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Across multicavity thin-wall injection moulding cells producing container closures and food packaging from high-fluidity linear low-density polyethylene (LLDPE), the simultaneous achievement of adequate knit-line mechanical resistance and dimensionally stable anisotropic shrinkage remains a process-controlled optimisation problem. High-fluidity LLDPE grades are defined here by melt mass-flow rate values between 20 g/10 min and 60 g/10 min when determined at 190 °C under a 2.16 kg load in accordance with ISO 1133-1:2022; densities typically lie between 0.918 g/cm³ and 0.935 g/cm³ according to ISO 1183-1:2019. These resins are processed in 25 mm to 35 mm screw diameter reciprocating injection units with screw length-to-diameter ratios of 20:1 to 25:1 and compression ratios between 2.2:1 and 2.8:1, where melt temperatures are maintained between 190 °C and 230 °C. In such systems, the low zero-shear viscosity and pronounced shear-thinning permit fill speeds of 150 mm/s to 300 mm/s and wall sections of 1.2 mm to 2.5 mm; however, the same low average molecular weight distribution that enables high flow reduces the chain entanglement density at the advancing front, so the tensile strength retention at a weld line measured on double-gated ISO 527-2 type 1A specimens can fall to 52%–68% of the un-welded reference. Shrinkage is comparably direction-dependent: moulded 60 mm × 60 mm × 2.0 mm plaques prepared under ISO 294-3 frequently show flow-direction shrinkage of 1.8%–2.4% and transverse-direction shrinkage of 1.2%–1.6% after conditioning at 23 °C ± 2 °C and 50% ± 5% relative humidity per ISO 291, yielding an anisotropy ratio of 1.3–1.6. Pre-drying is not normally required if the material is stored in sealed containers; when exposure to relative humidity above 60% has occurred, drying at 80 °C for 2 h in a desiccant dryer is recommended to avoid surface splay. The following scenarios examine the interactions among melt temperature, injection speed, packing pressure, and mould temperature that control weld-line strength and shrinkage anisotropy in production-scale high-fluidity LLDPE injection moulding.
At weld lines generated by head-on collision of two melt fronts in a cold runner or multicavity hot-runner system, the strength of the knitted interface depends on the depth of chain interdiffusion relative to the radius of gyration of the polymer chains and on the number of load-bearing entanglements established before crystallisation. For linear polyethylene with an entanglement molar mass between 1,200 g/mol and 1,400 g/mol, un-welded tensile strength arises from a network that transmits stress across many entanglements; when MFR exceeds 40 g/10 min, the average molar mass decreases sufficiently to reduce the terminal relaxation time and the maximum attainable entanglement density at the interface. Capillary rheometry performed at 190 °C on a 50 g/10 min grade gives apparent shear viscosities of 35 Pa·s to 70 Pa·s at 1,000 s⁻¹ and often less than 20 Pa·s at 10,000 s⁻¹, which promotes rapid fountain-flow spreading across the mould surface. The consequence at the knit line is counterintuitive: the low viscosity permits the two fronts to conform mechanically and eliminate visible V-notches, but the shallow interpenetration distance after cooling and the low number of tie molecules per unit area produce a brittle failure path. Tensile testing of double-gated ISO 527-2 type 1A specimens at 50 mm/min shows that increasing melt temperature from 190 °C to 220 °C raises weld-line strength retention from approximately 48% to 58% for a 50 g/10 min grade, while increasing mould temperature from 20 °C to 45 °C adds another 5–10 percentage points. These process changes do not alter the molecular weight distribution; they act entirely by increasing the diffusion time before the advancing front solidifies. When the melt temperature exceeds 230 °C under extended residence in a hot-runner system, oxidative chain scission can offset the gains, particularly in stagnant resin zones. The measurement of weld-line strength retention must therefore be linked to the local thermal history of the front, not simply to the barrel setpoint.
