Fly-knife cutting, or “fly cutting,” is a fast, effective, and widely-used method for cutting smaller, softer extruded tubing and profiles. Fly cutters use one or more blades, mounted on a flywheel that provides cutting inertia. The flywheel is driven by a motor that transmits rotational force through planetary reducing gears. These increase the torque of the flywheel and cutting force of the blades, while isolating the motor and motor shaft from the shock of cutting.
Fly cutting blades work by “displacing” material, slicing into the extrusion and pushing all of the material to either side of the cut, so no material is lost. When extrusions are small, thin-walled, or soft, fly cutters may slice through smoothly in a single cut. For larger cuts or firmer materials, the rotating flywheel and blades can move progressively through the extrusion, similar to a circular saw blade, expanding the cut until it is completed. In either case, cuts are typically very quick.
The 2 Key Components of Extrusion Fly Cutters
To make quality cuts quickly, fly cutters must be properly equipped with:
- Cutter bushings
- Cutter blades
To hold the product and blade steady during cutting, bushings (see above) are positioned on either side of a slot where the cut takes place. The extruded tube, pipe or profile slides through the cutter bushings and the blade makes a straight cut between them.
For optimum cutting, the space between the two bushings must be adjusted relative to the thickness of the blade(s), ideally allowing a very slight drag on the side of the blade as it cuts through the product. This “drag” fit prevents relatively thin blades from flexing or curving under stress and keeps the fly cutter properly aligned, cut after cut.
Optimum cut quality demands fly knife blades that are “right” in four key areas:
Shape: To appreciate the difference that a blade’s shape can make, consider a few basic blade profiles and what they do to blade performance.
- The straight blade makes a chopping motion, similar to a cleaver. It is attempting to cut through the entire width and depth of the tube in an almost pure vertical motion. This cut requires a lot of force and puts a lot of stress on the blade. If the tube or profile is soft, it might part easily. But if it’s firmer—or if the blade dulls—the profile could deflect or distort—potentially causing cut quality problems.
- The inclined straight blade adds an angled slicing motion to the chopping motion, moving more of the blade through a smaller section of the tube. While it is still positioned to move through the entire width and depth of the tube in one cut, the cut is somewhat more gradual, so it reduces the amount of stress on the blade and the tube.
- The curved blade makes an almost pure slicing motion, moving a lot more of the blade’s edge against a fairly small section of the tube and making a more gradual cut. Curved blades are very common in extrusion fly cutting applications.
- The pierce blade combines vertical chopping and slicing motion. Its shape, starting with a pointed edge, focuses all of the cutting force onto a very small section of the tube. Then, its slicing motion spreads rapidly across the full width of the tube. Like a straight blade, it too works best when cutting softer materials.
Width, height, and thickness: To cut effectively, blades must be sized for the extrusion they will be cutting. The rule of thumb for a correct blade size is:
- width must be greater than the diameter of the tube or the cross section of the profile.
- height must be at least equal to tube diameter or to the height of the profile.
- thickness must be at least equal to the wall thickness of the tube.
Blade bevel: When it comes to edges, the best approach is pretty simple: a blade with a double bevel or V shaped edge (right, below) is always best, because the bevels balance out the reaction forces (F) on each side and help keep cuts straight. A single bevel (left) accumulates all the reaction forces on one side. That tends to push the blade—and the cut—toward the other side.
Blade material composition: Most extrusion cutting blades are made of stainless steel or spring steel. Choosing the right material involves some tradeoffs. Overall, stainless steel is better at holding an edge and better at cutting more abrasive materials, but is more brittle and therefore more prone to breaking. Spring steel is better at tolerating the stress and shock of cutting, but it doesn’t hold an edge as well as stainless does.
|Stainless steel||Spring steel|
|Superior edge holding||+|
|Better abrasion tolerance||+|
|Greater shock tolerance||+|
|Higher resistance to breakage||+|
As you would expect, cut quality requires proper blade selection, periodic blade sharpening and occasional blade replacement. Blade wear and lifespan ultimately vary based on material hardness, presence of filler, and the rate of cuts/minute.
