OEM and Private Label in Polyethylene Products: What to Lock Down is not a marketing checklist; it is a set of resin, process, and compliance boundaries that control whether a converter can change supplier, additive package, or regrind ratio without shifting finished-part performance. Polyethylene (PE) resin selection is governed by parameters that remain under-specified when a private-label document records only density and melt flow rate. Batch-to-batch variance in molecular weight distribution, comonomer type, catalyst residue, and stabiliser loading alters shrinkage, warpage, environmental stress crack resistance, and organoleptic migration. For OEM PE components, the controlling document locks the resin manufacturer, grade designation, density at 23 °C per ISO 1183-1:2019, melt mass-flow rate at 190 °C/2.16 kg per ISO 1133-1:2022, comonomer type where applicable, and the antioxidant/processing aid package by trade name and mass loading.

Which resin parameters remain under-specified in PE private-label contracts?

The base resin grade and additive package are locked before downstream machine parameters are adjusted. A change from a Ziegler-Natta HDPE to a metallocene-catalysed HDPE of identical density can lower the seal initiation temperature in film or reduce notched impact strength in injection-moulded caps because comonomer distribution differs, even when average comonomer content is unchanged. Supplier certificates of analysis are held at lot level. Melt flow rate tolerance of ±15% of nominal is a common supplier window, but high-speed cap moulding on a 64-cavity hot-runner tool frequently requires ±5% to control fill variation across cavities. Density, measured on a gradient column per ISO 1183-1:2019, is a necessary but insufficient lock: a lot with density 0.961 g/cm³ can pass a density window but fail a cap-cracking test when stored in contact with surfactant solutions if the ESCR floor of ASTM D1693 condition B is not separately specified.

For PE closures, freeze environmental stress crack resistance, notched Izod impact per ASTM D256, and flexural modulus per ISO 178. ESCR does not correlate linearly with melt flow rate or density. An HDPE grade with MFR 2 g/10 min and density 0.952 g/cm³ may exceed 1000 h in ASTM D1693 condition B, while a grade with MFR 8 g/10 min and density 0.956 g/cm³ falls below 100 h. A private-label agreement that allows any high-density grade within a density band is therefore not technically controlled.

In a cap moulding production run, a change in PE melt flow ratio from 17 to 21 increases the risk of dimensional variation in hot-runner fill. Melt flow ratio, measured as MFR at 21.6 kg divided by MFR at 2.16 kg, reflects molecular weight distribution broadness. A shift from 17 to 21 indicates a broader distribution, which can increase shear thinning, reduce injection pressure at constant melt temperature, but increase die swell and warpage after ejection. The moulder records fill pressure and cushion position on a 2500 kN clamp-force injection moulding machine. Acceptable fill-pressure variation across lots is limited to ±3% at a melt temperature of 220 °C; exceeding this limit triggers a resin-lot rejection even if MFR remains inside the nominal range. Published data for the exact interaction between melt flow ratio and cap ovality in a specific tool is limited because tool geometry and hot-runner balance remain tool-specific; the lock-down must rely on in-house process capability data tied to the resin lot.

Twin-screw compounding and pellet stabilisation locks for PE converters

Where a private-label PE product is compounded before conversion, the process specification includes a co-rotating twin-screw extruder with L/D ratio 40:1 and vacuum venting at -0.08 MPa to remove residual volatiles. The screw profile, barrel temperature profile, and die plate geometry are part of the locked process because changes in shear history alter dispersion of the processing-aid polymer and distribution of primary antioxidants. Melt temperature at the die is maintained between 210 °C and 240 °C depending on the PE grade; higher temperatures increase oxidative chain scission, while lower temperatures reduce homogenisation of pigments or slip agents. Oxidative stability of the pellet is checked by oxidative induction time per ASTM D3895 at 200 °C. A private-label composition for pipe or non-food industrial packaging may require an OIT value above 20 min, while a thin-wall food container may require an OIT above 5 min and a lower limit on volatile residues per VDA 277 or ISO 11890-2. Publication of exact peroxide formulation levels is often restricted by additive suppliers; the converter locks the additive masterbatch trade name and let-down ratio instead of the active ingredient percentage.

Blown film private-label agreements in food packaging control blend ratio, not only final tensile properties. On a 3-layer coex line with 400 mm die and 1000 kg/h output, an increase from 20 wt% to 35 wt% LLDPE in the middle layer changes frost line height and requires adjustment of air ring, blow-up ratio, and take-off speed to maintain film thickness at 50 µm within ±5%. A converter must not compensate for a resin change by raising melt temperature above 220 °C, because this accelerates oxidation and may push off-flavour profile beyond the limit of food-contact organoleptic tests. For non-food industrial film, gloss and haze per ASTM D1003 and coefficient of friction per ASTM D1894 are locked at 45° and 0.2–0.4 respectively. Machine-direction tear and transverse-direction tear may invert with blend changes, so the private-label document records the permitted resin blend ratio and additive masterbatch before converter start-up.

