How does a 0.7 g/10 min melt index change parison sag in small-part extrusion blow moulding?

PetroChina Daqing HDPE 5007 enters small-container extrusion blow moulding with a melt flow rate of 0.7 g/10 min at 190°C under a 2.16 kg load as determined by ISO 1133-1:2022 procedure A. The density of 0.958 g/cm³ measured by ISO 1183-1:2019 places the grade in the high-density ethylene homopolymer range. At this MFR, the zero-shear viscosity is higher than that of conventional 1.5–2.0 g/10 min blow moulding HDPE, which retards parison sag during transfer from the die head to the mould. A parison hang time of 2–4 s is achievable for containers with a shot mass below 250 g, provided the die gap is maintained at 0.8–1.5 mm and the melt temperature at the head does not exceed 205°C. Above 205°C, the melt viscosity drops sufficiently that visible draw-down of the parison exceeds 15% of the target wall thickness before mould closure, especially in bottles with a length-to-diameter ratio above 3:1.

Processing on production-scale shuttle or continuous rotary blow moulding machines typically uses a single-screw extruder with a diameter of 50–75 mm and an L/D ratio of 24:1–30:1. Barrel temperature zones are set progressively from 165°C at the hopper to 190–200°C at the metering section, with the die head held between 195°C and 205°C. The die gap is adjusted between 0.8 mm and 1.5 mm depending on container wall thickness, and the blow-up ratio is controlled within 2.5:1–3.5:1. Preblow pressure is typically limited to 0.15–0.30 MPa, while full inflation pressure is set at 0.50–0.75 MPa. These settings minimize melt fracture and avoid excessive die swell, which in this density range can produce gauge variability greater than ±0.1 mm if the head temperature is not stabilized.

Mould cooling for 1 L containers with a nominal wall thickness of 1.0 mm requires a mould temperature of 10–25°C and cooling time of 8–14 s. Cooling water at 8–15°C is circulated through conformal channels to reduce post-mould warpage. After de-moulding, containers exhibit volumetric shrinkage of 1.5–2.5% within 24 h as measured by ISO 294-4:2018. The final wall thickness distribution is influenced primarily by hang time, preblow delay, and die eccentricity. Tight die centring with a dial indicator to within 0.03 mm total indicated run-out is necessary to avoid thin spots below 0.6 mm in the pinch-off and shoulder areas.

End products from this segment include household chemical bottles, personal care containers, detergent packaging, and small automotive fluid reservoirs up to approximately 5 L. The higher melt viscosity of HDPE 5007 supports container sidewall stiffness and improved environmental stress crack resistance compared with lower-viscosity HDPE grades, but it also limits the use of excessively deep handle pinch-offs or very small neck finishes. For containers with neck diameters below 20 mm, the head tooling must be revised because the melt at 0.7 g/10 min may not fill fine thread details without raising the head temperature beyond the sag threshold.

Compliance for direct food-contact containers is evaluated under FDA 21 CFR 177.1520(c) for olefin polymers and EU Regulation (EU) No 10/2011 Annex I for plastic materials and articles. The converter must verify that the finished container meets overall migration limits of 10 mg/dm² according to EN 1186-1:2002 and that specific migration limits for any added processing aids are satisfied. Because HDPE 5007 is supplied as a natural resin, the colour concentrate or masterbatch used in blow moulding must itself carry separate food-contact documentation. A table of regulatory checkpoints is provided below.

The operational boundary for blow moulding HDPE 5007 is set by melt temperature and moisture. Surface moisture above 0.05 wt% can produce surface splay and dimensional inconsistency, particularly in humid environments above 60% RH; a dehumidifying hopper or vented barrel is not normally required for virgin resin but becomes necessary when high levels of post-industrial regrind are introduced. Regrind addition should be limited to 10–15 wt% for containers exposed to aggressive surfactants because repeated thermal history raises the gel content and reduces environmental stress crack resistance below the threshold needed for long-term detergent packaging.

Blown film lines processing PetroChina Daqing HDPE 5007 at a melt temperature of 190–220°C through a die gap of 1.2–1.8 mm produce high-density film for industrial liners, agricultural greenhouse covering, and carrier packaging. The bubble is operated in the stalk configuration typical of high-molecular-weight HDPE: a stalk height of 6–9 die diameters is maintained before the frost line, and the blow-up ratio is set between 3:1 and 5:1. Film thickness from 12 μm to 100 μm is achievable on spiral mandrel dies with a diameter of 150–300 mm. The lower MFR of 0.7 g/10 min increases bubble stability relative to 2.0 g/10 min HDPE film grades because melt extensional viscosity is higher, but output at a given extruder diameter is generally lower.

