On fibre optic loose-tube cable production lines, Borealis HDPE HE3497-LS is extruded as an outer sheath directly over aramid yarn layers, water-swellable tapes, ripcords, and central strength members. The compound is supplied pre-compounded with carbon black; adding a separate black masterbatch is generally unnecessary and can reduce dispersion quality if the screw is not designed for masterbatch let-down. A single-screw extruder with a grooved feed section, 25:1 to 30:1 L/D, and a barrier screw with 3:1 compression ratio is typical for this jacket. Barrel zones from feed to die are set in a rising profile from 180 °C to 220 °C; melt temperature measured at the adapter is kept below 225 °C. Exceeding 230 °C for more than 30 min raises the carbonyl index and produces micro-roughness on the cooled jacket surface. The die is normally a pressure-type tube-on die with die land length to annular gap ratio above 10:1; draw-down ratio is maintained between 1.1:1 and 1.5:1 to limit frozen-in orientation. Cooling is carried out in a segmented water trough with first-stage water temperature held between 25 °C and 40 °C; rapid quenching below 20 °C increases free shrinkage. Shrinkback after 24 h at 100 °C is an acceptance criterion in many fibre optic cable specifications; depending on cable construction, the sheath movement is normally specified as less than 1% relative to a stripped cable end. Carbon black dispersion is checked according to ISO 18553:2002; poor dispersion appears as black specks in thin jacket sections. For outdoor installation, qualification for environmental stress cracking resistance is usually performed according to ASTM D1693-15 condition A or B using 10% Igepal CO-630 at 50 °C. Cable lubricants based on glycol or mineral oil require the jacket to be tested after chemical exposure; published data for this specific configuration is limited and should be verified per lot. If the cable is installed by blowing into duct, the jacket surface friction matters; HE3497-LS can be co-extruded with a thin lubricant-rich skin or surface-treated after extrusion, but the base compound itself is not a dedicated low-friction product. When the same jacket is considered for riser-rated indoor fibre optic cables, flame performance remains outside the base grade’s capability, and the addition of halogen-free flame-retardant fillers is not recommended without full revalidation because melt viscosity and ESCR balance shift simultaneously.

What Prevents Low-Temperature Jacket Cracking in Aerial Copper Drop Cables?

Aerial self-supporting copper drop cables impose a different stress field on the jacket than buried cable. The jacket is extruded over a corrugated or smooth aluminium shield that is bonded with an adhesive copolymer; the HDPE must maintain mechanical integrity without losing tensile elongation after thermal ageing. HE3497-LS is selected in these constructions because the bimodal molecular weight distribution provides a combination of slow crack growth resistance and stiffness under wind-induced cyclic bending. The relevant test for cold impact is not a single elongation number; the sheath is subjected to low-temperature impact at -20 °C or -40 °C in accordance with IEC 60811-506, with no visible cracks and no dielectric failure. ESCR performance under aerial conditions is evaluated after the jacket has been wrapped around a mandrel and exposed to a weathering solution; the test duration depends on the regional cable standard and is not fixed by the resin supplier. At the processing level, the main failure mode observed on production lines is screw slip in the feed zone when regrind levels exceed 15%. The grooved feed section of the extruder can generate feed pressures above 400 bar with virgin compound, but adding fluff or low-bulk-density regrind disturbs this compaction and causes melt-pressure oscillation. This oscillation is visible as periodic jacket wall-thickness variation, particularly at cable line speeds above 60 m/min. To maintain a stable melt cushion, the hopper loader should maintain a consistent resin level and the feed-throat temperature should not exceed 50 °C; otherwise pellet bridging occurs. The addition of colour masterbatch to convert the black compound to another colour is not recommended because the carbon black already present limits the achievable colour. If a non-black jacket is required, a different grade should be used. Jacket thickness for aerial copper drop cables is commonly in the range 0.8 mm to 1.2 mm; thin spots below 0.6 mm are a rejection criterion because they reduce the time to slow crack growth at cable attachment points. Thermal ageing at 100 °C for 7 days should not reduce elongation at break by more than 25% from the unaged value when tested according to ISO 527-2:2012. Published data for the exact retention of HE3497-LS in this configuration is limited; the value depends on the antioxidant package and carbon black dispersion.

