When high density polyethylene crates are injection moulded on horizontal clamp force machines ranging from 4 000 kN to 16 000 kN, the collision of separate melt streams at handle apertures, lattice ribs, gussets, and valve-gated flow boundaries forms weld lines that are structural discontinuities rather than cosmetic marks. The impact strength of an HDPE crate under cold-room or rough-handling conditions is frequently limited by these weld lines, because the impact fracture initiates at the point of lowest local macromolecular entanglement and propagates along the interface. Unfilled HDPE is a semi-crystalline polyolefin with a density between 0.940 g/cm³ and 0.970 g/cm³, a crystallinity commonly between 60 % and 85 %, and a broad melting range that begins near 120 °C and ends near 135 °C; these thermal characteristics control the available time for interdiffusion after two melt fronts contact. In production, weld lines are rarely eliminated by design. They are instead managed through gate count, gate sequencing, melt temperature, mould wall temperature, injection speed, packing pressure, and vent location. The strength loss at the weld line is typically quantified by comparing notched Charpy or Izod impact values from specimens prepared according to ISO 179-1 or ISO 180 with values from geometrically equivalent unwelded specimens. For crate grades of HDPE, reported weld-line impact strength retention is strongly process-dependent, with industrial and published laboratory data commonly spanning 55 % to 90 % of the unwelded reference; published data for a specific commercial crate-tool geometry is often limited because weld-line location and local cooling history are not transferable between moulds.

The impact-relevant weld line in a crate should not be interpreted as a simple crack. It is a three-dimensional region containing a surface groove, a zone of flow-front skin material, a core interface, and sometimes trapped volatiles or gas-venting residue. When a notched Charpy specimen is cut from a crate wall and tested according to ISO 179-1/1eA, the notch is machined into the bulk polymer, but the weld line may still introduce an independent stress concentrating feature if the notch path intersects the interface. Conversely, when the weld line itself is used as the notch without additional machining, the measured impact energy reflects the synergy of poor chain entanglement, surface roughness, and thermal history rather than the intrinsic bulk toughness of HDPE. This is why weld-line impact testing is critically dependent on specimen preparation protocols described in ISO 294-1. The standard specifies the injection moulding of multipurpose and bar test specimens under controlled conditions, including tool temperature, melt temperature, injection velocity, hold pressure, and hold time. When weld-line specimens are produced using a double-gate tool, the weld line is generated at the centre of the test bar; this configuration approximates the flow-front collision in a crate lattice but does not reproduce the local wall thickness, corner radius, or cooling asymmetry of a real crate handle or rib junction. Therefore, laboratory weld-line impact data must be used as a comparative ranking tool rather than as an absolute prediction of whole-crate impact performance. Whole-crate impact data are better correlated with instrumented puncture impact according to ISO 6603-2 or drop testing according to ASTM D5276-19.

What Limits Interfacial Healing Time in Cold Runner Crate Tools?

In a cold runner or hot runner crate tool, the two flow fronts that form a weld line have been cooled, sheared, and oriented before they meet. The interfacial healing of HDPE depends on the contact time during which polymer chains can diffuse across the interface, build bridging entanglements, and co-crystallise before the local temperature falls below the crystallisation onset. For high density polyethylene, the rate of chain self-diffusion is strongly molecular-weight dependent; reptation time scales with molecular weight to the power of 3.4, so a high molecular weight crate grade with a melt flow rate near 3 g/10 min at 190 °C under 2.16 kg load will require far longer for complete interfacial healing than a lower molecular weight grade with a melt flow rate near 8 g/10 min under the same ISO 1133-1 condition. However, the gain in interdiffusion speed obtained with a lower molecular weight grade is partially offset by a reduction in bulk impact strength, because fewer high molecular weight chains and tie molecules remain to bridge crystallite boundaries. Commercial HDPE crate grades are therefore balanced between flow length, weld-line healing, and impact resistance. The thermal contact time at the weld interface is often shorter than the terminal relaxation time of the high molecular weight fraction, so the interface retains a deficit of load-bearing entanglements. This deficit is amplified when the mould wall temperature is held at conventional cold-mould settings between 20 °C and 40 °C, because the flow front surface cools rapidly to the crystallisation onset. If the melt temperature at the front is 210 °C to 220 °C at fill completion, the outer skin may be below 125 °C within fractions of a second; the core may remain hotter and allow partial healing, but the surface remains weak. Mechanical impact loading initiates cracks at the surface groove, and the crack then propagates through the partially healed core. The result is that room-temperature notched Charpy impact values for weld-line specimens are lower than unwelded specimens even when the bulk crystallinity of the weld region is not dramatically different from the surrounding material.

