High-density polyethylene (HDPE) is a semi-crystalline polyolefin with density 0.941–0.967 g/cm³ when measured by ISO 1183-1 or ASTM D792. In injection moulding, the first article check is not a sampling formality; it is the gate through which tool, melt, and part geometry must pass before a full run. For HDPE, solidification is governed by crystallisation kinetics, not by a glass transition in the conventional processing range. First article data therefore must capture dimensions after post-mould shrinkage has stabilised, not immediately upon ejection. The workflow for HDPE first article checks before a full moulding run compresses multiple verification tasks—material lot rheology, tool thermal balance, hold-pressure transfer, gate freeze, and dimensional conformance—into a single release event.

Why does the first article check precede full moulding runs for HDPE?

Injection moulding grades of HDPE typically exhibit a melt mass-flow rate of 4–10 g/10 min at 190 °C under a 2.16 kg load when tested according to ISO 1133-1. The first article check verifies that the selected grade can fill the cavity under the available injection pressure and clamp force without exceeding the material’s oxidative or shear limits. On a 1,200 kN hydraulic injection moulding machine with a 25 mm screw diameter, a first article part with 2.0 mm nominal wall and 120 mm flow length can show short shots at melt temperature 195 °C even when calculated fill pressure is met, because gate freeze may prevent adequate packing of the core. The first article procedure therefore includes a deliberate short-shot series, increasing fill volume from 95 % to 100 % in 1 % increments, to isolate gate freeze from venting defects and insufficient shot weight.

Mould temperature setpoints below 20 °C accelerate skin freezing and can reduce sink, but they may increase warpage in parts with wall thickness transitions greater than 1.5 mm. The first article check records core and cavity surface temperatures using contact thermocouples or thermal imaging. A temperature difference above 5 °C across the face of a single cavity is diagnostic of a blocked bubbler, insufficient coolant flow, or an unbalanced multi-cavity circuit. Turbulent flow in coolant channels with a Reynolds number below 10 000 produces uneven heat removal, and cavity-to-cavity mass variation can exceed 0.5 % in tools without uniform cooling.

First article dimensional mapping for HDPE requires stabilisation at 23 ± 2 °C and 50 ± 10 % RH for a minimum of 24 h after ejection, or accelerated conditioning per ISO 291. Measurements taken immediately after demoulding commonly overstate length by 0.3–1.2 % for unfilled HDPE, depending on mould temperature and part thickness. Using a coordinate measuring machine with ±0.005 mm volumetric accuracy, features such as snap-fit beams, sealing bosses, and datum holes are checked against the tool drawing. A first article cannot be approved from caliper data alone when critical dimensions fall below ±0.05 mm. ISO 294-4 and ASTM D955 specify shrinkage determination on standard plaques, but production parts with varying wall thickness do not shrink uniformly. The first article should therefore record length, width, and hole-to-hole dimensions at three locations per cavity. Because HDPE crystallises along the direction of melt shear, longitudinal shrinkage can be lower than transverse shrinkage by 0.2–0.8 percentage points in gate-sealed parts.

Gate Freeze, Hold-Pressure Decay, and Short-Shot Boundaries Shape the Approval Window

Gate freeze time is a critical first article variable. For a side-gated part with wall thickness 1.5 mm, mould temperature 40 °C, and melt temperature 220 °C, gate freeze times are commonly observed in the range 3–8 s. A cavity pressure sensor positioned near the gate should show a hold-pressure plateau above 40 MPa for the entire gate-freeze interval. A pressure decay of more than 15 % before 5 s indicates premature gate sealing or insufficient screw cushion. Screw cushion at transfer should be maintained at 3–6 mm; below 2 mm the machine cannot maintain hold pressure, while above 8 mm residence time increases and molecular weight degradation risk rises. Transfer from velocity to pressure by screw position is preferred over timer-based transfer. Position repeatability of ±0.5 mm is required to keep shot mass within ±0.2 % of the approved first article value.

For HDPE, injection melt temperature is generally set between 190 °C and 260 °C. The lower bound is set by unmelted or partially melted particles, and the upper bound is set by oxidation-induced yellowing, odour, and possible melt-flow shift. A first article hold-time trial at 240 °C for 10 min can reveal whether the grade is stable under the planned hot-runner residence time. If melt mass-flow rate shifts by more than ±10 % from the virgin certificate of analysis value, the hot-runner temperature profile should be rebalanced before the full run is approved. Vent depth for polyolefin first article evaluation is typically 0.01–0.03 mm. Gas burn at the end of fill is separated from short shot or insufficient cushion by examining the short-shot progression; burn marks appearing at 98 % fill indicate inadequate venting, not a melt-temperature or fill-pressure defect.