| Specimen condition | Melt temperature | Mould temperature | Knit-line tensile strength retention |
|---|---|---|---|
| Double-gated ISO 527-2 type 1A, 20 g/10 min LLDPE | 200 °C | 20 °C | 62% |
| Double-gated ISO 527-2 type 1A, 50 g/10 min LLDPE | 200 °C | 20 °C | 49% |
| Double-gated ISO 527-2 type 1A, 50 g/10 min LLDPE | 220 °C | 45 °C | 61% |
| Double-gated ISO 527-2 type 1A, 50 g/10 min LLDPE | 230 °C | 60 °C | 66% |
On a 1,600 kN hydraulic injection moulding machine fitted with a 35 mm diameter three-zone screw and shut-off nozzle, a 1.8 mm wall-thickness rectangular lid was moulded with a single edge gate at one end and a merging flow front created by an insert at the opposite end. The machine was operated at 220 °C barrel setpoint, 60 °C water-circulated mould temperature, 180 mm/s injection speed, and 65 MPa final packing pressure. In this production-scale configuration, the cavity pressure at the melt front fell from 55 MPa near the gate to less than 12 MPa at the weld line, as recorded with a piezo-electric cavity-pressure transducer. This pressure collapse is significant because the interfacial chains can diffuse only while the local temperature remains above the crystallisation onset of approximately 115 °C; the pressure drop at the knit line reduces the local melting point elevation and accelerates solidification. The measured tensile strength retention at the weld line was 61% of the single-gate reference bar, while the elongation at break was only 18% of the un-welded value. The difference between strength and elongation retention indicates that the weld line behaves as a low-extension notch even when the visual appearance is acceptable. Such data, derived from a production-scale water-circulated mould and servo-electric injection unit, illustrates that high melt fluidity does not compensate for inadequate local pressure at the point of front coalescence.
When two melt fronts meet in a thin-wall LLDPE cavity, the region behind each front has spent several tenths of a second in contact with the mould wall and has developed a solidified skin, while the core remains molten. The fountain flow at the front deposits skin material from the centre outward, so the contact angle and melt temperature at the cavity wall determine whether a sharp notch or a diffuse blend forms. In a 2.0 mm thick plaque moulded at 30 °C wall temperature, the transcrystalline layer at the weld-line surface can be 0.2 mm–0.4 mm thick, according to polarised light microscopy of microtomed sections. This layer contains highly oriented lamellae and very few tie chains crossing the interface. The holding pressure applied after changeover reaches the weld line only if the core is not frozen; for high-fluidity LLDPE the core freeze time in a 2.0 mm wall at 30 °C is approximately 4–6 s, while the gate-freeze time may be 6–9 s under a 60 MPa holding pressure. If the packing pressure is too low or the holding time is shorter than the core freeze time, the central region contracts during cooling and a micro-void or notch develops at the weld line. The result is a reduction in notched impact strength of weld-line-containing plaques from 9 kJ/m² to 3 kJ/m² when tested at −30 °C according to ISO 179-1/1eA, compared with un-welded sections. These operational thresholds define a processing window of ±5 °C for the mould temperature and ±10 MPa for packing pressure at the weld line when the specification requires at least 55% weld-line tensile strength retention.
For a 240 mm × 180 mm × 2.0 mm rectangular slab injection-moulded from a 50 g/10 min high-fluidity LLDPE with a film gate along one short edge, the shrinkage measured after 24 h at 23 °C ± 2 °C and 50% ± 5% relative humidity is strongly anisotropic. The flow-direction shrinkage reaches 2.2% while the transverse-direction shrinkage is only 1.4%, producing an anisotropy ratio of 1.57. When the same slab is conditioned for 48 h at 80 °C, post-mould shrinkage increases flow-direction shrinkage to 2.7% and transverse-direction shrinkage to 1.8%, but the anisotropy ratio remains nearly constant at 1.50. This indicates that the principal anisotropy is locked in during filling and packing, not generated by post-mould relaxation alone. Differential scanning calorimetry at 10 °C/min under nitrogen shows a crystallinity of 42%–48% for the skin layer and 38%–42% for the core; the flow-induced oriented skin has a higher crystallinity and a higher linear thermal expansion coefficient perpendicular to the chain axis, which contributes to the measured directional difference. The shrinkage anisotropy is also influenced by gate type: a film gate produces a more uniform flow front and lower anisotropy than a point gate, while a tunnel gate with high shear at the gate can increase flow-direction shrinkage by 0.4–0.6 percentage points. Mould compensation factors for such high-fluidity LLDPE grades in production tooling are therefore specified as 1.8% parallel to flow and 1.2% transverse to flow, rather than a single isotropic value; these compensation factors are validated against shrinkage measured on ISO 294-3 plaques and on the actual moulded part after 72 h of ageing.
