Rotomoulding and Injection Moulding with LLDPE: Where It Actually Works

Linear low-density polyethylene (LLDPE) is a copolymer of ethylene with 1-butene, 1-hexene, or 1-octene, with a density classified under ASTM D792 between 0.915 g/cm³ and 0.940 g/cm³. The distribution of short-chain branching governs crystallisation rate, flexural modulus, and environmental stress crack resistance, the latter measured by ASTM D1693. Two fabrication routes dominate the downstream conversion of LLDPE: rotational moulding of a 35-mesh powder and injection moulding of pelletised feedstock. Rotational moulding relies on biaxial rotation and zero internal pressure to sinter the powder into a monolithic wall, while injection moulding uses a reciprocating screw to plasticate and deliver the melt under pressures of 55–100 MPa. Where the material actually works in each process is not determined solely by density or melt index, but by particle size distribution, oxidative stability, tool thermal history, and gate-to-wall solidification timing.

Why Does Rotational Moulding Select LLDPE With a Melt Index From 3 g/10 min to 6 g/10 min?

Rotational moulding is a low-shear process. The polymer powder is heated inside a closed tool while the tool rotates biaxially; densification occurs by particle coalescence, melt sintering, and surface tension rather than pressure flow. The melt index, measured at 190 °C under 2.16 kg load according to ASTM D1238 or ISO 1133-1, is therefore held between 3 g/10 min and 6 g/10 min for most rotomoulding LLDPE grades. Below 3 g/10 min, zero-shear viscosity is high enough that trapped air bubbles cannot escape before peak internal air temperature is reached, producing pinholes and voids. Above 6 g/10 min, the melt film may sag or drain from vertical mould walls during heating, causing wall-thickness variation on a 3-arm carousel machine with swing capacities from 3.5 m to 5.0 m. Oven set temperatures of 260–320 °C and peak internal air temperatures of 190–210 °C are commonly used, with dwell times of 15–30 min. Rotomoulding LLDPE grades typically use 1-hexene or 1-octene comonomers rather than 1-butene because the longer side chains raise ESCR above 1,000 h under ASTM D1693 Condition B and improve instrumented impact performance under ISO 6603-2. Powder passing 35 mesh, equivalent to a particle size at or below 500 µm, is critical for uniform tack and densification.

Injection moulding operates under high shear, so pelletised LLDPE grades are formulated at melt indices from 20 g/10 min to 50 g/10 min at 190 °C/2.16 kg when tested to ASTM D1238. A general-purpose PE reciprocating screw with an L/D ratio of 20:1 to 24:1 and a compression ratio of 2.5:1 to 3.5:1 plasticates the material at melt temperatures of 190–230 °C; mould temperatures are generally held at 15–50 °C. Because LLDPE is highly shear thinning but has lower die swell than LDPE due to limited long-chain branching, gate sealing and packing transfer differ from high-pressure LDPE tool behaviour. Injection pressures of 55–100 MPa are common, and thin-wall packaging with wall sections of 0.8–1.5 mm can require pressures above 80 MPa. Dimensional stability is evaluated using ISO 294-4 shrinkage plaques; LLDPE mould shrinkage typically falls between 0.015 mm/mm and 0.025 mm/mm, depending on wall section, melt temperature, and mould temperature. Tensile yield stress ranges from 10 MPa to 20 MPa under ASTM D638 Type IV, with elongation at break above 800%.

When LLDPE Replaces HDPE in an Existing Rotomoulding Tool Set

When LLDPE is substituted into a tool qualified for high-density polyethylene, the lower flexural modulus—typically 250–400 MPa for LLDPE at 0.935 g/cm³, measured by ISO 178, versus 900–1,200 MPa for HDPE—must be compensated by ribbing or wall-thickness increases. The lower crystallinity reduces the melting range to approximately 120–125 °C, which widens processing latitude but also increases post-mould cooling shrinkage. Rotational moulding of LLDPE in thin-wall HDPE tooling can produce warpage because the cooling rate across the tool surface is non-uniform; production-scale trials on shuttle machines have shown that oven dwell times may need to be reduced by 10–20% relative to HDPE to prevent over-sintering. In injection moulding, direct substitution into hot-runner tools can cause filament stringing because LLDPE has higher melt elongation than HDPE at equivalent melt index. ESCR is typically superior: LLDPE with 1-octene comonomer can exceed 1,000 h under ASTM D1693 Condition A, while an HDPE homopolymer may fail below 100 h in the same test.