Equipment configuration also influences healing time. General-purpose injection screws with L/D ratios between 20:1 and 24:1 and compression ratios between 2.5:1 and 3.5:1 are commonly used for HDPE crate production. Inadequate screw recovery time, excessive screw speed, or low back pressure can generate a non-uniform melt temperature distribution. When the melt temperature is not homogeneous, one flow front may arrive at the junction with a lower average temperature, reducing the interfacial diffusion distance. Thermolators or oil-based mould temperature control units set to 40 °C or below provide fast cycle times but narrow the healing window. When mould water temperature is raised to 60 °C or 70 °C, weld-line impact strength usually improves because the interface remains above the crystallisation onset longer, but cycle time increases, and demoulding distortion can occur in large crates. Some crate moulds use differential temperature control, with higher mould temperatures near handle weld lines and lower temperatures in flat wall regions, to obtain an acceptable balance of cycle time and impact performance. Published data for specific combinations of HDPE grade and crate tooling is limited, but the relationship between higher mould temperature and improved weld-line strength is well established for semi-crystalline polyolefins.

Across multi-cavity crate tools with hot runner valve gates, weld-line position and severity are frequently adjusted by changing gate opening sequence. If all valve gates open simultaneously, the melt front from each gate meets the adjacent front halfway between the gates, creating a linear weld line that may run across the bottom of the crate or through a high-stress handle region. If the gates are opened sequentially, the later melt stream fills behind the earlier melt front, shifting the weld line toward a lower stress area or reducing the head-on meeting angle. Sequential valve gating can improve weld-line impact strength, but it increases hot runner complexity and may create a visible flow transition line if the delay is too long. The connection between gate sequencing and impact strength should be validated using instrumented puncture impact on sections cut from the crate, because laboratory double-gate Charpy specimens do not capture the effect of sequential fill patterns. In industrial practice, the weld-line impact strength of HDPE crates is also influenced by the amount and quality of regrind. HDPE crates are frequently moulded from virgin material diluted with in-house regrind. Oxidative degradation during repeated heat histories reduces the high molecular weight fraction and may lower the zero-shear viscosity, which can improve weld-line surface wetting but reduces bulk impacts. The net effect depends on the regrind fraction and the original stabiliser package.

Rheological, Thermal, and Surface-Notch Contributions to Weld Line Impact Loss in HDPE

At the weld-line interface, the impact strength loss in HDPE crates is better described by three superimposed contributions: reduced chain entanglement density, altered crystallinity and tie-molecule distribution, and surface stress concentration. Reduced entanglement density arises because high molecular weight chains must reptate across the interface, and the available time is governed by the cooling rate at the junction. The relevant self-diffusion coefficient of HDPE in the melt at 200 °C is orders of magnitude lower than that of short alkanes, and the diffusive penetration depth grows only as the square root of time. For high density polyethylene, the time required to form a significant number of entanglements across a flowing interface is often longer than the fill-and-hold interval; therefore, the weld line remains a plane of weakness. Crystallisation adds another kinetic constraint. As the flow front cools, row nuclei may form along the interface, and growth fronts of lamellae advance from both sides. When the two fronts crystallise before substantial chain interpenetration, the interface contains fewer tie molecules crossing the plane. Tie molecules are the chains that link adjacent lamellae and transmit stress during impact; a deficit of tie molecules produces brittle failure even if the local spherulite size is small. The surface stress concentration arises because the weld line often produces a shallow groove at the outer surface of the crate wall. Even a groove with a depth of only a few micrometres and a small root radius can act as a crack initiator under impact loading. This effect is measured indirectly by differentiating between notched and unnotched impact tests. In notched Charpy tests according to ISO 179-1, the machined notch may dominate the stress concentration, while in unnotched tests the natural weld-line groove can initiate fracture. In crate service, the weld-line groove is more important than the machined notch because impact loading occurs at the free surface.

