User guide

B-3 l Appendix
Cutter Blade Selection and Use
Blade Thickness
Because material is displaced rather than removed in rotary knife cutting, think of
the blade as a wedge. The thicker the blade, the greater the displacement. This dis-
placement can cause fracture in rigid profiles and tubing, which is often observed
as a whitening on all or a portion of the cut. You can reduce this fracturing by
reducing the thickness of the blade.
(This effect can also be minimized by heating the profile or tube. However, if heat
is used to enhance cut quality, the bushings must be supportive enough to mini-
mize distortion.)
If the cutting blade is too thin, it may actually deflect within the bushing bore. This
can lead to "S" shaped cuts or premature blade breakage.
Optimizing Blade Speed
Flexible extrudates generally require a very fast blade speed with a slicing action
for best results. This is due to the fact that even minimal interruption can cause a
blade jam on a product that has little or no internal strength.
On the other hand, rigid extrudates may require different blade speeds to obtain the
desired cut quality. What's needed for a particular application depends on blade
style, internal heat, and blade thickness. Speeds as slow as 300 rpm may be
required if a curved blade is used with little or no heat.
Improving Cut Quality by Adding Heat to
Certain Materials
All rigid extrudates can have their cut quality improved by the addition of heat. A
few of the most common materials and the respective temperatures are listed
below:
Rigid PVC 110°-125° F
Styrene ABS 120°-135° F
Polypropylene 160°-200° F
It is important to remember that as the temperature approaches the glassification
zone, the degree of support offered by the bushing becomes more important.
(continued)