Blow-moulded HDPE containers require a locked parison swell specification because resin lot changes alter extrudate swell at a given shear rate. On a shuttle blow moulding machine with 80 mm extruder and 6-cavity mould, a change from a broad molecular weight distribution HDPE to a narrow distribution grade at the same MFR can reduce die swell by 10–15%, requiring pin adjustment. The lock-down includes parison length, pinch-off flash thickness, and drop impact at -20 °C per ASTM D2463. A grade substitution that increases density from 0.952 g/cm³ to 0.958 g/cm³ may improve top-load strength but lower drop impact; both properties are locked jointly rather than in isolation.

These ranges are industrial synthesis windows, not specification limits. A private-label product must set a narrower internal band based on the tool and application. For a HDPE blow-moulded container with density 0.950 g/cm³ and MFR 2 g/10 min, the internal lock may require a density band of ±0.002 g/cm³ and MFR band of ±0.3 g/10 min to maintain drop impact and top-load performance.

When PE regrind ratios exceed 20 wt% in injection moulding

Incorporation of post-industrial regrind into OEM polyethylene parts is controlled by a formal regrind protocol because regrind reheating reduces molecular weight and antioxidant reserves. At 20 wt% regrind, the mixture is gravimetrically blended and fed to a screw with vacuum venting; melt pressure before the non-return valve is monitored. The allowable regrind fraction is lower for thin-wall food containers because melt viscosity shift alters fill length: a 0.5 mm wall container can lose more than 5% flow length when 30 wt% regrind with the same nominal MFR is used, owing to gel content and additive depletion. When regrind content exceeds 20 wt%, notched Izod impact per ASTM D256 can fall by 20–40% in HDPE injection-moulded parts, and ESCR per ASTM D1693 may drop below 100 h. The lock-down records regrind source, maximum ratio, maximum reuse cycles, sieve residue after melt filtration, and OIT after blending. A 200-mesh screen pack is used to remove particulate contamination; pack pressure increase above 25 bar triggers filter change. Published data for regrind limits in specific PE compounds is limited because part geometry, hot-runner shear, and original stabiliser package dominate degradation.

Sterile-barrier PE seal strength falls outside the lock when jaw temperature varies by ±5 °C

For PE films and pouches used in sterile barrier systems, the private-label specification locks sealant layer resin, seal temperature window, and peel force after ageing. ISO 11607-1:2019 requires that the packaging system maintain sterility through distribution; PE seal layers are selected for low-seal initiation and broad hot-tack window. Seal strength is measured per ASTM F88/F88M-21 after sealing at 0.2 MPa and 120–160 °C, with dwell times of 0.5 s to 2.0 s. The crystalline melting point of the film is measured by differential scanning calorimetry per ISO 11357-3; a shift in melting peak from 106 °C to 112 °C indicates a blend change from LDPE to HDPE-rich skin and must be rejected for a specified tray lidding process. The sealing window is not a resin property alone: on a 300 mm width sealing station with coated steel jaws, jaw pressure distribution across the width must be verified by pressure-indicating film, because PE seal strength can vary ±1 N/15 mm when jaw temperature gradient exceeds ±5 °C.

For an injection-moulded HDPE industrial tote or reservoir, the private-label dimensional lock controls crystallisation-dependent shrinkage after demoulding. PE shrinkage in the flow direction can range from 1.5% to 3.0% in HDPE and 0.8% to 2.0% in LDPE, depending on wall thickness, melt temperature, holding pressure, and filler loading. The tool is cut to a specific shrinkage factor; a resin lot change that increases viscosity and reduces holding-pressure transmission can produce parts outside drawing tolerance even if the resin specification is nominally unchanged. As-moulded dimensions are measured at 24 h and 48 h after ejection because post-mould crystallisation continues at 23 °C. Holding pressure is set between 60 MPa and 100 MPa on a 3000 kN clamp-force machine, with screw forward time maintained until gate freeze. A change in MFR from 6 g/10 min to 9 g/10 min may require holding-pressure compensation; if the contract permits that change without approval, the tool steel can no longer be guaranteed to produce parts within a dimensional capability index Cpk ≥ 1.33. The OEM lock-down therefore pairs resin viscosity data with specific volume data generated per ISO 17744 so that pressure-volume-temperature behaviour remains traceable to the approved grade.