The plastic materials are Non-Newtonian material and they are defined Pseudo-Newtonian materials. They are considered Viscoelastic materials. This can be explained by Kelvin and Maxwell model so by system of springs and viscous dumpers.
The viscoelasticity of the polymers plays a fundamental role in the extrusion operation and process:
- Die Drool;
- Die Swell;
- Pressure and Melt Temperature;
- Negative pressure that can accelerate the die drool and die swell formation;
- Surface quality of the cable, tube, pipe, etc. such as smoothness or roughness, melt fracture aspect, shark skin, orange surface effect, etc.;
- Head Tooling type (Tube, Semi-tube, Pressure, Semi-Pressure, Special, etc.);
- Die dimensions in terms of: land length, land angle, die diameter, angle, DDR-DRB for tube or semi-tube tools, etc.
The die drool as well as the elastic behaviour must be reduced, or eliminated when/if it is possible, in order to reduce or avoid:
- Opaqueness and roughness of the cable surface
- Lump on the cable due to the drool rupture.
The die swell around the die mainly depends on the viscoelastic behaviour of the polymers; we can say that as the material is getting into the die, through the extrusion head, the macromolecules are stretched on the flow direction and a force orthogonal to the flow direction is applied on the material.
As the material is getting out from the die this force is suddenly missing and the material is relaxing expanding its volume.
Moreover, higher is the elastic effect than the viscous effect and bigger is the die Swell effect and consequently the Die Drool effect.
For example, we can consider 2 dies with different land length. On the die having longer cylindrical land the material has more time to follow the flow direction, to stretch the molecules and relaxes. It means that to the die exit the polymer will have higher viscous behaviour against the elastic behaviour and it means lower Die Swell.
The die land length as well as the die shape, with stable shear rate, stable extruder output and melt temperature, is acting on:
- Viscoelastic Behaviour of the polymer;
- Pressure level and consequently shear stress value into the tool rooms;
- Quality of surface of the cable. The quality depends on other factors as well, such as: die diameter, tools design and type, extruder and head processing parameters, later analyzed.
The Die Drool lumps on the cable are connected, even if not strictly, with material swell, and their creation depends on many other factors:
1. Compound characteristics
The characteristics of the polymers processed, in terms of its chemical formulation: Additives contained, Molecular weight, macro-molecules length, density, etc.
The die drool occurs on the die exit, where a negative pressure is generating.
The negative pressure acts as “Driving Force” for the drool creation and, combined with die shape, can create different type of drool.
Moreover, higher is the incompatibility between Polymer and additives, higher is the probability of their separation. The incompatible additives can migrate outside from the compound and starting the formation of the drool.
In the same way the molecular weight value, its distribution as well as the molecules length act on the viscoelastic behaviour and then on the die drool and die swell.
2. Back pressure
The pressure value acts on the viscoelastic effect. The pressures to be considered are:
- Extrusion head, its distributor;
- Tool chamber (material passage);
- Extruder Back-Pressure.
3. Line speed
By increasing the line speed and consequently the extruder Output/Rpm, the back pressure is increasing, even if not linearly, and consequently the shear stress increases.
As the shear stress and speed rate are achieving certain values they can generate an irregular cable surface, like the “Melt Fracture effect” and a not shine cable surface that shall facilitate the die droll, lump and die swell formation.
4. Extrusion process mode and tools design
Sheathing, Filling, Insulation, Jacketing, etc. i.e. whether tube tools or semi-tube tools instead of compression or semi-compression tools.
5. Extruder output and screw RPM
The screw rpm defines the peripheral speed given to the material and its speed rate.
The Material Speed and Shear rate act on the “melt fracture phenomenon” and then on the cable quality, mainly for speed and shear rate sensitive compounds.
The combination of “Extruder Output- Screw Rpm” and then the value “dm3/rpm”, for many compounds (XLPE, XLPE-FR or HFX, HFFR, Etc.) acts on:
- Die Drool;
- Lumps on the cable;
- Residence time into the extruder, compound Overheating, pre-reticulation, etc.
Moreover, considering 2 extruders that give the same output [lit/h], one that works at lower rpm, gives to the compound:
- Higher Viscous behaviour
- Lower Peripheral speed of the material into the extruder
- Longer residence time of the compound into the extruder, which can act on the quality of cable mainly for cross linkable compound. Moreover, the residence time will define the minimum rpm to process Cross-linkable materials avoiding pre-crosslinking.
- Lower probability of melt fracture
- Lower probability of Lumps on the cable
6. Die temperature
Reducing the die temperature, for many polymers, and of course when allowed, the die drool and consequently the lump on the cable are reduced (with stable Extruder Output and screw rpm).
The Lumps creation is even connected to the extruder output, screw rpm (extruder peripheral speed), Temperature profile and temperature of the compound to the exit.
Moreover, for many compounds such as XLPE-FR, XLPE cross-linkable by steam (Automotive cables, etc), etc., lower extruder temperature profile, lower and controlled die temperature as well as lower melt temperature is reducing the Die Drool and lump effect. Too low Extruder Temperature Profile and/or Melt temperature can generate other problems on the cable (Low Adhesion on the conductor, etc.).
