by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES Toughness, KIC [ MPa vm ] Hard Chrome plate Anilox cylinder (CR2O3) Steel blade Tool steel (blade tip) Ceramic blade tip TiO2 Pigment (white ink) 6-7 3-4 50 8 3-5 3-4 Hardness,HRC (Rockwell) 67-70 74-78 50-55 60-64 65-76 67 Table 1. Material properties of typical ink train components. Blade material Max temperature reached (ºC) Ceramic 1 112 Ceramic 2 133 Ceramic 3 112 Table 2. Maximum blade temperatures achieved under laboratory conditions. the lower-hardness chrome cylinder would be a ceramic blade tip with its hardness on the lower end of the scale, whereas for a ceramic anilox cylinder a much harder ceramic material can be chosen to take advantage of its greater wear-resistance but yet still be in a safe range for the cylinder surface. In the actual printing process we do not have a simple sys- tem of one dry surface running against another. The ink or coating being used adds a third and very critical component. The ink fi lm that is dragged along the cylinder surface by shear forces at high speed forms a thin fi lm that acts as a bar- rier between the blade tip and cylinder surface, keeping these two surfaces a very small distance apart. This thin fi lm can be lubricating in nature or, in the opposite extreme, it can be extremely abrasive in itself. Due to the intimate contact of the ink fi lm with the surfaces of both the blade tip and the cylinder it can be said that this is the primary mechanism for abrasive wear for both components. Using the example of white ink, where the primary pigment component is titanium dioxide, and noting its physical properties in the above table, it is easy to see how it can be the most critical factor in wear. FRICTION (ADHESIVE WEAR) Contrary to earlier explanations, kinetic friction is now under- stood not to be caused simply by surface roughness alone but by chemical bonding between the surfaces. The more chemically similar two surfaces are, the greater the mutual solubility of the materials and the greater the bonding. This phenomenon is most prevalent in gravure printing where two metallic surfaces, the steel blade tip and the chrome cylinder surface, come together intimately, particularly in the non-image areas of the cylinder. On a microscopic scale (Figure 2), there are high spots and low spots on both the blade tip surface and the cylinder surface. Where the high spots meet, there is an extremely high specifi c pressure generated at this point which causes a bond. As the surfaces momentarily bond and then instantaneously break free, material is transferred and energy released. This is frictional wear. Friction is therefore not a fundamental force and so cannot 38 FLEXO APRIL 2009 www. f le xography. org FACT #2 Ink or Coating is a Critical Component Because it: 1. Forms a thin fi lm that acts as a barrier between the blade and cylinder. 2. Can be either lubricating or abrasive. Remember: Due to the intimate contact of the fi lm with the surfaces, ink is the primary mechanism for abrasive wear. Steel 180 Hardness,HRC (Vickers) 900-1100 1200-1300 513-594 700-800 800-1300 900