Low-Voltage Power Cable Sheath Adhesion and Fill Ratio Limits

For low-voltage power cables built to IEC 60502-1, the outer HDPE sheath is not the primary insulation; it protects the underlying PVC bedding or XLPE insulation from moisture, abrasion, and sunlight. HE3497-LS can be used as an outer sheath only when the maximum conductor operating temperature does not exceed 70 °C, because HDPE has a low thermal softening point relative to XLPE. Short-circuit conductor temperatures above 100 °C can soften the sheath locally where it touches metal tape or armour and can lead to indentation and permanent deformation. The sheath is normally applied using a tube-on extrusion process so that the HDPE does not penetrate the interstices of the underlying layer; pressure extrusion would increase adhesion but also make stripping more difficult. Strippability is a key field installation requirement; the sheath removal force is measured on a 200 mm cable sample at 23 °C and should remain below the maximum specified in IEC 60811-100 or the cable purchaser’s specification. If the sheath is applied directly over corrugated steel armour, a flooding compound may be used to prevent water migration; this flooding compound must be compatible with the HDPE and must not contain high levels of aromatic oil that reduce ESCR. In low-voltage aerial bundled cables, the sheath is exposed to direct sunlight; UV stabilisation is provided by the carbon black dispersion in the compound, and carbon black content is controlled between 2.0% and 3.0% by mass according to ISO 6964:2019. Dispersion should be assessed by ISO 18553:2002; agglomerates create local stress concentrations that can start cracks under cyclic vibration from wind or traffic. The compound should not be blended with high-density PE pipe regrind or other waste streams as a cost-saving measure, because the molecular weight distribution and stabiliser package are changed unpredictably. That practice may pass a tensile test but fail ESCR after several months of field exposure. In production, the water trough length for power cable sheathing should be sufficient to cool the cable below 60 °C before coiling; otherwise the sheath deforms on the reel due to residual heat and the cable may block. Published processing data for this specific grade in power-cable sheath construction is limited; the use is more common in telecom and fibre optic cables.

In microduct extrusion lines, HE3497-LS is processed as a thin-wall tube with an outer diameter of 3.0 mm to 16.0 mm and a wall thickness from 0.3 mm to 1.0 mm, depending on the fibre count and blowing distance. The screw configuration is critical for thin-wall stability: a single-screw extruder with a 24:1 to 30:1 L/D barrier screw, a screen pack of 40/60/80 mesh, and a spiral annular die with a die gap of 0.8 mm to 1.2 mm is used. Melt temperature is held at 200 °C to 220 °C; lower melt temperature increases die pressure and may create melt fracture, while higher melt temperature degrades the polymer and increases ovality after vacuum calibration. The tube is calibrated in a vacuum tank with first-stage water at 25 °C to 35 °C; the vacuum level is adjusted to control outer diameter without collapsing the tube. The main production defect is wall-thickness eccentricity, which is caused by uneven die temperature distribution or insufficient melt distribution in the spiral mandrel. A nitrogen purge through the inside of the tube during start-up prevents collapse until the tube enters the vacuum tank; after calibration, the line speed can be raised to 30–80 m/min for small-diameter microducts. Because the duct may be installed by cable blowing, the inner surface must have a low friction coefficient; a co-extruded slip layer containing a migrating slip agent is often applied on the inner wall. HE3497-LS itself provides a stable outer wall and crush resistance, but the slip layer becomes part of the final microduct structure and must be compatible with the HDPE melt. Crush resistance is evaluated by a parallel-plate compression test at 23 °C; the duct must not crack at 25% deformation in typical microduct specifications. No single universal test standard governs all microducts; regional specifications may refer to IEC 61386 for conduits or to operator-specific blow-in tests. During blow-in tests, the duct is subjected to air pressure of 10 bar to 15 bar and a high-speed air flow; internal surface roughness and dimensional stability are more important than tensile strength. The carbon black content provides outdoor UV resistance; however, if microduct is installed inside an existing occupied duct, UV resistance is less important and the carbon black dispersion still contributes to melt homogeneity. Field failures often occur at joints where the microduct has been crushed or kinked during installation; the HDPE outer wall should resist kinking at a bend radius below 10 times the outer diameter.

