Low-density polyethylene (LDPE) is a branched, semi-crystalline polyolefin with a density of 0.910–0.940 g/cm³, produced by high-pressure autoclave or tubular free-radical polymerization. The injection grades LDPE MG70 and 132I are specified for caps and closures where melt flow, gate design, hinge endurance, organoleptic neutrality, and environmental stress-crack resistance interact. MG70 is classified as a high-fluidity injection grade, while 132I occupies a lower-flow, higher-molecular-weight region of the portfolio. Selection between these grades is not governed by a single datasheet value; it is driven by the cavity fill pattern, the closure tether and hinge configuration, the target removal torque, and the regulatory boundary applicable to the packaged contents.

How Does Melt Flow Rate Shape Injection Pressure and Fill Time?

Melt mass-flow rate is measured according to ISO 1133-1:2022 or ASTM D1238-20 at 190 °C under a 2.16 kg static load. MG70 is conventionally associated with an MFR near 70 g/10 min, whereas 132I is typically reported in the range of 1.8–2.5 g/10 min; the numeric portion of 132I is a supplier product code and does not encode a universally standardized melt index. These rheological differences alter injection pressure demand. In a 120-ton hydraulic injection molding machine with a 35 mm screw diameter, a 0.8 mm-thick cap skirt can fill with MG70 at estimated injection-specific pressures of 60–90 MPa, while 132I may require 90–120 MPa under the same gate and runner configuration. Higher MFR reduces melt viscosity at gate shear rates of 10²–10³ s⁻¹, shortening fill time and permitting thinner wall sections. Lower-flow 132I provides greater melt strength and lower tendency for drool at open nozzles, but it demands wider gates, shorter flow lengths, or elevated melt temperature to prevent short shots.

Processing on a reciprocating screw machine with a general-purpose polyolefin screw, L/D 20:1–25:1, compression ratio 2.5:1–3.0:1, and a shut-off nozzle is standard for both grades. Barrel-temperature profiles typically begin at 160–180 °C in the feed zone and rise to 210–230 °C at the metering zone and nozzle. Melt temperature should remain between 180 °C and 220 °C for thin-wall closures. LDPE does not require drying under normal closed-loop dry-air conveying at relative humidity below 60%; if surface condensation is observed on cold pellets in humid plants, pre-drying at 60–70 °C for 1–2 h is applied to prevent splay. Mold temperature is usually held between 10 °C and 35 °C. Lower mold temperatures reduce cycle time but increase flow-direction shrinkage and anisotropic residual stress. Cooling time for a 1.2 mm nominal wall closure is often 4–8 s, limited by gate seal and ejection robustness rather than full crystallization because LDPE crystallinity is only 40–55%.

The values in the table are representative class ranges compiled from supplier technical bulletins and are not a certificate of analysis. Lot-specific MFR, density, and additive package must be verified from the producer’s release documentation before tool transfer or process validation.

Thermal Degradation Pathways and Gel Formation in Hot Runner Systems

Multi-cavity closure molds, typically configured with 24–48 cavities for polyolefin caps, use valve-gated hot runner systems to reduce cold-runner scrap. In hot runner manifolds, LDPE melt residence time at 220 °C should remain below 10 min. Longer hold times at temperature promote chain scission, peroxide-assisted crosslinking, and gel-particle formation. Gel particles accumulate at gate inserts and cause gate blush or intermittent pinning. For MG70, the higher MFR permits lower nozzle temperatures, often 200–220 °C, which lowers degradation risk. For 132I, the higher melt viscosity may force nozzle and hot runner set points toward 220–230 °C, reducing the thermal safety margin. In both cases, thermocouple calibration drift greater than ±2 °C across hot runner zones can produce localized stagnation and black specks. Operators monitor purged melt at start-up for gels larger than 0.10 mm; visual inspection is supplemented by pressure-drop trend data from the machine transducer. A rising pressure drop across the hot runner at constant throughput indicates progressive gate blockage rather than a feedstock change.

Gate design is the main variable separating successful MG70 processing from 132I processing in closure applications. High-flow MG70 can fill through edge gates of 0.5–0.8 mm diameter, producing minimal gate vestige on the cap skirt. Lower-flow 132I often requires gate diameters of 0.8–1.2 mm and wider fan gates to prevent jetting and surface flow marks. In multi-gated caps, weld lines form where melt fronts merge at the hinge or tether region. The durability of a closure hinge under repeated flexing is not governed by a universal ISO test; manufacturers typically apply product-specific cycling tests at 23 °C and 45% relative humidity, often cycling the hinge between 200 and 500 cycles. Published data for this specific configuration is limited, so validation on production tooling is required. Melt temperature at the leading edge of the weld line is the strongest processing predictor of hinge failure: a drop below 180 °C at the weld line produces visible V-notch weakness. Molders consequently raise injection velocity for 132I rather than increasing melt temperature alone.

