LDPE Granules Buying Guide: MFI, Density and Film vs Injection Grades

Selecting low-density polyethylene (LDPE) granules for film extrusion or injection moulding begins with a procurement specification that distinguishes melt flow index, density, molecular weight distribution, and additive package. These parameters are not interchangeable across conversion routes. A film grade with an MFR of 0.3 g/10 min and an injection grade with an MFR of 20 g/10 min may share a density of 0.922 g/cm³, yet their rheological and thermal-mechanical responses differ by more than an order of magnitude. The following technical reference anchors purchasing decisions to standardised test methods, production equipment parameters, and known operational thresholds.

LDPE is produced by high-pressure radical polymerisation in autoclave or tubular reactors at pressures commonly in the range of 150–350 MPa and temperatures of 130–330 °C. Long-chain branching density, branch length distribution, and molecular weight distribution influence extensional viscosity, shear thinning, and melt strength. Procurement evaluations should not rely on density as a single grade descriptor because a 0.920 g/cm³ grade may be formulated for blown film, cast film, injection moulding, or extrusion coating. The conversion-route requirement dictates the acceptable MFI bandwidth and additive loading.

How Do Melt Flow Index and Density Specifications Interpret LDPE Grade Behaviour?

Melt flow index is determined at 190 °C with a 2.16 kg piston load under ISO 1133-1:2022 procedure A or ASTM D1238-20 procedure A. It is reported as mass-flow rate in g/10 min. The value is an inverse indicator of average molecular weight but is not a complete viscosity curve. Two LDPE lots with the same MFR can differ in molecular weight distribution, extruder torque, die swell, and blown-film bubble stability. Purchasing specifications should therefore combine MFR with density, and where available, shear viscosity data at 100 s⁻¹ and 1,000 s⁻¹. Density is measured by ISO 1183-1:2019 Method D or ASTM D792-20 Method B and reflects short-chain branching. LDPE homopolymer density typically ranges from 0.912 g/cm³ to 0.930 g/cm³, with lower values providing higher impact and flexibility but reduced tensile strength and moisture barrier.

Incoming QC laboratories should verify MFR under 2.16 kg load as the default for LDPE; higher loads are used only when the grade sheet specifies them. The test requires at least two cuts after a stabilising preheat, and the result is rounded to three significant figures per ISO 1133-1:2022 clause 8. Density conditioning should follow ISO 291:2008, at 23±2 °C and 50±10 % RH. A density deviation of ±0.001 g/cm³ can shift seal initiation temperature and coefficient of friction on automated packaging lines. Different density methods can produce additional variation. ISO 1183-1:2019 Method D gradient-column and ASTM D792-20 Method B water-displacement values can differ by 0.001–0.002 g/cm³ because of surface wetting and thermal equilibration effects; the purchase specification should name the controlling method.

Because LDPE is non-hygroscopic, predrying is not normally required for conversion of unopened bags. If condensation occurs after cold storage, drying at 60 °C for 2 h in a desiccant dryer is sufficient to remove surface moisture. High-humidity exposure at RH > 85 % without adequate bag sealing can introduce dust, but moisture uptake remains below 0.01 % by mass after 24 h at 23 °C and 85 % RH.

Rheometric testing beyond MFR can discriminate between lots with similar melt flow index. Capillary rheometry at 190 °C across shear rates from 1 s⁻¹ to 10,000 s⁻¹ reveals shear-thinning differences. Film extrusion is governed by extensional viscosity at Hencky strain rates of 0.5–5 s⁻¹ during bubble formation; injection moulding is more dependent on shear viscosity at gate shear rates above 1,000 s⁻¹. Melt strength is not an ISO grade parameter, but suppliers occasionally report maximum draw-down speed. Extrusion-coating grade selection additionally requires draw-down speeds above 200 m/min and neck-in values below 80 mm at 20 µm coating thickness. Published data for specific configurations varies widely across supplier and die geometry.