On a 75 mm single-screw extruder with L/D ratio 25:1–30:1, output typically remains in the range of 120–250 kg/h depending on die diameter and internal bubble cooling availability. The extruder screw should include a barrier section and a Maddock mixing section to homogenize the melt without exceeding a melt temperature of 220°C. Melt pressure at the screen pack is usually maintained between 15 MPa and 25 MPa with a 60/80/100 mesh screen pack; pressure rises of 5–8 MPa over a 72 h run indicate gel accumulation or unmelts and require a screen change to avoid melt fracture and bubble instability.

Film properties are tested according to ASTM D882-18 for tensile properties, ASTM D1922-15 for Elmendorf tear, and ASTM D1709-16ae1 for dart impact. Machine-direction tear is typically one-third to one-half of transverse-direction tear because the stalk configuration imposes strong transverse orientation. The final film stiffness is a direct function of the 0.958 g/cm³ density; high-density film made from this grade exhibits lower water vapour transmission rate than low-density PE film and is used where moisture barrier is the primary packaging function. The frost line height should be kept at 4–7 die diameters above the air ring to balance impact strength and tear resistance. Raising the frost line generally reduces MD tear but increases dart impact, while lowering it increases MD tear and reduces TD tear.

The application limit in blown film is the combination of high density and low MFR. Film below 25 μm may show more haze than expected from HDPE of identical density because the high melt viscosity can restrict stress relaxation before the frost line. For agricultural film requiring UV stability above 1,000 h of accelerated ageing per ISO 4892-2:2013, the converter must incorporate a U.V. stabilizer masterbatch at the dosage recommended by the stabilizer supplier. The base resin does not provide weatherability, and no single-layer HDPE film should be specified for long-term external exposure without carbon black or hindered amine light stabilizer systems.

Drainage pipe extrusion stabilises only when head pressure remains below 25 MPa

Non-pressure drainage pipe and corrugated conduit conversion uses HDPE 5007 at a melt temperature of 180–210°C. The density of 0.958 g/cm³ gives higher pipe stiffness than lower-density polyethylene, but the grade is not assigned a PE80 or PE100 minimum required strength classification under ISO 12162. It is therefore applied in gravity drainage, agricultural water disposal, landfill leachate collection, and cable conduit rather than potable or gas pressure pipe. The extruder is a single-screw machine with a diameter of 60–120 mm and L/D ratio of 25:1–33:1. Pipe dies are either spider-support or spiral mandrel designs, and the melt is passed through a breaker plate and screen pack before the die body.

Head pressure is a critical variable on pipe lines. Stable extrusion is observed when head pressure remains below 25 MPa; excursions above this value can distort the spider legs and produce weld-line weakness in the finished pipe. Typical melt pressure at the adapter for a 110 mm pipe die is 15–22 MPa at an output of 150–350 kg/h. The barrel profile from feed throat to die is generally 175°C, 190°C, 200°C, 205°C, and 195°C at the die, with minor adjustments based on screw speed. Screw speed is typically limited to 40–80 rpm to prevent viscous shear heating from pushing the melt above 210°C, where chain scission and gel formation accelerate.

Vacuum calibration is performed with a water-cooled calibrating sleeve at a vacuum level of -0.03 to -0.06 MPa. Spray cooling tanks hold water at 15–30°C to freeze the outer wall without causing excessive residual stress. For a pipe with an outside diameter of 110 mm and a wall thickness of 3.5 mm, haul-off speed is adjusted to maintain dimensional tolerance of ±0.1 mm on wall thickness and ±0.3 mm on outside diameter. Post-extrusion shrinkage of 1–2% over 24 h is typical and must be accounted for in the sizing sleeve.

Ring stiffness for drainage pipe is tested according to ISO 9969:2016. Pipes made from HDPE 5007 can achieve ring stiffness values necessary for buried drainage installations, but the exact value depends on wall thickness and diameter. A 110 mm solid-wall pipe with a 3.5 mm wall generally falls within the SN4 to SN8 range; published data for this specific grade and pipe configuration are limited, so plant trials with a specific die and cooling train must confirm the final ring stiffness class. The strain hardening modulus of HDPE 5007 is sufficient for short-term stiffness but does not by itself demonstrate long-term creep resistance under load.

The primary incompatibility for HDPE 5007 in pipe extrusion is with high levels of contaminated post-consumer recyclate. Calcium carbonate filler added too rapidly can block the screen pack and raise head pressure above the 25 MPa ceiling. If the pressure limit is exceeded, the process window narrows and the pipe may show internal pitting or dimensional fluctuation. The use of recycled HDPE should be controlled to 20–30 wt% in non-pressure drainage pipe and should be sourced from clean post-industrial scrap rather than mixed municipal waste unless a dedicated washing and melt filtration line with a screen changer is installed.