The crystallisation behaviour of HDPE also means that weld-line strength can be affected by nucleating agents, pigments, and other processing additives. Some inorganic pigments can act as heterogeneous nucleation sites, increasing crystallisation temperature and reducing the time window for interfacial healing. The same pigment may also increase local viscosity, changing the shape of the flow front and the depth of the surface groove. Therefore, a black or green crate grade may exhibit a different weld-line impact retention than an unpigmented natural grade even if the base resin is identical. The melt flow rate determined according to ISO 1133-1 at 190 °C and 2.16 kg is an important incoming quality control parameter because an upward shift in melt flow rate may indicate a lower molecular weight fraction or chain scission. A crate grade that drifts from 5 g/10 min to 7 g/10 min may show better weld-line surface healing but poorer whole-crate drop performance. Conversely, a downward shift may improve bulk impact strength but worsen weld-line retention because of slower interdiffusion. Batch-to-batch variation in molecular weight distribution cannot be detected by melt flow rate alone; dilute solution viscosity or rheological dynamic frequency sweep data are needed to assess the high molecular weight tail. These measurements are not commonly performed on the moulding floor, so processors often rely on instrumented impact and whole-crate drop testing as indirect indicators of weld-line quality. When regrind is added, the melt flow rate may increase because the average chain length decreases after repeated shear and thermal stress. The resulting weld-line appearance may improve, but impact strength at low temperatures may fall. This trade-off is especially visible at -20 °C, where HDPE loses much of its ductility and the weld-line interface behaves as a brittle plane.

When Melt Fronts Meet at Angles Below 90°, the Weld Line Root Radius Controls Crack Initiation

In a crate lattice, two melt fronts do not always meet perpendicular to the wall. At the intersection of a bottom rib and a side wall, or at the junction between a handle boss and the main surface, the angle of approach may be shallow or highly asymmetric. When flow fronts meet at an angle below 90°, the weld line tends to form a sharp V-shaped groove with a small root radius. When the meeting angle exceeds 135°, the flow fronts merge more gradually and the surface groove is shallower. This geometric effect is particularly important in HDPE crates because the impact load is rarely uniaxial. A crate dropped on a corner experiences mixed-mode loading: the weld line at a handle aperture may experience tension perpendicular to the interface along with shear. A sharp weld-line root radius promotes crack initiation at lower energy. Mould-fill simulation can identify these low-angle meeting points during tool design, but the simulation must incorporate accurate heat transfer coefficients, melt rheology, and crystallisation kinetics to be useful. Short-shot studies on the tool are used to visualise the actual meeting fronts and to measure the weld-line angle. When an unacceptable weld line is found, tool modifications include adding a melt trap, relocating the gate, changing wall thickness, or increasing local mould temperature. In some cases, a small radius or land is machined near the meeting point to convert a sharp V-groove into a less severe U-groove. The effect of these modifications on impact strength should be verified using instrumented puncture impact according to ISO 6603-2 on specimens cut from the crate, because standard laboratory bars cannot reproduce the local geometry.

Venting is another variable that changes the weld-line root radius and the local oxygen environment. At the last point of fill, trapped air and volatiles can be compressed against the weld line, producing a burn mark, a gas void, or a degraded polymer film at the interface. A vacuum assist or porous vent insert near the weld-line location can remove these gases and improve impact strength. Vent placement is especially critical in thin-wall crate sections where the flow front velocity is high and air displacement is rapid. Poor venting can also cause the melt front to cool locally due to the insulating effect of trapped gas, creating a cold slug that is incorporated into the weld line. That cold slug may not bond to the surrounding polymer and can become a disk-like defect visible only after impact fracture. The size and frequency of such defects are affected by regrind level, because recycled material may contain small amounts of moisture, printing ink, labels, or oxidative degradation products. While HDPE itself is non-hygroscopic and does not require drying at relative humidity below 60 %, surface moisture on regrind or wet pellets can produce steam at the vent and interfere with weld-line consolidation. If moisture is suspected, a desiccant or hot-air hopper dryer set to 70 °C to 80 °C for 2 hours to 4 hours may be used, although for clean virgin HDPE this is usually unnecessary. The introduction of incompatible contaminants such as polypropylene caps or labels can reduce weld-line strength if those contaminants concentrate at the interface. Melt filtration and source control are therefore part of weld-line management in crate production.

Instrumented Puncture Testing and Whole-Crate Drop Performance Correlation

Because the ultimate performance of an HDPE crate is judged by surviving dropping, stacking, and cold-room handling, the translation of weld-line Charpy data into whole-crate impact rating requires a hierarchy of tests. At the material level, notched Charpy impact strength is determined according to ISO 179-1/1eA. The test uses a type 1 specimen with thickness of 4 mm, width of 10 mm, and a V-notch with tip radius of 0.25 mm. Weld-line variants are produced in a double-gate mould according to ISO 294-1. At the intermediate level, sections cut from a crate wall or handle are subjected to instrumented puncture impact according to ISO 6603-2. This test measures peak force, energy at peak force, and total energy under biaxial loading, and it can be performed at 23 °C and -20 °C. The biaxial loading mode is more representative of a crate wall struck by a blunt object than a uniaxial notched Charpy test. At the product level, whole crates are tested under drop conditions according to ASTM D5276-19 or internal company specifications that derive from this standard. A loaded crate is dropped from a defined height onto a rigid surface, and the presence of cracks, hinge breaks, or complete separation at weld lines is recorded. The drop height, load, conditioning temperature, and impact orientation vary by application; no single pass/fail criterion is defined globally because crate designs and service conditions differ.

