MOPLEN PP HP552R is a high-flow polypropylene homopolymer supplied under the Moplen trade designation by LyondellBasell. It is specified for thin-wall injection molding, high-cavitation closures, and rigid packaging where melt fluidity and stiffness govern tooling productivity. The manufacturer’s technical data sheet lists a nominal melt flow rate of 25 g/10 min when determined at 230 °C with a 2.16 kg load in accordance with ISO 1133-1:2022. The density is reported as 0.900 g/cm³ under ISO 1183-1:2019. As a homopolymer, the backbone contains no ethylene comonomer; the crystalline fraction and flexural modulus are therefore higher than in random copolymers, while sub-ambient impact and optical transmission are lower. The grade belongs to the controlled-rheology class, in which post-reactor chain scission is used to raise MFR and narrow molecular weight distribution. This structural difference reduces melt elasticity and die swell, improves dimensional control, and requires tighter control of melt residence time during processing.

What Are the Core Property Boundaries and Standards Codes?

The following values are typical manufacturer-reported properties, not batch specification limits. They should be reconfirmed using the listed methods on critical lots because controlled-rheology grades can exhibit viscosity drift with uncontrolled regrind addition.

Processors should treat the notched Izod values as the limiting mechanical property. At 23 °C, the homopolymer absorbs approximately 2.0 kJ/m² under ISO 180/A:2020; at 0 °C the value falls to 1.2 kJ/m². Applications with drop loads near 0 °C should not be specified without end-use impact testing. The grade is not a substitute for impact copolymer grades in deep-freeze containers, luggage shells, or structural battery cases. Controlled-rheology grades may show batch-to-batch MFR variation of ±2 g/10 min; processors should request certificate of analysis values and use capillary rheometry to ISO 11443:2021 for gate sizing.

Thermal Degradation Margins and Screw Recovery Limits in Thin-Wall Tooling

On production injection molding machines with three-zone general-purpose screws of 18:1 to 24:1 L/D ratio, the melt temperature measured at the nozzle is maintained between 220 °C and 260 °C. Barrel set point selection should not be used as a substitute for direct melt pyrometry. At temperatures above 280 °C, oxidative chain scission accelerates; the resulting viscosity drop produces inconsistent fill weight, weak knit lines, and yellowing, particularly in hot-runner systems with prolonged residence. Mold temperature is typically set between 20 °C and 50 °C using water-cooled or oil-heated circuits. Raising mold temperature from 20 °C to 40 °C improves surface gloss and relaxes oriented skin stresses but may add 1.5–3.0 s of cooling time in wall sections above 1.5 mm. For thin-wall packaging below 0.8 mm, the cooling-time penalty is much smaller and mold temperature control is used primarily to reduce warpage. Injection velocity is normally set between 100 mm/s and 250 mm/s. At speeds above 300 mm/s, the risk of jetting, gate blush, and flashing at vented parting lines increases, especially in center-gated cup tools without flow inversion.

Gate sizing for HP552R should be based on apparent shear rate at the gate, not solely on part mass. For a 0.8 mm wall cup with a center gate, gate diameter should be 60–80% of wall thickness to limit shear rate to 1000–2000 s⁻¹; above 3000 s⁻¹, melt fracture and micro-fissures can appear. The exact shear-rate limit is influenced by melt temperature and additive package. Hot-runner systems with internal valve pins require a minimum holding pressure of 50–80 MPa hydraulic to maintain pin closure; if the holding pressure decays below that range, stringing and gate drool occur. Screw recovery should be completed without excessive back pressure; values of 0.5–2.0 MPa are typical, with higher back pressure increasing melt temperature and reducing MFR stability.

Pre-drying is not mandatory for the base resin under normal dry conditions. When ambient relative humidity exceeds 60%, or when the material is blended with hygroscopic masterbatches, drying at 80 °C for 2–4 h in a dehumidifying dryer prevents splay and surface silver streaks. Uncontrolled regrind addition above 30% can shift the effective MFR upward by repeated thermal shear history and alter the packing-pressure response; regrind levels should be held constant to maintain consistent cushion and part weight. The grade is incompatible with high loadings of low-molecular-weight external lubricants, which can produce screw slippage and variable melt temperature. Amine-based additives are not a known incompatibility for polypropylene homopolymer, but high levels of certain hindered amine light stabilizers may affect color and long-term oxidative stability if not predispersed.

When Low-Temperature Impact Is Not the Controlling Variable in Packaging

In dairy cups, delicatessen containers, overcap systems, and disposable housewares, the primary failure modes are top-load deformation, stacking stability, and cap stripping torque rather than cold-temperature fracture. HP552R is used where a higher MFR permits wall sections below 0.6 mm without short shots and without excessive injection pressure. Under ISO 75-2:2013 at 0.45 MPa, the grade retains heat-deflection temperature near 95 °C; this supports hot-fill or microwave reheat applications where the short-term surface temperature may reach 80–90 °C. However, the notched Izod impact at -20 °C falls below 1.0 kJ/m², so the grade should not be selected for frozen-food tubs or ice-cream containers that undergo drop loading. This distinction, rather than MFR alone, determines whether a homopolymer or an impact copolymer is appropriate.