When HDPE 5007 is processed on a 2,000 kN clamp injection moulding machine with a barrel temperature profile of 210–245°C, the low melt index confines the process to thick-walled industrial components. The resin is not suitable for thin-wall food containers, closures, or high-cavitation houseware tools because the spiral flow length at 0.5 mm wall thickness is considerably shorter than that of a 7.0 g/10 min HDPE. The injection moulding window supported by this grade includes industrial crates, pallet feet, thick-walled battery boxes, agricultural trays, and heavy-duty storage containers with nominal wall thickness from 3 mm to 12 mm.

Melt temperature is set between 220°C and 250°C, measured at the nozzle with a needle pyrometer. The injection speed is held in the range of 30–80 mm/s to balance flow length and shear heating. Hydraulic holding pressure is programmed at 60–100 MPa, and holding time for a 6 mm wall may extend beyond 20 s to compensate for the high volumetric shrinkage of HDPE. Backpressure is set to 5–10 MPa to homogenize the melt without excessive shear. Mould temperature should be maintained at 20–50°C; higher mould temperatures improve weld-line integrity but increase cooling time and reduce productivity.

Gate design must account for the low melt fluidity. Direct sprue gates or edge gates with a diameter of 1.5–2.5 mm are preferred. Gate diameters below 1.0 mm are not recommended because the pressure drop can exceed the capability of the machine and produce short shots or burn marks. Runner systems should be full-round with a minimum diameter of 4 mm for secondary runners. The flow length to wall thickness ratio for this grade is generally kept below 60:1; above this limit, filling stresses increase and the moulded part can exhibit visible flow lines and uneven shrinkage.

Mould shrinkage is measured according to ASTM D955-08 and typically falls between 1.5% and 3.0% in the flow direction and 1.0% and 2.0% in the transverse direction for wall thicknesses between 3 mm and 8 mm. These values depend on holding pressure, mould temperature, and gate freeze time. If the gate freezes before the holding pressure has fully packed the cavity, uncontrolled shrinkage can exceed 3.0% and produce sink marks. Frozen-layer thickness at the end of holding should be monitored by short-shot studies and weight consistency. A shot-to-shot weight variation above 0.5% indicates insufficient cushion or inconsistent plastication.

The injection moulding limitation for HDPE 5007 is the conflict between high melt viscosity and fine feature reproduction. Textured surfaces with grain depth below 25 μm may not reproduce fully unless the mould temperature is raised above 40°C and the melt temperature is pushed to the upper limit. At the same time, melt temperatures above 250°C can increase the formation of oxidation-induced gel particles that become visible on the surface of dark-colour parts. For applications requiring high surface gloss, the converter should use a melt temperature of 235–245°C and a mould temperature of 40–50°C, with a fast injection speed above 60 mm/s to reduce premature freeze-off at the cavity surface.

Monofilament drawing ratios, crystalline orientation, and tenacity ceilings

Production of HDPE 5007 monofilament begins on a single-screw extruder with a diameter of 45–65 mm and an L/D ratio of 24:1–30:1. The melt is delivered through a gear pump to a spinneret plate with hole diameters from 1.2 mm to 2.0 mm. The melt temperature at the spinneret is held between 190°C and 215°C. After leaving the die, the filaments enter a water quench bath at 30–50°C with an immersion length of 0.5–1.0 m. The quench temperature controls the initial crystalline morphology; an excessively high bath temperature above 50°C produces large spherulites that reduce the maximum attainable draw ratio.

The first godet speed is set between 10 m/min and 20 m/min, and the final draw is applied in one or two stages. In a two-stage oven drawing line, the first stage operates at 80–100°C with a draw ratio of 4:1–6:1, and the second stage operates at 100–110°C with an additional draw ratio of 1.5:1–2.0:1. Total draw ratios of 7:1 to 10:1 are typical for HDPE of this density and MFR. Published data for this specific grade are limited; the draw ratio ceiling should be established by plant trials because subtle differences in molecular weight distribution and catalyst residues can shift the maximum orientation by ±1.0:1.

Drawing above 11:1 commonly causes fibrillation and splitting because the tie chains become fully extended and the fibrillar structure can no longer distribute stress evenly. The monofilament surface becomes rough when the draw ratio exceeds the natural extension limit, a failure mode visible on production spools as white streaks or broken filaments. Tenacity is measured according to ASTM D2256-10 and is generally expected in the range of 0.35–0.55 N/tex for HDPE monofilament after optimum drawing. Elongation at break under the same standard typically falls between 15% and 30%. If relaxation is not applied, residual shrinkage in hot air at 85°C can exceed 5%; a relaxation stage of 5–10% between the final godet and the winder is therefore introduced.

