What governs extrusion stability when Borstar LE8707 replaces unimodal LLDPE in 6–36 kV distribution cable outer sheaths?

Borstar LE8707 is specified as the outer sheath compound for XLPE-insulated, copper-wire-screened cables with nominal voltages of 6/10 kV, 12/20 kV, and 18/30 kV when the sheath is not required to provide flame retardancy. The applicable product standard is IEC 60502-2:2014 in IEC markets and HD 620 S2 in European distribution practice; material acceptance tests are carried out under IEC 60811-501:2012 for tensile and elongation, IEC 60811-401:2012 for thermal ageing, and ISO 1133-1:2022 for melt flow rate. At the formulation stage, the compound is charged at 100 wt% as-received granules; no additional carbon black masterbatch or silane crosslinking system is required, and rework of clean in-house jacket scrap is limited to 0–5 wt% to avoid shifting viscosity and die-lip behaviour. In downstream extrusion, a single-screw jacketing line with screw diameter 60–150 mm, L/D 25:1–30:1, and compression ratio 2.5:1–3.5:1 is used; a 80/120/80 mesh screen pack is positioned ahead of a pressure-compensated tube-on die. Barrel temperatures are set from 150–170 °C at the feed zone up to 200–215 °C at the die, with melt temperature held at 190–215 °C. The bimodal molar mass distribution suppresses drawdown sag on thick sheath walls of 1.8–4.5 mm, but the processing window is narrower than for unimodal LLDPE: a melt temperature above 215 °C for more than 10–15 min can generate thermo-oxidative degradation and die-lip plate-out, while insufficient screw shear below 185 °C leaves carbon black agglomerates visible as surface die lines. Production-scale failure modes include pinholes from residual moisture adsorbed onto the black compound surface when stored in unheated warehouses, and layer thickness oscillation when the extruder is operated below 40% of its maximum screw speed with very high compression screw geometries. The finished construction is an underground or ducted medium-voltage energy cable with an XLPE insulation layer, semiconductive screens, copper wire or tape screen, and a black polyethylene outer sheath that is suitable for direct burial in soil where the sheath surface is not exposed to continuous conductor temperatures above 80 °C.

In low-voltage energy cable manufacture governed by IEC 60502-1:2021, the outer sheath is extruded at 0.8–1.8 mm wall thickness over a polyvinyl chloride or halogen-free bedding layer, and Borstar LE8707 is added at 100 wt% as the sole jacket polymer; clean factory regrind from the same grade is metered at 5 wt% maximum, and no flame-retardant additive is used because the compound is intended for outdoor or ducted runs where reaction-to-fire classification under EN 50575:2014+A1:2016 is not the qualifying criterion. The production line employs a single-screw extruder with L/D 20:1–28:1, a low- to medium-shear screw with compression ratio 2.0:1–3.0:1, and 60/80/60 mesh filtering before the die; melt temperatures are maintained at 180–210 °C, and line speeds of 30–150 m/min are typical depending on sheath thickness and downstream water bath length. At these settings, the compound enters a straight-through die and is drawn onto the cable core after an air gap of 0.5–1.5 m; inaccurate centring at this point produces radial eccentricity that is detected by in-line X-ray diameter gauges. Material qualification for this construction is anchored to ISO 527-2:2012 tensile testing, ISO 868:2003 Shore hardness, and IEC 60811-401:2012 thermal ageing, while the melt flow rate is controlled by ISO 1133-1:2022 to ensure batch-to-batch viscosity is within the specified envelope. The finished product is a 0.6/1 kV unarmoured or galvanized-steel wire armoured cable used for outdoor site distribution, street lighting, pump stations, and temporary power supply; the sheath is not a primary insulation layer and must not be exposed to continuous surface temperatures above 80 °C.

Underground and aerial fibre optic cable sheathing demands: ESCR, UV weathering, and low post-extrusion shrinkage

Outdoor fibre optic cable jackets are exposed to installation lubricants, water-blocking gels, and soil-borne surfactants; the high-molecular-weight component of Borstar LE8707 is relied upon to resist slow crack growth in these environments. Product compliance is assessed under IEC 60794-1-1:2015 for generic cable requirements and Telcordia GR-20-CORE Issue 4 for telecommunications fibre outside plant; the material-specific tests include ASTM D1693-15 environmental stress crack resistance in 10% Igepal CO-630, ISO 4892-2:2013 xenon-arc weathering, and IEC 60811-501:2012 tensile testing. The compound is dosed at 100 wt% as the outer sheath layer, with clean rework limited to 0–5 wt% and no external processing aids; a silicone-based slip agent is not necessary for typical ducted pulls, but dry-film lubricants may be applied downstream if a low-friction installation is specified. The cable core—central loose tube, stranded loose tube, or ribbon stack—is jacketed on a rotating head extruder with L/D 24:1–28:1, compression ratio 2.0:1–3.0:1, and melt temperature 180–200 °C. Thin-wall die design and a 60/80/60 mesh screen pack are used to produce sheath walls of 0.8–1.5 mm without fibre PMD degradation because the buffer layer remains below 60 °C. Post-extrusion shrinkage is controlled by setting the first water trough temperature to 30–45 °C and the second to 15–25 °C; too aggressive cooling at 10 °C can freeze-in orientation that later relaxes in outdoor heat and produces jacket retraction at cable ends. The terminal finished products are outdoor fibre optic cables for direct burial, ducting, aerial lashed or self-supporting installations, and optical ground wire jacketing where high UV endurance and low-temperature impact are required.

