May we use a PVC or PE screw for these materials?
PVC or PE screw can be used with limitation on output. The high compression ratio will cause a strong friction and material overheating that cannot be avoided other than by reducing the screw speed. Anyway, in case, the flight depth in the metering zone must be at least 1/10 of the screw diameter, and must be avoided mixing zones, pins, Maddok zones and similar on the screw profile
What can we do if the current load of the extruder motor is too high?
Current load higher than PVC or PE is due to the higher viscosity of these materials, that require a high torque on the screw; if the extruder is equipped with a gear box, then switch to a slower speed; otherwise you should act on the gear box pulleys in order to reduce the screw speed and increase the torque
What can we do if the back pressure on collar and cross head is too high?
This is due to the higher viscosity of these materials. If you are using a breaker plate, replace it with a ring, in order to reduce as much as possible any flux constraint; channels of the collar and at the entrance of the crosshead must be as larger and shorter as possible in order to reduce resistance to flux
Should we expect same throughput as with PVC or PE from my extruder?
No, at the same screw speed the throughput is higher, from 50 to 100% more, particularly if you are using a low compression screw for HFFR; this means that typically, you must run at 30-50 rpm to get satisfactory cable line speed
What is the maximum actual melt temperature the material can reach during processing?
The maximum temperature measured at the exit of the die is 200°C (392°F); the optimal is 170-180°C (338-356°F)
What happens if actual melt temperature is too high?
The FR filler starts to decompose generating water vapor, that give place to porosity and pre-crosslinking that appears typically with an “orange peel” effect
What happens if we use a screw with pins or Maddok elements or other kind of mixing zones?
One effect is due to the high friction that increases the actual melt temperature and limits the screw speed; a second drawback is the possible stagnation in these zones, and subsequent release of hard pre-crosslinked particles in the melt that will appear as lumps on the cable surface
What happens if we use an extruder longer than 25 L/D?
First, a higher torque on the screw will be required, with a significant increase of the motor current load; then, the transit time of the material in the screw will be longer, and some pre-crosslinking may occur. In case, you can try to overcome the problem by keeping the temperature setting of the first 5 diameters of the screw (feeding zone) at ambient temperature
How does the screw diameter impact material processing?
Normally, the smaller is the diameter of the screw, the better is the control on melt temperature; extruders are suggested to be not larger than 120 mm
Can we use cooling of the screw to control melt temperature?
Screw cooling may determine a gradient of temperature and viscosity of the material along the depth of the flight, with possible decrease of mechanicals and increase of friction; cooling of the screw is normally not suggested
Can we grind and recover scraps during extrusion?
No, scraps cannot be recovered at any time
Can we run at low screw speed or stop the extruder with the material in?
To long residence time in the extruder may induce material pre-crosslinking; at the beginning of the extrusion, it is suggested to run slowly (5-8 rpm) just for 5-10 minutes, then speed up over 10-12 rpm; never interrupt the extruder running for more than 2-3 minutes; in case, use a by-pass valve on collar to purge continuously the material at low speed
Should we observe some difference in processing the material with and without the Catalyst MB?
Yes, adding the Catalyst MB means a decrease of the current load (10-15%) and of the bac pressure on collar and cross head (10-15%). It has the effect of fluidizing, easing the material melting and decreasing viscosity and friction.
What should we do if the color of the cable is not homogeneous, and lighter stripes appear?
If you are using an EVA based colour MB, increase the dosage up to 1.5-1.6%. If you are using a PE based MB switch to an EVA based. Some help can come also by using the breaker plate instead of the ring, and, exceptionally, you can try also with a very raw filter net (mesh light around 500 μm).