Regrind in HDPE Moulding: Allowable Percentage and How to Test It

Regrind in HDPE moulding refers to size-reduced high-density polyethylene scrap generated from sprues, runners, rejected parts, or edge trim that is reintroduced into the injection moulding feed stream. Allowable addition is not a single fixed value; it is governed by molecular-mass retention, contamination history, end-use compliance, and the test methods used to release each regrind lot. The keyword “Regrind in HDPE Moulding: Allowable Percentage and How to Test It” defines a quality-control boundary rather than a formulation constant. For non-load-bearing general-purpose mouldings, 20 wt% to 30 wt% in-house regrind is commonly accepted when melt-flow rate and density remain within ±15 % of virgin lot reference values, while food-contact and pressure-pipe applications may restrict regrind to 0 wt% or ≤10 wt% under audit-controlled streams.

HDPE moulding grades typically fall within a density range of 0.950 g/cm³ to 0.965 g/cm³ measured to ISO 1183-1:2019 and an MFR range of 4 g/10 min to 20 g/10 min at 190 °C/2.16 kg tested to ISO 1133-1:2022. Reprocessing history shifts these values through chain scission, oxidative carbonyl formation, and additive depletion. On production lines using 45 mm diameter, 22:1 L/D general-purpose screws, a regrind fraction with an MFR elevated by more than 25 % relative to virgin material produces lower injection-pressure demand, flash in low-clearance vents, and gate-stringing at the same shot size. Conversely, oxidatively thickened regrind can reduce MFR, causing short shots, screw overcushion instability, and non-return valve leakage. The operational boundary for demanding moulds is therefore tighter than a nominal blend percentage: the melt-flow ratio and the melt viscosity curve must be demonstrated, not assumed.

What Regrind Addition Does to Moulding-Grade Melt Rheology

Melt rheology is the most immediate process variable because HDPE injection moulding grades are selected for their MFR window. A virgin grade rated at 8 g/10 min will not behave the same after one regrind cycle if the MFR moves to 10 g/10 min or 12 g/10 min. Mould fill pressure responds approximately linearly to viscosity at constant shear rate, but the relationship is complicated by regrind particle-size distribution and irregular feed geometry. Irregular regrind particles with bulk density 0.40 g/cm³ to 0.55 g/cm³ feed less consistently than cylindrical virgin pellets with bulk density 0.58 g/cm³ to 0.62 g/cm³. This causes screw-starved feeding, metering-zone pressure oscillation, and shot-weight drift in machines with 60 mm screw diameter and clamp force 3500 kN or larger. To keep shot weight within ±0.3 % on a 1200 g shot, the regrind-virgin blend should be homogenized in a low-shear tumbling mixer for 15 min to 20 min before hopper loading, and the hopper throat should be maintained above 40 °C when plant relative humidity exceeds 60 % to prevent surface-moisture splay.

When a regrind lot is blended into virgin HDPE, the resulting MFR is not a simple arithmetic average. For miscible polyolefin melts, the log-additive rule often gives a better approximation: the logarithm of the blend MFR is assumed to scale with the mass-weighted logarithm of component MFRs. However, published data for HDPE regrind in this specific configuration is limited because molecular-weight distribution, long-chain branching, and residual antioxidant content alter the slope. A moulding operation performing production-scale validation should therefore measure the blend MFR directly on a capillary rheometer or melt indexer after dry blending and after screw plastication. A maximum shift of ±15 % from the virgin MFR is a common release criterion for close-tolerance mouldings, while ±20 % may be acceptable for thick-wall bins and pallets with no living hinge or snap-fit features.

Allowable Percentage Depends on the Moulded Part End-Use

The allowable regrind percentage is not set by material chemistry alone; it is set by the mechanical, thermal, and regulatory demands of the moulded part. A living-hinge closure moulded from a high-molecular-weight HDPE with virgin notched Izod impact above 8 kJ/m² may tolerate only 10 wt% regrind because repeated hinge flex cycles concentrate topographical discontinuities introduced by regrind carbon specks and gel particles. A storage bin or automotive wheel-arch liner may accept 30 wt% to 50 wt% of controlled in-house regrind when the tensile yield strength remains above 20 MPa and the elongation at break remains above 100 % under ISO 527-2:2012. A crate or pallet application may use up to 100 wt% regrind only if the regrind stream is segregated, stabilized, and tested to retain a notched Izod impact of at least 5 kJ/m² at 23 °C under ISO 180/A.

Post-consumer HDPE regrind differs from in-house regrind by an order of magnitude in contamination variability. In-house scrap generated from a single moulding cell is classified as clean when it contains less than 0.05 wt% foreign matter and is not exposed to outdoor ultraviolet radiation. Post-consumer HDPE flake can contain polypropylene, ethylene-vinyl acetate, barrier layers, pigments, and mineral fillers from closures. Even after sink-float separation and optical sorting, its melt flow and density are less stable. For injection moulding, post-consumer regrind should be limited to 0 wt% to 15 wt% unless the part is a non-critical thick-wall item and the supplier provides lot-wise ash, density, and MFR data with certificates. When post-consumer regrind is used above 15 wt%, the mould should be designed with wider gates, shorter flow paths, and vent depths in the range 0.02 mm to 0.05 mm to vent volatiles.