Meeting Special Fly Cutting Challenges
While attention to the above are vital, they might not be enough to achieve flawless fly cutting. Below are a few common challenges and some suggested solutions:
Angel hair or fines on knife surface
When cutting softer materials such as flexible PVC, fly knife cutters tend to pull or carry out fine pieces of material as they exit the cut. To eliminate the formation of these fines, heat the blade between cuts. Add a heater at the blade’s home position, with a temperature set-points based on the material being cut.
Material buildup on knife surface
In most situations, processors can get multiple shifts out of a sharp blade. But even on a sharp blade, any material buildup can affect cut quality because it tends to grab or stick to the extruded product as the cut is made. Material buildup typically occurs after a few hours of cutting softer, stickier extruded materials.
There’s not a perfect solution to this problem: In some cases, it is possible to equip the cutter with a felt pad—usually wetted with alcohol—that gently wipes the cutting blade on every revolution.
Another solution is to add an engineered system to spray out a controlled amount of isopropyl alcohol on the blade prior to each cut. The same system can also spray nearby cutter bushings to protect against build-up and rub-off of particulates onto the finished product. Spray systems don’t eliminate the need for periodic removal and thorough cleaning of particulate from blades, bushings, cutting chambers, and drip trays, but they do reduce frequency while maintaining better cut quality.
Product binds or sticks in cutter bushings
It’s not uncommon for warm extrudate to bind or stick somewhat as it passes through steel cutter bushings. If the problem becomes significant, it can affect the smooth operation of the line and cause inconsistencies in product and cut quality. The best options for managing this problem are:
- Lubricate cutter bushings – Many cutters offer optional lubricant reservoirs that use pulses of compressed air to inject small amounts of lubricating fluid into diagonal, forward-facing holes located on the top of the cutter bushings. Alcohol is the most common lubricant, particularly in medical applications, though water-soluble silicone may also be used.
- Lined cutter bushings – Cutter bushings with inner linings or sleeves of Delrin® or Teflon® material offer another good way to keep extrudate moving smoothly. If you use this solution, note that sleeves or liners must allow ¼ to ½-inch of clearance at the interior ends of the bushings so that only clean steel edges butt against the cutting blades.
- Air-feed bushings – Using a controllable stream of air as a lubricant may also help tacky extrusions to slide more easily through cutter bushings. In fact, air-feed bushings sometimes work so well that the processor can hold tighter cut-length tolerances.
Material chips or cracks
When fly cutting semi-rigid to rigid materials like rigid PVC or polypropylene, you may see “breakaway” cuts—cuts with edges that break, crack, or chip off before a clean cut is completed. Obviously, you’ll want to address this problem very quickly, since it’s going to cause reject parts or require secondary finishing operations.
However, there’s no single, simple answer for breakaways, cracks, or chips. You may need to try several approaches to achieve success:
- Reduce cutter RPM—There’s a rule of thumb for fly cutting: For the cleanest cuts, cut softer materials at higher RPMs and harder materials at lower RPMs. So, if cutting semi-rigid or rigid extrusions results in chips or cracks, start by slowing down the RPM of the blade to reduce or eliminate the problem. Sometimes, especially with rigid PVC, using a slightly duller blade actually provides a better cut.
- Cut extrudate at higher temperatures—Chipping or cracking can also be related to the temperature of the extruded product. Many processors get better results by cutting at higher temperatures so that the material is a bit softer and less likely to fracture. The most efficient way to achieve a higher material temperature is to bring the extrudate out of the cooling process and into the cutter while it still retains some process heat, typically at a temperature of about 120-140° Using retained process heat is better than the alternative, reheating fully-cooled products, because process heat is more evenly distributed and costs nothing extra. Cutting extrudate that is warmer often enables the blade to travel cleanly through, free of cracks.