Film-grade LDPE requires controlled rheology, low gel count and narrow bubble stability margins

Blown-film LDPE grades are frequently specified with MFR from 0.2 g/10 min to 0.8 g/10 min. MFR below 0.2 g/10 min increases melt temperature and head pressure on 24:1–30:1 L/D single-screw extruders, which can exceed barrel set-points of 180–220 °C and produce oxidised gel defects. Film grades above 1.0 g/10 min often lose bubble stability at blow-up ratios above 2.5:1 and generate gauge bands due to insufficient melt strength. A production blown-film line typically uses a die gap of 1.0–2.5 mm, a blow-up ratio of 2.0:1–3.0:1, and a frost line height of 3–8 die diameters. Screw design should include a low-shear barrier section and a Maddock mixer to homogenise melt temperature. Specific energy input on modern 60–90 mm extruders is typically 0.20–0.35 kWh/kg.

Film mechanical properties are evaluated according to ISO 527-3:2018 for tensile properties and ISO 7765-1:2004 Method A or ASTM D1709-20 Method A for dart drop impact. At 25 µm thickness, LDPE film with density 0.918–0.924 g/cm³ typically exhibits machine-direction tensile strength of 18–30 MPa and transverse-direction elongation at break above 400 %. A shift in density from 0.918 g/cm³ to 0.924 g/cm³ can reduce dart drop impact by 10–25 % and increase tensile modulus by approximately 20–30 % based on typical commercial datasheet comparisons.

Gel contamination is an operational boundary for critical film. Optical surface defect scanners on cast film lines classify gel particles above 200 µm as defects. No universal ISO threshold exists; grade-specific limits must be agreed bilaterally. Slip and antiblock additive packages influence surface coefficient of friction. Erucamide slip at 500–1,500 ppm and silica antiblock at 500–2,000 ppm are common in applications where a coefficient of friction below 0.4 is required. Migration kinetics of erucamide in LDPE follow temperature-dependent diffusion; coefficient of friction often reaches practical equilibrium within 24–72 h at 40 °C. Additive loadings must be checked against food-contact migration limits under EU Regulation 10/2011.

LDPE film optical properties such as haze and gloss are measured by ISO 14782:2021 and ISO 2813:2014. Haze below 5 % at 25 µm is typical for high-clarity LDPE film, but the value depends on die gap, blow-up ratio, and frost line height. Lower melt temperature and faster quenching reduce crystal size and improve haze, but can reduce modulus and raise blocking if antiblock loading is too low.

When Downgauging Film Below 40 µm, Bubble Cooling and Die Lip Velocity Become Limiting Factors

Below 40 µm thickness, LDPE film processing enters a process window where melt strength, die lip deposits, and cooling air turbulence interact more strongly. Conventional single-lip air rings on a 1.8 m die with 2.5:1 BUR can typically sustain line speeds between 80 m/min and 120 m/min. Above 120 m/min, bubble flutter and machine-direction gauge variation of ±8 % can occur unless internal bubble cooling and automatic gauge control are used. Internal bubble cooling systems reduce variation to approximately ±4 % on multi-layer lines but add capital cost and maintenance complexity. For downgauged LDPE film, frost line height should be held at 3–6 die diameters. Frost line heights below 2 die diameters quench the bubble too rapidly, increasing haze and reducing transverse tear, while heights above 8 die diameters create excessive machine-direction orientation and reduced transverse tear.

Melt fracture onset is a further limitation. Published data for specific LDPE grades indicates sharkskin melt fracture at wall shear stresses above 0.14 MPa, but the threshold varies with molecular weight distribution, die temperature, and lip geometry. Die lip build-up is influenced by slip agent migration and low molecular weight fractions. When output exceeds 200 kg/h on large blown-film dies, die lip build-up can occur within 4–8 h of continuous operation. This build-up produces die lines and weak points in the film. The use of fluoropolymer polymer processing aids at 200–800 ppm can reduce accumulation on steel die lips, but compatibility with food-contact declarations must be verified under EU Regulation 10/2011 and FDA 21 CFR 177.1520. Published data for LDPE films below 25 µm with high-MFI grades is limited; pilot-line trials remain necessary for those grades.

Injection-moulding LDPE grades are selected at MFR values commonly between 4 g/10 min and 50 g/10 min. Higher MFR reduces filling pressure and allows longer flow-length-to-wall-thickness ratios, but it increases shrinkage anisotropy and reduces notched impact strength. Melt temperatures from 160 °C to 260 °C and mould temperatures from 20 °C to 50 °C are typical for unfilled LDPE. Screw L/D ratios of 18:1–22:1 with a general-purpose metering screw are sufficient. Compression ratios above 3.0:1 increase screw slip and reduce plasticising rate. Injection pressures of 500–1,500 bar are observed depending on part geometry and gate size. For a cavity pressure of 300–600 bar, a projected area of 500 cm² requires 150–300 tonnes clamp force. Insufficient clamp force causes flash in thin-walled closures; excessive clamp force plus high holding pressure can cause mould breathing and dimensional variation.

