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FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES Why is it OK to Use Ceramic Blades on Ceramic Rolls and Cylinders? By Michael Paczkowski precedent in many industrial processes for the wear-resistant na- ture of ceramics and the longevity of such components. The tre- mendous economic benefi t of increased productivity that results from more consistent printing quality and less waste has been addressed in previous papers and presentations; but what about the less tangible eff ect of these blades on the cylinders? For fl exo printing, the longevity and integrity of the laser-en- I graved anilox cylinder is of critical importance. In this paper we will address the nature of wear and discuss the interaction of the doctor blade with the cylinders to which they are applied. In the printing process, where a sta- tionary doctor blade is used against a rotating cylinder, there is inevitably a degradation of both the blade and the cylinder surface. We casually call this phenomenon wear. There are, however, two distinct aspects of wear; abrasive wear and friction. Understanding these diff erent mechanisms and the interactions of various materials is the key to understanding observed results when at times these results are almost counterintuitive. n recent years, some “exotic” wear-resistant doctor blades have been introduced to the printing industry based on the mate- rial behavior of ceramics. There is certainly a great deal of FACT #1 Abrasive Wear Is Dependent Upon 1. Roughness of the two surfaces. 2. Hardness of the materials. 3. Fracture toughness (brittleness). Remember: Even if material is very hard, and it fractures when contacting an opposing surface, it cannot cause abrasion. ABRASIVE WEAR The fi rst type of wear to be addressed is that of abrasion. The component of wear that is due to abrasion can be thought of as the “cutting” of one surface by the other (Figure 1). Particularly with respect to fl exo printing and the laser engraved ceramic anilox cylinder, this is the primary wear mechanism. 36 FLEXO APRIL 2009 www. f le xography. org The degree to which this mechanism occurs is dependent on the roughness of the two surfaces, the hardness of the materi- als, as well as their “fracture toughness.”Hardness is, of course, a concept most of us can relate to; however, fracture toughness is perhaps not so clear. In the fi eld of material sciences; Fracture Toughness = KIC (MPavm). Without going into the mathematics of this term, the easiest way to address this concept is to think of it like brittleness. For example, a fi ne china cup can be very hard but if it is dropped on the fl oor it readily shatters; it is not very “tough.” So even if a material is very hard, and it fractures when contacting an opposing surface, it cannot cause abrasion. When designing a ceramic doctor blade tip, it is therefore quite impor- tant to take into consideration the nature of the cylinder surface it will run against. In Table 1, several materials are listed along with their hardness and tough- ness values. The term “ceramic” in the table is being used generically and actu- ally refers to a family of metal oxides. The wide range of hardness refl ects this. Being able to select an appropriate ceramic material for the specifi c appli- cation is the key to producing a doctor blade that is safe for the cylinder surface should there be a direct contact or clash of the two surfaces. The safest choice for FIGURE 2