Testing a HDPE regrind lot for moulding release begins with particle-size distribution and bulk density because both alter feed consistency. Sieve analysis under ASTM D1921-18 is used to quantify the fraction retained on 8 mm and 4 mm screens. A moulding blend should contain less than 5 wt% of particles retained on an 8 mm screen because oversize particles bridge in the hopper and impair screw intake. Bulk density according to ASTM D1895-17 Method A should be recorded for every lot; a lot with bulk density below 0.35 g/cm³ requires longer screw-feed residence or regranulation. Moisture content is then measured by ISO 15512:2019 Method B or ASTM D6980-17. HDPE is not strongly hygroscopic, but surface water above 0.10 wt% from outdoor storage or washing introduces splay at injection speeds above 200 mm/s.

The next stage is melt characterization. ISO 1133-1:2022 procedure A at 190 °C with 2.16 kg is the default polyethylene condition. The measured MFR is compared with the virgin lot certificate; a shift greater than 15 % triggers a viscosity-curve check on a capillary rheometer at apparent shear rates from 100 s⁻¹ to 10,000 s⁻¹. Incoming regrind should also be tested for mineral contamination by ash content using ISO 3451-1:2019 or ASTM D5630-17, with a maximum ash of 0.10 wt% for clean in-house HDPE scrap. Ash above 0.30 wt% indicates filler, pigment, or dirt contamination and requires magnetic separation or air-wash classification before use.

Incoming Regrind QC Bench Sequence for Injection Moulders

A release sequence for regrind entering a moulding cell should be structured around specific instruments rather than visual acceptance alone. A typical sequence uses a melt indexer (ISO 1133-1:2022), a density gradient column or immersion balance (ISO 1183-1:2019), a universal tensile machine (ISO 527-2:2012 or ASTM D638-14), a pendulum impact tester (ISO 180/A at 23 °C), and a muffle furnace for ash (ISO 3451-1:2019). Each lot is dried at 80 °C for 2 h if moisture exceeds 0.10 wt%. Five moulded specimens are prepared from each blend ratio. The first test is MFR because it is the fastest predictor of process drift; the second is density because a density shift above 0.005 g/cm³ changes melt compressibility and part weight. Tensile yield and impact are run only for structural or safety-relevant parts because they consume material and time. Colour stability is evaluated against a master plaque under a D65 illuminant using a spectrophotometer and CIE L*a*b* coordinates; a ΔE greater than 2.0 in a natural HDPE grade signals pigment carryover or polymer degradation.

On the moulding floor, the most common batch-to-batch failure associated with regrind is not a mechanical property drop but a fill-pressure oscillation. When a 30 wt% regrind blend contains regrind from two different virgin grades with MFR values of 5 g/10 min and 18 g/10 min, the resulting broad molecular-weight distribution can produce pressure traces that vary by more than 8 bar between shots. This creates gate blush, sink marks, and ejection-force variation. The corrective path is not a blanket no-regrind rule but a segregation rule: regrind from high-flow HDPE must be kept in labelled gaylords and added only to high-flow virgin lots with a blending ratio not exceeding 10 wt%.

When Contamination and Gel Defects Override Percentage Limits

Even 5 wt% contaminated regrind can reject a moulding line, so contamination testing overrides the nominal percentage. If the regrind contains polypropylene caps, paper labels, or metal leaf, the melt will contain unmelts and foreign inclusions that block gates below 0.5 mm. A melt-filtration test with a 25 mm single-screw extruder fitted with a 100-mesh/200-mesh screen pack can expose gel and particle contamination. The pressure rise across the screen pack is recorded for 20 min at 230 °C; a pressure rise above 2.0 MPa indicates unacceptable gel load for technical mouldings. This test is not defined by a single ISO code, so the method must be documented in the plant QC procedure and calibrated with the virgin HDPE reference lot. Incompatible polymeric contamination, such as PET or PVC, is more severe because it degrades at HDPE processing temperatures of 220 °C to 260 °C, releasing acidic or gaseous by-products that corrode mould surfaces and create surface splay.

Repeated heat history is the principal hidden variable in HDPE regrind. A part reground once from virgin has experienced one additional melt residence; a part that contains 30 wt% regrind and is then rejected and reground again has experienced multiple cumulative heat cycles. On a 40 mm diameter, 20:1 L/D injection screw running at 220 °C barrel temperature and 15 MPa back pressure, each pass through the plastication unit adds approximately 2 min to 3 min of residence time. The consequences are measurable through oxidative induction time under ISO 11357-6:2018. Virgin HDPE stabilizer packages are formulated for a defined melt residence; regrind that has consumed antioxidants shows a reduction in OIT. A loss of more than 50 % relative to the virgin reference OIT is a common release cutoff for high-temperature moulding above 240 °C. Below that threshold, the blend can still exhibit yellowing, carbonyl formation, and reduced weld-line strength.

Blend-ratio validation is performed on the production injection moulding machine, not only on laboratory specimens. A machine with clamp force 4000 kN and screw diameter 55 mm is purged with virgin HDPE until stable, then the regrind-virgin blend is run for 30 consecutive shots. Shot weight is recorded with a forced-removal balance to ±0.1 g. The coefficient of variation of shot weight must remain below 0.3 % for closure moulds and below 0.8 % for thick-wall industrial parts. If shot-weight CV exceeds the threshold, the regrind particle-size distribution is rechecked on a 6.3 mm sieve and the hopper magnet is inspected for ferrous particles. The mould should not be accepted until the same lot demonstrates stable fill at the programmed transfer position with injection pressure within ±5 bar of the virgin reference.