Thin-wall injection molding of dairy cups at wall stock 0.5–0.8 mm benefits from the low shear viscosity. In such tools, short shots are generally avoided by increasing injection velocity before increasing melt temperature, because shear heating at the gate can produce local temperatures above 270 °C and accelerate degradation. For closures with continuous thread features, the low die swell permits consistent thread fill and sealing diameter. In multicavity cap molds, valve-gated hot runners are used to prevent stringing; the narrow molecular weight distribution of HP552R reduces gate vestige variation across cavities. Housewares and DVD packaging use the grade where rigidity and scratch resistance are valued more than optical clarity. The material is not intended for deep-freeze impact or for clear contact packaging; for those applications, an ethylene random copolymer or an impact copolymer is preferred.

Comparative Positioning Against Lower-Flow and Copolymer Grades

Relative to a lower-flow homopolymer control with MFR near 12 g/10 min, the 25 g/10 min grade lowers fill pressure in thin-wall tools. At a melt temperature of 230 °C and apparent shear rate of 1000 s⁻¹, the apparent viscosity is typically 25–40% lower; the exact shift is tool-specific and must be confirmed by capillary rheometry to ISO 11443:2021. The lower-flow homopolymer retains a higher notched Izod impact of 2.5 kJ/m² at 23 °C under ISO 180/A:2020, and is preferred for thick-wall industrial parts with longer flow paths and fewer cavity numbers. Compared with ethylene-random copolymers of similar MFR, HP552R exhibits flexural modulus near 1600 MPa under ISO 178:2019, whereas random copolymers typically fall between 800 MPa and 1000 MPa. The stiffness advantage is accompanied by lower optical transparency and a greater loss of impact at 0 °C; notched Izod values for random copolymers in that range are often 5–10 kJ/m², so HP552R is not recommended where cold-chain impact or flexural hinges are required.

For food-contact parts, the manufacturer’s regulatory documentation cites FDA 21 CFR 177.1520 and European Union Regulation (EU) No 10/2011 for polyolefin food-contact materials. These citations refer to the base polymer; the finished article must be tested for overall and specific migration using the intended food simulant and contact conditions. Compliance under EU 10/2011 requires documentation of additives, colorants, and processing aids. For electrical or electronic accessory applications, conformity to RoHS Directive 2011/65/EU is typically provided by the supplier through a declaration. REACH registration in the European Union is maintained by the manufacturer. The grade is not supplied with a specific UV-stabilization package for prolonged outdoor exposure; outdoor applications require additional hindered amine light stabilizer and ultraviolet absorber masterbatch.

Mold shrinkage for HP552R is reported in the range 0.8–1.2% under ISO 294-4:2018, with the higher values occurring in the flow direction for oriented thin-wall parts. Tooling compensation should account for shrinkage anisotropy; for a round closure, the diameter may shrink less than the sidewall thickness, so pilot-tool validation is required. Color concentrates, especially phthalocyanine blue or green, can nucleate crystallization and increase mold shrinkage by 0.1–0.3%. The coefficient of linear thermal expansion is approximately 1.0–1.5 × 10⁻⁴ K⁻¹ under ISO 11359-2:2021; this creates a post-mold dimensional change of 0.1–0.3% when parts are annealed or exposed to 60 °C. Water absorption is below 0.03% after 24 h, so dimensional change due to moisture is negligible. Chemical resistance is typical for polypropylene homopolymer: resistance to aqueous acids, alkalis, and polar solvents is good, but chlorinated hydrocarbons and hot oxidizing acids cause swelling or surface attack.

Closure systems with continuous thread features and integral tamper-evident bands are a high-volume application for HP552R. The material’s low die swell and controlled rheology permit consistent gate vestige height and thread fill. Dimensional tolerance on the closure diameter after mold shrinkage is typically held to ±0.05 mm when tooling compensation for 0.8–1.2% linear shrinkage under ISO 294-4:2018 is applied. Color concentrates containing high levels of phthalocyanine pigments can further increase mold shrinkage by 0.1–0.3%; tooling trials are required to verify roundness and sealing-torque loss. In hot-runner tools, valve-gate sequencing must be timed to avoid flow hesitation in adjacent cavities; failure to balance the hot runner below 10 °C temperature variance causes cavity-to-cavity shrinkage variation and cap ovality. Published data for this specific configuration is limited, but the processing sensitivity is consistent with measured molecular weight distribution.