized by a "dry" or "crackly" feel. The effect is attributed to the requirement for papers with very low moisture content to enable ink fusing. As a result, you are usually restricted to using laser-approved papers, which limits your choice of substrate and costs more overall.
The Absorbency Factor
Conventional printing supports thousands of different substrates, even beyond paper. Depending on the press and substrate combination, however, there may still be issues. Non-absorbent substrates typically take a long time to dry, which prohibits fast duplex printing or finishing. Highly absorbent papers, such as watercolor papers that soak up ink, may also increase dot gain and reduce image sharpness. HP ElectroInk dries instantaneously on contact with the substrate. It doesn't get absorbed to any great extent and thus maintains highly defined, sharp dots - delivering superb results on either non-absorbent or highly absorbent materials. In most cases the printed sheets can be handled and finished immediately. As with all printing processes, there are always exceptions and it is worth confirming your substrate requirements with your Sales Rep.
Coated and uncoated papers Paper types may be classified as coated or uncoated. A coated paper is one where the paper surface is pigmented, or coated, with a special mixture of clay or chalk along with other additives and agents. Papers that do not have this coating are called uncoated. Coating formulations, as well as the amount of coating, vary according to the paper type. Coating weights are generally placed in four categories: pigmented, medium coated, fully coated and art papers. Properties such as smoothness, gloss, printability and opacity generally improve with increasing coat weight. The degree of calendering that is applied in the paper manufacturing process will also determine these properties. Coated papers are typically used for applications that require a higher quality look and feel, such as "coffee table" books, specialty magazines and high end product brochures. Uncoated papers are typically used for single-color books (e.g., black only) or forms, where a higher absorption capacity makes them more suitable for writing on.
Approved and Optimized Substrates
RIT substrate evaluation. HP subscribes to an independent substrate approval process undertaken at Rochester Institute of Technology in order to assure media suitability for HP Indigo digital presses. The RIT Worldwide Media Center (WWMC) operates a fullscale, controlled environment and includes a range of stringent tests that examine a substrate's runnability, ink transferability, blanket compatibility and other factors, such as ink-media interaction and flaking. The RIT WWMC pays particular attention to factors that have a significant impact on digital print quality, such as ink adhesion. While not all substrates are submitted for approval, it is not an indication that they are unsuitable for use on an HP Indigo digital press. Your Sales Rep should be able to advise you on the range of papers they keep in stock and the availability of any specialized substrates, if needed. Some substrates may experience problems in printing, but often a Sales Rep can apply a top-coating which alters the surface characteristics and enables a suitable key for the ink. This is referred to as optimization or treatment and should not be confused with the term "coated paper." Visually the substrate will look the same in appearance as the original sheet, but an optimized or treated substrate will give better results. The optimizing or treating process may come at an additional cost, so be sure to inquire about alternative substrates that meet the needs of the printed material.