Making the Cut: Pick the Right Cutting Technology

Jul 1, 2020

We came across in interesting article in Plastic Technology that we thought we would share:

Tubing, pipe, or profiles are extruded, sized, cooled, and processed in a continuously moving process, so it’s important to choose a cutting technology that keeps pace and is appropriate for the size of the product and characteristics of the materials that you’re cutting. This article will discuss available cutting technologies and provide some insights on how to handle typical problems associated with each.

Four Basic Types of Cutters

Fly-Knife Cutters: 

Fly-knife cutters are very popular, made to cut smaller, softer tubing and profiles quickly and cleanly, using one or more blade types in a chopping or slicing motion. The simplest fly-knife cutters employ a blade mounted on a flywheel that provides cutting inertia. Fly-knife cutters are driven by a motor that transmits rotational force through planetary reducing gears. The gears increase the torque and cutting force of the blades, while also isolating and protecting the motor and motor shaft from the shock of cutting.

Fly cutting blades work by “displacing” material, literally pushing all of the material to one side of the cut or the other, so no material is lost. When extrusions are small, thin-walled, or soft, a fly cutter blade may slice through in a single cut.Blade shape and angle are vitally important to successful cutting. Straight blades (1) chop vertically through the entire width and depth of the tube. That’s ok for softer extrusions, but can cause a lot of blade stress and wear on harder materials. Increasing blade angle (2) or adding curvature (3) creates a more gradual, slicing motion that cuts less material at once and reduces stress. Pierce blades (4) may use a variety of edges and angles to combine chopping and slicing motion.

To hold the product and blade steady during cutting, bushings are positioned on either side of the cutting blade. The extruded tube, pipe or profile slides through the cutter bushings and the blade makes a straight cut between them. The space between the two bushings is adjusted relative to the thickness of the blade. Ideally, the spacing should allow for a very slight “drag” on the side of the blade when it cuts through the product. This “drag” fit prevents blades from flexing or curving under stress and keeps the fly cutter properly aligned, cut after cut.

Guillotine or Traveling Guillotine Cutters:

Similar to their name, guillotine cutters use a vertical blade to slice or chop downward through extruded products. These cutters are often seen as fitting “in between” fly-knife cutters and traveling-saw cutters (see below), where they fill a more specialized niche: cutting very soft or sticky extrusions (e.g., thermoplastic elastomers, polyurethanes) that could “gum up” rotating fly knives or saw blades. While smaller guillotine cutters can process small-diameter products very quickly, larger-diameter products may be guillotine-cut on a moving table because cuts take longer and the extruded product is continually flowing.

Saw Cutters: 

Traveling-saw cutters are used on larger pipes or extrusions that are made of relatively hard, thick, or brittle materials, such as rigid PVC. Essentially, these are large circular saws adapted for cutting plastics. To ensure straight cuts of a continuously flowing extruded product, saw cutters are often mounted on traveling tables that move at the speed of the extrusion line. The extrudate is momentarily clamped to the table so that all three elements—extrusion, saw and table—move together while the cut is made. Then the table saw retracts to make a new cut. Unlike the other cutting types, which displace extruded material to either side of the cut, saw cutters remove a narrow swath of the material, creating “sawdust” which must be collected using a vacuum system.Single-bevel or chisel-edge blades (left) tend to concentrate the reaction force (F) of cutting on one side, so they tend to veer in the opposite direction.  A better choice are double-bevel cutting blades (right), which balance cutting force on both sides and therefore tend to move straight and square through cuts.

Planetary Cutters:

Planetary cutters make extremely high-quality cuts—square, distortion- and particle-free—on high-value rigid tubing products used in medical, high-purity, or automated-assembly applications. Closely resembling a plumber’s pipe-cutting tool, planetary cutters hold a circular cutting wheel on the inside of a rotating ring that surrounds the pipe. The ring spins around the circumference of the tubing, pressing the cutting wheel inward so that it gradually splits the tube apart without any loss of tube material. Like other cutters, planetary cutters are often mounted on moving tables so they can make square cuts on moving product.

See the entire article HERE

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