There are several methods of slitting aluminum foil, including shear, razor and crush (score) slitting. This discussion focuses on the shear slitting method, since it is capable of slitting under a far wider range of calipers, tempers, and speeds than razor or crush methods.
The most common complaints when slitting aluminum foil are: a distorted edge, seen as a small crease or longitudinal bend along the slit edge, or the edge may appear to have a “scalloped” appearance. Wound rolls may have “bell ends”, indicating that the slit edge, being stretched, is longer than the web itself. Thicker aluminum products may also have a burr on either the top or bottom edge of the slit.
Metals are permanently deformed when they are subjected to stresses that exceed their yield strengths. The lower the yield strength (“softer”), the easier the metal is permanently deformed. (Lead, annealed copper, and “dead soft” aluminum are examples of “soft” metals, compared to spring steel or highly tempered aluminum as “hard” metals). Aluminum is the most common of the metal foils, having typical yield strengths of 5,000 to 30,000 psi, as compared to 50,000+ psi for steel. Notice, however, that the range for aluminum can be quite broad, depending on temper. The harder tempers generally slit with fewer of the above described problems than the softer tempers.
Typical Current Methods
“Gap” (Clearance) Slitting
Metals are slit by being forced through a nip which stresses the metal beyond its ultimate yield. For sheet and plate metals, the slitter rings are not necessarily in contact, but are usually separated by precise “clearances”. The tensile strength of the metal determines the fracture angle along which the failure will occur. The goal is to align the slitter rings so that their edges lie along the fracture angle, thus creating a clean fracture in the material. If the blade edges are not aligned properly, a burr or metal sliver will form. Therefore, it is quite evident that high tolerance accuracy in the clearances between slitter rings is required, especially when gap slitting thinner metals. In practical reality, however, aluminum foils can be cleanly shear slit without resorting to the rigors of gap slitting, but slit edge distortion is a challenge.
Waste Strip Method
To sidestep the complications of two-shaft gap slitting, two narrow rimmed male blades may placed back-to-back to slit out a narrow strip of metal. The distorted slit edges are confined to this waste strip which is discarded. Needless to say, the collection and recycling of the waste strip is a problem of its own, as are minimum slit width limitations .
Two-shaft, “Metal Slitting” Method.
Where the extreme accuracy of gap slitting is required, square edged slitter rings must be assembled on upper and lower slitter mandrels, and the run-out and inter–ring clearances must be controlled to very high tolerances. Slit size changes and tooling costs are very expensive. The benefit is a distortion–free slit edge, since the shear effort and the web path through the slitters are symmetrical when square edge slitter rings are used. It is this principle of symmetrical shear geometry that we can use to our benefit when using individual knifeholders.
Applying the Principles
The following list of do’s and don’ts assumes that you are going to shear slit, using upper knifeholders that hold the top blade against the lower slitter ring, and that you are not going to slit using the “gap” method. For foils, contact shear slitting is most practical.
- To avoid distortion, metal must not be subjected to any web path or operation which imposes bending or stretching beyond its yield strength. Exceeding yield will result in permanent deformation. It will not recover to its former shape. Some “don’ts” to avoid the causes of bending include:
Don’t use too much top blade overlap. The web is forced to detour laterally around an obstacle in its path. Multiple, narrow slits have the additive effect of multiple blade tips thrust into a web that is intolerant of lateral deformation.
Don’t use too much top blade setback. This means the top blade will have to be lowered into the web line even further, creating an even larger obstacle for the web to bend around.
Don’t use extremely narrow lower slitter grooves, in conjunction with too much overlap, or too thick top blade. The web is deflected downward at an angle that is too acute, exceeding yield.
Don’t wrap slit under the above conditions. The edge is forced into a compound bend. Again, yield is exceeded, a scalloped edge will result. (See Photo #1)
Don’t wrap slit on too small diameter lower slitters. The compound bend problem is even worse.
- Take a tip from the two-shaft heavy metal slitting industry: use a blade profile which avoids forcing the edge to scrub past the side of the blade tip. They direct the web “under and over” square edge slitters… lateral bending cannot be tolerated. Some specific suggestions are:
Do use wide rim top blades. That means a rim width sufficient to positively deflect the web vertically, not laterally. Widths of 1/4” to 3/8” should be about minimum. The grind angle should be very obtuse, almost square… 5 degrees works very well. (See photo’s 2 & 3)
Do slit tangent, minimum widths permitting. Arrange the web path to precisely bisect the overlapping arcs of the top and bottom slitters. This way, the web is precisely symmetrical through the slitters, and stretching is minimized.
Do use minimum cant angle (toe-in). Metals don’t need it, your blades will last longer without it. If your blade holders can’t hold the blades truly vertical, however, you’ll need more toe-in to keep the cutting nip point closed.
For a material that slits so effortlessly, metal foils can be devilishly difficult to slit without permanent distortion. Just remember; don’t bend it beyond its yield point, and you’ll have the problem well under control.