User guide

Appendix l D-1
Blade and Bushing Lubrication
Blade and bushing lubrication can nearly always improve the quality of cutting.
Description of the cutting process
Unlike sawing, a rotary knife cutter displaces material rather than removing it.
When the knife blade first contacts the extrudate, it pushes it against the opposite
side of the cutter bushings. If there is too much clearance the extrudate may crack
or distort before cutting even begins. Tubing may develop two distinctive marks
related to the compression of the tube.
Once the blade penetrates the part, material is displaced to either side of the blade.
This displacement will vary in degree and visibility depending on the type of mate-
rial, temperature, blade thickness, blade style, and blade speed. As the material is
displaced, heat is generated and passed to the blade surface.
Flexible materials (flexible PVC, urethanes, and even LDPE) will generally com-
press during cutting, leaving little or no sign of displacement. The cut will appear
uniformly glossy and free of fracture. However, a closer look will show very fine
lines on the cut face. With flexible materials, these lines will typically show an arc
or "S" pattern which can be attributed to compression of the part as the blade
passed through.
Rigid materials such as rigid PVC and styrene will tend to fracture during cutting.
The cut surface changes from glossy to dull, and finally becomes whitened and
rough. Whitening occurs when cutting changes to fracturing: the cut begins to
extend in front of the cutting blade, which acts as a wedge. At this point, you can
only hope the fracture is controlled, allowing for a square cut.
Friction and heat during cutting
Because most rotary knife cutters don't travel with the flow of the extrusion line,
forward motion is interrupted as the blade passes through the plastic tube or pro-
file. This interruption causes friction, which generates heat in the cutting blade. As
the temperature of the blade increases, plastic is melted at the cut site. This melted
plastic can adhere to and coat the cutting blade, especially on the upstream side,
and be transferred to the next part in the form of hairs or tissue-like film. This will
be especially noticeable on the top inside of the tube or profile.