Of course, reducing the die temperature we could get:
- Higher back pressure and consequently higher friction between polymer and tools and then higher shear stress;
- More opaqueness of the cable surface.
7. Die diameter for tube and semi-tube mode
By Increasing the die diameter, keeping constant other parameters such as extruder output, line speed, etc., we are consequently increasing the DDR value. This consequently reduces the speed of the compound through the tools. It helps on the Die Swell formation, because the plastic needs more time to cross the tools and this increases its viscous component against the elastic one.
Too low DDR can act on the insulation aspect like the opacity or transparency (FEP, PVDF, HDPE, Etc.)
High DDR values can even act on:
- Higher transparency (FEP, ETFE, PVDF, LLDPE, Etc.);
- Less opacity;
- Lower Material Speed Rate through the tools, shifting the Melt Fracture effect on higher line speed values.
The DDR has to be chosen in according with the compound to be processed, cable dimensions, thickness of plastic and line speed (higher is the line speed and higher could be the DDR); too high DDR Value can generate:
- Longer Cone of the Plastic;
- Eventual Loss of Cable Concentricity;
- Higher Material Stretching/Elongation.
8. Die diameters for pressure and semi-pressure tools
By reducing the die diameter, it will increase the back pressure and the compound speed as well on the wall of the die. This will consequently reduce the elastic behaviour of the polymer.
In few word, reducing the die diameter comparing with the cable diameter, the cable will be shiner and with low powder emission around the die (Type of powdered drool).
Considering the HFFR-LS 0H material, defining with “C” the final cable diameter and “D” the die diameter, we can write:
|D = C – K||0,05 < K* < 0,20mm|
* The length of the die land act on the choice of K value and on the roundness of the surface.
The “D” or “K” values biunivocally should be chosen even according to:
- Line Speed, Extruder Output during the production, Rpm and then Consequently Back Pressure;
- Type of Compound and its additives;
- Conductor Construction, Conductor dimension and Cable Dimensions;
- Distance between Tip and die and then pressure on the conductor, its eventual elongation, etc.;
- Plastic adhesion, Cable peeling force, etc.
The Die Swell and Die Drool, and then the Viscoelastic behaviour, depends, on the physical point of view, on:
- Physical Property of the compound (Density, Hardness, etc.) and additives used;
- Viscosity e its variation with rheological parameters (Temperature, Pressure, Speed);
- Back Pressure;
- Line Speed and the ratio Output/Rpm;
- Extrusion Process Mode (Insulation, Sheathing, Filling, etc.);
- Extruder Output and Screw Rpm (Shear Rate, Shear Stress, Peripheral Speed, etc.);
- Tools Design: Dimensions, angle, final land length, die shape and channel design for material passage;
- Die diameter for Tube (DDR), semi-tube, semi-pressure, Pressure Mode, etc.;
- Die temperature and Melt temperature;
- Polymer cooling and re-crystallization and its viscoelastic behavior.
The die droll, if present during the production, can be eliminated or reduced by using:
- Flame/Burner on the die (According with UE Role, Country/Special role and Internal role) which will increase the brightness of the cable surface as well.
- Compressed air Flow around the die. Working mainly for powdered Drool (XLPE Monosil Process, some type of HFFR, etc).
Obviously, the type and design of tools will help. For HDPE, HFFR Semi-Tube Tools or Special Tools are preferred instead of Tube tools.
9. Semi-tube and tube tools design
The tools used can be classified in different modes, such as:
- Pressure Tools;
- Semi-Pressure Tools;
- Tube Tools;
- Semi-Tube Tools, single or multi angle;
- Special Tools, etc.
The Tools type must be chosen even according to:
- Process: Sheathing, Insulation, Filling, jacketing, Tandem, Coextrusion, etc.;
- Compound to be processed, adhesion required on the cable;
- Type of Cable, design, geometry, etc.
The design of tools, their choice, their process factors and calculation will be faced on other articles, for the moment we are briefly explaining the tube and semi-tube tools.
10. Tube tools
The die Dimension and outer Tip diameter are defined according to the DDR and DRB, thickness of tip land, as well as its length.
As we have analysed, the DDR and Land Length (nozzle), etc., act on the viscoelastic behaviour, on the cable surface, etc.
For LS 0H compounds, the tip land length should be shorter than 4 mm, if tube tools must be used. In Fact, many compounds, such as HFFR, LS 0H, XLPE FR, etc., Semi-Tube Tools are preferred to be used.
11. Semi-tube tools
The tip, even with 2 angles, dosen’t have the cylindrical nozzle. The die can have a land length of 0.2-1.0 mm.
This semi-tube shape reduces the cone length and the plastic flow direction and cable direction are not parallel as well as the plastic direction and die land direction, with all advantages.
The Die/Tip position has to be settled. On the basis of their relative position “x”, the DDR and DRB, the length of the cone, Inner drool, the adhesion and shape of the compound on the cable change consequently.
For many compounds, this shape, if properly calculated, can help to:
- Increase the brightness of the cable surface (HDPE, LLDPE, etc.);
- Reduce the “Ring Effect” or “Vibration Effect” on the big cable surface, mostly for LS 0H processing;
- Reduce External Drool, etc.;
- According to the inner tip diameter value, the Inner drool can be even reduced.