Part 1 of 3
One of the most common problems regarding slitting, whether it’s crush, shear, or razor is dust generation. Although the amount of dust is highly dependent on the material being slit, there could also be some aspects of the set-up and equipment being used, which can worsen this problem even further and start hurting the production line, especially when it comes to materials such as tissue, paper, sandpaper, non-wovens, and fiber-glass.
Something that needs to be mentioned before we continue with this topic is that due to the nature and mechanics involved in slitting, the dust generation will never be completely eliminated, regardless of the material and slitting method.
But, if you follow the guidelines and tips presented in this article, the dust generation can and will be minimized. This article is the first of a three-part series on dust generation. Given that this is one of the most common problems in the industry and a topic that can be talked to in great lengths, our goal is to cover this topic as in-depth as possible, explaining why and how some of the aspects of the slitting operation can impact dust generation.
The First Thing that is Important to Know: “What Causes Dust?”
The answer: There are many contributing factors for dust generation. Blade set-up errors, poor-design, mechanical characteristics, and the web material itself, can lead to excessive dust generation. The starting point regarding slitting dust is to adhere to the geometric fundamentals: Cut Point, Overlap, Shear (cant) Angle, and Run-out. In this section, we will now see how they each affect dust generation, except Overlap, as this fundamental will have a section of its own, as it greatly affects dust generation.
Below are pictures showing these 4 fundamentals of slitting:
designed in all shear slitting systems, based upon the knife diameters and the web path; mounting locations are set in the manufacturing and assembly drawings. Unfortunately, operators have no control over the offset, as this is defined in the design process, yet this is a critical element of controlling dust generation.
Although, the amount of web tension also plays a key role in limiting dust. If a web is loose or floats into the slitter’s, the material will come in contact with the top (male) knife before the actual Cut Point. The web is then sawed as the blade “bursts” through it.
So, just as tension control is important to winding a perfect roll, it is as important for web slitting, controlling a clean roll edge, and limiting dust generation.
In turn, the shear angle contributes to slitting dust generation by the amount of planned angle, the unplanned decreasing angle, and as a cause of blade dulling. One side of the slit material must travel across and around the angled blade. This by itself will cause damage to the web edge and contribute to dust. The angle is set by the knife holder and should be kept to a minimum; ¼, ½, or ¾ degrees, depending on the material to be cut. The steeper the angle, the more the web is disrupted after it is cut.
Various angles, shapes, and blade holder clearances can, and do, affect when and how much dust will occur. Knife geometry is the shape of the knife cutting edge. The materials the knife is made from can also affect slitting performance. How effective the knife holder is in maintaining the proper geometry of the cutting tools relates to the design and ruggedness of the holder itself. Accuracy and consistency of setup, forces applied, and safety are affected by the setup procedures and tools themselves.
If a knife holder is manufactured with loose internal clearances, it can not maintain the planned shear angle because, as side load is applied when contacting the bottom knife, the loose components shift until the clearance is taken up. A 0.0065″ looseness can remove a full ¼ degree of unplanned shear angle from some holders. Unfortunately, not much can be done about this except for maintaining the minimal shear angle, by not using loose blade head holders, and possibly looking at high chromium knife blade materials to keep the sharp edge as long as possible.
Finally, there are two types of run-outs affecting slitting and indirectly, dust: Excessive axial run-out and radial run-out cause cyclic changes in overlap depth and blade thickness. Both will increase knife blade wear or dulling.
These run-out conditions can be the result of poorly designed, improperly manufactured, inadequately mounted, or improperly reground tools. Inadequate or worn-out holder bearings, worn-out vertical or horizontal pneumatic piston rods (guides), and poorly designed or worn-out knife mounting surfaces can cause run-out.
Shaft driven bottom knives are susceptible to run-out based on knife O.D., width, and shaft mounting clearance. Cocking should always be avoided. A simple axial run-out check is to mount and lock your bottom knives to the driven shaft. Then place a 0 to 0.010″ dial indicator against the bottom knife-edge and slowly rotate the shaft while noting the maximum dimension change. The dial indicator can also be placed on the bottom knife O.D. to measure radial run-out.
As we mentioned earlier in this article, this is only the first part of a three-part series. Make sure to come back for the second and most extensive Part 2: Dust generation caused by an incorrect overlap.