LyondellBasell HDPE LP390-01 NARROW is a high-density polyethylene grade with a narrowed molecular weight distribution. The material is designated PE-HD under ISO 1043-1 and is supplied as pellets for injection-moulded articles in which dimensional tolerance, part flatness, and consistent mould filling are ranked higher than melt strength or parison sag resistance. The narrow distribution reduces the concentration of very high-molecular-mass chains, which lowers the zero-shear viscosity and reduces the elastic recovery contribution to post-moulding warpage. In a multicavity tool with restricted gates, the pressure required to fill the last cavity is therefore less sensitive to small changes in melt temperature than in a broad-molecular-weight-distribution HDPE of the same density. The grade should not be confused with bimodal or broad-MWD high-density polyethylene intended for blow moulding or pressure pipe. Exact lot values for melt flow rate, density, and thermal properties are documented on the certificate of analysis according to ISO 1133-1:2022 and ISO 1183-1.

The resin is produced with a controlled additive package. The narrow molecular weight distribution is not a filler or processing aid; it is a structural feature of the polyolefin chain population. This feature influences the shear-viscosity curve, the die swell behaviour, and the frozen-in orientation after solidification. Users who transfer tooling from a broad-MWD HDPE should not expect identical short-shot limits, because the pressure drop across a hot-runner manifold responds differently when the high-molecular-weight tail is absent.

What Specification Profile Applies to the LP390-01 NARROW Grade?

The following values are manufacturer-published typical data, not release limits. Lot-specific certificates control the actual values. The table is provided for preliminary tool design and material substitution screening only.

These values place the material in the medium-flow HDPE range. The narrow distribution means that capillary-viscosity measurements do not extrapolate to blow-moulding melt strength; the zero-shear viscosity may be lower than expected from the melt flow rate alone. A processed part may therefore exhibit less frozen-in stress, but it will also have less parison stability or melt strength when evaluated outside injection moulding. Published data for this specific grade do not include a complete pvT diagram, so Moldflow-style simulation inputs should be generated by capillary rheometry and pvT measurement under ISO 17744 rather than transferred from a broad-MWD HDPE.

Melt Temperature and Mould Cooling Conditions Influence Final Degree of Crystallinity

For LP390-01 NARROW, the recommended melt temperature is 200–230 °C. Barrel settings below 190 °C can produce a steep rise in viscosity because the low shear-rate plateau shortens as the crystalline melting region is approached. Barrel settings above 240 °C accelerate thermo-oxidative chain scission and can generate local aldehyde and carboxylic acid species that contribute to plate-out on the mould surface. The mould wall temperature should be kept within 10–30 °C, with the lower half of the range used for fast-cycle thin-wall articles and the upper half used where reduced frozen-in stress and improved gloss are specified. Since HDPE does not absorb water at a rate requiring desiccant drying under normal indoor conditions, the resin can be processed directly. If surface condensation occurs after storage at relative humidity above 60%, a hopper dryer at 80 °C for 2–4 h removes surface moisture. Drying beyond this interval does not increase physical properties and may increase energy carryover into the feed throat.

On a general-purpose polyolefin screw with length-to-diameter ratio between 20:1 and 25:1 and compression ratio 2.5:1–3.5:1, the melt benefits from back pressure of 0.5–1.0 MPa. Back pressure above 2.0 MPa can generate shear heating at the non-return valve and produce melt temperatures higher than the barrel set-point. A cushion of 2–4 mm is appropriate to avoid gas entrapment and to stabilize hold pressure. Shrinkage should be measured according to ISO 294-4 after conditioning at 23 °C/50% RH for 48 h. Because the narrow distribution reduces orientation-induced shrinkage anisotropy, the final dimension is governed mainly by packing pressure and gate freeze time.

The solid density of 0.949 g/cm³ and the approximate melt density at 190 °C of 0.754 g/cm³ imply a volumetric shrinkage from melt to solid of approximately 20%. That volume loss must be compensated by packing pressure before the gate freezes. In articles with wall thickness below 1.0 mm, gate freeze times can be as short as 0.1–0.3 s. If pack pressure is released too early, the unfilled volume fraction may remain as microvoids that are not visible on the surface but reduce burst strength and increase oxygen transmission. Differential scanning calorimetry under ISO 11357-3 at a cooling rate of 10 K/min typically shows a non-isothermal crystallization peak near 116 °C for high-density polyethylene, but the rapid cooling in a cold mould shifts solidification to lower temperatures and reduces the final degree of crystallinity. The resulting modulus and Vicat softening temperature may therefore be slightly lower than values obtained from slowly cooled compression-moulded plaques.

Cooling channel design has a larger effect on warpage than barrel settings once the melt is inside the cavity. Cooling water at 10 °C should be circulated with sufficient velocity to maintain turbulent flow, conventionally described by a Reynolds number above 10,000. Laminar flow in cooling channels creates hot spots and differential shrinkage. For closure moulds, conformal channels with diameters of 6–8 mm and centre-to-centre spacing of 2–3 times the channel diameter provide more uniform heat removal than straight drilled channels. Demoulding while the part surface is above 70 °C can cause ejection distortion because the part is near the deflection temperature region and has low rigidity at that moment.