When Hybrid Fibre-Copper Cables Require a Single Common Jacket

When hybrid fibre-copper cables combine loose-tube fibre optic elements with copper power pairs or coaxial elements in a single cable core, the outer jacket must satisfy two sometimes conflicting requirements: low shrinkback for the optical elements and adequate crush resistance for the copper elements. HE3497-LS is used in such constructions because its high molecular weight and carbon black stabilisation give the jacket enough modulus to prevent the copper conductor bundle from printing through the outer surface without making the jacket so stiff that the optical fibres are bent during cold installation. The cable core is normally filled or flooded; the fill compound must be selected for compatibility with HDPE. High aromatic mineral oils can plasticise the HDPE jacket over time and reduce tensile strength; qualification includes an oil immersion test at 70 °C for 7 days followed by tensile testing according to ISO 527-1:2019. A loss in tensile strength greater than 20% after oil ageing is considered a red flag in cable development. The jacket is usually applied with a tube-on die because the core has an irregular profile; pressure extrusion would force HDPE into the gaps between the optical tubes and the copper pairs, creating hard spots that pinch the optical fibres during bending. The minimum jacket thickness is set by the cable standard and the diameter over the core, but values below 0.8 mm are rarely used for hybrid cables due to the risk of copper print-through. During extrusion, the lay length of the copper pairs and the binding tape tension must be controlled so that the core diameter variation at the crosshead is below 0.2 mm; larger variation causes cyclical changes in wall thickness. Because hybrid cables are often pulled through conduits, the jacket is tested for abrasion resistance by a reciprocating abrasion test; the exact test method depends on the operating company specification and no single ISO test applies. Published data for the abrasion loss of HE3497-LS in hybrid constructions is limited; design verification should include field-pull trials rather than relying on laboratory test coupons alone.

Coaxial Feeder Lines Benefit from a Two-Layer HDPE Jacket Structure

Coaxial feeder cables for base stations and broadcast installations use a foam-skin polyethylene insulation over a copper or copper-clad aluminium inner conductor. In some designs the outer HDPE jacket is co-extruded as a two-layer system: a thin pigmented skin for identification is laid over the main HE3497-LS jacket body. The base compound forms the structural layer over the corrugated copper or aluminium outer conductor. The jacket is applied at a total thickness of 0.8 mm to 1.5 mm, depending on cable diameter, using a tube-on die that leaves a small air gap between the jacket and the outer conductor. This air gap allows the jacket to expand and contract independently of the metal tape, reducing the risk of jacket cracking at low temperature and preventing the jacket from being punctured by the corrugation peaks during bending. The plain HDPE layer is not bonded to the outer conductor; if bonded jacket construction is required, a different co-extruded adhesive layer must be used because plain HDPE adhesion to aluminium or copper is poor. The co-extruded skin must be selected from a compatible polyolefin with a lower melt temperature than the base HDPE; processing at the same die temperature can cause the skin to degrade if the temperature is above 220 °C. The main HE3497-LS layer provides mechanical strength and UV resistance; the skin layer is too thin to contribute meaningfully to crush resistance and is used only for marking or colour coding. During processing, the corrugated outer conductor enters the crosshead from a payoff arrangement; the line speed is often set between 20 m/min and 50 m/min, and the extruder screw speed is adjusted to maintain a die-head pressure of 150 bar to 250 bar. Die-head pressure above 300 bar indicates excessive screen blockage or melt temperature that is too low. The jacket is cooled in a water trough with a first-stage temperature of 30 °C to 40 °C; rapid cooling below 20 °C can create radial thermal residual stress that causes the jacket to split when the cable is bent at low temperature. Low-temperature bend testing is performed by conditioning a cable sample at -20 °C for 4 h and bending it around a mandrel of 10 times the cable diameter without cracking. The HDPE jacket is not flame-retardant; where riser or plenum flame ratings are required, the base grade must not be used unless a fire-resistant barrier tape is added to the cable core, and the jacket itself still contributes fuel load in vertical burn tests. The carbon black in the base layer provides UV resistance and weathering stability, but the black surface temperature can exceed 70 °C in direct sunlight in some regions; this reduces the mechanical safety margin and should be considered when specifying maximum conductor temperature.