End products in this segment include rope, agricultural netting, industrial braided cord, geotextile reinforcement, and slit film for artificial grass backing. The 0.958 g/cm³ density contributes to higher tensile modulus than low-density PE monofilament. However, the grade does not contain anti-UV additives as supplied, and external rope or netting must be compounded with carbon black or HALS stabilizers before extrusion. The compliance framework for these products is less food-contact-oriented and more focused on mechanical performance and REACH Article 33 reporting of any intentionally added substances above 0.1 wt%.

The primary equipment bottleneck in monofilament extrusion is the die shear rate. With a spinneret hole diameter of 1.5 mm and low output, the apparent shear rate can remain below 500 s⁻¹. Above that level, sharkskin melt fracture becomes visible as periodic surface roughness after the godet stand. Melt pump slip and die head pressure fluctuations above ±0.5 MPa cause denier variation and subsequent draw resonance. The use of a melt pump is therefore strongly preferred over direct screw discharge. Screen pack configuration of 40/60/80 mesh provides sufficient filtration without raising melt temperature excessively.

If sheet extrusion is held above 240°C, edge bead oxidation becomes the limiting variable

Sheet extrusion trials with HDPE 5007 are typically performed on a single-screw extruder of 90–120 mm diameter with an L/D ratio of 30:1–33:1. The melt is delivered to a flat sheet die with a width of 800–1800 mm and a die gap of 1.5–3.0 mm. Melt temperature at the die entrance is normally set between 200°C and 235°C. Above 240°C, the production risk changes from normal viscosity management to thermo-oxidative degradation at the sheet edges, where the residence time is longest. The edge bead oxidizes first because the exposed surface area to process air is highest and cooling is non-uniform. Reintroduction of that oxidized edge trim as regrind then accelerates gel formation and reduces environmental stress crack resistance in the next pass.

The roll stack is operated with a top roll temperature of 80–100°C, a middle roll temperature of 70–90°C, and a bottom roll temperature of 50–70°C. These settings control sheet gloss, flatness, and residual stress. For sheet thicknesses between 2 mm and 12 mm, the line speed is adjusted to maintain a consistent roll gap and embossing pattern. The melt bank between the die exit and the roll nip is kept small, typically no more than 10–15 mm in diameter, to prevent excessive residence time and pre-shear. If the bank size fluctuates more than 5 mm, the sheet may show thickness variation beyond ±0.05 mm.

Sheet produced from HDPE 5007 is tested under ISO 178:2019 for flexural modulus and ISO 527-2:2012 for tensile yield. Typical HDPE of this density range exhibits a flexural modulus above 900 MPa and a tensile yield stress above 25 MPa, but grade-specific data for this exact sheet configuration are limited and must be verified on the production line. The high density of 0.958 g/cm³ gives the sheet enough stiffness for reusable industrial dunnage, automotive trunk mats, luggage shells, and formwork panels. The sheet can be thermoformed at a surface temperature of 125–135°C with vacuum levels of 0.06–0.09 MPa. Plug-assisted forming is recommended for draw depths greater than 50 mm to maintain wall thickness above 1.5 mm.

Regrind loading is the central process variable in sheet extrusion. Virgin HDPE 5007 can tolerate edge-trim regrind at 15–20 wt% when the melt temperature remains below 235°C. If the melt temperature exceeds 240°C, even 10 wt% oxidized regrind can generate enough gel particles to produce visible blemishes on embossed or thermoformed surfaces. A vented twin-screw compounding step can strip some volatile degradation products from reclaimed sheet, but this adds a second heat history and is generally not economical for a mono-material HDPE sheet plant. The practical boundary is therefore to process at the lowest melt temperature that still delivers adequate die lip flow: 200–225°C for thin sheet and 220–235°C for thick sheet.

The sheet line should be equipped with a screen changer using a 60/100 mesh pack for early detection of gel build-up. A pressure rise across the screen pack from 8 MPa to 14 MPa over 48 h indicates that gels or oxidized regrind are accumulating. Under such conditions, the screen change interval must be shortened and the regrind fraction reduced. Sheet conversion of HDPE 5007 is also sensitive to moisture at storage; although HDPE is not hygroscopic, surface condensation above 60% RH can introduce moisture into the feed throat and create steam-induced pinholes in the melt web. Pre-drying is not required below 0.02 wt% moisture content but becomes necessary when wet regrind or outdoor-stored pellets are introduced.