To understand the benefits of high volume anilox structures from a printer’s point of view, first we must understand how contemporary high volume anilox (CHVA) works from a fluid dynamic prospective.
For some 40 years, the laser engraving of a ceramic anilox roller has used the same traditional engraving technique. Carefully placed pulses of a laser beam with varying energy and focused spot size, produces beautifully geometric, tessellated cell structures. However, the hexagon or diamond anilox shape we have all seen are an accident of nature. Melted ceramic solidifies on the roller to give the familiar anilox cell shape.
Traditional anilox cells have a tendency toward a pointed bottom because the cell below the roller surface is formed by a round pointed beam. In turn, this means it is harder for liquids to come out of a pointed cell, as compared to a flatter bottomed cell, so shape matters when it comes to fluid dynamics.
An anilox cell should be thought of as having two parts, one above the surface of the roller and the other below. The shape below the surface is largely controlled by the shape of the laser beam itself. A laser beam used for traditional anilox engraving, is round and Gaussian in its energy distribution (Figure 1). However, the shape of the cell above the roller surface is formed by resolidified ceramic.
All photos courtesy of ALE
The reason a ceramic anilox roller has a larger diameter after engraving is due to the re-solidified ceramic. It would be fair to say that the increase in roller diameter, after engraving, is a measure of the anilox cell structure above the surface of the roll.
Shape Matters
Sorry to remind you of your school days, but most of us had to learn the volume of some shapes. The one shape I would like you to consider, is a cube. If a box, height (H) of one meter, and width (W) of one meter, and depth (D) of one meter, the volume of the box would be one cubic meter. I like to think of this as a reference volume or the Maximum Theoretical Volume (MTV = H x W x D), for that space.
- A cube has 100 percent the MTV
- A pyramid has about 33 percent of the MTV
- A cone will have about 26 percent
The point I am making is that the shape of a cell matters (Figure 2). In particular, the flatter the bottom of a cell, the more volume it will have. Another way to think of this is to know the depth necessary for all shapes to have the same volume.
- The cube is reference, so depth is 100 percent
- The pyramid must be three times deeper
- The cone about four times deeper
Do not get me wrong, I am not suggesting it is practical to engrave cubic cells. However, I am suggesting the flatter the bottom of a cell, the more closely its volume will approach the MTV. This also means the cell has less depth for the same volume. The lesser depth will also help the cell release liquid more readily. This, I would argue, is a virtuous circle.
What part of a cell has the greatest volume? This, at first, sounds a very strange question. However, if a cell were like a test tube, the same cross section all the way to a deep bottom, the answer would have to be the top of the cell must have more realizable volume than the bottom.
If liquid cannot be released from the bottom of a cell, the volume of the bottom of the cell is wasted. In the case of a mechanical engraved pyramid or quad cell, there is much more effective volume at the top of a cell than at the bottom. Traditional laser engraved cells are the same, the nearer to the top of the cell, the greater effective volume there is.
Traditional Engraving
Mechanical engraving tools need a point to push their way into metal. It is true to say that a degree of flatness can be incorporated in a mechanical tool offering higher volume. Traditional anilox laser engraving also uses a pointed tool in order to give the cell its shape. CHVA has set aside the historic link between the shape of a laser beam and the shape of a cell.
CHVA technology dynamically shapes the beam’s impact on the ceramic being engraved. The CHVA approach is to dynamically change the laser power and laser beam position throughout a cell in order to change the cell shape or profile.
This facilitates the engraving of flatter and wider bottomed cells, offering higher volume at a shallower depth and good release characteristics. The ease in obtaining a required cell volume also means cell walls can be polished and as a result lesson doctor blade ware.
Below the Surface
The traditional hexagonal and diamond form of a laser engraved ceramic cell are created by the melting and solidification of the ceramic. However, the hole in the ceramic roller is the shape of the laser beam…. round! With CHVA engravings, quad cells make diamond shaped holes into the roller and hex cells make hexagonal holes into the roller (Figure 3).
Differences noticeable to a printer utilizing the CHVA approach, as compared to traditional methods, will notice:
- Shallower flatter-bottomed cells give higher volume for a given depth
- Shallower flatter-bottomed cells release more of the ink held in a cell
- Shallower flatter bottomed cells are easier to clean
- Shallower flatter bottomed cells are less prone to plug
- Longer printruns before cleaning
- Anilox roller cleaning is more efficient—less time and effort spent
- Due to improvement in ink release, higher print density can be achieved at higher press speed
Figure 4 shows an extensive range of cell types, all exhibiting a characteristic flatness, which is the hallmark of CHVA. The fact that different shaped cells are now an essential part of contemporary engraving is also well illustrated.
The underlying new concept that CHVA imposes is that cell volume to depth ratio is a very good indicator of how well a cell will release liquid. The lower this ratio is, the better off you are. This ration alludes to the shape of a cell and “the shape of a cell matters.”
A test tube and a saucer would have a very different volume to depth ratio. The lower the depth for a fixed volume, the more like a saucer the cell is, the flatter the cell bottom will be and consequently the more ready the cell is to release ink. This is not only important for printing, but it is also important when it comes to cleaning a roller.
The traditional depth to opening ratio for anilox cells was useful and meaningful for traditional cell engraving. However, the more contemporary, volume to depth ratio may prove to be very useful and could easily be provided by anilox volume measurement equipment suppliers.
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