The correlation between laboratory weld-line impact data and whole-crate drop performance is not linear. A weld-line retention of 70 % in a double-gate Charpy bar may be acceptable in a crate design where the weld line is located in a low-stress area. The same retention may be unacceptable in a handle web that experiences tensile impact during a corner drop. The location and orientation of the weld line relative to the loading direction are often more important than the absolute material strength. Crate tool designers therefore use structural simulation to map stress during drop events and to avoid placing weld lines in areas of high principal tensile stress. If weld lines cannot be moved, the processor increases mould temperature, adds sequential valve gating, or increases the meeting angle. The burden of verification falls on instrumented puncture testing and whole-crate drop testing rather than on Charpy bars alone. For HDPE crates containing recycled content, the weld-line impact strength may vary from lot to lot even when the virgin resin and regrind ratio are constant, because contaminant concentration and oxidative degradation products vary with the source of recycled material. Published data for this specific configuration is limited; production plants typically generate an internal correlation matrix from at least 30 drop-tested crates per mould and lot, then track weld-line impact retention as a process alarm. Without such data, statements about weld-line safety in HDPE crates based solely on a generic polymer grade are unreliable. The role of weld lines in the impact strength of high density injection moulded crates is therefore a combined function of material molecular architecture, tool geometry, thermal history, and impact test severity, and each variable must be controlled if the weld line is to be relegated from a fracture origin to a benign flow transition.

Across cold-room crate installations, the impact of weld-line weakness becomes more severe as temperature decreases. At -20 °C, the brittle fracture energy of HDPE is substantially lower than at 23 °C, and the weld-line interface may fail before the bulk polymer yields. Crate designers who specify impact performance at room temperature but use the crates in freezers or unheated logistics chains may observe field failures that are not reproduced by ambient laboratory drop tests. Conditioning of specimens and whole crates at the intended service temperature is therefore necessary, and the conditioning time should allow full wall-thickness equilibration. A crate wall of 4 mm thickness may require several hours at -20 °C to reach thermal equilibrium. In practice, whole-crate tests at low temperature are more demanding than laboratory specimens because of thermal gradients and local stress concentrations at handle openings. Instrumented puncture testing at low temperature provides an intermediate step, because it applies biaxial loading at controlled strain rates to specimens cut from the actual wall. This method can distinguish between a weld line that fails by brittle cracking from a well-formed weld line that yields and dissipates energy. The resulting data, when plotted against drop height and load, give the plant an empirical boundary for acceptable weld-line strength in a specific crate design. Without such data, a specification based only on notched Charpy impact strength may pass material that fails in a whole crate because the test notch dominates the fracture process and masks the contribution of the natural weld-line groove. Conversely, a Charpy weld-line specimen may under-predict real performance when the crate weld line is located in a residual compressive zone or is oriented parallel to the principal impact stress. The interpretation of weld-line effects on HDPE crate impact strength therefore depends on the exact stress state, temperature, and geometry of the weld line in the moulded article, not on a single universal retention factor.

Operational boundaries must also be respected. If the melt temperature exceeds the upper limit recommended by the HDPE supplier, oxidative degradation may produce carbonyl species that embrittle the weld line even though initial surface appearance improves. If the mould temperature is raised too aggressively in a thick section, cycle time can increase beyond economic viability and the crate may exhibit sink marks or warpage. If the regrind fraction is too high, batch-to-batch weld-line impact strength may vary unpredictably because of thermal history and contamination. If venting is inadequate, no amount of melt temperature increase can fully compensate for gas-void defects at the weld line. These boundary conditions apply specifically to high density polyethylene crates and may not transfer to glass-filled or mineral-filled polyolefins, which have different weld-line sensitivities due to filler orientation at the interface. For unfilled HDPE, the weld line is generally less catastrophic than in a random glass-fibre reinforced polyolefin, because unfilled HDPE does not lose fibre-bridging reinforcement at the interface. Nevertheless, the weld line remains the most frequent site of impact failure in crates because it combines low entanglement, surface stress concentration, and often a low-temperature service condition.