The aluminum material is a very “dead” material compared to stainless steel and it has a much more “dampening” effect on the fluctuations in cutting force caused by the variations in the cutting designs. After an impulse, like the punch that is generated when a cutting line engages with the substrate during rotation, aluminum will go back to it’s relaxed state almost twice as fast as stainless steel.
The final process in manufacturing of a magnetic cylinder is the grinding process. This is where the accuracy is most critical. In traditional magnetic cylinders the surface is a mix of brittle and hard magnets and relatively soft stainless steel. The HMT cylinders have a much more homogeneous surface because the magnets are not exposed. This makes it a lot easier to grind to theoretically perfect dimensions, surface quality and geometry. This is especially important for achieving the near zero tolerance level that is required by a modern magnetic cylinder. All HMT magnetic cylinders are cylindrically ground in ONE grinding process. This ensures the maximum possible precision in concentric grinding of body, bearers and journals with zero run out error. Due to the HMT design with no magnets exposed on the surface of the magnetic body, it is possible to achieve extreme accuracy between the bearer diameter and the body diameter (drop).
The area on the surface of a magnetic cylinder that is most responsible for attracting the flexible die is the thin steel lines between the magnets. You can check this out by holding the edge of a die (or another thin piece of magnetic material) parallel to the line and try and move it from line to line. You will see that the die edge will jump from line to line and not stay on the magnets. That active area of high power magnetic force is between five and ten times larger on the HMT cylinder than on a traditional magnetic cylinder giving the HMT cylinder a superior grip of the flexible die
Because these lines are across the width of the magnetic cylinder on a HMT cylinder, any of these lines can be used to help align the flexible die during mounting of a new die. Simply let the edge of your flexible die catch one of the magnetic lines of the HMT cylinder and the flexible die will be aligned when it’s wrapped around the cylinder. The accuracy of the alignment is of course depending on how straight the edge of the flexible die is cut from the manufacturers side but in most cases this is more than sufficient for the alignment of the dies
Stress on the press:
On top of that you can add less stress to your press due to the lower weight of the
cylinders. The mass in a traditional magnetic cylinder is putting significant load on the drive
for the press especially during acceleration and deceleration. This usually has a bad
influence on the register between colors and die cutting.
The HMT magnetic cylinders comes with both hardened steel journals and hardened steel bearers for maximum precision and durability. Aluminum is extremely corrosion resistant and in this environment even more so than stainless steel.
The use of light weight, high strength aluminum in the HMT design is lowering the total
weight of the magnetic cylinder to approximately only the half of what a traditional magnetic cylinder is weighing. This not only makes the HMT cylinder much easier to handle for the operator, but they are also much easier on the printing machines due to less stress on the drives.
In some cases, traditional magnet cylinders might have bleeding problems caused by the glue used to fix the magnets in the body. Bleeding is caused by insufficient curing of the glue and will occur when one or more magnets are pushed into the groove that it is located in. This will make the magnet(s) sink and the glue is squeezed out between the groove and the magnet(s). The HMT cylinders does not have this problem because the magnets and their “container” are resting on the bottom of the grove. There is no room for it to move in and it acts as if the cylinder is completely solid.
Will it work with my flexible dies?:
All HMT magnetic cylinders are made to exact measurements and has TRUE repeat size.
No special distortion ratios or special plate lengths are required. You simply state your repeat length to your flexible die manufacturer and you have a perfect fit between your HMT cylinder and your flexible die.
“But aluminum is too soft”:
Aluminum IS softer than steel generally. What does that mean for the longevity of the HMT cylinder? The surface area on any magnetic cylinder that carries the load from the flexible die punching through substrates during production is many times greater than the tip of the cutting edge going through the material being cut. If the flexible die was made from aluminum, that would be a problem but the actual stress on the surface of a magnetic cylinder is reduced to a fraction of what is necessary to cut through any material. Even if the cutting edge is hardened on the flexible die, the flexible die would still give up before the aluminum on the surface of a magnetic cylinder simply because the load is spread out on a much larger area than what the tip of a cutting edge has to endure.
No magnetic cylinder is immune to excessive mechanical abusive. The most fragile part of a traditional magnetic cylinder are the exposed magnets. If that’s not a problem, HMT cylinders are even more durable.
Steel is three times more rigid than aluminum. That means that a magnetic cylinder made from aluminum will deflect three times more than steel for the same load. So why use aluminum?
In most cases the deflection of the magnetic cylinder during operation caused by the pressure required to cut through the substrate is a fraction of the total deflection of all elements that determine the combined “lift”. The total “lift” is a combination of deflection of the magnetic cylinder, the anvil cylinder and the unit that is keeping the cylinders together. For cutting across in full width a certain diameter to width ratio is required. A traditional magnetic cylinder will typically give up in the worst case scenario (cutting full width in a tough material) when the D/W (diameter to width) ratio is less than .32 (assuming that the anvil cylinder and cutting unit has sufficient resistance to the same load)
The same cylinder in aluminum will give up at D/W .37 for the same load. For a ten inch wide cylinder D/W .32 is equivalent to a diameter of 3.2” or ten inch in repeat (remember this is worst case scenario). D/W of .37 is equivalent to 11.5 inch repeat for a ten inch wide cylinder.
IF the D/W ratio for your job is too small for an aluminum body we make the HMT cylinder with stainless steel body and you have the same capability as in a traditional magnetic cylinder only the weight is not reduced but you still get all the benefits of the HMT technology.