In injection-moulded closures, the resin is used for tamper-evident bands and lids where controlled flexural modulus and roundness after high-speed ejection are necessary. The reduced elastic recovery of the narrow-MWD polyolefin allows the part to release from the core with less ovality. In high-cavitation hot-runner tools, the decrease in melt-pressure variation between the first and last cavities reduces the tendency for the last cavities to short-shot when the fill time is shortened below 0.5 s. If the last cavities fail to fill, raising the melt temperature within the recommended range is preferred over raising injection pressure alone because the narrow distribution responds more uniformly to temperature than to shear in the upper shear-rate region. For thin-wall packaging and housewares, sink marks over bosses remain sensitive to pack pressure and gate diameter; the narrow MWD does not eliminate packing requirements.

Regulatory acceptability is not a physical property. High-density polyethylene grades in this class are typically formulated to permit compliance with FDA 21 CFR 177.1520 for olefin polymers in food-contact use and with EU 10/2011 overall migration requirements, provided the finished article is tested under its actual thickness and service conditions. The additive package and colourant system should be confirmed from the current product stewardship bulletin because processing aids and pigments can alter migration behaviour. Compliance with RoHS 2011/65/EU restrictions on heavy metals should also be verified at the finished-article level.

When Narrow Molecular Weight Distribution Alters Environmental Stress Crack Resistance Relative to Broad-MWD HDPE

The difference between LP390-01 NARROW and broad-MWD HDPE becomes significant in applications with sustained environmental stress. Broad-MWD and bimodal HDPE grades develop a high-molecular-weight tail that increases the number of tie molecules connecting adjacent crystallites; those tie molecules slow crack growth in the presence of polar stress-cracking agents. The narrow distribution has fewer tie molecules, so the environmental stress crack resistance is lower than a comparable broad-MWD grade of the same density. This trade-off is acceptable in injection-moulded closures and housewares where service loads are intermittent, but it excludes the material from continuous tensile stress applications such as pressure pipe. The resin is not positioned as a PE100 pressure pipe compound under ISO 9080 or as a gas-pipe resin under ISO 4437. Published data for the exact loss of failure time under specific stress-cracking agents is limited, and users should verify ESCR by ASTM D1693-B if the service environment includes detergents, alcohols, or surface-active agents.

The same molecular weight distribution feature reduces melt strength and sag resistance in blow moulding and thermoforming. A broad-MWD HDPE can sustain a molten parison with less drawdown because the high-molecular-weight tail contributes extensional strain hardening. LP390-01 NARROW does not provide the same parison hang-time stability in accumulator-head machines, and long parison hang times can produce wall-thinning at the lower end of the parison. If the tooling requires a parison with large diameter or heavy shot mass, a broad-MWD or bimodal HDPE is preferred. The narrow-MWD grade is therefore not a universal replacement for high-density polyethylene; it is selected specifically for controlled flow, low warpage, and reduced differential shrinkage in injection-moulded articles.

How Does Regrind History Affect the Processing Window?

Regrind from LP390-01 NARROW can be re-introduced into the feed stream if the particle size is controlled to prevent bridging at the feed throat. The narrow molecular weight distribution makes the melt viscosity sensitive to chain scission caused by repeated extrusion. Each pass through a high-shear screw at melt temperatures above 230 °C can break chains in the presence of oxygen, lowering melt viscosity and shifting the processing window toward the lower end of the temperature range. A regrind fraction up to 30 wt% is generally tolerated in non-food applications, but the exact limit should be established by measuring melt flow rate and density on the blended material. If the melt flow rate of the blend exceeds the upper control limit established for the original pellets, pack pressure should be reduced to prevent flash. Conversely, if the regrind contains coarse particles that do not melt completely, unmelted material can appear as fish-eye defects at the surface of thin-wall parts.

Residence time should be monitored during start-up and colour changeovers. The narrow-MWD HDPE can overheat in the screw flights if the screw speed is high and the back pressure is set above 2.0 MPa. Long residence times above 6 min at melt temperatures above 220 °C can produce brown streaks and an acrid odour. The correction is to reduce shot size to match the production rate, lower screw speed, and purge with a viscous HDPE or LDPE purge compound. Polypropylene contamination above 0.5 wt% can create visible splay and weak weld lines because the two polyolefins are not fully miscible at the molecular level and solidify at different rates.

Dimensional testing should include both as-moulded and conditioned measurements. Because HDPE undergoes post-moulding shrinkage and may continue to relax for many hours, a minimum conditioning interval of 48 h at 23 °C/50% RH is required before final dimensional release. For closure roundness, ovality should be measured at multiple heights because the gate area and the weld line opposite the gate may shrink differently. The narrow MWD reduces this difference, but it does not eliminate the need for correct packing and cooling uniformity.