Blown film lines processing LDPE for heavy-duty sacks and industrial liners operate within a narrow bubble stability window defined by melt strength, die pressure, and the temperature gradient from the die lip to the frostline. Grades selected for this sector typically exhibit a melt index of 0.5–2.0 g/10 min when measured per ISO 1133-1:2022 at 190 °C/2.16 kg and a density of 0.918–0.925 g/cm³ per ISO 1183-1:2019. On a 45 mm grooved-feed extruder with an L/D ratio of 30:1 and a dual-lip air ring, barrel temperatures are profiled from 150 °C in the feed zone to 180 °C at the die, while the die temperature is maintained at 165–175 °C to stabilise the bubble. Blow-up ratios between 2.2:1 and 3.0:1 are standard for sacks requiring balanced machine-direction and transverse-direction tensile properties; die gap settings of 1.2–2.0 mm are used to control orientation. Frostline height is set between 400 mm and 800 mm depending on cooling demand. The long-chain branching in autoclave LDPE provides higher melt tension than linear low-density polyethylene at equivalent melt index, allowing the bubble to withstand high-speed haul-off without draw resonance. Surface treatment and additive let-down are specified from the converter end. The addition of 0.5–1.5 wt% slip agent and 0.2–0.8 wt% antiblock concentrate prevents blocking after 4–8 weeks of storage without dropping dart impact below the converter specification. Dart impact is measured per ASTM D1709-22 Method A or ISO 7765-1:2004. Surface tension after corona treatment reaches 38–42 mN/m for flexographic or rotogravure printing, measured per ISO 8296:2003. Film line output is limited by bubble cooling capacity rather than screw speed; a 45 mm line running 120 kg/h at 2.5:1 blow-up ratio and 50–70 µm gauge is common, but output varies with die diameter and air ring design. Published data for specific production-scale configurations is limited because converter conditions are proprietary.

Extrusion Coating Adhesion and Neck-In Control on Liquid-Packaging Board

In liquid-packaging board converting, LDPE is applied as a heat-seal layer or barrier tie layer using autoclave grades with melt indices of 4.0–8.0 g/10 min and densities of 0.915–0.923 g/cm³. Melt temperature at the slot die is maintained between 290 °C and 320 °C; this elevated temperature promotes chemical adhesion to aluminium foil and improves wettability on paperboard surfaces. Neck-in is the limiting variable in this process, with typical neck-in values of 25–60 mm per side on a 600 mm die depending on die gap, air gap, and melt temperature. Coating weight is controlled via substrate speed and extruder throughput; liquid-packaging structures commonly use 12–25 g/m² for barrier layers and 8–15 g/m² for heat-seal layers. Adhesion is measured on 15 mm strips per ASTM D1876-08 or ISO 8510-2:2006, with converter specifications often requiring fibre tear on paper and a minimum peel force of 2.0–6.0 N/15 mm on foil-based structures. Ozone treatment of the melt curtain at 0.06–0.12 kW/m² increases oxygen-containing functional groups and improves adhesion on paper. The chill roll finish controls gloss and back-side roughness; matte finishes with Ra 6–10 µm are used for downstream lamination, while gloss finishes with Ra <2 µm are required for high-print-quality surfaces.

Processing conflict arises when melt index is adjusted for draw-down. Low-MI resins reduce neck-in but are more susceptible to draw resonance and edge tearing at line speeds above 300 m/min. High-MI resins improve draw-down but increase penetration into porous paper and reduce barrier uniformity. The converter therefore sets the die gap at 0.6–0.9 mm, the air gap at 100–250 mm, and the chill roll temperature at 10–18 °C; these parameters are interdependent and cannot be optimised independently. When aluminium foil is the substrate, the oxide layer must be free of water stains and rolling oil, because adhesion failure occurs cohesively in the LDPE at peel angles above 90°. For food-contact applications, the coated board must comply with FDA 21 CFR 177.1520 and EU Regulation 10/2011, including specific migration limits for any added slip agents or processing aids.

What Governs Low-Temperature Impact and Warpage in LDPE Closures?