The standardised measurement of shrinkage anisotropy in injection-moulded thermoplastics is anchored to ISO 294-3, which specifies the mould cavity geometry for 60 mm × 60 mm × 2.0 mm plaques, and to ASTM D955-08 for parallel and normal-to-flow shrinkage after specified conditioning. The calibration of a production mould therefore begins with measuring the as-moulded plaque dimensions at 23 °C ± 2 °C after 24 h and again after 48 h, then computing the percentage dimensional change relative to the cold cavity dimension. For high-fluidity LLDPE, moulded dimensions are not the sole factor: part ejection temperature, post-ejection cooling fixture, and stacking of thin-wall parts can introduce additional anisotropy that is invisible in a free-standing plaque. When rectangular plaques are measured for a 50 g/10 min grade, the flow-direction shrinkage is commonly 1.9%–2.3% at a 20 °C mould temperature, while the transverse-direction shrinkage is 1.3%–1.5%; raising the mould temperature to 60 °C reduces the flow-direction shrinkage to 1.6%–1.8% and the transverse-direction shrinkage to 1.2%–1.4%. The corresponding anisotropy ratio therefore falls from approximately 1.5 to 1.3. The same grade moulded in a 200 mm diameter circular housing with a central gate can exhibit radial shrinkage of 1.6% but tangential shrinkage of 1.0%, causing an out-of-roundness of 0.3 mm to 0.7 mm over the diameter. Mould compensation factors must therefore be assigned separately for the flow and cross-flow axes, and the cavity dimensions must be verified with a coordinate measuring machine at 23 °C ± 1 °C. Published data for very thin sections below 1.0 mm are limited, but industrial experience indicates that anisotropy increases with decreasing wall thickness because of higher shear rates and more rapid solidification.
| Melt temperature | Mould temperature | Flow-direction shrinkage | Transverse-direction shrinkage | Anisotropy ratio |
|---|---|---|---|---|
| 200 °C | 20 °C | 2.35% | 1.55% | 1.52 |
| 200 °C | 60 °C | 1.90% | 1.40% | 1.36 |
| 230 °C | 20 °C | 2.10% | 1.50% | 1.40 |
| 230 °C | 60 °C | 1.70% | 1.30% | 1.31 |
A seven-cavity hot-runner tool for thin-wall food-container lids was operated with valve gates set to open sequentially to control air traps and move knit lines away from the sealing surface. The high-fluidity LLDPE grade had an MFR of 35 g/10 min and was run at 210 °C melt temperature, 30 °C mould temperature, 240 mm/s injection speed, and 60 MPa holding pressure. In the original tool, the lids exhibited visible weld lines near the hinge and a measured weld-line tensile strength retention of 44% relative to the un-welded lid material, as tested on specimens cut from the lids and tested at 50 mm/min under ISO 527-2. The sequential valve gating improved the retention to 58% by shifting the weld line to a thicker section and by reducing the amount of cold front that met at the hinge. However, the anisotropic shrinkage also shifted: the lids dropped from a circularity tolerance of 0.2 mm to an out-of-roundness of 0.6 mm because the sequential opening created a new flow-direction bias. The final processing solution required a mould-temperature increase to 45 °C, a packing-pressure increase to 70 MPa, and a holding time extension to 5.5 s. The resulting lids passed a seal-tightness test at 0.3 bar internal air pressure and showed weld-line strength retention of 66% while maintaining circularity within 0.25 mm. This production-scale example demonstrates that weld-line strength and shrinkage anisotropy cannot be optimised independently: moving the knit line to improve mechanical integrity changes the local orientation field and therefore the shrinkage direction and magnitude.
When a converter substitutes high-fluidity LLDPE for LDPE in injection-moulded thin-wall containers, the reduction in melt strength and the change in crystallisation behaviour alter both weld-line integrity and shrinkage anisotropy, even if the MFR values are matched at 190 °C. LDPE has long-chain branching that increases strain hardening during melt expansion and stabilises the advancing front, whereas high-fluidity LLDPE is substantially linear and exhibits a lower melt strength, so the front is more sensitive to fountain-flow instability and jetting. In a 1.5 mm wall container moulded on a 1,000 kN machine with a single top gate, the un-welded tensile strength of the LLDPE grade may be 14 MPa against 12 MPa for LDPE, but the weld-line tensile strength measured on specimens cut transverse to the front is only 6.5 MPa for LLDPE compared with 8.0 MPa for LDPE. The corresponding weld-line strength retention is 46% for LLDPE and 67% for LDPE under the same mould temperature of 35 °C. The shrinkage anisotropy is also more pronounced in LLDPE: flow-direction shrinkage increases from 1.6% for LDPE to 2.2% for LLDPE, while transverse-direction shrinkage increases from 1.2% to 1.4%, yielding an anisotropy ratio of 1.57 for LLDPE and 1.33 for LDPE. These differences imply that the mould compensation factors and the gate location cannot be copied from an LDPE tool when the material is changed to high-fluidity LLDPE. The replacement is nevertheless feasible if the melt temperature is raised by 10 °C to 15 °C, the mould temperature is increased by 15 °C to 25 °C, and the packing pressure is increased by 10 MPa to 20 MPa; otherwise the weld line becomes the limiting feature for drop-impact and seal performance.
