1. Bi-Metal Knife Procedures
PKI Procedure 2.01 Knife Grinding and Start Up:
Overview- Not all Underwater Pelletizer’s are the same as not all knives
are the same, as they vary in length, thickness, land area, and operators
that use them. This is a start up procedure in order to look for signs of
problems before they get out of hand. This is an extension to the OEM’s
recommendations.
Various resins produced are:
Polypropylene Polyethylene
a. Random Copolymers a. High Density
b. Homopolymers b. Linear Low Density
c. Impact Copolymers c. Low Density
Polymer viscosity ranges from fractional to upwards of 1500 MI.
Step 1. After the knives have been mounted properly(PKI procedure
1.01) the knives should be brushed with a silicone grease or spray to
make sure polymer does not stick for the beginning of the grind in
process.
Step 2. Most all the new equipment is equipped with a hydraulic system,
both forward and backward. These pelletizers also have a “locking”
system so that the shaft can be “locked” into a position, so that no
movement is made without un-locking this system. Most of the older
pelletizers use a wheel to move the knives into the die face.
Step 3. Select the Auto position, to start the initial grind, fill the die with
polymer, fill the chamber with water and move the knives into the die at
a pressure of 20 bar at 750-800 RPM’s for approx. 1 minute.
Step 4. Lower the forward pressure to 10 bar and run for 15 minutes
with Nikro-128 knives, around 1 hour with Nikro-143 material.

2. Step 5. Stop and uncouple the Pelletizer and inspect each knife visually
for grind marks from the die face. This should be across the entire
cutting edge. Below is an example of a poor knife grind.
If this happens, restart the Pelletizer and run for another 15 minutes or until all the
knives are seated in to the die face. In crease the forward pressure to 25 bar, which
may be needed from older dies.
Also, PKI can offer knife designs to help with a faster start up if one is needed.
Note: 1 bar = 14.5 psi or 0.9 kg/cm2
Step 6. A dial indicator should be mounted on top of the Pelletizer so
that a measurement of knife travel can be monitored and noted in a
station log book. We suggest a movement to start with of .001” per 24
hours or .01mm. Adjustments can be made to this depending on your
circumstances.
Step 7. PKI recommends the lower the forward pressure the better.
Ideal forward pressure with the Bi-metal Nikro-128 knives should be
around 3 bar. This could be affected by any surging and polymer flow.
When running at 600 RPM’s versus 900 RPM’s will also affect the knife
wear.
Step 8. If you encounter tail problems, PKI recommends that you do not
increase the forward pressure. First unlock the stop for 5 seconds, to see
if this clears the problem. If not, next increase the RPM’s by 30-50 in
the unlock position, then lock and see if they are clear. If this becomes a
chronic problem the knives may need to be inspected for evaluation.
This is just an outline to go by. PKI offers round table discussions to
evaluate the problem and offer suggestions to help with designs and
procedures. This is not a guarantee to extend knife life as there may be
other issues involved causing premature knife life.