Low-Density Polyethylene (LDPE) Melting Point: 105–115 °C and Why Processors Should Care

Low-density polyethylene (LDPE) is supplied at a density of 0.910–0.925 g/cm³ and is polymerized in tubular or autoclave reactors that create both short-chain and long-chain branches along the ethylene backbone. The melting point commonly reported as 105–115 °C is a differential scanning calorimetry (DSC) endotherm peak, not a single crystal-to-liquid transition. It is typically measured on a second heating scan at 10 K/min under nitrogen according to ISO 11357-3 or ASTM D3418. That lower melting range relative to high-density polyethylene (130–137 °C) arises from the short-chain branches limiting lamellar thickness. For food-contact applications, LDPE grades are covered by FDA 21 CFR 177.1520 and by EU Regulation (EU) No 10/2011. Processors should care because the melting endotherm defines the minimum thermal energy required to erase crystallinity but does not prescribe the optimum barrel, die, or mould temperature.

Thermal Transitions That Precede the LDPE Melting Endotherm

Before the DSC melting peak is reached, LDPE can show melting onset around 85–95 °C because the lamellar thickness distribution is broad. Some crystallites may survive until 120–125 °C in branched grades. Specimen mass is normally 5–10 mg, and the second heat is used to remove prior thermal history. The measured heat of fusion is compared with the reference value for 100% crystalline polyethylene, commonly 293 J/g, yielding crystallinity of 35–55% for LDPE. Vicat softening temperature is lower—typically 80–100 °C under 10 N or 50 N load per ISO 306 or ASTM D1525—because Vicat measures penetration under load rather than complete melting. The Melt Flow Index of LDPE is measured at 190 °C under 2.16 kg using ISO 1133-1 or ASTM D1238 and ranges from 0.2 g/10 min for high-molecular-weight film grades to 70 g/10 min for injection grades. The combination of a low melting point and a broad MFI range explains why one thermal endpoint cannot predict process behavior.

On production single-screw extruders with length/diameter ratios of 24:1 to 30:1, the DSC melting point is only the lower limit of a stable melt-flux window. Representative barrel profiles for LDPE place the feed zone at 120–150 °C, the compression zone at 150–180 °C, the metering zone at 170–200 °C, and the die at 175–200 °C. The actual melt temperature is not equal to the barrel setpoint because viscous dissipation converts mechanical energy into heat. At screw speeds above 60 rpm in a 30:1 extruder, the melt can exit 8–15 °C above the final barrel setpoint. If the metering zone is held too close to 105–115 °C, the melt seal is incomplete; symptoms include screw surging, pressure oscillation, unmelted granules, and low output. Conversely, melt temperatures above 230 °C can initiate oxidative chain scission, and extended residence time may generate gel particles. The exact gel-onset temperature depends on antioxidant package, screw geometry, residence time distribution, and oxygen ingress, so published data for a specific line configuration is limited. Barrier screws with Maddock mixers reduce unmelt content but also increase shear heating, so a drop in screw speed may be necessary when melt temperature approaches the upper limit. Screen-pack pressure upstream of the breaker plate is often monitored to detect melt-temperature drift: a steady pressure rise at constant screw speed can indicate gel accumulation or insufficient melting rather than a simple temperature setpoint error.

What Controls Bubble Stability in LDPE Blown Film Above 105–115 °C?

In tubular blown film, die temperature is not set at 105–115 °C. LDPE film grades are commonly extruded at 160–200 °C through spiral mandrel dies with a die gap of 0.8–1.2 mm and a blow-up ratio of 2:1 to 3:1. The melting range matters because the bubble is cooled by air rings while the interior remains molten for a defined frost-line height. At die melt temperatures below 160 °C, high melt viscosity increases die pressure and can produce sharkskin or melt fracture on the visible surface. Above 210 °C, long-chain branching still exists but the extensional viscosity of the melt may not suppress bubble sag and draw resonance under high take-off speeds. The frost-line height then controls residence time in the molten state and influences film haze, gloss, and impact strength. LDPE has a broad molecular weight distribution, and this polydispersity contributes to the melt strength that processors rely on; narrow-distribution LLDPE grades require more precise temperature control. The 105–115 °C endotherm is therefore necessary for bubble formation but insufficient to define the stable operating window.

During heat sealing, the interface between two LDPE films must reach 105–115 °C to destroy the original film boundary and allow chain interdiffusion. Seal initiation for LDPE packaging film is reported at approximately 100–110 °C, but destructive seal strength generally requires jaw temperatures of 140–170 °C with dwell times of 0.5–1.5 s and jaw pressures of 0.5–4.0 bar. Seal strength and hot tack are evaluated under ASTM F88/F2029. If seal jaws are set too close to the melting range, only a thin molten skin forms and the seal can peel below the package specification. If seal jaws exceed 180 °C, film shrinkback, seal thinning, and blocking may occur. The melting point is therefore the minimum thermodynamic threshold for a seal, not the optimum sealing condition. On form-fill-seal lines, the effect of dwell time is asymmetric: increasing bar temperature can compensate for a short dwell up to a point, but excessive temperature damages the film even when the measured jaw temperature remains stable.

When Extrusion Coating Operates at 300 °C with a 105–115 °C Melting Point

Above 280 °C, extrusion coating lines place LDPE in a thermal regime nearly 200 °C above its melting point. Slot-die temperatures of 280–325 °C are used to lower elongational viscosity for high draw-down and to promote penetration into paper, paperboard, or corona-treated polymer webs. The air gap between die exit and pressure roll deliberately exposes the molten web to oxygen, which generates polar carbonyl groups at the surface and improves adhesion to substrates. The process conflict is sharp: temperatures above 315 °C accelerate chain scission and may form gels, yet some thermal oxidation is required for bonding. Antioxidant packages, short transfer lines, and fast screw recovery are used to limit oxidation without eliminating the polar-surface functionality. For LDPE, the melt temperature is not chosen because of the 105–115 °C melting range; it is chosen because adhesion and draw-down demand low viscosity. Published adhesion data for exact melt-temperature settings vary by substrate and line configuration, but the operating window is consistently far above the DSC melting peak.

With a mould temperature of 15–40 °C, injection moulding of LDPE depends more on cooling rate and holding pressure than on the melting peak alone. Barrel temperatures are normally set from 170 °C to 230 °C, while the nozzle is held above 105 °C to prevent freeze-off. If the hot runner or nozzle falls below the melting range, short shots and erratic cushion control can occur. Mould shrinkage for LDPE is typically 1.5–3.5% depending on part thickness, melt temperature, and packing time; dimensional evaluation is often performed after conditioning under ISO 291 or ASTM D618. The low melting point of LDPE does not allow low barrel temperatures because viscosity must be reduced for filling of thin sections.

Rotational Moulding Cycle Time and Melting Point for LDPE Powder

Rotational moulding exposes LDPE powder to a condition where sintering, not just melting, controls part quality. The melting point of 105–115 °C marks the beginning of particle coalescence, but peak internal air temperature must typically reach 180–230 °C to remove trapped air and consolidate the melt. If the peak internal air temperature is too low, the part retains porosity, poor impact strength, and poor edge definition. If the temperature is too high or the hold time is too long, oxidative yellowing and embrittlement occur. Oven temperature is often set at 250–300 °C, but the controlling variable is the internal air temperature inside the closed mould. Processors should care because the DSC endotherm alone underestimates the thermal energy required for densification; sintering time is governed by particle-size distribution, oven heat transfer, and mould wall thickness.