Injection-moulded LDPE closures and overcaps are produced from grades with melt indices of 10–70 g/10 min and densities of 0.917–0.924 g/cm³. The polymer is plasticised in a 25–40 mm screw with a compression ratio of 2.5:1–3.5:1 and a non-return valve; melt temperatures are set at 180–240 °C. Clamp force is calculated on projected area, with LDPE requiring 2.0–4.0 kN/cm², and vented moulds are seldom necessary because moisture absorption is <0.01 % per ISO 62:2008. Injection pressure typically falls between 60 MPa and 120 MPa, with holding pressure at 40–70 MPa and holding time of 1.5–3.0 s per mm of wall thickness. Shrinkage after 48 h is 1.5–3.0 % in the flow direction and 1.0–2.5 % transverse, measured per ASTM D955-08. Warpage in closures is driven mainly by differential shrinkage between thick hinge sections and thin membrane areas; this is mitigated by balanced gate placement and by increasing the melt index from 10 g/10 min to 35 g/10 min, which reduces shear-induced orientation but also lowers environmental stress crack resistance.

ESCR is the principal limitation for high-MI closure grades. Measured per ASTM D1693-15 Condition B in 10 % Igepal CO-630, a 35 g/10 min LDPE may show F50 values in the range of 10–100 h, whereas a 10 g/10 min grade with identical density often exceeds 500 h. Food-contact closure grades comply with FDA 21 CFR 177.1520 and EU Regulation 10/2011, with overall migration limits of 10 mg/dm² for plastics in contact with aqueous and acidic simulants. Impact resistance at −20 °C is tested per ASTM D256-23 or ISO 180:2023; partial break or non-break results are typical for 2 mm notched specimens, but this property degrades sharply when density exceeds 0.930 g/cm³. The processing window is narrower than for polypropylene because LDPE displays lower rigidity; demoulding angles below 0.5° lead to ejection marks and stress whitening at the gate.

For low-voltage power cable insulation and jacketing, LDPE is selected for its dielectric constant below 2.35 at 1 MHz and dissipation factor below 0.001, both measured per ASTM D150-22 and IEC 62631-2-1:2018. The polymer is compounded with antioxidant at 0.1–0.3 wt% and, for crosslinked systems, 1.2–2.0 phr dicumyl peroxide; the compound is extruded onto copper or aluminium conductors on a continuous vulcanisation line with a 90 mm extruder and L/D 25:1–30:1. Barrel temperatures in the screw are held at 120–160 °C to prevent premature peroxide decomposition, and die temperature is maintained at 130–150 °C. After extrusion, the insulated core enters a continuous vulcanisation tube at 180–260 °C and 0.6–1.5 MPa steam pressure; residence time is controlled so that hot-set elongation under 0.2 MPa at 200 °C remains below 175 % and permanent elongation remains below 15 %, as specified in IEC 60811-2-1.

Jacketing grades with melt indices of 0.4–0.8 g/10 min are applied over metallic screens at a wall thickness of 0.8–2.0 mm; carbon black loading of 2.5–3.0 wt% is used for UV resistance per ASTM D1248-12. A critical process boundary is the conductor preheat temperature: conductor preheat above 160 °C before the crosshead can trigger peroxide scorch, while preheat below 100 °C causes inadequate adhesion to the copper. Extrudate filtration through 40–60 µm screens is standard before the crosshead because volume resistivity for clean LDPE insulation is above 10¹⁵ Ω·cm at 23 °C per ASTM D257-14, and this value declines with water-tree growth and contamination.

When ESCR Governs Squeeze Bottle Grade Selection in LDPE Blow Moulding

Blow-moulded LDPE squeeze bottles for pharmaceutical and personal-care products are manufactured from grades with melt indices of 0.3–1.0 g/10 min and densities of 0.919–0.922 g/cm³. The low melt index is required because parison sag must be controlled during the open-mould transfer; at a melt temperature of 160–200 °C, a 0.5 g/10 min grade exhibits significantly higher zero-shear viscosity than a 1.0 g/10 min grade, allowing longer parison hang times. The parison is extruded through a diverging die with a die gap of 1.5–2.5 mm; die swell in LDPE is typically 40–70 %, which must be compensated in die and core geometry. Blow pressure is set at 0.4–0.8 MPa and mould temperature is maintained at 20–40 °C to achieve a smooth surface while retaining adequate drop impact.