Dimensional stability after demoulding is specified through ISO 294-4:2018; unfilled LDPE mould shrinkage after 48 h typically ranges from 1.5 % to 3.0 %. Tensile properties follow ISO 527-2:2012, and notched Izod impact follows ISO 180:2023 or ASTM D256-23. A processing temperature above 260 °C can induce thermal degradation of additive packages, although LDPE homopolymer is chemically stable below 280 °C under short residence times. Colour masterbatch dilution requires high-shear dispersion with a screw mixing section; poor dispersion on short L/D injection screws manifests as pigment agglomerates larger than 10 µm in thin-walled parts. Gate-freeze time must exceed holding-pressure time. If holding pressure is removed before gate solidification, melt backflow from the cavity produces sink marks, voids, and post-mould warpage.

High-cycle closures and caps using LDPE may require cycle times below 8 s in multi-cavity tools. These conditions require MFR near 20–50 g/10 min, mould cooling channels with Reynolds numbers above 10,000, and conformal cooling. Melt temperature should remain below 230 °C to reduce odour generation and mould plate-out. Ejector force and cam action must account for high shrinkage; deep undercuts increase rejection rate. Shrinkage anisotropy in injection-moulded LDPE depends on molecular orientation and cooling rate. For a rectangular plaque 2 mm thick, machine-direction shrinkage can be 1.8–2.8 % while transverse-direction shrinkage is 1.2–2.0 %. Differential shrinkage above 0.6 % causes warpage; gate location and holding pressure can reduce anisotropy.

The values in the table are typical for unfilled LDPE homopolymer. Specific grades may lie outside these limits depending on comonomer content, molecular architecture, and additive loading.

Additive and Compliance Specifications for Food-Contact Film and High-Cycle Moulding

Additive packages modify processing and end-use performance. Film grades can contain slip agents, antiblock agents, antioxidants, and polymer processing aids. Injection grades may contain nucleating agents, mould-release agents, and UV stabilisers. The exact formulations are not always disclosed; therefore, a purchasing specification should require a compliance statement aligned to the intended application. For food-contact film, LDPE granules must comply with FDA 21 CFR 177.1520, which permits olefin polymers for food contact within specified density and extractables limits, and with EU Regulation 10/2011, which imposes overall migration limits of 10 mg/dm². Medical packaging may require ISO 10993-5:2009 for cytotoxicity and ISO 10993-10:2021 for skin sensitisation, but these are packaging-end-use validations rather than raw-material grade tests.

Primary antioxidants such as hindered phenols and secondary antioxidants such as phosphites are present at 300–1,500 ppm total in many LDPE formulations. They are consumed during processing; reprocessing of LDPE film scrap beyond 20 % by mass can increase carbonyl index and reduce dart impact. Incoming lots should be evaluated for oxidation induction time per ISO 11357-6:2018 where long-term thermal stability is required. For high-cycle injection moulding, mould-release agents such as calcium stearate or zinc stearate are used at 0.05–0.2 % by mass. Overdosing above 0.3 % can lead to plate-out on mould surfaces and loss of surface adhesion for printing or secondary assembly.

Batch-to-batch variance in LDPE granules is controlled through incoming QC sampling of 25 kg or 1,000 kg lots. A minimum of 2 kg of representative granules is required. Retention samples should be stored below 25 °C and dry conditions for 24 months. If MFI varies by more than ±0.2 g/10 min in low-MFI film grades or ±1.5 g/10 min in high-MFI injection grades from the grade certificate, the lot should trigger a production-scale trial at minimum or return. Density variation greater than ±0.001 g/cm³ can shift seal initiation temperature and mould shrinkage. Converters should request the supplier’s batch chart for molecular weight distribution, though many suppliers do not provide full GPC curves. When published data for a specific packaging configuration is limited, pilot-line validation at 1:1 width and at least 5,000 m of film or 1,000 shots of injection moulding is the only reliable acceptance method.