Gate positioning around circular housings and rectangular bases frequently determines whether a weld line forms in a low-stress cosmetic area or in a load-bearing snap-fit. In a 160 mm diameter cylindrical container base moulded from a 25 g/10 min high-fluidity LLDPE, a central sprue gate produced radial flow and a single meld line at the last filling point on the rim. When the part was subjected to a 0.8 m drop test at −20 °C, failure occurred at the meld line in 80% of samples, even though the visual appearance of the meld line was inconspicuous. Moving the gate to an edge diaphragm created two head-on weld lines across the base, shifting the failure location but not eliminating it. The final tooling used a two-drop hot-runner system with valve gates positioned at 55% and 95% of the radius; this reduced the weld-line length from 160 mm to 45 mm and raised the drop impact survival rate to 90%. Shrinkage measurement after 24 h at 23 °C ± 2 °C showed radial shrinkage of 1.9% with the central gate but a maximum radial shrinkage of 2.6% near the gate and 1.7% at the rim with the two-drop system, producing a radial variation of 0.9 percentage points. This variation was corrected by changing the cavity insert dimensions by 0.8% in the gate region and 0.4% at the rim, a level of compensation that is only possible when the processor has prior shrinkage-anisotropy data from the same grade and the same wall thickness. The interaction between gate location, weld-line position, and shrinkage anisotropy therefore requires a tooling iteration, and the first-article dimensional inspection must include a coordinate-measuring-machine comparison of the flow and cross-flow axes.
Tensile property measurement on weld-line-containing specimens is most reproducible when the specimens are injection-moulded in a two-gate cavity that creates a head-on collision at the centre of the gauge length, then conditioned and tested according to ISO 527-2. The un-welded reference is moulded from the same cavity configuration with one gate blocked, so that the material, thickness, and thermal history are otherwise identical. For a 50 g/10 min high-fluidity LLDPE moulded at 220 °C and 60 °C mould temperature with 65 MPa packing pressure, the un-welded tensile strength may be 13.0 MPa to 14.5 MPa, while the weld-line strength remains at 7.8 MPa to 8.6 MPa, placing retention between 60% and 66%. If the same grade is moulded at 190 °C and 20 °C mould temperature, the weld-line strength falls to 6.2 MPa to 6.9 MPa, a retention of only 43%–49%. The dynamic packing phase is decisive because the pressure applied during the first 2–4 s after fill determines whether molten core material is forced across the knit line. A packing-pressure ramp from 40 MPa to 70 MPa over 1.5 s can improve weld-line tensile strength by 10–15% compared with a step change to the same final pressure, because the gradual ramp avoids premature gate freeze and allows the front to re-open under pressure. However, a packing pressure above 70 MPa on a high-fluidity LLDPE can produce flash in low-viscosity regions and can exacerbate flow-direction orientation, increasing the anisotropy ratio by up to 0.1. The operational boundary is therefore narrow, and the processing window around the optimum packing profile is approximately ±10 MPa for weld-line strength retention above 60%.
An eight-cavity thin-wall container tool running a 45 g/10 min high-fluidity LLDPE grade at 215 °C melt temperature and 40 °C mould temperature experienced intermittent brittle cracking at a hinge weld line during a 0.6 m drop test at 4 °C. The tool was equipped with sequential valve gates, and the cracking was initially attributed to inadequate gate timing. Cavity-pressure recordings revealed that the gate at the hinge position opened 0.4 s later than the adjacent gates, causing the hinge region to fill from two cold fronts that had already lost 18 °C relative to the main front. When the valve-gate timing was advanced by 0.35 s and the packing pressure at that gate was reduced from 65 MPa to 55 MPa, the weld-line strength retention increased from 41% to 63% and the drop-test failure rate fell from 35% to 5%. The same tool also showed a shrinkage anisotropy problem at the base rim: the flow-direction diameter shrank 1.9% while the cross-flow diameter shrank 1.3%, producing an ovality of 0.4 mm. This ovality was corrected by increasing the local cooling time at the rim from 3.0 s to 4.2 s and by adjusting the rim cavity insert in the cross-flow direction by 0.6%. The case confirms that high-fluidity LLDPE weld-line robustness and shrinkage anisotropy are controlled by gate timing, local cavity pressure, and cooling rate; published data for very high-MFR grades above 60 g/10 min in valve-gated multicavity tools remains limited, so processors must rely on direct cavity instrumentation for these configurations.
