User guide

Appendix l B-2
Cutter Blade Selection and Use
Blade Design
• Straight-edge knives have a straight cutting surface. A chopping action (which
has cutting forces parallel to the cut) is typically obtained with straight-edge
blades.
Because the blade is mounted on a rotary arm, some slicing action (which has
additional force vectors at various angles to the cutting edge) is obtained, but
generally not through the entire cutting action. If a slicing action is required, the
angle of attack can be modified by mounting the blade on a 30-45 degree angle
as close to the cut site as possible. In many cases the bushings must be modified
to allow the blade holder to have close proximity to the cut site. This offers the
steepest angle of attack throughout the entire cutting process. Generally straight
knives can be obtained in thicknesses from 0.004-0.060 inch depending on the
application.
• Curved-edge knives offer increased slicing throughout the entire cutting action.
They are generally used for cutting rubber preforms, rubber hose, flexible foams,
and other materials that require slicing. Blade lubrication is often used to
enhance the cut and minimize blade and bushing buildup.
As a general rule, curved-edge knives offer improved cut quality on rigid materi-
als if additional heat can be used. However if used on cold rigid materials,
curved knives have a tendency to produce wavy or angled cuts.
A curved edge knife can sometimes cut larger cross section profiles and tubing
with the same horsepower as a straight edge blade. However, the use of a curved
blade increases product interruption. To overcome this effect, use a variable
speed rotary knife cutter to vary the blade speed to obtain the desired cut quality.
• Piercing blade (bat-wing, woodpecker) knives are specifically designed for
cutting thin wall tubing. Their shape minimizes penetration marks caused by the
flattening action of the blade prior to penetration of the extrudate. These are the
most expensive type of blade, and the most susceptible to breakage. Because the
point is exposed and not fully supported by the bushings, it may deflect into the
bottom of the bushing bore and break off. For these reasons, piercing blade
knives are usually used as a last resort.
Some rigid materials require warming when this type of blade is used because the
impact of the point can cause cracking or whitening.
(continued)