Before a first article run begins, the material lot is characterised for melt flow rate and density, but HDPE blends containing regrind introduce additional variance. A production mixture of 80 wt% virgin grade with 20 wt% hot-runner regrind will show lower melt viscosity if the regrind has been exposed to multiple heat histories. The first article shot must therefore use exactly the same regrind ratio intended for production. If the MFR of the blend differs from the virgin CoA by more than ±15 %, the hold-pressure profile is re-optimised rather than accepting the first article. HDPE is generally regarded as non-hygroscopic, but surface condensation on cold granules stored at 5 °C and introduced to a 190 °C feed throat can generate splay. Predrying is warranted only when ambient dew point exceeds 15 °C or hopper residence time exceeds 4 h. Polypropylene contamination is immiscible with HDPE; a first article thermal analysis can detect a PP melting endotherm near 160–165 °C and should trigger reject of the material lot if contamination is suspected.

The following first article inspection checklist consolidates the release criteria for a typical HDPE injection moulded component. It is not a substitute for part-specific drawing requirements, but it establishes the minimum industrial data set.

When mould temperature deviates by more than ±10 °C from the first article setpoint

HDPE shrinkage is highly sensitive to mould-temperature history. A mould temperature change from 30 °C to 50 °C can increase total shrinkage by 0.3–0.8 % because slower cooling increases crystallinity in the solidifying layer. If the mould temperature deviates by more than ±10 °C from the approved first article setpoint, the first article approval is void. A new first article must be moulded because part mass, crystallinity, and post-mould dimensions shift even when machine settings remain unchanged. This boundary is especially important in multi-cavity tools with hot-runner drops. Cavity-to-cavity wall temperature variation above 5 °C can produce part mass variation above 0.5 % and differential shrinkage that cannot be corrected by hold-pressure adjustment alone.

Cooling time for HDPE first article evaluation is commonly assessed at 8–20 s/mm of nominal wall thickness. The lower end risks dimensional instability, while the upper end is limited by production economics and tool residence time. Cooling channels with diameter below 8 mm can show excessive pressure drop in the coolant circuit, reducing flow rate below the turbulent regime. The first article check should record flow rate, supply temperature, and return temperature for each circuit. A return-to-supply temperature difference below 2 °C often indicates short-circuiting or insufficient heat transfer from the steel.

Weld Lines, Knit Lines, and Low-Temperature Impact Signatures in HDPE

Weld-line strength is a first article concern when the part contains holes, inserts, or multi-gated flow paths. HDPE weld-line tensile strength can retain only 60–80 % of the non-weld strength, depending on melt temperature, mould temperature, and gas-vent location. Tensile bars cut from first article plaques and tested according to ASTM D638 across the weld line are compared against solid reference bars. A weld factor below 0.6 triggers rejection for load-bearing components. Low-temperature notched Izod impact testing according to ISO 180/A at -30 °C is more sensitive to crystalline tie molecules and molecular orientation than room-temperature tests. For cold-chain or outdoor parts, first article impact specimens are conditioned at the service temperature before testing. Published comparative datasets for HDPE weld-line impact at -30 °C in production tools are limited; first article decisions should default to part-specific qualification rather than generic literature values.

Environmental stress cracking resistance per ASTM D1693 is generally not included in first article checks because full ESCR testing can exceed 1,000 h for high-density grades. Where chemical exposure is part of the application specification, the first article record references the ESCR test start but does not delay moulding release. Any change in cooling rate, regrind content, or pigment dispersion can alter the crystalline morphology and therefore weld-line toughness, even when tensile yield is unaffected.

Approved first article records include injection velocity profile, switch-over position, hold pressure and time, plasticising speed, back pressure, screw cushion, hot-runner manifold temperature, nozzle temperature, and part mass. The records are retained against the material batch and mould revision. If any parameter changes by more than 5 %, if the tool is moved to another press with a different screw L/D ratio, or if the regrind ratio is altered, the first article is repeated before the full moulding run is authorised.