Comparative Processing Data: Rotomoulding Versus Injection Moulding LLDPE

On a 3-arm carousel rotational moulding machine with a swing capacity of 4.5 m, pinholes are observed when venting ports are below 12 mm diameter or when the vent tube is blocked by melt, because decomposition gases from the stabiliser package cannot escape before the inner surface fuses. Batch-to-batch variance in powder particle size distribution becomes visible as orange peel when the fraction retained above 35 mesh exceeds 5%. In injection moulding, a 1,500 kN clamp machine processing a 25 g/10 min LLDPE can short-shot below 210 °C; raising the melt temperature above 250 °C can cause surface smear, screw recovery drift, and stabiliser depletion. Gate blush appears when injection speed exceeds 150 mm/s through a 1.0 mm cold-runner gate. These are process boundaries, not material defects per se, and they shift with comonomer type, molecular weight distribution, and tool geometry.

Thermal Stability Boundaries Are Set by Oxidative Induction Time

Rotational moulding LLDPE is exposed to oxygen at 260–320 °C for 15–30 min, so the stabiliser package must protect the melt during sintering. Oxidative induction time measured by ASTM D3895 at 200 °C is commonly specified above 30 min; grades intended for outdoor tanks are additionally evaluated by xenon arc exposure under ASTM D2565 or ISO 4892-2. For chemical tank service, LLDPE alone has high permeation to hydrocarbon vapour; automotive fuel tanks therefore use multi-layer rotomoulding with an EVOH barrier layer because the LLDPE layer alone would fail evaporative emission requirements. Rotomoulding powder should be dried before charging if ambient relative humidity is above 60%; moisture pickup as low as 0.05 wt% can produce steam pinholes at peak internal air temperature.

Gate Freeze, Packing Pressure, and Post-Mould Shrinkage in Injection Moulding

Gate diameter must be at least 0.8–1.0 mm for easy-flow LLDPE; smaller gates freeze before pack pressure is transferred. Packing pressure is typically applied at 30–50% of injection pressure for 0.5–1.0 s. The crystalline melting range of LLDPE is approximately 120–125 °C when measured by differential scanning calorimetry at 10 °C/min under ISO 11357-3, so gate freeze occurs quickly. A 0.918 g/cm³ LLDPE shows a broad melting endotherm from 90 °C to 125 °C; this broad transition reduces the sharpness of solidification and contributes to post-mould shrinkage. Shrinkage may continue for 24–48 h after demoulding, so dimensional inspection should be delayed accordingly. Adding a sorbitol-based nucleating agent at 0.05–0.10 wt% can raise crystallisation temperature by 3–5 °C and reduce cycle time, but combination with amine-based additives should be avoided because of discolouration and phenolic antioxidant interactions.

If a 50 g/10 min LLDPE Is Run on a General-Purpose PE Screw, These Limitations Appear

Injection moulding LLDPE with a melt index of 50 g/10 min at 190 °C/2.16 kg can fill flow length-to-thickness ratios above 150:1 at a melt temperature of 220–240 °C and an injection speed of 100–150 mm/s. On a general-purpose PE screw with a compression ratio of 2.5:1, the shallow feed section may deliver an unstable melt stream because LLDPE pellets soften early and slip in the feed zone; screw recovery time can drift by 10–15% batch to batch. Increasing screw speed beyond 200 rpm may introduce unmelted particles, while nozzle pressure drops above 80 MPa can cause parting line flash. The clamp force requirement is projected area times cavity pressure; for LLDPE thin-wall packaging, cavity pressure is often 25–40 MPa, so a four-cavity tool with 200 cm² projected area per cavity requires 2,000–3,200 kN. If the available clamp is below this range, flash occurs because the low-viscosity melt enters the parting line before solidification.

Rotomoulded LLDPE parts such as agricultural tanks, kayaks, and portable fuel containers commonly use wall thicknesses from 2 mm to 6 mm. At 2 mm, the powder particle size must remain below 500 µm; larger particles require longer sintering time and create porosity. Rock-and-roll machines for kayaks use a swing angle of 360° and a tilt angle of 45°; wall thickness distribution depends on the rotational ratio, typically near 4:1 for symmetrical parts. Published data for specific LLDPE wall thickness distributions is limited because tool geometry and oven airflow dominate the result. High-flow LLDPE with a melt index above 6 g/10 min can produce acceptable surface gloss but increases the risk of thick lower corners and thin upper walls when the rotational ratio is not adjusted for the lower melt viscosity.

Food-contact applications for rotomoulded and injection moulded LLDPE are governed by FDA 21 CFR 177.1520 for olefin polymers and by EU Regulation (EU) No 10/2011 migration limits. Potable water tanks require NSF/ANSI 61 certification. LLDPE is not suitable for continuous service above 70 °C under load unless crosslinked or reinforced; for hot-water pressure applications, crosslinked PE or PP-R grades are used instead. ASTM D4976 classification of LLDPE covers density, melt index, and copolymer composition, but it does not guarantee rotomoulding or injection moulding processability.