For utility-scale photovoltaic and onshore wind balance-of-plant cable runs, the sheath layer is exposed to sustained ultraviolet irradiance, sand abrasion, and temperature swings from −30 °C to +70 °C, so the material is processed at 185–205 °C on a single-screw jacket line with L/D 24:1–28:1 and a compression ratio 2.0:1–3.0:1 to produce a 1.2–2.0 mm sheath over XLPE insulation and aluminium tape or copper wire screens. The product standard for these AC collection and tracker/tower auxiliary circuits is IEC 60502-1:2021, with material qualification under IEC 60811-501:2012 for tensile, IEC 60811-505:2012 for low-temperature impact, and ISO 4892-2:2013 for weathering; the compound is charged at 100 wt%, and only 0–5 wt% of clean in-house rework is permitted in the hopper blend. In practice, production batches fed at high ambient humidity may develop pinholes if condensed moisture is carried into the compression zone, so the granulate is pre-dried at 50–60 °C for 2–3 h when pellet surface condensation is visible or when relative humidity exceeds 70%. Borstar LE8707 is not a substitute for the crosslinked halogen-free sheath compounds required by EN 50618:2014 for direct-current photovoltaic string cables; its use in solar installations is limited to AC collection, inverter output, tracker power, and balance-of-plant circuits where the cable product standard does not require thermoset sheath performance. The finished product is an outdoor-rated multicore or single-core energy cable with a black polyethylene sheath that retains tensile elongation and low-temperature ductility after prolonged field exposure in desert and coastal solar farms.

When sub-zero installation conditions dominate the qualification of industrial tray cable jackets

Industrial tray cable installations in cold-storage, surface mining, and arctic oilfield service require the sheath to be pulled through steel tray supports at temperatures at which conventional LDPE exhibits brittle fracture; LE8707 is therefore evaluated with IEC 60811-505:2012 low-temperature impact and IEC 60811-504:2012 low-temperature elongation rather than ambient tensile strength alone. In IEC markets, the cable product standard remains IEC 60502-1:2021 for 0.6/1 kV construction, while North American evaluations reference UL 1277 Type TC and the test methods of UL 1581; the jacket is extruded at 1.0–2.4 mm thickness on a single-screw machine with L/D 25:1–30:1, compression ratio 2.5:1–3.5:1, and 80/120/80 mesh screening. The formulation ratio is 100 wt% as-supplied Borstar LE8707, with 0–5 wt% clean rework; no low-temperature impact modifier is added because the bimodal comonomer distribution itself contributes the required ductility. A key process conflict occurs at high line speed: increasing melt temperature to 215–230 °C improves die-lip surface finish and reduces extrusion head pressure, but it also raises the risk of thermo-oxidative chain scission in the high-molecular-weight tail; this can produce a measurable drop in environmental stress crack resistance, so the alarm limit on melt temperature is set at 215 °C and residence time at melt is kept below 10–15 min. Published data for the exact loss gradient of this grade at elevated residence time in high-compression barrier screws is limited, so line trials are required before locking the temperature profile. At the downstream side, the cable enters a gradient water trough at 20–40 °C followed by a second trough at 10–20 °C; quenching the sheath below 10 °C is avoided because frozen-in orientation reduces low-temperature impact resistance in subsequent bending. Finished products are tray and control cables for industrial machinery, cold-weather pumping stations, and outdoor instrumentation loops where installers require a jacket that does not crack at −40 °C during winter cable pulls.

Where thin-wall high-speed jacketing is specified for outdoor microcable and last-mile optical drop cable constructions, the extruder is configured with a 20:1–25:1 L/D screw, a 2.0:1–2.5:1 compression ratio, and a 50/70/50 mesh screen pack; Borstar LE8707 is metered at 100 wt%, and no separate slip additive is required for tube-on processing at 180–205 °C. The applicable cable family specification is within IEC 60794-1-1:2015 and Telcordia GR-20-CORE Issue 4, while material acceptance is based on IEC 60811-501:2012 tensile testing, ISO 1133-1:2022 melt flow rate, and ASTM D1693-15 environmental stress crack resistance. The sheath wall is held at 0.5–0.8 mm; this thin section places a premium on melt filtration, because a single gel or agglomerate above 250 µm in diameter can rupture the wall during blown-fibre installation or aerial cable sway. Line speeds of 100–300 m/min are used with an air gap below 1 m and a two-stage water trough at 25–40 °C followed by 15–25 °C; eccentricity is monitored with in-line laser diameter gauges, and jacket ovality above 0.05 mm is rejected because it causes air leakage in blown microduct systems. The terminal constructions are outdoor microcables and fibre-to-the-home drop cables that are installed by air blowing, direct burial, or lashed aerial deployment; the black sheath provides ultraviolet protection without the addition of a separate UV masterbatch.