When Environmental Stress Crack Resistance Governs Material Selection

Environmental stress-crack resistance is evaluated by ASTM D1693, using a bent notched specimen immersed in 10% Igepal CO-630 at 50 °C. The test reports F50 time for 50% of specimens to fail. LDPE injection grades generally show better ESCR than higher-crystallinity high-density polyethylene because the branched structure concentrates tie molecules and reduces crystalline lamellae thickness. Within the LDPE portfolio, lower-flow 132I is expected to give a longer F50 than high-flow MG70 when both are tested on compression-molded plaques. This does not make 132I universally superior: high-flow MG70 is preferred for snap-on closures and overcap tapers that require long flow lengths in thin sections, while 132I is selected for aggressive liquid packaging where cap hoop stress remains elevated during storage. Closure torque retention and sealability are application-specific; no universal ISO procedure exists for all geometries, so test development must reproduce the bottle finish dimensions and the container’s top-load behavior.

Food-contact compliance is evaluated under FDA 21 CFR 177.1520(c) for olefin polymers and under European Commission Regulation 10/2011, where overall migration limits are 10 mg/dm² or 60 mg/kg of food simulant. LDPE closure grades are usually supplied with compliance statements for aqueous, acidic, and fatty food simulants, but converters must verify the additive package, color concentrate, and regrind ratio do not raise migration above the specified limit. REACH Annex XVII restricts phthalates in plasticized articles; LDPE caps are typically unplasticized, but recycled content can introduce restricted substances. RoHS Directive 2011/65/EU applies to electromechanical packaging and restricts lead, mercury, hexavalent chromium, PBB, and PBDE at 0.1% by weight in homogeneous material, with cadmium at 0.01%.

In closure manufacturing, batch-to-batch MFR variation of ±3 g/10 min for MG70 and ±0.5 g/10 min for 132I can shift fill time by 0.2–0.5 s in thin-wall overcaps. On a 150-ton electric injection machine, operators observe short shots when the as-delivered MFR falls at the lower specification limit and cavity pressure drops below 35–45 MPa at the end of fill. Higher back pressure of 0.5–1.5 MPa improves melt homogenization but reduces recovery capacity and increases melt residence time. Screw-recovery time should be longer than cooling time to prevent starving the hot runner; for closure cycles of 6–10 s, recovery time is typically set to 4–7 s with a screw speed of 100–150 rpm for 35 mm general-purpose screws. Process stabilization is achieved through cavity-pressure transfer rather than timer-based transfer.

Process Control Limits and Incompatibilities for LDPE Closure Grades

Melt temperature above 230 °C is the primary operational boundary for both grades. At these temperatures, oxidative degradation accelerates even with antioxidant stabilizers, and the melt may develop an odor that persists in the closure headspace. Molders should not raise barrel temperatures as a substitute for proper gate sizing with 132I. The acceptable melt-temperature window is wide by polyolefin standards, but the practical upper limit for prolonged operation is 220–230 °C. Regrind use is common at ratios up to 20% by weight, but dryer or color additive changes can alter crystallization and dimensional stability. Avoid combining these LDPE grades with high levels of peroxide masterbatches in closed-loop regrind streams because free-radical residual activity can initiate long-chain branching and shift the MFR unpredictably. Silicone-based mold release agents, if used excessively, can interfere with cap print adhesion and with ultrasonic welding of fitments; concentrations should be controlled below the supplier’s recommended range for polyethylene closures.

Post-mold shrinkage for LDPE closures proceeds over 24–48 h at ambient storage. Dimensional checks performed immediately after ejection may not reflect the final fit on bottle finishes. For a 28 mm closure, radial shrinkage of 0.5–1.2% can differentiate a seal that passes initial torque from one that loosens after pallet stacking. Molders of MG70 observe faster stress relaxation and lower part internal stress, but tooling must compensate for higher overall shrinkage from low melt viscosity. For 132I, stress relaxation is slower, and molded-in stress near the gate can remain elevated. Ejection force is higher for 132I because of greater melt strength and deeper undercut engagement. Ejector pins and stripper plates should be sized to avoid deflection greater than 0.03 mm on the cap rim. Where packers observe ovality after destaticization, the cause is usually uneven cooling in the core rather than a deficiency in grade selection. The final process window for either LDPE MG70 or 132I is therefore established on the production tool with cavity-pressure sensors, not from laboratory plaque data alone.