The quality of the cable depends not only on the head and tools used but even on the other process parameters:
Extruder, Screw design, Barrel, thermoregulation, etc.
Temperature profile into the extruder and head
Cooling Mode of the cable after the X-Head, water temperature, etc.
Tensioning Device/mode of the cable, Etc.
The extrusion head is among the components that act on the quality of the cable and on the production performance.
The extrusion heads must be properly designed to:
Reduce the friction between compounds and all components of the head
Reduce the back pressure as much as possible in according with extruder output and consequently the line Speed, in according with cable dimensions, etc.
Compounds to be processed
Avoid static and dynamic overheating of the compound during the extrusion process
Reduce the residence time of the compound into the head (flow distributor, tools chamber, etc.) and avoiding the material stagnation.
Setting Repeatability in order to get the same quality of the cable at any time
Reach highest cable concentricity
The tools to be used must be chosen according to:
Compound to be extruded
Extrusion Process Mode, Insulation, Filling, Sheathing, Jacketing, etc
Aspect of the cable surface required
Construction and Dimension of the cable as well as the line speed (Extruder Output and Shear rate)
The tools can be classified in:
EXTRUDER PRESSURE AND PRESSURE ON THE CONDUCTOR
Keeping constant the cable dimensions as well as the processing parameters (Line Speed, Extruder speed, etc.): Higher is the extruder back pressure and consequently lower is the pressure on the conductor; Lower is the extruder back-pressure and higher is the pressure of the compound on the conductor. Shorter is the distance between tip and die and higher is the extruder back-pressure and consequently lower is the pressure on the conductor.
The Tip has normally an angle of 60°. The die has normally 2 angles. The first is 60° and the second, at the end of the die, can be between 5°-10°. The last angle is increasing the compression of the compound and consequently the pressure on the conductor. Obviously, in order to increase the plastic adhesion on the conductor, semi-compression die with horizontal long land length are advised.
The adhesion of the plastic on the conductor can be even increased warming up the conductor, scratching it or acting on the temperature profile.
The long cylindrical length to the end of the die calibrates the cable diameter for longer time.
The pressure tools are mainly used for insulation of:
PE, PE Foam, HDPE, LDPE, PP insulation
Insulation with some Technopolymers
Virgin XLPE Insulation, even if semi-compression tools with long length die are preferred.
The choice of the final diameter of the die depends on the:
Cable surface and its roundness. Smaller the die is, respect to the final cable diameter, and higher is the back-pressure and brightness. Too small die diameter can cause diameter unstability.
Compound to be extruded: For PVC is less or equal to the final cable diameter. For HDPE is equal to the Hot Cable Diameter.
PRESSURE TOOLS FOR PVC
The die diameter “D” can be calculated as below described:
D = C – k
0,17 < k < 0
D: die diameter [mm]C: final cable diameter [mm]k: correcting factor [mm]K depends on Cable and conductor Diameter, Type of compound, Line Speed, adhesion, etc.PRESSURE TOOLS FOR HDPE – PE – PP
The die diameter corresponds to the hot cable diameter C’. The die diameter “D” can be calculated as below described:
D = E + k’ oppure D = C’
D: die diameter [mm]C’: hot cable diameter [mm]E: cold/final cable diameter
k’: correcting factor according to compound shrinkage [mm]
The shrinkage factor depends on different factors:
Cable Dimension and Thickness of compound
Type of compounds
Temperature profile into the extruder and screw design
Cooling of the cable after the extrusion head, water temperature, distance between crosshead and cooling through. This acts on the polymer re-crystallization as well as on it mechanical property.
The cooling mode has an important effect on the final cable surface (Roughness, Fish Eyes, etc.)
If “D ≤ E”, we can get:
High Back Pressure
Not Stable Diameter, Sausage effect, etc.
Not Stable Process Parameters, etc.
The tip has normally one angle only. It is generally smaller than 60°. The die has 1 angle only. It has normally smaller than the tip angle.
The lower angle than the pressure tools act on the cable adherence and elongating force direction on the conductor. Less is the die angle and less is the orthogonal component of the force “Fc” on the cable and consequently higher is the horizontal component of the force “Fa”, parallel to the cable, that is pulling it out from the die.
They are used when low pressure of the plastic on the cable or conductor is required.
The final surface aspect depends mainly on:
Die Diameter “D”. The “D” value could be Smaller, Equal or Bigger Than final cable diameter.
Length of the final cylindrical land of the die “X”.
D = C – k
-0.25 < k* < 0,20
* The length of the die land act on the choice of k value and on the roundness of the surface and Process. C is the final cable diameter
If k < 0 mm means that the die diameter is bigger than the final cable diameter.
SHEATING ON BUNCHED CABLE OR INSULATION ON BUNCHED CONDUCTOR
During the sheathing on the bunched cable or insulation on bunched conductor, some problem unwanted can arise:
Rough Surface and not round surface
Feeling of the cable and conductor under sheath, etc.
In order to get a rounder cable surface without feeling the conductor or cable shaper the die with long land must be used. The “X” value is chosen according to:
Type of Compound to be extruded
Cable Diameter and Plastic thickness
Roundness required and adherence on the cable
The pressure on the cable can be even reduced by using the “Special Tip” with 2 angles for low pressure.
The semi-pressure tools can be normally used for:
SHEATHING (preferred: tube & semi-tube)
The Die diameter “D” and land length “X” change in according with the compound processed, Process, surface aspect.
The tube tools are normally used for sheathing process, but can be even used for insulation or jacketing process:
Insulation on Sector conductor
Insulation or Jacketing in FEP, ETFE, X-ETFE, Etc.
Insulation of XLPE Monosil o Visico (Lands with 2 angles)
Insulation or jacketing in PA
HDPE (Semi-Tube Tools are preferred)
HFFR, LS0H (Semi-Tube Tools are preferred)
The Tube tools must be calculated according to:
Extrusion Process (Sheathing, Insulation, etc.)
Cable Construction and dimensions
The tube tools must be calculated in according with:
The DDR, Draw Down Ratio is the ratio between the speed of the Line and speed of plastic through the tools.
DDR = C2/C1 = S1/S2 = [D2 – T2]/ [F2 – C2]
DRB = D*C/F*T
C1: Speed of compound through the toolsS1: Section of plastic passage through the toolsC2: Line SpeedS2: Section of Insulation on the cable
The tube tools can even be realized with not horizontal final land.
This particular shape is generating, in according with relative position between tip and die, a bit more pressure and shorter cone.
0.94 < DRB < 1.1
1.15 < DDR < 200
* According to the compound to be processed, Cable Dimension, plastic thickness and line Speed.
Wrong DDR Values, even according to the production Speed, can generate:
Loss of concentricity, cable ovalization, etc.
Loss of transparency and of cable brightness.
Cone breakage, Cable Surface not shine, Defects on the cable surface, etc.
SHEATHING – JACKETING
TPE / HYTREL
The semi-Tube tools, as we have already explained on the previous articles, are particular tube tools where the tip has not the final cylindrical land; the tip and die have normally one angle only. They must be calculated according to the DDR and DRB value set.
This shape can be mainly used for sheathing or insulation process on conductor that have not a cylindrical section:
HFFR, LS 0H Sheathing
The advantages of the semi tube tools are:
Lower Die Drools
Shorter Cone Length
More Brilliant surface
No Ring or Vibration Effect on HFFR Cable Surface.
The disadvantages are:
If the Relative position “X” changes, the DDR and DRB is consequently changing