The pinch-off insert must be kept sharp and cooled; LDPE melt with low crystallinity may extrude into the pinch-off weld, creating a weak tail that reduces drop impact. ESCR is the limiting specification, measured per ASTM D1693-15 Condition B in 10 % Igepal at 50 °C; pharmaceutical squeeze bottles often require F50 above 200 h, which excludes high-MI grades and recycled content above 20 %. Density above 0.925 g/cm³ is avoided because it raises modulus but reduces ESCR. Biaxial orientation is not used for LDPE bottles because the long-chain branched architecture inhibits uniform stretch; instead, the mould is designed with a deep pinch-off and an oval sidewall to concentrate flexure away from the weld line. Food-contact and pharmacopoeial compliance is verified under FDA 21 CFR 177.1520 and EU 10/2011, with migration testing according to EN 1186-1:2002.

High-dispersion masterbatch produced with LDPE as carrier resin operates in a different rheological envelope from film or moulding. Carrier grades with melt indices of 5–30 g/10 min and densities of 0.917–0.924 g/cm³ are let down into saturated and unsaturated polyolefins; the low melting point of LDPE, 105–115 °C by differential scanning calorimetry per ISO 11357-3:2018, allows a co-rotating twin-screw extruder to run with a flat temperature profile of 140–180 °C. The extruder typically has L/D 40:1–48:1 and screw speed 300–1200 rpm for pigment loadings of 20–70 wt%; for carbon black, specific energy input of 0.15–0.30 kWh/kg is effective, while melt temperature at the die must remain below 200 °C to limit carrier degradation. Dispersion quality is assessed by pressure rise across a 14 µm or 25 µm screen pack under ISO 4577:2020 conditions; pressure rise above 0.25 MPa/min usually indicates undispersed pigment agglomerates.

The carrier melt index is selected by the let-down ratio and the target final melt index: a 20 g/10 min carrier at 50 wt% loading may raise the base polyolefin melt index by 1.5–3.0 g/10 min, which must be accounted for in the final part specification. LDPE is preferred over LLDPE as carrier when wax loadings of 3–10 wt% are present, because the branched architecture tolerates the viscosity drop without severe screw slip. However, LDPE carrier resins have lower thermal stability than HDPE carriers; batch-to-batch variation in melt flow at 190 °C is typically controlled within ±0.5 g/10 min by blending, measured per ISO 1133-1:2022. Published data for specific twin-screw configurations is limited, but the process window above is consistent with equipment manufacturer technical bulletins.

Rotational Moulding Drop Impact and Porosity Are Controlled by Peak Internal Air Temperature

Rotational moulding grades of LDPE are produced with melt indices of 3–8 g/10 min and densities of 0.922–0.932 g/cm³, with a particle size distribution of 150–500 µm. The powder is charged into a mould that is rotated biaxially at a ratio of 4:1–10:1 in an oven heated to 280–350 °C. The key control variable is peak internal air temperature, usually 190–230 °C, because the melt coalescence and sintering of LDPE powder depend on zero-shear viscosity and surface tension. If the peak internal air temperature is below 180 °C, incomplete fusion leaves pinholes and porosity that reduce drop impact; if it exceeds 240 °C, thermo-oxidative degradation increases yellowness and shifts the melt index upward.

Cooling is controlled at 4–10 °C/min in air before water spray; rapid cooling lowers crystallinity and improves impact, but may warp thick sections. Drop impact is measured per ASTM D5276-19 or ISO 6603-2:2000; LDPE rotational moulds for storage tanks and outdoor playground components are commonly specified to pass 200 J at −20 °C when wall thickness is 4–6 mm, but published data for specific mould geometries is limited. ESCR per ASTM D1693-15 is specified for chemical storage tanks holding aggressive surfactants; LDPE grades with density below 0.930 g/cm³ and melt index below 5 g/10 min show longer failure times than higher-MI variants. UV-stabilised LDPE rotomoulding compounds contain 0.5–2.0 wt% hindered amine light stabiliser and 0.1–0.3 wt% antioxidant, with weatherability tested per ASTM D2565-23 and colour change measured per ISO 4582:2017.